microRNA 166: an evolutionarily conserved stress biomarker in land plants targeting HD-ZIP family

MicroRNAs (miRNAs) are significant class of noncoding RNAs having analytical investigating and modulatory roles in various signaling mechanisms in plants related to growth, development and environmental stress. Conserved miRNAs are an affirmation of land plants evolution and adaptation. They are a proof of indispensable roles of endogenous gene modulators that mediate plant survival on land. Out of such conserved miRNA families, is one core miRNA known as miR166 that is highly conserved among land plants. This particular miRNA is known to primarily target HD ZIP-III transcription factors. miR166 has roles in various developmental processes, as well as regulatory roles against biotic and abiotic stresses in major crop plants. Major developmental roles indirectly modulated by miR166 include shoot apical meristem and vascular differentiation, leaf and root development. In terms of abiotic stress, it has decisive regulatory roles under drought, salinity, and temperature along with biotic stress management. miR166 and its target genes are also known for their beneficial synergy with microorganisms in leguminous crops in relation to lateral roots and nodule development. Hence it is important to study the roles of miR166 in different crop plants to understand its defensive roles against environmental stresses and improve plant productivity by reprogramming several gene functions at molecular levels. This review is hence a summary of different regulatory roles of miR166 with its target HD-ZIP III and its modulatory and fine tuning against different environmental stresses in various plants.