The Populus alba cationic cell-wall-bound peroxidase (CWPO-C) regulates plant growth, lignin content and composition in poplar

Cationic cell-wall-bound peroxidase (CWPO-C) from Populus alba is the only Class III peroxidase that has been shown to be able to oxidize high molecular weight lignin polymers from sinapyl alcohol and previously, has been believed to be a lignin polymerization-specific peroxidase. However, using an Arabidopsis heterologous expression system, we showed recently that CWPO-C contributes to differentiation or early growth and is involved in auxin catabolism. In this study, to clarify the function of CWPO-C in poplar, we analyzed CWPO-C gene expression and phenotypic changes with CWPO-C overexpression and suppression. Real-time PCR and monitoring promoter activity of CWPO-C using β-glucuronidase (GUS) assay revealed that CWPO-C was strongly expressed in immature tissues, such as the upper stem, axillary buds, and young leaves, in addition to expression in developing xylem. In transgenic poplars in which the expression of CWPO-C was upregulated or suppressed, changes in stem growth, gravitropism bending time, lignin content and syringyl/guaiacyl (S/G) composition were observed. Overexpressing CWPO-C enhanced stem growth and gravitropic response (shorter bending time). With suppressed CWPO-C expression, the lignin content was reduced approximately 45% and the S/G ratio decreased by half. These results strongly suggest that CWPO-C plays a role in differentiation and early growth, as well as in lignin polymerization.