Different photorespiratory mechanisms in conifer leaves where peroxisomes have intrinsically low catalase activity.

Photorespiration is an essential metabolism associated with photosynthesis; however, little is known about the photorespiratory pathway of conifer gymnosperms. Metabolite analyses of the leaves of 27 tree species showed that the mean glycerate content in conifer leaves was lower than that in angiosperm leaves. We performed experiments of 13 C-labeled serine 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 tree (Populus nigra), because glycerate is produced from serine via hydroxypyruvate in peroxisomes. In P. nigra, hydroxypyruvate, glycerate and glycine were labeled with 13 C while, in C. japonica, glycolate and a non-canonical photorespiratory metabolite, formate, were also labeled, suggesting that H 2 O 2 -mediated non-enzymatic decarboxylation (NED) reaction occurs in C. japonica. We analyzed changes in the metabolite contents of the leaves kept under dark and exposed to illuminated photorespiration-promoted conditions: a positive relationship between the formate and serine contents in C. japonica implied the active C1-metabolism pathway synthesizing serine from formate. Leaf gas exchange analyses revealed that CO 2 produced through NED was re-captured by the chloroplasts. Database analysis of the peroxisomal targeting signal motifs of an H 2 O 2 -scavenging enzyme, catalase, derived from various species including nine coniferous species, as well as analyses of peroxisomal fractions isolated from C. japonica and P. nigra leaves indicated that conifer peroxisomes had less catalase activity. These results suggest that NED and the subsequent C1-metabolism are involved in the photorespiratory pathway of conifer leaves where peroxisomes have intrinsically low catalase activity.