Aggregation during coniferyl alcohol polymerization in pectin solution: a biomimetic approach of the first steps of lignification

Pectin modifications promote haustoria development in the parasitic plant Phtheirospermum japonicum

Parasitic plants are globally prevalent pathogens with important ecological functions but also potentially devastating agricultural consequences. Common to all parasites is the formation of the haustorium which requires parasite organ development and tissue invasion into the host. Both processes involve cell wall modifications. Here, we investigated a role for pectins during haustorium development in the facultative parasitic plant Phtheirospermum japonicum. Using transcriptomics data from infected Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), we identified genes for multiple P. japonicum pectin methylesterases (PMEs) and their inhibitors (PMEIs) whose expression was upregulated by haustoria formation. Changes in PME and PMEI expression were associated with tissue-specific modifications in pectin methylesterification. While de-methylesterified pectins were present in outer haustorial cells, highly methylesterified pectins were present in inner vascular tissues, including the xylem bridge that connects parasite to host. Specifically blocking xylem bridge formation in the haustoria inhibited several PME and PMEI genes from activating. Similarly, inhibiting PME activity using chemicals or by overexpressing PMEI genes delayed haustoria development. Our results suggest a dynamic and tissue-specific regulation of pectin contributes to haustoria initiation and to the establishment of xylem connections between parasite and host.

The calcium-mediated homogalacturonan pectin complexation in cell walls contributes the firmness increase in loquat fruit during postharvest storage

INTRODUCTION

Postharvest textural changes in fruit are mainly divided into softening and lignification. Loquat fruit could have severe lignification with increased firmness during postharvest storage. Pectin is mainly associated with the postharvest softening of fruit, but some studies also found that pectin could be involved in strengthening the mechanical properties of the plant.

OBJECTIVES

This study focused on characterizing the dynamics of pectin and its complexation in the cell wall of lignified loquat fruit during postharvest storage, and how these changes could influence fruit firmness.

METHODS

The homogalacturonan (HG) pectin in the cell wall of loquat fruit was identified using monoclonal antibodies. An oligogalacturonide (OG) probe was used to label the egg-box structure formed by Ca²⁺ cross-linking with low-methylesterified HG. An exogenous injection was used to verify the role of egg-box structures in the firmness increase in loquat fruit.

RESULTS

The JIM5 antibody revealed that low-methylesterified HG accumulated in the tricellular junctions and middle lamella of loquat fruit that had severe lignification symptoms. The pectin methylesterase (PME) activity increased during the early stages of storage at 0°C, and the calcium-pectate content and flesh firmness constantly increased during storage. The OG probe demonstrated the accumulation of egg-box structures at the cellular level. The exogenous injection of PME and Ca²⁺ into the loquat flesh led to an increase in firmness with more low-methylesterified HG and egg-box structure signals.

CONCLUSION

PME-mediated demethylesterification generated large amounts of low-methylesterified HG in the cell wall. This low-methylesterified HG further cross-linked with Ca²⁺ to form egg-box structures. The pectin-involved complexations then contributed to the increased firmness in loquat fruit. Overall, besides being involved in fruit softening, pectin could also be involved in strengthening the mechanical properties of postharvest fruit. This study provides new ideas for obtaining a better texture of postharvest loquat fruits based on pectin regulation.

Dynamics of pectic homogalacturonan in cellular morphogenesis and adhesion, wall integrity sensing and plant development

Homogalacturonan (HG) is the most abundant pectin subtype in plant cell walls. Although it is a linear homopolymer, its modification states allow for complex molecular encoding. HG metabolism affects its structure, chemical properties, mobility and binding capacity, allowing it to interact dynamically with other polymers during wall assembly and remodelling and to facilitate anisotropic cell growth, cell adhesion and separation, and organ morphogenesis. HGs have also recently been found to function as signalling molecules that transmit information about wall integrity to the cell. Here we highlight recent advances in our understanding of the dual functions of HG as a dynamic structural component of the cell wall and an initiator of intrinsic and environmental signalling. We also predict how HG might interconnect the cell wall, plasma membrane and intracellular components with transcriptional networks to regulate plant growth and development.

