title: Small glycosylated lignin oligomers are stored in Arabidopsis leaf vacuoles.
abstract: Lignin is an aromatic polymer derived from the combinatorial coupling of monolignol radicals in the cell wall. Recently, various glycosylated lignin oligomers have been revealed in Arabidopsis thaliana. Given that monolignol oxidation and monolignol radical coupling are known to occur in the apoplast, and glycosylation in the cytoplasm, it raises questions about the subcellular localization of glycosylated lignin oligomer biosynthesis and their storage. By metabolite profiling of Arabidopsis leaf vacuoles, we show that the leaf vacuole stores a large number of these small glycosylated lignin oligomers. Their structural variety and the incorporation of alternative monomers, as observed in Arabidopsis mutants with altered monolignol biosynthesis, indicate that they are all formed by combinatorial radical coupling. In contrast to the common believe that combinatorial coupling is restricted to the apoplast, we hypothesized that the aglycones of these compounds are made within the cell. To investigate this, leaf protoplast cultures were cofed with 13C6-labeled coniferyl alcohol and a 13C4-labeled dimer of coniferyl alcohol. Metabolite profiling of the cofed protoplasts provided strong support for the occurrence of intracellular monolignol coupling. We therefore propose a metabolic pathway involving intracellular combinatorial coupling of monolignol radicals, followed by oligomer glycosylation and vacuolar import, which shares characteristics with both lignin and lignan biosynthesis.
authors.affiliation: Department of Plant Systems Biology, VIB, B-9052 Gent, Belgium Department of Plant Biotechnology and Bioinformatics, Ghent University, B-9052 Gent, Belgium.
authors.forename: Oana
authors.initials: O
authors.lastname: Dima
authors.affiliation: Department of Plant Systems Biology, VIB, B-9052 Gent, Belgium Department of Plant Biotechnology and Bioinformatics, Ghent University, B-9052 Gent, Belgium.
authors.forename: Kris
authors.initials: K
authors.lastname: Morreel
authors.affiliation: Department of Plant Systems Biology, VIB, B-9052 Gent, Belgium Department of Plant Biotechnology and Bioinformatics, Ghent University, B-9052 Gent, Belgium.
authors.forename: Bartel
authors.initials: B
authors.lastname: Vanholme
authors.affiliation: Departments of Biochemistry and Biological Systems Engineering, and the DOE Great Lakes Bioenergy Research Center, The Wisconsin Energy Institute, University of Wisconsin, Madison, Wisconsin 53726.
authors.forename: Hoon
authors.initials: H
authors.lastname: Kim
authors.affiliation: Departments of Biochemistry and Biological Systems Engineering, and the DOE Great Lakes Bioenergy Research Center, The Wisconsin Energy Institute, University of Wisconsin, Madison, Wisconsin 53726.
authors.forename: John
authors.initials: J
authors.lastname: Ralph
authors.affiliation: Department of Plant Systems Biology, VIB, B-9052 Gent, Belgium Department of Plant Biotechnology and Bioinformatics, Ghent University, B-9052 Gent, Belgium wout.boerjan@psb.vib-ugent.be.
authors.forename: Wout
authors.initials: W
authors.lastname: Boerjan
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D053898:Biosynthetic Pathways; D002853:Chromatography, Liquid; D004952:Esters; D006031:Glycosylation; D008031:Lignin / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism*; D008293:Malates / Q000378:metabolism; D013058:Mass Spectrometry; D008954:Models, Biological; D010636:Phenols / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism*; D011523:Protoplasts / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism*
chemical_list: D004952:Esters; D008293:Malates; D010636:Phenols; C030298:malic acid; D008031:Lignin