Small RNA metabolism and function in Arabidopsis

Abstract

The transcriptome of plants includes diverse species of 21‐24 nucleotide small RNAs, which are composed of microRNAs (miRNAs) and endogenous small interfering RNAs (siRNAs). miRNAs regulate target mRNAs through transcript cleavage or translational inhibition and play crucial roles in various developmental and physiological processes. Endogenous siRNAs trigger and maintain DNA and histone methylation leading to heterochromatin formation.We have been studying the biogenesis, degradation, and function of small RNAs in Arabidopsis. We showed that plant small RNAs are 2′‐O‐methylated by the small RNA methyltransferase HEN1 on the 3' terminal ribose and that the methyl group protects small RNAs from a 3′‐5' exonuclease that degrades small RNAs and from a so‐called uridylation activity that adds a short U tail to small RNAs. Subsequently, animal siRNAs and piRNAs were found to be 2′‐O‐methylated by HEN1 homologs. We have also identified a family of exonucleases that degrades small RNAs and demonstrated that small RNA turnover is crucial for plant development. Finally, we studied the developmental function of one miRNA, miR172, and revealed that it plays a crucial role in the temporal regulation of floral stem cells. These findings in small RNA metabolism and function will be discussed.