ALTERED LIGNIN COMPOSITION IN TRANSGENIC TOBACCO EXPRESSING O-METHYLTRANSFERASE SEQUENCES IN SENSE AND ANTISENSE ORIENTATION

The monomeric composition of tobacco lignin has been modified by genet ic engineering. Sense or antisense expression of sequences encoding O- methyltransferase (OMT), a lignin biosynthetic enzyme, was shown to mo dulate enzyme activity. Ten constructs harboring the entire or a parti al OMT cDNA were used. Populations of 20 transgenic plants per constru ct were analyzed for OMT activity and compared with untransformed cont rols. 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 sh own to be stably integrated into the tobacco genome and to be transmit ted to the progeny. By using gene-specific probes, OMT inhibition in s tems was correlated to a parallel disappearance of OMT transcripts ori ginating from both the resident gene and the transgene. In contrast, t ransgene 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-hy droxy guaiacyl units were demonstrated. These two structural features are also characteristic of natural mutants of maize with an improved d igestibility compared with wild lines. These data demonstrate the feas ibility and the potential benefits of lignin manipulation.