Orchid fleck dichorhavirus movement protein shows RNA silencing suppressor activity

To counteract RNA interference-mediated antiviral defence, virus genomes evolved to express proteins that inhibit this plant defence mechanism. Using six independent biological approaches, we show that orchid fleck dichorhavirus citrus strain (OFV-citrus) movement protein (MP) may act as a viral suppressor of RNA silencing (VSR). By using the alfalfa mosaic virus (AMV) RNA 3 expression vector, it was observed that the MP triggered necrosis response in transgenic tobacco leaves and increased the viral RNA (vRNA) accumulation. The use of the potato virus X (PVX) expression system revealed that the cis expression of MP increased both the severity of the PVX infection and the accumulation of PVX RNAs, further supporting that MP could act as an RNA silencing suppressor (RSS). From the analysis of the RSS-defective turnip crinkle virus (TCV), we do not find local RSS activity for MP, suggesting a link between MP suppressor activity and the prevention of systemic silencing. In the analysis of local suppressive activity using the GFP-based agroinfiltration assay in Nicotiana benthamiana (16 c line), we do not identify local RSS activity for the five OFV RNA1-encoded proteins. However, when evaluating the small interfering RNA (siRNA) accumulation, we find that the expression of MP significantly reduces the accumulation of GFP-derived siRNA. Finally, we examine whether the MP can prevent systemic silencing in 16c plants. Our findings show that MP inhibits the long-distance spread of RNA silencing, but does not affect the short-distance spread. Together, our findings indicate that MP is part of OFV's counter-defence mechanism, acting mainly in the prevention of systemic long-distance silencing. This work presents the first report of a VSR for a member of the genus Dichorhavirus.