Effects of H2O2 Micropipette Application on Photosynthetic Electron Transport in Chara Chloroplasts With Unimpaired and Short-Circuited H+ Flows.

Hydrogen peroxide is a byproduct of oxygenic photosynthesis. While the ways of H2O2 disposal in chloroplasts are mostly known, the influence of H2O2 on photosynthetic electron transport received less attention. According to various reports, H2O2 indirectly promotes electron transport on the acceptor side of photosystem I and activates cyclic electron transport (CET). Here the microinjection technique and PAM microfluorometry were applied to study the influence of H2O2 on linear and cyclic electron flows in internodal Chara cells. First, the cells loaded with a fluorogenic ROS-sensitive probe DCF were used to show that microinjection of H2O2 is followed by rapid intracellular penetration of H2O2 and its distribution with the streaming cytoplasm. Next, the regulatory influence of H2O2 on electron transport was examined in chloroplasts with unimpaired and short-circuited H+ flows. Linear electron transport (LET) was assessed with a Microscopy PAM fluorometer, whereas the inference on CET was based on measurements of the post-illumination rise of the chlorophyll fluorescence yield. The local application of H2O2 was followed by a temporary development of non-photochemical quenching (NPQ). In chloroplasts uncoupled with nigericin, the NPQ development after H2O2 microinjection was eliminated, while the polarity and amplitude of H2O2-induced F' fluorescence changes were reverted and enhanced, respectively. Post-illumination fluorescence responses (PIFR) in Chara were large and comparable in extent to changes induced by intense light. The development of PIFR after preillumination was instantly collapsed upon H2O2 injection, indicating that the reaction product arising in the presence of H2O2 redirected electron flow from the cyclic route along the competitive linear pathway.