Effect of NO(-3) transport and reduction of intracellular pH: an in vivo NMR study in maize roots
2004
Espen, L. | Nocito, F.F. | Cocucci, M.
The effect of NO3(-) uptake on cellular pH was studied in maize roots by an in vivo 31P-NMR technique. In order to separate the effects on cytoplasmic pH due to NO3(-) uptake from those due to NO3(-) reduction, tungstate was used to inhibit nitrate reductase (NR). The results confirm that in maize roots tungstate inhibited NR activity. 15N-NMR in vivo experiments demonstrated the cessation of nitrogen flux from nitrate to organic compounds. Tungstate affected neither NO3(-) uptake nor the levels of the main phosphorylated compounds. Slight changes in cytoplasmic pH were observed during NO3(-) 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 NO3(-) uptake is an acidifying process and suggest a possible involvement of NO3(-) reduction in pH homeostasis. In the presence of NO3(-), a transient depolarization of transmembrane electric potential difference (E(m)) was observed in all the conditions analysed. However, in tungstate-treated roots, a lesser depolarization accompanied by a greater ability to recover E(m) was found. This was related to a higher activity of the plasma membrane (PM) H(+)-ATPase. When NO3(-) was administered as potassium salt, its uptake increased and a greater depolarization of E(m) 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.
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