1. Preface
2. Executive Summary
3. Introduction and Overview
4. Goals and Mechanisms of Plant Breeding for Resistance or Tolerance to Pests
5. The Scope of Plant Biotechnology
6. Interpretations of the Term "Pesticide"
7. Pest Defense Mechanisms in Plants: Current State of Knowledge
Plant Pathogens: Fungi, Bacteria, and Viruses
Arthropods: Insects and Mites
Plant Parasitic Nematodes
8. Equivalency and Conservation of Pest-Defense Mechanisms Within and Beyond the Plant Kingdom
9. Effects of Plants on Associated Flora and Fauna in the Environment
10. Potential for New Environmental or Dietary Exposures Due to Genetic Modifications
11. Procedures in Use to Assure Safety of New Crop Plant Varieties to People and the Environment
Responsibilities of the Plant Breeder
Plant Variety Peer Review
Opportunities for Strengthening Current Procedures
12. Federal Oversight of New Crop Varieties
USDA Review
FDA Review
EPA Review Based on Proposed Rule
13. Principles for Appropriate Oversight
14. References
15. Appendix 1 -- List of participating scientific societies that responded to EPA
16. Appendix 2 -- Participants in January 29-30, 1996, workshop and their professional society affiliations and contact addresses
17. Figures
Box 1
Box 2
18. Footnotes

Crop plants and plants used in gardens and for recreational and aesthetic purposes have been subject to genetic manipulation by plant breeding methods for more than 100 years. One of the great achievements of plant breeding has been the development and distribution of plant varieties resistant to pests, including viruses, bacteria, fungi, nematodes, insects, and mites. Crops protected by resistance genes reduce the need for pesticides and are generally uniformly protected throughout the life of the crop. Plant breeding has relied upon several methods to exploit genetic resources for pest resistance, including selection of resistant plants from varieties or farmer's landraces, from hybrids obtained from resistant and susceptible parents, and from populations after treatment with chemical or physical mutagens.

The modern era of molecular biology has given plant breeders a new tool, genetic transformation, that provides the means to capture genes from sources beyond the realm of immediate progenitors and near-relative species. The ability for plant breeders to use "any gene from any species" is nearing reality. Obviously, the genetic resource base for crop improvement has been greatly expanded. At the same time, genetic transformation can be used to transfer genes from one variety to another within the same species with much greater precision than by traditional methods of selection following sexual hybridization. Because of the overwhelming record of safety, there has been little concern for plant varieties developed by modern plant breeders and the hundreds of generations of farmers before them. The new methods of gene transfer, and especially the potential for introduction of genes into crops from sources unrelated to the crop, have raised questions about the safety of new crop varieties to consumers and the environment. Assurance of public and environmental safety has therefore become a matter for the federal and state governments with oversight of new products of plant breeding.

This report was prepared in response to a proposed rule by the U.S. Environmental Protection Agency (EPA) under authority of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), namely, that the substances produced by plants for their defense against pests, and the necessary genes, are "plant-pesticides" subject to regulation under FIFRA. The EPA- proposed policy and rule, announced November 23, 1994, was open for public comment. Representatives from eight professional scientific societies that participated in drafting this report provided point-by-point statements individually to EPA during the comment period. These societies are listed in Appendix 1. The concern in the scientific community for the implications of the proposed rule was uniformly so great that it was agreed that a consortium of scientific societies should prepare a scientific review of the proposed rule, focusing on the definition of "plant-pesticide" and the implications of this definition. An informal workshop was held January 29-30, 1996, in Washington, DC, by representatives of the scientific societies mentioned in Appendix 2. It was agreed that the assembled scientists at the workshop would prepare an analysis of the EPA proposed rule as well as the status of scientific understanding of the mechanisms of defense used by plants against pests. This report is a product of that effort. It has been reviewed by the elected leadership of each of the collaborating societies. We do not claim that this report is endorsed by all of the approximately 80,000 members of the societies represented, but we hope that the scientific integrity of the report will be accepted as a step toward rationale oversight of new products for food and agriculture. We also hope that research and development will be stimulated to continue the advance of biological understanding of plant pest resistance and the role of this phenomenon in the quest for sustainable and environmentally friendly agricultural and other uses of plants.

We extend our special appreciation to Joyce Nettleton (Institute of Food Technologists) and Robert Barnes (American Society of Agronomy) for their support in arranging the workshop and to Linda Murphy (American Society of Microbiology) for making conference facilities available. Thanks also to Keith Menchey (AESOP) for his support in facilitating the meeting and Patrick E. McGuire (California Genetic Resources Conservation Program) for technical editing.

The following persons are the authors of this report:

R. James Cook
Calvin O. Qualset
Workshop co-chairs and principal report editors
July 1996

Historically, plants have not been subjected to risk assessment and management in the manner applied to pesticides. Oversight for the safety of food, including food from new plant varieties developed with the aid of recombinant DNA (rDNA) techniques (genetic engineering), is provided by the Food and Drug Administration (FDA). Similarly, oversight for protection of the environment from the release of plants genetically modified by rDNA techniques is provided by the U.S. Department of Agriculture (USDA).

To complement and extend this regulatory framework, the Environmental Protection Agency (EPA) proposed, on November 23, 1994, to regulate the inherited traits of plants that confer resistance to pests under statutes developed for chemical pesticides: Proposed Policy: Plant-Pesticides Subject to the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and the Federal Food, Drug and Cosmetic Act (FFDCA). Based on this proposal, the substances produced by plants for their defense against pests and diseases, together with the necessary genes for production of these substances, would fall under a new category of pesticide termed "plant-pesticide," subject to the statutes of FIFRA and FFDCA.

Concern among plant, food and microbiological scientists for the scientific basis and unnecessary regulatory burdens of this proposed policy was so great that a consortium of scientific societies convened a group of scientists to examine the scientific basis of the proposed rule and develop principles for appropriate oversight of the inherited traits in plants for resistance to pests and diseases. Plant breeding has been used with great success for most of this century to develop plant varieties with inherited traits for resistance to pests, including to viruses, bacteria, fungi, nematodes, insects and mites, with an overwhelming record of safety.

Eleven scientific societies representing more than 80,000 members endorsed the consortium report. Each of these societies shares a common mission -- to disseminate scientific information. The scientists representing these societies bring diverse knowledge and experience about plants, foods from plants, plant pests and diseases, plant defense mechanisms, and techniques to develop and use new disease- and pest-resistant varieties of plants for agriculture, forestry, landscapes, gardens, and other environments.

Scientific Concerns

The primary scientific concern with the proposed rule is the creation of a new category of pesticide, called "plant-pesticide," solely for the purposes of regulation under existing statutes. EPA proposes to designate as "plant-pesticides" all substances responsible for pest resistance in plants, as well as the genes needed for production of these substances. Under its proposed policy, however, EPA singles out for possible registration as "plant-pesticides" only those traits introduced into plants using rDNA techniques. This happens as follows: under the statutes of FFDCA, the EPA establishes tolerances for the formulations and intended uses of pesticides. Under the new category of "plant-pesticide," EPA proposes to exempt from requirement for a tolerance those pest-defense substances and the genes necessary for the production of those substances, if they evolved naturally or were transferred to the plant by traditional plant breeding methods. Thus, tolerances for "plant-pesticides" would be established specifically for pest-defense traits that could not be transferred to the variety by traditional breeding methods and were therefore transferred to the variety using rDNA methods. A tolerance would also be required for varieties of plants developed by traditional breeding methods if one of the parents carried an inherited trait for pest resistance originally transferred to that parent by rDNA methods.

The consortium reached the following conclusions:

• It is scientifically indefensible to regulate the inherited traits of plants for pest and disease resistance under statutes developed specifically for chemical pesticides applied externally to plants;
• All plants are able to prevent, destroy, repel or mitigate pests. Further, all plants are resistant to most potential pests (susceptibility is the exception), although the actual mechanisms of pest defense are complex and the roles of any specific substances remain largely unknown;
• While pest resistance can be determined by specific genes, the ability to respond to and resist pests is a characteristic of the plant and cannot be separated for regulatory purposes from the plant itself; and
• Evaluation of the safety of substances in plants should be based on the toxicological and exposure characteristics of the substance and not on whether the substance confers protection against a plant pest.

Economic and Public Policy Concerns

The consortium recognizes the role of the federal government in assuring environmental and public safety of plants new to the environment and of plant products used as food. However, the EPA proposal designating the inherited traits of plants for resistance to pests as "plant pesticides" would, if implemented, have several negative consequences for agriculture and consumers. The EPA proposal will:

• Erode public confidence in the safety of the food supply by sending the message that all plants contain "pesticides;"
• Discourage the development of new pest-resistant crops, thereby prolonging the use of synthetic chemical pesticides;
• Increase the regulatory burden for those developing pest-resistant varieties of crops, while also increasing federal and state bureaucracy;
• Limit the use of rDNA technology for the development of pest-resistant plants to those applications that can pay the increased costs associated with additional regulation, and deny the benefit of this technology to applications for niche markets likely to be developed by small companies and public plant breeding programs;
• Handicap the United States in competition for international markets because of U.S. government policy that new pest-resistant varieties, or products from these varieties, be identified as containing their own "pesticides;" and
• Limit the use of valuable genetic resources and new technologies to improve crop protection from pests and disease.

