A genetically modified (GM) food is a plant that has a genetic change in each of its cells that a researcher has introduced. The modification may add a gene from a different species and thereby create a transgenic plant, or it may overexpress or silence a preexisting plant gene. Overexpression is accomplished by altering the promoter region of a gene, which controls how rapidly and in which cells the encoded protein is synthesized, thus directing a plant to manufacture more of a natural product. Conversely, a gene may be "silenced" (directed not to synthesize a protein) through the use of antisense technology, which applies a complementary nucleic acid to messenger RNA, halting expression of the encoded protein.
Genetic Modification in Animals and Plants
Animals have not yet been genetically modified to provide foods. Transgenic animals can, however, produce certain pharmaceuticals, but this approach is still experimental. One possible future use of transgenic animals is to create herds of cattle or sheep that are genetically resistant to developing transmissible spongiform encephalopathies, such as scrapie in sheep and "mad cow disease" in cattle.
Genetic modification in plants produces the same types of changes that result from traditional agricultural techniques, such as controlled breeding. However, genetic modification alters one gene at a time in a controlled manner, and typically has faster results than breeding plants with particular combinations of traits. With standard breeding techniques, it may take a generation to introduce, or remove, a single gene. Breeding a polygenic trait (a trait that involves more than one gene) into apples, which have a generation time of four years, could take two decades or longer.
GM traits that have already been introduced into plants include resistance to insects, insecticides, and herbicides; larger fruits; salt tolerance; slowed ripening; additional nutrients; easier processing; insecticide production; and the ability to take its own nitrogen from the air, lowering reliance on fertilizer. Specific products of genetic manipulation include insect-resistant corn, frost-resistant strawberries, rice that makes beta-carotene (a vitamin precursor), frost and salt-tolerant tomatoes, delayed-ripening pineapples and bananas, canola with a healthier oil profile, and cotton and trees altered to make it easier to process fabric and paper. Some transgenic combinations are strange. Macintosh apples that have been given a gene from a Cecropia moth that encodes an antimicrobial protein, for example, are resistant to a bacterial infection called fire blight.
Regulatory Concerns
Whether a new variety of crop plant presents a hazard to human health depends upon the nature of the trait, not how the plant received that trait. For example, the U.S. Department of Agriculture found that a variety of potato obtained through conventional breeding was very toxic, and so it was never developed as a food. However, a potato developed through genetic modification at about the same time did not contain the toxin and was apparently safe to eat. This is why U.S. government regulatory agencies do not evaluate crops on how they were developed, but on their effects on the digestive tracts of animals.
Even after government agencies approve the marketing of a GM crop, consumer acceptance is crucial to its success. The FlavrSavr tomato, for example, was introduced in the 1980s. It ripened later, while in the supermarket, which extended its shelf life while providing an attractive product. However, the developers had focused only on this characteristic, and the tomatoes just did not taste very good. Consumer objection to GM foods also contributed to the FlavrSavr's failure. However, a high-solids GM tomato sold in England before the anti-GM movement began was popular with consumers, largely because it was priced lower than other tomatoes.
The Technique of Genetic Modification
The first step in developing a transgenic plant is to identify a trait in one type of organism that would make a useful characteristic if transferred to the experimental plant. The components of an experiment to create a transgenic plant are the gene of interest, a piece of "vector" DNA that delivers the gene of interest, and a recipient plant cell. Donor genes are often derived from bacteria, and are chosen because they are expected to confer a useful characteristic, such as resistance to a pest or pesticide.
To begin, the donor DNA and vector DNA are cut with the same restriction enzyme. This creates hanging ends that are the same sequence on both of the DNA molecules. Some of the pieces of donor DNA are then joined with vector DNA, forming a recombinant DNA molecule. The vector then introduces the donor DNA into the recipient plant cell, and a new plant is grown.
