Abstract
Intracellular Ca(2+) controls various cellular functions, and local Ca(2+) dynamics are tightly regulated upon environmental cues. How cellular Ca(2+) balance is maintained during stress remains unresolved. Here, we identify an intracellular phosphoregulatory module comprising calcium-dependent protein kinase (CPK)-ER-type Ca(2+)-ATPases 1 (ECA1) that integrates [Ca(2+)](cyt) flux with abscisic acid (ABA) homeostasis to modulate root growth under osmotic stress. Pharmacological and genetic disruption of ECA1-mediated Ca(2+) efflux triggered enhanced [Ca(2+)](cyt) transients and [ABA](cyt), exacerbating root growth hypersensitivity to osmotic stress. Biochemical assays reveal that CPK2/6/11 directly bind and phosphorylate ECA1, enhancing its Ca(2+)-pumping activity to restore cytosolic Ca(2+) equilibrium and subsequently prevent ABA(cyt) overaccumulation. Genetic analysis shows that eca1cpk11 phenocopied eca1 hypersensitivity, while triple and quadruple mutants amplified osmotic sensitivity, demonstrating that CPKs and ECA1 function as a cooperative phosphoregulatory hub regulating Ca(2+) signaling and root growth. Our work resolves a critical gap by elucidating how plants couple ionic and hormonal messengers to balance plant growth and stress resilience.