Cytoplasmic calcium regulates voltage-dependent ion channels in plant vacuoles

Abstract

Vacuoles, the storage organelles of plants, can constitute up to 90% of the cell volume. Transport of solutes such as sugars and organic and inorganic ions across the vacuolar membrane constitutes one of the major cellular achievements during diurnal and annual cycles of carbon metabolism. Recent investigations by patch-clamp techniques¹ identified ion channels and pumps as pathways for the movement of ions and metabolites^2–4. The mechanisms by which these transport proteins are regulated remain unknown. Changes in the cytoplasmic concentration of Ca²⁺ have been postulated to be involved in the physiology of various ion transport processes in plant cells^5–9. But in contrast to animal cells¹⁰, to date no direct evidence exists for the modulation of ion-transport proteins by Ca²⁺ in plant cells. Here we report that in plant vacuoles, changes in cytoplasmic Ca²⁺ regulate the activation of two distinct types of voltage-dependent ion channels. A 60–80 pS channel appeared at Ca²⁺ levels above 10⁻⁷ M, activating slowly (SV—slow vacuolar type) upon application of hyperpolarizing voltage pulses. The voltage-threshold of appearance changed with the cytoplasmic Ca²⁺ concentration. At low Ca²⁺ a smaller channel (30–40 pS) activated instantaneously (FV—fast vacuolar type) upon application of both depolarizing and hyperpolarizing voltages. The two channel types modulated by physiological changes of cytoplasmic Ca²⁺ provide for versatile regulation of ion and metabolite fluxes between the cytoplasm and the vacuole.