Essential role of caffeoyl coenzyme A O-methyltransferase in lignin biosynthesis in woody poplar plants

Caffeoyl coenzyme A O-methyltransferase (CCoAOMT) has recently been shown t o participate in lignin biosynthesis in herbacious tobacco plants. Here, we demonstrate that CCoAOMT is essential in lignin biosynthesis in woody popl ar (Populus tremula x Populus alba) plants. In poplar stems, CCoAOMT was fo und to be expressed in all lignifying cells including vessel elements and f ibers as well as in xylem ray parenchyma cells. Repression of CCoAOMT expre ssion by the antisense approach in transgenic poplar plants caused a signif icant decrease in total lignin content as detected by both Klason lignin as say and Fourier-transform infrared spectroscopy. The reduction in lignin co ntent was the result of a decrease in both guaiacyl and syringyl lignins as determined by in-source pyrolysis mass spectrometry. Fourier-transform inf rared spectroscopy indicated that the reduction in lignin content resulted in a less condensed and less cross-linked lignin structure in wood. Repress ion of CCoAOMT expression also led to coloration of wood and an elevation o f wall-bound p-hydroxybenzoic acid. Taken together, these results indicate that CCoAOMT plays a dominant role in the methylation of the 3-hydroxyl gro up of caffeoyl CoA, and the CCoAOMT-mediated methylation reaction is essent ial to channel substrates for 5-methoxylation of hydroxycinnamates. They al so suggest that antisense repression of CCoAOMT is an efficient means for g enetic engineering of trees with low lignin content.