Two spatially and temporally distinct Ca 2+ signals convey Arabidopsis thaliana responses to K + deficiency

Summary In plants, potassium (K + ) homeostasis is tightly regulated and established against a concentration gradient to the environment. Despite the identification of Ca 2+ ‐regulated kinases as modulators of K + channels, the immediate signaling and adaptation mechanisms of plants to low‐K + conditions are only partially understood. To assess the occurrence and role of Ca 2+ signals in Arabidopsis thaliana roots, we employed ratiometric analyses of Ca 2+ dynamics in plants expressing the Ca 2+ reporter YC 3.6 in combination with patch‐clamp analyses of root cells and two‐electrode voltage clamp (TEVC) analyses in Xenopus laevis oocytes. K + deficiency triggers two successive and distinct Ca 2+ signals in roots exhibiting spatial and temporal specificity. A transient primary Ca 2+ signature arose within 1 min in the postmeristematic stelar tissue of the elongation zone, while a secondary Ca 2+ response occurred after several hours as sustained Ca 2+ elevation in defined tissues of the elongation and root hair differentiation zones. Patch‐clamp and TEVC analyses revealed Ca 2+ dependence of the activation of the K + channel AKT 1 by the CBL 1– CIPK 23 Ca 2+ sensor‐kinase complex. Together, these findings identify a critical role of cell group‐specific Ca 2+ signaling in low K + responses and indicate an essential and direct role of Ca 2+ signals for AKT 1 K + channel activation in roots.