Search

Thermal treatment is one of the widely used methods for feedstuff preservation. The aim of this study was to evaluate possible cell wall alterations during the industrial-scale drying of alfalfa. Alfalfa is mainly dehydrated and pelleted using two drying treatments. First a thermal treatment at 70 °C is applied to the chopped raw material, prior to the main dehydration procedure which involves a temperature of about 600 °C for a restricted time (15 min). The different steps of the dehydration process did not cause significant changes in alfalfa cell wall composition, but the proportion of labile ether-linked lignin structure (β-O-4) increased after the thermal shock. Chemical fractionation of cell wall carbohydrates showed that pectins were less accessible to extraction with ammonium oxalate after alfalfa drying. Moreover, xylanase (endo enzyme) was more effective on cell walls obtained from heated material. These results suggested that heat processing causes some changes in interpolymer bonds (covalent and hydrogen bonds) between pectins and the lignin/hemicellulose matrix, whereby the cell wall network is modified. © 2002 Society of Chemical Industry

Oxidants formed through the interaction between MnO2 and oxalate accumulated in rotten wood have been evaluated as abiotic agents possibly involved in wheat straw ligninolysis. The hemicellulose and cellulose content of straw remained unchanged, and no release of free soluble materials from lignin or polysaccharides could be evidenced. Structural analysis of oxidized lignin in situ has revealed up to 30% and 10% decrease of β-O-4 linked Guaiacyl and Syringyl units, respectively. Lignin phenolic moieties only were directly oxidized by MnO2/oxalate as no structural alteration was observed within extensively permethylated wheat straw. Modifications were also evidenced at the level of the cell wall linked cinnamic acids present in wheat straw. Esterified phenolic acids were more readily oxidized by Mn complexes than ethers analogues, and disappearance of ferulic moieties was always more pronounced than that of p-coumaric acids. The abiotic Mn oxidants generated from MnO2 and oxalate may therefore significantl...

Lignins definition, potential applications, (macro)molecular structures and properties, analytical characterisations and natural versus induced variability are reviewed with reference to both the biological diversity of lignification patterns in ecotypes, mutants and transgenic plants and the levels of organisation of the lignins polymeric networks. A discrimination between a common primary, guaiacyl type metabolism and a more specific secondary, syringyl type is suggested as a base of this variability and properties.

The monomeric composition of in situ lignins in sorghum, maize and wheat straw stems is investigated by two chemical degradation methods: thioacidolysis and alkaline nitrobenzene oxidation. It is shown that the structural information obtained in the case of lignin characterisation by alkaline nitrobenzene oxidation is strongly dependent on the reaction conditions, namely the temperature and the reaction time, as indicated by the significant variations in the yields of aromatic aldehydes that correspond to the three main constitutive lignin units, p -hydroxyphenyl (H), guaiacyl (G) and syringyl (S). Moreover, the complementary character of the two methods that stems from their different specificities is discussed in view of the molecular heterogeneity of lignins. Finally, the results obtained indicate a significant structural variability of the lignins of the three crops examined.

In plant cell walls, lignins, the second most abundant biopolymers, form 3D crosslinked molecular structures which interact with polysaccharides to form lignin-carbohydrate complexes. The incorporation of these phenolic polymers into synthetic polysaccharide-based composites is currently under investigation. The properties of the final material (mechanical strength, degradability,…) strongly depend on the degree of compatibility between such hydrophilic and hydrophobic molecules. This phenomenon is expected to be in part related to the surface properties of lignins. Organosolve dioxane lignins were isolated from wheat straw and characterized with respect to their HPSEC elution profile ( M n =3.6×10 3 ), total sugar and esterified phenolic acid content (3 and 3.5%, respectively) and to the degradation yield of thioacidolysis (750 μ moles g −1 ). These lignins were then dissolved in a dioxane–water mixture and used for Langmuir trough experiments at the air–water interface. Compression isotherms (pressure/concentration) were obtained at 20°C by compressing the surface deposited layers at specified rates. A particular feature of the interfacial layers of lignin is that surface pressure relaxes strongly when compression is stopped. Thus, the compression isotherms exhibit a surface pressure which is roughly three times higher than the value reached after 48 h relaxation. These data suggest that previously reported isotherms have been determined in conditions far from equilibrium. Dilational moduli, measured at several surface concentrations, range from 13.5 to 72 mN m −1 . These special surface properties should determine a part of the behavior of lignins in blends.

