Strong decrease in lignin content without significant alteration of plant development is induced by simultaneous down-regulation of cinnamoyl CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) in tobacco plants

Different transgenic tobacco lines down-regulated for either one or two enz ymes of the monolignol pathway were compared for their lignin content and c omposition, and developmental patterns. The comparison concerned CCR and CA D down-regulated lines (homozygous or heterozygous for the transgene) and t he hybrids resulting from the crossing of transgenic lines individually alt ered for CCR or CAD activities. Surprisingly, the crosses containing only o ne allele of each antisense transgene, exhibit a dramatic reduction of lign in content similar to the CCR down-regulated parent but, in contrast to thi s transgenic line, display a normal phenotype and only slight alterations o f the shape of the vessels. Qualitatively the lignin of the double transfor mant displays characteristics more like the wild type control than either o f the other transgenics. In the transgenics with a low lignin content, the transformations induced other biochemical changes involving polysaccharides , phenolic components of the cell wall and also soluble phenolics. These re sults show that the ectopic expression of a specific transgene may have a d ifferent impact depending on the genetic background and suggest that the tw o transgenes present in the crosses may operate synergistically to reduce t he lignin content. In addition, these data confirm that plants with a sever e reduction in lignin content may undergo normal development at least in co ntrolled conditions.