Characterization of the Transcriptome and Proteome of Brassica napus Reveals the Close Relation between DW871 Dwarfing Phenotype and Stalk Tissue

Rapeseed is a significant oil-bearing cash crop. As a hybrid crop, Brassica napus L. produces a high yield, but it also has drawbacks such as a tall stalk, easy lodging, and is not suitable for mechanized production. To address these concerns, we created the DW871 rapeseed dwarf variety, which has a high yield, high oil content, and is suitable for mechanized production. To fully comprehend the dwarfing mechanism of DW871 and provide a theoretical foundation for future applications of the variety, we used transcriptome and proteome sequencing to identify genes and proteins associated with the dwarfing phenotype, using homologous high-stalk material HW871 as a control. By RNA-seq and iTRAQ, we discovered 8665 DEGs and 50 DAPs. Comprehensive transcription and translation level analysis revealed 25 correlations, 23 of which have the same expression trend, involving monolignin synthesis, pectin-lignin assembly, lignification, glucose modification, cell wall composition and architecture, cell morphology, vascular bundle development, and stalk tissue composition and architecture. As a result of these results, we can formulate a hypothesis about the DW871 dwarfing phenotype: plant hormone signal transduction, such as IAA and BRs, is linked to the formation of dwarf phenotypes, and metabolic pathways related to lignin synthesis, such as phenylpropane biosynthesis, also play a role. Our works will contribute to a better understanding of the genes and proteins involved in the rapeseed dwarf phenotype, and we will propose new insights into the dwarfing mechanism of Brassica napus L.

Synthesis of deuterium-labeled cinnamic acids: Understanding the volatile benzenoid pathway in the flowers of the Japanese loquat Eriobotrya japonica

Cinnamic acids are widely distributed in plants, including crops for human use, and exhibit a variety of activities that are beneficial to human health. They also occupy a pivotal position in the biosynthesis of phenylpropanoids such as lignins, anthocyanins, flavonoids, and coumarins. In this context, deuterium-labeled cinnamic acids have been used as tracers and internal standards in food and medicinal chemistry as well as plant biochemistry. Therefore, a concise synthesis of deuterium-labeled cinnamic acids would be highly desirable. In this study, we synthesized deuterium-labeled cinnamic acids using readily available deuterium sources. We also investigated a hydrogen-deuterium exchange reaction in an ethanol-d₁ /Et₃ N system. This method can introduce deuterium atoms at the ortho and para positions of the phenolic hydroxy groups as well as at the C-2 position of alkyl cinnamates and is applicable to various phenolic compounds. Using the synthesized labeled compounds, we demonstrated that the benzenoid volatiles, such as 4-methoxybenzaldehyde, in the scent of the flowers of the Japanese loquat Eriobotrya japonica are biosynthesized from phenylalanine via cinnamic and 4-coumaric acids. This study provides easy access to a variety of deuterium-labeled (poly)phenols, as well as to useful tools for studies of the metabolism of cinnamic acids in living systems.

ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1) releases latent defense signals in stems with reduced lignin content

There is considerable interest in engineering plant cell wall components, particularly lignin, to improve forage quality and biomass properties for processing to fuels and bioproducts. However, modifying lignin content and/or composition in transgenic plants through down-regulation of lignin biosynthetic enzymes can induce expression of defense response genes in the absence of biotic or abiotic stress. Arabidopsis thaliana lines with altered lignin through down-regulation of hydroxycinnamoyl CoA:shikimate/quinate hydroxycinnamoyl transferase (HCT) or loss of function of cinnamoyl CoA reductase 1 (CCR1) express a suite of pathogenesis-related (PR) protein genes. The plants also exhibit extensive cell wall remodeling associated with induction of multiple cell wall-degrading enzymes, a process which renders the corresponding biomass a substrate for growth of the cellulolytic thermophile Caldicellulosiruptor bescii lacking a functional pectinase gene cluster. The cell wall remodeling also results in the release of size- and charge-heterogeneous pectic oligosaccharide elicitors of PR gene expression. Genetic analysis shows that both in planta PR gene expression and release of elicitors are the result of ectopic expression in xylem of the gene ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1), which is normally expressed during anther and silique dehiscence. These data highlight the importance of pectin in cell wall integrity and the value of lignin modification as a tool to interrogate the informational content of plant cell walls.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.

