Induction of Alkaline Bands in Chara by External Electric Current Reveals the Influence of Plasma Membrane H+ Fluxes on Cyclosis-Mediated Signaling

Abstract

Long-distant communications in plants and giant plant cells are essential for optimal cell functioning under variable environmental conditions. In characean internodal cells exposed to spotted or flickering light, the redox balance in dimly lit chloroplasts is sensitive to the reducing power produced in brightly illuminated chloroplasts of a distal cell region located upstream in the directed cytoplasmic flow. The distant communication is mediated by excessive production in the high-light region of reducing metabolites that are exported into the streaming cytoplasm and delivered by cytoplasmic flow to light-starving chloroplasts. These target chloroplasts perceive the transportable metabolic signal, which is reflected in the transient increase of modulated chlorophyll fluorescence F '. Previous studies of cyclosis-mediated F ' transients revealed that they are sensitive to natural H+ fluxes across the plasma membrane and that the signal transduction is suppressed in cell areas with massive H+ influx. In this study with Chara australis R. Br., we show that the injection of inward electric current is accompanied by proton influx sufficient for the creation of artificial alkaline band. The combined application of PAM chlorophyll microfluorometry, local light stimuli, and measurements of pericellular pH showed that the microfluidic signal transduction is notably suppressed under artificially produced alkaline band where H+ influx acidifies the cytoplasm. It is hypothesized that changes in cytoplasmic pH regulate the rates of electron flows directed to NADP reduction and O2 reduction, which may disturb long-distance redox control system involving production and consumption of reducing equivalents.