that any unique insect vectoring properties of a transcapsidated virus genome will not be inherited.

Synergism Between Viral Transgenes and Heterologous Viruses

Mixed infections by viruses can sometimes lead to a synergistic disease syndrome that is more severe than that caused by either virus individually (Matthews 1991). In cases of synergism involving the potyvirus family of viruses, the region of the viral genome that causes exacerbation of disease codes for a protein termed HC-Pro (Pruss et al. 1997). The synergism effect appears due to the natural role of HC-Pro as a suppressor of the gene-silencing response (Scheid 1999). Thus, plants are unable to mount an effective defense response to infection. Indeed, transgenic plants that express potyvirus genome segments that include HC-Pro exhibit more severe symptoms when inoculated with heterologous viruses (Pruss et al. 1997). No data indicate that expression of viral coat protein or replicase proteins enhances the virulence of heterologous viruses.

The problem of synergism is manageable through avoiding the use of functional transgenes that encode defense-suppressor substances or pathogenicity-enhancer substances. In addition, the normal process of testing in breeding programs that seek to incorporate natural or transgenic resistance traits will reveal the extent to which the virulence of heterologous viruses is exacerbated. Thus, it is highly unlikely that transgenic plants with general hyper susceptibility characteristics will pass through a breeding program to commercialization.

2.8.3 Summary

In light of the above analysis, the committee found that

Most virus-derived resistance genes are unlikely to present unusual or unmanageable problems that differ from those associated with traditional breeding for virus resistance.

Case studies of virus resistant squash and papaya are presented in chapter 3 (section 3.1.4).

2.9 PEST RESISTANCE TO PEST-PROTECTED PLANTS AND RESISTANCE MANAGEMENT

In this section, the ability of pests to adapt and develop resistance to transgenic or conventional pest-protected plants will be discussed, and resistance management strategies to abate this development and their scientific basis will be presented.



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