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Impact of membrane excitability on fluidic transport of photometabolites and their cell-to-cell passage via plasmodesmata was examined by pulse-modulated chlorophyll (Chl) microfluorometry in Chara australis internodes exposed to dim background light. The cells were subjected to a series of local light (LL) pulses with a 3-min period and a 30-s pulse width, which induced Chl fluorescence transients propagating in the direction of cytoplasmic streaming along the photostimulated and the neighboring internodes. By comparing Chl fluorescence changes induced in the LL-irradiated and the adjoining internodes, the permeability of the nodal complex for the photometabolites was assessed in the resting state and after the action potential (AP) generation. The electrically induced AP had no influence on Chl fluorescence in noncalcified cell regions but disturbed temporarily the metabolite transport along the internode and caused a disproportionally strong inhibition of intercellular metabolite transmission. In chloroplasts located close to calcified zones, Chl fluorescence increased transiently after cell excitation, which indicated the deceleration of photosynthetic electron flow on the acceptor side of photosystem I. Functional distinctions of chloroplasts located in noncalcified and calcified cell areas were also manifested in different modes of LL-induced changes of Chl fluorescence, which were accompanied by dissimilar changes in efficiency of PSII-driven electron flow. We conclude that chloroplasts located near the encrusted areas and in the incrustation-free cell regions are functionally distinct even in the absence of large-scale variations of cell surface pH. The inhibition of transnodal transport after AP generation is probably due to Ca 2+ -regulated changes in plasmodesmal aperture., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.)

Characean internodal cells develop alternating patterns of acid and alkaline zones along their surface in order to facilitate uptake of carbon required for photosynthesis. In this study, we used a pH-indicating membrane dye, 4-heptadecylumbiliferone, to study the kinetics of alkaline band formation and decomposition. The differences in growth/decay kinetics suggested that growth occurred as an active, autocatalytic process, whereas decomposition was due to diffusion. We further investigated mutual interactions between internodal cells and found that their alignment parallel to each other induced matching of the pH banding patterns, which was mirrored by chloroplast activity. In non-aligned cells, the lowered photosynthetic activity was noted upon a rise of the external pH, suggesting that the matching of pH bands was due to a local elevation of membrane conductance by the high pH of the alkaline zones of neighboured cells. Finally, we show that the altered pH banding pattern caused the reorganization of the cortical cytoplasm. Complex plasma membrane elaborations (charasomes) were degraded via endocytosis, and mitochondria were moved away from the cortex when a previously acid region became alkaline and vice versa. Our data show that characean internodal cells react flexibly to environmental cues, including those originating from neighboured cells.

Cytoplasmic streaming occurs in most plant cells and is vitally important for large cells as a means of long-distance intracellular transport of metabolites and messengers. In internodal cells of characean algae, cyclosis participates in formation of light-dependent patterns of surface pH and photosynthetic activity, but lateral transport of regulatory metabolites has not been visualized yet. Hydrogen peroxide, being a signaling molecule and a stress factor, is known to accumulate under excessive irradiance. This study was aimed to examine whether H2O2 produced in chloroplasts under high light conditions is released into streaming fluid and transported downstream by cytoplasmic flow. To this end, internodes of Chara corallina were loaded with the fluorogenic probe dihydrodichlorofluorescein diacetate and illuminated locally by a narrow light beam through a thin optic fiber. Fluorescence of dihydrodichlorofluorescein (DCF), produced upon oxidation of the probe by H2O2, was measured within and around the illuminated cell region. In cells exhibiting active streaming, H2O2 first accumulated in the illuminated region and then entered into the streaming cytoplasm, giving rise to the expansion of DCF fluorescence downstream of the illuminated area. Inhibition of cyclosis by cytochalasin B prevented the spreading of DCF fluorescence along the internode. The results suggest that H2O2 released from chloroplasts under high light is transported along the cell with the cytoplasmic flow. It is proposed that the shift of cytoplasmic redox poise and light-induced elevation of cytoplasmic pH facilitate the opening of H(+)/OH(-)-permeable channels in the plasma membrane.