Several principles are appropriate to guide federal oversight for novel kinds of plant varieties intended for use in agriculture, gardens, urban landscapes, or other managed ecosystems:

• Federal oversight of plants should be based on accepted standards of practice for recombinant DNA research and field testing of plants. These include nongovernmental peer review and recommendations/guidelines provided by the U.S. National Academy of Sciences, the Organization for Economic Cooperation and Development, and the U.S. Department of Agriculture;
• Regulatory oversight should focus on high-probability risk rather than hypothetical or unrecognizable risk and should be sufficiently flexible to keep pace with new scientific developments;
• The level of risk of a plant variety to the environment or human safety is determined by the characteristics of the plant, not by novelty or initial lack of familiarity, the source of the gene(s) that produce a pest-defense substance or initiate a pest-defense reaction, or the method by which a gene for pest defense is transferred into the variety;
• Genes and the substances encoded by them that confer resistance characteristics to plants are not the equivalent of "pesticides" as defined by FIFRA; and
• GRAS, the FFDCA concept of "generally regarded as safe," applies only to foods and food additives, but is an appropriate concept for environmental risk management. Mechanisms should be developed for conferring the environmental equivalent of GRAS status to new varieties of plants as we gain experience and familiarity.

The major recommendations of the report are:

• Ensure that federal oversight for plants with inherited traits for resistance to pests is based on the Principles for Oversight stated in this document;
• Strengthen and make greater use of non-regulatory oversight mechanisms developed for plants, such as plant variety review boards and scientific peer review, that have served effectively during this century; and
• Simplify the federal oversight of plants genetically modified to express novel traits, including those for pest defense, to be consistent with the risk-based policies of the FDA for food safety and the USDA for environmental protection.

On November 23, 1994, the U.S. Environmental Protection Agency (EPA) proposed a new policy and rule (EPA, 1994), hereafter referred to as the EPA-proposed rule, to establish a rationale and procedures for the regulation of inherited traits expressed in plants for pest resistance. EPA proposes to derive its regulatory authority from the broad definition of pesticide contained in the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), namely: "(1) any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest, and (2) any substance or mixture of substances intended for use as a plant regulator, defoliator or desiccant."

EPA has proposed that all substances produced by plants for their protection against pests, together with the necessary genes for production of these substances, fall under a new category of pesticide termed "plant-pesticide." A "plant-pesticide" is defined in the proposed rule as "a pesticidal substance that is produced in a living plant and the genetic material necessary for the production of the substance, where the substance is intended for use in the living plant."

The report Field Testing Genetically Modified Organisms: Framework for Decisions (NRC, 1989a) presents an excellent description of the historical and current processes of plant genetic modification for the improvement of desired traits such as resistance to pests (Footnote 1). For centuries, farmers, gardeners, and later plant scientists have modified plants, through a wide variety of procedures, to develop and select new pest-resistant phenotypes (Footnote 2). In each case the selection was based on field observations or assays but did not define or determine the basis of resistance. Genes for pest resistance have been identified by appropriate genetic tests and incorporated into the genotypes of virtually every major food and fiber crop and most ornamental and garden plants grown in the United States and around the world today. In nearly every case the mechanism of action of these genes remains largely unknown or obscure.

People have consumed food products derived from plants expressing genes for pest resistance with confidence that the products are safe. Similarly, farmers, foresters, gardeners, and other practitioners have planted and cared for these plants with the confidence that they are at least as safe and usually safer than their wild relatives and with no known or measurable effect on environmental quality due to these added pest-resistance genes. Even though FIFRA has been in effect since 1947, no government agency, including EPA, has demonstrated any prior interest in regulating the inherited traits of plants for pest resistance as pesticides.

The EPA-proposed rule includes in its definition of "plant-pesticides" the inherited traits naturally present in the plant or transferred to plants by traditional plant breeding, including resistance expressed by a nontoxic mechanism, but it exempts those traits from requirement for tolerance under FFDCA. Apparently, the intent is to regulate only those pest-defense traits conferred by genes deployed using recombinant DNA (rDNA) techniques, even though these techniques are least likely to introduce extraneous genes into the new plant variety. The EPA- proposed rule is based on the assumption that traits transferred from outside the normal range of sexual compatibility of the recipient plant will increase the likelihood of novel exposures or hazards to human health and the environment, even though the same resistance gene or a homologue (Footnote 3) may be present in other crops used as food. This would include, for example, the transfer of resistance genes from tomato to corn to protect corn.

Two critical questions are raised and addressed in this report:

• Does the body of scientific information on genes and their products for pest defense, when transferred from outside the normal range of sexual compatibility of the recipient plant, suggest an increase in the likelihood of significant new hazards or exposures to humans or the environment?
• Does the body of scientific information on the genetic and biochemical mechanisms of plant defense against pests support the EPA proposal that these genes and substances produced in plants having these genes, regardless of whether these traits are naturally present or transferred by plant breeding, pose unreasonable risks and therefore are "plant-pesticides?"

• There is no evidence that unique hazards exist either in the use of rDNA techniques or in the movement of genes between unrelated organisms;
• The risks associated with the introduction of rDNA-engineered organisms are the same in kind as those associated with the introduction of unmodified organisms and organisms modified by other methods; and
• Assessment of the risks of introducing rDNA-engineered organisms into the environment should be based on the nature of the organisms and the environment into which they are introduced, not on the method by which they were produced.

Given: (1) that the principles set forth in the 1987 NAS and 1989 National Research Council (NRC) reports still hold; (2) that additional formal federal regulation of genes for pest resistance in plants was not considered necessary before the widespread utilization of rDNA techniques; and (3) that available information on the genetic and biochemical mechanisms by which plants do or can be made to defend themselves, demonstrate no known or undue risks to people or the environment; therefore, we cannot support the fundamental premise of the EPA- proposed rule which is that substances that plants produce for their protection against pests, and the genetic material necessary to produce these substances, are pesticides subject to oversight separate from the plant itself.

This report has been developed through the efforts of representatives of 11 scientific societies, with more than 80,000 members having expertise and experience in the scientific and practical aspects of growing, breeding, protecting, and using plants for food, fiber, ornamental, and many other purposes. The new tools of biotechnology provide the means to develop plants for many kinds of uses heretofore not possible or possible only with great difficulty using conventional techniques. The report is focused on genes conferring pest resistance to plants, but the principles apply for any trait conferred by gene transfer to the plant.

Breeding for pest resistance has been a time-honored tradition in crop improvement, and is one of the primary reasons why agricultural productivity has reached unprecedented levels in the world today. Historically, public-sector plant breeding programs have produced the majority of crop varieties grown in the United States, especially of perennial crops and self-pollinated annual crops, and private-sector plant breeding programs have produced mainly the hybrid seeds and some vegetable seeds. The components of a typical plant breeding program are outlined in Box 1.

Genetic resistance as a pest-control strategy is a desirable developmental approach of plant breeding because resistant varieties provide a cost-effective alternative to pesticides. Control of certain pests in some crops, such as the small grains (wheat, barley, millet, etc.), is not economical by chemical pesticides because of the relatively low per-acre value of the crop. In addition, a pest-resistant variety generally provides more uniform plant-to-plant protection throughout the life of the plant than protection realized by pesticide applications. In some cases, the use of resistant varieties has even resulted in the reduction of the pest population for a particular crop, e.g., Hessian fly in wheat (NRC, 1989a). Other advantages of innate resistance in plants to their pests include: (1) it is expressed and inherently present as the plant grows; (2) it is not known to contribute to environmental pollution; (3) except for a very few well-documented cases, it poses no hazard to consumers of plant foods and feeds; and (4) it provides a means of control for problems for which chemical control is not possible, e.g., viruses.

The successes in breeding for plant resistance have been achieved despite the fact that often little or nothing is known about the biochemical, physiological, and/or morphological mechanisms that underlie the genetic resistance. Many pest resistance traits are attributed to genes that have been mapped to specific sites on specific chromosomes. Although these genes have been in use and the resistance they confer has remained effective for years, their functions in terms of gene control, mode-of-action, and expression product have remained largely uncharacterized.

Incorporation of genes from alien germplasm (Footnote 4), including from non-food sources, has been used for many decades with economically beneficial results. Langford (1937) and Bohn and Tucker (1940) were apparently the first plant breeders to use a wild species to transfer disease resistance into a crop variety. In both cases, resistance to fungal pathogens of tomato was transferred from Lycopersicon pimpinellifolium into the cultivated tomato L. esculentum. The intergeneric transfer of resistance to wheat leaf rust, mediated by X-ray irradiation, was reported in the early 1950s (Sears, 1956). Since then, with steady incremental improvements and refinements in hybridization and tissue culture methodologies, resistance genes have been transferred from an increasingly wide range of genera and species to cultivated crops (Goodman et al., 1987; Vaeck et al., 1987). None of the derived cultivated varieties has subsequently proven harmful to people, livestock, or the environment. Nor have increased risks been incurred from varieties developed through mutational techniques such as ionizing radiation, chemical mutagens, or somaclonal variation.

One of the safeguards in breeding pest-resistant plants is that the very process of domestication substantially reduces the probability that a cultivated plant will become an environmental hazard. Common features associated with the domestication of plants result in plants that are less, rather than more, capable of survival outside of the grower's field. These features include reduced seed dormancy, reduced seed dispersal mechanisms, more synchronous ripening, greater dependence upon a managed environment, including soil fertility and irrigation, and larger seeds. Thus, while pest-resistant varieties may have enhanced survival capabilities as crop plants by virtue of resistance, most other traits introduced in the breeding process make them poor competitors outside of the agriculture production arena for which they are intended. Another safeguard in the breeding process is the extensive testing and review of varieties prior to the release to growers for field production (see "Procedures in Use to Assure Safety of New Crop Plant Varieties to People and the Environment").

Crop plant improvement through plant breeding seeks to introduce new genetic variation into plants to make them more tolerant to an environmental stress or more resistant to some pests (see Box 2). Plant breeding also seeks to introduce new genetic variation into plants to produce new food or non-food products, enhance yield, or to make plants or the harvested products more attractive, uniform, safer, or more nutritious when consumed as food. The extensive safety record of the products of plant breeding has meant that food from these plants has been generally recognized as safe (GRAS) according to FDA standards, and there has been little need for the formal regulation of plants apart from the approval process invoked by the developers themselves, such as the public sector State Agricultural Experiment Stations and USDA Agricultural Research Service and the private sector plant breeding companies (see "Procedures in Use to Assure Safety of New Crop Plant Varieties to People and the Environment").

