Effect of NO3- transport and reduction on intracellular pH: an in vivo NMR study in maize roots.
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 (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 NO3- 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.
Espen
Luca
L
Dipartimento di Produzione Vegetale, University of Milan, Via Celoria 2, I-20133 Milano, Italy.
Nocito
Fabio F
FF
Cocucci
Maurizio
M
D001692:Biological Transport; D003593:Cytoplasm / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D009566:Nitrates / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism; D010084:Oxidation-Reduction; D010766:Phosphorylation; D018517:Plant Roots / Q000187:drug effects / Q000502:physiology*; D017973:Tungsten Compounds / Q000494:pharmacology; D003313:Zea mays / Q000502:physiology*
D009566:Nitrates; D017973:Tungsten Compounds; D009584:Nitrogen; C045951:tungstate