RNA Silencing and its Suppressors in the Plant‐virus Interplay

Abstract

Abstract Viruses are obligate intracellular parasites that need to evade the host defences targeted against them. In plants, activation of ribonucleic acid (RNA) silencing is among the most powerful responses against viruses. To overcome this defence, both deoxyribonucleic acid (DNA) and RNA plant viruses make use of viral products able to interfere with silencing mechanisms, and known collectively as RNA silencing suppressors (RSSs). RSSs present an extraordinary diversity in modes of action, and therefore, their study is providing valuable information about the different silencing pathways in plants. This article reviews our knowledge on RSSs, including the experimental systems used to identify them, and the mechanisms by which they act. Finally, the additional effects that they cause in the network of silencing pathways, and hence the need of a tight control of their activity, are also considered. Key concepts: Argonaute: Family of proteins that contain certain conserved domains, such as PIWI domain (found in Piwi, or P‐element‐induced wimpy testis in Drosophila , and related proteins) and PAZ domain (found in Piwi, Argonaute and Zwille proteins). Proteins of this family are the main components of RNA silencing effector complexes. Dicer: Ribonuclease belonging to the RNase III family. Dicer‐like enzymes cleave hairpin or double‐strand RNA molecules into short RNA duplexes of 20–24 nucleotides with a 2‐nucleotides overhang at the 3′ ends, which are key components of the RNA silencing pathways. microRNA (miRNA): Endogenous single‐strand RNA of 20–21 nucleotides, which derives from a hairpin precursor. miRNAs are incorporated into RISC, guiding it, by base complementary, to silence endogenous mRNAs. RNA‐dependent RNA polymerase (RdRP, RDR): Enzyme able to synthesize RNA using RNA as template. These enzymes are involved in amplification steps of RNA silencing. RNA‐induced silencing complex (RISC): Multicomponent complex including an Argonaute protein and a short RNA. RISC can silence mRNAs by endonucleolytic cleavage, decay acceleration and translation inhibition. RNA‐induced transcriptional silencing complex (RITS): Multicomponent complex including an Argonaute protein and a short RNA that silence gene expression by inhibiting or downregulating the transcription activity. RNA silencing: General term used to describe different events triggered by small RNAs to induce transcriptional or posttranscriptional gene downregulation in a sequence‐homology manner. RNA silencing suppressor (RSS): Factor (usually a protein) with the capacity to interfere with the onset of RNA silencing or with its maintenance. The expression of viral RSSs is the most common strategy that plant viruses use to escape from RNA silencing. Short interfering RNA (siRNA): Small RNA of 20–24 nucleotides derived from Dicer‐mediated cleavage of a double‐stranded RNA precursor. One of the strands of the siRNA is incorporated in RISC or RITS to guide their RNA silencing effector activities.