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