Lignin-polysaccharide interactions in plant secondary cell walls revealed by solid-state NMR

Lignin is a complex aromatic biopolymer that strengthens and waterproofs plant secondary cell walls, enabling mechanical stability in trees and long-distance water transport in xylem. Lignin removal is a key step in paper production and biomass conversion to biofuels, motivating efforts to re-engineer lignin biosynthesis. However, the physical nature of lignin's interactions with wall polysaccharides is not well understood. Here we show that lignin self-aggregates to form highly hydrophobic and dynamically unique nanodomains, with extensive surface contacts to xylan. Solid-state NMR spectroscopy of intact maize stems, supported by dynamic nuclear polarization, reveals that lignin has abundant electrostatic interactions with the polar motifs of xylan. Lignin preferentially binds xylans with 3-fold or distorted 2-fold helical screw conformations, indicative of xylans not closely associated with cellulose. These findings advance our knowledge of the molecular-level organization of lignocellulosic biomass, providing the structural foundation for optimization of post-harvest processing for biofuels and biomaterials.

Genetic engineering of grass cell wall polysaccharides for biorefining

Grasses represent an abundant and widespread source of lignocellulosic biomass, which has yet to fulfil its potential as a feedstock for biorefining into renewable and sustainable biofuels and commodity chemicals. The inherent recalcitrance of lignocellulosic materials to deconstruction is the most crucial limitation for the commercial viability and economic feasibility of biomass biorefining. Over the last decade, the targeted genetic engineering of grasses has become more proficient, enabling rational approaches to modify lignocellulose with the aim of making it more amenable to bioconversion. In this review, we provide an overview of transgenic strategies and targets to tailor grass cell wall polysaccharides for biorefining applications. The bioengineering efforts and opportunities summarized here rely primarily on (A) reprogramming gene regulatory networks responsible for the biosynthesis of lignocellulose, (B) remodelling the chemical structure and substitution patterns of cell wall polysaccharides and (C) expressing lignocellulose degrading and/or modifying enzymes in planta. It is anticipated that outputs from the rational engineering of grass cell wall polysaccharides by such strategies could help in realizing an economically sustainable, grass-derived lignocellulose processing industry.

Characterization and Functional Analysis of the Poplar Pectate Lyase-Like Gene PtPL1-18 Reveal Its Role in the Development of Vascular Tissues

Pectin is a major component of plant cell walls, and the structure of pectin impacts on the properties of wood. Although we know that pectate lyase (PL, EC 4.2.2.2) has a major influence on the structure of pectin, our knowledge of Pectate lyase-like genes (PLL) in tree species remains limited. To better understand the characteristics of PLL genes in trees and to identify novel PLL genes that are potentially involved in the development of wood, we performed comprehensive analyses of gene structures, phylogenetic relationships, chromosomal locations, gene duplication events, conserved protein motifs, and gene expression patterns of 30 PLLs in Populus trichocarpa (PtPL1s). We performed an in silico gene expression profiling and quantitative real-time PCR analysis and found that most of the PtPL1 genes from subgroups Ia and Ib were highly expressed in xylem. PtPL1-18 from subgroup Ia was preferentially expressed in developing primary xylem and in xylem cells that were developing secondary walls. Overexpression of PtPL1-18 in poplar reduced plant growth and xylem development. Reduced secondary cell wall thickening and irregular xylem cells were observed in the transgenic trees, probably due to their lower pectin content. Although pectin is not a major component of plant secondary cell walls, our results are consistent with the PtPL1 genes performing important functions during wood formation.

Activation tagging of Arabidopsis POLYGALACTURONASE INVOLVED IN EXPANSION2 promotes hypocotyl elongation, leaf expansion, stem lignification, mechanical stiffening, and lodging

