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JXB Advance Access published online on December 20, 2004
Journal of Experimental Botany, doi:10.1093/jxb/eri053
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Journal of Experimental Botany © Society for Experimental Biology 2004; all rights reserved
Received May 25, 2004
Accepted October 19, 2004

RESEARCH PAPER

Physiological evidence for a sodium-dependent high-affinity phosphate and nitrate transport at the plasma membrane of leaf and root cells of Zostera marina L

L. Rubio 1*, A. Linares-Rueda 1, M. J. García-Sánchez 1, and J. A. Fernández 1

1 Departamento Biología Vegetal, Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos s/n, S-29071, Málaga, Spain

* To whom correspondence should be addressed.
L. Rubio, E-mail: Lrubio{at}uma.es


  Abstract

Zostera marina L. is an angiosperm that grows in a medium in which inorganic phosphate (Pi) and nitrate (NO3-) are present in micromolar concentrations and must be absorbed against a steep electrochemical potential gradient. The operation of a Na+-dependent NO3- transport was previously demonstrated in leaf cells of this plant, suggesting that other Na+-coupled systems could mediate the uptake of anions. To address this question, Pi transport was studied in leaves and roots of Z. marina, as well as NO3- uptake in roots. Electrophysiological studies demonstrated that micromolar concentrations of Pi induced depolarizations of the plasma membrane of root cells. However, this effect was not observed in leaf cells. Pi-induced depolarizations showed Michaelis-Menten kinetics (Km=1.5±0.6 µM Pi; Dmax=7.8±0.8 mV), and were not observed in the absence of Na+. However, depolarizations were restored when Na+ was resupplied. NO3- additions also evoked depolarizations of the plasma membrane of root cells only in the presence of Na+. Both NO3-- and Pi-induced depolarizations were accompanied by an increase in cytoplasmic Na+ activity, detected by Na+-sensitive microelectrodes. Pi net uptake (measured in depletion experiments) was stimulated by Na+. These results strongly suggest that Pi uptake in roots of Z. marina is mediated by a high-affinity Na+-dependent transport system. Both NO3- and Pi transport systems exploit the steep inwardly directed electrochemical potential gradient for Na+, considering the low cytoplasmic Na+ activity (10.7±3.3 mM Na+) and the high external Na+ concentration (500 mM Na+).

Keywords: Nitrate uptake; phosphate uptake; sodium-dependent transport; sodium-selective microelectrodes; Zostera marina.
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