Molecular and functional characterization of the barley yellow striate mosaic virus genes encoding phosphoprotein, P3, P6 and P9

Plants rely on RNA silencing (RS) as a major layer of innate defense to neutralize invading viruses. In an opposite way, plant viruses evolved a survival mechanism, based on viral suppressors of RS (RSS), able to hinder the host RS activation system. In the present study, the sequences of the barley yellow striate mosaic virus (BYSMV) proteins P3, P6, P9 and of Phosphoprotein (P), from Chahar Mahall (Iran) isolate, were determined. Furthermore, we carried out agroinfiltration assays in leaves of Nicotiana benthamiana (16c line) to explore the RS suppression mechanism of these proteins. Analyses showed that P and P3 act as weak local and limited short-term systemic inducers of RSS. A reduction of RNA-dependent RNA polymerase 6 (RDR6) gene expression was observed in presence of BYSMV P and P3 proteins, which imply their putative role in interfering with systemic RS, in which RDR6 is involved. In addition, P and P3 RSS activity were reduced when co-expressedwith other BYSMVaccessory proteins, indicating that, in the system used, only free P or P3 proteins lead to RSS. The BYSMV P protein lacked any detectable Glycine-Tryptophan (WG/GW) or F-box motifs, known to interfere with the AGO1 slicing activity. On the opposite, two WG/GW were identified in P3 sequence.