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Genetically modified crops

Article by: Prakash, C. S. College of Agriculture, Tuskegee University, Tuskegee, Alabama. Publication year: 2005 DOI: http://dx.doi.org/10.1036/1097-8542.YB051540 (http://dx.doi.org/10.1036/1097-8542.YB051540)

Content

• Traditional breeding • Genetic modification technology • transfer • GM plant development • Today's GM crops • Tomorrow's GM crops • Benefits • Safety of GM foods and crops • Conclusion • Bibliography • Additional Readings

Genetic modification is the newest scientific tool for developing improved crop varieties. Such crops can help to enhance agricultural productivity, boost food production, reduce the use of farm chemicals, and make our food healthier. Genetically modified (GM; also called transgenic, genetically engineered, or bioengineered) crops represent the fastest-adopted technology in the history of agriculture, yet they are not universally accepted because of perceived concerns about their safety. Skeptics believe that such crops may pose unrecognized risks to human and animal health and could damage the environment.

Traditional

Humans have been modifying crop ever since farming began 10,000 years ago, when wild plants were first domesticated to provide food, feed, and fiber. Traditionally, this modification was accomplished primarily through selection of desirable plant types but more recently by plant breeding, which involves the crossing of plants with desirable traits, followed by many generations of selection to eliminate undesirable traits acquired from wild plants. Thus, every crop is a product of repeated genetic adjustment by humans over the past few millennia. While this process has provided the current crop varieties, it is slow and arduous. In addition, the traits for which plants can be bred are limited to those that occur in the wild in closely related crop species; it has not been possible to incorporate characteristics that occur only in nonrelated species.

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Genetic modification technology

Recombinant deoxyribonucleic acid (DNA) technology is the most recent tool employed by crop breeders to improve traditional methods of incorporating desirable traits and to eliminate undesirable characters. Genetic modification technology, while providing greater precision in modifying crop plants, enables scientists to use helpful traits from a wider pool of species to develop new crop varieties quickly. Genetic modification involves a clear-cut transfer of one or two known into the plant —a surgical alteration of a tiny part of the crop's genome compared with the sledgehammer approaches of traditional techniques, such as wide-cross hybridization or breeding, which bring about gross genetic changes, many of which are unknown and unpredictable. Furthermore, unlike traditional varieties, modern GM crops are rigorously tested and subjected to intense regulatory scrutiny for safety prior to commercialization.

Gene transfer

The direct transfer of genes into plants is achieved through a variety of means, but the vector and the “” methods are the most common.

Argobacterium vector

Agrobacterium tumefaciens is a soil-borne natural that causes tumors in plants by transferring a piece of its DNA to plant cells. The bacterial cells harbor a large plasmid, called Ti (tumor-inducing) plasmid, which carries the tumor-causing genes in a region of the plasmid called T-DNA. Scientists have modified this bacterium to eliminate disease-causing genes from the T-DNA region, enabling the disarmed bacterium to deliver desirable bacterial genes, such as those for insect resistance, to the plant cells. The resulting plants grown from such cells are healthy but contain the newly introduced gene in every cell expressing the desired trait (Fig. 1).

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Fig. 1 Agrobacterium vector method. The Ti plasmid of the plant bacterium Agrobacterium tumefaciens is used in plant . (Reprinted with permission from P. H. Raven and G. B. Johnson, , 6th ed., McGraw-Hill, New York, 2002)

The Agrobacterium approach is the most popular method used to deliver genes to plant cells because of the clean insertion and low-copy number of the inserted genes. However, this bacterium does not readily infect monocotyledonous crops, such as , rice, and corn. Scientists have genetically engineered new plasmids to help overcome this problem, and the Agrobacterium method is now being employed to transfer genes into cereal crops such as rice.

Gene gun

In the gene gun technique (also called particle bombardment or the Biolistic® approach), microscopic particles coated with the DNA fragment representing the desired gene are shot into the plant cells using a special device. A small proportion of the DNA which enters the cells becomes incorporated into the of the plant cell. The gene gun technique helps overcome some of the deficiencies of the Agrobacterium method (such as bacterial contamination, low-efficiency transfer to cereal crops, and inconsistency of results).

