Abstract
The development of virus-resistant transgenic crops has widened the horizons of virus control. A common approach to confer virus resistance relies on the transfer and expression of viral genes in susceptible plants. The successful application of this strategy is illustrated by the commercialization of virus-resistant transgenic squash and papaya in the USA. Since virus-resistant transgenic plants express viral sequences, environmental safety issues have been raised, in particular on the potential for recombination and creation of new viruses. It is conceivable that recombinant viruses can arise from exchange of genetic information as a consequence of RNA recombination between transgene transcripts and the genome of challenging viruses. Resulting chimeric RNA may lead to viable recombinant viruses with identical biological properties as parental lineages or altered biologic al properties, including increased pathogenicity, expanded host range and changes in vector relationship. The development of recombinant viruses has been extensively documented in transgenic plants expressing viral genes, primarily under conditions of high to moderate selective pressure in confined environments. Under field conditions with limited, if any, selective pressure, no recombinant virus has been found at detectable levels even in transgenic perennial plants established in experimental sites over extended periods of time. So far, although the potential for recombination is real, extensive research indicates that the creation of recombinant viruses in transgenic crops expressing viral genes does not seem to exceed baseline events in conventional plants, thus providing valuable insights into the safe release of virus-resistant transgenic plants and suggesting a reasonable certainty of limited or no hazard.