RNA silencing amplification in plants: Size matters

Abstract

Plant microRNAs (miRNAs) regulate endogenous gene expression by recruiting silencing factors to complementary binding sites in target transcripts. Small-interfering RNAs (siRNAs) from transgenes or viruses also guide silencing effector complexes to their targets. But unlike most miRNAs, siRNAs can additionally initiate the production of a second wave of siRNAs, called secondary (2°) or “transitive” siRNAs, from the regions surrounding their primary target sites. This process ultimately enhances the silencing response, both locally and non-cell autonomously (1). Given their key roles in establishing cell identities and precise boundaries between tissues, 2° siRNA synthesis is generally not a desired feature of miRNAs, yet a small fraction of these molecules do trigger transitivity in Arabidopsis. Thus, 2° siRNA synthesis is not confined to highly abundant, exogenous RNAs (e.g., viruses) because it may also operate on a subset of endogenous targets. Very defined determinants must therefore entail the recruitment of enzymes required for 2° siRNA production to only the intended target RNAs, especially because the same Argonaute effector protein, AGO1, mediates the action of both siRNAs and miRNAs in Arabidopsis.