Pectin is the most abundant component of primary cell walls in eudicot plants. The modification and degradation of pectin affects multiple processes during plant development, including cell expansion, organ initiation, and cell separation. However, the extent to which pectin degradation by polygalacturonases affects stem development and secondary wall formation remains unclear. Using an activation tag screen, we identified a transgenic Arabidopsis thaliana line with longer etiolated hypocotyls, which overexpresses a gene encoding a polygalacturonase. We designated this gene as POLYGALACTURONASE INVOLVED IN EXPANSION2 (PGX2), and the corresponding activation tagged line as PGX2^AT . PGX2 is widely expressed in young seedlings and in roots, stems, leaves, flowers, and siliques of adult plants. PGX2-GFP localizes to the cell wall, and PGX2^AT plants show higher total polygalacturonase activity and smaller pectin molecular masses than wild-type controls, supporting a function for this protein in apoplastic pectin degradation. A heterologously expressed, truncated version of PGX2 also displays polygalacturonase activity in vitro. Like previously identified PGX1^AT plants, PGX2^AT plants have longer hypocotyls and larger rosette leaves, but they also uniquely display early flowering, earlier stem lignification, and lodging stems with enhanced mechanical stiffness that is possibly due to decreased stem thickness. Together, these results indicate that PGX2 both functions in cell expansion and influences secondary wall formation, providing a possible link between these two developmental processes.

Rice pectin methylesterase inhibitor28 (OsPMEI28) encodes a functional PMEI and its overexpression results in a dwarf phenotype through increased pectin methylesterification levels

Pectin methylesterases (PMEs, EC 3.1.1.11) belonging to carbohydrate esterase family 8 cleave the ester bond between a galacturonic acid and an methyl group and the resulting change in methylesterification level plays an important role during the growth and development of plants. Optimal pectin methylesterification status in each cell type is determined by the balance between PME activity and post-translational PME inhibition by PME inhibitors (PMEIs). Rice contains 49 PMEIs and none of them are functionally characterized. Genomic sequence analysis led to the identification of rice PMEI28 (OsPMEI28). Recombinant OsPMEI28 exhibited inhibitory activity against commercial PME protein with the highest activities detected at pH 8.5. Overexpression of OsPMEI28 in rice resulted in an increased level of cell wall bound methylester groups and differential changes in the composition of cell wall neutral monosaccharides and lignin content in culm tissues. Consequently, transgenic plants overexpressing OsPMEI28 exhibited dwarf phenotypes and reduced culm diameter. Our data indicate that OsPMEI28 functions as a critical structural modulator by regulating the degree of pectin methylesterification and that an impaired status of pectin methylesterification affects physiochemical properties of the cell wall components and causes abnormal cell extensibility in rice culm tissues.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Homogalacturonan-modifying enzymes: structure, expression, and roles in plants

Understanding the changes affecting the plant cell wall is a key element in addressing its functional role in plant growth and in the response to stress. Pectins, which are the main constituents of the primary cell wall in dicot species, play a central role in the control of cellular adhesion and thereby of the rheological properties of the wall. This is likely to be a major determinant of plant growth. How the discrete changes in pectin structure are mediated is thus a key issue in our understanding of plant development and plant responses to changes in the environment. In particular, understanding the remodelling of homogalacturonan (HG), the most abundant pectic polymer, by specific enzymes is a current challenge in addressing its fundamental role. HG, a polymer that can be methylesterified or acetylated, can be modified by HGMEs (HG-modifying enzymes) which all belong to large multigenic families in all species sequenced to date. In particular, both the degrees of substitution (methylesterification and/or acetylation) and polymerization can be controlled by specific enzymes such as pectin methylesterases (PMEs), pectin acetylesterases (PAEs), polygalacturonases (PGs), or pectate lyases-like (PLLs). Major advances in the biochemical and functional characterization of these enzymes have been made over the last 10 years. This review aims to provide a comprehensive, up to date summary of the recent data concerning the structure, regulation, and function of these fascinating enzymes in plant development and in response to biotic stresses.

Effect of ionic liquid treatment on the structures of lignins in solutions: molecular subunits released from lignin

The solution structures of three types of isolated lignin--organosolv (OS), Kraft (K), and low sulfonate (LS)--before and after treatment with 1-ethyl-3-methylimidazolium acetate were studied using small-angle neutron scattering (SANS) and dynamic light scattering (DLS) over a concentration range of 0.3-2.4 wt %. The results indicate that each of these lignins is comprised of aggregates of well-defined basal subunits, the shapes and sizes of which, in D(2)O and DMSO-d(6), are revealed using these techniques. LS lignin contains a substantial amount of nanometer-scale individual subunits. In aqueous solution these subunits have a well-defined elongated shape described well by ellipsoidal and cylindrical models. At low concentration the subunits are highly expanded in alkaline solution, and the effect is screened with increasing concentration. OS lignin dissolved in DMSO was found to consist of a narrow distribution of aggregates with average radius 200 ± 30 nm. K lignin in DMSO consists of aggregates with a very broad size distribution. After ionic liquid (IL) treatment, LS lignin subunits in alkaline solution maintained the elongated shape but were reduced in size. IL treatment of OS and K lignins led to the release of nanometer-scale subunits with well-defined size and shape.

