Two spatially and temporally distinct Ca 2+ 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 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 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 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 AKT1 by the CBL1-CIPK23 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 AKT1 K ⁺ channel activation in roots.
(© 2016 The Authors. New Phytologist © 2016 New Phytologist Trust.)