*5.5.1 ABA pathway*

Many genes responsible for response to stress are triggered under abiotic stress conditions. Abscisic acid (ABA) is a main plant stress-signaling hormone and its accumulation automatically increases as the harsh conditions are faced by plant to fight the stress effect. Two pathways are triggered in plant under osmotic stress condition, that is, ABA-dependent and ABA-independent pathways. In ABA-dependent pathway, a mixture of transcription factors, ABRE binding protein/ABRE binding factors (AREB/ABFs) demonstrate critical functions. A cis-element, dehydrationresponse element/C-repeat (DRE/CRT) and DRE-/CRT-binding protein 2 (DREB2) transcription factors play a main part in the expression of ABA-independent genes in response to osmotic stress. Continuous increase in expression of AREB1/ABF2, AREB2/ABF4 and ABF3 is triggered by drought and salinity in vegetative tissues. Over-expression studies indicate that in conditions of drought stress, these three AREB/ABFs are useful signals from ABA regulators. As shown in the figure, AREB/ ABF transcription factors result in gene expression of the genes involved in abiotic stress reaction and tolerance [80].
