A voltage-dependent Ca 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 signalling in plants. While the function of the plasma membrane in cytosolic Ca 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 and pH homeostasis of bulging root hair cells of Arabidopsis. Depolarisation of the vacuolar membrane caused a rapid increase in the Ca concentration and alkalised the cytosol, while hyperpolarisation led to the opposite responses. The relationship between the vacuolar membrane potential, the cytosolic pH and Ca concentration suggests that a vacuolar H /Ca exchange mechanism plays a central role in cytosolic Ca homeostasis. Mathematical modelling further suggests that the voltage-dependent vacuolar Ca homeostat could contribute to calcium signalling when coupled to a recently discovered K channel-dependent module for electrical excitability of the vacuolar membrane.