Gibberellic Acid lnduces Vacuolar Acidification in Barley Aleurone

The roles of gibberellic acid (GA3) and abscisic acid (ABA) in the regulation of vacuolar pH (pH,) in aleurone cells of barley were investigated using the pH-sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). BCECF accumulated in vacuoles of aleurone cells, but sequestration of the dye did not affect its sensitivity to pH. BCECFloaded aleurone cells retained their ability to respond to both GA3 and ABA. The pH, of freshly isolated aleurone cells is 6.6, but after incubation in GA3, the pH, fel1 to 5.8. The pH, of cells not incubated in hormones or in the presence of ABA showed little orno acidification. The aleurone tonoplast contains both vacuolar ATPase and vacuolar pyrophosphatase, but the levels of pump proteins were not affected by incubation in the presence or absence of hormones. We conclude that GA3 affects the pH, in aleurone cells by altering the activities of tonoplast H+ pumps but not the amounts of pump proteins.