The mitochondrial thioredoxin system regulates the TCA cycle-derived metabolic fluxes toward the GS/GOGAT cycle in illuminated leaves

Previous studies suggest that the synthesis of glutamate/glutamine is regulated by the mitochondrial thioredoxin (TRX) system. However, the mechanisms behind it remain unclear. Here, we demonstrated that the level of citrate and glutamate was higher in illuminated leaves from Arabidopsis mutants lacking the mitochondrial TRX o1 (trxo1) or both NADPH-dependent TRX reductases A/B (ntrab), that are found in the nucleus, cytosol, and mitochondria, when compared with the wild type (WT). Increased 13C-labelling in glutamate derived from [13C]pyruvate was observed in illuminated trxo1 and ntrab leaves, but not in the WT or in the microsomal trxh2 mutant. The lack of TRX o1 decreased the content and activity of glutamine synthetase (GS), which leads to a lower level of glutamine, and exacerbated the increases in GS activity triggered by high light, when compared with the WT. The level of glutamine was positively correlated with the percentage of the oxidized GS band. However, the GS redox status was unaltered in all mutants. Our results indicate that mitochondrial TRX mutants have higher metabolic fluxes from the tricarboxylic acid (TCA) cycle to the GS/glutamate synthase (GOGAT) cycle in vivo, probably associated with an increased substrate availability and by direct and indirect TRX-mediated mechanisms that regulate enzymes of both the TCA and GS/GOGAT cycle.