A voltage‐dependent Ca 2+ homeostat operates in the plant vacuolar membrane

Summary Cytosolic calcium signals are evoked by a large variety of biotic and abiotic stimuli and play an important role in cellular and long distance signalling in plants. While the function of the plasma membrane in cytosolic Ca 2+ signalling has been intensively studied, the role of the vacuolar membrane remains elusive. A newly developed vacuolar voltage clamp technique was used in combination with live‐cell imaging, to study the role of the vacuolar membrane in Ca 2+ and pH homeostasis of bulging root hair cells of Arabidopsis. Depolarisation of the vacuolar membrane caused a rapid increase in the Ca 2+ concentration and alkalised the cytosol, while hyperpolarisation led to the opposite responses. The relationship between the vacuolar membrane potential, the cytosolic pH and Ca 2+ concentration suggests that a vacuolar H + /Ca 2+ exchange mechanism plays a central role in cytosolic Ca 2+ homeostasis. Mathematical modelling further suggests that the voltage‐dependent vacuolar Ca 2+ homeostat could contribute to calcium signalling when coupled to a recently discovered K + channel‐dependent module for electrical excitability of the vacuolar membrane.