Syringyl lignin is unaltered by severe sinapyl alcohol dehydrogenase suppression in tobacco
Barakate, Abdellah, A. | Stephens, Jennifer, J. | Goldie, Alison, A. | Hunter, William N., W. N. | Marshall, David, D. | Hancock, Robert D., R. D. | Lapierre, Catherine, C. | Morreele, Kris, K. | Boerjane, Wout, W. | Halpin, Claire, C. | Division of Plant Sciences, College of Life Sciences ; University of Dundee | Division of Biological Chemistry and Drug Discovery, College of Life Sciences ; University of Dundee | Institut Jean-Pierre Bourgin (IJPB) ; Institut National de la Recherche Agronomique (INRA)-AgroParisTech | Department of Plant Systems Biology ; Flanders Institute for Biotechnology | Department of Plant Biotechnology ; Universiteit Gent = Ghent University = Université de Gand (UGENT) | We thank Vincent Chiang for the gift of the SAD antiserum, Peter M. Waterhouse for the gift of the pHellsgate8 vector, Frederic Legee for running the wood extractions and the Klason determinations, Laurent Cezar for running the thioacidolysis experiments, Brigitte Pollet for supervising these analyses, Frank Wright for help with phylogenetic analysis, and Ingo Hein for help with real-time RT-PCR. This work was funded by the Biotechnology and Biological Sciences Research Council of the United Kingdom (P18182).
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.
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