The functions of plant small RNAs in development and in stress responses.

Like metazoans, plants use small regulatory RNAs (sRNAs) to direct gene expression. Several classes of sRNAs, which are distinguished by their origin and biogenesis, exist in plants. Among them, microRNAs (miRNAs) and trans-acting small interfering RNAs (ta-siRNAs) mainly inhibit gene expression at post-transcriptional levels. In the past decades, plant miRNAs and ta-siRNAs have been shown to be essential for numerous developmental processes, including growth and development of shoots, leaves, flowers, roots and seeds, among others. In addition, miRNAs and ta-siRNAs are also involved in the plant responses to abiotic and biotic stresses, such as drought, temperature, salinity, nutrient deprivation, bacteria, virus and others. This review summarizes the roles of miRNAs and ta-siRNAs in plant physiology and development.