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 describe d (P. Pouliquin, I.-P. Grouzis, R. Gibrat [1999] Biophys J 76: 360-373). J( Ni) in control liposomes responded to membrane potential or to NO3- as expe cted from the Goldman-Hodgkin-Katz diffusion theory, J(Ni) in reconstituted PM vesicles exhibited an additional component (J(Nif)), which was saturabl e (K-m for NO3- approximately 3 mM, with J(Nifmax) corresponding to 60 x 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. I,ii was voltage dependent, with an optimum voltage at 105 mV at pH 6.5. The activation ene rgy of J(Nif) was high (129 kJ mol(-1)), close to that of the H+-ATPase (15 5 kJ mol(-1), and J(Nif) displayed the same acidic optimal pH (pH 6.5) as t hat of the H+ pump. This is the first example, to our knowledge, of a secon dary transport at the plant PM with such a feature. Several properties of t he NO3- uniport seem poorly compatible with that reported for plant anion c hannels and to be attributable instead to a classical carrier. The physiolo gical relevance of these findings is suggested.