Timm S, Nunes-Nesi A, Pärnik T, Morgenthal K, Wienkoop S, Keerberg O, Weckwerth W, Kleczkowski LA, Fernie AR, Bauwe H
A cytosolic pathway for the conversion of hydroxypyruvate to glycerate during photorespiration in Arabidopsis
The Plant Cell: 2008 20:2848-2859

Abstract
Deletion of any of the core enzymes of the photorespiratorycycle, one of the major pathways of plant primary metabolism,results in severe air-sensitivity of the respective mutants.The peroxisomal enzyme hydroxypyruvate reductase (HPR1) representsthe only exception to this rule. This indicates the presenceof extraperoxisomal reactions of photorespiratory hydroxypyruvatemetabolism. We have identified a second hydroxypyruvate reductase,HPR2, and present genetic and biochemical evidence that theenzyme provides a cytosolic bypass to the photorespiratory corecycle in Arabidopsis thaliana. Deletion of HPR2 results in elevatedlevels of hydroxypyruvate and other metabolites in leaves. Photosyntheticgas exchange is slightly altered, especially under long-dayconditions. Otherwise, the mutant closely resembles wild-typeplants. The combined deletion of both HPR1 and HPR2, however,results in distinct air-sensitivity and a dramatic reductionin photosynthetic performance. These results suggest that photorespiratorymetabolism is not confined to chloroplasts, peroxisomes, andmitochondria but also extends to the cytosol. The extent towhich cytosolic reactions contribute to the operation of thephotorespiratory cycle in varying natural environments is notyet known, but it might be dynamically regulated by the availabilityof NADH in the context of peroxisomal redox homeostasis.

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