Backgrounder—From Gene Shuffling to Directed Precision:
The Sophistication of Plant Breeding
Modern biotechnology refers to a group of techniques that have wide application in research and commerce. One of these applications, recombinant deoxyribonucleic acid (rDNA) biotechnology, has become the focus of intense public scrutiny and debate.
Commonly known as genetic modification or gene splicing, rDNA biotechnology allows for the effective and efficient transfer of genetic material from one organism to another. Instead of cross-breeding plants for many generations or introducing mutations to introduce a desired trait—processes that are imprecise and that sometimes introduce unwanted changes—scientists can identify and insert one or more genes responsible for a particular trait into a plant or microorganism with greater precision and speed. These transferred genes, or transgenes, do not have to come from a related species in order to be functional, and can be moved virtually at will among different living organisms.
Recombinant DNA biotechnology can be applied to microorganisms, plants, and animals to develop useful products and processes in industries as diverse as drug development, fish farming, forestry, crop development, fermentation, and oil-spill clean up. This report focuses on rDNA biotechnology-derived foods, food ingredients, and animal feed of plant origin, and on the use of rDNA biotechnology-derived microorganisms such as yeasts and enzymes in food production. While milk from cows that have received rDNA biotechnology-derived hormones is discussed, transgenic animals resulting from the application of rDNA biotechnology techniques to animal reproduction are beyond the scope of this report. Health and medical benefits associated with rDNA biotechnologyderived plants are discussed briefly.
Plant Breeding over the Millennia
Although biotechnology is widely viewed as new, the plants and animals that modern agriculture produces today to feed the world’s people are the result of over 10,000 years of continuous genetic modification and refinement. Genetic modification of plants began approximately 10,000 years ago when people first used what is referred to as selective breeding. This technique simply involved saving seeds from the most vigorous plants in an environment for replanting at a later time. Over a period of many years, this selection resulted in higher yielding varieties of the crop. This type of selection turned a wild plant with almost no usefulness to humans into what we know today as corn, an important human food and animal feed crop in the United States. This process in itself runs counter to natural selection. Breeding involves selection for optimal growth for human purposes or other characteristics in an agricultural setting. In many cases, breeding is inconsistent with nature and the ability of the organism to survive under evolutionary pressure. Therefore, human intervention has involved a primitive type of directed genetic engineering from the outset.
The history of cultivated wheat serves as an excellent example of breeding versus natural selection. The seeds of wild wheat relatives disperse when their brittle seed heads shatter. In the earliest stages of domestication, 10,000 years ago, forms that do not shatter emerged, enabling gatherers to collect the ripe seeds rather than pick them up from the ground. Such a mutation in nature would prevent seed dispersal and lead to rapid extinction of those plants in the wild.
Microorganisms also have been used in food technology for thousands of years. As early as 6000 B.C., Sumerians and Babylonians used yeast to brew beer. Although the ancients knew nothing about microorganisms and could not knowingly culture them, they nevertheless systematically selected those with desirable fermentation characteristics to improve their food. In modern times, the increasingly powerful science of genetics has been systematically applied to produce many valuable variants of yeast and bacteria.
Breeding for Diversity, Diversity for Breeding
Selective breeding relies principally on genetic diversity in a starting population. As the available, unused genetic diversity of the species diminishes, the potential for improvement also decreases. Since crop improvement relies on genetic diversity, continued improvement has required and will continue to require even greater diversity. Farmers discovered that crosses between certain closely related species would produce fertile offspring. Cross-breeding (also known as interspecies or inter-generic breeding), either fortuitous or intentional, permitted recombination and selection among genes at a whole new level to provide new sources of genetic diversity and desirable traits.
Interspecies or cross-breeding offers two possible outcomes. First, new species that contain all of the genes from multiple parents can be created. Triticale, a wheatrye hybrid, is a product of this type of cross-breeding. Second, the resulting plant contains all the genes from one of the original plants, as well as randomly chosen copies of genes from either of the parents. This type of breeding in a sense is the precursor to modern rDNA biotechnology; however, it is highly imprecise. Large segments of chromosomes containing thousands of individual genes have been introduced from one species into another in this way. Breeders today employ this type of technology for many crops, including tomato, soybean, canola, and cotton, which are all products of extensive genetic modification and selection.
The products of naturally occurring interspecies crosses have been employed for thousands of years, and many of the foods eaten today are derived from such crosses. Thus, it is considered natural. Many interspecific crosses, however, produce infertile offspring, limiting the available gene pool. In order to further increase the potential for gene diversity, scientists developed a technique known as embryo rescue. This involves recovery of the embryo shortly after fertilization and placing it in an in vitro tissue culture system. In this unnatural setting, the embryo can develop into a mature, fertile plant. Tissue culture has, then, greatly expanded the potential genetic diversity by saving crosses that would never survive outside of a laboratory.
