Possible role of cytoplasm pH in the induction of nitrate uptake systems in maize roots

After a period of nitrogen deprivation, maize seedlings were resupplied with nitrate, and the processes involved in the uptake of this anion were studied. Addition of nitrate to the growth medium was effective in inducing both nitrate uptake and PM H+-ATPase activity. In this condition, an increase in the expression of ZmNRT2.1 occurred. Measurements of plasma membrane electric potential difference showed that the entity of membrane depolarisation increased as nitrate uptake capability increased. Moreover, cytoplasmic pH (pHc) declined sharply during the first hour of root contact with nitrate, and subsequently returned to more alkaline values. In accordance with the pH-stat theory, increased pHc was accompanied by increased dark fixation of CO2. To investigate on the physiological role of pHc changes, butyric acid (BA) was used to induce a decrease in pHc similar to that occurring after nitrate addition. In maize seedlings grown in the absence of nitrogen, treatment with BA induced an increase in both activity and expression of H+-ATPase as well as in the expression of ZmNRT2.1, suggesting an involvement of pHc in the signal transduction pathway.