RNA-dependent direct polymerase involved in antiviral silencing. Required for the production of some small RNAs (mainly 21 and some 22 nucleotides) derived from the crucifer-infecting tobamovirus (TMV-cg). Required for turnip mosaic virus (TuMV) silencing and accumulation of viral siRNAs. Involved in cucumber mosaic virus (CMV) silencing. Required for the biogenesis of viral secondary siRNAs, process that follows the production of primary siRNAs derived from viral RNA replication. Specifically targets the positive-strand of the 3 RNA genomes of CMV and preferentially amplifies the 5'-terminal siRNAs of each viral genomic RNA. Not involved in the production of siRNAs derived from a single-stranded 336-nucleotide satellite RNA of CMV. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) By salicylic acid (SA) and infection by a crucifer-infecting tobamovirus (TMV-cg). Upon viral infection, increased levels of TMV-cg, tobacco rattle virus (TRV) and CMV genomic viral RNAs. Reduced levels of TMV-cg-derived small RNAs and CMV-derived siRNAs. Belongs to the RdRP family.