Plant genetic engineering has promised researchers improved speed and flexibility with regard to the introduction of new traits into cultivated crops. A variety of approaches have been applied to produce virus-resistant transgenic plants, some of which have proven to be remarkably successful. Studies on transgenic resistance to Cucumber mosaic virus probably have been the most intense of any plant virus. Several effective strategies based on pathogen-derived resistance have been identified; namely, resistance mediated by the viral coat protein, the viral replicase, and post-transcriptional gene silencing. Techniques using non-pathogen-derived resistance strategies, some of which could offer broader resistance, generally have proven to be much less effective. Not only do the results obtained so far provide a useful guide to help focus on future strategies, but they also suggest that there are a number of possible mechanisms involved in conferring these resistances. Further detailed studies on the interplay between viral transgene-derived molecules and their host are needed in order to elucidate the mechanisms of resistance and pathogenicity.

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One possible environmental risk related to the utilization of virus-resistant transgenic plants expressing viral sequences is the emergence of new viruses generated by recombination between the viral transgene mRNA and the RNA of an infecting virus. This hypothesis has been tested recently for cucumber mosaic virus (CMV) by comparing the recombinant populations in transgenic and non-transgenic plants under conditions of minimal selection pressure in favour of the recombinants. Equivalent populations were observed in transgenic and non-transgenic plants but, in both, there was a strongly dominant hotspot recombinant which was shown recently to be nonviable alone in planta, suggesting that its predominance could be reduced by applying an increased selection pressure in favour of viable recombinants. Partially disabled I17F-CMV mutants were created by engineering 6 nt deletions in five sites in the RNA3 39-non-coding region (39-NCR). One mutant was used to inoculate transgenic tobacco plants expressing the coat protein and 39-NCR of R-CMV. A total of 22 different recombinant types were identified, of which 12 were, as expected, between the transgene mRNA and the mutated I17F-CMV RNA3, while 10 resulted from recombination between the mutated RNA3 and I17F-CMV RNA1. Twenty recombinants were of the aberrant type, while two, including the dominant one detected previously under conditions of minimal selection pressure, were homologous recombinants. All recombinants detected were very similar to ones observed in nature, suggesting that the deployment of transgenic lines similar to the one studied here would not lead to the emergence of new viruses.