UC Davis

P0 protein of some polerovirus members can target ARGONAUTE1 (AGO1) to suppress RNA silencing. Although P0 harbors an F-box-like motif reported to be essential for interaction with S phase kinase-associated protein 1 (SKP1) and RNA silencing suppression, it is the autophagy pathway that was shown to contribute to AGO1 degradation. Therefore, the role of P0-SKP1 interaction in silencing suppression remains unclear. We conducted global mutagenesis and comparative functional analysis of P0 encoded by Brassica yellows virus (BrYV) (P0^Br ). We found that several residues within P0^Br are required for local and systemic silencing suppression activities. Remarkably, the F-box-like motif mutant of P0^Br , which failed to interact with SKP1, is destabilized in vivo. Both the 26S proteasome system and autophagy pathway play a role in destabilization of the mutant protein. Furthermore, silencing of a Nicotiana benthamiana SKP1 ortholog leads to the destabilization of P0^Br . Genetic analyses indicated that the P0^Br -SKP1 interaction is not directly required for silencing suppression activity of P0^Br , but it facilitates stability of P0^Br to ensure efficient RNA silencing suppression. Consistent with these findings, efficient systemic infection of BrYV requires P0^Br . Our results reveal a novel strategy used by BrYV for facilitating viral suppressors of RNA silencing stability against degradation by plant cells.