Host-Virus-Vector Interactions of Wheat Viruses

Wheat (Triticum aestivum L.) is the principal cereal grown in the United States, whose production ranks third in the world. High Plains wheat mosaic emaravirus (HPWMoV) and Wheat streak mosaic tritimovirus (WSMV) are two of the economically important viruses transmitted by wheat curl mite (WCM; Aceria tosichella Keifer) in the Great Plains. Majorly, this study deals with the host-virus-vector interactions of these organisms. HPWMoV genome was screened for viral suppressor of RNA silencing (VSR) and found that P7 and P8 proteins suppress local and systemic silencing induced by single- or double-stranded RNAs. Electrophoretic mobility shift assays revealed that P7 binds to dsRNA and also protects them from in vitro dicing, while P8 binds to PTGS-like small ds-siRNA. The C-terminal end of P7 harbors conserved dsRNA binding domain and AGO-binding GW motif. Pull-down of haemagglutinin (HA)-tagged P7 co-immunoprecipitated AGO4. P7 or P8 complemented the functions of P1 by rescuing the RNA silencing suppressor (P1)-deficient WSMV in wheat. P7 or P8 enhanced pathogenicity of Potato virus X in Nicotiana benthamiana. However, mutational disruption of P7-GW motif resulted in loss of RNA silencing suppression, pathogenicity enhancement, and ability to rescue WSMV-ΔP1. Additionally, we determined the silencing suppression mechanism of WSMV P1. RNA interaction studies showed that P1 binds to dsRNA in a size and sequence independent manner and protects long dsRNA from activity of dicer. P1 has a conserved N-terminal dsRNA binding and C-terminal GW-type AGO binding domain. Co-immunoprecipitation assay of HA-tagged P1 transiently expressed in N. benthamiana showed NbAGO4 interaction. Mutagenesis of GW-motif resulted in the loss of not only silencing suppression activity and pathogenicity enhancement but also viability of WSMV. Furthermore, WCM transcriptome was analyzed from WSMV-viruliferous and aviruliferous mites to know whether WSMV persistence and coevolution influenced WCM gene expression to prolong viral association and increase its transmission efficiency. Among 7,785 de novo-assembled unigenes, 1,200 were differentially-expressed in viruliferous WCMs and were categorized into 251 different gene ontology terms. Overall, these transcriptional changes may inhibit WCM immune response to prolong viral association and alter WCM development to expedite population expansion, both of which could enhance viral spread and transmission