Mechanisms of calcium homeostasis orchestrate plant growth and immunity.

Calcium (Ca) is an essential nutrient for plants and a cellular signal, but excessive levels can be toxic and inhibit growth. To thrive in dynamic environments, plants must monitor and maintain cytosolic Ca homeostasis by regulating numerous Ca transporters. Here we report two signalling pathways in Arabidopsis thaliana that converge on the activation of vacuolar Ca/H exchangers (CAXs) to scavenge excess cytosolic Ca in plants. One mechanism, activated in response to an elevated external Ca level, entails calcineurin B-like (CBL) Ca sensors and CBL-interacting protein kinases (CIPKs), which activate CAXs by phosphorylating a serine (S) cluster in the auto-inhibitory domain. The second pathway, triggered by molecular patterns associated with microorganisms, engages the immune receptor complex FLS2-BAK1 and the associated cytoplasmic kinases BIK1 and PBL1, which phosphorylate the same S-cluster in CAXs to modulate Ca signals in immunity. These Ca-dependent (CBL-CIPK) and Ca-independent (FLS2-BAK1-BIK1/PBL1) mechanisms combine to balance plant growth and immunity by regulating cytosolic Ca homeostasis.