Absorbance at 280 nm was used to determine lignin concentrations in solutions of cell walls from winter wheat whole-crop silage, stems, and straw. Three different cell wall preparations (hot water−organic solvent fiber, ethanol−benzene residue, neutral detergent fiber) were prepared and dissolved in either HCl−triethylene glycol or acetyl bromide. Three phenolic standard solutions were used to calculate lignin concentrations in the wheat materials from absorbance readings: mixtures of phenolic monomers based on aldehydes (HIGUCHI), aldehydes and acids (REEVES), and washed Indulin (WIND). Gravimetric Klason lignin determination on the cell wall preparations consistently gave higher values than spectrophotometric lignin analysis. Material recovered as cell wall was slightly greater for hot water−organic solvent fiber than for neutral detergent fiber and was lowest for ethanol−benzene residue, probably due to partial dissolution of cell wall components with the latter procedure. Acetyl bromide shifted UV sp...

The chemical structure of lignins in Buxus sempervirens wood samples showing a progressive grading from normal wood to reaction wood was investigated by thioacidolysis. The mechanical states of the various samples were assessed quantitatively by measuring the growth strains at the stem surface. Sample woods with the more pronounced compression character showed the highest lignin contents. Buxus lignins are composed of guaiacyl and syringyl units, similar to more evolved angiosperm lignins. The proportion of syringyl units, together with that of labile inter-unit ether bonds, was, however, lower in boxwood lignins than in poplar or birch lignins. More importantly, lignins in boxwood samples with a pronounced compressed character were found to have structural similarities with gymnosperm compression wood lignins, i.e. a higher content of carbon-carbon inter-unit bonds and of p-hydroxyphenyl units, compared with normal wood lignins.

Summary Lignin is a polymeric constituent of the cell wall that needs to be removed during the paper making process. Bi-specific caffeic acid/5-hydroxyferulic acid O-methyltransferase (COMT) catalyses the O-methylation of caffeic acid and 5-hydroxyferulic acid to ferulic acid and sinapic acid, respectively. These compounds are intermediates in the biosynthesis of the lignin precursors. Therefore, COMTs are potential target enzymes for reducing the amount, or modifying the composition, of lignin in plants. Different antisense and sense constructs have been expressed of a gene encoding a COMT from poplar (Populus trichocarpa x P. deltoides) in a P. tremula x P. alba clone under the control of the cauliflower mosaic virus 35S promoter. From all analysed transformants, four lines transformed with an antisense construct had a reduced COMT activity. Two showed a 50% reduction of COMT activity, which altered only slightly the monomeric composition. In the two other transformants, the COMT activity was reduced by 95%. In the latter case, the syringyl/ guaiacyl ratio (S/G) was reduced by sixfold (due to a decrease of S and an increase of G), as analysed by thioacidolysis. A new component of lignin, the 5-hydroxyguaiacyl residue, was detected among the thioacidolysis products. Moreover, in contrast to the white/yellow colour of wild-type wood, the xylem of the transgenic lines with a 95% reduction of COMT activity was pale rose. A similar phenotype was observed in brown-midrib mutants of maize and sorghum, known for their altered lignification. Although the lignin composition was consistently modified, the lignin content of the transgenic poplars was similar to that of the controls.