These conventional techniques for crop improvement share the disadvantage that they are by nature imprecise and unpredictable, and only occasionally useful. Many thousands of genes can be and routinely are transferred across the normal species boundaries using interspecies and even inter-generic crossing and embryo rescue. In spite of the unnatural and undefined nature of these changes, many years of experience have affirmed the safety and usefulness of genetically improved varieties. Plant breeders, farmers, food manufacturers, and consumers all have routine, frequent and extensive exposure to these genetically improved varieties.
Moments of Scientific Discovery
Twentieth century plant breeding, even before the advent of modern rDNA biotechnology methods, sought ways to take advantage of useful genes and gradually has found an ever-wider range of plant species and genera on which to draw. Breeders have long used interspecies hybridization, transferring genes between different, but related, species. Subsequently, plant geneticists found ways to perform even wider crosses between members of different genera using tissue culture techniques. Crops resulting from such wide crosses are commonly grown and marketed in the United States and elsewhere. They include familiar and widely used varieties of tomato, potato, corn, oat, sugar beet, bread and durum wheat, rice, and pumpkin.
By the 1940s, scientists understood that chromosomes were linear structures composed of DNA and protein and that chromosomes carried genes determining inherited characteristics. Genes were conceived of as beads on a string. It was also known that many organisms have two sets of chromosomes, one inherited from each parent. Further, those pairs of chromosomes were known to be present in all the cells of an organism. When cells divided, the chromosomes also divided equally by a process called mitosis.
In 1944, Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty established the key role played by DNA in determining the mechanism of inheritance in all living organisms. These scientists demonstrated how bacterial cells from one organism could take up DNA from another organism. This demonstration was the first observation of transformation, a phenomenon that is central to an understanding of rDNA biotechnology.
In 1953, James D. Watson and Francis H.C. Crick described the structure of the double-stranded DNA molecule. Their work helped scientists better understand how DNA not only functions but also replicates. Then in 1968, Stewart Linn and Werner Arber learned how to cut DNA at precise sites to insert a length of DNA from another organism, and rejoin them. This made it possible to produce many copies of a known DNA fragment to transform other organisms such as plants.
At this point, it was understood that DNA is the universal code used by all living things and, even more significantly, that the central functions of all organisms are nearly identical. Molecular biologists soon learned to determine the sequences of genes that encoded these properties. As more and more genes were sequenced and compared, scientists found that the products of the genes that encode similar traits in very diverse organisms are often very similar in protein sequence. It also became apparent that most genes do not have characteristics specific to the organism in which they are found. In fact, it is impossible to determine the organism from which a gene arises by inspecting the gene sequence alone. Put another way, there is no way to identify “fish genes,” “tomato genes,” “or broccoli genes.” The uniqueness of organisms lies not only in the DNA sequences of their genes, but also the organization of the genes which are present, and at what time and to what extent they are expressed.
It is against this experience base that rDNA biotechnology must be examined and compared. Recombinant DNA techniques involve the introduction of one or a few defined genes into a plant much more precisely than the techniques breeders have employed for millennia. Recombinant DNA techniques have provided both an important new set of tools for microbiologists and access to a broader range of markets. They enable researchers to precisely identify, characterize, enhance, and transfer the appropriate individual genes rather than uncontrolled and randomly assorted groups of genes, hoping the desirable ones were included. With precision, researchers can now readily move selected and well-characterized genetic material from virtually any source in nature, greatly increasing the diversity of useful genes available for crop and microbe improvement.
Enormous quantities of DNA have now been sequenced for a wide range of organisms. The genomes (the totality of genetic material) of several bacteria and small organisms have already been fully sequenced, and the genome sequences of higher organisms such as plants, animals, and even humans will soon be available. In fact, about 40 genomes are expected to have been sequenced by the end of 2000. Sequencing of the human genome is now more than 90 percent complete. In the course of determining DNA sequences, identical genes are regularly found in organisms that are only remotely related. This observation has provided direct evidence that genetic transfer systems operate in nature to produce natural rDNA-containing organisms.
Two methods of plant transformation are in use at the present time: free-DNA and T-DNA. In the free-DNA method, DNA carrying the gene of interest is literally shot into cultured plant cells. This method allows the introduction of precise sequences of DNA, but it is difficult to predict exactly where the sequence will be integrated. In the T-DNA method, nonpathogenic DNA from a bacterial plant pathogen, Agrobacterium tumefaciens, carries the genes of interest into the host-cell chromosome. The T-DNA method greatly increases the precision of DNA insertion. In both cases, the precision of rDNA biotechnology permits accurate determination of the location and number of copies of the inserted DNA, even if the location of DNA insertion cannot be controlled.
