A Ca ²⁺ signaling pathway regulates a K ⁺ channel for low-K response in <i>Arabidopsis</i>

<jats:p> 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 <jats:sup>+</jats:sup> ) in soils becomes a limiting factor for plant growth. In response to low-K conditions, plants enhance their capability of K <jats:sup>+</jats:sup> uptake through an unknown signaling mechanism. Here we report the identification of a Ca <jats:sup>2+</jats:sup> -dependent pathway for low-K response in <jats:italic>Arabidopsis</jats:italic> . 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 <jats:sup>2+</jats:sup> 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 <jats:sup>+</jats:sup> channel (AKT1) required for K <jats:sup>+</jats:sup> acquisition in <jats:italic>Arabidopsis</jats:italic> . In the <jats:italic>Xenopus</jats:italic> 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 <jats:sup>2+</jats:sup> -dependent manner, connecting the Ca <jats:sup>2+</jats:sup> signal to enhanced K <jats:sup>+</jats:sup> uptake through activation of a K <jats:sup>+</jats:sup> channel. Disruption of both <jats:italic>CBL1</jats:italic> and <jats:italic>CBL9</jats:italic> or <jats:italic>CIPK23</jats:italic> gene in <jats:italic>Arabidopsis</jats:italic> reduced the AKT1 activity in the mutant roots, confirming that the Ca <jats:sup>2+</jats:sup> -CBL-CIPK pathway functions to orchestrate transporting activities <jats:italic>in planta</jats:italic> according to external K <jats:sup>+</jats:sup> availability. </jats:p>