Plant DICER-LIKE, dual measurement RNA rulers

Small RNA (sRNA)-mediated gene silencing plays critical roles in eukaryotic gene regulation. The biogenesis of the two major types of sRNA, microRNA (miRNA) and small interfering RNA (siRNA), relies on the type III endoribonuclease Dicer. Unlike animals, which typically harbor fewer Dicer proteins, seed plants possess a greatly expanded Dicer family of DICER-LIKE (DCL) proteins that produce multiple types of sRNAs with diverse lengths, structures, and functions. Accordingly, plant DCLs must both specifically select substrate RNAs and measure product RNAs with defined lengths. In this review, we summarize recent progress about the molecular basis underlying the substrate selectivity and product specificity of plant DCLs, revealing that these enzymes not only act as molecular rulers for product RNAs, as do their animal counterparts, but also serve as rulers for substrate RNAs, enabling precise selection of appropriate targets.