Plant RNA-dependent RNA polymerase 1(RDR1) is a key component of antiviral RNA silencing pathway contributing to the biogenesis of the virus-derived small interfering RNAs (vsiRNAs). This enzyme is also responsible for producing virus-activated endogenous siRNAs (vasiRNAs) to stimulate the broad-spectrum antiviral activity through silencing host genes. The expression of RDR1 orthologs in various plants is usually induced by virus infection. However, the molecular mechanisms of activation of RDR1 expression in response to virus infection remain unknown. Here, we show that a monocot-specific miRNA, miR444, is a key factor to relay the antiviral signaling from virus infection to OsRDR1 expression. Expression of miR444 is enhanced by infection of Rice stripe virus (RSV) and overexpression of miR444 improves rice resistance against RSV infection accompanied by upregulation of OsRDR1 expression. We further show that three miR444 targets, the MIKCC-type MADS-box proteins OsMADS23, OsMADS27a and OsMADS57, form homodimers and heterodimers between them to repress the expression of OsRDR1 by directly binding to the CArG motifs of its promoter. Consequently, increased level of miR444 diminishes the repressive roles of OsMADS23, OsMADS27a and OsMADS57 on OsRDR1 transcription thus to activate OsRDR1-dependent antiviral RNA silencing pathway. We also show that overexpression of miR444-resistant OsMADS57 reduced OsRDR1 expression and rice resistance against RSV infection and knockout of OsRDR1 reduced rice resistance against RSV infection. In conclusion, our results unveil a molecular cascade in the rice antiviral pathway in which miR444 and its MADS-box targets directly control OsRDR1 transcription.

MiR444 promotes phytohormone brassinosteroid biosynthesis, conferring ammonium-triggered root elongation inhibition in rice.

Receptor-like kinases (RLKs) reside on the cell surface and recognize apoplastic colonization by plant-infecting microbes to initiate immune responses. Whether RLKs can also recognize intracellular colonization by viruses to activate antiviral defense mechanisms in plants remains unknown. Here, we report the identification and characterization of a trans -Golgi network/early endosome (TGN/EE)-localized RLK that recognizes viral proteins and inhibits infection in rice. OsVIRK1, a cysteine-rich receptor-like kinase, promotes rice resistance to rice stripe virus (RSV), one of the most devastating viruses of rice. OsVIRK1 transcription is induced in RSV-infected rice, and its protein accumulates through autophosphorylation and redox-mediated regulation. OsVIRK1 physically interacts with the RSV coat protein (CP), a known immune elicitor, and nonstructural protein 3 (NS3), an antiviral RNA-silencing suppressor, at the TGN/EE. OsVIRK1 is required for CP-triggered defense gene expression. It phosphorylates NS3, reducing NS3 accumulation in the cytoplasm and thus repressing its activity as an RNA-silencing suppressor. Our findings suggest that OsVIRK1 recognizes viral proteins at the TGN/EE to inhibit infection by activating plant antiviral immunity and dampening viral counterdefense.