AC fields of low frequency and amplitude stimulate pollen tube growth possibly via stimulation of the plasma membrane proton pump

Abstract

Growing pollen tubes of Lilium longiflorum Thunb. were exposed to weak AC fields (1.75–100 mV cm −1) of low frequency (1–100 Hz). A stimulation of tube growth was observed in a narrow range of AC fields (1.75–30 mV cm −1 and 1–20 Hz) with a maximum effect (1.42-fold stimulation) at 20 mV cm −1 and 10 Hz. Pollen tubes did not change their growth direction during field application. The stimulation was independent from the orientation of the tubes to the field, but depended on the initial growth rate. Slowly growing pollen tubes (1–3 μm min −1) were more stimulated than fast growing tubes (> 6 μm min −1). In search for a possible target interacting with the AC field we focused on the P-type H + ATPase of the plasma membrane. H + transport activity of a microsomal fraction obtained from pollen tubes was also stimulated by AC fields showing a maximum stimulation at similar AC fields as tube growth (10 mV cm −1, 10 Hz). The stimulation of H + transport was still observed in the presence of azide and bafilomycin inhibiting the mitochondrial, F-type H + ATPase and the vacuolar, V-type H + ATPase, respectively. No stimulation was observed in the presence of the P-type H + ATPase inhibitor vanadate. We, therefore, suggest that an increased activity of the plasma membrane H + ATPase causes the increase in tube growth when pollen tubes were exposed to AC fields. These results underline the importance of the H + ATPase in pollen tube growth; regulation of its activity may also regulate tube elongation in vivo.