Localization of cell wall polysaccharides in normal and compression wood of radiata pine: relationships with lignification and microfibril orientation

The distribution of noncellulosic polysaccharides in cell walls of tracheids and xylem parenchyma cells in normal and compression wood of Pinus radiata, was examined to determine the relationships with lignification and cellulose microfibril orientation. Using fluorescence microscopy combined with immunocytochemistry, monoclonal antibodies were used to detect xyloglucan (LM15), β(1,4)-galactan (LM5), heteroxylan (LM10 and LM11), and galactoglucomannan (LM21 and LM22). Lignin and crystalline cellulose were localized on the same sections used for immunocytochemistry by autofluorescence and polarized light microscopy, respectively. Changes in the distribution of noncellulosic polysaccharides between normal and compression wood were associated with changes in lignin distribution. Increased lignification of compression wood secondary walls was associated with novel deposition of β(1,4)-galactan and with reduced amounts of xylan and mannan in the outer S2 (S2L) region of tracheids. Xylan and mannan were detected in all lignified xylem cell types (tracheids, ray tracheids, and thick-walled ray parenchyma) but were not detected in unlignified cell types (thin-walled ray parenchyma and resin canal parenchyma). Mannan was absent from the highly lignified compound middle lamella, but xylan occurred throughout the cell walls of tracheids. Using colocalization measurements, we confirmed that polysaccharides containing galactose, mannose, and xylose have consistent correlations with lignification. Low or unsubstituted xylans were localized in cell wall layers characterized by transverse cellulose microfibril orientation in both normal and compression wood tracheids. Our results support the theory that the assembly of wood cell walls, including lignification and microfibril orientation, may be mediated by changes in the amount and distribution of noncellulosic polysaccharides.

Supramolecular self-assembled chaos: polyphenolic lignin's barrier to cost-effective lignocellulosic biofuels

Phenylpropanoid metabolism yields a mixture of monolignols that undergo chaotic, non-enzymatic reactions such as free radical polymerization and spontaneous self-assembly in order to form the polyphenolic lignin which is a barrier to cost-effective lignocellulosic biofuels. Post-synthesis lignin integration into the plant cell wall is unclear, including how the hydrophobic lignin incorporates into the wall in an initially hydrophilic milieu. Self-assembly, self-organization and aggregation give rise to a complex, 3D network of lignin that displays randomly branched topology and fractal properties. Attempts at isolating lignin, analogous to archaeology, are instantly destructive and non-representative of in planta. Lack of plant ligninases or enzymes that hydrolyze specific bonds in lignin-carbohydrate complexes (LCCs) also frustrate a better grasp of lignin. Supramolecular self-assembly, nano-mechanical properties of lignin-lignin, lignin-polysaccharide interactions and association-dissociation kinetics affect biomass deconstruction and thereby cost-effective biofuels production.

Supramolecular self-assembled chaos: polyphenolic lignin's barrier to cost-effective lignocellulosic biofuels

Phenylpropanoid metabolism yields a mixture of monolignols that undergo chaotic, non-enzymatic reactions such as free radical polymerization and spontaneous self-assembly in order to form the polyphenolic lignin which is a barrier to cost-effective lignocellulosic biofuels. Post-synthesis lignin integration into the plant cell wall is unclear, including how the hydrophobic lignin incorporates into the wall in an initially hydrophilic milieu. Self-assembly, self-organization and aggregation give rise to a complex, 3D network of lignin that displays randomly branched topology and fractal properties. Attempts at isolating lignin, analogous to archaeology, are instantly destructive and non-representative of in planta. Lack of plant ligninases or enzymes that hydrolyze specific bonds in lignin-carbohydrate complexes (LCCs) also frustrate a better grasp of lignin. Supramolecular self-assembly, nano-mechanical properties of lignin-lignin, lignin-polysaccharide interactions and association-dissociation kinetics affect biomass deconstruction and thereby cost-effective biofuels production.

