Roles of siRNAs and miRNAs in host responses to virus infection: Identification and characterization of a novel viral suppressor of RNA silencing

Abstract

Small interfering RNAs (siRNAs) are processed by the ribonuclease Dicer from distinct precursors, double-stranded RNA (dsRNA) and hairpin RNAs, respectively, although either may guide RNA silencing via a similar complex. The siRNA pathway is antiviral, whereas an emerging role for miRNAs is in the control of development. In this thesis, I describe a virulence factor encoded by turnip yellow mosaic virus, p69, which suppresses the siRNA pathway but promotes the miRNA pathway in Arabidopsis. p69 suppression of the siRNA pathway is upstream of dsRNA and is as effective as genetic mutations in Arabidopsis genes involved in dsRNA production. Possibly as a consequence of p69 suppression, p69-expression plants contained elevated of a Dicer mRNA and miRNAs as well as a correspondingly enhanced miRNA-guided cleavage of four host mRNAs. Because p69-expressing plants exhibited disease-like symptoms in the absence of viral infection, the finding suggest a novel mechanism for viral virulence by promoting the miRNA-guided inhibition of host gene expression.