An intracellular CPK-ECA1 phosphoregulatory circuit couples calcium signatures to ABA homeostasis for plant osmosensivity.

Intracellular Ca2+ controls various cellular functions, and local Ca2+ dynamics are tightly regulated upon environmental cues. How cellular Ca2+ balance is maintained during stress remains unresolved. Here, we identify an intracellular phosphoregulatory module comprising calcium-dependent protein kinase (CPK)-ER-type Ca2+-ATPases 1 (ECA1) that integrates [Ca2+]cyt flux with abscisic acid (ABA) homeostasis to modulate root growth under osmotic stress. Pharmacological and genetic disruption of ECA1-mediated Ca2+ efflux triggered enhanced [Ca2+]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 Ca2+-pumping activity to restore cytosolic Ca2+ equilibrium and subsequently prevent ABAcyt 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 Ca2+ 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.