Sodium-dependent nitrate transport at the plasma membrane of leaf cells of the marine higher plant Zostera marina L.

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 mu M 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.