A Ca 2+ 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 Ca 2+ -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 Ca 2+ 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 Ca 2+ -dependent manner, connecting the Ca 2+ 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 Ca 2+ -CBL-CIPK pathway functions to orchestrate transporting activities in planta according to external K + availability.