Effect of NO23 transport and reduction on intracellular pH: an in vivo NMR study in maize roots

The effect of NO23 uptake on cellular pH was studied in maize roots by an in vivo P-NMR technique. In order to separate the effects on cytoplasmic pH due to NO23 uptake from those due to NO23 reduction, tungstate was used to inhibit nitrate reductase (NR). The results confirm that in maize roots tungstate inhibited NR activity. N-NMR in vivo experiments demonstrated the cessation of nitrogen flux from nitrate to organic compounds. Tungstate affected neither NO23 uptake nor the levels of the main phosphorylated compounds. Slight changes in cytoplasmic pH were observed during NO23 uptake and reduction (i.e. control). By contrast, in the presence of tungstate, a consistent decrease in cytoplasmic pH occurred. The vacuolar pH did not change in any of the conditions tested. These data show that NO23 uptake is an acidifying process and suggest a possible involvement of NO23 reduction in pH homeostasis. In the presence of NO23 , a transient depolarization of transmembrane electric potential difference (Em) was observed in all the conditions analysed. However, in tungstate-treated roots, a lesser depolarization accompanied by a greater ability to recover Em was found. This was related to a higher activity of the plasma membrane (PM) H-ATPase. When NO23 was administered as potassium salt, its uptake increased and a greater depolarization of Em took place, whilst the changes in cytoplasmic pH were remarkably reduced, according to the central role played by K in the control of plasma membrane activities and cell pH homeostasis. A possible involvement of cytoplasmic pH in the control of PM H-ATPase expression during nitrate exposure is suggested.