The molecular structure of lignins (occurrence of condensed and non-condensed monomeric units and associations with phenolic acids by ether and ester linkages) have been reviewed in this paper. Critical aspects of chemical determination of lignin content and monomeric composition have been discussed in the case of nitrobenzene oxidation and thioacidolysis. As an example of assessment of lignin biodegradation by peroxidases, lignin peroxidase and horseradish peroxidase have been compared, indicating that horseradish peroxidase is a ligninolytic enzyme, confirming the influence of the physical state of lignins on their biodegradability and the general difficulty to characterize biomacromolecules.

Formation of standard guaiacyl-syringyl lignin has been shown, by thioacidolysis, during healing and suberization of wound potato tuber discs. Recovery of p -hydroxybenzaldehyde after nitrobenzene oxidation has been related to the presence of tyramine. Variations in the relative proportion of hydroxyphenyl, guaiacyl and syringyl degradation products obtained after nitrobenzene oxidation, thioacidolysis and alkaline hydrolysis suggest the formation, during suberization, of a complex phenolic network linked to the cell wall.

The rate of oxidation of spruce milled wood lignin by Phanerochaete chrysosporium was investigated with precipitated lignin as well as lignin finely dispersed in water (colloidal lignin). H2O2 consumption rates in the course of lignin oxidation in the presence of purified lignin peroxidase were much higher with colloidal lignin compared with precipitated lignin. Characterization of reaction products and molecular-size distribution confirms that reaction rate of lignin peroxidase with 14C-lignin is strongly dependent on the physical state of lignin. The lignin peroxidase catalyse mainly repolymerization of colloidal lignin. However, in the presence of veratryl alcohol, both polymerization and depolymerization were observed.

Lignocellulosic fractions from wheat straw were used as natural fillers in composites of a polyolefin (a copolymer of polyethylene and polypropylene) and a biodegradable polyester [poly(butylene adipate-co-terephthalate)]. The mechanical properties of these injected composites were investigated with tensile and impact testing. A reinforcing effect of wheat-straw residues was found for both types of composites. Compared with the polyester-based composites, the polyolefin composites were more brittle. The addition of compatibilizing agents (γ-methacryloxypropyltrimethoxysilane, maleic anhydride modified polypropylene, and stearic acid) did not improve the properties of the polyolefin composites. The surface properties were studied with contact-angle measurements, and poor interfacial adhesion was found between the hydrophilic lignocellulosic filler and the hydrophobic polyolefin matrix. Thermal characterization revealed the formation of low intermolecular bonds between the polyester matrix and the lignocellulosic filler, in agreement with the surface tensions results and scanning electron microscopy observations. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 428–436, 2004

The Arabidopsis thaliana sam1 gene encoding S-adenosylmethionine synthetase (EC 2.5.1.6) was transferred to flax (Linum usitatissimum) cells via Agrobacterium tumefaciens. This enzyme catalyses the conversion of methionine to S-adenosylmethionine (SAM), the major methyl group donor in living cells. The aim of this work was to study the consequences of an increased SAM-synthetase (SAM-S) activity in transgenic cell lines on both the production of mono- and dimethoxylated lignin monomers and the degree of methylesterification of pectins. Hypocotyls were cocultivated with Agrobacterium tumefaciens strain GV3101 (pGV2260) harbouring the pO35SSAM binary vector carrying the sam1 gene under the control of the 35S promoter and the nptII gene for selection of putative transformed cells. Most of the transgenic cell lines exhibited a significant (up to 3.2-fold) increase in SAM-S activity compared to the controls. The results showed that for the cell lines analysed this transformation had no effect on caffeic acid O-methyltransferase (COMT, EC 2.1.1.68) in vitro activity, degree of methoxylation of lignin precursors or lignin deposition, pectin methyltransferase (PMT, EC 2.1.1) in vitro activity, but led to an increase of pectin methylesterification in friable and fast-growing transgenic cell lines.

