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Published on August 6, 2008; 10.1104/pp.108.122622

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Received May 7, 2008
Accepted July 26, 2008

Peroxisomal malate dehydrogenase is not essential for photorespiration in Arabidopsis but its absence causes an increase in the stoichiometry of photorespiratory CO2 release

Asaph B. Cousins *, Itsara Pracharoenwattana , Wenxu Zhou , Steven M. Smith , and Murray R. Badger

School of Biological Sciences, Washington State University; Pullman, WA 99164-4236; 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; Australian Research Council Centre of Excellence in Plant Energy Biology, and Centre of Excellence for Plant Metabolomics, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

* Corresponding author; email: acousins{at}wsu.edu.

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 dehydrogenases (PMDH). PMDH, which is encoded by two genes in Arabidopsis, 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 wild-type (WT). 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 wild type (WT) 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 WT plants indicating chloroplast linear electron transport and photorespiratory O2 uptake were similar between genotypes. The CO2 post illumination burst and the rate of CO2 released during photorespiration were both greater in the pmdh mutants compared to WT, suggesting the ratio of photorespiratory CO2 release to Rubisco oxygenation was altered in the pmdh mutants. 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 at 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 the HPR reaction is altogether circumvented.






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