ABSTRACT
Thioredoxins (TRXs) are important proteins involved in redox regulation of metabolism. In plants, it has been shown that the mitochondrial metabolism is regulated by the mitochondrial TRX system. However, the functional significance of TRX h2, which is found at both cytosol and mitochondria, remains unclear. Arabidopsis plants lacking TRX h2 showed delayed seed germination and reduced respiration alongside impaired stomatal and mesophyll conductance, without impacting photosynthesis under ambient O2 conditions. However, an increase in the stoichiometry of photorespiratory CO2 release was found during O2 -dependent gas exchange measurements in trxh2 mutants. Metabolite profiling of trxh2 leaves revealed alterations in key metabolites of photorespiration and in several metabolites involved in respiration and amino acid metabolism. Decreased abundance of serine hydroxymethyltransferase and glycine decarboxylase (GDC) H and L subunits as well as reduced NADH/NAD+ ratios were also observed in trxh2 mutants. We further demonstrated that the redox status of GDC-L is altered in trxh2 mutants in vivo and that recombinant TRX h2 can deactivate GDC-L in vitro, indicating that this protein is redox regulated by the TRX system. Collectively, our results demonstrate that TRX h2 plays an important role in the redox regulation of mitochondrial photorespiratory metabolism.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/metabolism , Mitochondria/metabolism , Thioredoxin h/metabolism , Arabidopsis/genetics , Arabidopsis Proteins/genetics , Carbon Dioxide/metabolism , Cell Respiration/physiology , Chlorophyll A , Gene Expression Regulation, Plant , Glycine Dehydrogenase (Decarboxylating)/metabolism , Glycine Hydroxymethyltransferase , Oxidation-Reduction , Photosynthesis/physiology , Plant Leaves/metabolism , Thioredoxin h/genetics , TranscriptomeABSTRACT
Photorespiration sustains photosynthesis in the presence of oxygen due to rapid metabolization of 2-phosphoglycolate, the major side-product of the oxygenase activity of Rubisco that also directly impedes carbon assimilation and allocation. Despite the fact that both the biochemical reactions and the underlying genetics are well characterized, information concerning the regulatory mechanisms that adjust photorespiratory flux is rare. Here, we studied the impact of mitochondrial-localized thioredoxin o1 (TRXo1) on photorespiratory metabolism. The characterization of an Arabidopsis (Arabidopsis thaliana) transfer DNA insertional line (trxo1-1) revealed an increase in the stoichiometry of photorespiratory CO2 release and impaired Gly-to-Ser turnover after a shift from high-to-low CO2 without changes in Gly decarboxylase (GDC) gene or protein expression. These effects were distinctly pronounced in a double mutant, where the TRXo1 mutation was combined with strongly reduced GDC T-protein expression. The double mutant (TxGT) showed reduced growth in air but not in high CO2, decreased photosynthesis, and up to 54-fold more Gly alongside several redox-stress-related metabolites. Given that GDC proteins are potential targets for redox-regulation, we also examined the in vitro properties of recombinant GDC l-proteins (lipoamide dehydrogenase) from plants and the cyanobacterium Synechocystis species strain PCC6803 and observed a redox-dependent inhibition by either artificial reducing agents or TRXo1 itself. Collectively, our results demonstrate that TRXo1 potentially adjusts photorespiration via redox-regulation of GDC in response to environmental changes.
