The plasma membrane (PM) proton (H+)-ATPase plays an important role in regulating plant growth, development, and acclimation to environmental stress. The inhibition of PM H+-ATPase promotes stomatal closure and reduces transpirational water loss under drought stress. Although calcium (Ca2+) signals modulate PM H+-ATPase activity, the underlying mechanism remains unknown. In this study, we found that Arabidopsis (Arabidopsis thaliana) CALCIUM-DEPENDENT KINASE 8 (CPK8) physically interacts with and phosphorylates PLASMA MEMBRANE PROTON ATPASE 1 (AHA1) at the conserved serine residue Ser-899 in the C-terminal region. CPK8-mediated phosphorylation of AHA1 significantly inhibits its H+-ATPase activity. CPK8 overexpression significantly ameliorated the excessive transpirational water loss while concurrently elevating leaf temperature in open stomata 2-2D (ost2-2D), a mutant with constitutively activated PM H+-ATPase. In addition, drought-induced Ca2+ signal activates CPK8, which further modulates PM H+-ATPase activity under drought stress. Our findings indicate that CPK8 phosphorylates PM H+-ATPase to fine-tune stomatal closure and optimize plant drought tolerance.
Liu et al. (Fri,) studied this question.