Antiviral RNA interference in plants: increasing complexity and intertwining with other biological processes.

RNA interference (RNAi, also known as RNA silencing) is one of the most important plant defense responses to combat viral invasions. Although the major components of the RNAi pathway, the steps leading to viral small interference RNA (siRNA) biogenesis, and the strategies of viral counter-defense via RNAi suppressors have been well studied, the broader roles of RNAi in virus infection and seed transmission remain less thoroughly reviewed. In particular, the increasing complexity of RNAi-associated mechanisms and their integration with other biological processes have not been comprehensively summarized. Meanwhile, an increasing number of papers report on the identification of non-canonical RNAi pathways, novel host factors involved in RNAi, and the possibility of small RNAs acting cross-kingdom and modulating plant-virus-vector tritrophic interactions. In this review, we briefly overview the roles of RNAi in plant viral infections, and describe recent advances with an emphasis on the discoveries of novel positive and negative RNAi regulators, potential upstream and downstream signaling pathways of antiviral RNAi, and the prospects and challenges of double-stranded RNA applications, either expressed from transgenes or exogenously provided by a spray. We will also discuss how these novel findings reshape our views on (antiviral) RNAi, highlight remaining knowledge gaps and examine how these advances influence plant-virus co-evolution and inform the strategies that can be utilized to manage plant virus diseases to reduce their impact.