Viral suppression of RNA silencing in plants

RNA silencing is a conserved eukaryotic pathway involved in suppression of gene expression via sequence-specific interactions that are mediated by nt 21-24-long RNA molecules. In plants, cell-autonomous and noncell-autonomous steps of RNA silencing form the basis of an elaborate immune system that is activated by, and targeted against, viruses. As a counter-defensive strategy, viruses have evolved suppressor proteins that inhibit various stages of the silencing process. These suppressors are diverse in sequence and structure and appear to be encoded by virtually any type of plant viruses. In this review, we consider the impact of silencing suppression on virus infections and its possible contribution to symptom development. We examine the presumed mode of action of some silencing suppressors and discuss their value as molecular probes of the RNA silencing mechanism. Finally, the biotechnological applications of silencing suppression are considered.