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.

A Key Role for Apoplastic H2O2 in Norway Spruce Phenolic Metabolism

“…An analogous situation to that in the spruce cell culture has been observed in flax (Linun usitatissimum) plants, whose outer stem tissues contain hypolignified bast fibers with increased levels of glycosylated oligolignols compared with the inner stem, which contains lignified xylem and a higher content of less polar oligolignols (Huis et al, 2012). The data obtained from flax mutants with lignified bast fibers suggest that, similar to that in the KI-treated spruce cell cultures ( Fig.…”

Small Glycosylated Lignin Oligomers Are Stored in Arabidopsis Leaf Vacuoles

“…The stems of these mutants also accumulated a series of 5-hydroxyguaiacyl-containing dilignol and trilignol hexosides (Vanholme et al, 2010). Recently, a wide range of oligolignol hexosides has been detected in wild-type Arabidopsis leaves , and some were also found in flax stems (Huis et al, 2012;Chantreau et al, 2014).…”

“…Recently, oligolignol derivatives have been found in the stems of flax (Huis et al, 2012) and in the stems and rosette leaves of Arabidopsis (Vanholme et al, 2012b;Morreel et al, 2014). To …”

Ectopic Lignification in the Flax lignified bast fiber1 Mutant Stem Is Associated with Tissue-Specific Modifications in Gene Expression and Cell Wall Composition

“…We have previously shown that lignification in flax appears to be partially modulated through transcriptional regulation of genes encoding lignin monomer biosynthesis and polymerization (Fenart et al, 2010;Huis et al, 2012). More recently, we generated and characterized a flax ethyl methanesulfonate (EMS) mutant population that has allowed us to obtain mutants for the CAD and C3H lignin genes through a TILLinG reverse genetics approach (Chantreau et al, 2013).…”