Down-regulation of cinnamoyl-CoA reductase induces significant changes of lignin profiles in transgenic tobacco plants

Taking advantage of the recent characterization of the cDNA encoding cinnamoyl-CoA reductase (CCR, E.C. 1.2.1.44) in Eucalyptus gunnii, antisense constructs have been introduced in tobacco plants in order to down regulate this key enzyme of lignification. Primary transformants exhibiting reduction in CCR activity were obtained and a good correlation was observed between the decrease in the steady state level of CCR mRNA and the level of CCR activity. Lignin content and composition were examined in the progeny of primary transformants down-regulated for CCR activity and exhibiting one single T-DNA integration locus. All CCR down-regulated lines display common features, such as an orange brown coloration of the xylem cell walls, an increase in the syringyl over guaiacyl (S/G) ratio, and the presence of unusual cell wall bound phenolics. Moreover, the less severely depressed lines exhibited a normal phenotype and a very slight reduction of the thioacidolysis yield, which is an indication of the abundance of the beta-O-4 linkages in lignin. The new lignin profiles observed in these lines support a role for CCR down-regulation in improving wood properties of forest trees used in the pulp industry. On the other hand, the line with the most severely depressed CCR activity exhibited a strong reduction in lignin content together with altered development (reduced size, abnormal morphology of the leaves, collapsed vessels). This new lignin mutant offers an unique opportunity to explore the various roles of lignins in plant development.