Diversity of viral RNA silencing suppressors and their involvement in virus-specific symptoms.

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RNA silencing is an evolutionarily conserved and homology-dependent gene inactivation system that regulates most biological processes at either the transcriptional or post-transcriptional level. In plants, insects and certain mammalian systems, RNA silencing constitutes the basis of the antiviral defense mechanism. To counteract RNA silencing-based antiviral responses viruses adopt strategies of replication and host invasion that include mechanisms of RNA silencing suppression. Indeed, viruses can express proteins known as RNA silencing suppressors (RSSs). Over the last two decades, silencing studies in plant virology have been largely devoted to the discovery and description of RSSs. The result has been exciting and these studies have revealed (i) an incredible diversity of proteins and mechanisms of RSSs belonging to various viral taxonomic groups, (ii) the multifunctionality of RSSs: they can fulfill several functions during viral infection and target one or more key points in the RNA silencing machinery. Some RSSs of model viral systems have been the subject of exceptional in-depth studies; they have proven to be real molecular tools for studying plant physiology, plant biology and virus-plant interactions, even in some cases extending the knowledge of the response of plants to other biotic and abiotic stressors. RSS diversity in phylogenesis, in mechanism of action and the frequent presence of more than one RSS in a single viral genome all suggest that they are extremely plastic in evolving to overcome host defenses. In this chapter, we present and discuss the most recent findings related to the well-studied RSSs of four viral taxonomic groups: geminiviruses, potyviruses, tombusviruses and cucumoviruses.