A GMO may worsen an existing pest or pathogen problem in a variety of ways. Currently the most common genetic engineering approach to increase plant resistance to insect pests is the "Bt strategy." This is based on the discovery that strains of a soil-dwelling bacterium, Bacillus thuringiensis (Bt), produce a class of proteins selectively toxic to many insect species that attack crops. Farmers and gardeners have used microbial sprays of Bt for many years to control insect pests as part of integrated pest-management programs. Bt insect control proteins have been engineered into major commodity crops and a growing list of vegetable, fruit, and tree species. The potential consequences of extensive and long-term use of Bt crops are one of the most widely discussed environmental issues associated with transgenic crops. The concern is that as insect pest populations increas ingly are exposed to high levels of Bt proteins over long periods, emergence of resistant individuals within the pest population will be accelerated. This concern with pest resistance to transgenic pesticides is the same as that with resistance to chemical pesticides as a result of overexposure. Many experts agree that the question of pest resistance to Bt is not "if" but "when." This is particularly important in organic farming where chemical alternatives are not acceptable.
The de novo generation of new viruses from virus-resistant (VR) engineered crops has also been raised as a potential risk. To date, the most widely used biotechnology approach to controlling plant virus diseases has been the use of genes derived from the plant viruses themselves. For a number of important virus pathogens, expression of the viral coat protein gene in the host plant inhibits replication of that virus. In addition to being the structural component of virus particles, coat proteins also play a role in determining the host range of the virus and serve other functions as well. For some virus groups, other viral genes have been used successfully to limit disease.
The presence of viral sequences in major crop plants may increase the likelihood of creating novel viruses through molecular recombination between the transgenes and the genomes of other viruses that infect the plant. Such exchange of genetic information encoding coat proteins genes, for instance, could lead to the production of a new recombinant virus that has a unique coat protein that alters its host range. Similarly, recombination between other transgenes and infecting viruses could yield new virus strains with novel characteristics. Multiple plant viruses simultaneously infect many crops, and there is strong molecular evidence that virus evolution has proceeded rapidly through the exchange of large blocks of genetic information via recombination. Ongoing studies are exam ining the frequency of recombination events in naturally infected plants compared with transgenic VR plants.
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