Sodium-Dependent Nitrate Transport at the Plasma Membrane of Leaf Cells of the Marine Higher Plant Zostera marina L.

NO(3)(−) is present at micromolar concentrations in seawater and must be absorbed by marine plants against a steep electrochemical potential difference across the plasma membrane. We studied NO(3)(−) transport in the marine angiosperm Zostera marina L. to address the question of how NO(3)(−) uptake is energized. Electrophysiological studies demonstrated that micromolar concentrations of NO(3)(−) induced depolarizations of the plasma membrane of leaf cells. Depolarizations showed saturation kinetics (K(m) = 2.31 ± 0.78 μm NO(3)(−)) and were enhanced in alkaline conditions. The addition of NO(3)(−) did not affect the membrane potential in the absence of Na(+), but depolarizations were restored when Na(+) was resupplied. NO(3)(−)-induced depolarizations at increasing Na(+) concentrations showed saturation kinetics (K(m) = 0.72 ± 0.18 mm Na(+)). Monensin, an ionophore that dissipates the Na(+) electrochemical potential, inhibited NO(3)(−)-evoked depolarizations by 85%, and NO(3)(−) uptake (measured by depletion from the external medium) was stimulated by Na(+) ions and by light. Our results strongly suggest that NO(3)(−) uptake in Z. marina is mediated by a high-affinity Na(+)-symport system, which is described here (for the first time to our knowledge) in an angiosperm. Coupling the uptake of NO(3)(−) to that of Na(+) enables the steep inwardly-directed electrochemical potential for Na(+) to drive net accumulation of NO(3)(−) within leaf cells.