Transport of photorespiratory intermediates between cellular compartments

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Photorespiratory metabolism is a highly compartmentalized pathway, comprising chloroplasts, peroxisomes, mitochondria and cytosol. At least 14 different transport steps are necessary to allow for effective shuttling of photorespiratory intermediates in between the different compartments. In contrast to the well-characterized photorespiratory enzymes the situation for the metabolite transporters is rather poor. The aim of this project was and is to identify those transporters and thus close a major gap in knowledge about photorespiration. We could establish a crucial function of the metabolite transporters BOU1 and MEP1 in photorespiration. Corresponding Arabidopsis mutants showed a typical photorespiratory phenotype. We suggest that the mitochondrial carrier protein BOU1 transports a substrate that is essential for glycine decarboxylase complex activity. The gene mep1 encodes for a chloroplast metabolite transporter required for the transport of glycolate and glycerate across the plastid envelope membrane. Since transporter activities could not be demonstrated by in vitro assays, we will follow an in vivo approach in the next funding period. Additionally, we could recently show that the Arabidopsis dicarboxylate transporter DiT1 is required for photorespiration. To identify further novel photorespiratory transporters we are continuing with screening Arabidopsis mutants selected on the basis of transcript co-expression analyses. With At1g78620 and ucp1-3 four new candidate genes could be detected. Mutants in these genes are currently under investigation. The aim of the PROMICS joined “Central experiment” is to unravel new insights into photorespiratory metabolism, associated pathways and C/N signalling. We could establish the workflow for successful sample preparation and transcriptomic analysis. In the next round we will expand the sample input and concentrate on the evaluation of the full dataset. We believe that we can use the integrated transriptomic and metabolic data to run a computational model aimed at improving the efficiency of the photosynthetic pathway.