Cytosolic calcium regulates ion channels in the plasma membrane of Vicia faba guard cells

Abstract

THE molecular mechanisms by which Ca²⁺ controls a variety of ion transport-associated cellular functions in higher plant cells¹, including movements of stomatal pores, remain unknown. Stomatal pores regulate the gas exchange in leaves. Openings of stomatal pores are mediated by an increase in the intracellular potassium and anion content of guard cells^2,3. Voltage-dependent K⁺ channels^4–7 and hyperpolarizing H⁺ pumps^8,9 have been identified as mechanisms controlling stomatal movements. But depolarizing mechanisms required for K⁺ efflux during stomatal closing remain unknown. Indirect evidence suggests that Ca²⁺ triggers stomatal closing and inhibits stomatal opening^10–12. Using patch-clamp techniques¹³, we show here that elevated (micromolar) concentrations of cytosolic Ca²⁺ in guard cells block inward rectifying K⁺ channels. Furthermore, elevation of cytosolic Ca²⁺ leads to the activation of a voltage-dependent depolarizing conductance with a permeability to anions. The Ca²⁺-induced modulation of ion channels reported here could provide a molecular basis for Ca²⁺-dependent regulation of stomatal movements in leaves.