The plant vacuole: Emitter and receiver of calcium signals

This review portrays the plant vacuole as both a source and a target of Ca2+ signals. In plants, the vacuole represents a Ca2+ store of enormous size and capacity. Total and free Ca2+ concentrations in the vacuole vary with plant species, cell type, and environment, which is likely to have an impact on vacuolar function and the release of vacuolar Ca2+. It is known that cytosolic Ca2+ signals are often generated by release of the ion from internal stores, but in very few cases has a role of the vacuole been directly demonstrated. Biochemical and electrophysical studies have provided evidence for the operation of ligand- and voltage-gated Ca2+-permeable channels in the vacuolar membrane. The underlying molecular mechanisms are largely unknown with one exception: the slow vacuolar channel, encoded by TPC1, is the only vacuolar Ca2+-permeable channel cloned to date. However, due to its complex regulation and its low selectivity amongst cations, the role of this channel in Ca2+ signalling is still debated. Many transport proteins at the vacuolar membrane are also targets of Ca2+ signals, both by direct binding of Ca2+ and by Ca2+-dependent phosphorylation. This enables the operation of feedback mechanisms and integrates vacuolar transport systems in the wider signalling network of the plant cell.