A drought stress-responsive metabolite malate modulates stomatal responses through G-protein-dependent pathway

Drought poses a significant environmental threat to global agricultural production and distribution. Plant adaptation to dehydration stress involves intricate biological processes with substantial changes in metabolite composition. In this study, we investigated the role of tricarboxylic acid (TCA) cycle metabolites in drought tolerance in grapevine and Arabidopsis. Metabolome analysis revealed that malate, citrate, and isocitrate were upregulated over time in detached grapevine leaves. Five out of 10 organic acids tested, including TCA cycle metabolites, elicited increases in cytosolic free Ca ²⁺ concentration ([Ca ²⁺ ] _cyt ) in guard cells. Eight out of 10 directly activated a major anion channel in guard cell SLAC1, suggesting that SLAC1 is a general sensor of carboxylic acids. Furthermore, malate alone remarkably induced stomatal closure, which required increases in [Ca ²⁺ ] _cyt in guard cells and activation of SLAC1. Pharmacological experiments suggested that cyclic ADP-ribose (cADPR), cAMP and inositol trisphosphate (IP ₃ ) act as second messengers essential for malate-induced [Ca ²⁺ ] _cyt elevation and stomatal closure. G-proteins, which regulate second messenger production, were identified as components of malate signaling. These results indicate that malate plays a key role in connecting metabolic regulation and drought tolerance. As TCA cycle intermediates are key players in stress responses in mammals as well, we propose that they are common stress-responsive signal molecules mediated by G-protein-dependent signal cascades in both the animal and plant kingdoms.