Plant Physiol, January 2000, Vol. 122, pp. 265-274

Passive Nitrate Transport by Root Plasma Membrane Vesicles Exhibits an Acidic Optimal pH Like the H⁺-ATPase¹

Biochimie et Physiologie Moléculaire des Plantes, Agro-M/Centre National de la Recherche Scientifique (Unité Mixte de Recherche 5004)/Institut National de la Recherche Agronomique/Université de Montpellier 11, 2, Place Viala, F-34060 Montpellier cedex 1, France.

The net initial passive flux (J_Ni) in reconstituted plasma membrane (PM) vesicles from maize (Zea mays) root cells was measured as recently described (P. Pouliquin, J.-P. Grouzis, R. Gibrat [1999] Biophys J 76: 360-373). J_Ni in control liposomes responded to membrane potential or to NO₃ as expected from the Goldman-Hodgkin-Katz diffusion theory. J_Ni in reconstituted PM vesicles exhibited an additional component (J_Nif), which was saturable (K_m for NO₃ approximately 3 mM, with J_Nifmax corresponding to 60 × 10⁹ mol m² s¹ at the native PM level) and selective (NO₃ = ClO₃ > Br > Cl = NO₂; relative fluxes at 5 mM: 1:0.34:0.19). J_Nif was totally inhibited by La³⁺ and the arginine reagent phenylglyoxal. J_Nif was voltage dependent, with an optimum voltage at 105 mV at pH 6.5. The activation energy of J_Nif was high (129 kJ mol¹), close to that of the H⁺-ATPase (155 kJ mol¹), and J_Nif displayed the same acidic optimal pH (pH 6.5) as that of the H⁺ pump. This is the first example, to our knowledge, of a secondary transport at the plant PM with such a feature. Several properties of the NO₃ uniport seem poorly compatible with that reported for plant anion channels and to be attributable instead to a classical carrier. The physiological relevance of these findings is suggested.