Induction of the Na+/Pi cotransport system in the plasma membrane of Chara corallina requires external Na+ and low levels of Pi

It was shown in previous studies that the giant freshwater alga Chara corallina does not control its Na⁺‐dependent Pi uptake by monitoring the internal Pi concentration and it was hypothesized that Chara may instead detect changes in Pi supply from the environment. The present work investigated the conditions that control the induction and inactivation of high affinity Na⁺/Pi influx in Chara. Withdrawal of Pi from the external medium resulted in a gradual increase in the rate of uptake measured immediately after Pi was resupplied. The increase continued for at least 7 d of starvation. In the initial stages, 0·5 or 1 µm Pi were more effective at inducing transport activity than no Pi, suggesting that low levels of Pi are actually required for induction. The high Na⁺‐dependent Pi uptake observed in Pi‐starved cells was inactivated by treatment with as little as 1 µm Pi over 6 d. External Na⁺ plays a major role in controlling the capacity for Na⁺/Pi cotransport activity, and in the absence of Na⁺, both induction and inactivation were either delayed or abolished. Na⁺ starvation stimulated Na⁺ uptake even though there were no measurable changes in the concentrations of Na⁺, or of K⁺ or Pi in either the vacuole or cytoplasm. It was concluded that both substrate (Pi) and driver ion (Na⁺) are required at adequate concentrations for the induction of the cotransporter. In the case of Pi, it was suggested that passive leakage of Pi from the cell into the apoplast is sufficient for this purpose but that supplementation by up to 1 µm Pi is more effective at the earlier stage. A mechanism for sensing the external supply of Pi is proposed.