Mechanism Of Begomovirus Satellite DNA Replication And Mediated Virus-vector Mutualism

The Geminiviridae is the largest family of plant viruses with circular, single-stranded DNA genomes that cause devastating diseases of economically important crops world-wide. To get a better insight into the pathogenesis and epidemics of geminiviruses, the molecular mechanism of begomovirus satellite DNA replication specificity and the mechanism of begomovirus-vector whitefly mutualism were studied.1. The molecular mechanism of begomovirus satellite DNA replication specificityTomato yellow leaf curl China virus (TYLCCNV) and Tobacco curly shoot virus (TbCSV), both obtained from Yunnan Province, were each found to be associated with a distant species of betasatellite molecule. Both tomato yellow leaf curl China betasatellite (TYLCCNB) and tobacco curly shoot betasatellite (TbCSB) can be trans-replicated stably by the noncognate begomovirus. When either TYLCCNV or TbCSV was associated with both betasatellite molecules simultaneously, the cognate betasatellite became dominant and the noncognate betasatellite became undetectable at late stages of infection. In the present work, we showed that the interaction between the LCR region of betasatellite and Rep protein of begomovirus determined the replication specificity. Furthermore, utilizing biochemistry assays in vitro, we found a novel Rep binding motif (RBM) in the LCR region and proved that the binding activity of Rep protein to its cognate RBM is stronger than to its noncognate RBM. Infectivity tests using infectious clones of RBM deletion mutant showed the RBM was involved in betasatellite infection and replication specificity. Finally we inoculated the Tomato leaf curl China virus (ToLCCNV) and its associated betasatellite together with TYLCCNB or TbCSB and found that the noncognate betasatellite containing the same RBM was detected even in late stage of infection while same the noncognate betasatellite containing the different RBM was undetectable in late stage of infection, indicating that the molecular mechanism of betasatellite replication specificity was ubiquitous. 2The mechanism of Begomovirus-vector whitefly mutualismThe whitefly can establish mutualism with the begomovirus TYLCCNV via crop plants. Here, we show that TYLCCNV and betasatellite co-infection suppresses jasmonic acid defenses in the plant. Using VIGS assays and transgenic plants to impair or enhance defenses mediated by jasmonic acid in the plant enhances or depresses the performance of the whitefly. We further generate the βC1transgenic tobacco plants, and demonstrate that the pathogenicity factor βC1encoded in the betasatellite is responsible for the initiation of suppression on plant defenses and contributes to the realization of the virus-vector mutualism. Our findings reveal that a begomovirus pathogenicity factor can suppress the plant defences and provide a mechanism whereby reduced defences result in increased whitefly vector multiplication. Our study also provides an interesting model to study the plant-pathogen-vector interactions through an integration of ecological, physiological and molecular approaches.We also found that TYLCCNV and betasatellite co-infection or whitefly infested down or up regulated a key gene NtEAS involved in the terpenoid synthesis pathway, respectively. VIGS or transgenic silence NtEAS enhances the performance of the whitefly. Our study demonstrates that virus infection depletes the terpenoid-mediated plant defense against whiteflies, thereby favoring vector-virus mutualism. Our findings suggest that plant terpenoids play a key role in shaping vector-pathogen relationships. This is an evidence of secondary metabolic pathway mediated virus-vector mutualism on tobacco.