(CS-25-3) The link between Photorespiration and one carbon metabolism - Lightning Talk

Body of Abstract: Photorespiratory intermediates such as glycine, serine and formate are potential sources of one-carbon (C1) metabolites. There is a well-established link between photorespiratory intermediate and C1 metabolism in mitochondria through serine hydroxymethyl transferase (SHMT). Additionally, cytosolic SHMT could feed serine produced by photorespiration into C1 metabolism. Under this scenario, rather than being converted to hydroxypyruvate, serine could exit the photorespiratory pathway and be converted into C1 intermediates. Another alternative pathway allowing the flux of photorespiratory carbon towards C1 metabolism is the biosynthesis of formate through the non-enzymatic decarboxylation (NED) of glyoxylate. It has been often observed that high CO₂ release under high photorespiratory conditions is due to NED of hydroxypyruvate or glyoxylate by H₂O₂. This NED would produce formate which could then be oxidized to CO₂ in the mitochondria by formate dehydrogenase or recovered by entering cytosolic C1 metabolism. Catalase in peroxisomes can decrease NED’s by disproportionating H₂O₂ resulting from glycolate oxidation into O₂ and water. We hypothesize that the absence of catalase increases carbon flux from photorespiratory pathway towards cytosolic C1 metabolism in the form of formate. We test this hypothesis by ¹³CO₂ labeling approach on Arabidopsis thaliana mutants lacking peroxisomal catalase (cat) and wild type (WT). Our results showed that glycolate and glyoxylate seem to be labeled relatively faster in the absence of catalase. Nevertheless, glycine and serine were highly labeled in WT compared to cat knockout plants, indicating reduced flux from glycolate to glycine in cat. Surprisingly, C1 metabolites such as 5-methyl-THF, and its downstream metabolite (methionine and S-adenosine methionine) were less labeled in cat plants compared to WT, suggesting that most the of the formate produced during photorespiration is not entering to C1 metabolism. Formate dehydrogenase activity is currently being analyzed to test if large portion of formate is oxidized to CO₂ in the cat plants.