Expressions and functions of poplar peroxidase related to lignin biosynthesis and stress responses.

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In this study, expressions and functions of peroxidases in poplar were exhaustively investigated. Firstly, poplar peroxidase genes were screened from Populus Genome Database, and 83 genes were selected as complete peroxidase sequences. Based on the selected sequences, 31 of individual peroxidase cDNA were isolated, and their transcriptional profiles were investigated by Real-time PCR technique. Eight peroxidases were extensively expressed in many organs and other 7 peroxidase genes were strongly induced in the leaf by hydrogen peroxide treatment.Secondary, proteomic analysis of peroxidase proteins in poplar organs was performed. The highly expressed peroxidase proteins were acidic isoforms and belong to a small claster. Two peroxidase isoforms were specifically expressed in the developing xylem. Other two peroxidases were highly induced by stress treatments, suggesting that these two peroxidases may be involved in the suberin synthesis or in the elimination of reactive oxygen species (ROS).An isolated peroxidase isoenzyme, CWPO-C, showed high oxidation ability toward sinapyl alcohol and synthetic lignin polymer. These kinetic data strongly suggest that CWPO-C is a lignification specific peroxidase. Therefore, localization of CWPO-C in the developing xylem of poplar stem was investigated by an immunohistochemical technique. The intensity of the CWPO-C labeling increased gradually from the cell wall thickening stage to mature stage of fiber cells, which is very consistent with the increase of lignin content in the developing xylem. These results strongly support that CWPO-C is responsible for the lignification of the secondary xylem. Interestingly, immuno-labeling of CWPO-C was also observed inside of the ray parenchyma cells instead no signals were detected within the developing fiber cells. This suggests that CWPO-C is biosynthesized in the parenchyma cells and provided to the middle lamellae, the cell corners, and the cell walls to achieve lignin polymerization.