Until the advent of rDNA techniques, plant breeders were limited to selecting spontaneous or induced mutants or the progeny of hybrids for new combinations of genes resulting in plants superior to existing varieties or their progenitors. Because of the constraints of sexual compatibility, the sources of new traits introduced by hybridization are those from closely related species or genera where cross pollination results in the formation of offspring that can be used for further breeding and multiplication. In spite of these limitations, plant breeding has made enormous contributions toward supporting human food needs through increases in yield, improvements in quality of the harvested product, and the introduction of genes that confer tolerance or resistance to a range of environmental constraints that include pests and diseases.

New genetic information can now be introduced into plant cells from virtually any other organism by any one or a combination of several methods known collectively as genetic transformation (Footnote 5), rDNA methods, or genetic engineering. These include: a disarmed plasmid from the plant pathogen Agrobacterium tumefaciens, which delivers foreign DNA to random sites within the host plant genome; delivery of DNA as a coating on small metal particles (microprojectiles) that are forcefully bombarded into the recipient plant cells, some of which penetrate nuclei; and the direct uptake of DNA by plant cell protoplasts.

Because the frequency of genetic transformation is low, selectable markers are commonly used to aid the identification and recovery of transformed cells or tissues that are then regenerated into plants. Unlike the source material in conventional plant breeding, the transforming DNA can be completely characterized so that the entire DNA base sequence is known.

In addition to improvements in plant resistance to pests, crop plants are being developed with improved tolerance to drought, low temperatures, and herbicides. Plants are also being engineered to produce pharmaceutical chemicals, biodegradable plastics, more desirable fats and oils, fruits and vegetables with extended shelf life, and cut flowers with new colors.

The EPA proposal that inherited traits for pest resistance in plants be subject to review and approval as pesticides under FIFRA and FFDCA is based on an interpretation of the 1947 definition of pesticide in FIFRA as: (1) any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest, and (2) any substance or mixture of substances intended for use as a plant regulator, defoliator, or desiccant. The fact that inherited traits for pest resistance are exempted from requirement for tolerance if they occur naturally in the plant or are transferred to or enhanced in the plant by traditional breeding may reduce the regulatory burden, but does not change the EPA interpretation that these substances and necessary genes for expression of these traits are pesticides. Moreover, such exemption from requirement for tolerance could become moot as more and more varieties of plants are developed with genes for resistance transferred from outside the range of sexual compatibility of the plant by rDNA techniques. For example, the EPA-proposed rule would not exempt varieties of pest resistant plants developed by traditional breeding if one or more of the traits for resistance was transferred originally into one of the parental lines from outside the normal range of sexual compatibility of that line, depending on the final option(s) adopted by EPA for implementation.

The great majority of inherited traits for pest resistance are too poorly characterized biochemically or genetically to suggest a "pesticidal" mode of action (see "Pest Defense Mechanisms in Plants: Current State of Knowledge"). Many examples show that disease resistance is the result of a cascade of genetically controlled events and is expressed as a hypersensitive or localized response not unlike the rejection of transplanted organs (Staskawicz et al., 1995). Even when resistance can be partially explained in terms of the genes that account for production of one or more specific substances, it is doubtful that scientifically meaningful toxicological or exposure data could be obtained for these substances or the genes singled out from among the many inherited traits for pest resistance likely to characterize that plant (see "Effects of Plants on Associated Flora and Fauna in the Environment"). This is true for constitutive (preformed) or induced expression of a substance associated with pest resistance of the plant.

We conclude that for a product of a resistance gene or biosynthetic pathway to be characterized as a pesticide, it should be possible to isolate it in pure form from the resistant plant and demonstrate its biological activity by assay outside the plant, before any mechanism can be developed to determine environmental fate and consequence. The biological activity of some substances produced by unimproved plants (e.g., pyrethrin), and by transgenic plants expressing chemically definable molecules or mixtures of molecules [e.g., plants expressing Bt toxin (Vaeck et al., 1987)], can be isolated in chemically pure form and toxicity demonstrated by assay outside the plant. Plants with these and similar mechanisms of pest defense, whether naturally occurring or introduced by gene transfer, require the same degree of experimentation and review for assessing safety to consumers, workers, or the environment as other traits intended to improve the performance or utility of plants or their products. Even in these cases, the concern is for the characteristics and performance of the plant itself, as a phenotype and not as a pesticide. Products of pest-resistance genes and biosynthetic pathways are part of the plant's defense inseparable from the plant itself.

It is now and always has been possible to develop pest-resistant plants that may be harmful to organisms other than the targeted pest species, including humans, by virtue of the substances they may form prior to, or in response to, infection (see "Effects of Plants on Associated Flora and Fauna in the Environment"). Nevertheless, these resistance mechanisms are not pesticides and cannot be separated for purposes of risk analysis from the plant itself.

The inclusion of inherited traits for pest resistance within the FIFRA definition of pesticide raises the additional practical matter that all food from such resistant plants will, because of the definition, contain pesticides whether or not the plants or products from those plants were treated with a substance intended to prevent, destroy, repel, or mitigate pests. This could unnecessarily erode public confidence in the safety of our food supply, which is already among the safest in the world, and will become even safer as more and more pests are controlled through the use of inherited traits for resistance. The designation of plant-defense traits as "pesticides" could also affect the ability of the U.S. agricultural and food industries to compete in global markets, if or when it became necessary to report that the commodity or food came from a plant that "produces its own pesticide." The requirement to label U.S. plant products with such information would only discourage pest control through plant breeding while providing no useful information to the customer or consumer on the safety of that commodity. The "plant-pesticide" definition proposed by EPA could even perpetuate the continued use of synthetic pesticides and hinder the use of novel genes for pest resistance.

We conclude that genes should not be included in the definition of "plant-pesticides." Obviously genes, being composed of nucleic acids, are "substances or mixtures of substances," and those that confer the resistance phenotype are "intended for preventing, destroying, repelling, or mitigating a pest." However, the inclusion of genes within the FIFRA definition of pesticide serves only to illustrate how far removed this regulatory proposal is from both scientific and practical understanding of genes and gene action. It also raises the possibility that the FIFRA definition of pesticide has outlived its usefulness.

The question arises as to whether genes that confer pest resistance should be regulated as pesticides because of the potential for gene flow between crop varieties and with closely related compatible species. Examples of such gene flow have been reported (Mikkelsen et al., 1996), although no gene for pest resistance has been shown to have moved from a domesticated plant to a wild relative and become incorporated into the populations of wild plants. While this potential exists, it is still not possible or practical to separate potential new exposures caused by the trait from potential new exposures caused by the "new" plant. Progeny that result from outcrossing may be sterile, but, if fertile, will be hybrids with genomes containing not only the new gene but also millions of other genes transferred with the gamete from the crop plant. Whether such hybrids could then survive and establish or create new hybrids within the wild population is possible but highly unlikely. Further, the practical and ecological importance of such gene flow must be assessed, including whether the gene transfer would be advantageous or disadvantageous to the wild plant.

It is now possible to transfer genes into plants that confer the ability to produce an entire virus that is pathogenic to a targeted insect pest of that plant (Service, 1996). Such plants obviously must be subject to careful study, evaluation, and formal or informal oversight, but as plants and not as pesticides. We would point out that microbial biocontrol agents delivered or vectored to a targeted insect pest or weed by a nematode or arthropod are currently exempt from regulation as microbial pesticides under FIFRA on the basis that the nematode or arthropod released to deliver the microorganism is already subject to adequate oversight by the USDA.

Plants have evolved in complex ecosystems with a wide variety of other organisms, some beneficial, some detrimental, and some neither beneficial nor detrimental. The detrimental or potentially detrimental organisms include insects, mites, nematodes, microorganisms (fungi, bacteria, viruses), birds, snails, herbivorous mammals, and others -- the same kinds of organisms that make up the pests of plants grown for commercial, aesthetic, or personal purposes today. Species or subspecies of plants unable to survive the attack of the predatory, herbivorous, sucking, parasitic, and pathogenic organisms under natural conditions were replaced through natural selection by those that could survive. For this reason, plant breeders have repeatedly returned to the geographic "homes" (centers of origin) of our domesticated plants to obtain new sources of genes for resistance to pests for incorporation into these plants.

This section reviews the current state of knowledge on the natural mechanisms of defense used by plants against pests, including the limited information on the chemical and physical nature of the many defense mechanisms. The information presented concerns three major categories of pests for which there has been the greatest use of pest defense traits, namely, the plant pathogens (fungi, bacteria, and viruses), arthropods (insects and mites), and plant-parasitic nematodes.

Plant Pathogens: Fungi, Bacteria, and Viruses

Resistance in plants to the pathogenic fungi, bacteria, and viruses is a natural genetic attribute of the plant, nearly universal, mechanistically obscure, and inseparable as a "substance or mixture of substances" from the plant as an organism. Susceptibility to a given pathogen is a combination of attributes of both the plant and the compatible pathogen. Specific resistance genes have been selected and deployed by conventional plant breeding in variety improvement as a cost-effective and environmentally sound strategy to control pathogens for more than 100 years.

What follows from these observations is a fundamental principle of plant pathology: most plants are resistant to most potential pathogens; susceptibility to disease is the exception. For example, there are more than 8,000 species of fungi known to be pathogenic on plants but only about 80 can infect tomato, among which fewer than 12 are economically serious. (Of course, even one economically serious pathogen can put a grower or industry out of business.) Similar pathogenic specialization exists in an estimated 180 species of bacteria and 1,200 viruses able to infect plants, and 500 species of nematodes able to parasitize plants. The generally accepted explanation for these phenomena is either that: (1) plants express basic or non-host genetic resistance to all potential pathogens, except to those to which they are susceptible (i.e., the rare pathogen has the ability to evade the plant's defense mechanisms) or (2) potential pathogens lack the necessary factors required to invade (penetrate) and establish a parasitic relationship (infect) except for specific plant species.

