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

NO3- is present at micromolar concentrations in seawater and must be absorb ed by marine plants against a steep electrochemical potential difference ac ross the plasma membrane. We studied NO3- transport in the marine angiosper m 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. Depol arizations showed saturation kinetics (K-m = 2.31 +/- 0.78 mu M NO3-) and w ere enhanced in alkaline conditions. The addition of NO3- did not affect th e membrane potential in the absence of Na+, but depolarizations were restor ed when Na+ was resupplied. NO3--induced depolarizations at increasing Naconcentrations showed saturation kinetics (K-m = 0.72 +/- 0.18 mM Na+). Mon ensin, an ionophore that dissipates the Na+ electrochemical potential, inhi bited NO3--evoked depolarizations by 85%, and NO3- uptake (measured by depl etion from the external medium) was stimulated by Na+ ions and by light. Ou r results strongly suggest that NO3- uptake in Z. marina is mediated by a h igh-affinity Na+-symport system, which is described here (for the first tim e to our knowledge) in an angiosperm. Coupling the uptake of NO3- to that o f Na+ enables the steep inwardly-directed electrochemical potential for Na to drive net accumulation of NO3- within leaf cells.