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 cel ls was studied in relation to its apparent uncontrolled uptake using P -31-nuclear magnetic resonance spectroscopy. Kinetics of Pi uptake by the cells indicated that apparent K-m and V-m were about 7 mu M and 20 mu mol 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 wit h abundant Pi, cells absorbed Pi at a constant rate (V-m) for the firs t hours and accumulated it in the vacuole. As the endogenous pool expa nded, the rate of Pi uptake gradually decreased to nil. Maximum Pi acc umulation was 100 to 120 mu mol g(-1) fresh weight if cell swelling du ring Pi uptake (about 2-fold in cell volume) was not considered. Resul ts 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 mu M) unless the cells were pr eloaded with excess Pi, and (b) there was no apparent feedback control over the Pi uptake process in the plasma membrane to avoid Pi toxicit y. The importance of the tonoplast Pi transport system in cytoplasmic Pi homeostasis is discussed.