Conifer leaves have a peroxisomal oxidative decarboxylation path in the photorespiratory pathway
The photorespiratory pathway consists of enzymes operating in chloroplasts, mitochondria, and peroxisomes. Conifer leaves lack one of them, chloroplastic Gln synthetase, which questioned the current belief that the photorespiratory mechanism is identical between angiosperm C3 species and conifers. A photorespiratory-metabolite analysis of the leaves of 13 conifer and 14 angiosperm tree species revealed significant differences in the mean metabolite concentrations between the two taxonomic groups: the glycerate content on chlorophyll basis in conifer leaves was <1/10 that detected in angiosperm leaves, whereas the glycolate content was 1.6 times higher in conifer leaves. Glycerate is produced from Ser through an intermediate, hydroxypyruvate. To investigate the lower glycerate levels observed in conifer leaves, we performed experiments of 13C-labeled Ser feeding to the detached shoots of a conifer (Cryptomeria japonica) via the transpiration stream, and compared the labeling patterns of photorespiratory metabolites with those of an angiosperm (Populus nigra). Glycerate was most labeled in P. nigra, whereas glycolate was more labeled than glycerate in C. japonica. The photorespiration pathway involves H2O2-scavenging and H2O2-generating enzymes, catalase (CAT) and glycolate oxidase (GLO), respectively, which are the peroxisomal targeting enzymes in angiosperms. In contrast, database analyses of the peroxisomal targeting signal motifs and analyses of the peroxisomal fractions isolated from C. japonica leaves indicated that the conifer peroxisomes were not a major localization of CAT. These results suggest that the conifer photorespiration pathway has a bypass from Ser to glycolate via the decarboxylation of hydroxypyruvate, because of an imbalance between CAT and GLO activities in peroxisomes. One sentence summaryConifer peroxisome is not a major localization of catalase and yields a unique oxidative decarboxylation path in the photorespiratory pathway.