Notes: Duckweed plants (Spirodela oligorrhiza) grown under phosphate (Pi)-deficient conditions (– P plants) exhibited more than 50-fold higher Pi uptake activity than plants grown under Pi-sufficient conditions (+ P plants). The Pi uptake activity of – P plants measured using 32Pi was significantly inhibited by carbonylcyanide m-chlorophenylhydrazone, indicating that Pi uptake is energized by the electrochemical proton gradient across the plasma membrane (PM). When Pi uptake was examined at various concentrations of Pi, more active uptake of Pi was observed in – P plants than in + P plants, irrespective of the Pi concentrations. An immunoblot analysis of the PM proteins using antiserum against the conserved sequence of the high-affinity Pi transporter recognized the occurrence and large accumulation of a novel protein band at 48 kDa in – P plants. The protein was almost completely extracted with chloroform-methanol (2:1, v/v), but only a trace amount of the protein was detected in + P plants. Immunohistochemical studies of plant roots using the same antiserum demonstrated a large accumulation of high-affinity Pi transporters at the outermost cortical cells of – P plants, but not of + P plants. When an immunoblot analysis of PM proteins was performed using antiserum against the PM H+-ATPase, a positive band of about 96 kDa was detected in both plants with a similar signal intensity. Furthermore, ATP-hydrolytic and ATP-dependent H+-transporting activities of PM H+-ATPase in – P plants were not higher than those in + P plants. However, kinetic analyses showed that the PM H+-ATPase in – P plants had a lower Km value and a higher coupling efficiency between ATP hydrolysis and H+ pumping than the corresponding values in + P plants. These results suggest that the significant stimulation of Pi uptake in – P plants may be due mainly to the induction and accumulation of the high-affinity Pi transporter in the PM, and that the electrochemical proton gradient across the PM may be generated by the high-ATP-affinity and energy-efficient H+ pump in – P plants. This would facilitate the acquisition of Pi in S. oligorrhiza under Pi-depleted conditions.