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 NO(3)(−) 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 NO(3)(−) approximately 3 mm, with J(Nifmax) corresponding to 60 × 10(−9) mol m(−2) s(−1) at the native PM level) and selective (NO(3)(−) = ClO(3)(−) > Br(−) > Cl(−) = NO(2)(−); relative fluxes at 5 mm: 1:0.34:0.19). J(Nif) was totally inhibited by La(3+) 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 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 NO(3)(−) 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.