| Abstract
| - Zostera marina L. is an angiosperm that grows in a medium in which inorganic phosphate (Pi) and nitrate \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \((\mathrm{NO}_{3}^{{-}})\) end{document} are present in micromolar concentrations and must be absorbed against a steep electrochemical potential gradient. The operation of a Na+-dependent \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} 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 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} 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. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} additions also evoked depolarizations of the plasma membrane of root cells only in the presence of Na+. Both \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document}- 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 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} 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+).
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