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

Cytosolic calcium signals are evoked by a large variety of biotic and abiotic stimuli and play an important role in cellular and long distance signaling in plants. While the function of the plasma membrane in cytosolic Ca2+ signaling 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 Ca2+ and pH homeostasis of bulging root hair cells of Arabidopsis. Depolarization of the vacuolar membrane caused a rapid increase of the Ca2+ concentration and alkalized the cytosol, while hyperpolarization led to the opposite responses. The relationship between the vacuolar membrane potential, the cytosolic pH and Ca2+ concentration suggests that a vacuolar H+ /Ca2+ exchange mechanism plays a central role in cytosolic Ca2+ homeostasis. Mathematical modelling further suggests that the voltage-dependent vacuolar Ca2+ homeostat could contribute to calcium signaling when coupled to a recently discovered K+ channel-dependent module for electrical excitability of the vacuolar membrane.