Resistance to a pathogen occurs in a variety of ways, but often is associated with a localized hypersensitive response (HR) that is detected as induced cell death at the site of attempted infection (Dietrich et al., 1994; Staskawicz et al., 1995). Alternatively, the vast majority of plants manifest no visible response to potential pathogens, often referred to as non-host resistance. Such resistance may mean that there are unknown mechanisms of resistance, that the plant may lack essential susceptibility factors, or that the pathogen may lack necessary determinants of pathogenicity.

Research on the mechanisms associated with both the HR and non-host resistance has identified many physiological changes that are induced in the plant upon challenge by pathogens. These include a rapid oxidative burst, ion fluxes characterized by a K⁺:H⁺ exchange, cellular decompartmentalization, cross-linking and strengthening of cell walls, production of antimicrobial compounds (phytoalexins), and the induction of pathogenesis-related proteins such as chitinases and glucanases. While these events characterize a plant's defense response regardless of the pathogen or the plant, the response to a specific pathogen can vary with respect to timing, cell autonomy, or intensity. Taken together, these observations indicate that resistance genes, while not directly limiting, enable the expression of inherent multistep defense signals.

The actual mechanism by which any one or a combination of these physiological changes deter a pathogen remains unknown, although HR is thought in many cases to restrict the growth of pathogens, because of the death of individual plant cells. The functional process by which plant cell death is manifested may be even more widely conserved between plants and animals, as suggested by recent reports that link programmed cell death in plant disease, HR, and normal plant development to the well-characterized process of programmed cell death in animals, called apoptosis (Ryerson and Heath, 1996; Wang et al., 1996).

Simple expression of a resistance gene cannot be assumed to be lethal to a potential invading pathogen; there are examples where the expression of single resistance genes only restricts but does not kill the potential pathogen (Nelson and Ullstrup, 1960; Odvody et al., 1977; Sulzinski and Zaitlin, 1982; Klittich and Bronson, 1986). Often, application of external chemicals to nullify resistance has lead to the resumption in growth of the quiescent pathogen (Yoder and Scheffer, 1969; Cantone and Dunkle, 1990; Kohomoto and Otani, 1991). Again, the direct chemical or biological effect of natural resistance genes on microorganisms is unclear and cannot be separated from the living plant system.

In spite of a lack of information on the mode of action of resistance genes, cultivated or domesticated plants susceptible to pathogens and pests have been made more resistant by grower selection and classical methods of plant breeding for decades if not centuries. Further, the record in terms of progress, safety, and protection of plants is a major contributor to the improved quantity and quality of current food, fiber, and forage plants. Standard practices used in deployment and evaluation of genetic resistance to pests have generally proved sufficient and effective in providing safe and predictable plants. This includes the movement of new disease- resistant varieties into environments where they were not grown previously.

Arthropods: Insects and Mites

As is the case with pathogens, resistance in plants to insects and mites is the rule and susceptibility is the exception. In natural systems, plants use varying combinations of four basic strategies to mitigate the effects of insects and mites. These strategies include life cycles that: (1) enable plants to minimize (escape) exposure to insects and mites; (2) result in associations with other species that limit insect and mite feeding; (3) compensate for injury inflicted by insect and mite feeding (tolerance); and (4) deter (resist) pest attack due to the presence of chemical and physical attributes that interfere with the ability of insects and mites to use the plants as a source of food (Painter, 1951; Atsatt and O'Dowd, 1976; Feeny, 1976; Rhoades and Cates, 1976; Harris, 1979).

In some cases, a particular trait or combination of traits may confer resistance to more than one arthropod (Krischik, 1991). This is the case with a methyl ketone, 2-tridecanone, which is produced in glandular trichomes of the foliage and stems of the wild tomato species, Lycopersicon hirsutum f. glabratum. Because of its acute toxicity, the presence of high levels of 2-tridecanone confers resistance to several pest species, including tobacco hornworm and Colorado potato beetle (Kennedy, 1986; Farrar and Kennedy, 1991), and it has been implicated in resistance to several other insects, including tomato pinworm and beet armyworm (Lin et al., 1987). In other cases, traits conferring resistances to one organism may contribute to enhanced susceptibility of the plant to other pest organisms. For example, the tetracyclic triterpenoids, cucurbitacins, present in some cucurbit genotypes, confer resistance to spider mites but act as both developmental arrestants and feeding stimulants for cucumber beetles (DaCosta and Jones, 1971).

Resistance due to a plant's chemical or physical attributes is ubiquitous in the plant kingdom and has been exploited extensively for crop protection through plant breeding (Painter, 1951; Ehrlich and Raven, 1964; Whittaker and Feeny, 1971; Levin, 1976; Harris, 1979; Rosenthal and Janzen, 1979; Maxwell and Jennings, 1980; Smith, 1989; Fritz and Simms, 1992). Resistance in a plant may be associated with the lack of sufficient nutritional or physical properties necessary to elicit egg laying or feeding behavior, or that are required for normal growth and development by plant-feeding insects and mites. Resistance may also involve the presence of physical structures, such as trichomes (Norris and Kogan, 1980), or chemicals, such as alkaloids, saponins, furanocoumarins, terpenes, tannins, glucosinolates, and others (Rosenthal and Janzen, 1979; Harbourne, 1988; Smith, 1989; Fritz and Simms, 1992) that adversely affect insects and mites physiologically and cause delayed development, reduced fecundity, and elevated mortality (Smith, 1989). While most chemical defenses against insects and mites thus far studied occur constitutively within plants (i.e., they are synthesized as normal plant constituents), some are produced specifically in response to injury by arthropods or other agents (e.g., Karban and Carey, 1984; Karban, 1985; Karban. et al., 1987; Paxton, 1991, Schaller and Ryan, 1995).

The chemical diversity within even a single plant is tremendous. Furthermore, these chemical constituents of plants may act individually, synergistically, or antagonistically in ways that affect arthropod physiology and behavior (Duffey and Bloem, 1986; Duffey and Felton, 1991; Berenbaum and Zangerl, 1992). Thus, the contribution of any particular plant constituent to resistance may be dependent on the chemical and physical milieu of the plant in which it occurs. The expression of resistance to arthropods, just as to other pests, is an emergent property of that milieu, and, as such, is inseparable from the plant itself.

In natural systems, all four of the defensive strategies mentioned above can contribute to the total defense of any particular plant species. However, the ability to exploit particular strategies for defense against insects and mites in crop plants may be severely constrained in many instances by the production and marketing requirements of particular crops (Kennedy and Barbour, 1992). For example, in natural systems, the options for escape from insects and mites may be limited only by the occurrence of unfavorable environmental conditions for the plant or the occurrence of other organisms with ability to attack the plant. However, in agricultural systems, market requirements may impose additional restrictions on when a crop can be planted to be profitable in a particular area. Similarly, the ability of a plant to produce a second seed crop following destruction of the first crop by an insect (tolerance) may provide an effective defense for a plant growing in a natural ecosystem but may be of no value in a crop that is harvested when the seeds are immature (e.g., green beans) or that must meet a time-restricted market.

In like fashion, chemical and physical factors that confer resistance in plants in natural systems may not be suitable for use in particular crop plants. For example, high levels of glycoalkaloids in the foliage of potato species are associated with resistance to Colorado potato beetle, but glycoalkaloid levels in foliage are often correlated with high levels in tubers. Since glycoalkaloids are toxic to humans, the use of resistance based on high glycoalkaloid levels in the foliage is feasible only to the extent that it does not result in unacceptably high levels in the tubers (Sinden et al., 1984; Tingey, 1984). Today, the tubers of potential new potato varieties are routinely screened for glycoalkaloid content to assure that no variety is released that would be unsafe as food (Maga, 1994; Hellanas et al. 1995).

Because of the mechanistic complexity responsible for the expression of an insect- or mite-resistant plant phenotype, the chemical and/or physical mechanisms of resistance in crop varieties specifically selected for resistance in conventional plant breeding programs are rarely known. Nonetheless, plant breeders have developed numerous insect and mite resistant crop varieties that have contributed immensely to crop protection (Painter, 1951; Maxwell and Jennings, 1980; Smith, 1989). To date, there is no evidence that these pest resistant crops have resulted in adverse health or environmental effects.

Plant Parasitic Nematodes

Nematodes are microscopic roundworms that live in all soils. As with other kinds of pests or potential pests, resistance of plants to nematodes is the rule rather than the exception. Still, the relatively small proportion of all nematodes with ability to parasitize plants cause an estimated $8 billion in crop losses annually in the United States (Barker et al., 1994).

Plant resistance to nematodes may take the form of a physical barrier that prevents some species of nematodes from penetrating roots. In other cases, resistance to nematodes is due to the presence of preformed chemical constituents that are toxic to nematodes. The use of marigolds by home gardeners to control soil-inhabiting nematodes is the most widely known example of a plant thought to be chemically toxic towards nematodes by virtue of constitutive and/or secreted compounds -- polythienyls in this specific case. Thus plants produce over 100 distinct chemicals, including acetylenes, alkaloids, quassinoids, fatty acids, phenols, terpenoids, and many others (Chitwood, 1992). Although it may be tempting to speculate that such compounds are functionally active in plants as antinematodal substances, the concentrations of these compounds within plants or their cellular or subcellular locations are unknown. Thus, the association of preformed chemicals in plants with nematode resistance is largely correlative rather than determined.

The nematode resistance deployed in crops by plant breeders for most of this century is still another form by which different varieties of the same crop plant differ in resistance to the same nematode species. This kind of resistance is not the result of a physical barrier to nematode penetration or preformed chemical constituents. Instead, the plant's biochemistry or physiology determines this type of resistance. The post-penetration response can be very similar to the hypersensitive response (HR) of plants to microbial pathogens, although little is known about the steps that mediate the response (Kaplan and Keen, 1980). Indeed, although plant breeders have been identifying and transferring genes for nematode resistance for decades, the underlying biochemical basis for this kind of nematode resistance remains obscure. As with plant resistance to other kinds of pests, direct involvement of biochemical substances in nematode resistance is possible, but the actual mechanisms are not resolved.

