Mechanisms that impact microRNA stability in plants.

microRNAs (miRNAs) are 20-24 nucleotide RNAs that regulate a variety of developmental and metabolic processes. The accumulation of miRNAs in vivo can be controlled at multiple levels. In addition to miRNA biogenesis, mechanisms that lead to RNA degradation, such as 3' uridylation and 3' truncation, also affect the steady-state levels of miRNAs. On the other hand, 2'-O-methylation in plant miRNAs protects their 3' ends from truncation and uridylation. The recent identification of HESO1 as the key enzyme responsible for miRNA uridylation in Arabidopsis was a first step toward a full understanding of the mechanisms underlying miRNA turnover. Analyses of the heso1 mutant predicted the existence of another uridylation activity and a previously unknown nuclease that act on miRNAs. The future identification of these enzymes will enrich our understanding of miRNA turnover.