RNA silencing pathways exist in most eukaryotes and regulate endogenes to integrate development and environmental responses, and protect organisms against foreign nucleic acids including viruses and transposons. In plants, small interfering RNA (siRNA) and microRNA (miRNA) duplexes of 21-24bp are produced from double-stranded RNA (dsRNA) precursors by Dicer-like RNaseIII enzymes (DCL1-4). Selected si/miRNA strands are then loaded into Argonaute proteins (AGO1-10), which guide an RNA-induced silencing complex (RISC) to mediate sequence-specific post-transcriptional gene silencing (PTGS) at the RNA level, and transcriptional gene silencing (TGS) at the chromatin level. Plant resistance to RNA viruses requires these same components, notably DCL2 and 4 for production of viral-derived siRNAs, AGO1 and 2 to guide viral-targeted RISCs, and an RNA-dependent RNA polymerase (RDR6) and the RNA binding protein SGS3 for amplification of viral dsRNA. While the central role of these factors is well-established, the molecular mode of action that enables these proteins to restrict viral replication is unknown; in fact, the field of RNA silencing is actively studying the extent to which endogenous mRNAs are silenced by AGO-mediated cleavage (“slicing”) or by translational repression (TR) coupled to mRNA decay. In addition, many questions regarding the cell biological and genetic basis for viral silencing have yet to be addressed in a satisfactory manner. This proposal outlines a series of interdisciplinary and mutually-reinforcing experiments to 1) test the mode of action of viral silencing, specifically the relative contributions of slicing and TR during resistance; 2) discover cell biological aspects of viral silencing, particularly where in cells RNA silencing components target viral RNA, and which forms of viral RNA are targeted; and 3) conduct the first large-scale forward genetic screen designed exclusively to reveal components required for viral silencing and its suppression.