Roles of RNA silencing in viral and non-viral plant immunity and in the crosstalk between disease resistance systems

RNA silencing is a well-established antiviral immunity system in plants, in which small RNAs guide Argonaute proteins to targets in viral RNA or DNA, resulting in virus repression. Virus-encoded suppressors of silencing counteract this defence system. In this Review, we discuss recent findings about antiviral RNA silencing, including the movement of RNA through plasmodesmata and the differentiation between plant self and viral RNAs. We also discuss the emerging role of RNA silencing in plant immunity against non-viral pathogens. This immunity is mediated by transkingdom movement of RNA into and out of the infected plant cells in vesicles or as extracellular nucleoproteins and, like antiviral immunity, is influenced by the silencing suppressors encoded in the pathogens' genomes. Another effect of RNA silencing on general immunity involves host-encoded small RNAs, including microRNAs, that regulate NOD-like receptors and defence signalling pathways in the innate immunity system of plants. These RNA silencing pathways form a network of processes with both positive and negative effects on the immune systems of plants.