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

Abstract

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 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 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.