Cytoplasmic Acidification Induced by Inorganic Phosphate Uptake in Suspension Cultured Catharanthus roseus Cells: Measurement with Fluorescent pH Indicator and P-Nuclear Magnetic Resonance.

Cytoplasmic acidification during inorganic phosphate (Pi) absorption by Catharanthus roseus cells were studied by means of a fluorescent pH indicator, 2',7'-bis-(2-carboxyethyl)-5 carboxyfluorescein (acetomethylester) (BCECF), and (31)P-nuclear magnetic resonance spectroscopy. Cytoplasmic acidification measured by decrease in the fluorescence intensity started immediately after Pi application. Within a minute or so, a stable state was attained and no further acidification occurred, whereas Pi absorption was still proceeding. As soon as Pi in the medium was exhausted, cytoplasmic pH started to recover. Coincidentally, the medium pH started to recover toward the original acidic pH. The Pi-induced changes in the cytoplasmic pH were confirmed by (31)P-nuclear magnetic resonance study. Maximum acidification of the cytoplasm induced by 1.7 millimolar Pi was 0.2 pH units. Vacuolar pH was also affected by Pi. In some experiments, but not all, pH decreased reversibly by 0.2 to 0.3 pH units during Pi absorption. Results suggest that the cytoplasmic pH is regulated by proton pumps in the plasma membrane and in the tonoplast. In addition, other mechanisms that could consume extra protons in the cytoplasm are suggested.