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.

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. P-i-induced depolarizations showed Michaelis-Menten kinetics (K-m=1.5 +/- 0.6 muM P-i; D-max =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 P-i-induced depolarizations were accompanied by an increase in cytoplasmic Na+ activity, detected by Na+-sensitive microelectrodes. P-i 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+).