BORIC-ACID STIMULATES THE PLASMA-MEMBRANE H-ATPASE OF UNGERMINATED LILY POLLEN GRAINS()

The stimulation of the plasma membrane (PM) H+-ATPase by boric acid wa s studied on a microsomal fraction (MF) obtained from ungerminated, bo ron-dependent pollen grains of Lilium longiflorum Thunb. which usually need boron for germination and tube growth. ATP hydrolysis and H+ tra nsport 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 sti mulation was detected when vanadate was added to the MF, whereas an in crease 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-sens itive increase in ATP hydrolysis activity was also observed in partial ly permeabilized vesicles (0.001% [w/v] Triton X-100) suggesting a dir ect interaction between berate and the PM H+-ATPase rather than a weak acid-induced stimulation. Additionally, we measured the effect of bor on on membrane voltage (V-m) of ungerminated pollen grains and observe d small hyperpolarizations in 48% of all experiments. Exposing pollen grains to a more acidic pH of 4 caused a depolarization, followed in s ome experiments by a repolarization (21%). In the presence of 2 mM bor on such hyperpolarizations, perhaps caused by an enhanced activity of the H+-ATPase, were measured in 58% of all tested pollen grains. The e ffects of boron on V-m may be reduced by additional stimulation of a K + inward current of opposite direction to the H+-ATPase. All experimen ts indicate that boron stimulates an electrogenic transport system in the plasma membrane which is sensitive to vanadate and has a pH optimu m around 7, i.e. the plasma membrane H+-ATPase. A boron-increased PM H +-ATPase activity in turn may stimulate germination and growth of poll en tubes.