Plant viruses against RNA silencing-based defenses: Strategies and solutions

Abstract Plants defend themselves from pathogenic virus infections by deploying a complex defense system. Fundamental in the plant antiviral immune system is the RNA silencing (RS) pathway, which consists in a multilayered set of enzymatic components. The RS machinery targets the double-stranded RNA, generating 21–24 nucleotides (nt) small interfering RNAs (siRNAs) that guide the cleavage of viral RNA in a sequence-specific manner. RS signals are then amplified and transmitted cell-to-cell and systemically, in a process that is provided with self-regulatory feedback mechanisms. All the steps of the RS pathway were revealed to be targeted by viral counterattack strategies that allow pathogens to escape host defenses and establish a successful infection. Viral suppressors of RNA silencing (VSRs) are virus-encoded proteins that interfere with RS by blocking or destabilizing the host antiviral functions. This chapter reviews the current knowledge on VSRs mechanisms of action, and describes the latest discoveries in this fascinating evolutionary arms race between plants and viruses.