The role of xylem class III peroxidases in lignification.

Lignification is a cell wall fortifying process which occurs in xylem tissue in a scheduled manner during tissue differentiation. In this review, enzymes and the genes responsible for lignin biosynthesis have been studied with an emphasis on lignin polymerizing class III secretable plant peroxidases. Our aim is to understand the cell and molecular biology of the polymerization of lignin especially in tracheids and vessels of woody species but much of the experimental evidence comes from herbaceous plants. Class III peroxidases pose many problems for empirical work as their encoding genes are variable, their substrate specificities are wide and the half-life of many of the isozymes is very long. However, there is some evidence for the role of specific peroxidases in lignin polymerization through antisense mutants in tobacco and poplar and from tissue and cell culture lines of Picea abies and Zinnia elegans. Peroxidase enzyme action has been shown by substrate specificity studies and, for example, RT-PCR results have pointed out that many peroxidases have tissue-specific expression patterns. Tissue-level location of gene expression of some peroxidases has been studied by in situ hybridization and their cellular localization with antibodies and using EGFP-fusion genes. From these, it can be concluded that, although many of the xylem class III peroxidases have the potential for functioning in the synthesis of the lignin polymer, the combined information of catalytic properties, expression, and localization can reveal differences in the significance of different peroxidases in the lignification process.