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Abstract: 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. [Copyright &y& Elsevier]

Multicellular eukaryotes produce small RNA molecules (approximately 21–24 nucleotides) of two general types, microRNA (miRNA) and short interfering RNA (siRNA). They collectively function as sequence-specific guides to silence or regulate genes, transposons, and viruses and to modify chromatin and genome structure. Formation or activity of small RNAs requires factors belonging to gene families that encode DICER (or DICER-LIKE [DCL]) and ARGONAUTE proteins and, in the case of some siRNAs, RNA-dependent RNA polymerase (RDR) proteins. Unlike many animals, plants encode multiple DCL and RDR proteins. Using a series of insertion mutants of Arabidopsis thaliana, unique functions for three DCL proteins in miRNA (DCL1), endogenous siRNA (DCL3), and viral siRNA (DCL2) biogenesis were identified. One RDR protein (RDR2) was required for all endogenous siRNAs analyzed. The loss of endogenous siRNA in dcl3 and rdr2 mutants was associated with loss of heterochromatic marks and increased transcript accumulation at some loci. Defects in siRNA-generation activity in response to turnip crinkle virus in dcl2 mutant plants correlated with increased virus susceptibility. We conclude that proliferation and diversification of DCL and RDR genes during evolution of plants contributed to specialization of small RNA-directed pathways for development, chromatin structure, and defense. [ABSTRACT FROM AUTHOR]