The most economically important plant parasitic nematodes establish permanent feeding sites within plant roots. These feeding sites are produced when plant tissues differentiate into specialized cells that transfer food to the nematodes. Although little is known of these plant cells, the current belief is that salivary proteins secreted by the nematodes into the plant cells induce a cascade of steps that results in altered gene expression within the plant (Opperman and Conkling, 1994). Many genes within the plant allow the plant to recognize the nematode by unknown means and thereby express resistance to the nematode. Breeders have successfully exploited plants that carry these resistance genes to create numerous commercially viable nematode- resistant crop cultivars, even though most of the specific chemical events in this complex interaction are unknown.

Plant scientists are using various novel strategies based on rDNA techniques to develop new generations of crop plants resistant to nematodes. In some cases, genes for resistance from related plant species but of unknown function are being identified, cloned, and inserted into the genome of susceptible plants, thereby conferring the resistance phenotype (Yaghoobi et al., 1995). In other cases, methods are being developed to turn genes on and off; no foreign DNA is inserted from other plant species, but rather, the method uses altered genes obtained from the host plant (Opperman and Conkling, 1994). As another strategy, a "genetic switch" or gene promotor turned on by nematode feeding has been characterized and used to produce nematode- resistant plants. By this approach, the genetic switch is either fused to a bacterial phytotoxin (barnase) gene, to kill the nematode, or to one of the critical plant genes, but in the reverse orientation (antisense) to starve the nematode (Opperman et al., 1994).

8. EQUIVALENCY AND CONSERVATION OF PEST-DEFENSE MECHANISMS WITHIN AND BEYOND THE PLANT KINGDOM

The availability of the new tools of biotechnology, and especially the ability to compare genes based on the nucleotide sequences of their DNA, is beginning to reveal that certain genes or gene families occur repeatedly within the plant kingdom and even across kingdoms. Further, it is becoming clear that resistance genes themselves are only part of a complex signaling process that triggers a resistance response and shares functions with other plant processes not unique to a given pest and widely conserved. This section briefly reviews the status of knowledge on equivalency and conservation of pest defense mechanisms used or available for use in plants.

Pest-resistant strategies -- as exemplified by animals, plants, and other organisms -- have certain similarities as well as vast differences. For example, animals defend themselves against invasion by means of several types of strategies, including specific cells (e.g., macrophages) that recognize foreign materials and destroy them in a cytolytic manner. In other cases, the immune system is activated to recognize foreign materials, and antibodies bind to and neutralize the foreign or invading materials, including disease-causing organisms; this limits their activity and prevents the spread of the invader. By contrast, plants produce no antibodies, but rather, they produce certain antibiotic-like materials, cell wall constituents, a variety of proteins, and small molecules in response to attack by pests. The synthesis of these compounds follows recognition of the invader and activation of a mechanism that triggers a response that may kill the cells or tissues that are invaded or the invader itself. This effectively limits the spread of the invader. This type of "hypersensitivity" to certain pathogens can be highly effective and has been exploited through plant breeding for many years.

Relatively little is known about how plants respond to infections or pest attack. The signaling mechanism studied in most detail reveals for tomato that defense genes are activated systemically throughout the plant within just a few hours after an insect begins to eat on a leaf of the plant (Schaller and Ryan, 1995). The systemic response results from a peptide hormone produced in cells of the damaged leaf. Likewise in animals, tissues exposed to an invading pathogen produce peptide hormones that signal cells in other parts of the body to activate the immune system.

Some of the mechanisms for limiting spread of pathogens in animals and plants are also similar. For example, there is growing evidence that the small molecule, hydrogen peroxide (H₂O₂), is involved in the early steps in the "oxidative burst" that marks the early stages of a resistance reaction in both plants and animals (Wu et al., 1995). Likewise, there are similar enzymes in both plants and animals that can protect the organism from attack. For example, lysozyme, the enzyme responsible for destroying certain bacteria and fungi, is found in rice and bean plants, chickens, cattle, and humans. While each form of the enzyme has somewhat different activities, each is important in defending the host organism against disease-causing agents. It logically follows that to develop a rice variety protected with resistance to certain bacteria by virtue of carrying a lysozyme gene from a different organism does not require legal regulation of the gene or the lysozyme as a pesticide.

Plants support complex communities of microorganisms on their leaves and roots (NRC, 1989b), many of which produce antibiotics as their mechanism of defense against their competitors and predators. Some of these antibiotic-producing plant-associated microorganisms also have been shown to protect their host plants against infections (Weller, 1988). The phenazine antibiotics inhibitory to wheat root pathogens are produced by a metabolic pathway similar to the process used by plants to produce the antimicrobial compounds known as phytoalexins (Thomashow et al., 1993). One of the genes for production of a phloroglucinol antibiotic by bacteria in the rhizosphere is homologous with the plant gene, chalcone synthase (Bangera and Thomashow, 1996). Thus, microorganisms and plants have the same or similar genes for production of substances used for their defense against microorganisms.

Since 1992, several plant genes controlling resistance have been characterized (Jones, 1994a; Song et al., 1995; Staskawicz et al., 1995). The evidence indicates that these genes fall into several classes, including those whose gene products degrade toxins produced by pathogens (Johal and Briggs, 1992), those that encode for enzymes involved in signaling the plant to respond to pathogens (kinases) (Martin et al., 1993), and those whose gene products may act as receptors for pathogen signals that subsequently lead to triggering of host defenses through the physiological changes indicated earlier (Bent et al., 1994; Jones et al., 1994; Whitman et al., 1994; Ellis et al., 1995). In all cases, it is clear that these genes are highly conserved in individual plants and may have functional homologues in many other related and unrelated plants.

The community of organisms that exists in association with any particular plant, whether the plant is naturally occurring or under cultivation, includes mammals, birds, arthropods, nematodes, fungi, bacteria, and viruses. The community is comprised of organisms with ability to eat different parts of the plant (herbivores and predators); parasitize the plant (parasites); live harmlessly on or within tissues or organs of the plant (epiphytes, endophytes) or on microorganisms, pollen or other substrates provided by the plant (parasitoids, pollinators, other visiting arthropods, saprophytic microorganisms, and saprophagus nematodes); provide nutrients for the plant (e.g., Rhizobium spp. responsible for nitrogen fixation in root nodules on legumes and mycorrhizal fungi responsible for uptake of phosphorus); or begin the decomposition process of dead plants (saprophytes and detritivores).

Many of the associations involving a particular plant species or variety are the result of similar habitat requirements; others are more intimate or specialized. These interactions result because the plant provides the associated flora and fauna with shelter, substrates (nutrients) for growth, and a suitable physical and chemical environment.

A pest-resistant phenotype -- attributable to the plant genotype, environment, or genotype by environment interaction -- can significantly influence the community structure associated with a particular plant species (Maxwell and Jennings, 1980; Heinrichs, 1988; Barbosa et al., 1991; Fritz and Simms, 1992). Such effects may be mediated through direct effects on plants and herbivores or direct effects on interspecific competition among components of the plant-associated microbiota as well as among herbivores (Whittaker and Feeny, 1971; Denno and McClure, 1983; Boethel and Eikenbary, 1986; Benedict et al., 1991; Fritz, 1992; Hare, 1992).

Plant exudates and leachates are important determinants of the microflora that occur on aerial portions of the plant, as well as roots (NRC, 1989b; Rovira et al., 1990). These microbial communities affect host suitability to insects (Berenbaum, 1988; Dicke, 1988; Benedict et al., 1991) and to plant pathogens (Blakeman and Fokkema, 1982; Blakeman, 1985; Benedict et al., 1991). Genetic modification of crop plants to achieve pest resistance, whether through conventional breeding or by other means, can alter the communities of plant-associated organisms and indirectly affect other components of the agroecosystem (Benedict et al., 1991).

The plant defenses may influence predators, parasitoids, and pathogens of the plant's herbivores whether or not the herbivores are directly affected by those defenses (Price et al., 1980; Boethel and Eikenbary, 1986; Barbosa and Letourneau, 1988; Hare, 1992; Farrar et al., 1994; Eigenbrode and Espelie, 1995). There are many examples of direct negative effects of plant resistance traits on natural enemies of insect pests (Obrycki, 1986; Kauffman and Kennedy, 1989; Kashyap et al., 1991; Farrar et al., 1994). There also are many examples of successful integration of plant resistance and biological control with natural enemies (Starks et al., 1972; Pimentel and Wheeler, 1973; Casagrande and Haynes, 1976; Lambert et al., 1983). The successful integration has been variously attributed to: (1) lower rates of increase of pest populations on resistant plants, (van Emden and Wearing 1965, van Emden 1966, 1986); (2) prolonged development times or altered behavior of the pest on resistant plants, thereby increasing exposure to natural enemies (Dahms, 1972; Rhoades, 1983; Shultz, 1983; Feeny et al., 1985); or (3) increased attraction of natural enemies to resistant plants (Way and Murdie, 1965; Chandler, 1968; Hagen, 1986; Williams et al., 1988).

