DIFFERENTIAL EXPRESSION OF 2 O-METHYLTRANSFERASES IN LIGNIN BIOSYNTHESIS IN ZINNIA-ELEGANS

Caffeic acid 3-O-methyltransferase (CAOMT) and caffeoyl-coenzyme A 3-O -methyltransferase (CCoAOMT) are involved in different methylation pat hways in lignin biosynthesis. We previously showed that only the CCoAO MT was markedly induced during lignification in in vitro differentiati ng tracheary elements (TEs) of Zinnia elegans. To further examine the expression patterns of CAOMT in lignification, we isolated a cDNA clon e for Zinnia CAOMT. RNA gel blot analysis showed that the expression o f the CAOMT gene did not correlate well with lignification during in v itro TE differentiation from Zinnia-isolated mesophyll cells. Tissue-p rint hybridization showed that, in the young internodes, the CAOMT mRN A signal was much more evident in phloem fibers than in xylem, whereas the CCoAOMT mRNA signal was predominantly present in differentiating xylem regions. In the older internode, both the CAOMT and CCoAOMT mRNA s markedly accumulated in phloem fibers and differentiating xylem regi ons. Immunocytochemical localization showed that the CAOMT protein sta ining was much more evident in phloem fibers and xylem fibers than in xylem TEs. These results indicate that the expression of these two O-m ethyltransferases is differentially regulated during lignification in different cell types in Zinnia. We suggest that all the intermediates in the CAOMT-mediated methylation pathway might become substrates for the CCoAOMT-mediated methylation pathway after coenzyme A ligation whe n these two pathways occur in the same lignifying cell types.