Suspension-cultured cells of mangrove [Bruguiera sexangula (Lour.) Poir.] showed a rapid increase in vacuolar volume under salt stress, although there was no change in the cell volume. The rapid increase in the vacuolar volume was an active process, which followed the activation of the tonoplast H + -ATPase and the vacuolar acid phosphatase. The same phenomenon was observed in barley (Hordeum vulgare L. cv. Doriru) root meristematic cells under salt stress but not in pea (Pisum sativum L.). Increases in vacuolar volume could potentially protect the cytoplasm by decreasing the cytoplasmic volume during the initial phases of salt stress.

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Inorganic phosphate (Pi) uptake across the vacuolar membrane of intact vacuoles isolated from Catharanthus roseus suspension-cultured cells was measured. Under low Pi status, Pi uptake into the vacuole was strongly activated compared to high Pi status. Since Pi uptake across the vacuolar membrane is correlated with H+ pumping, we examined the dependency of H+ pumping on plant Pi status. Both H+ pumping and the activities of the vacuolar H+-pumps, the V-type H+-ATPase and the H+-PPase were enhanced under low Pi status. Despite this increase in H+ pumping, Western blot analysis showed no distinct increase in the amount of proton pump proteins. Possible mechanisms for the activation of Pi uptake into the vacuole under low Pi status are discussed.