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JXB Advance Access originally published online on December 20, 2004
Journal of Experimental Botany 2005 56(412):613-622; doi:10.1093/jxb/eri053
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Journal of Experimental Botany, Vol. 56, No. 412, © Society for Experimental Biology 2004; all rights reserved

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

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

* To whom correspondence should be addressed. Fax: +34 952 131944. E-mail: Lrubio{at}uma.es

Zostera marina L. is an angiosperm that grows in a medium in which inorganic phosphate (Pi) and nitrate are present in micromolar concentrations and must be absorbed against a steep electrochemical potential gradient. The operation of a Na+-dependent 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 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. additions also evoked depolarizations of the plasma membrane of root cells only in the presence of Na+. Both - 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 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+).

Key words: Nitrate uptake, phosphate uptake, sodium-dependent transport, sodium-selective microelectrodes, Zostera marina


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