Plant Physiol, March 2000, Vol. 122, pp. 879-886

Sodium-Dependent Nitrate Transport at the Plasma Membrane of Leaf Cells of the Marine Higher Plant Zostera marina L.¹

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

NO₃ 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 NO₃ transport in the marine angiosperm Zostera marina L. to address the question of how NO₃ uptake is energized. Electrophysiological studies demonstrated that micromolar concentrations of NO₃ induced depolarizations of the plasma membrane of leaf cells. Depolarizations showed saturation kinetics (K_m = 2.31 ± 0.78 µM NO₃) and were enhanced in alkaline conditions. The addition of NO₃ did not affect the membrane potential in the absence of Na⁺, but depolarizations were restored when Na⁺ was resupplied. NO₃-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 NO₃-evoked depolarizations by 85%, and NO₃ uptake (measured by depletion from the external medium) was stimulated by Na⁺ ions and by light. Our results strongly suggest that NO₃ 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 NO₃ to that of Na⁺ enables the steep inwardly-directed electrochemical potential for Na⁺ to drive net accumulation of NO₃ within leaf cells.