Instant rerouting of photosynthetic electron transport to O2 reduction after the plasma membrane excitation of Chara in the presence of methyl viologen

—Action potential (AP) of excitable plant cells is an important signaling event that can differentially alter physicochemical and physiological processes in various parts of the same cell. In giant cells of characean algae, the AP propagation has minor effect on photosynthetic electron transport in areas with high activity of plasmalemmal H⁺-pump but inhibits linear electron flow in regions featuring high passive H⁺/OH⁻ conductance of the plasma membrane (PM). Uneven spatial distributions of local periplasmic and cytoplasmic pH facilitate the operation of distinct (CO₂-dependent and O₂-mediated) pathways of photoinduced electron flow, which presumably accounts for differential influence of AP on photosynthesis. The excitation of Chara australis cell in the presence of methyl viologen (MV), a redox mediator with the prooxidant action, provides a convenient model system to clarify the influence of voltage-dependent ion fluxes across PM on photosynthetic activity of chloroplasts. This study shows that permeation of MV to their target sites in chloroplasts is restricted by PM in resting cells, but MV easily passes through ionic channels opened during the PM depolarization. This gated permeation of MV gives rise to strong non-photochemical quenching, decrease in the effective quantum yield of linear electron flow, apparent O₂ uptake, and, finally, the enhanced ROS production, as detected by the fluorescent probe dichlorofluorescein. Taken together, the results indicate that the AP generation in the presence of MV acts as trigger for instant redirection of photosynthetic linear electron flow from CO₂-dependent route to the path of O₂ reduction with the eventual formation of H₂O₂ as a dominant and most stable ROS form.