Updated project metadata. (1) While it is known that redox-regulation in chloroplasts is vital for functional photosynthesis, the interplay of different redox-cascades in balancing reactive oxygen species (ROS) with metabolic regulation via thiol switching is still partly unresolved. We investigate the importance of the glutathione reductase isoform maintaining a highly reducing stromal glutathione redox potential (EGSH). (2) Using the model moss Physcomitrella patens, we knocked-out the plastid/mitochondrial GR isoform. We generated a sensor line expressing plastid-targeted redox-sensitive GFP to monitor stromal EGSH in vivo, and compared protein abundances between wildtype (WT) and Δgr1 knock-out mutants by a metabolic labelling approach. (3) On the organelle level, we find that the absence of PpGR1 leads to a shift of stromal EGSH to less reducing values that is not rescued in the light, whereas WT EGSH was clearly responsive to light. On the plant level, growth and photosynthetic performance were decreased with increasing light intensities while ascorbate and zeaxanthin levels were elevated. Proteomics showed an induction of proteins involved in plastid protein repair and degradation. (4) Our results indicate a limited cross-talk between plastid thioredoxin and glutathione redox systems and show that glutathione reductase is necessary for efficient photosynthesis under stress and non-stress conditions.