Resolving the dynamics of photosynthetically produced ROS by high-resolution monitoring of chloroplastic EGSH in Arabidopsis

Plants are naturally subjected to fluctuations in light intensity, causing unbalanced photosynthetic electron fluxes and overproduction of reactive oxygen species (ROS). While high rates of ROS production are harmful, moderate levels play a signaling role, coordinating photosynthetic activity and downstream metabolism. Here, we explore the dynamics of light-dependent oxidant production by high-temporal-resolution monitoring of chloroplastic glutathione redox potential (chl-EGSH) using chloroplast-targeted-roGFP2-expressing Arabidopsis lines, over several days, under dynamic environmental conditions and in correlation with PSII operating efficiency. Peaks in chl-EGSH oxidation during light-darkness transitions, when light harvesting is not balanced with downstream metabolism, were observed. Increasing light intensities triggered a binary oxidation response, with a threshold around the light saturating point, pointing for two regulated oxidative states of the chl-EGSH. These patterns were not affected in npq1 plants which are impaired in non-photochemical quenching. Frequency-dependent oscillations between the two oxidation states were observed under fluctuating light in WT and npq1 plants, but not in pgr5 plants, suggesting a role for PSI photoinhibition in regulation of oxidant production. Remarkably, pgr5 plants showed an increase in chl-EGSH oxidation during the nights following light stresses, linking between day photoinhibition and night redox metabolism. This work provides a comprehensive view on the balance between photosynthesis-dependent ROS production and antioxidant activity during light acclimation.