Extracellular Ca²⁺ Ameliorates NaCl-Induced K⁺ Loss from Arabidopsis Root and Leaf Cells by Controlling Plasma Membrane K⁺-Permeable Channels

Calcium can ameliorate Na⁺ toxicity in plants by decreasing Na⁺ influx through nonselective cation channels. Here, we show that elevated external [Ca²⁺] also inhibits Na⁺-induced K⁺ efflux through outwardly directed, K⁺-permeable channels. Noninvasive ion flux measuring and patch-clamp techniques were used to characterize K⁺ fluxes from Arabidopsis (Arabidopsis thaliana) root mature epidermis and leaf mesophyll under various Ca²⁺ to Na⁺ ratios. NaCl-induced K⁺ efflux was not related to the osmotic component of the salt stress, was inhibited by the K⁺ channel blocker TEA⁺, was not mediated by inwardly directed K⁺ channels (tested in the akt1 mutant), and resulted in a significant decrease in cytosolic K⁺ content. NaCl-induced K⁺ efflux was partially inhibited by 1 mM Ca²⁺ and fully prevented by 10 mM Ca²⁺. This ameliorative effect was at least partially attributed to a less dramatic NaCl-induced membrane depolarization under high Ca²⁺ conditions. Patch-clamp experiments (whole-cell mode) have demonstrated that two populations of Ca²⁺-sensitive K⁺ efflux channels exist in protoplasts isolated from the mature epidermis of Arabidopsis root and leaf mesophyll cells. The instantaneously activating K⁺ efflux channels showed weak voltage dependence and insensitivity to external and internal Na⁺. Another population of K⁺ efflux channels was slowly activating, steeply rectifying, and highly sensitive to Na⁺. K⁺ efflux channels in roots and leaves showed different Ca²⁺ and Na⁺ sensitivities, suggesting that these organs may employ different strategies to withstand salinity. Our results suggest an additional mechanism of Ca²⁺ action on salt toxicity in plants: the amelioration of K⁺ loss from the cell by regulating (both directly and indirectly) K⁺ efflux channels.