Regulation of cytoplasmic pH under extreme acid conditions in suspension cultured cells of Catharanthus roseus: a possible role of inorganic phosphate

Changes in cytoplasmic pH of suspension-cultured cells of Catharanthus rose us under extreme acid conditions were measured with the pa-dependent fluore scence dye; 2',7'-bis-(2-carboxyethyl)-5 (and-6) carboxyfluorescein (-ace-t oxymethylester) (BCECF). When cells were treated with 1 mM HCl (pH 3 soluti on), the cytoplasmic pH first decreased then returned to the original level . Treatment with 10 mM HCl (pH 2 solution) acidified the cytoplasm to a gre ater extent, and the acidification continued at a constant level throughout the measurement. Treatment with a pH 2 solution resulted in a gradual decr ease of the malate content, indicating the operation of biochemical pH regu lation mechanism. The pH 2 treatment also caused a sudden decrease of the i ntracellular level of Pi. The cellular content of total phosphorus did not change during the acidification. The Pi was converted to the organic phosph ate form. The ATP level was not increased by the pH 2 treatment, but slight ly decreased. The role of Pi, which might be functioning as a regulatory fa ctor of cytoplasmic pH, a non-competitive inhibitor of the H+-pumps of both the plasma membrane and tonoplast is discussed.