Ca²⁺ signaling pathway regulates a K⁺ channel for low-K response in Arabidopsis

Nutrient sensing is critical for plant adaptation to the environment. Because of extensive farming and erosion, low content of mineral nutrients such as potassium (K⁺) in soils becomes a limiting factor for plant growth. In response to low-K conditions, plants enhance their capability of K⁺ uptake through an unknown signaling mechanism. Here we report the identification of a Ca²⁺-dependent pathway for low-K response in Arabidopsis. We are not aware of any other example of a molecular pathway for a nutrient response in plants. Earlier genetic analyses revealed three genes encoding two Ca²⁺ sensors (CBL1 and CBL9) and their target protein kinase (CIPK23) to be critical for plant growth on low-K media and for stomatal regulation, indicating that these calcium signaling components participate in the low-K response and turgor regulation. In this study, we show that the protein kinase CIPK23 interacted with, and phosphorylated, a voltage-gated inward K⁺ channel (AKT1) required for K⁺ acquisition in Arabidopsis. In the Xenopus oocyte system, our studies showed that interacting calcium sensors (CBL1 and CBL9) together with target kinase CIPK23, but not either component alone, activated the AKT1 channel in a Ca²⁺-dependent manner, connecting the Ca²⁺ signal to enhanced K⁺ uptake through activation of a K⁺ channel. Disruption of both CBL1 and CBL9 or CIPK23 gene in Arabidopsis reduced the AKT1 activity in the mutant roots, confirming that the Ca²⁺-CBL-CIPK pathway functions to orchestrate transporting activities in planta according to external K⁺ availability.