Elaboration of effects of plant resistance traits in natural ecosystems at the population and community association levels is difficult and complex because small effects are difficult to measure and may be interactive (Fritz, 1992; Karban, 1992). Even in agricultural systems, the task can be daunting because of interspecific and intercommunity interactions that are complicated by the array of farming practices that are used (e.g., pesticides, soil amendments, irrigation practices, tillage practices, crop rotations and crop sequences, planting patterns, planting and harvest dates, etc.) that change over time and with the crop variety grown. Rotating a field from corn to soybeans, for example, will undoubtedly have significant effects on birds and insects likely to visit that field and the flora and fauna in the soil of that field. Likewise, a shift from conventional to conservation tillage practices, made possible in some cases by a new variety, including a new pest-resistant variety, will greatly affect the communities of organisms that thrive or do not thrive in that field. In addition, there is the possibility that pest-resistance genes may be transferred by natural hybridization to weedy relatives of crop plants. In the unlikely event that resistance genes become incorporated into the weed population, it might affect the agriculture system (see "Interpretations of the Term Pesticide"). Thus, it is important that the significance of any shifts in the associated flora and fauna that may be attributed to plant resistance be evaluated within the context of the larger agricultural enterprise and the impact of alternative practices the resistance may displace.

The United States grows some 250 or more crops (USDA, 1995), the great majority of which are not native to North America. Instead, they were imported as seeds, root stocks, tubers, or other propagation materials, first by early colonists and more recently by scientists and others through official USDA plant introduction stations. Individuals can privately introduce plants into the U.S. with approval of the USDA Animal and Plant Health Inspection Service (APHIS). The U.S. National Plant Germplasm System contains more than 400,000 plant introductions that are maintained both as a public service and as potential new crops or useful new sources of genetic variation for improvement of existing crops. Most U.S. crops have been subjected to extensive breeding and selection to develop varieties adapted to U.S. conditions, resistant to at least some U.S. pests, and acceptable to U.S. consumers and foreign customers.

Crops developed from introduced plants create the potential for new environmental and dietary exposures, as for example, when the plant species is new to the region and is a new food or feed. Historically, each new plant introduction (species, variety, or landrace) has been grown first in a greenhouse or small plots in the field, and then gradually over a larger area as more seed has become available and more uses have been found for the plant. With each plant species or variety grown in any given area for the first time, previous experience suggests that the populations and activity of organisms favored by cultivation of that plant can be expected to increase, while populations and activity of those organisms not favored can be expected to decrease.

Some organisms that interact with crop plants thrive when the crop is under stress while others thrive mainly in response to the lush growth typical of crops not under stress (Baker and Cook, 1974). Likewise, plants damaged by a pest may have different effects on the associated flora and fauna than plants that remain healthy. For example, wheat roots damaged by the take- all disease caused by the soilborne fungal pathogen Gaeumannomcyes graminis var. tritici favor higher populations of antibiotic-producing Pseudomonas spp. in the rhizosphere (soil surrounding the roots) than do healthy roots (Cook and Weller, 1987). These antibiotic- producing bacteria, in turn, suppress the take-all disease by inhibiting the fungal pathogen, and they are thought to account for the spontaneous "take-all decline" observed following two or more outbreaks of the disease and continued monoculture of wheat susceptible to take-all.

While all plants have genes for defense against pests, different kinds of resistance could have different effects on the associated flora and fauna in addition to the target pest. Resistance expressed with no visible plant response (non-host resistance), typical of the vast majority of plant-microbe, plant-nematode, and plant-arthropod interactions, may be different from the exposure created by resistance expressed as a hypersensitivity response or other types of resistance. In these cases, the potential for exposure to chemical constituents is created by the nature of the pest interaction with the plant and not just the plant itself.

The resistance genes transferred by conventional breeding into an otherwise susceptible plant genotype are typically specific for a target pest or group of closely related pests. Other organisms are probably prevented from attacking these plants by different mechanisms of resistance functioning in the plant. Because of the nature of most forms of resistance, exposure of organisms interacting with the plant typically requires penetration of the cells or tissues where the resistance is expressed. With the exception of certain alkaloids produced by plants, or by fungi within plants (endophytes) (Seigel et al., 1987), and structural organs such as thorns, there are no known problematic substances in resistant lines that can be related specifically to an inherited trait for pest resistance.

Most concerns for new environmental or dietary exposures are based on examples best described as currently unfamiliar. For example, it is possible to transfer bacterial genes controlling production of proteins toxic to insect pests to plants to provide a defense mechanism against insect attack (Vaeck et al., 1987). Obviously, these transformed plants present a new situation, and questions appropriately are raised about environmental impacts and safety for organisms that may consume the transformed plants. The question of risk in this new situation can be assessed by proper experimentation, just as it must be for plants that produce biodegradable plastics, pharmaceuticals, vaccines, and other products for which we lack experience and familiarity, or for which familiarity would predict a risk to people or the environment.

The potential for new environmental exposures is created by the plant in the environment where grown. Similarly, the potential for new dietary exposures to humans, livestock, or wildlife is created by the part(s) of the plant intended for use as food or feed or likely to become a source of feed for wildlife. The same principles for assessing exposure should be in effect regardless of the source of genes transferred into the plant or the method used to transfer genes into the plant.

A diversity of formal and informal mechanisms of oversight have evolved in the United States that have ensured ever higher standards for safety of new varieties of crops to the environment and of the harvested products as food for people or feed for livestock. For public- sector plant breeding programs, approval for commercial use is based on peer review of data on performance and quality factors and approval of release by the director of the agricultural experiment station where the variety is produced (ESCOP, 1988). Private-sector plant breeding programs follow similar procedures; some private-sector varieties are observed along with public-sector varieties in regional performance and evaluation trials conducted by public-sector scientists. Oversight for the scale-up of seed for commercial production is usually provided by a state crop improvement association working under the authority of the state department of agriculture, but this is not mandatory, especially for privately developed varieties.

The peer-review and approval procedures that have evolved for crop varieties are carried out by professionals representing the plant, food, microbiological, and entomological sciences. These procedures are voluntary but elegantly simple and efficient. Plant varieties developed through newer molecular techniques are subject to these same peer review procedures, but are also subject to formal review and approval by one or more federal agencies before release for scale-up and use in commercial agriculture (see "Federal Oversight of New Crop Varieties").

In this section we review the current procedures used during research and variety development to assure safety. This includes responsibilities of the plant breeder, plant variety peer-review, and opportunities for strengthening these procedures.

Responsibilities of the Plant Breeder

The plant breeder (representing, in some cases, a crop variety development team of pathologists, entomologists, agronomists, or horticulturists and, more recently, applied molecular biologists) has the responsibility for development of new varieties. A "new" variety may represent a marked departure from currently grown varieties, such as the dwarfed varieties of wheat and rice introduced in the 1960s. A "new" variety may also be a familiar variety with a single gene or genetic construct (gene of interest plus a marker and/or gene promoter) introduced by rDNA techniques, such as the century-old and very popular Russet Burbank potato with a gene from Bacillus thuringiensis introduced for protection against the Colorado potato beetle.

The crop improvement team identifies sources of pest resistance from among the world's genetic resources and develops a strategy for transferring the resistance into locally useful varieties. The breeding strategy depends upon the breeding system of the crop (self-pollinated or outcrossing) and the mode of reproduction (seed or asexual). The strategy also includes the chosen method of gene transfer (see Box 2) -- by sexual hybridization, as in traditional breeding, or by asexual methods of gene transformation using vectors to transfer cloned DNA having only the gene of interest. With either strategy, progenies from the hybridization or transformation are regrown and observed for the traits of interest and selected for another generation of observation. This process has multiple goals, but mainly this establishes: (1) the genetic stability of the trait by ensuring that it is a permanent and predictable characteristic, and (2) the stability of the degree of expression under varying conditions in the target environments. Breeding procedures are well established for the major crops and vary according to specifics of each crop (OECD, 1993b).

After observing entire families or progenies of new plant lines through successive generations in the greenhouse and in small-scale plots, eliminating undesirable types and replanting desirable types with each generation, the plant breeder then selects one or a few lines for testing over a larger scale. Performance trials are carried out at several locations within the area where the crop variety may be grown, including by other plant breeders, under conditions more typical of commercial production. A plant line selected for release as a new variety will normally be the final choice from tens or hundreds of thousands of lines available at the start of the process. Furthermore, the new variety will commonly have been observed for 50 and sometimes 100 or more site-years (number of test sites × number of years) before consideration for release as a new variety.

Throughout this process, it is the responsibility of the plant breeder or crop development team to consider agronomic factors and environmental fate, including: yielding ability, reproductive stability, uniformity of characters, tendencies for weediness, potential vulnerability to attack by pests for which resistance has not been targeted, sensitivity to environmental stresses, and any other significant, unexpected, or undesirable characteristics of the new line. For varieties having new traits with potential risk to people and the environment, the breeder's evaluation methods will account for this in the design of the evaluation experiments. For example, ecologists have expressed concern (e.g., Seidler and Levin, 1994) that small-scale plots do not adequately represent the situation presented when the new variety is grown on thousands of hectares. The breeder's procedures, using sequential evaluation at larger and larger scale during the development process, provide considerable information on the potential for environmental effects associated with the new variety. This is especially true for pest resistance where environmental risks are extremely low.

The plant breeder or breeding team is also responsible for evaluation of new pest-resistant plant lines for end-use quality of the harvested product intended for use as food, feed, or fiber. For example, the tubers of new lines of potato are evaluated very early in the process for solanine content (Maga, 1994; Hellanas et al., 1995), the toxic glycoalkaloid. Other plant products with known toxicities and intended for food are similarly evaluated early in the variety development process based on established threshold concentrations and other quality standards. The more information available about the parents and genes transferred to produce the new variety, the more predictable the end-use quality characteristics of that variety.

Plant Variety Peer Review

New varieties developed by public plant breeding programs, such as those of the USDA Agricultural Research Service (ARS), and the State Agricultural Experiment Stations (SAES), are approved for release based on data submitted by the plant breeder and evaluated by a committee appointed specifically for that purpose. Often ARS and several SAES within a region will join in announcing the release of a new variety. Private companies conduct their own performance reviews; for some crops, this includes the results of trials conducted by the extension service or public plant breeders. Varieties are released when the owners of the varieties are satisfied that there is sufficient value and benefit in making them available to growers. For both public and private programs there are market forces and peer reviews that have strong influence on the decision for release of a variety. If varieties are to be protected by the Plant Patent Act (for clonal species), the Plant Variety Protection Act (for seed crops), or the Patent Act (for special characters qualifying for utility patents), there is careful review for novelty (that a variety is not the same as a previously released one) and evidence to meet performance and descriptive claims made by the developer(s).

