Beet Curly Top Iran Virus Rep and V2 Suppress Post-Transcriptional Gene Silencing via Distinct Modes of Action.

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 Mastrevirus, whereas the virion strand organization is similar to the Curtovirus genus. 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) levels. Multiple geminivirus genes have been identified as viral suppressors of RNA silencing (VSR) but VSR activity remains mostly elusive in becurtoviruses. We found that BCTIV-V2 and -Rep could suppress specific Sense-PTGS mechanisms with distinct efficiencies depending on the nature of the silencing inducer and the target gene. Local silencing induced by GFP inverted repeat (IR) could not be suppressed by V2 but was partially reduced by Rep. Accordingly, we documented that Rep but not V2 could suppress systemic silencing induced by GFP-IR. In addition, we showed that the VSR activity of Rep was partly regulated by RNA-dependent RNA Polymerase 6 (RDR6), whereas the VSR activity of V2 was independent of RDR6. Domain mapping for Rep showed that an intact Rep protein was required for the suppression of PTGS. In summary, we showed that BCTIV-Rep and -V2 function as silencing suppressors with distinct modes of action.

The Rep and C1 of Beet curly top Iran virus represent pathogenicity factors and induce hypersensitive response in Nicotiana benthamiana plants.

Beet curly top Iran virus (BCTIV) is a member of the genus Becurtovirus (Family Geminiviridae) with a circular single-strand DNA genome. BCTIV causes leaf curling and vein swelling symptoms in plants. However, the potential pathogenicity factor/s in BCTIV is/are not known. This study presents characterization of complementary-sense transcripts of BCTIV and the viral factors in directing the pathogenicity and hypersensitive response (HR) in Nicotiana benthamiana plants. In both local and systemic infection, splicing of the complementary transcripts of BCTIV was observed. Notably, a small number (8.3%) of transcripts were spliced to produce Rep (C1:C2) transcripts after deletion of 155 nt (position 1892-2046 from BCTIV). Expression of BCTIV genes in N. benthamiana using tobacco rattle virus (TRV)-based vector showed that Rep together with C1 are the main pathogenicity factors which cause typical viral leaf curling symptoms. In addition, the V2 caused a mild leaf curling, thickening, and asymmetric leaves, while the V1, V3, and C2 had no clear effect on the plant phenotype. Transient expression of individual viral genes showed that both the C1 and Rep trigger a HR response in N. benthamiana. The higher expression of HR marker genes, harpin-induced 1 (Hin1) and hypersensitivity-related (Hsr203JI), supported the role of C1 and Rep in HR response in plants. It is concluded that Rep and C1 are the main pathogenicity factors that also trigger HR response in plants.