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

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, withJ  Nifmax corresponding to 60 × 10−9 mol m−2 s−1 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 mol−1), close to that of the H+-ATPase (155 kJ mol−1), and JNif 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.