AN ALTERNATIVE METHYLATION PATHWAY IN LIGNIN BIOSYNTHESIS IN ZINNIA

S-Adenosyl-L-methionine:trans-caffeoyl-coenzyme A 3-O-methyltransferas e (CCoAOMT) is implicated in disease resistant response, but whether i t is involved in lignin biosynthesis is not known. We isolated a cDNA clone for CCoAOMT in differentiating tracheary elements (ifs) induced from Zinnia-isolated mesophyll cells. RNA gel blot analysis showed tha t the expression of the CCoAOMT gene was markedly induced during TE di fferentiation from the isolated mesophyll cells. Tissue print hybridiz ation showed that the expression of the CCoAOMT gene is temporally and spatially regulated and that it is associated with lignification in x ylem and in phloem fibers in Zinnia organs. Both CCoAOMT and caffeic a cid O-methyltransferase (COMT) activities increased when the isolated Zinnia mesophyll cells were cultured, whereas only CCoAOMT activity wa s markedly enhanced during lignification in the in vitro-differentiati ng TEs. The induction pattern of the OMT activity using 5-hydroxyferul oyl CoA as substrate during lignification was the same as that using c affeoyl CoA. Taken together, the results indicate that CCoAOMT is asso ciated with lignification during xylogenesis both in vitro and in the plant, whereas COMT is only involved in a stress response in vitro. We propose that CCoAOMT is involved in an alternative methylation pathwa y In lignin biosynthesis. In Zinnia in vitro-differentiating TEs, the CCoAOMT mediated methylation pathway is dominant.