Genes for resistance to plant viruses

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Abstract
Although many cases are known of plant genes conferring resistance to particular viruses, in no case is the mechanism of gene action fully understood. The complementary phenomenon of virulence in the virus, whereby an altered form may overcome a particular resistance gene, also awaits explanation. This article reviews current knowledge of genetic aspects of control of resistance and virulence, and draws attention to common features of mechanisms in different species. The evidence for polygenic resistance and “modifier” genes is critically assessed, and possible overlap between resistance and mechanisms of nonhost immunity examined. Theoretical models of resistance gene action and of the interaction between resistance and virulence genes are described and developed. The extent to which known mechanisms fit these models is tested. Current and future possibilities for exploiting resistance genes in crop protection using techniques of cell and genetic manipulation are discussed.