Lignin plays an important role in plant cell walls, enhancing the rigidity, conferring resistance toward pathogens and mechanical stress, and enabling solute transport in the xylem. Among woody plants, such as poplar, the amount of lignin varies between 15% and 36% of the dry weight. During Kraft pulping, the most used chemical paper pulping process. lignin is removed from the cellulose. This process is toxic, energy consuming, and results in a low biomass utilization. For the pulp industry, it would be very beneficial to process trees with either less lignin or with modified lignin that is more easily separated from the cellulose. To obtain this goal, a basic knowledge of the biosynthesis of lignin is necessary. We have cloned cDNAs for caffeic acid/5-hydroxyferulic acid-O-methyltransferase (COMT), caffeoyl-CoA-O-methyltransferase (CCoAOMT), cinnamoyl-CoA-reductase (CCR), and cinnamoyl-alcohol-dehydrogenase (CAD). In addition, we cloned DNAs for xylem-specific peroxidases putatively involved in lignification. By the antisense strategy, it is possible to unravel the in vivo role of these enzymes in regulating the amount and composition of lignin. Inhibition of the COMT activity in transgenic plants resuits in a change in the composition of lignin with the simultaneous appearance of a novel lignin unit. Inhibition of CAD activity in poplar results in an increased lignin extracubility during chemical pulping. Experiments with transgenic trees expressing CCR, CCoAOMT, or a combination of several constructs are underway. Furthermore, we have cloned CAD, CCR. and CCoAOMT genes from poplar and identified cis elements in the promoter that could be involved in xylem-specific gene expression.