The risk of generating new viruses and viral diseases through the use of virus-resistant, genetically engineered crop plants is likely to be no greater than the risk posed by traditional plant-breeding methods and should not inhibit development and evaluation of plants carrying genes engineered for virus resistance, according to researchers at the University of California, Davis. Their risk-assessment perspective will appear in Friday's issue of the journal Science. This potential risk has become an issue in light of the development of a new technique for introducing viral disease resistance into plants by inserting certain portions of the virus's genes into plants. The viral genetic material provides the plant with a repellent genetic shield against that specific virus. Recent studies, however, indicate that once in the plant, the virus-derived genetically engineered material can recombine with the genetic material of an infecting mutant virus, creating a virus now able to infect the plant. "Under typical agricultural conditions, plant viruses have many opportunities to interact genetically as viral genes are distributed over vast acreages by insects, seeds and plant cuttings," said Bryce W. Falk, a professor of plant pathology and an authority on viral diseases of plants. "There is no reason to believe that insertion of viral genetic material into plants to create disease resistance will result in any greater number of new viruses than already occur under normal processes of viral infection in the field." "We have seen the emergence of virulent strains of plant viruses resulting from traditional breeding of virus-resistant crop plants, yet we would not consider abandoning plant breeding," said co-author George E. Bruening, a professor of plant pathology and director of the UC Davis Center for Engineering Plants for Resistance Against Pathogens. "It would be equally foolish to abandon development of genetically engineered virus-resistant crops in light of a vanishingly small risk of creating new and harmful viruses." Previous studies have demonstrated that there is ample opportunity for viruses to interact genetically in the field, with single and multiple viral infections commonly occurring in both crop plants and weeds. As many as five different viruses can be commonly found in one plant, according to Falk. Furthermore, the genetic material of viruses -- known as RNA or ribonucleic acid -- has been shown to be able to recombine in four groups of plant viruses. However, such recombination seems to be detected only when there is strong environmental pressure on the virus to either change or be eliminated in the evolutionary process, said Falk. In addition to this selective pressure, RNA recombination seems to require similar patterns in the sequences of the chemical components of the recombining RNAs. Current evidence suggests that while approximately 100 identical components are needed for recombination between related viral RNAs, about 1,000 identical components are necessary for recombination between viral RNA that has been transferred into a plant and the RNA of a related co-infecting virus. Even if recombination occurs in the transgenic crop plant, it is unclear whether it would result in a viral disease that would be economically or environmentally significant. "Although we can't yet definitively determine whether plant-virus RNA recombinations will result in the evolution of more virulent and difficult-to-control viruses, we do understand enough about RNA-RNA recombinations and about virus-derived resistance genes to make some good guesses," Falk said. He notes that the potency of the recombination depends on the recombination occurring more frequently than do normal mutations and recombinations, and on the resultant virus being competitive and producing a significant disease. "It is unlikely that viruses resulting from recombinant transgenic RNA and viral RNA will occur more frequently than they already are occurring in natural infections," Falk said. "And it is doubtful that any new virus would be more viable than existing competing viruses." This overview of the viral risk of genetically engineered crop plants was funded in part by the UC Davis Center for Engineering Plants for Resistance Against Pathogens, which is sponsored by the National Science Foundation.