GM plant development

With any method of gene transfer, the plant cells containing the introduced gene are allowed to develop into full plants under -culture conditions. To ensure that only the modified cells are grown into full plants, scientists include an -resistant marker gene along with the desired gene to be introduced into plant cells. When plant tissues are cultured in a medium containing phytotoxic , such as kanamycin,

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only those genetically modified cells containing the marker gene are able to survive and proliferate. This helps scientists to selectively allow a few genetically transformed plant cells (among a mass of millions of untransformed cells) to develop into full plants.

Today's GM crops

GM plants were first developed in the laboratory in 1983. In 1994, the first commercial crop was released for cultivation by farmers and for public consumption. The ® , developed by Calgene in Davis, California, had a delayed ripening trait so that the fruits stayed firm after harvest. This was achieved by the suppression of the enzyme polygalacturonase, which occurs naturally in the cell walls and causes ripe tomatoes to soften. Thus, Flavr Savr® tomatoes, with lower levels of the enzyme than other varieties, can remain on the vine longer before being picked so that full flavor development can occur, because they soften more slowly and remain firm in the supermarket. Fruits and vegetables with improved shelf life are also being developed by silencing other genes related to ripening such as the ethylene receptor gene. In developing countries, nearly half of all the fresh produce perishes because of poor storage and transportation conditions. Thus, may help both farmers and consumers in these countries through development of fruits and vegetables with longer shelf lives.

Today, GM crops are grown on 70 million hectares across 18 countries. While the United States, Argentina, and Canada dominate the list of countries growing these crops, farmers in less developed countries now plant almost one-third of the world's transgenic crops on more than 20.4 million hectares. , corn, , and canola account for most of these crops, and they have been modified for pest resistance, tolerance, and disease resistance.

Herbicide tolerance

Much of the grown in the United States and Argentina is genetically modified to tolerate the herbicide glyphosate (Round Up®). This herbicide inhibits the production of certain essential amino acids in plants by blocking the action of one enzyme (EPSP synthase) in the biosynthetic pathway of these compounds, and thus is toxic to both weeds and crops. To develop tolerance to this herbicide, scientists introduced into crops a soil bacterium gene encoding an altered EPSP synthase enzyme that eludes binding by the herbicide. Thus, glyphosate will kill the weeds but leave the herbicide-resistant crops unharmed, providing extremely high selectivity between the weed and the crop. Herbicide-resistant crops have proved very popular with farmers, as they simplify the weed management operation, promote no-tillage or low-tillage farming which helps conserve the fertile topsoil, and allow the use of less toxic .

Pest resistance

GM cotton and corn with enhanced internal pest resistance are also being grown. These were developed through the transfer of genes encoding natural proteins that serve as a natural defense against pests. By transferring the gene for a protein (known as Bt) that blocks digestion in caterpillars from the bacterium

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Bacillus thuringiensis to a plant, scientists have created crops producing novel pesticidal proteins that help control the insect pest when it feeds on the plant. The advent of GM crops with improved pest resistance has led to considerable reduction in pesticide usage on farms and has also increased crop productivity.

Tomorrow's GM crops

Rapid advances in plant , , and bioinformatics are enabling scientists to develop novel GM crops with direct consumer benefits.

Nutritionally enhanced foods

Many nutritionally enhanced foods, such as healthier oils, low-calorie sugars, and vitamin-enriched vegetables and fruits, are under development. A good example of a nutritionally enhanced GM crop is . This rice is enriched with provitamin A (beta carotene) and, more recently, with elevated levels of digestible iron (Fig. 2). The diet of more than 3 billion people worldwide has inadequate levels of essential vitamins and minerals, such as vitamin A and iron. Deficiency in just these two micronutrients can result in severe anemia, impaired intellectual development, blindness, and even death. Many countries, including India, the Philippines, and Vietnam, are now developing and testing golden rice. Scientists in India have also developed high-protein potatoes by transferring a gene from an amaranth plant.

