Small RNAs in plants

Abstract

Small RNAs 20-30 nucleotides in length are sequence-specific regulatory RNAs that guide nucleic acid-based processes in eukaryotic organisms. In plants, small RNAs are classified as microRNAs (miRNAs) or small interfering RNAs (siRNAs) based on differences in their precursors and biogenesis. miRNAs are generated from precursors with an imperfect intramolecular hairpin structure and silence their target genes at the posttranscriptional level through mRNA cleavage or translational inhibition. Endogenous siRNAs are processed from long double-stranded RNAs with perfect base-pair complementarity and are classified as heterochromatic siRNAs (hc-siRNAs) or trans-acting siRNAs (ta-siRNAs). Heterochromatic siRNAs (hc-siRNAs) are endogenous siRNAs generated from transposons and repeats and guide cytosine methylation, which induces heterochromatin formation and transcriptional gene silencing. Trans-acting siRNAs (ta-siRNAs) are endogenous siRNAs whose biogenesis is triggered by specific miRNAs. Like miRNAs, ta-siRNAs repress their targets in trans at the posttranscriptional level and are important for plant development. Exogenous siRNAs are generated in both virus-infected plants and transgenic plants. The steady-state levels of small RNAs are precisely regulated through their biogenesis and turnover. A specific modification of small RNAs protects them from uridylation and truncation, processes associated with small RNA turnover.