Lack of control in inorganic phosphate uptake by Catharanthus roseus (L.) G. Don cells: Cytoplasmic inorganic phosphate homeostasis depends on the tonoplast inorganic phosphate transport system?

Inorganic phosphate (Pi) uptake by Catharanthus roseus (L.) G. Don cells was studied in relation to its apparent uncontrolled uptake using ³¹P-nuclear magnetic resonance spectroscopy. Kinetics of Pi uptake by the cells indicated that apparent K_m and V_m, were about 7 μm and 20 μmol g^-1 fresh weight h^-1, respectively. Pi uptake in Murashige-Skoog medium under different Pi concentrations and different initial cell densities followed basically the same kinetics. When supplied with abundant Pi, cells absorbed Pi at a constant rate (V_m) for the first hours and accumulated it in the vacuole. As the endogenous pool expanded, the rate of Pi uptake gradually decreased to nil. Maximum Pi accumulation was 100 to 120 μmol g^-1 fresh weight if cell swelling during Pi uptake (about 2-fold in cell volume) was not considered. Results indicated that (a) the rate of Pi uptake by Catharanthus cells was independent of initial cell density and was constant over a wide range of Pi concentrations (2 mM to about 10 μm) unless the cells were preloaded with excess Pi, and (b) there was no apparent feedback control over the Pi uptake process in the plasma membrane to avoid Pi toxicity. The importance of the tonoplast Pi transport system in cytoplasmic Pi homeostasis is discussed.