Abscisic acid flux alterations result in differential ABA signalling responses and impact assimilation efficiency in barley under terminal drought stress

ABA is a central player in plants response to drought stress. How variable levels of ABA under short-term versus long-term drought stress impact assimilation and growth in crops is unclear. We addressed this through comparative analysis, using two elite breeding lines of barley that show senescence or stay-green phenotype under terminal drought stress and by making use of transgenic barley lines that express AtNCED (Arabidopis thaliana 9-cis-epoxycarotenoid dioxygenase) coding sequence or an RNAi sequence of ABA8’-OH (ABA 8’-hydroxylase) under the control of a drought-inducible barley promoter. The high levels of ABA and its catabolites in the senescing breeding line under long-term stress were detrimental for assimilate productivity whereas these levels were not perturbed in the stay-green type that performed better. In transgenic barley, drought-inducible AtNCED expression afforded temporal control in ABA levels such that the ABA levels rose sooner than in wild-type (WT) plants but also subsided, unlike as in WT, to near-basal levels upon prolonged stress treatment due to downregulation of endogenous HvNCED genes. The interdiction of ABA catabolism with RNAi approach of ABA8’-OH had lower ABA flux during the entire period of stress. These transgenic plants performed better than the WT under stress to maintain a favourable instantaneous water use efficiency and better assimilation. Gene expression analysis, protein structural modeling, and protein-protein interaction analyses of the members of the PYR/PYL/RCAR, PP2C, SnRK2, and ABI5/ABF family identified specific members that could potentially impact ABA metabolism and stress adaptation in barley.