The Regulatory Activities of Plant MicroRNAs: A More Dynamic Perspective

Twenty years have passed since the first discovery of the microRNA (miRNA) genes in Caenorhabditis elegans. Based on the growing research progress, we are approaching an understanding of this small RNA species, which seemed to be mysterious before. The regulatory activities of miRNAs have been extensively studied through target identification, physiological and phenotypic assays by using bioinformatic, genetic, and biochemical approaches. However, recent evidences indicate that the effective levels of miRNAs are determined by transcription, processing, miRISC (microRNA-induced silencing complex) loading, action, turnover use, and decay. Each process is affected by certain factors, such as genomic modifications, RNA editing, miRISC loading competition, target abundance and complementarity, and spatio-temporal effects, thus conferring a highly dynamic feature to the miRNA activities. To maintain the steady-state levels of the functional miRNAs, thus ensuring normal physiological and biochemical status, plants employ several exquisite strategies, such as feedback regulation and buffering system, to minimize the influence of external signal fluctuations. In this review, we raised the notion that a more dynamic picture of miRNA activities should be drawn to construct comprehensive miRNA-mediated networks in plants.