Potassium nutrient status drives posttranslational regulation of a low-K response network in Arabidopsis

Under low-potassium (K⁺) stress, a Ca²⁺ signaling network consisting of calcineurin B-like proteins (CBLs) and CBL-interacting kinases (CIPKs) play essential roles. Specifically, the plasma membrane CBL1/9-CIPK pathway and the tonoplast CBL2/3-CIPK pathway promotes K⁺ uptake and remobilization, respectively, by activating a series of K⁺ channels. While the dual CBL-CIPK pathways enable plants to cope with low-K⁺ stress, little is known about the early events that link external K⁺ levels to the CBL-CIPK proteins. Here we show that K⁺ status regulates the protein abundance and phosphorylation of the CBL-CIPK-channel modules. Further analysis revealed low K⁺-induced activation of VM-CBL2/3 happened earlier and was required for full activation of PM-CBL1/9 pathway. Moreover, we identified CIPK9/23 kinases to be responsible for phosphorylation of CBL1/9/2/3 in plant response to low-K⁺ stress and the HAB1/ABI1/ABI2/PP2CA phosphatases to be responsible for CBL2/3-CIPK9 dephosphorylation upon K⁺-repletion. Further genetic analysis showed that HAB1/ABI1/ABI2/PP2CA phosphatases are negative regulators for plant growth under low-K⁺, countering the CBL-CIPK network in plant response and adaptation to low-K⁺ stress.