Elaboration of spin-coated cellulose-xyloglucan multilayered thin films

In the context of developing a biomimetic model of the primary cell wall, our aim was to produce multilayered thin films composed of cellulose nanocrystals (CN) and xyloglucan (XG). We investigated the effect of XG concentrations ranging from 0.5 g/L to 10 g/L. The choice of concentration was based on rheological investigation of the XG solutions which indicated that the two lower concentrations (0.5 and 1 g/L) correspond to a semidilute regime where the polymer chains are not entangled, whereas they are entangled at the highest concentrations (5 and 10 g/L). Several processes of film preparation were tested (dipping or spin-coating, with or without a rinsing step). The film growth profiles obtained for different XG concentrations by mechanical profilometry showed that spin-coating without rinsing was the most efficient process. Results showed that at high XG concentrations (XG = 5 g/L and XG = 10 g/L) plateau values were reached after the formation of 3 or 4 bilayers, whereas growth of the multilayer structure was linear at the lower XG concentrations (XG = 0.5 g/L and XG = 1 g/L). The thickness of one CN/XG bilayer corresponded to a single layer of CN covered by a thin XG layer, despite the absence of a rinsing step between successive coatings. The importance of the XG concentration was confirmed by determining by neutron reflectivity the film architecture obtained from four XG solutions after eight successive paired coatings. The results are discussed in relation to the role of XG in the plant cell wall.

Elaboration of extensin-pectin thin film model of primary plant cell wall

With the aim of mimicking the plant cell wall, a layer by layer approach was used to build a thin film consisting of successive adsorption of pectin and extensin. Elaboration of the thin film was monitored by surface plasmon resonance, quartz crystal microbalance, and ellipsometry. All data indicate that formation of the film was successful and that growth occurred according to a nonuniform growth. It is likely that diffusion of the polymers occurred within the multilayer structure and that the final structure is not constituted by layered individual pectin and extensin films. Polymer rearrangements were also supported by the atomic force microscopy images that show a smoother surface after extensin adsorption than after pectin deposition.

Effect of reaction media concentration on the solubility and the chemical structure of lignin model compounds

In plant cell walls, lignin polymerization occurs in concentrated polysaccharide gel. The effect of high polysaccharide concentrations on the structure of lignin-like polymers (DHPs=dehydrogenation polymers), were investigated by running lignification-like polymerization under three reaction conditions in which the concentrations of all reactants (xylan/coniferyl alcohol (CA)/oxidising system) were gradually increased. Control experiments were also run in similar conditions but without polysaccharides. DHPs showed increased solubility with increased concentrations of reactants in the presence of xylans but were mostly insoluble in buffer control experiments. The structures of DHPs were characterized by thioacidolysis and size exclusion chromatography (SEC). Results indicated that the frequency of beta-alkyl aryl ether bonds and DHP molecular weight increased with increasing concentration of the reaction mixture in the presence of xylans whereas those of control DHPs decreased slightly under the same conditions. This emphasizes the role of the pre-existing polysaccharide gel and high concentrations existing in the cell wall during construction of the lignin polymer.

Studies of xylan interactions and cross-linking to synthetic lignins formed by bulk and end-wise polymerization: a model study of lignin carbohydrate complex formation

The mechanism of lignin carbohydrate complex formation by addition of polysaccharides on quinone methide (QM) generated during lignin polymerisation was investigated using a model approach. Dehydrogenation polymers (DHPs, lignin model compounds) were synthesized from coniferyl alcohol in the presence of a glucuronoarabinoxylan (GAX) extracted from oat spelts, by Zutropfverfahren (ZT) and Zulaufverfahren (ZL) methods. The methods ZT and ZL differed in their distribution of QM over the reaction period but generated roughly the same QM amount. Steric exclusion chromatography of the ZT and ZL reaction products showed that only the ZT reaction produced high molar mass compounds. Covalent linkages in the ZT reaction involving ether bonds between GAX moiety and alpha carbon of the lignin monomer were confirmed by (13)C NMR and xylanase-based fractionation. The underlying phenomena were further investigated by examining the interactions between GAX and DHP in sorption experiments. GAX and DHPs were shown to interact to form hydrophobic aggregates. In the ZT process, slow addition permitted polymer reorganisation which led to dehydration around the lignin-like growing chains thereby limiting the addition of water on the quinone methide formed during polymerisation and thus favoured lignin-carbohydrate complex (LCC) formation.

