RNA-silencing suppressors of geminiviruses

Geminiviruses are single-stranded (ss) DNA viruses that not only cause devastating diseases of important food and fiber crops worldwide, but also are important models to study fundamental aspects of virus-induced gene silencing and RNA interference. As a counterdefense mechanism, viruses have evolved various antisilencing strategies that are being progressively unraveled. The geminiviruses, ssDNA molecules that replicate inside the nucleus and therefore have no dsRNA phase during replication, can both induce and become targets of gene silencing. Proteins AC2 (encoding the transcriptional activator protein) and AC4 of bipartite geminiviruses and protein C2, a positional homolog of AC2 of monopartite viruses, have been identified as suppressors of posttranscriptional gene silencing. The majorities of geminiviral suppressors characterized to date do not share any obvious structural or sequence similarity across families and groups except that they have been identified as pathogenicity determinants. This review mainly focuses on the geminivirus-encoded suppressors of RNA-silencing-the beta C1 and V2 proteins-and their possible role in the interference of silencing at different steps in the pathways.