Cysteine biosynthesis acts as a regulatory hub for ABA-mediated stomatal closure

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Stomatal closure is the fastest and most critical response of plants towards the drought-associated stresses soil drying, high light, and decreased air humidity. The phyto¬hormone absicic acid (ABA) is the crucial trigger for stomatal closure. Comprehensive studies identified the ABA receptor complex and the ABA signal transduction cascade in guard cells. In contrast, the knowledge about the localization and the dominant triggers of stress-induced ABA synthesis is only beginning to emerge. Our published work established xylem-transported sulfate as a new long-distance signal that allows the stomata to perceive the soil water status. We showed that in the leaf sulfate promotes the synthesis of cysteine by triggering de novo ABA production and that in principle this process is sufficient to promote stomatal closure. In addition, we found that high light stress-induced stomatal closure also depends on the ability to synthesize cysteine and ABA de novo. Here we will investigate the general significance of this mechanism by screening a spectrum of water deficit related stresses for their dependence on sulfate/cysteine triggered stomatal closure. With respect to the physiological response of the plant to drought-related stresses we will investigate if, how and in which cell types the two limiting steps of the ABA pathway are triggered by sulfate. Signaling by the oxylipin OPDA is known to mediate a distinct set of stress responses, among them stomatal closure in reaction to soil drying. However, any OPDA-specific receptor and signal transduction cascade is unknown. OPDA binds to the chaperone CYP20-3 in the chloroplast stroma where is promotes association and activity of the cysteine synthase complex. We propose that the CYP20-3/pCSC relay functions as a receptor complex for OPDA that stimulates ABA biosynthesis by controlling cysteine biosynthesis and will investigate this function using combinations of Arabidopsis mutants and stress conditions.If these hypotheses hold, plants would be able to coordinate different drought-associated stresses by the inclusion of these diverse stimuli into one endogenous signal: cysteine-induced ABA production.