PARALLEL PATHWAYS FOR INTRACELLULAR CA2+ RELEASE FROM THE VACUOLE OF HIGHER-PLANTS

A novel pathway for intracellular Ca2+ release via a voltage-gated channel has been discovered in the tonoplast of Beta vulgaris (sugar beet) tap roots. The channel is characterized electrophysiologically in isolated vacuoles and radiometrically in tonoplast-enriched microsomes. Single channel properties were studied in excised membrane patches. With 5-20 mM Ca2+ as a charge carrier on the vacuolar side, the unitary current saturates to a maximal value of 0.59 +/-0.05 pA as membrane voltage approaches +30 to +50 mV. The maximal slope conductance at non-saturating voltages is 12.45 +/- 1.06 pS. Open-state probability increases markedly with positive-going voltage changes in the physiological range. Channel activity is also increased by vacuolar, but not by cytosolic Ca2+. The lanthanide Gd3+ is alone among a large range of Ca2+ channel antagonists in acting as an effective inhibitor of the channel, primarily as a result of a dramatic effect on the open-state probability. We conclude that this voltage-gated channel could constitute an alternative, inositol (1,4,5)-trisphosphate-independent pathway for vacuolar Ca2+ release during signal transduction in plants.