A calcium signalling network activates vacuolar K⁺ remobilization to enable plant adaptation to low-K environments

Potassium (K) is an essential nutrient, but levels of the free K ions (K⁺) in soil are often limiting, imposing a constant stress on plants. We have discovered a calcium (Ca²⁺)-dependent signalling network, consisting of two calcineurin B-like (CBL) Ca²⁺ sensors and a quartet of CBL-interacting protein kinases (CIPKs), which plays a key role in plant response to K⁺ starvation. The mutant plants lacking two CBLs (CBL2 and CBL3) were severely stunted under low-K conditions. Interestingly, the cbl2 cbl3 mutant was normal in K⁺ uptake but impaired in K⁺ remobilization from vacuoles. Four CIPKs-CIPK3, 9, 23 and 26-were identified as partners of CBL2 and CBL3 that together regulate K⁺ homeostasis through activating vacuolar K⁺ efflux to the cytoplasm. The vacuolar two-pore K⁺ (TPK) channels were directly activated by the vacuolar CBL-CIPK modules in a Ca²⁺-dependent manner, presenting a mechanism for the activation of vacuolar K⁺ remobilization that plays an important role in plant adaptation to K⁺ deficiency.