Plant miRNAs: biogenesis, organization and origins

MicroRNAs, or miRNAs, are posttranscriptional regulators of gene expression. A wealth of observations and findings suggest highly complex, multicomponent, and intermingled pathways governing miRNA biogenesis and miRNA-mediated gene silencing. Plant miRNA genes are usually found as individual entities scattered around the intergenic and-to a much lesser extent-intragenic space, while miRNA gene clusters, formed by tandem or segmental duplications, also exist in plant genomes. Genome duplications are proposed to contribute to miRNA family expansions, as well. Evolutionarily young miRNAs retaining extensive homology to their loci of origin deliver important clues into miRNA origins and evolution. Additionally, imprecisely processed miRNAs evidence noncanonical routes of biogenesis, which may affect miRNA expression levels or targeting capabilities. Majority of the knowledge regarding miRNAs comes from model plant species. As ongoing research progressively expands into nonmodel systems, our understanding of miRNAs and miRNA-related pathways changes which opens up new perspectives and frontiers in miRNA research.