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,Murray R Badger,Steven M Smith

doi:10.1104/pp.108.122622

Abstract

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 CO(2) to grow. The pmdh mutant had a higher O(2)-dependent CO(2) compensation point than the wild type, implying that either Rubisco specificity had changed or that the rate of CO(2) released per Rubisco oxygenation was increased in the pmdh plants. Rates of gross O(2) evolution and uptake were similar in the pmdh and wild-type plants, indicating that chloroplast linear electron transport and photorespiratory O(2) uptake were similar between genotypes. The CO(2) postillumination burst and the rate of CO(2) released during photorespiration were both greater in the pmdh mutant compared with the wild type, suggesting that the ratio of photorespiratory CO(2) release to Rubisco oxygenation was altered in the pmdh mutant. Without PMDH in the peroxisome, the CO(2) 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.

Highlights

Peroxisomes are important for recycling carbon and nitrogen that would otherwise be lost during photorespiration
Rates of net CO2 assimilation measured at approximately air CO2 concentrations (373 mbar) in both the wild-type and pmdh1 plants were significantly higher than in the pmdh2 and double pmdh1pmdh2 lines (Table I)
When rates of net CO2 assimilation were measured at elevated CO2 (1,515 mbar), there was no significant difference between the lines (Table I)

Summary

Introduction

Peroxisomes are important for recycling carbon and nitrogen that would otherwise be lost during photorespiration. The oxygenation of ribulose-1,5-bisphosphate by Rubisco initiates the photorespiratory metabolic pathway (Badger, 1985) This reaction competes with photosynthetic carbon assimilation, producing CO2 and NH3 while consuming ATP and reducing equivalents, minimizing the efficiency of net CO2 assimilation (Tolbert, 1997; Wingler et al, 2000; Reumann and Weber, 2006). It was proposed that an alternative pathway may metabolize hydroxypyruvate during photorespiration in these mutants, such as a cytosolic HPR or being linked to simultaneous b-oxidation in the peroxisome (Murray et al, 1989; Kleczkowski et al, 1990; Pracharoenwattana et al, 2007) This alternative pathway may provide an overflow metabolic route of hydroxypyruvate reduction if the commonly recognized reaction becomes limiting when rates of photorespiration are high (Murray et al, 1989)

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