MicroRNA—the Promising Molecular Tool for Engineering Stress Resistance in Crop Plants

miRNA are endogenous regulatory molecules comprising 20–24 nucleotides playing a crucial role in the growth and development of various organs in animals as well as plants. In plants, miRNAs regulate biotic and abiotic stress responses conferring tolerance by functioning as negative and positive regulators. Down-regulation of the target genes is accomplished by repressing translation or by cleavage of a transcript or by inhibiting transcription. Various studies on crop plants like rice, wheat, and sugarcane have revealed the importance of miRNA in conferring abiotic stress tolerance against salinity and drought and biotic resistance against pests such as plant pathogens and insects. The biotic and abiotic stress regulates the target gene expression through the differential expression of specific miRNAs. An in-depth study of miRNA as a molecular marker in plants under stress conditions, profiling miRNA expressions and thorough understanding of its role in gene regulation may help in enhancing crop productivity and crop management against various stress conditions. So, the main objective of this review is to summarize recent advances in miRNA regulation in plants with special emphasis on its role in stress tolerance.