RNA-dependent RNA polymerase involved in post-transcriptional gene silencing (PTGS). Possesses ssRNA and ssDNA-dependent polymerase activity, but does not have priming activity. Possesses in vitro 3' nucleotidyltransferase activity in the presence of UTP as single nucleotide. Required for the production of 21 nucleotide trans-acting small interfering RNAs (ta-siRNAs) derived from TAS1, TAS2 and TAS3 endogenous transcripts. Acts in the RDR6/SGS3/DCL4/AGO7 ta-siRNA pathway involved in leaf developmental timing. Required for the production of natural siRNAs (nat-siRNAs) derived from cis-natural antisense transcripts. Required for the production of 24 nucleotide nat-siRNAs derived from the stress-related P5CDH-SRO5 antisense gene pair. Required for PTGS induced by transgene direct repeats. Plays an essential role in transitive silencing of transgenes by processing secondary siRNAs. This pathway, which requires DCL2 and DCL4, amplifies silencing by using the target RNA as substrate to generate secondary siRNAs, providing an efficient mechanism for long-distance silencing. Involved in the biogenesis of secondary siRNAs which require 22 nucleotide miRNAs associated to AGO1. Participates synergistically with AS1 and AS2 to proper plant development by repressing the miR165 and miR166 microRNAs (independently of AGO10) that may lead to mRNA degradation of genes in the class III HD-ZIP family. Required for the production of some small RNAs derived from the crucifer-infecting tobamovirus (TMV-cg). Required for sense virus-induced post-transcriptional gene silencing (S-PTGS). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Interacts with SGS3. Colocalize with SGS3 in cytoplasmic granules distinct from P-bodies. Widely expressed. First leaves are elongated and curl downward. Presence of stigmatic tissue in the middle of the septum at the apical end of the carpels. Altered post-transcriptional gene silencing (PTGS). Accumulation of ARF4 mRNA (targeted by TAS3-derived siRNAs). Upon infection, over-accumulation of CMV RNA and enhanced susceptibility to cucumber mosaic virus (CMV). Reduced levels of TMV-cg-derived small RNAs. Belongs to the RdRP family.