In addition, plant variety review boards have been established by the Association of Official Seed Certifying Agencies (AOSCA) for several crops, including alfalfa, grass, small grains, soybean, forage legumes other than alfalfa, and sunflower. The purpose of these review boards is to advise state seed certifying agencies on the eligibility of varieties for certification. It is optional for breeders to submit new varieties for review to these boards. However, for crops eligible for Plant Variety Protection, this review is important for both public and private breeders because the option of implementing variety protection is exercised only through the seed certification process. Certification by AOSCA agencies assures that the seed is free of contaminants from other crops, other varieties of the same crop and of noxious weed seeds. Information contained in an application for certification includes a statement of the breeding history used to develop the variety, a description of morphological and physiological characteristics of the variety, a profile of susceptibility or resistance to diseases and insects, agronomic performance and targeted area of adaptation, and procedures for maintaining seed stocks. The certification agency is required to keep reference seed stocks for a specified period of time for later comparison to other samples.

Extensive use is also made of peer-reviewed journals to register the release of new plant lines as varieties, germplasm (available as parents for breeding because of novel characteristics), or genetic stocks having specific genes or combinations of genes or chromosomal types. The published registrations include critical information about the characteristics of the lines and, in the case of a variety, agronomic performance. Each scientific society has its own criteria and procedures for acceptance of such reports based on their professional standards. As with the variety review boards, this is also optional for breeders, and release of a variety is not conditional upon acceptance through the peer review process of the scientific journal.

The above review processes are not mandated by law and do not specifically address environmental or human safety issues. The products derived from varieties for human or animal consumption are subject to the FDA GRAS standards. However, formal review by FDA has seldom been required because of insignificant risk introduced by new varieties, which for the most part have contained no traits that had not been observed in prior varieties; any risks were therefore insignificant.

Opportunities for Strengthening Current Procedures

Regardless of oversight, formal or nonformal, the safety of new varieties of crop plants depends on the procedures actually used by scientists to test and evaluate the new variety and hence the kind and validity of the information available for that variety. The newer molecular methods of plant breeding are more precise, and commonly much more is known about the genetics, biochemistry, and mode of action of traits introduced as single genes or gene constructs by rDNA technology compared with varieties produced by mutagenesis, somaclonal variation and intra- and interspecific hybridization. The new molecular methods also allow scientists to answer questions pertaining to food and environmental safety that previously could not be answered, or could be answered only with difficulty, such as the precise protein products of genes and the frequency of gene transfer by outcrossing. As a result, the new tools of rDNA technology as an aid to plant variety development are more likely to increase the safety rather than the risk of new varieties of crop plants to people and the environment.

The procedures used by scientists to assure safety of new varieties must and will continue to change in the same way that the procedures used to produce these new varieties are changing as our knowledge and experience grows. It is a natural progression of science to adopt ever-better techniques and establish ever-higher standards of performance in research and development. Clearly, the availability of the new tools of molecular biology has elevated the standards of performance in all of the life and environmental sciences, in addition to increasing the ability of scientists to solve problems or pursue new opportunities for food, agriculture, and the environment. Agricultural experiment stations and private companies with plant variety development and testing programs are aware of safety issues and continue to upgrade their methods and standards.

While the review process outlined above does not specifically address environmental and human safety issues, these issues are considered as a matter of good scientific practice when performance claims are made by the developers for varieties having traits that are initially unfamiliar, and can readily be included in the current review process as appropriate. This has worked effectively for the FDA where plant breeders are provided clear guidance on safety issues and new products have been subject to regulation only when a potentially significant departure from the norm for a crop could be expected (see "Federal Oversight of New Crop Varieties").

Reasonable and continued assurance of safety of each new variety to people or the environment does not require addressing every question that might be asked or every hypothetical concern that might be raised about that variety. The focus must be on high- probability risk rather than hypothetical or unrecognizable risk. Even with the most thorough review based on the best science, not all risks can be anticipated and problems can still emerge after the variety is grown and used commercially

USDA Review

Since 1987, the USDA's Animal and Plant Health Inspection Service (APHIS) has regulated plants with genes introduced by rDNA techniques under authority of the Federal Plant Pest Act (7USC 150 aa jj) and the Plant Quarantine Act (7 USC 151-164a,166-167). Under these statutes, the USDA regulates the importation, interstate movement, and release into the environment of genetically modified plants if the donor organism, recipient organisms, vector, or vector agent used to develop the new plant genotype belongs to one of the taxa of organisms considered to be a plant pest. The USDA-APHIS reviews these "regulated articles" to prevent the introduction, spread, or establishment of plant pests new to or not widely prevalent in the United States.

In all cases subject to USDA regulation to date, plants considered to be "regulated articles" have been genetically transformed using DNA from certain plant pathogens to introduce or control expression of the gene. As an example, plants with genes introduced along with genes of the disarmed plasmid from Agrobacterium tumefaciens (see "The Scope of Plant Biotechnology") are subject to regulation under the Federal Plant Pest Act on the basis that A.tumefaciens is a plant pathogen (causal agent of the crown gall disease in a wide range of plant species). As another example, plants with genes introduced together with DNA of the 35S promoter from cauliflower mosaic virus is subject to regulation as a "regulated article." The promoter controls expression of the introduced gene. There is no evidence, however, that plants are potential plant pests when transformed to express new genes with the aid of DNA from plant pathogens. Scientific analyses of the USDA's regulation of rDNA-engineered plants have been published (Miller et al., 1990; Huttner et al., 1992).

Genes can also be introduced into plants by rDNA techniques that do not require the aid of DNA from a plant pathogen. Occasionally researchers have voluntarily requested USDA review and approval of these genetically modified plants.

The USDA carries out an environmental assessment prior to issuing a permit for importation, interstate movement, or release of a "regulated article," based on information supplied by the applicant and information available in the scientific literature. In preparing the environmental assessment, USDA examines the potential for impacts of the release on the physical environment, human health, wildlife, endangered and threatened species, and other nontarget flora and fauna. If the available information supports a finding of no significant impact on the environment, the USDA will authorize the release.

The USDA has gained considerable experience during the nine years of conducting environmental assessments for field tests of plant lines and varieties developed by rDNA techniques. This has led to the adoption of performance standards rather than case-by-case environmental assessments for field testing of six crop species (USDA, 1993). In August, 1995, the USDA proposed to extend its streamlined notification process to the majority of crop species.

An important aspect of the USDA approach is that novel plants are examined and approved early in developmental phases at the time of the first field trials. A determination of no significant risk exempts the new plant line(s) from further regulation.

FDA Review

The FDA is the primary agency responsible for ensuring the safety of commercial food and food ingredients. The FDA regulates food safety under authority of the Federal Food, Drug and Cosmetic Act (21 USC 301 et seq.). U.S. food laws place the burden of proving the safety of all new food products on developers. The safety of most food is regulated primarily under FDA's postmarket authority, whereas substances that are not generally recognized as safe require premarket approval as "food additives."

FDA has authority to regulate food from new plant varieties intended for commercial use regardless of the genetic methods or source of genes used to develop that variety. Since the advent of crop variety development by rDNA techniques, FDA has issued a policy statement to help developers determine whether foods are subject to regulations under the FFDCA (FDA, 1992). The policy statement says:

• Genetically modified food products will be regulated in the same way as foods produced by other means;
• Genetically modified food products will be judged on their individual safety, allergenicity, or toxicity, rather than on the methods or techniques used to produce them; and
• FDA will evaluate the safety of new ingredients added to foods through biotechnology in the same way it now evaluates a new food additive, i.e., on the basis of a history of safe use in the food supply.

The FDA provides guidance to developers for assessing the safety of foods derived from new plant varieties and has established a process by which developers can consult with the agency on questions of safety and regulatory status.

EPA Review Based on the Proposed Rule

The EPA regulates the registration, manufacture and use of all pesticides under authority of FIFRA and FFDCA. Under these statutes, with the authority under FFDCA, EPA proposes to review and approve the use of substances in crop plants with traits introduced specifically for pest resistance from outside the range of sexual compatibility of that plant prior to conducting any field test greater than 10 acres (EPA, 1994). The review under the EPA-proposed rule is currently voluntary but will become mandatory if or when the rule is approved.

Under the proposed rule, before the EPA approves such a field test, the agency must issue an Experimental Use Permit (EUP). To get such a permit, the applicant must supply the EPA with documentation describing in detail the:

• Genetic makeup of both the host and donor organisms
• Genetic modification
• Stability of the genetic modification
• Proposed field test design and monitoring procedure
• Available health and environmental information on the host and donor organisms
• Results of tests performed in the laboratory and growth chambers

• Identity and genetic makeup of the host and donor organisms
• Genetic modification that took place on these organisms
• Genetic stability and expression of the "plant-pesticide"
• Chemical characterization of the "plant-pesticide"
• Environmental fate (e.g., biological, biochemical) of the "plant-pesticide"
• Ecological effects on non-target organisms
• Human health effects

13. PRINCIPLES FOR APPROPRIATE OVERSIGHT

It is not the intention of this report to make specific recommendations on how or whether the federal government should regulate the development and use of new varieties of crop plants with inherited traits for pest resistance. Nor is it the intention of this report to comment on which federal agency or agencies should be responsible for regulation, if any, of new varieties of crop plants. Similarly, we have not addressed the role of state regulatory agencies with oversight responsibilities for the products of plant biotechnology.

However, we do recommend that the EPA-proposed rule on "plant-pesticides" be further reviewed in consideration of the information and evaluations presented in this report. This recommendation is supported by the recent concept paper Ecologically Based Pest Management: New Solutions for a New Century (NRC, 1996), that resistant crop varieties, when compared to synthetic chemical pesticides, present different kinds of risks.

