Abstract
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 (VSRs). TRIPLE GENE BLOCK PROTEIN1 (TGBp1) of potexviruses is a well-characterized VSR, but the detailed mechanism by which it suppresses RNA silencing remains unclear. We demonstrate that transgenic expression of TGBp1 of plantago asiatica mosaic virus (PlAMV) induced developmental abnormalities in Arabidopsis thaliana similar to those observed in mutants of SUPPRESSOR OF GENE SILENCING3 (SGS3) and RNA-DEPENDENT RNA POLYMERASE6 (RDR6) required for the trans-acting small interfering RNA synthesis pathway. PlAMV-TGBp1 inhibits SGS3/RDR6-dependent double-stranded RNA synthesis in the trans-acting small interfering RNA pathway. TGBp1 interacts with SGS3 and RDR6 and coaggregates with SGS3/RDR6 bodies, which are normally dispersed in the cytoplasm. In addition, TGBp1 forms homooligomers, whose formation coincides with TGBp1 aggregation with SGS3/RDR6 bodies. These results reveal the detailed molecular function of TGBp1 as a VSR and shed new light on the SGS3/RDR6-dependent double-stranded RNA synthesis pathway as another general target of VSRs.
Highlights
Sequence-specific RNA degradation in RNA silencing plays an important antiviral role in plants and invertebrates (Ding and Voinnet, 2007)
Viral RNA fragments generated by primary virus-derived small interfering RNAs (vsiRNAs)- and RNA-induced silencing complex (RISC)-mediated cleavage serve as a template for the host SUPPRESSOR OF GENE SILENCING3 (SGS3)/ RNA-DEPENDENT RNA POLYMERASE6 (RDR6) complex to produce de novo double-stranded RNA (dsRNA), which is subsequently processed into 21-nucleotide secondary vsiRNAs by DCL4 (Wang et al, 2011)
Downward curling of leaf margins is a characteristic phenotype of trans-acting small interfering RNA (tasiRNA)-deficient mutants such as sgs3, rdr6, dcl4, and ago7 (Figure 1A; Adenot et al, 2006), suggesting that TRIPLE GENE BLOCK PROTEIN1 (TGBp1) may prevent some specific steps in the tasiRNA pathway
Summary
Sequence-specific RNA degradation in RNA silencing plays an important antiviral role in plants and invertebrates (Ding and Voinnet, 2007). Once RNA silencing is initiated in a plant cell with the production of primary vsiRNAs from dsRNAs, it can be amplified through a process referred to as transitive silencing During this amplification phase, viral RNA fragments generated by primary vsiRNA- and RISC-mediated cleavage serve as a template for the host SUPPRESSOR OF GENE SILENCING3 (SGS3)/ RNA-DEPENDENT RNA POLYMERASE6 (RDR6) complex to produce de novo dsRNA, which is subsequently processed into 21-nucleotide secondary vsiRNAs by DCL4 (Wang et al, 2011). Amplification of RNA silencing has been implicated in the spread of an RNA silencing signal (Himber et al, 2003; Schwach et al, 2005; Kalantidis et al, 2008) This signal can move between cells via plasmodesmata and over long distances through phloem, triggering sequence-specific RNA silencing in distant tissues (Voinnet and Baulcombe, 1997; Voinnet et al, 1998). The requirement of RDR6 for systemic movement of a silencing signal suggests that amplification of RNA silencing is involved in antiviral defense in uninfected systemic tissues
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