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 [Ca 2+] 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 Ca 2+ 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 Ca 2+ and fully prevented by 10 mM Ca 2+. This ameliorative effect was at least partially attributed to a less dramatic NaCl-induced membrane depolarization under high Ca 2+ conditions. Patch-clamp experiments (whole-cell mode) have demonstrated that two populations of Ca 2+-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 Ca 2+ and Na + sensitivities, suggesting that these organs may employ different strategies to withstand salinity. Our results suggest an additional mechanism of Ca 2+ action on salt toxicity in plants: the amelioration of K + loss from the cell by regulating (both directly and indirectly) K + efflux channels. © 2006 American Society of Plant Biologists.