Plant breeding is also being changed by applications that do not involve transformation. For example, scientists have almost completely sequenced the entire genome of Arabidopsis thaliana, a small plant in the cabbage family. Due to the great similarities among plants in general, DNA sequences from Arabidopsis can be used to identify sequences that have the same function in economic crops. Yield, maturity, baking quality, flavor, and aroma are subject to improvement with this new knowledge.
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The first rDNA biotechnology-derived food plant marketed in the United States was the FlavrSavrTM tomato, introduced in 1994. Produced using T-DNA, this tomato carried an “antisense” gene for the enzyme that is responsible, in large part, for fruit softening. The antisense gene reduced the production of the ripening enzyme. It was expected that the tomato would have an extended shelf life since it would not soften as rapidly as regular tomatoes. In fact, the FlavrSavrTM tomato was not a commercial success as a retail product, but a processing variety proved to be useful to processors because the ripe fruit had a higher solids content, resulting in economic and quality advantages.
Following the introduction of the rDNA biotechnologyderived tomato in 1994, other rDNA biotechnologyderived crops that contained modified agronomic traits soon followed. These plants included squash resistant to some strains of zucchini yellows and watermelon mosaic viruses in 1994, insect-resistant potato and corn in 1995 and 1996, and herbicide-tolerant soybean and canola in 1996. Although consumers’ awareness is largely limited to these products, many others under development are expected to appeal more directly to consumers. These include fruits, root and leaf vegetables, and grains with enhanced nutritional and healthpromoting properties.
Neither Different Nor Dangerous
Even though food biotechnology is hardly new, some have nevertheless theorized that biotechnology may result in different and dangerous organisms. Considering that there are tens of thousands of the host organism’s own genes, the introduction by precise techniques of one or a few additional, well-characterized genes does not create an organism that is more likely to be changed in gross physical properties or wholesomeness than an organism derived through a traditional breeding program. Indeed, because of the greater precision in selecting the desired trait, an adverse result is unlikely. A corn plant with a newly inserted bacterial gene that confers increased resistance to the European corn borer (a commercially important insect predator) is still a corn plant. Likewise, a microorganism long used for food production is not altered in any fundamental way by the insertion of additional copies of a gene-encoded, rate-limiting enzyme. Aided by the recent voluminous data from the DNA sequencing of various genomes and other basic research on plants, such questions have been widely discussed and reported by an array of national and international scientific groups.
In a 1989 report, the National Research Council stated that “no conceptual distinction exists between genetic modification of plants and microorganisms by classical methods or by molecular techniques that modify DNA and transfer genes. . . . The same physical and biological laws govern the response of organisms modified by modern molecular and cellular methods and those produced by classical methods.”
A 1991 joint Food and Agriculture Organization/World Health Organization consultation, addressing the question of the safety of rDNA biotechnology-derived foods, came to similar conclusions:
Biotechnology has a long history of use in food production and processing. It represents a continuum embracing both traditional breeding techniques and the latest techniques based on molecular biology. The newer biotechnological techniques, in particular, open up very great possibilities of rapidly improving the quantity and quality of food available. The use of these techniques does not result in food which is inherently less safe than that produced by conventional ones.
Regulatory oversight over biotechnology spans three major federal agencies: the Food and Drug Administration (FDA), the Environmental Protection Agency (EPA) and the U.S. Department of Agriculture (USDA). Jurisdiction over the various products of biotechnology is determined by their use. More than one agency may be involved in regulating different aspects of a single biotechnologyderived product.
The Coordinated Framework for Regulation of Biotechnology, prepared by the Office of Science and Technology Policy (OSTP) and published in the June 26, 1986, Federal Register, is the comprehensive federal policy for ensuring the safety of biotechnology research and products. It explains the coordination among federal agencies. Subsequently, the OSTP prepared and published in the February 24, 1992, Federal Register, the document, Exercise of Federal Oversight within Scope of Statutory Authority: Planned Introductions of Biotechnology Products into the Environment. This notice describes a risk-based, scientifically sound approach to the oversight of planned introductions of biotechnology products into the environment. It focuses on the characteristics of the biotechnology product and the environment into which it is introduced, not the process by which the product is created. The ultimate goal of the regulatory agencies is to ensure the overall safety of foods, food ingredients, and feeds produced using biotechnology.
FDA is responsible for ensuring the safety and proper labeling of food products for human consumption. FDA also regulates the labeling and safety of animal feed, taking into account both the safety to human consumers of animal-derived food products, as well as safety to the animal. FDA’s primary statutory authority is provided by the Federal Food, Drug, and Cosmetic Act (FFDCA).
