Regulation of Pi,uptake by Acer pseudoplatanus cells

In view of the importance of P_i in the control of cell metabolism, it was of interest to study the mechanism and regulation of P_i uptake by Acer pseudoplatanus cells grown as cell suspensions. At low external P_i concentrations up to 10 mm, sycamore cells incorporate phosphate against a concentration gradient, by a process which is energy dependent. Under these conditions the intracellular P_i concentration is maintained constant (2–3 mm). On the contrary at high external P_i concentrations, higher than that which counterpoises the cytoplasmic P_i concentration (approximately 10 mm), P_i enters the cell by slow diffusion and the intracellular P_i concentration increases continuously as the extracellular P_i concentration increases from 15 to 50 mm. When sycamore cells are transferred to a phosphate-deficient medium, growth slows down considerably and ceases after 4–5 days. During this time, intracellular P_i concentration falls from 3 to 0.1 mm and phosphate esters from 8 to 2 mm. Phosphate starvation stimulates the uptake indicating that phosphate uptake depends on the intracellular phosphate and/or cytoplasmic ester-P pool. P_i uptake by P_i-starved cells is strongly dependent on the pH of the medium.