REDUCED LIGNIN CONTENT AND ALTERED LIGNIN COMPOSITION IN TRANSGENIC TOBACCO DOWN-REGULATED IN EXPRESSION OF L-PHENYLALANINE AMMONIA-LYASE OR CINNAMATE 4-HYDROXYLASE

We analyzed lignin content and composition in transgenic tobacco (Nico tiana tabacum) lines altered in the expression of the early phenylprop anoid biosynthetic enzymes L-phenylalanine ammonia-lyase and cinnamate 4-hydroxylase (C4H). The reduction of C4H activity by antisense expre ssion or sense suppression resulted in reduced levels of Klason lignin , accompanied by a decreased syringyl/guaiacyl monomer ratio as determ ined by pyrolysis gas chromatography/mass spectrometry. Similar reduct ion of lignin levels by down-regulation of L-phenylalanine ammonia-lya se, the enzyme preceding C4H in the central phenylpropanoid pathway, d id not result in a decreased syringyl/guaiacyl ratio. Rather, analysis of lignin methoxyl content and pyrolysis suggested an increased syrin gyl/guaiacyl ratio. One possible explanation of these results is that monolignol biosynthesis from L-phenylalanine might occur by more than one route, even at the early stages of the core phenylpropanoid pathwa y, prior to the formation of specific monolignol precursors.