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

Thiol-based redox-regulation is vital to coordinate chloroplast functions depending on illumination and is well investigated for thioredoxin-dependent processes. In parallel, glutathione reductase (GR) maintains a highly reduced glutathione pool, enabling glutathione-mediated redox buffering. Yet, how the redox-cascades of the thioredoxin and glutathione redox machineries integrate metabolic regulation and detoxification of reactive oxygen species (ROS) remains largely unresolved, as null mutants of plastid/mitochondrial GR are embryo-lethal in Arabidopsis thaliana. To investigate if maintaining a highly reducing stromal glutathione redox potential (E_GSH ) via glutathione reductase (GR) is necessary for functional photosynthesis and plant growth, we created knock-out lines of the homologous enzyme in the model moss Physcomitrella patens. In these viable mutant lines, we find decreasing photosynthetic performance and plant growth with increasing light intensities, while ascorbate and zeaxanthin/antheraxanthin levels are elevated. By in vivo monitoring stromal E_GSH dynamics, we show that stromal E_GSH is highly reducing in wildtype and clearly responsive to light while absence of GR leads to a partial glutathione oxidation, which is not rescued by light. By metabolic labelling, we reveal changing protein abundances in the GR knock-out plants, pinpointing the adjustment of chloroplast proteostasis and induction of plastid protein repair and degradation machineries. Our results indicate that the plastid thioredoxin system is not a functional backup for the plastid glutathione redox systems, while GR plays a critical role in maintaining efficient photosynthesis.