Photorespiratory and Respiratory Carbon Isotope Fractionation in Leaves

Abstract

The photorespiratory pathway in plants is initiated with the uptake of O2 instead of CO2 by the photosynthetic enzyme Rubisco. This generates a compound in the leaf that must then be recycled through enzymatic reactions distributed over the leaf chloroplasts, peroxisomes, and mitochondria, resulting in a net loss of CO2. The pathway was thought to be well understood, but recent studies using mutants lacking expression of the genes coding for peroxisomal malate dehydrogenase (PMDH) have suggested that there are additional mechanisms that alter the stoichiometry of photorespiratory CO2 release to O2 uptake. Any changes in this stoichiometry will likely cause a change in the isotopic composition of the CO2 being released since it would involve additional enzymes or carbon sources. The isotopic composition of the CO2 being released was measured under both photorespiratory and non-photorespiratory conditions, as well as under different isotopic conditions. Measurements were taken using a tunable diode laser spectroscopy system, which allows high frequency in vivo gas exchange measurements of different CO2 isotopologues. Comparing the PIB isotopic signature of wild type Arabidopsis to that of PMDH mutants, may improve our understanding of alternate metabolic pathways in plants. The decarboxylation fractionation seen is significantly higher in PMDH mutant plants relative to wild-type, which can be partially explained by higher rates of photorespiratory decarboxylation seen in previous studies. The photorespiratory fractionation also varied significantly between wild-type and mutant plants.