Diverse small RNA-directed silencing pathways in plants

Small silencing RNAs of 21- to 24-nucleotide (nt) in length are essential regulatory components expressed in most eukaryotic organisms. These regulatory small RNAs are produced through pathways that involve several evolutionarily conserved protein families, including DICER (DCR) or DICER-LIKE (DCL), ARGONAUTE (AGO), and RNA-DEPENDENT RNA POLYMERASE (RDR). Plants possess multiple functional DCL, RDR, and AGO proteins. Genetic analyses in the model plant Arabidopsis thaliana have revealed multiple small RNA pathways, each utilizing a distinct set of RNA silencing factors. In this short review, mainly based on the work done on A. thaliana, we give a brief overview on the multiple small RNA-directed silencing pathways in plants, which includes the biogenesis and function of microRNAs (miRNAs), trans-acting siRNAs (ta-siRNAs), natural cis-antisense transcripts-associated siRNAs (nat-siRNAs), and heterochromatic siRNAs.