Beet curly top Iran virus Rep and V2 gene work as suppressors of post-transcriptional gene silencing through separate mechanisms

Abstract

Beet curly top Iran Virus (BCTIV) is a yield-limiting geminivirus belonging to the becurtovirus genus. The genome organization of BCTIV is unique such that the complementary strand of BCTIV resembles mastreviruses, whereas the virion strand organization is close to curtoviruses. Geminiviruses are known to avoid the plant defense system by suppressing the RNA interference mechanisms both at the transcriptional gene silencing (TGS) and post-transcriptional gene silencing (PTGS) level. Multiple geminivirus genes have been identified as viral suppressors of RNA silencing (VSR) but VSR activity remains elusive in becurtoviruses. By screening all verified open reading frames in the BCTIV genome, we found that only V2 and Rep were able to suppress specific PTGS mechanisms, triggered by the expression of a partial or full-length sense-strand transcript of the target gene (S-PTGS). BCTIV-V2 could suppress S-PTGS more efficiently than BCTIV-Rep when then the target GFP gene is transiently expressed. On the other hand, S-PTGS is suppressed by Rep but not V2 when target GFP is only stably expressed. Deletional mutagenesis of BCTIV-Rep implicated that multiple domains are required for its VSR activity. Furthermore, neither V2 nor Rep could fully suppress local PTGS induced by inverted repeat targeting GFP (GFP-IR). Also, in a closer look at the spread of local silencing by GFP-IR, we observed that V2 or Rep are not able to suppress the movement of sRNAs. Nevertheless, Rep suppressed the systemic silencing induced by GFP-IR in 16C plants. Northern blot analyses showed that BCTIV-Rep inhibits silencing by mitigating sRNA production, whereas BCTIV-V2 does not alter sRNA levels. In summary, both the silencing phenotype and the molecular signatures of silencing implicate distinct modes of VSR activity of BCTIV-Rep and -V2.