Magnesium sensitizes slow vacuolar channels to physiological cytosolic calcium and inhibits fast vacuolar channels in fava bean guard cell vacuoles

摘要

Vacuolar ion channels in guard cells play important roles during stomatal movement and are regulated by many factors including Ca superior 2 superior +, calmodulin, protein kinases, and phosphatases. We report that physiological cytosolic and luminal Mg superior 2 superior + levels strongly regulate vacuolar ion channels in fava bean (Vicia faba) guard cells. Luminal Mg superior 2 superior + inhibited fast vacuolar (FV) currents with a K inferior 1 of approximately 0.23 mM in a voltage-dependent manner at positive potentials on the cytoplasmic side. Cytosolic Mg superior 2 superior + at 1 mM also inhibited FV currents. Furthermore, in the absence of cytosolic Mg superior 2 superior +, cytosolic Ca superior 2 superior + at less than 10 mu M did not activate slow vacuolar (SV) currents. However, when cytosolic Mg superior 2 superior + was present, submicromolar concentrations of cytosolic Ca superior 2 superior + activated SV currents with a K(d) of approximately 227 nM, suggesting a synergistic Mg superior 2 superior +-Ca superior 2 superior + effect. The activation potential of SV currents was shifted toward physiological potentials in the presence of cytosolic Mg superior 2 superior + concentrations. The direction of SV currents could also be changed from outward to both outward and inward currents. Our data predict a model for SV channel regulation, including a cytosolic binding site for Ca superior 2 superior + with an affinity in the submicromolar range and a cytosolic low-affinity Mg superior 2 superior +-Ca superior 2 superior + binding site. SV channels are predicted to contain a third binding site on the vacuolar luminal side, which binds Ca superior 2 superior + and is inhibitory. In conclusion, cytosolic Mg superior 2 superior + sensitizes SV channels to physiological cytosolic Ca superior 2 superior + elevations. Furthermore, we propose that cytosolic and vacuolar Mg superior 2 superior + concentrations ensure that FV channels do not function as a continuous vacuolar K superior + leak, which would prohibit stomatal opening.