Chlorophyll fluorescence images demonstrate variable pathways in the effects of plasma membrane excitation on electron flow in chloroplasts of Chara cells

Abstract

Chlorophyll fluorescence Imaging and Microscopy PAM fluorometry were applied to study spatial dynamics of photosystem II quantum yield (ΔF/F'(m)) and non-photochemical quenching (NPQ) in resting and electrically stimulated Chara corallina cells in the absence and presence of the hydrophilic electron acceptor methyl viologen (MV) in the external medium. Electrical excitation of the plasma membrane temporarily enhanced the heterogeneity of photosynthetic patterns under physiological conditions (in the absence of MV), but irreversibly eliminated these patterns in the presence of MV. These findings suggest that the action potential (AP) of the excitable plant cell affects the spatial patterns of photosynthesis and chlorophyll fluorescence through different pathways operated in the absence and presence of MV. Based on the extent of NPQ as an indicator of MV-dependent electron flow, it is supposed that MV cannot permeate into the chloroplasts of photosynthetically active "acid cell regions" but gains an immediate access to the stroma of these chloroplasts after triggering of an AP. The AP-triggered MV-dependent non-photochemical quenching in the chloroplasts of acidic cell regions was routinely observed at 0.1 mM Ca(2+) in the medium but not at elevated (2 mM) external Ca(2+) concentration. The results are interpreted in terms of competition between two permeant divalent ion species, Ca(2+) and MV(2+), for their passage through the voltage-gated calcium channels of the plasma membrane. It is proposed that the herbicidal activity of MV in characean cells, here serving as model object, can be manipulated by triggering AP and varying Ca(2+) concentration in the environmental medium.

Photoinduction of cyclosis-mediated interactions between distant chloroplasts

“…This is in line with previous reports that MV does not permeate through the plasma membrane of Chara and remains inaccessible for chloroplasts, at least in the areas with slightly acid external pH [41]. However, MV was shown to permeate immediately into chloroplasts and switch over the electron transport routes after a single elicitation of the action potential [41,42]. Our present experiments confirm that MV becomes immediately accessible to chloroplasts after a single AP generation.…”

“…8B). Strong light-induced quenching of F m ′ under low-irradiance illumination is typical of MV-treated cells [41,42]. In the presence of MV the rapid drops in F′ and F m ′ were coincident, indicating that the drop in F′ was largely determined by non-photochemical quenching.…”

Regulation of electron transport in microalgae

“…In the alkaline bands, the quantum yield of PSII-driven electron flow is lower than in acid regions, reflecting the lower CO 2 concentration. The periodic profile of extracellular pH follows the profile of the photoprotective response, while PSII-driven electron flow presents an opposite trend [64]. After triggering the action potential, the pH banding temporarily disappears while the differences in the PSII-driven electron flow and photoprotection remain.…”