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 cbl3mutant 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.