Introduction Historical Outline Occurrence in the Plant Kingdom According to Type of Plant According to Plant Tissue Contribution to Dry Matter Annual Global Synthesis Ultrastructure of Wood and Lignocellulose Role of Lignin in the Biopolymer Matrix Interactions with Other Cell-wall Polymers Functions of Lignin Stability of Plant Tissues Protection against Biodegradation Regulation of Water Transport Other Functions Related to Plant Evolution and Systematics Precursors of Biosynthesis Biosynthesis Transport Established Pathways Radical Polymerization Labeling Experiments Recent Findings, Hypotheses, and Controversies Regulation of Lignification Expression of Enzymes and Genes Effects of Genetic Manipulation Patents Keywords: algae; angiosperm; biodegradation; biomechanic; plant cell wall; cellulose; cryptogram; plant evolution; fern; grass; gymnosperm; hemicellulose; humification; lignins; lignin–polysaccharide complex; monocotyledon; moss; phenolic acids; plant systematic; pectin; structural protein; suberin; water transport; wood

International audience; Coniferyl alcohol was polymerised in the presence of pectin and a pectin-DHP complex was isolated. Characterisation of the complex has demonstrated that DHP (20% wt) was covalently linked by ester bonds to the pectin. The water sorption behaviour of the pectin-DHP complex was determined at several relative vapour pressures and compared with those obtained on pectin, DHP and a blend of both polymers in the same proportion as in the complex. The complex exhibited a lower hydrophilicity resulting from three associated phenomena: grafting, pectin-DHP interactions and the formation of a chemical network.

In addition to morphological factors (height of the plant, proportion of stems), alfalfa quality is related to several biochemical factors, especially the content and monomeric composition of the cell wall components. This work was aimed at studying internode development in relation to the chemical composition and fractionation of cell walls. Analyses were performed on the fourth apical internodes (elongating), the two following ones (onset of cambial activity), and bottom internodes (mature). Deposition of guaiacyl type lignin occurred in the pectin rich cell walls of apical internodes. The onset of cambial activity corresponded to cell wall accumulation of syringyl-guaiacyl lignin, xylans and/or glucuronoxylans, and cellulose. Such events were related to the thickening of secondary cell walls which proportion increased in mature internodes. Chemical fractionation of the cell walls allowed the release of a high content of water soluble pectins from apical internodes, whereas alkalis were more efficient in extracting heteroxylans from bottom internodes. A partial release of lignin from the cell walls also occurred during the extraction steps. Lignin solubilization was enhanced by the cleavage of labile-ether lignin structures and appeared closely related to the extent of internode differentiation along the stem. Data gained from chemical fractionations evidenced that lignin-xylan-pectic complexes accumulated progressively in secondary wall rich tissues.

Peroxidases and laccases are key enzymes in the lignin biodegradation process. They oxidize phenolic and non-phenolic lignin model compounds into their phenoxy and cation radicals, respectively. Further non-enzymatic evolution lead then to various C-C and ether bond cleavages. Nevertheless, almost no information on the structural alterations undergone in vitro or in situ by lignin after enzymatic catalysis is available. We report here on the molecular structure of lignin oxidized by various (per)oxidasic systems. The oxidizability of phenolic and non-phenolic structures of the guaiacyl and syringyl type in the lignin network will be discussed as well as the modification of the macromolecular properties of the polymer oxidized in situ or in isolated state.

In the present study, we have performed chemical investigations of the stem cell walls during internode maturation in order to study the growth dynamics of alfalfa and the deposition of the main cell wall components (polysaccharides and lignins). Internode cell walls were analysed by chemical fractionation using a mild delignification step aiming at sequential removal of polysaccharides and lignins. Delignification facilitated the subsequent removal of the xylose-rich polysaccharides by NaOH extraction as previously shown. This trend was more pronounced in the case of older internodes which have a larger proportion of secondary tissues containing syringyl type lignins in contrast to younger ones which are mainly composed of primary tissues containing guaiacyl type lignins and pectin rich cell walls.

