Immunocytochemical localization of enzymes involved in lignification of the cell wall

A knowledge of the localization of the enzymes involved in lignification is essential to better understand the dynamics of lignin biosynthesis in the cell, as well as the regulation of lignification in woody plants. Though most of the enzymes involved in lignification have been identified, their localization in the cell and cell wall remains equivocal. Immunocytochemistry is a powerful tool with which to investigate the localization of a target substance such as an enzyme in cells. Our recent immunolabeling experiments revealed that phenylalanine ammonia-lyase, 4-coumalate: CoA ligase, caffeate O-methyltransferase, and cinnnamyl alcohol dehydrogenase were localized to differentiating xylem. These enzymes are particularly abundant during secondary wall formation. Immunolabeling was observed on polysomes and in the cytosol of the cells during secondary wall formation, indicating that these enzymes are synthesized in the polysomes and released in the cytosol. The synthesis of monolignols might occur in the cytosol. Immunolabeling of anionic peroxidase was also localized to the differentiating xylem, particularly during secondary wall formation. The labeling, however, was observed in the rough endoplasmic reticulum (r-ER), the Golgi apparatus, and the plasma membrane, indicating that peroxidase is synthesized in the r-ER, transported to the Golgi apparatus, and localized on the plasma membrane by fusion of the Golgi vesicles to the membrane.