A Mutant of <italic>Arabidopsis thaliana</italic> That Defines a New Locus for Glycine Decarboxylation

A novel photorespiratory mutant of Arabidopsis thaliana, designated gld2, was isolated based on a growth requirement for abnormally high levels of atmospheric CO2. Photosynthetic CO2 fixation was inhibited in the mutant following illumination in air but not in atmosphere containing 2% O2. Photosynthetic assimilation of 14CO2 in an atmosphere containing 50% O2 resulted in accumulation of 48% of the soluble label in glycine in the mutant compared to 9% in the wild type. The rate of glycine decarboxylation by isolated mitochondria from the mutant was reduced to 6% of the wild type rate. In genetic crosses, the mutant complemented two previously described photorespiratory mutants of A. thaliana that accumulate glycine during photosynthesis in air due to defects in glycine decarboxylase (glyD, now designated gld1) and serine transhydroxymethylase (stm). Because glycine decarboxylase is a complex of four enzymes, these results are consistent with a mutation in a glycine decarboxylase subunit other than that affected in the gld1 mutant. The two gld loci were mapped to chromosomes 2 and 5, respectively.