Cinnamyl alcohol dehydrogenase (CAD) catalyzes the last step in the biosynthesis of the lignin precursors, the monolignols. We have down-regulated CAD in transgenic poplar (Populus tremula X Populus alba) by both antisense and co-suppression strategies. Several antisense and sense CAD transgenic poplars had an approximately 70% reduced CAD activity that was associated with a red coloration of the xylem tissue. Neither the lignin amount nor the lignin monomeric composition (syringyl/guaiacyl) were significantly modified. However, phloroglucinol-HCl staining was different in the down-regulated CAD plants, suggesting changes in the number of aldehyde units in the lignin. Furthermore, the reactivity of the cell wall toward alkali treatment was altered: a lower amount of lignin was found in the insoluble, saponified residue and more lignin could be precipitated from the soluble alkali fraction. Moreover, large amounts of phenolic compounds, vanillin and especially syringaldehyde, were detected in the soluble alkali fraction of the CAD down-regulated poplars. Alkaline pulping experiments on 3-month-old trees showed a reduction of the kappa number without affecting the degree of cellulose degradation. These results indicate that reducing the CAD activity in trees might be a valuable strategy to optimize certain processes of the wood industry, especially those of the pulp and paper industry.

Stem bark and pith of sweet sorghum were analysed with reference to their sucrose, simple reducing sugars, cellulose, hemicelluloses, lignin and associated phenolic acids contents. Moreover, lignin monomeric units (guaiacyl and syringyl) engaged in non-condensed structures were characterized by thioacidolysis, whereas cell wall associated phenolic acids (p-coumaric and ferulic acids) were estimated by alkaline hydrolysis at 170°C. The results obtained showed that bark and pith are heterogeneous as far as their chemical composition and the structure of their chemical components are concerned. In particular, the pith content in water soluble sugars is twice as high compared to the one in the bark, whereas bark is enriched in lignocellulosic fibres. Bark lignin is twice more important in content and less condensed in structure compared to pith lignin. P-coumaric acid is the predominant p-hydroxycinnamic acid associated to the cell walls, whereas ferulic acid is present in significant quantities.

The removal of lignin in paper manufacture is a toxic and energy-requiring process. For the paper industry and for the environment, it would be very beneficial to process trees with less lignin or with lignin that is more easily ex tractable. To achieve this goal, we are down-regulating the activity of a number of lignin biosynthesis enzymes in poplar by genetic engineering and studying their role in determining the quantity and quality of lignin. Here, our recent results obtained in this field are summarized with emphasis on bispecific caffeic acid/5-OH-ferulic acid-O-methyltransferase, cinnamoyl-CoA-reductase, caffeoyl-CoA-O-methyltransferase, and peroxidase.

Cinnamyl alcohol dehydrogenase (CAD) catalyses the last step in the biosynthesis of the lignin precursors, which is the conversion of cinnamaldehydes into cinnamyl alcohols. A poplar (P. trichocarpa x P. deltoides)CAD cDNA was introduced in sense and in antisense orientation into P. tremula x P. alba via an Agrobacterium- mediated transformation. The CAD activity was reduced by 70% in the xylem tissue of three antisense and two co-suppressed lines. A red coloration of the xylem was detected in all poplar lines with a residual CAD activity of less than 60 %. The lignin content and the lignin composition (syringyl/guaiacyl) were similar in co-suppressed, antisense CAD and control lines. However, chemical properties of the cell wall were different. First, a phloroglucinol staining showed a different coloration in the xylem of the down-regulated CAD poplars, being red-brown as compared to the typical pink staining of the control xylem. Second, after NaOH treatment, lignin could be precipitated by acidification of the alkali-soluble fraction of the cell wall of the red poplars but not of that of the white poplars. A spectral characterisation of the alkali fraction from red poplars showed an increased level of vanillin and syringaldehyde. Third, pulping experiments showed less residual lignin in the pulp of CAD down-regulated poplars.

