The syringaldazine-oxidizing peroxidase PXP 3-4 from poplar xylem: cDNA isolation, characterization and expression

The cell wall polymer lignin is believed to be condensed by specific cell w all-localized oxidoreductases. In many plants species, including poplar, th e peroxidase-directed oxidation of the lignin analogue syringaldazine (SYR) has been localized to cells that undergo secondary wall formation, a proce ss that includes lignification. As a first step to analyse the correspondin g peroxidases, we have isolated previously two anionic isoenzymes (PXP 3-4 and PXP 5) from poplar xylem (Populus trichocarpa), which use SYR as a subs trate. Here, we demonstrate that these enzymes are responsible for the visu alized SYR oxidation in the developing xylem. The cDNA that corresponds to PXP 3-4 was isolated and the deduced protein was found closely related to t he other SYR-oxidizing peroxidase PXP 5 (ca. 98% of identity). PXP 3-4 was expressed in a baculovirus expression system yielding high levels of active peroxidase (3 mg/l medium). The heterologously produced protein showed cha racteristics similar to those of the corresponding protein from poplar xyle m (enzymatic properties, isoelectric point, and migration in a native gel). PXP 3-4 was expressed in the stem and in the root xylem. The data demonstr ate that PXP 3-4 (and/or PXP 5) are present in differentiating xylem, suppo rting a function in secondary cell wall formation.