Lignin in lignin-biosynthetic-pathway mutants and transgenics: Structural studies

Structural work on lignin-biosynthetic-pathway mutant and transgenic plants has suggested that down- or up-regulating the production of lignin monomers may result in increased incorporation of other plant phenols into the lignin polymer. Some of the incorporated monomers are not associated with the normal monolignol pathway; consequently, lignification may have considerably more plasticity than previously realized. Mutants and transgenics provide a rich source of insights into biosynthetic pathways and the metabolic plasticity of lignification.; This research structurally examines lignins from natural mutants and a variety of transgenic plants deficient (or in one case, upregulated) in phenylpropanoid pathway enzymes. These enzymes include: (1) cinnamoyl-coenzyme A reductase (CCR); (2) cinnamyl alcohol dehydrogenase (CAD); (3) caffeic acid O-methyltransferase (COMT); (4) caffeoyl-coenzyme A 3-O-methyltransferase (CCOMT); (5) ferulate 5-hydroxylase (FSH).; Diagnostic NMR studies revealing the compositional and structural changes that have occurred in the plant phenolic polymers provide insight into metabolic changes that are occurring in these plants. The major findings include the following: (1) FSH down- and up-regulation can alter lignification to produce ∼100% guaiacyl lignin to ∼3% guaiacyl lignin (far lower than in any plant reported to date). (2) COMT down-regulation results in significant incorporation of 5-hydroxyconiferyl lcohol into the lignin, producing novel benzodioxane structures. (3) Down-regulation of CCR or CAD incorporates greater quantities of previously minor components such as tyramine ferulates, hydroxycinnamaldehydes, or hydroxybenzaldehydes. A double transformant (hemizygous for CCR and CAD transgenes) resulted in an intermediate plant with lower lignin content similar to the antisense-CCR parent but with higher lignin extractability as in the antisense-CAD parent. (4) CCOMT down-regulation results in a general down-regulation of the lignin pathway similar to effects that occur following the down-regulation of enzymes early in the phenylpropanoid pathway. The presence of benzodioxane units suggests an interaction between both O-methyltransferases, CCOMT and COMT. (5) The cell wall composition, and the lignin particularly, of the bm corn mutants did not reveal anticipated dramatic changes except for bm3. Both the bm1 (CAD) and bm3 (COMT) mutational effects (elevated aldehyde levels and the presence of benzodioxane units) were evident in their lignins.