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

Abstract

Abstract 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 angiospermZostera 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 mmNa+). 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.