Résumé
Summary Faced with declining soil‐water potential, plants synthesize abscisic acid ( ABA ), which then triggers stomatal closure to conserve tissue moisture. Closed stomates, however, also create several physiological dilemmas. Among these, the large CO 2 influx required for net photosynthesis will be disrupted. Depleting CO 2 in the plant will in turn bias stomatal opening by suppressing ABA sensitivity, which then aggravates transpiration further. We have investigated the molecular basis of how C3 plants resolve this H 2 O– CO 2 conflicting priority created by stomatal closure. Here, we have identified in A rabidopsis thaliana an early drought‐induced spermidine spermine‐ N 1 ‐ acetyltransferase homolog, which can slow ABA ‐mediated stomatal closure. Evidence from genetic, biochemical and physiological analyses has revealed that this protein does so by acetylating the metabolite 1,3‐diaminopropane ( DAP ), thereby turning on the latter's intrinsic activity. Acetylated DAP triggers plasma membrane electrical and ion transport properties in an opposite way to those by ABA . Thus in adapting to low soil‐water availability, acetyl‐ DAP could refrain stomates from complete closure to sustain CO 2 diffusion to photosynthetic tissues.