title: Sodium-dependent nitrate transport at the plasma membrane of leaf cells of the marine higher plant Zostera marina L.
abstract: 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 microM 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.
authors.affiliation: Departamento de Biología Vegetal, Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos s/n, 29071 Málaga, Spain.
authors.forename: M J
authors.initials: MJ
authors.lastname: García-Sánchez
authors.forename: M P
authors.initials: MP
authors.lastname: Jaime
authors.forename: A
authors.initials: A
authors.lastname: Ramos
authors.forename: D
authors.initials: D
authors.lastname: Sanders
authors.forename: J A
authors.initials: JA
authors.lastname: Fernández
mesh_terms: D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000187:drug effects; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D019684:Magnoliopsida / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D008985:Monensin / Q000494:pharmacology; D009566:Nitrates / Q000378:metabolism*; D018515:Plant Leaves / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D012623:Seawater; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D007476:Ionophores; D009566:Nitrates; D008985:Monensin; D012964:Sodium