Coniferyl Alcohol Oxidase Activity in the Cell Wall of Callisthephus sinensis

The oxidation of coniferyl alcohol by cell walls isolated from lignifying hypocotyls of Callistephus sinensis was studied in the absence and in the presence of H2O2. The results showed that the oxidation of coniferyl alcohol by cell walls in the absence of H2O2 (coniferyl alcohol oxidase activity) was approximately 3% of that occurring in the presence of H2O2. The in vivo significance of this cell wall-located coniferyl alcohol oxidase activity was studied in C. sinensis hypocotyls by histochemical tests, which revealed that coniferyl alcohol oxidase was located in the H2O2-producing lignifying xylem cells. The results obtained from these histochemical probes strongly suggest that coniferyl alcohol oxidase activity of C. sinensis cell walls is physiologically irrelevant in tissues that accumulate H2O2, such as the lignifying xylem, where the peroxidatic activities favorably compete with the oxidase activities during the polymerization of cinnamyl alcohols to lignins.