Schematic diagram of silencing pathways in plants. An overview of the (a) microRNA, (b) trans-acting siRNA, (c) viral siRNA and (d) heterochromatic siRNA pathways. sRNAs are processed from partially or perfectly double-stranded RNA (dsRNA) precursor molecules by Dicer-like (DCL) proteins. They are incorporated into Argonaute (AGO) complexes to target nucleic acids. RNA-dependent RNA polymerases (RDRs) convert transcripts generated by DNA-dependent RNA polymerase II (POL II) and IV (POL IV) into dsRNA. The associated RDR, DCL and AGO proteins that function in each pathway and the sizes and functions of the sRNAs produced are indicated. RISC refers to the RNA-induced silencing complex, a multiprotein complex that consists of an AGO protein, an sRNA and other protein factors. The different compositions of RISC complexes are indicated in different colors.