Studies on H+-Translocating ATPases in Plants of Varying Resistance to Salinity

Abstract

Membrane vesicles were isolated from the roots of the halophyte Atriplex nummularia Lindl. H⁺-translocating Mg²⁺-ATPase activity was manifested by the establishment of a positive membrane potential (measured as SCN⁻ accumulation); and also by the establishment of a transmembrane pH gradient (measured by quinacrine fluorescence quenching). H⁺-translocation was highly specific to ATP and was stable to oligomycin. Growing the plants in the presence of 400 millimolar NaCl doubled the proton-translocating activity per milligram of membrane protein and otherwise modulated it in the following ways. First, the flat pH profile observed in non-salt-grown plants was transformed to one showing a peak at about pH 6.2. Second, the lag effect observed at low ATP concentration in curves relating SCN⁻ accumulation to ATP concentration was abolished; the concave curvature shown in the double reciprocal plot was diminished. Third, sensitivity to K-2 (N-morpholino)ethanesulfonic acid stimulation was shown in salt-grown plants (about 40% stimulation) but was absent in non-salt-grown plants. Fourth, the KCl concentration bringing about 50% dissipation of ATP-dependent SCN⁻ accumulation was 20 millimolar for salt-grown plants and 50 millimolar for non-salt-grown plants. Vanadate sensitivity was shown in both cases. No clear NO₃⁻ inhibition was observed.