title: Syringyl lignin is unaltered by severe sinapyl alcohol dehydrogenase suppression in tobacco.
abstract: The manipulation of lignin could, in principle, facilitate efficient biofuel production from plant biomass. Despite intensive study of the lignin pathway, uncertainty exists about the enzyme catalyzing the last step in syringyl (S) monolignol biosynthesis, the reduction of sinapaldehyde to sinapyl alcohol. Traditional schemes of the pathway suggested that both guaiacyl (G) and S monolignols are produced by a single substrate-versatile enzyme, cinnamyl alcohol dehydrogenase (CAD). This was challenged by the discovery of a novel sinapyl alcohol dehydrogenase (SAD) that preferentially uses sinapaldehyde as a substrate and that was claimed to regulate S lignin biosynthesis in angiosperms. Consequently, most pathway schemes now show SAD (or SAD and CAD) at the sinapaldehyde reduction step, although functional evidence is lacking. We cloned SAD from tobacco (Nicotiana tabacum) and suppressed it in transgenic plants using RNA interference-inducing vectors. Characterization of lignin in the woody stems shows no change to content, composition, or structure, and S lignin is normal. By contrast, plants additionally suppressed in CAD have changes to lignin structure and S:G ratio and have increased sinapaldehyde in lignin, similar to plants suppressed in CAD alone. These data demonstrate that CAD, not SAD, is the enzyme responsible for S lignin biosynthesis in woody angiosperm xylem.
authors.affiliation: Division of Plant Sciences, College of Life Sciences, University of Dundee at the James Hutton Institute, Invergowrie, Dundee, United Kingdom.
authors.forename: Abdellah
authors.initials: A
authors.lastname: Barakate
authors.forename: Jennifer
authors.initials: J
authors.lastname: Stephens
authors.forename: Alison
authors.initials: A
authors.lastname: Goldie
authors.forename: William N
authors.initials: WN
authors.lastname: Hunter
authors.forename: David
authors.initials: D
authors.lastname: Marshall
authors.forename: Robert D
authors.initials: RD
authors.lastname: Hancock
authors.forename: Catherine
authors.initials: C
authors.lastname: Lapierre
authors.forename: Kris
authors.initials: K
authors.lastname: Morreel
authors.forename: Wout
authors.initials: W
authors.lastname: Boerjan
authors.forename: Claire
authors.initials: C
authors.lastname: Halpin
mesh_terms: D000171:Acrolein / Q000031:analogs & derivatives / Q000378:metabolism; D000429:Alcohol Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D003001:Cloning, Molecular; D004789:Enzyme Activation; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D008031:Lignin / Q000235:genetics / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D010636:Phenols / Q000378:metabolism; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D018547:Plant Stems / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified / Q000201:enzymology / Q000235:genetics / Q000502:physiology; D034622:RNA Interference; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity; D014026:Nicotiana / Q000201:enzymology* / Q000235:genetics / Q000502:physiology; D014934:Wood / Q000235:genetics / Q000378:metabolism
chemical_list: D010636:Phenols; D010940:Plant Proteins; D011994:Recombinant Proteins; C434468:sinapyl alcohol dehydrogenase; C434469:syringyl monolignol; C075386:sinapaldehyde; D000171:Acrolein; D008031:Lignin; D000429:Alcohol Oxidoreductases; C018656:cinnamyl alcohol dehydrogenase