We recognize the role of the federal government in assuring environmental and public safety and thereby helping to gain public acceptance of plants new to the environment and of new plant products used as food. We are concerned, however, with the confusion created by the unusual definitions and interpretations of federal statutes intended to selectively capture plants modified by rDNA techniques (Huttner et al., 1992). We have attempted to describe the elegance and simplicity of the nonstatutory oversight mechanisms and urge that these mechanisms be recognized more widely and strengthened. The scientific societies represented in the development of this report can contribute to the realization of workable oversight processes and would welcome the opportunity to be of service to the producers and the state and federal agencies responsible for ensuring the safety of new crops and new foods. We support cooperative oversight by the relevant agencies of the federal government.

The principles stated below are offered as a guide to developing appropriate oversight for novel plants, including plants with inherited traits for pest resistance for which there is a lack of familiarity at present, or for which familiarity would predict a potential hazard to people or the environment at present.

• The oversight should be based on accepted standards of practice, including guidelines, regulations, statements of policy, and reports provided by the National Institutes of Health, U.S. Food and Drug Administration, the U.S. Department of Agriculture (USDA, 1996), the Organization for Economic Cooperation and Development (OECD, 1993a), and the National Academy of Sciences (NAS, 1987; NRC, 1989a, 1996);
• The focus should be on high-probability risk rather than hypothetical or unrecognizable risk, and the oversight should be sufficiently flexible to keep pace with new scientific developments;
• The level of risk of a plant variety is not determined by novelty or lack of familiarity, the source of the gene or genes that produce a pest-defense substance or initiate a pest-defense reaction, nor the method by which a gene for pest defense is transferred into the variety; and
• GRAS, the FFDCA concept of "generally recognized as safe", should be considered for environmental risk management, and mechanisms should be developed for conferring the equivalent of GRAS status to new or substantially modified varieties of plants as we gain experience and familiarity.
  
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List of participating scientific societies that responded to EPA

American Institute of Biological Sciences
American Phytopathological Society
American Society for Horticultural Science
American Society for Microbiology
American Society of Plant Physiologists
Crop Science Society of America
Entomological Society of America
Institute of Food Technologists

Participants in January 29-30, 1996, Workshop and Professional Society Affiliations and Addresses:

Robert F Barnes
American Society of Agronomy
677 S. Segoe Road
Madison, WI 53711
Tel: (608)272-8090
FAX: (608)273-2021
email: rbarnes@agronomy.org

Roger N. Beachy
[American Phytopathological Society]
Scripps Research Institute
Molecular Biology Department
10666 North Torrey Pines Road
La Jolla, CA 92037
Tel: (619)554-2550
FAX: (619)554-6330
email: beachy@scripps.edu

John Bode
Olsson Frank and Weeda, P.C.
1400 16th Street, Suite 400
Washington, DC 20036
Tel: (202) 234-3555
FAX: (202)234-3537
email: ofw@ix.netcom.com

Ronald P. Cantrell
[Crop Science Society of America]
Iowa State University
Department of Agronomy
2101 Agronomy Hall
Ames, IA 50011
Tel: (515)294-1360
FAX: (515)294-3163
email: rcantrel@iastate.edu

David J. Chitwood
[Society of Nematologists]
USDA-ARS
Nematology Laboratory
Bldg. 011A, Rm. 165B, BARC-West
Beltsville, MD 20705-2350
Tel: (301)504-8634
FAX: (301)504-5589
email: dchitwoo@asrr.arsusda.gov

R. James Cook
[American Phytopathological Society]
USDA-ARS
Washington State University
Pullman, WA 99164
Tel: (509)335-3722
FAX: (509)335-7674
email: rjcook@wsu.edu

Perry B. Cregan
[Crop Science Society of America]
USDA-ARS
Bldg. 011, Headhouse - 19, BARC-West
Beltsville, MD 20705
Tel: (301)504-5070
FAX: (301)504-5728
email: pcregan@asrr.asrusda.gov

Peter R. Day
[American Phytopathological Society]
Rutgers University
Cook College
P.O. Box 231
New Brunswick, NJ 08903-0231
Tel: (908)932-8165
FAX: (908)932-6535
email: day@mbci.rutgers.edu

Thomas E. Devine
[Crop Science Society of America]
USDA-ARS
Plant Molecular Biology Laboratory
Bldg. 006, BARC-West
Beltsville, MD 20705
Tel: (301)504-6375
FAX: (301)504-5320
email: reisingr@asrr.arsusda.gov

Elisabeth Gantt
[American Society of Plant Physiologists]
University of Maryland
Department of Plant Biology
HJ Patterson Hall
College Park, MD 20742-5815
Tel: (301)405-1625
FAX: (301)314-9082
email: eg37@umail.umd.edu

David G. Gilchrist
[American Phytopathological Society]
University of California
CEPRAP and
Department of Plant Pathology
Davis, CA 95616
Tel: (916)752-3045
FAX: (916)753-2697
email: dggilchrist@ucdavis.edu

Brian Hyps
American Society of Plant Physiologists
15501 Monona Drive
Rockville, MD 20855-2768
Tel: (301)251-0560
FAX: (301)279-2996
email: bhyps@aspp.org

George G. Kennedy
[Entomological Society of America]
North Carolina State University
Department of Entomology
Research Annex West
Raleigh, NC 27695
Tel: (919)515-1655
FAX: (919)515-3748
email: gkennedy@unity.ncsu.edu

Keith E. Menchey
AESOP Enterprises Ltd.
236 Massachusetts Ave., NE, Suite 400
Washington, DC 20002
Tel: (202)675-4511
FAX: (202)675-4512
email: aesop@access.digex.net

Linda Murphy
American Society for Microbiology
Office of Public Affairs
1325 Massachusetts Avenue, NW
Washington, DC 20005-4171
Tel: (202)942-9302
FAX: (202)942-9335
email: lmurphy@asmusa.org

Joyce A. Nettleton
Institute of Food Technologists
221 North LaSalle Street
Suite 300 Chicago, IL 60601-1291
Tel: (312)782-8424
FAX: (312)782-8348
email: janettleton@ift.org

Timothy J. Ng
[American Society for Horticultural Science]
University of Maryland
Department of Horticulture
College Park, MD 20742-5611
Tel: (301)405-4345
FAX: (301)314-9308
email: tng@deans.umd.edu

Calvin O. Qualset
[American Society of Agronomy]
Genetic Resources Conservation Program
University of California
Davis, CA 95616
Tel: (916)754-8502
FAX: (916)754-8505
email: coqualset@ucdavis.edu

J. Scott Thenell
[Institute of Food Technologists]
DNA Plant Technology Corporation
6701 San Pablo Avenue
Oakland, CA 94608
Tel: (510)450-9310
FAX: (510)547-2817
email: thenell@dnap.com

Sue A. Tolin
[American Phytopathological Society]
VPI and State University
Department of Plant Pathology
Blacksburg, VA 24061
Tel: (540)231-5800
FAX: (540)231-5755
email: stolin@vt.edu

Anne K. Vidaver
[American Society for Microbiology]
University of Nebraska
Department of Plant Pathology
406 Plant Sciences Hall
Lincoln, NE 68583-0722
Tel: (402)472-2858
FAX: (402)472-2853
email: path001@unlvm.unl.edu

Box 1. Components of a Plant Breeding Program

This diagram represents a simplified sequence of the elements of a typical plant breeding program (adapted from Qualset (1982)). The major points relevant to modern plant breeding are:

(1) multiple sources of genetic variability as native or induced diversity within the crop species, its relatives, or other species (see Box 2); (2) access to variability from induced mutation, sexual hybridization, or parasexual methods of gene transformation; (3) selection of plants for desired traits through several sexual generations or clonal propagations under rather close containment to assure genetic and reproductive stability of expression of the trait(s) under selection; (4) multi- environment testing in field, greenhouse, and laboratory to assure stability of expression of the targeted traits, acceptable agronomic and end-use performance, potential for undesirable traits, such as weediness or hybridization with other crops or species, and range of adaptation or limitations in adaptation due to genotype x environment interactions; (5) official review and release of a new variety; and (6) scale-up of seed or clonal propagules under specified conditions of isolation from other varieties of the same crop and from weedy relatives that may hybridize with the new variety.

The primary gene pool in the center of the diagram includes varieties and landraces within the same biological species as the crop plant that is subject to improvement by plant breeding. Hybridization within this gene pool is generally not restricted, although sexual incompatibility genes prevent occurrence of certain hybrid combinations in some crops. The secondary gene pool consists of the crop progenitors; generally these can be hybridized readily with the crop plant. The third circle includes wild relatives or domesticated species more distantly related to the crop; these usually can be hybridized with the crop plant, but often embryo rescue on artificial growth media or other special culture conditions must be applied to produce a viable hybrid progeny. The next circle shows any other plant species; these can be gene resources for the crop plant transferred rarely by sexual hybridization, and practically by parasexual methods using cloned DNA transferred by a vector to cells of the crop plant. There are several methods to accomplish gene transformation, as this process is known. The next circle represents genes from any other organism, including animals, that can only be transferred to the crop plant by rDNA technology. Finally, the outside circle is a view of "designer genes" being synthesized from known nucleotide sequences, or from sequences having predicted desirable properties. Only gene transformation methods can be used to introduce these genes.

It should be emphasized that gene transformation is also used increasingly for gene transfer among individuals that can be hybridized by sexual means. This permits transfer of only genes of known characters from the gene source to the target crop variety. Gene transfer is completed after one or two progeny generations. In contrast, sexual hybridization combines all of the genes of each of the parents into the hybrid; desirable combinations are then obtained by selecting from progeny produced after self- or sib-pollination. This process requires several sexual generations to recover the desired combination of traits. For greater efficiency, the hybrid can be backcrossed one or more times to the parent having the more desirable combination of traits.