Ribonuclease (RNase) III involved in RNA-mediated post-transcriptional gene silencing (PTGS). Involved in the processing of natural small interfering RNAs (nat-siRNAs, derived from cis-natural antisense transcripts) by cleaving small dsRNAs into 24 nucleotide nat-siRNAs. Plays an essential role in transitive silencing of transgenes by processing secondary siRNAs. This pathway, which requires DCL4 and RDR6, amplifies silencing by using the target RNA as substrate to generate secondary siRNAs, providing an efficient mechanism for long-distance silencing. May participate with DCL3 in the production of 24 nucleotide repeat-associated siRNAs (ra-siRNAs) which derive from heterochromatin and DNA repeats such as transposons. Plays a role in antiviral RNA silencing. Involved in the production of viral siRNAs derived from the turnip crinkle virus (TCV) and tobacco rattle virus (TRV). Targeted by the viral silencing suppressor (VSR) protein 2b of the cucumber mosaic virus (CMV) that inactivates DCL2 function in RNA silencing. Does not seem to be involved in microRNAs (miRNAs) processing. Belongs to the helicase family. Dicer subfamily. The predicted gene At3g03300 has been split into 2 genes: At3g03300 and At3g03305. Truncated N-terminus.