Oxidation of spruce milled-wood lignins in colloidal state catalyzed by horseradish peroxidase (HRP) and lignin peroxidase (LiP) in the presence of H 2 O 2 was compared in nonbuffered dimethyl-formamide/water. Lignins were characterized for their hydrodynamic properties by size-exclusion chromatography and structural bonding pattern by analysis after thioacidolysis. In contrast to LiP, HRP did not induce modification of lignin hydrodynamic properties. The lignin content in β-O-4-linked guaiacyl monomers and dimeric structures, however, decreased after oxidation by the two enzymes, indicating large structural changes in the polymer. Preferential degradation by both LiP and HRP of β-5 and β-1 lignin dimeric units was also observed. This suggests a greater susceptibility to enzyme oxidation, as compared to the 5-5′ and 4-O-5 lignin substructures. In conclusion, the action of HRP on the lignin polymer is similar in many respects to that of fungal LiP, but is distinctly different in its inability to cause a net destructuring of the macromolecular three-dimensional network.

Cell wall-linked phenolics were investigated in maize internodes located at three positions of the stem (top-middle-bottom). While the lignin content did not change drastically with position, the amount of cell wall-ester linked p-coumaric acid sharply increased from the top to the bottom internodes of the stem. Conversely, the saponified ferulic acid content remained relatively unchanged along the stem. Moreover, the highest syringyl content of the β-O-4-lignin structures was found in the basal (most mature) internode. Therefore, enhanced p-coumaric esterification of the cell wall and preferential deposition of syringyl units in the lignin polymer might indicate an extended maturity stage of the cell wall of maize internodes. The bm3 mutation in the three maize hybrids is expressed by lignin reduction and ester-bound p-coumaric decrease in the mutant lines. Furthermore, all bm3 hybrids synthesised lignin polymers which were characterised by a very low S/G molar ratio (0.16-0.43). This ratio originates from the substantial reduction of the syringyl unit content in β-O-4-lignin structures compared with the normal lignin. The occurrence in the same range of the 5-hydroxyguaiacyl unit in bm3 lignin from each type of hybrids was noteworthy, demonstrating the high heritability of the bm3 mutation at the molecular level. The alkaline solubility of lignin was greater for the three mutant lines compared to the normal cultivars. Furthermore, the alkali-labile fraction of lignin of both normal and mutant lines had a monomeric composition which was consistent with the non-condensed structures of in-situ lignin.

The hypothesis that auxin (IAA) and gibberellic acid (GA(3)) control the formation of lignin is confirmed for the primary phloem fibers and for the secondary xylem in the stem of Coleus blumel Benth. Indoleacetic acid alone, or a combination of high IAA/low GA(3) (w/w), induced short phloem fibers with thick secondary walls, that contained lignin rich in syringyl units (high ratio of syringyl/guaiacyl). On the other hand, a combination of high GA(3)/low IAA (w/w), which promoted the differentiation of long phloem fibers with thin walls, decreased the relative content of the syringyl units (low syringyl/guaiacyl ratio). In the secondary xylem, these hormonal treatments yielded only slight changes in the noncondensed monomeric guaiacyl units, confirming the relative stability of the guaiacyl lignification pattern in this tissue. In the xylem, indoleacetic acid alone, or a combination of high IAA/low GA(3) induced lignin poor in syringyl units (low syringyl/guaiacyl ratio). A combination of high GA(3)/low IAA promoted a relatively slight increase in syringyl yield, indicating greater responsiveness of the syringyl lignification pattern to growth regulators. The possible functional and technological significance of our results is discussed.

Extractive-free wheat straw, and the corresponding samples issued from its alkaline treatment — i.e. saponified wheat straw and alkali-soluble wheat lignin- were submitted to three chemical procedures of lignin characterization: nitrobenzene oxidation, acidolysis and thioacidolysis. The various performances of the three techniques were comparatively evaluated. The thioacidolysis procedure was found to have higher performances with regards to the two following criteria: the prominent alkyl aryl ether lignin structures were the almost exclusive source for the recovered degradation products; these products were obtained with a high yield. These advantages make the thioacidolysis procedure an unambiguous way to characterize grass lignins, even when present in low amounts.

