title: Reduced Lignin Content and Altered Lignin Composition in Transgenic Tobacco Down-Regulated in Expression of L-Phenylalanine Ammonia-Lyase or Cinnamate 4-Hydroxylase.
abstract: We analyzed lignin content and composition in transgenic tobacco (Nicotiana tabacum) lines altered in the expression of the early phenylpropanoid biosynthetic enzymes L-phenylalanine ammonia-lyase and cinnamate 4-hydroxylase (C4H). The reduction of C4H activity by antisense expression or sense suppression resulted in reduced levels of Klason lignin, accompanied by a decreased syringyl/guaiacyl monomer ratio as determined by pyrolysis gas chromatography/mass spectrometry Similar reduction of lignin levels by down -regulation of L-phenylalanine ammonia-lyase, the enzyme preceding C4H in the central phenylpropanoid pathway, did not result in a decreased syringyl/guaiacyl ratio. Rather, analysis of lignin methoxyl content and pyrolysis suggested an increased syringyl/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 pathway, prior to the formation of specific monolignol precursors.
authors.affiliation: Plant Biology Division, Samuel Roberts Noble Foundation, P.O. Box 2180, Ardmore, Oklahoma 73402 (V.J.H.S., J.W.B., P.A.H., S.A.M., R.A.D.).
authors.forename: VJH.
authors.initials: V
authors.lastname: Sewalt
authors.forename: W.
authors.initials: W
authors.lastname: Ni
authors.forename: J. W.
authors.initials: JW
authors.lastname: Blount
authors.forename: H. G.
authors.initials: HG
authors.lastname: Jung
authors.forename: S. A.
authors.initials: SA
authors.lastname: Masoud
authors.forename: P. A.
authors.initials: PA
authors.lastname: Howles
authors.forename: C.
authors.initials: C
authors.lastname: Lamb
authors.forename: R. A.
authors.initials: RA
authors.lastname: Dixon