P1/HC-Pro of the Potyvirus targets RISC assembly (Brigneti et al., 1998; Anandalakshmi et al., 1998 ; Kasschau et al., 2003), 2b of Cucumber mosaic virus (CMV) binds to dsRNA and interferes with spread of silencing signal (Brigneti et al.,1998 ; Qi et al., 2004), p19 of tombusviruses binds to siRNAs (Voinnet et al., 1999; Silhavy et al., 2002 ; Qi et al., 2004), p38 encoded by the Turnip crinkle virus binds AGO1 and inhibits the activity of DCL-4 (Thomas et al., 2003; Azevedo et al., 2010), P25 of Potato Virus X interferes with the spread of silencing signal (Voinnet et al., 2000), P23, S, γb, P15, P0, P1, P30, P69, NS and coat protein of closterovirus (Reed et al., 2003 ; Lu et al., 2004), comovirus (Liu et al., 2004), hordeivirus…show more content… Contrary to RNA viruses that can be controlled only by PTGS, geminiviruses may be manipulated by both PTGS and TGS. TGS is introduced when siRNAs corresponding to the promoter regions are produced that direct methylation of the promoter followed by inhibition of transcription (Mette et al., 2000). In a transient assay, TGS was reported to be effective against the begomovirus Mungbean yellow mosaic virus (Pooggin et al., 2003).
Viral invasiveness is reported to be promoted by the ability of C1 to suppress PTGS (Cui et al., 2005). It has been shown that C4 gene of Cotton leaf curl Multan virus and βC1gene of Cotton leaf curl Multan betasatellite bind short RNAs, with a preference for the double stranded and single stranded forms, respectively, suggesting that these suppressors sequester siRNAs and prevent their incorporation into the RNA-induced silencing complex (RISC) involved in sequence specific mRNA degradation (Hammond et al., 2000).
V2, a unique protein of monopartite begomoviruses has been shown to suppress PTGS in transient assays (Zrachya et al., 2007). It was demonstrated that ability of V2 to interact with SISG3, the tomato homologue of Arabidopsis SGS3 leads to suppression of PTGS (Glick et al., 2008). Furthermore, it was shown that the Rep proteins encoded by two alphasatellites, Gossypium darwinii symptomless alphasatellite and Gossypium mustelinium symptomless alphasatellite possess suppressor activity (Nawaz-ul-Rehman et