**Abstract**

This chapter describe the possibilities of MicroRNAs (miRNAs) in crop plants gene expression regulation in different metabolic pathways. Several current researches have shown different environmental stresses induce abnormal expression of miRNA, thus signifying that miRNAs may be an appropriate tool for genetical improvement in plant for stress tolerance. These miRNAs mainly control gene expression through translational inhibition. Generally, stress induce miRNAsbased inhibition of their target mRNAs, however, positive transcription factors accumulated and become more active after mRNA inhibition. Initially, researchers were mainly focused on miRNA identification, appropriate to specific or multiple environmental condition, expression profiling and recognize their roles in stress tolerance. Transformed miRNA expression studied in some plant species for better understanding of plant development and stress tolerance such as heavy metal, salinity, temperature, drought and nutrient deficiency. All these findings indicate that miRNAs act as a potential tool for genetic engineering and to enhance stress tolerance in crop plants.

**Keywords:** RNA, miRNA, siRNA, biotic and abiotic stresses, plant development
