Metabolic flux analysis in leaf metabolism quantifies the link between photorespiration and one carbon metabolism

Photorespiration is the second largest carbon flux in most leaves and is integrated into metabolism broadly including one-carbon (C₁) metabolism. Photorespiratory intermediates such as serine and others may serve as sources of C₁ units, but it is unclear to what degree this happens in vivo, whether altered photorespiration changes flux to C₁ metabolism, and if so through which intermediates. To clarify these questions, we quantified carbon flux from photorespiration to C₁ metabolism using ¹³CO₂ labelling and isotopically non-stationary metabolic flux analysis in Arabidopsis thaliana under different O₂ concentrations which modulate photorespiration. The results revealed that ~5.8% of assimilated carbon passes to C₁ metabolism under ambient photorespiratory conditions, but this flux greatly decreases under limited photorespiration. Furthermore, the primary carbon flux from photorespiration to C₁ metabolism is through serine. Our results provide fundamental insight into how photorespiration is integrated into C₁ metabolism, with possible implications for C₁ metabolic response to climate change.