Regulation of transpiration hinges on spatial differences in abscisic acid perception

The plant hormone abscisic acid (ABA) is best known for its role as a regulator of the responses to abiotic stressors. Components of ABA signaling pathway are thus considered as promising targets for securing yield under stress. ABA levels rise in response to abiotic stress, mounting a number of physiological and metabolic responses that promote plant survival under unfavorable conditions. ABA elicits its effects by binding to a family of soluble receptors called PYR/PYL/RCAR. Arabidopsis genome encodes 14 PYL proteins, knowledge on differential biological functions of these members is scarce. In this work, we took a gain-of-function approach to predict receptor-specific functionality. We introduced a set of activating mutations in the mobile gate and α-helix of sub family II ABA receptors. These mutations constitutively enforce the active ABA-bound conformation. We then transformed ABA deficient mutants with the constitutive receptors and monitored suppression of ABA deficiency phenotype. Our findings suggest that subfamily II receptors have differential activity in regulating transpiration and transcription of oxidation and stress response genes. These differences partly derived from differential accumulation of PYLs in the guard cells. Data mining revealed that only a small subset of PYL receptors are transcribed in the guard cells in a steady state. Because the guard cell specific receptors have different affinity to ABA and to the PP2C co-receptor we propose that, a combined partial receptor redundancy facilitate a gradual activation of the ABA output signal as its cellular concentration rises via biochemically different receptors. Whole PYL/PYR seedlings mock treated, and whole Col-0 and aba2-1 seedlings mock treated or treated with 10nM ABA or 50nM ABA.