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BIOENERGETICS AND PHOTOSYNTHESIS

Peroxisomal Malate Dehydrogenase Is Not Essential for Photorespiration in Arabidopsis But Its Absence Causes an Increase in the Stoichiometry of Photorespiratory CO₂ Release^1,[W],[OA]

School of Biological Sciences, Washington State University, Pullman, Washington 99164–4236 (A.B.C.); Australian Research Council Centre of Excellence in Plant Energy Biology, Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia (A.B.C., M.R.B.); and Australian Research Council Centre of Excellence in Plant Energy Biology and Centre of Excellence for Plant Metabolomics, University of Western Australia, Crawley, Western Australia 6009, Australia (I.P., W.Z., S.M.S.)

Peroxisomes are important for recycling carbon and nitrogen that would otherwise be lost during photorespiration. The reduction of hydroxypyruvate to glycerate catalyzed by hydroxypyruvate reductase (HPR) in the peroxisomes is thought to be facilitated by the production of NADH by peroxisomal malate dehydrogenase (PMDH). PMDH, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana), reduces NAD⁺ to NADH via the oxidation of malate supplied from the cytoplasm to oxaloacetate. A double mutant lacking the expression of both PMDH genes was viable in air and had rates of photosynthesis only slightly lower than in the wild type. This is in contrast to other photorespiratory mutants, which have severely reduced rates of photosynthesis and require high CO₂ to grow. The pmdh mutant had a higher O₂-dependent CO₂ compensation point than the wild type, implying that either Rubisco specificity had changed or that the rate of CO₂ released per Rubisco oxygenation was increased in the pmdh plants. Rates of gross O₂ evolution and uptake were similar in the pmdh and wild-type plants, indicating that chloroplast linear electron transport and photorespiratory O₂ uptake were similar between genotypes. The CO₂ postillumination burst and the rate of CO₂ released during photorespiration were both greater in the pmdh mutant compared with the wild type, suggesting that the ratio of photorespiratory CO₂ release to Rubisco oxygenation was altered in the pmdh mutant. Without PMDH in the peroxisome, the CO₂ released per Rubisco oxygenation reaction can be increased by over 50%. In summary, PMDH is essential for maintaining optimal rates of photorespiration in air; however, in its absence, significant rates of photorespiration are still possible, indicating that there are additional mechanisms for supplying reductant to the peroxisomal HPR reaction or that the HPR reaction is altogether circumvented.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Steven M. Smith (ssmith{at}cyllene.uwa.edu.au).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.122622

^* Corresponding author; e-mail acousins{at}wsu.edu.

Received May 7, 2008; accepted July 26, 2008; published August 6, 2008.