The monomeric composition of tobacco lignin has been modified by genetic engineering. Sense or antisense expression of sequences encoding O-methyltransferase (OMT), a lignin biosynthetic enzyme, was shown to modulate enzyme activity. Ten constructs harboring the entire or a partial OMT cDNA were used. Populations of 20 trensgenic plants per construct were analyzed for OMT activity and compared with untransformed controls. As expected, expression of only the full-length sense construct led to an increase in OMT activity. An important reduction of activity was found in a variable number of plantlets from all other transgenic populations but the inhibition was sustained through the adult stage only in plants transformed with the complete cDNA. T-DNA genes were shown to be stably integrated into the tobacco genome and to be transmitted to the progeny. By using gene-specific probes, OMT inhibition in stems was correlated to a parallel disappearance of OMT transcripts originating from both the resident gene and the transgene. In contrast, transgene transcripts were detected in leaf tissues where the resident gene is poorly expressed, thus indicating that relative expression of the two OMT genes controls transcript turnover. In stems of inhibited plants, a marked decrease of syringyl units and the appearance of 5-hydroxy guaiacyl units were demonstrated. These two structural features are also characteristic of natural mutants of maize with an improved digestibility compared with wild lines. These data demonstrate the feasibility and the potential benefits of lignin manipulation.