Un protocole de dosage de la lignine insoluble dans l’acide sulfurique concentré, lignine Klason, est décrit : il comporte une étape de préhydrolyse par l’acide sulfurique 5 % et un lavage, en milieu chlorhydrique concentré, du résidu de préhydrolyse. Ce lavage permet d’améliorer la reproductibilité du dosage de la lignine après préhydrolyse et d’atteindre une reproductibilité comparable à celle obtenue par le dosage Klason standard. L’analyse élémentaire des préparations de lignines isolées au cours de ces dosages indique des variations complexes qui dépendent de la nature du produit analysé., A procedure for estimating lignin insoluble in concentrated acids, Klason lignin, is described ; it includes a 5 % sulfuric acid prehydrolysis step and washing in concentrated hydrochloric acid. This procedure allowed better reproducibility than without washing, of the same order as that given by the standard Klason procedure. Elemental analysis showed complex changes depending on the plant analysed.

To mimic the lignin polymerization process, mats of bacterial cellulose and of a pectin/cellulose composite were used as a host matrix for in vitro polymerization of coniferyl alcohol. A diffusion cell was used to allow the diffusion of both hydrogen peroxide and coniferyl alcohol into the peroxidase impregnated cellulose mats through dialysis membranes. The results indicate that significant polymerization occurs within the mats. The resulting binary and ternary blends were imaged by scanning electron microscopy (SEM) and characterized by chemical means. The presence of pectin induces a better dispersion of the synthetic lignin in the cellulose network and enhances the proportion of alkyl-aryl-ether in the polymer.

The cell wall phenolic components in the internodes of three maize genotypes, namely normal, bm2 and bm3 maize, were determined. The bm2 and bm3 brown midrib mutations lowered the lignin content of the bottom, middle and top internodes to a similar extent. However, unlike bm3, the bm2 trait did not induce a sharp reduction of the level of ester-bound p-coumaric acid in maize internodes. The other main alkali labile phenolic acid, ferulic acid, reached similar levels in the three genotypes. The main difference between bm2 and bm3 mutations occurred in the alkyl aryl ether linked structures of the lignin component. In contrast to bm3 lignins, which are characterised by a low syringyl content, the bm2 lignin had a lower content of guaiacyl units than lignin of normal maize internode. Consequently, the syringyl/guaiacyl molar ratio of bm2 lignin gave higher values (2.7–3.2) than those from either normal (0.9–1.5) or bm3 lignins (0.3). The alkali solubility of lignin was also compared between the three genotypes. Incorporation of the bm3 trait in maize led to a high recovery of alkali soluble lignin whereas the bm2 lignin had a similar solubility to the normal one in 2 M NaOH. The monomeric composition of the alkali soluble lignins was consistent with the non-condensed structures of the in-situ polymer. Although the bm3 and bm2 mutations had different effects on lignification, the modification of the cell wall phenolic level was also found in the bm2 maize stem as previously studied.

For the production of high-quality paper, lignin needs to be removed from cellulose. This process is energy requiring, expensive and toxic. Molecular biology offers tools to generate trees with a modified lignin composition to facilitate the extractability of lignin. We have cloned several genes encoding enzymes in the methylation of the lignin monomers [bi-specific caffeic acid/5-hydroxyferulic acid O -methyltransferase (COMT) and caffeoyl-CoA O -methyltransferase], and encoding enzymes specific for the lignin biosynthesis pathway [cinnamoyl-CoA reductase and cinnamyl alcohol dehydrogenase (CAD)]. The antisense strategy is used to study the effect of a down-regulation of these enzymes on the lignin content and the lignin composition of poplar wood. Our results demonstrate that the monomeric composition of lignin was dramatically affected by down-regulation of COMT. Transgenic poplars with a reduced CAD activity have better pulping properties, due to a higher extractability of the lignin.