Abstract

The giant internodal cells of the alga Chara serve as an established model for investigating fundamental processes in plant cell biology, including membrane excitability and intracellular transport. This study investigates the complex interplay between plasma membrane excitability, cytoplasmic streaming, and photosynthetic activity, as monitored by changes in chlorophyll (Chl) fluorescence in Chara chloroplasts. The research objective was to elucidate the mechanism by which electrical excitation of the plasma membrane, a transient event, propagates its effect to the internal chloroplasts and influences their function. Using microfluorometry, we observed that plasma membrane excitation induces multiphasic transients in Chl fluorescence, indicating a rapid, non-photochemical effect on the photosynthetic electron transport chain. Crucially, this excitation is accompanied by a transient arrest of cytoplasmic streaming (cyclosis). We propose that the cessation of streaming leads to a localized, transient depletion of photosynthetic metabolites, such as triose phosphates, in the immediate vicinity of the chloroplasts. This microfluidic effect, rather than a direct electrical signal, acts as a key signaling pathway linking the plasma membrane's excitable state to the chloroplast's photosynthetic response. These findings highlight the critical role of cytoplasmic streaming in maintaining the metabolic homeostasis of large plant cells and provide a novel mechanism for rapid, long-distance signaling between the cell periphery and internal organelles.