An intracellular calcium sensory complex CPKs/ECA1 controls cytosolic calcium homeostasis for plant osmosensitivity

Intracellular Ca2+ controls various cellular functions and local Ca2+ dynamics is tightly regulated upon environmental cues. Maintaining cellular Ca2+ balance is essential for plant survival. Here we report a calcium-dependent protein kinases (CPKs)-mediated signaling pathway, in conjunction with the ER membrane-resident Ca2+-ATPase ECA1, acts influentially for cytosolic Ca2+ homeostasis and osmotic stress tolerance. We show that targeting cytosolic Ca2+ efflux via specific inhibitors or eca1 mutation results in augmented [Ca2+]cyt spikes, elevated cytoplasmic ABA ([ABA]cyt) level and ultimately hypersensitive to osmotic stress. Screening of Arabidopsis CPKs revealed direct binding of CPK2/6/11 to ECA1. Moreover, CPK2/6/11 phosphorylate the N-terminal of ECA1 at Ser5, thereby enhancing its activity for cytosolic Ca2+ efflux into ER and subsequently lower [ABA]cyt. The cumulative effect of ECA1 and CPKs mutation on Arabidopsis plant sensitivity to osmotic stress further illustrates that CPKs/ECA1 acts an intracellular sensory module for plant stress tolerance via regulating [Ca2+]cyt and [ABA]cyt homeostasis. One-sentence summary: CPKs/ECA1 acts an intracellular sensory module for plant osmotic stress tolerance via regulating cytosolic Ca2+ and ABA homeostasis.