An alternative methylation pathway in lignin biosynthesis in Zinnia.

S-Adenosyl-L-methionine:trans-caffeoyl-coenzyme A 3-O-methyltransferase (CCoAOMT) is implicated in disease resistant response, but whether it is involved in lignin biosynthesis is not known. We isolated a cDNA clone for CCoAOMT in differentiating tracheary elements (TEs) induced from Zinnia-isolated mesophyll cells. RNA gel blot analysis showed that the expression of the CCoAOMT gene was markedly induced during TE differentiation from the isolated mesophyll cells. Tissue print hybridization showed that the expression of the CCoAOMT gene is temporally and spatially regulated and that it is associated with lignification in xylem and in phloem fibers in Zinnia organs. Both CCoAOMT and caffeic acid O-methyltransferase (COMT) activities increased when the isolated Zinnia mesophyll cells were cultured, whereas only CCoAOMT activity was markedly enhanced during lignification in the in vitro-differentiating TEs. The induction pattern of the OMT activity using 5-hydroxyferuloyl CoA as substrate during lignification was the same as that using caffeoyl CoA. Taken together, the results indicate that CCoAOMT is associated 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 pathway in lignin biosynthesis. In Zinnia in vitro-differentiating TEs, the CCoAOMT mediated methylation pathway is dominant.