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 (Ca 2+ )-dependent signalling network, consisting of two calcineurin B-like (CBL) Ca 2+ 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 Ca 2+ -dependent manner, presenting a mechanism for the activation of vacuolar K + remobilization that plays an important role in plant adaptation to K + deficiency. Potassium, one of the most important ions, is stored in the vacuole. A new mechanism for potassium homeostasis and release from this vacuolar store relies on the CBL–CIPK calcium sensing module, which activates the tonoplastic two-pore K + channel.