Fig. 2 Golden rice, a GM grain, enriched with beta carotene to address vitamin A deficiency. Light- colored regular rice is interspersed among the grains of golden rice. (Courtesy of Ingo Potrykus)

Other healthful products

Hypoallergenic , soybean, wheat, and even ryegrass are also on the horizon. Trees requiring fewer chemicals during paper making and even trees that can decontaminate heavy metals from polluted soils are under development. The incorporation of vaccine proteins into commonly eaten foods may provide a way of eliminating diseases in developing countries, where normal inoculation is cost-prohibitive. See also: Pharmaceutical crops (/content/pharmaceutical-crops/YB051820)

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Hardier crops

In many parts of the world, especially in sub-Saharan Africa and South Asia, agricultural productivity continues to be low because of environmental stresses such as drought, heat, soil salinity, and flooding. Hardier crops resilient to these factors are being developed which may make food production possible under marginal conditions and thus may help ensure increased food security.

Benefits

The world population is growing at a rapid pace and will reach 8 billion by 2025. While the available farmland will remain about the same, improving agricultural productivity, especially in the developing world, can ensure food security for all. Improving such farm productivity—while conserving the natural resource base—is a daunting task. The judicious use of genetic modification technology in combination with classical approaches can help improve the food production, reduce the impact of plant diseases or insect pests, tailor crops to harsh environmental conditions, and enhance the nutritive value of foods while reducing the environmental impact of farming.

In 2001, GM soybeans, corn, cotton, , squash, and canola increased U.S. food production by 4 billion pounds, saved $1.2 billion in production costs, and decreased pesticide use by about 46 million pounds. Such crops have also helped lower the pressure on natural resources used in farming: In 2000, U.S. farmers growing transgenic cotton used 2.4 million fewer gallons of fuel and 93 million fewer gallons of water, and were spared some 41,000 ten-hour days needed to apply pesticide sprays. In China, GM cotton varieties have lowered the amount of pesticides used by more than 75% and reduced the number of pesticide poisonings by an equivalent amount.

Safety of GM foods and crops

Despite the success of GM crops, there is still apprehension among some consumers, especially in Europe, about the safety of such foods. However, there has not been any documented case of harm from GM foods since their introduction in 1994. The safety of biotech crops and food has been affirmed by hundreds of studies, including a 15-year study by the European Commission that involved more than 400 teams on 81 projects. The scientific community has thus repeatedly demonstrated that biotech crops and foods are safe for human and animal consumption.

Antibiotic resistance

Concerns were expressed initially about the safety of antibiotic-resistance marker genes introduced along with desired genes into plants during genetic modification. Drawing from extensive scientific studies, the U.S. Food and Drug Administration and regulators from the European Union have concluded, however, that antibiotic-resistance markers in GM crops do not pose any significant new risks to human health or the environment. Critics have argued that antibiotic-resistance genes might be inadvertently transferred from GM plant cells to bacteria in the guts of animals and humans, making the bacteria resistant to antibiotics and thus rendering some antibiotics less useful for treating bacterial diseases. Research by various world

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agencies has consistently concluded that these genes have never been shown to be transferred from crops derived through biotechnology to bacteria in nature. Further, there are no antibiotics in these plant foods to cause in the bacteria that would cause them to become resistant to antibiotics, as occurs through excess antibiotic use in much meat.

Environmental toxicity of herbicide-resistant crops

Crop resistance to the herbicide glyphosate enables glyphosate to kill all weeds without harming crops. It makes growing the crop easier for farmers and eliminates the need to use high dosages of far more toxic compounds. Glyphosate has no effect on animals (unlike more toxic compounds often used) and breaks down rapidly (much more than other pesticides) in the environment. One of the main advantages of GM crops is that they enable a decrease in the use of a wide range of pesticides that are damaging to human health and the environment.

