Small RNA sequences in transgenic Arabidopsis plants expressiong a viral RNA silencing suppressor of plantago asiatica mosaic virus.

RNA silencing plays an important antiviral role in plants and invertebrates. To counteract antiviral RNA silencing, most plant viruses have evolved viral suppressors of RNA silencing. Triple gene block protein 1 (TGBp1) of potexviruses is a well-characterized viral suppressor of RNA silencing, but the detailed mechanism by which it suppresses antiviral RNA silencing remains unclear. We demonstrated that transgenic expression of TGBp1 of plantago asiatica mosaic virus (PlAMV) induced developmental abnormalities in Arabidopsis similar to those observed in mutants of the trans-acting small interfering RNA (tasiRNA) synthesis pathway. tasiRNAs are plant-specific endogenous siRNAs generated from noncoding TAS transcripts after miRNA/AGO-mediated cleavage. The cleavage fragment is converted into double-stranded RNA (dsRNA) by SUPPRESSOR OF GENE SILENCING 3 (SGS3) and RNA-DEPENDENT RNA POLYMERASE 6 (RDR6), which is sequentially processed by DCL4 into tasiRNA. We demonstrate here that PlAMV TGBp1 inhibits the SGS3/RDR6-dependent dsRNA synthesis in the tasiRNA pathway. Furthermore, TGBp1 interacts with SGS3 and RDR6 and co-aggregates with SGS3-containing bodies (SGS3/RDR6-bodies) that are normally dispersed in the cytoplasm. In addition, TGBp1 forms homooligomers, whose formation coincides with TGBp1 aggregation with SGS3/RDR6-bodies. In conclusion, we identified a novel mechanism of RNA silencing suppression mediated by a viral suppressor of RNA silencing.