Apigenin and quercetin promote ΔpH-dependent accumulation of IAA in membrane vesicles

Flavonoids may act as regulators of polar auxin transport. In the presence of a pH gradient (pH 8in/6out) the flavonoids quercetin and apigenin, as well as the synthetic herbicide napthylphthalamic acid (NPA), promote the accumulation of IAA in membrane vesicles from dark-grown zucchini hypocotyls. Simultaneous accumulation of 3H-IAA (10 nM) and 14C-butyric acid (5 μM; included as a pH probe) was determined by a filtration assay after incubating the vesicles with 3 nM to 100 μM quercetin, apigenin, NPA or unlabeled IAA. Maximal stimulation (% of Control) was observed with 3 μM NPA (130%), 1 μM quercetin (120%), or 3 μM apigenin (115%); ΔpH was not affected by these concentrations. As reported by others, IAA uptake was saturable: 1 μM unlabeled IAA eliminated ΔpH-dependent uptake of 3H-IAA without altering ΔpH. However, at 30 to 100 μM, every compound tested collapsed the imposed pH gradient and therefore abolished specific 3H-IAA uptake