Boric acid stimulates the plasma membrane H+‐ATPase of ungerminated lily pollen grains

Abstract

The stimulation of the plasma membrane (PM) H+‐ATPase by boric acid was studied on a microsomal fraction (MF) obtained from ungerminated, boron‐dependent pollen grains of Lilium longiflorum Thunb. which usually need boron for germination and tube growth. ATP hydrolysis and H+ transport activity increased by 14 and 18%, respectively, after addition of 2‐4 mM boric acid. The optimum of boron stimulation was at pH 6.5‐8.5 for ATP hydrolysis and at pH 6.5‐7.5 for H+ transport. No boron stimulation was detected when vanadate was added to the MF, whereas an increase of 10‐20% in ATP hydrolysis and H+ transport was still measured in the presence of inhibitors specific for V ‐type ATPase (nitrate and bafilomycin) and F‐type ATPase (azide), respectively. A vanadate‐sensitive increase in ATP hydrolysis activity was also observed in partially permeabilized vesicles (0.001%[w/v] Triton X‐100) suggesting a direct interaction between borate and the PM H+‐ATPase rather than a weak acid‐induced stimulation. Additionally, we measured the effect of boron on membrane voltage (Vm) of ungerminated pollen grains and observed small hyperpolarizations in 48% of all experiments. Exposing pollen grains to a more acidic pH of 4 caused a depolarization, followed in some experiments by a repolarization (21%). In the presence of 2 mM boron such hyperpolarizations, perhaps caused by an enhanced activity of the H+‐ATPase, were measured in 58% of all tested pollen grains. The effects of boron on Vm may be reduced by additional stimulation of a K+ inward current of opposite direction to the H+‐ATPase. All experiments indicate that boron stimulates an electrogenic transport system in the plasma membrane which is sensitive to vanadate and has a pH optimum around 7, i.e. the plasma membrane H+‐ATPase. A boron‐increased PM H+‐ATPase activity in turn may stimulate germination and growth of pollen tubes.