The Photorespiratory Metabolite 2-Phosphoglycolate Regulates Photosynthesis and Starch Accumulation in Arabidopsis

The Calvin-Benson cycle and its photorespiratory repair shunt are in charge of nearly all biological CO₂ fixation on Earth. They interact functionally and via shared carbon flow on several levels including common metabolites, transcriptional regulation, and response to environmental changes. 2-Phosphoglycolate (2PG) is one of the shared metabolites and produced in large amounts by oxidative damage of the CO₂ acceptor molecule ribulose 1,5-bisphosphate. It was anticipated early on, although never proven, that 2PG could also be a regulatory metabolite that modulates central carbon metabolism by inhibition of triose-phosphate isomerase. Here, we examined this hypothesis using transgenic Arabidopsis thaliana lines with varying activities of the 2PG-degrading enzyme, 2PG phosphatase, and analyzing the impact of this intervention on operation of the Calvin-Benson cycle and other central pathways, leaf carbohydrate metabolism, photosynthetic gas exchange, and growth. Our results demonstrate that 2PG feeds back on the Calvin-Benson cycle. It also alters the allocation of photosynthates between ribulose 1,5-bisphosphate regeneration and starch synthesis. 2PG mechanistically achieves this by inhibiting the Calvin-Benson cycle enzymes triose-phosphate isomerase and sedoheptulose 1,7-bisphosphate phosphatase. We suggest this may represent one of the control loops that sense the ratio of photorespiratory to photosynthetic carbon flux and in turn adjusts stomatal conductance, photosynthetic CO₂ and photorespiratory O₂ fixation, and starch synthesis in response to changes in the environment.