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

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American Society of Plant Physiologists

RESUMO

The net initial passive flux (JNi) 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). JNi in control liposomes responded to membrane potential or to NO3− as expected from the Goldman-Hodgkin-Katz diffusion theory. JNi in reconstituted PM vesicles exhibited an additional component (JNif), which was saturable (Km for NO3− approximately 3 mm, with JNifmax 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). JNif was totally inhibited by La3+ and the arginine reagent phenylglyoxal. JNif was voltage dependent, with an optimum voltage at 105 mV at pH 6.5. The activation energy of JNif 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.

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