Plant microRNAs and stress response The microRNAs (miRNAs) are small (20 24 nt) sized single stranded regulatory RNA molecule or gene regulators of critical transcriptional or post transcriptional gene regulation in plants in sequence specific manner to respond to numerous abiotic stresses and animals non coding highly evolutionarily conserved and widely distributed throughout the plant kingdom. The miRNAs are master regulators of plant growth and development growth and development attenuation under various environmental stresses by stress responsive miRNAs and plant stress responses and tolerance. The drought salinity heat cold UV radiation heavy metal pathogens and pests and other microbial infections dismay survival growth and development quality yield and production of plants. Stress induced miRNAs down regulate their target mRNAs whereas their down regulation leads to accumulation and function of positive regulators recognizing their roles in stress responses and tolerance. Plant miRNA mediated modifications include overexpression or repression of stress responsive miRNAs and or their target complementary or partially complementary gene products miRNA resistant target genes target mimics and artificial miRNAs. Thus miRNAs may serve as genomic gold mines key novel potent and potential targets in plant genetic manipulations and miRNA based biotechnology will enhance plant improvement and crop plant tolerance to different environmental stresses. This book summarizes recent know how on plant microRNAs biogenesis and functions computational tools and bioinformatics regulation of plant growth and development expression studies and role of plant miRNAs in various biotic and abiotic stress response regulation in plants