Abstract
The duckweed Spirodela oligorrhiza grown under Pi-deficient conditions (-P plants) exhibited more than 50-fold higher Pi-transporting activity than did plants grown under Pi-sufficient conditions (+P plants). The Pi-transporting activity of -P plants was inhibited by CCCP, indicating that this Pi transport is energized by the electrochemical proton gradient across PM. From kinetic analyses of Pi transport and western-blot analysis of PM proteins, it was suggested that -P plants would have a high-affinity Pi transporter(s) induced by Pi-depletion. On the contrary, the amount and ATP-hydrolytic and ATP-dependent H+-transporting activities of PM H+-ATPase did not increase in -P plants. However, kinetic analyses showed that the PM H+-ATPase in -P plants had a lower Km value and higher H+/ATP coupling ratio than those in +P plants. Therefore, 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 PM.
