Lignification in the flax stem: evidence for an unusual lignin in bast fibers

Abstract

In the context of our research on cell wall formation and maturation in flax (Linum usitatissimum L) bast fibers, we (1) confirmed the presence of lignin in bast fibers and (2) quantified and characterized the chemical nature of this lignin at two developmental stages. Histochemical methods (Weisner and Maüle reagents and KMnO(4)-staining) indicating the presence of lignin in bast fibers at the light and electron microscope levels were confirmed by chemical analyses (acetyl bromide). In general, the lignin content in flax bast fibers varied between 1.5% and 4.2% of the dry cell wall residues (CWRs) as compared to values varying between 23.7% and 31.4% in flax xylem tissues. Immunological and chemical analyses (thioacidolysis and nitrobenzene oxidation) indicated that both flax xylem- and bast fiber-lignins were rich in guaiacyl (G) units with S/G values inferior to 0.5. In bast fibers, the highly sensitive immunological probes allowed the detection of condensed guaiacyl-type (G) lignins in the middle lamella, cell wall junctions, and in the S1 layer of the secondary wall. In addition, lower quantities of mixed guaiacyl-syringyl (GS) lignins could be detected throughout the secondary cell wall. Chemical analyses suggested that flax bast-fiber lignin is more condensed than the corresponding xylem lignin. In addition, H units represented up to 25% of the monomers released from bast-fiber lignin as opposed to a value of 1% for the corresponding xylem tissue. Such an observation indicates that the structure of flax bast-fiber lignin is significantly different from that of the more typical 'woody plant lignin', thereby suggesting that flax bast fibers represent an interesting system for studying an unusual lignification process.

Natural Hypolignification Is Associated with Extensive Oligolignol Accumulation in Flax Stems

“…Recently, a comprehensive study has provided detailed information on the structure of flax milled wood lignin from both stem inner tissues (shives) and bast fibers (del Rio et al, 2011). That study underlined the fact that flax lignin is rich in G-units and also contains high amounts of H-units, as reported previously in flax bast fibers (Day et al, 2005b). Analyses by pyrolysis-gas chromatography-MS indicated that H-units represented 5% and 13% of shive lignin and fiber lignin, respectively.…”

“…The presence of this monolignol in stems is not unexpected, since flax lignin is rich in G-lignin (Day et al, 2005b;del Rio et al, 2011). Of interest is the observation that coniferyl alcohol was nearly three times more abundant in outer stem tissues as compared with inner stem tissues, and this despite the fact that the majority of lignin genes are significantly underexpressed in outer stem tissues.…”

“…They are used to produce textiles (linen) and to reinforce composite materials (Summerscales et al, 2010). The secondary cell walls of flax bast fibers are unusual in that they are rich in cellulose but contain only low amounts of lignin (2%) as compared with most other plant secondary cell walls (Day et al, 2005b). In contrast, the secondary walls of flax xylem cells located in the inner stem tissues show a typical lignification profile containing around 25% lignin.…”

“…We did not see any evidence in our transcriptomic data to suggest that the flax HCT/C3H genes were underexpressed in comparison with other lignin biosynthesis genes; however, in the absence of suitable flax mutants/transgenics for comparison, it is difficult draw a conclusion on the likely explanation for the high H-unit content of flax lignin. Nevertheless, the high proportion of G-and H-units, as well as the high percentage of condensed bonds, render the flax lignin of both xylem and bast fibers particularly difficult to degrade and have an important impact on the industrial processing of this species (Day et al, 2005b;del Rio et al, 2011).…”

Biodegradation of Natural Textile Materials in Soil

“…Moreover, the band at 2900 cm -1 exhibits typical cellulose shape [25]. Flax befor composting Flax after 3 week composting Flax after 2 week composting Flax after 1 week composting Flax after 4 week composting Figure 11: FT-IR spectra of biodegraded ax samples ermogravimetric analysis of ax bres is carried out at 600 ºC, the temperature is raised because of the presence of polyester bres in the ax fabric but the temperature ramp rate is kept the same as for the other, which is 10 ºC per minute.…”