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

In plants, potassium (K ) homeostasis is tightly regulated and established against a concentration gradient to the environment. Despite the identification of Ca -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 signals in Arabidopsis thaliana roots, we employed ratiometric analyses of Ca dynamics in plants expressing the Ca reporter YC3.6 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 signals in roots exhibiting spatial and temporal specificity. A transient primary Ca signature arose within 1 min in the postmeristematic stelar tissue of the elongation zone, while a secondary Ca response occurred after several hours as sustained Ca elevation in defined tissues of the elongation and root hair differentiation zones. Patch-clamp and TEVC analyses revealed Ca dependence of the activation of the K channel AKT1 by the CBL1-CIPK23 Ca sensor-kinase complex. Together, these findings identify a critical role of cell group-specific Ca signaling in low K responses and indicate an essential and direct role of Ca signals for AKT1 K channel activation in roots.