Characterization of Arabinoxylan−Dehydrogenation Polymer (Synthetic Lignin Polymer) Nanoparticles

Coniferyl alcohol (G monomer) and a mixture of coniferyl alcohol/sinapyl alcohol (GS monomers, 1/1 ratio) were polymerized to dehydrogenation polymers (DHPs) in presence of two structurally related heteroxylans (HX) differing only in their phenolic substitution patterns. One (HX-40) was enriched in ferulate (FA) while the other (HX-90) was almost devoid of FA. The morphology of the resulting nanoparticles was studied by transmission electron microscopy whereas formation of particles was followed by size exclusion chromatography with online multiangle laser light scattering. HX-40-DHP-G- and HX-40-DHP-GS-derived particles display complex morphological patterns whereas HX-90-DHP-G and HX-90-DHP-GS present rather spherical shapes. The determination of particle sizes and molar masses showed that HX-90 samples formed denser particles than HX-40 ones. These differences are discussed in relation to the ferulate substitution level.

Peroxidase activity can dictate the in vitro lignin dehydrogenative polymer structure

The objective of this study was to assess the influence of the peroxidase/coniferyl alcohol (CA) ratio on the dehydrogenation polymer (DHP) synthesis. The soluble and unsoluble fractions of horseradish peroxidase (HRP)-catalyzed CA dehydrogenation mixtures were recovered in various proportions, depending on the polymerization mode (Zutropf ZT/Zulauf ZL) and HRP/CA ratio (1.6-1100purpurogallin U mmol(-1)). The ZL mode yielded 0-57%/initial CA of unsoluble condensed DHPs (thioacidolysis yields <200micromolg(-1)) with a proportion of uncondensed CA end groups increasing with the HRP/CA ratio (7.2-55.5%/total uncondensed CA). Systematically lower polymer yields (0-49%/initial CA) were obtained for the ZT mode. In that mode, a negative correlation was established between the beta-O-4 content (thioacidolysis yields: 222-660micromolg(-1)) and the HRP/CA ratio. In both modes, decreasing the HRP/CA ratio below 18Ummol(-1) favoured an end-wise polymerization process evidenced by the occurrence of tri-, tetra- and pentamers involving at least one beta-O-4 bond. At low ratio, the unsoluble ZT DHP was found to better approximate natural lignins than DHPs previously synthesized with traditional methods. Besides its possible implication in lignin biosynthesis, peroxidase activity is a crucial parameter accounting for the structural variations of in vitro DHPs.

Enzymatic Polymerization of Coniferyl Alcohol in the Presence of Cyclodextrins

The dehydrogenative polymerization of coniferyl alcohol by horseradish peroxidase was performed in 0.10 M phosphate buffer at 27 degrees C. Dehydrogenative polymer (DHP) from coniferyl alcohol was characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. The ratio of 8-O-4':8-5':8-8' linkages was determined by the 1H NMR spectrum of DHP acetate which had good solubility. In "end-wise like" polymerization (the slow addition of hydrogen peroxide), addition of alpha-cyclodextrin to the medium led to DHP with increased 8-O-4' content and a decrease in 8-5' linkages. Under higher pH conditions, DHP with higher 8-O-4' and 8-5' content was obtained in the presence of alpha-cyclodextrin. In the end-wise polymerization (the slow additions of coniferyl alcohol and hydrogen peroxide), using alpha-cyclodextrin also gave DHP with a 8-O-4' richer structure than that prepared in no additive system. The analysis of thioacidolysis products from DHP supported the results of the alpha-cyclodextrin effects on the 8-O-4'-rich structure of DHP. The 8-O-4' structure in DHP prepared in the presence of alpha-cyclodextrin had racemic form as shown by ozonation.