Bt pollen toxicity

Researchers at Cornell University found that monarch butterfly larvae that were fed milkweed leaves coated with high levels of pollen from Bt corn (corn genetically modified to produce the Bt protein) ate less, grew slower, and suffered a higher death rate than larvae that consumed milkweed leaves with pollen from non-Bt corn or with no pollen at all. However, field evaluations show that exposure of nontarget , such as monarch larvae, to Bt pollen would in fact be minimal. The Bt pollen toxicity is degraded rapidly by sunlight and is washed off leaves by rain. In addition, due to the enhanced pesticide resistance of the Bt corn, lower amounts of monarch-lethal insecticides can be sprayed.

Other concerns

Other concerns about genetically modified crops include the possibility that pollen from GM “supercrops” may spread to natural environments and become invasive or persistent weeds. However, a team of British scientists has recently found that genetically modified potatoes, beets, corn, and oilseed rape (canola) planted in natural habitats were as feeble at spreading and persisting in the wild as their traditional counterparts. Scientists said research should allay fears that genetic engineering per se would make plants more prone to becoming vigorous, invasive pests.

Another concern is that transferred genes may produce proteins that could cause allergic reactions. However, careful studies are performed to ensure that no such products are put on the market. Recently, a large company stopped developing a genetically engineered soybean that contained a protein from Brazil nuts because research showed that the new soybean triggered the same allergic reactions as does the Brazil nut.

Conclusion

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The governments of the United States and other nations have always maintained strict oversight over GM crops. In the United States, they are subject to the extensive, science-based regulations of the Department of Agriculture, the Food and Drug Administration, and the Environmental Protection Agency, which examine the safety of such crops on a case-by-case basis. The American Medical Association believes GM crops have the potential to improve nutrition as well as prevent and even cure disease. Moreover, the World Health Organization believes that GM crops can help developing nations overcome food security problems. Recently the National Academy of Sciences reported that foods from GM crops are as safe as any other foods in the supermarket. While public unease on any new technologies related to food is understandable, continued education on the safety of GM foods and continued oversight of their use may eventually quell such fears.

See also: Agricultural science (plant) (/content/agricultural-science-plant/015900); Biotechnology (/content/biotechnology/084350); Breeding (plant) (/content/breeding-plant/095100); Gene (/content/gene/284400); Genetic engineering (/content/genetic-engineering/285000); Herbicide (/content/herbicide/314900); Pesticide (/content/pesticide/501600)

C. S. Prakash

Bibliography

M. J. Chrispeels and D. E. Sadava, Plants, Genes and Crop Biotechnology, Jones and Bartlett, Boston, 2003

A. Hiatt, Transgenic Plants: Fundamentals and Applications, Marcel Dekker, New York, 1993

E. Galun and A. Breiman, Transgenic Plants, Imperial College Press, London, 1997

A. McHughen, Pandora's Picnic Basket: The Potential and Hazards of Genetically Modified Foods, Oxford University Press, 2000

H. I. Miller and G. Conko, The Frankenfood Myth: How Protest and Politics Threaten the Biotech Revolution, Praeger, 2004

Additional Readings

C. M. Benbrook, Troubled Times amid Commercial Success for Roundup Ready Soybeans, Northwest Science and Environmental Policy Center, 2001

J. E. Carpenter and L. P. Gianessi, Agricultural Biotechnology: Updated Benefit Estimates, National Center for Food & Agricultural Policy, 2001

J. Carpenter et al., Comparative Environmental Impacts of Biotechnology Derived and Traditional Soybean, Corn, and Cotton Crops, Council for Agricultural Science Technology, 2002

J. Fernandez Cornejo and W. D. McBride, Genetically Engineered Crops: U.S. Adoption and Impacts, ERS Agri. Econ. Rep. No. AER810, 2002

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NASS/USDA, Agricultural Chemical Usage 2001 Field Crops Summary, 2002

AgBio World (http://www.agbioworld.org/)

International Service for the Acquisition of Agri-biotech Applications (http://www.isaaa.org/)

National Center for Food and Agricultural Policy (http://www.ncfap.org/)

UCBiotech (http://www.ucbiotech.org/)

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