Plant NADPH-dependent thioredoxin reductases are crucial for the metabolism of sink leaves and plant acclimation to elevated CO 2 .

Plants contain three NADPH-thioredoxin reductases (NTR) located in the cytosol/mitochondria (NTRA/B) and the plastid (NTRC) with important metabolic functions. However, mutants deficient in all NTRs remained to be investigated. Here, we generated and characterised the triple Arabidopsis ntrabc mutant alongside with ntrc single and ntrab double mutants under different environmental conditions. Both ntrc and ntrabc mutants showed reduced growth and substantial metabolic alterations, especially in sink leaves and under high CO 2 (HC), as compared to the wild type. However, ntrabc showed higher effective quantum yield of PSII under both constant and fluctuating light conditions, altered redox states of NADH/NAD + and glutathione (GSH/GSSG) and lower potential quantum yield of PSII in sink leaves in ambient but not high CO 2 concentrations, as compared to ntrc, suggesting a functional interaction between chloroplastic and extra-chloroplastic NTRs in photosynthesis regulation depending on leaf development and environmental conditions. Our results unveil a previously unknown role of the NTR system in regulating sink leaf metabolism and plant acclimation to HC, while it is not affecting full plant development, indicating that the lack of the NTR system can be compensated, at least to some extent, by other redox mechanisms.