A LACCASE-LIKE PHENOLOXIDASE IS CORRELATED WITH LIGNIN BIOSYNTHESIS IN ZINNIA-ELEGANS STEM TISSUES

Stem tissues from different internodes of 4-6 week-old Zinnia elegans cv. Envy plants were sectioned and stained with chromogenic substrates previously used in studies of laccases (p-diphenol:O-2 oxidoreductases) isolated from tree tissues. The pattern of color development found when stem sections were stained in the presence and absence of H2O2 suggested that p-diphenol:O-2 oxidoreductase activity was tightly correlated spatially and temporally with the lignification of secondary cell walls in developing primary xylem. The correlation between this laccase-like phenoloxidase activity and lignification appeared tighter than that between lignification and peroxidases stained using the same substrates. Zymogram analysis of the phenoloxidase activities catalyzed by enzymes that were not boiled prior to separation by SDS-PAGE suggested that a single enzyme was predominantly responsible for the laccase-like phenoloxidase activity in Zinnia stems. Some of this enzyme was released from cell wall residue by washing with high ionic strength buffer; however, substantial amounts of the enzyme could only be recovered after treatment of the residue with cell wall-degrading enzymes. This phenoloxidase appears to share significant characteristics with the coniferyl alcohol oxidase isolated from developing secondary xylem in pines, which suggests that such enzymes may be widespread in vascular plants.