For plants that have two seed leaves (dicots), a naturally occurring ring of DNA called a Ti plasmid is a commonly used vector. Dicots include sunflowers, tomatoes, cucumbers, squash, beans, tomatoes, potatoes, beets, and soybeans. For monocots, which have one seed leaf, Ti plasmids do not work as gene vectors. Instead, donor DNA is usually delivered as part of a disabled virus, or sent in with a jolt of electricity (electroporation) or with a "gene gun" (particle bombardment). The monocots include the major cereals (corn, wheat, rice, oats, millet, barley, and sorghum).
Transgenesis in plants is technically challenging because the transgene must penetrate the tough cell walls, which are not present in animal cells. Instead of modifying plant genes in the nucleus, a method called transplastomics alters genes in the chloroplast, which is a type of organelle called a plastid. Chloroplasts house the biochemical reactions of photosynthesis. Transplastomics can give high yields of protein products, because cells have many chloroplasts, compared to one nucleus. Another advantage is that altered chloroplast genes are not released in pollen, and therefore cannot fertilize unaltered plants. However, it is difficult to deliver genes into chloroplasts, and expression of the trait is usually limited to leaves. This is obviously not very helpful in a plant whose fruits or tubers are eaten. The technique may be more valuable for introducing resistances than enhancing food qualities. Someday, transplastomics may be used to create "medicinal fruits" or edible vaccines.
Gm Beyond the Laboratory
After genetic modification, the valuable trait must be bred into an agricultural variety. Consider "golden rice," a grain that was given genes from daffodils and a bacterium to confer on it the ability to manufacture beta-carotene, a precursor to vitamin A. The first golden rice plants were created solely to show that the manipulation worked, and the modification of an entire biochemical pathway took a decade. The plant varieties were not edible, and the production of beta-carotene was low. In early 2002, however, researchers at the International Rice Research Institute in the Philippines began using conventional breeding to transfer the ability to produce beta-carotene from the inedible golden rice into edible varieties.
Genetic manipulation of plants can also focus on a particular species' own genes. This is the case for the potato, which has traditionally been difficult to cultivate because edible varieties must have an acceptable taste and texture, yet lack the alkaloid toxins that many natural strains produce. Breeding for so many characteristics is very time-consuming, and this is where genetic manipulation might speed the process. Researchers have identified a group of disease resistance genes on a region of one potato chromosome. The genes provide resistances to various insects, nematode worms, viruses, and Phytophthora infestans, which caused the blight infection that resulted in the nineteenth-century Irish potato famine. Being able to manipulate and transfer these genes will help researchers quickly breed safe and tasty new potato varieties, and perhaps transfer the potato's valuable resistance genes to related plants, such as tomatoes, peppers, and eggplants.
GM crops are widely grown in some countries, but are boycotted in others where many people object to genetic manipulation. As of 2001, 75 percent of all food crops grown in the United States were genetically modified, including 80 percent of soybeans, 68 percent of cotton, and 26 percent of corn crops. Farmers find that GM crops are cheaper to grow because their reliance on pesticides and fertilizer is less and a uniform crop is easier to harvest. Heavy reliance on the same varieties may be dangerous, however, if an environmental condition or disease should arise that targets the variety, but this dilemma also arises in traditional agriculture.
Because GM crop use is so pervasive in the United States, and because regulatory agencies evaluate the chemical composition and biological effects of crops rather than their origin, a consumer would not know that a fruit or vegetable has been genetically modified unless it is so labeled. Some people argue that these practices prevent consumers from having a choice of whether or not to use a genetically modified food.
Bibliography
Fletcher, Liz. "GM Crops Are No Panacea for Poverty." Nature Biotechnology 19, no. 9 (September 2001): 797-798.
Hileman, Bette. "Engineered Corn Poses Small Risk." Chemical and Engineering News 79, no. 38 (September 17, 2001): 11.
Maliga, Pat. "Plastid Engineering Bears Fruit." Nature Biotechnology 19, no. 9 (September 2001): 826-927.
Potrykus, I. "Golden Rice and Beyond." Plant Physiology 123 (March 2001): 1157-1161.
—Ricki Lewis






