| Genetic Modification of Plants |
New traits introduced to crop plants by genetic engineering have the potential to increase crop yields, improve agricultural practices, or add nutritional quality to products. For example, transgenic crop plants capable of degrading weed killers allow farmers to spray weeds without affecting yield. Use of herbicide-tolerant crops may also allow farmers to move away from pre-emergent herbicides and reduce tillage, thereby decreasing soil erosion and water loss. Transgenic plants that express insecticidal toxins resist attacks from insects. Crops engineered to resist insects are an alternative to sprays, which may not reach all parts of the plant. They are also cost effective, reducing the use of synthetic insecticides. Genetic engineering has also been used to increase the nutritional value of food; "golden rice" is engineered to produce beta-carotene, for example. Edible vaccines, present in the plants we eat, may be on the horizon.
The new traits expressed in such transgenic plants are derived from a variety of other organisms. Scientists have given a gene from the bacterium Salmonella to cultivars of soybeans, corn, canola, and cotton to degrade the pesticide glyphosphate (Roundup TM). The gene for the insecticidal toxin in transgenic cotton, potato, and corn plants comes from the bacterium Bacillus thuringiensis (Bt). One of the genes allowing vitamin A production in golden rice is derived from the bacterium Erwinia uredovora; others are from the daffodil.
The development of golden rice involved the introduction of several genes into a plant to provide a multistep biochemical pathway
(Fig 2). Rice grain, which serves as a food staple for much of the world, lacks vitamin A. An estimated 100 million to 200 million children worldwide have vitamin A deficiency, a condition that causes blindness; and increases susceptibility to diarrhea, respiratory infection, and childhood diseases, such as measles. Beta-carotene and other carotenes (the red, yellow, and orange pigments found in carrots and other vegetables) are the precursors of vitamin A. Rice synthesizes beta-carotene in its chloroplasts but not in the edible seed tissue.
Ingo Potrykus and his colleagues found that geranyl geranyl diphosphate (GGPP), a precursor to carotenoid production, is present in rice seed. They genetically engineered golden rice to express the enzymes necessary for the conversion of GGPP to beta-carotene. The synthesis of beta-carotene from geranyl geranyl diphosphate requires four biochemical reactions, each catalyzed by a different enzyme. A bacterium, Agrobacterium tumefaciens, containing three plasmids, was used to introduce all the genes necessary for the complete biochemical pathway for beta-carotene production. It was possible to use three enzymes instead of four because the bacterial enzyme phytoene desaturase accomplishes what two plant enzymes (phytoene desaturase and beta-carotene desaturase) do.
If transgenic plants can help prevent vitamin deficiencies, can they also produce vaccines? Edible vaccines available in crops could help people in developing nations where transportation, refrigeration, and disposable needle supplies are limited. Hugh Mason and his colleagues (Boyce Thompson Institute) have expressed a gene that encodes an E. coli protein in potatoes. Volunteers who ate raw, modified potatoes developed antibodies to the protein. Research is underway to see whether the antibodies will protect against diarrhea induced by disease-causing E. coli.