title: Natural hypolignification is associated with extensive oligolignol accumulation in flax stems.
abstract: Flax (Linum usitatissimum) stems contain cells showing contrasting cell wall structure: lignified in inner stem xylem tissue and hypolignified in outer stem bast fibers. We hypothesized that stem hypolignification should be associated with extensive phenolic accumulation and used metabolomics and transcriptomics to characterize these two tissues. (1)H nuclear magnetic resonance clearly distinguished inner and outer stem tissues and identified different primary and secondary metabolites, including coniferin and p-coumaryl alcohol glucoside. Ultrahigh-performance liquid chromatography-Fourier transform ion cyclotron resonance-mass spectrometry aromatic profiling (lignomics) identified 81 phenolic compounds, of which 65 were identified, to our knowledge, for the first time in flax and 11 for the first time in higher plants. Both aglycone forms and glycosides of monolignols, lignin oligomers, and (neo)lignans were identified in both inner and outer stem tissues, with a preponderance of glycosides in the hypolignified outer stem, indicating the existence of a complex monolignol metabolism. The presence of coniferin-containing secondary metabolites suggested that coniferyl alcohol, in addition to being used in lignin and (neo)lignan formation, was also utilized in a third, partially uncharacterized metabolic pathway. Hypolignification of bast fibers in outer stem tissues was correlated with the low transcript abundance of monolignol biosynthetic genes, laccase genes, and certain peroxidase genes, suggesting that flax hypolignification is transcriptionally regulated. Transcripts of the key lignan genes Pinoresinol-Lariciresinol Reductase and Phenylcoumaran Benzylic Ether Reductase were also highly abundant in flax inner stem tissues. Expression profiling allowed the identification of NAC (NAM, ATAF1/2, CUC2) and MYB transcription factors that are likely involved in regulating both monolignol production and polymerization as well as (neo)lignan production.
authors.affiliation: Université Lille Nord de France, Lille 1 UMR 1281, F-59650 Villeneuve d'Ascq cedex, France.
authors.forename: Rudy
authors.initials: R
authors.lastname: Huis
authors.forename: Kris
authors.initials: K
authors.lastname: Morreel
authors.forename: Ophélie
authors.initials: O
authors.lastname: Fliniaux
authors.forename: Anca
authors.initials: A
authors.lastname: Lucau-Danila
authors.forename: Stéphane
authors.initials: S
authors.lastname: Fénart
authors.forename: Sébastien
authors.initials: S
authors.lastname: Grec
authors.forename: Godfrey
authors.initials: G
authors.lastname: Neutelings
authors.forename: Brigitte
authors.initials: B
authors.lastname: Chabbert
authors.forename: François
authors.initials: F
authors.lastname: Mesnard
authors.forename: Wout
authors.initials: W
authors.lastname: Boerjan
authors.forename: Simon
authors.initials: S
authors.lastname: Hawkins
mesh_terms: D002473:Cell Wall / Q000235:genetics / Q000378:metabolism; D019597:Flax / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant / Q000235:genetics; D042845:Laccase / Q000235:genetics / Q000378:metabolism; D017705:Lignans; D008031:Lignin / Q000737:chemistry / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008954:Models, Biological; D009195:Peroxidase / Q000235:genetics / Q000378:metabolism; D010636:Phenols / Q000378:metabolism; D018547:Plant Stems / Q000235:genetics / Q000378:metabolism*; D058105:Polymerization; D017550:Spectroscopy, Fourier Transform Infrared; D014157:Transcription Factors / Q000378:metabolism; D052584:Xylem / Q000378:metabolism
chemical_list: D017705:Lignans; D010636:Phenols; D014157:Transcription Factors; D008031:Lignin; D042845:Laccase; D009195:Peroxidase