NMR CHARACTERIZATION OF ALTERED LIGNINS EXTRACTED FROM TOBACCO PLANTSDOWN-REGULATED FOR LIGNIFICATION ENZYMES CINNAMYL-ALCOHOL DEHYDROGENASE AND CINNAMOYL-COA REDUCTASE

Homologous antisense constructs were used to down-regulate tobacco cin namyl-alcohol dehydrogenase (CAD; EC 1.1.1.195) and cinnamoyl-Coa redu ctase (CCR; EC 1.2.1.44) activities in the lignin monomer biosynthetic pathway. CCR converts activated cinnamic acids (hydroxycinnamoyl-SCoA s) to cinnamaldehydes; cinnamaldehydes are then reduced to cinnamyl al cohols by CAD. The transformations caused the incorporation of nontrad itional components into the extractable tobacco lignins, as evidenced by NMR. Isolated lignin of antisense-CAD tobacco contained fewer conif eryl and sinapyl alcohol-derived units that were compensated for by el evated levels of benzaldehydes and cinnamaldehydes. Products from radi cal coupling of cinnamaldehydes, particularly sinapaldehyde, which wer e barely discernible in normal tobacco, were major components of the a ntisense-CAD tobacco lignin. Lignin content was reduced in antisense-C CR tobacco, which displayed a markedly reduced vigor. That lignin cont ained fewer coniferyl alcohol-derived units and significant levels of tyramine ferulate, Tyramine ferulate is a sink for the anticipated bui ld-up of feruloyl-SCoA, and may be up-regulated in response to a defic it of coniferyl alcohol. Although it is not yet clear whether the modi fied lignins are true structural components of the cell wall, the find ings provide further indications of the metabolic plasticity of plant lignification, An ability to produce lignin from alternative monomers would open new avenues for manipulation of lignin by genetic biotechno logies.