Changes in Chloroplast Fluorescence Related to Excitability and Metabolite Transport by Cytoplasmic Streaming in Chara Cells

Abstract

Propagation of action potentials and the transfer of metabolites with the streaming cytoplasm represent two main pathways of long-distance signaling in giant cells of characean algae. This work shows that the excitation of plasma membranes and the arrest of cytoplasmic streaming in Chara cells at low light (in the absence of large pH changes on the cell surface) is accompanied by a transient increase or a series of oscillations of chlorophyll F ' fluorescence in chloroplasts located near zones of external calcification but has no effect on fluorescence of chloroplasts in the predominant non-calcified cell areas. The different sensitivity to the action potential generation in chloroplasts located near and far from the incrustation zones is probably due to structural and biochemical differences of cellular domains and to the influence of cytoplasmic streaming on chloroplast environment in the area of fluorescence measurements. The kinetic curves of F ' changes upon cell excitation and the number of individual peaks in the F ' response depended on the illuminated area of plant sample. The number of peaks diminished when the overall illumination was replaced with illumination of a narrow cell region using flexible light guides with diameters of 2.0 and 0.4 mm. The results indicate that the initial stages of F ' changes induced by the action potential generation are caused by internal processes in chloroplasts, while the subsequent changes in F ' are due to the influx of metabolites from the recovering cytoplasmic flow into the chloroplasts of the studied zone. The amplitude of excitation-induced F ' changes decreased transiently after the transition from general to local illumination, which is explained by the depletion of reducing equivalents in the cytoplasm during postillumination CO₂ fixation under cessation of NADPH photoproduction outside the narrow illuminated zone.