Effect of phytohormones on the transmembrane translocation of protons in plasma membrane vesicles from tubers of Solanum tuberosum L.

Effects of phytohormones gibberellic acid (GA) and abscisic acid (ABA) on the ATP-dependent transmembrane transport of protons were studied in plasma membrane vesicles (PMVs) from non-dormant potato tubers. The uptake of H+ into PMVs was assessed by the fluorescence quenching of acridine orange (AO) after the addition of ATP to the incubation medium. Addition of ATP to the incubation medium led to the instantaneous rise of the AO fluorescence intensity followed by its decrease. The fluorescence quenching was not observed in the presence of either protonophore CCCP or inhibitors of the membrane-bound H+-ATPase. It is concluded that the ATP-induced quenching of the AO fluorescence resulted from the accumulation of protons in PMVs due to the function of the plasma membrane-bound H+-ATPase. Depending on their concentrations, GA and ABA either inhibited or stimulated the ATP-driven H+ translocation across the vesicle membrane. The growth-stimulating hormone GA at concentrations of 10(-9)-10(-5) M increased the initial rate of the fluorescence quenching, whereas 10(-4) M GA slightly inhibited the H+ translocation. The growth inhibitor ABA at a concentration of 10(-9) M slightly increased the rate of the proton accumulation in PMVs; at higher concentrations (10(-8)-10(-4) M), ABA inhibited the H+ translocation. Acetic acid, which has pK similar to pK of GA and ABA, did not influence the ATP-dependent H+ accumulation in PMVs, suggesting the hormone-specific action of GA and ABA on the H+-ATPase activity. In the presence of DCC, which completely inhibited the accumulation of H+, GA and ABA did not affect the passive proton efflux from PMVs. It is proposed that the mechanisms of the regulatory effects of phytohormones may involve modification of H+-ATPase activity leading to changes in the electrochemical gradient of H+ across the plasma membrane.