The discovery of short, non-protein-coding RNA molecules have revolutionized our understanding of the function of RNA molecules. Plant small RNAs such as microRNA and small-interfering RNA are usually 18–25 nucleotides long and play important regulatory roles in biological processes including the maintenance of genome integrity, developmental transitions and patterning, and responses to abiotic and biotic stresses. Although highly diversified in their biogenesis pathways, target sequences and targeting mechanisma, and in their degree of conservation among species, small RNA provides versatile and flexible epigenetic regulation with great specificity. In this chapter, we introduce the types of plant small RNAs, their biogenesis pathways, and their regulatory roles in biological processes. We also discuss recent advances in understanding the mechanisms of small RNA-directed gene silencing. Lastly, we discuss the origins and evolution of small RNAs, including a current model for small RNA biogenesis and evolution.

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