Expression of bifunctional caffeoyl-CoA 3-O-methyltransferase in stress compensation and lignification

Caffeate and caffeoyl-CoA O-methyltransferases (COMTs and CCoAOMTs) catalyze the formation of ferulic acid and feruloyl-CoA, respectively, in many plants, and their physiological significance is under investigation. CCoAOMT was proposed to play a pivotal role in cell wall reinforcement during the induced disease resistance response, as exemplified in elicitor-treated parsley cells, as well as in the formation of guaiacyl- and syringyl-type lignins. This requires selective substrate and tissue specificities. Parsley CCoAOMT expressed in E. coli methylated caffeoyl- or 5-hydroxyferuloyl-CoA to feruloyl- and sinapoyl-CoA, whereas neither caffeate nor 5-hydroxyferulate was accepted. Tissue print hybridizations of parsley stem and root sections revealed, furthermore, that CCoAOMT mRNA is constitutively associated with the vascular tissues, but is also expressed in the surface cell layers upon wounding. In order to study the promoter activity of the parsley CCoAOMT gene, tobacco plantlets were transformed with parsley CCoAOMT promoter-GUS reporter gene constructs; these transformants, at the very young stage, expressed GUS activity in a narrow subapical root zone only extending later to the vascular tissue at the onset of xylem differentiation. GUS activity of the mature transgenic tobacco plants was observed exclusively in the parenchyma lining the differentiated xylem elements and xylem ray cells of root, stem or leaf tissues. Thus, parsley CCoAOMT is a bifunctional enzyme which appears to serve in both stress compensation and lignification. This was supported by the ontogenetic activity profile of tobacco endogeneous CCoAOMT, which correlated closely with the GUS expression under the control of parsley CCoAOMT promoter, while the proportion of CCoAOMT vs. COMT activities varied substantially during growth of the transgenic tobacco plants.