JXB Advance Access originally published online on August 13, 2004
Journal of Experimental Botany 2004 55(405):2053-2061; doi:10.1093/jxb/erh231
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RESEARCH PAPER |
Effect of NO3– transport and reduction on intracellular pH: an in vivo NMR study in maize roots
Dipartimento di Produzione Vegetale, University of Milan, Via Celoria 2, I-20133 Milano, Italy
* To whom correspondence should be addressed. Fax: +39 (0)2 50316521. E-mail: maurizio.cocucci{at}unimi.it
The effect of 
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 uptake from those due to 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 uptake nor the levels of the main phosphorylated compounds. Slight changes in cytoplasmic pH were observed during 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 uptake is an acidifying process and suggest a possible involvement of reduction in pH homeostasis. In the presence of , 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 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.
Key words: Cytoplasmic pH, maize, nitrate reductase, nitrate uptake, NMR spectroscopy, tungstate
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