Roles of microRNAs in plant development and stress tolerance

Abstract Small RNAs are single-stranded noncoding RNA molecules of ~20–30 nucleotides known to play a significant role in several cellular mechanisms. These small RNAs comprised small-interference RNAs (siRNAs), microRNAs (miRNAs), and PIWI (P element–induced wimpy testis)-interacting RNAs. The small RNAs regulate gene expression at the transcriptional and posttranscriptional level, and the whole mechanism of gene regulation by small RNAs is referred to as RNA interference. Among small RNAs, miRNAs and siRNAs are found in majority of eukaryotes, while piRNAs are found only in animals. miRNAs are an evolutionarily conserved group of ~22 nucleotides long-endogenous noncoding RNAs, which control the gene expressions, while posttranscriptionally via either transcript cleavage, translation repression, or both. It is becoming apparent that miRNAs assist various regulatory mechanisms such as developmental timing, host–pathogen interaction, cell differentiation, cell proliferation, apoptosis, as well as tumorigenesis. miRNA acts as a regulatory element and is modulated by various effectors such as single nucleotide polymorphism (SNP), miRNA editing, methylation, and circadian clock.