In 1992, FDA established its current policy on foods and animal feed derived from new plant varieties developed by conventional and new breeding techniques. Fundamentally, the policy is that existing requirements mandate the same safety standards for foods, food ingredients, and feeds, regardless of the techniques used in their production and manufacture. Nevertheless, FDA has maintained a “voluntary consultation procedure,” in which producers of rDNA biotechnology-derived foods are asked to consult with the agency before marketing their products, and without exception they have done so. To date, almost 50 new rDNA biotechnology-derived foods have been evaluated successfully in FDA’s voluntary consultation process.
Pharmaceuticals and Human Vaccines. FDA regulates rDNA biotechnology-derived pharmaceutical products for human and animal use under the FFDCA and the Public Health Service Act (PHSA). FDA also regulates rDNA biotechnology-derived vaccines for human use under the PHSA, while USDA regulates vaccines for animal use. Under both the FFDCA and the PHSA, new products must be the subject of premarket approval, based on laboratory and clinical testing to show the safety and effectiveness of the products for their intended uses.
Meat and Poultry Safety. The Food Safety and Inspection Service (FSIS) of the USDA is responsible for regulating the safety and labeling of meat and poultry products for human consumption. FSIS does so under the Federal Meat Inspection Act and the Poultry Products Inspection Act. FSIS consults with FDA regarding the safety of food ingredients.
Field Testing and Permits. The Animal and Plant Health Inspection Service (APHIS) is the agency within the USDA charged with protecting American agriculture against pests and diseases. Under the Plant Quarantine Act and the Federal Plant Pest Act, APHIS regulates the importation and interstate movement of plants and plant products that may result in the entry into the United States of injurious plant diseases or insect pests.
The field-testing and the commercial sale of agricultural biotechnology crops are regulated by APHIS through a permit and notification system. USDA’s regulations cover the introduction of organisms and products altered or produced through genetic engineering which are plant pests or which there is reason to believe are plant pests. “Plant pest” includes agents that can directly or indirectly injure or cause disease or damage in or to any plant. A “regulated article” includes any organism or any product, which has been altered or produced through genetic engineering, which is a plant pest, or for which there is reason to believe is a plant pest. The permit and notification system does not apply to plants that are modified through traditional breeding methods. Thus, USDA’s regulatory protocol is process based.
The introduction of a regulated article is prohibited unless a permit authorizes the introduction. The regulation is intended to prevent the introduction, dissemination and establishment of plant pests in the United States. APHIS will grant a permit only if it determines that the plant poses no significant risk to other plants in the environment and is as safe to use as more traditional varieties. APHIS can authorize non-regulated status for an article through a petition for a determination of non-regulated status. Non-regulated status allows a plant to be widely grown and commercialized.
Animal Vaccines. APHIS regulates animal vaccines under the Virus-Serum-Toxin Act. In general, animal vaccines are subject to pre-market approval, based on testing to show their safety and effectiveness.
Pesticides. Regulating pesticides, setting environmental tolerances for pesticides, and establishing safe levels for pesticide residues in and on crops is the responsibility of EPA. A pesticide is any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest. The Food Quality Protection Act (FQPA) of 1996 established a single, health-based standard for assessing the risks of pesticide residues in food or feed. The standard measures the aggregate risk from dietary exposure and other non-occupational sources of exposure. EPA must now focus explicitly on exposures and risks to infants and children, assuming when appropriate, an additional safety factor to account for uncertainty in data. If EPA determines that there is a “reasonable certainty that no harm” to the public will result from aggregate exposure to a particular pesticide residue, then that residue level will be deemed “safe.”
In the case of pesticides produced by plants developed using rDNA biotechnology, EPA’s Nov. 23, 1994, proposed rule takes the view that its regulatory process is focused on the pesticide and not on the plant; plants are subject to regulation only if they produce plant pesticidal proteins as a result of modification with rDNA techniques. The National Research Council issued a report in April 2000 accepting EPA’s regulatory approach in its proposed policy. In contrast, 11 major scientific societies representing more than 80,000 biologists and food professionals published a report warning that the EPA policy would discourage the development of new pest-resistant crops and prolong and increase the use of synthetic chemical pesticides; increase the regulatory burden for developers of pestresistant crops; limit the use of biotechnology to larger developers who can pay the inflated regulatory costs; and handicap the United States in competition for international markets.
In an effort to contribute to a meaningful dialogue on scientific issues and consumer concerns about rDNA biotechnology, the Institute of Food Technologists, a non-profit society for food science and technology, conducted a comprehensive review of biotechnology
. IFT convened three panels of experts, consisting of IFT members and other prominent biotechnology authorities, to evaluate the scientific evidence and write a report divided into four sections: Introduction, Safety, Labeling, and Benefits and Concerns.