title: Metabolic engineering of novel lignin in biomass crops.
abstract: Lignin, a phenolic polymer in the secondary wall, is the major cause of lignocellulosic biomass recalcitrance to efficient industrial processing. From an applications perspective, it is desirable that second-generation bioenergy crops have lignin that is readily degraded by chemical pretreatments but still fulfill its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of some fraction of the traditional monolignols by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. However, successful engineering of lignin incorporating alternative monomers requires knowledge about phenolic metabolism in plants and about the coupling properties of these alternative monomers. Here, we review the current knowledge about lignin biosynthesis and the pathways towards the main phenolic classes. In addition, the minimal requirements are defined for molecules that, upon incorporation into the lignin polymer, make the latter more susceptible to biomass pretreatment. Numerous metabolites made by plants meet these requirements, and several have already been tested as monolignol substitutes in biomimetic systems. Finally, the status of detection and identification of compounds by phenolic profiling is discussed, as phenolic profiling serves in pathway elucidation and for the detection of incorporation of alternative lignin monomers.
authors.affiliation: Department of Plant Systems Biology, VIB, Technologiepark 927, 9052, Gent, Belgium.|Department of Plant Biotechnology and Bioinformatics, Ghent University, Technologiepark 927, 9052, Gent, Belgium.
authors.forename: Ruben
authors.initials: R
authors.lastname: Vanholme
authors.affiliation: Department of Plant Systems Biology, VIB, Technologiepark 927, 9052, Gent, Belgium.|Department of Plant Biotechnology and Bioinformatics, Ghent University, Technologiepark 927, 9052, Gent, Belgium.
authors.forename: Kris
authors.initials: K
authors.lastname: Morreel
authors.affiliation: Department of Plant Systems Biology, VIB, Technologiepark 927, 9052, Gent, Belgium.|Department of Plant Biotechnology and Bioinformatics, Ghent University, Technologiepark 927, 9052, Gent, Belgium.
authors.forename: Chiarina
authors.initials: C
authors.lastname: Darrah
authors.affiliation: Department of Plant Systems Biology, VIB, Technologiepark 927, 9052, Gent, Belgium.|Department of Plant Biotechnology and Bioinformatics, Ghent University, Technologiepark 927, 9052, Gent, Belgium.
authors.forename: Paula
authors.initials: P
authors.lastname: Oyarce
authors.affiliation: USDA-Agricultural Research Service, US Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI, 53706, USA.
authors.forename: John H
authors.initials: JH
authors.lastname: Grabber
authors.affiliation: Departments of Biochemistry and Biological Systems Engineering, the Wisconsin Bioenergy Initiative, and the DOE Great Lakes Bioenergy Research Center, University of Wisconsin, 433 Babcock Drive, Madison, WI, 53706, USA.
authors.forename: John
authors.initials: J
authors.lastname: Ralph
authors.affiliation: Department of Plant Systems Biology, VIB, Technologiepark 927, 9052, Gent, Belgium.|Department of Plant Biotechnology and Bioinformatics, Ghent University, Technologiepark 927, 9052, Gent, Belgium.
authors.forename: Wout
authors.initials: W
authors.lastname: Boerjan
mesh_terms: D056804:Biofuels; D018533:Biomass; D018556:Crops, Agricultural / Q000378:metabolism; D008031:Lignin / Q000096:biosynthesis / Q000737:chemistry* / Q000378:metabolism*; D060847:Metabolic Engineering / Q000379:methods*; D010636:Phenols / Q000378:metabolism; D010944:Plants / Q000235:genetics / Q000378:metabolism*
chemical_list: D056804:Biofuels; D010636:Phenols; D008031:Lignin