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Key Words agriculture, biosafety, ecological impact, GMO, VRTP
* Abstract Virus-resistant transgenic plants (VRTPs) hold the promise of enormous benefit for agriculture. However, over the past ten years, questions concerning the potential ecological impact of VRTPs have been raised. In some cases, detailed study of the mode of action of the resistance gene has made it possible to eliminate the source of potential risk, notably the possible effects of heterologous encapsidation on the transmission of viruses by their vectors. In other cases, the means of eliminating likely sources of risk have not yet been developed. When such residual risk still exists, the potential risks associated with the VRTP must be compared with those associated with nontransgenic plants so that risk assessment can fully play its role as part of an overall analysis of the advantages and disadvantages of practicable solutions to the problem solved by the VRTP.
INTRODUCTION
The demonstration by Beachy's group in the mid-1980s that the expression of a viral coat protein (CP) gene in a transgenic plant could confer resistance to the donor virus was immediately hailed as a major breakthrough (86). It opened the prospect of a virtually unlimited source of virus resistance genes that could be used in crops in which sources of natural resistance genes were inadequate. Since this first article, there have been enormous numbers of reports of the creation of virus-resistant transgenic plants (VRTPs), nearly all of which can be considered to be applications of the concept of pathogen-derived resistance described by Sanford & Johnston in 1985 (99), since they are based on the expression of viral sequences, most often the CP gene. This is the case for all the VRTPs that have been authorized for unrestricted release in the United States (Table 1). The importance of resistance based on a CP gene is also illustrated by the fact that among the 794 authorizations for field tests of VRTPs carried out in the United States through October 2001 (http://www.nbiap.vt.edu/cf docs/fieldtests3.cfm), the vast majority concern plants expressing a CP gene. However, there were also numerous field trials of plants expressing other viral sequences, as well as several in which resistance is due to transfer of nonviral virus-resistance genes (Table 2). Since use of nonviral genes...