Nitrogen assimilation plays a role in balancing the chloroplastic glutathione redox state under high-light conditions

Nitrate reduction and subsequent ammonium assimilation require reducing equivalents directly produced by the photosynthetic electron transport chain. Therefore, it has been suggested that nitrate assimilation provides a valuable sink for excess electrons under high-light (HL) conditions, which protects the photosynthetic apparatus from excessive harmful reactive oxygen species. This work experimentally tested this hypothesis by monitoring photosynthetic efficiency and the chloroplastic glutathione redox state (chl-EGSH) of plant lines with mutated glutamine synthetase 2 (GS2) and ferredoxin-dependent glutamate synthase 1 (GOGAT1), two key enzymes of the nitrogen assimilation pathway. Unlike wild-type (WT) plants, mutant lines incorporated significantly less isotopically-labeled nitrate into amino acids, demonstrating impaired nitrogen assimilation. When nitrate assimilation was compromised, photosystem II (PSII) proved more vulnerable to photodamage, as shown by the low PSII quantum yields recorded in the mutant lines. High temporal resolution monitoring of the redox state of chloroplast-targeted reduction-oxidation sensitive green fluorescent protein 2 (chl-roGFP2), expressed in the background of the mutant lines, enabled assessment of the effect of the nitrate assimilation pathway on the chl-EGSH. Remarkably, while oxidation followed by reduction of chl-roGFP2 was detected in WT plants in response to HL, oxidation values were stable in the mutant lines, suggesting that the relaxation of chl-EGSH after HL-induced oxidation is achieved by diverting excess electrons to the nitrogen assimilation pathway. Together, these findings indicate that the nitrogen assimilation pathway serves as a sustainable energy dissipation route, ensuring efficient photosynthetic activity and fine-tuning redox metabolism under light-saturated conditions.