Turnip crinkle virus-encoded suppressor of RNA silencing interacts with Arabidopsis SGS3 to enhance virus infection.

Most plant viruses encode suppressors of RNA silencing (VSRs) to protect themselves from antiviral RNA silencing in host plants. The capsid protein (CP) of Turnip crinkle virus (TCV) is a well-characterized VSR, whereas SUPPRESSOR OF GENE SILENCING 3 (SGS3) is an important plant-encoded component of the RNA silencing pathways. Whether the VSR activity of TCV CP requires it to engage SGS3 in plant cells has yet to be investigated. Here, we report that TCV CP interacts with SGS3 of Arabidopsis in both yeast and plant cells. The interaction was identified with the yeast two-hybrid system, and corroborated with bimolecular fluorescence complementation and intracellular co-localization assays in Nicotiana benthamiana cells. While multiple partial TCV CP fragments could independently interact with SGS3, its hinge domain connecting the surface and protruding domains appears to be essential for this interaction. Conversely, SGS3 enlists its N-terminal domain and the XS rice gene X and SGS3 (XS) domain as the primary CP-interacting sites. Interestingly, SGS3 appears to stimulate TCV accumulation because viral RNA levels of a TCV mutant with low VSR activities decreased in the sgs3 knockout mutants, but increased in the SGS3-overexpressing transgenic plants. Transgenic Arabidopsis plants overexpressing TCV CP exhibited developmental abnormalities that resembled sgs3 knockout mutants and caused similar defects in the biogenesis of trans-acting small interfering RNAs. Our data suggest that TCV CP interacts with multiple RNA silencing pathway components that include SGS3, as well as previously reported DRB4 (dsRNA-binding protein 4) and AGO2 (ARGONAUTE protein 2), to achieve efficient suppression of RNA silencing-mediated antiviral defence.