Sodium-Dependent Nitrate Transport at the Plasma Membrane of LeafCells of the Marine Higher Plant Zostera marinaL.

摘要

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 (Km = 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(Km = 0.72 ± 0.18 mmNa⁺). Monensin, an ionophore that dissipates theNa⁺ electrochemical potential, inhibitedNO3⁻-evoked depolarizations by 85%, andNO3⁻ uptake (measured by depletion from theexternal medium) was stimulated by Na⁺ ions and by light.Our results strongly suggest that NO3⁻ uptakein Z. marina is mediated by a high-affinityNa⁺-symport system, which is described here (for the firsttime to our knowledge) in an angiosperm. Coupling the uptake ofNO3⁻ to that of Na⁺ enables thesteep inwardly-directed electrochemical potential for Na⁺to drive net accumulation of NO3⁻ within leafcells.