Ribonuclease (RNase) III involved in RNA-mediated post-transcriptional gene silencing (PTGS). Functions in the microRNAs (miRNAs) biogenesis pathway by cleaving primary miRNAs (pri-miRNAs) and precursor miRNAs (pre-miRNAs). Functions with DRB1/HYL1 and SERRATE proteins for accurate pri-miRNAs to miRNAs processing. Indirectly involved in the production of trans-acting small interfering RNAs (ta-siRNAs) derived from the TAS1, TAS2 or TAS3 endogenous transcripts by participating in the production of their initiating miRNAs. Involved in the processing of natural siRNAs (nat-siRNAs, derived from cis-natural antisense transcripts) by cleaving 24 nucleotide nat-siRNAs into 21 nucleotide nat-siRNAs. Can produce RDR6-dependent endogenous ta-siRNAs derived from TAS1 and TAS2. Required for the production of 30-40 nucleotide bacterial-induced long siRNAs (lsiRNA). Acts redundantly with DICER-LIKE 3 (DCL3) to promote flowering via repression of FLOWERING LOCUS C (FLC). Represses antiviral RNA silencing through negative regulation of the expression of DCL4 and DCL3. Interacts (via N-terminus) with DDL. Interacts (via DRBM domains) with DRB1, DRB2 and DRB5. May interact with AGO1 or AGO10 through their common PAZ domains (By similarity). Localizes to nuclear dicing body (also named D body), a nuclear body distributed throughout the nucleoplasm and involved in miRNA processing. A number of isoforms are produced. According to EST sequences. Highly expressed in flowers and seeds and detected in leaves and stems. Found in ovule integuments, inflorescence and floral meristems, stigma of flowers until just before pollination, vasculature of the funiculus, and embryo. Detected in the embryo, but not in the suspensor, up to the globular stage. Embryonic lethality in sus-1 mutant. Weaker mutant (caf-1) also exists. Mutant caf-1 produces extra whorls of stamens, indefinite number of carpels and show an absence of axillary inflorescence meristems and abnormally shaped leaves and floral organs. Expression in the early embryo is from the maternally contributed genome. Belongs to the helicase family. Dicer subfamily.