Phosphate uptake in Lemna gibba G1: energetics and kinetics

Phosphate uptake was studied by determining [32P]phosphate influx and by measurements of the electrical membrane potential in duckweed (Lemna gibba L.). Phosphate-induced membrane depolarization (ΔEm) was controlled by the intracellular phosphate content, thus maximal ΔEm by 1 mM ${\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}^{-}$ was up to 133 mV after 15 d of phosphate starvation. The ΔEm was strongly dependent on the extracellular pH, with a sharp optimum at pH 5.7. It is suggested that phosphate uptake is energized by the electrochemical proton gradient, proceeding by a $2{\mathrm{H}}^{+}/{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}^{-}$ cotransport mechanism. This is supported also by the fusicoccin stimulation of phosphate influx. Kinetics of phosphate influx and of ΔEm, which represent mere plasmalemma transport, are best described by two Michaelis-Menten terms without any linear components.