Chloroplasts Require Glutathione Reductase to Balance Reactive Oxygen Species and Maintain Efficient Photosynthesis

Summary Thiol-based redox-regulation is vital to coordinate chloroplast functions depending on illumination. Yet, how the redox-cascades of the thioredoxin and glutathione redox machineries integrate metabolic regulation and reactive oxygen species (ROS) detoxification remains largely unresolved. We investigate if maintaining a highly reducing stromal glutathione redox potential ( E GSH ) via glutathione reductase (GR) is necessary for functional photosynthesis and plant growth. Since absence of the plastid/mitochondrial GR is embryo-lethal in Arabidopsis thaliana , we used the model moss Physcomitrella patens to create knock-out lines. We dissect the role of GR in chloroplasts by in vivo monitoring stromal E GSH dynamics, and reveal changes in protein abundances by metabolic labelling. Whereas stromal E GSH is highly reducing in wildtype and clearly responsive to light, the absence of GR leads to a partial oxidation, which is not rescued by light. Photosynthetic performance and plant growth are decreased with increasing light intensities, while ascorbate and zeaxanthin levels are elevated. An adjustment of chloroplast proteostasis is pinpointed by the induction of plastid protein repair and degradation machineries. Our results indicate that the plastid thioredoxin and glutathione redox systems operate largely independently. They reveal a critical role of GR in maintaining efficient photosynthesis.