The Ca2+ sensor SCaBP3/CBL7 Fine Tunes Arabidopsis Alkali Tolerance and Modulats Plasma Membrane H+-ATPase Activity.

Saline-alkali soil is a major environmental constraint impairing plant growth and crop productivity. In this study, we identify a Ca2+ sensor/kinase/plasma membrane H+-ATPase module as central component conferring alkali tolerance in Arabidopsis. We report that SCaBP3/CBL7 loss-of-function plant exhibits enhanced stress tolerance associated with increased PM H+-ATPase activity and provides fundamental mechanistic insights into the regulation of PM H+-ATPase activity. Consistent with the genetic evidence, interaction analyses, in vivo reconstitution experiments and determination of H+-ATPase activity indicate that interaction of the Ca2+ sensor SCaBP3 with the C-terminal RI domain of the PM H+-ATPase AHA2 facilitates intramolecular interaction of AHA2 C terminus with the Central loop region of the PM H+-ATPase to promote autoinhibition of H+-ATPase activity. Concurrently, direct interaction of SCaPB3 with the kinase PKS5 stabilizes kinase-ATPase interaction and thereby fosters the inhibitory phosphorylation of AHA2 by PKS5. Consistently, yeast reconstitution experiments and genetic analysis indicate that SCaBP3 provides a bifurcated pathway for coordinating intramolecular and intermolecular inhibition of PM H+-ATPase. We propose that alkaline stress-triggered Ca2+ signals induce SCaBP3 dissociation from AHA2 to enhance PM H+-ATPase activity. This work illustrates a versatile signaling module that enables the stress-responsive adjustment and fine-tuning of plasma membrane proton fluxes.