Cell Wall Deconstruction Research Articles

Plant cell wall enzymatic deconstruction: Bridging the gap between micro and nano scales

Understanding lignocellulosic biomass resistance to enzymatic deconstruction is crucial for its sustainable conversion into bioproducts. Despite scientific advances, quantitative morphological analysis of plant deconstruction at cell and tissue scales remains under-explored. In this study, an original pipeline is devised, involving four-dimensional (space + time) fluorescence confocal imaging, and a novel computational tool, to track and quantify deconstruction at cell and tissue scales. By applying this pipeline to poplar wood, dynamics of cellular parameters was computed and cellulose conversion during enzymatic deconstruction was measured. Results showed that enzymatic deconstruction predominantly impacts cell wall volume rather than surface area. Additionally, a negative correlation was observed between pre-hydrolysis compactness measures and volumetric cell wall deconstruction rate, whose strength was modulated by enzymatic activity. Results also revealed a strong positive correlation between average volumetric cell wall deconstruction rate and cellulose conversion rate. These findings link key deconstruction parameters across nano and micro scales.

Ectopic Production of 3,4-Dihydroxybenzoate in Planta Affects Cellulose Structure and Organization.

Lignocellulosic biomass is a highly sustainable and largely carbon dioxide neutral feedstock for the production of biofuels and advanced biomaterials. Although thermochemical pretreatment is typically used to increase the efficiency of cell wall deconstruction, genetic engineering of the major plant cell wall polymers, especially lignin, has shown promise as an alternative approach to reduce biomass recalcitrance. Poplar trees with reduced lignin content and altered composition were previously developed by overexpressing bacterial 3-dehydroshikimate dehydratase (QsuB) enzyme to divert carbon flux from the shikimate pathway. In this work, three transgenic poplar lines with increasing QsuB expression levels and different lignin contents were studied using small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS). SANS showed that although the cellulose microfibril cross-sectional dimension remained unchanged, the ordered organization of the microfibrils progressively decreased with increased QsuB expression. This was correlated with decreasing total lignin content in the QsuB lines. WAXS showed that the crystallite dimensions of cellulose microfibrils transverse to the growth direction were not affected by the QsuB expression, but the crystallite dimensions parallel to the growth direction were decreased by ∼20%. Cellulose crystallinity was also decreased with increased QsuB expression, which could be related to high levels of 3,4-dihydroxybenzoate, the product of QsuB expression, disrupting microfibril crystallization. In addition, the cellulose microfibril orientation angle showed a bimodal distribution at higher QsuB expression levels. Overall, this study provides new structural insights into the impact of ectopic synthesis of small-molecule metabolites on cellulose organization and structure that can be used for future efforts aimed at reducing biomass recalcitrance.

Insights into the poplar cell wall deconstruction following deep eutectic solvent pretreatment for enhanced enzymatic saccharification and lignin valorization

In this study, a combination of microcosmic and chemical analysis methods was used to investigate deep eutectic solvent (DES) pretreatment effects on cell wall's micromorphology and lignin's dissolution regular, in order to achieve high-performance biorefinery. The atomic force microscope observed that DES pretreatment peeled off non-cellulose components to reduced “anti-degradation barrier”, resulting to improve the enzymatic saccharification from 12.36 % to 90.56 %. In addition, DES pretreatment can break the β-O-4 bond between the lignin units resulting in a decline in molecular weight from 3187 g/mol to 1112 g/mol (0–6 h). However, long pretreatment time resulted regenerated lignin samples repolymerization. Finally, DES has good recoverability which showed saccharification still can reach 51.51 % at 6 h following four recycling rounds and regenerated lignin also had a typical and well-preserved structure. In general, this work offers important information for industrial biorefinery technologies and lignin valorization.

Pretreatment with ChOH/Urea under mild conditions for the efficient deconstruction of poplar cell wall

In this study, DES consisting of Choline hydroxide (ChOH) and Urea (Ur) was used to pretreat poplar under mild conditions (60 ℃). The 72 h saccharification efficiency of the pretreated samples for 10 min was significantly improved (69.97 %), which amounted to a 2.4-fold increase in sugar yield over the raw material (29.89 %). The remarkable enhancement could be correlated to the topochemical and morphological changes occurring in poplar cell walls, including the removal of lignin and hemicelluloses, decreased crystallinity and microstructural disruption. Noteworthy, X-ray diffraction analysis showed that cellulose I underwent a crystallographic transformation from cellulose I to cellulose II during DES pretreatment. Our study demonstrated that ChOH/Ur pretreatment was quite effective in deconstructing poplar cell walls and increasing the enzymatic hydrolysis efficiency of cellulose under mild conditions.

Relation of xylitol formation and lignocellulose degradation in yeast

One of the critical steps of the biotechnological production of xylitol from lignocellulosic biomass is the deconstruction of the plant cell wall. This step is crucial to the bioprocess once the solubilization of xylose from hemicellulose is allowed, which can be easily converted to xylitol by pentose-assimilating yeasts in a microaerobic environment. However, lignocellulosic toxic compounds formed/released during plant cell wall pretreatment, such as aliphatic acids, furans, and phenolic compounds, inhibit xylitol production during fermentation, reducing the fermentative performance of yeasts and impairing the bioprocess productivity. Although the toxicity of lignocellulosic inhibitors is one of the biggest bottlenecks of the biotechnological production of xylitol, most of the studies focus on how much xylitol production is inhibited but not how and where cells are affected. Understanding this mechanism is important in order to develop strategies to overcome lignocellulosic inhibitor toxicity. In this mini-review, we addressed how these inhibitors affect both yeast physiology and metabolism and consequently xylose-to-xylitol bioconversion. In addition, this work also addresses about cellular adaptation, one of the most relevant strategies to overcome lignocellulosic inhibitors toxicity, once it allows the development of robust and tolerant strains, contributing to the improvement of the microbial performance against hemicellulosic hydrolysates toxicity. KEY POINTS: • Impact of lignocellulosic inhibitors on the xylitol production by yeasts • Physiological and metabolic alterations provoked by lignocellulosic inhibitors • Cell adaptation as an efficient strategy to improve yeast's robustness.

Selfish uptake versus extracellular arabinoxylan degradation in the primary degrader Ruminiclostridium cellulolyticum, a new string to its bow

BackgroundPrimary degraders of polysaccharides play a key role in anaerobic biotopes, where plant cell wall accumulates, providing extracellular enzymes to release fermentable carbohydrates to fuel themselves and other non-degrader species. Ruminiclostridium cellulolyticum is a model primary degrader growing amongst others on arabinoxylan. It produces large multi-enzymatic complexes called cellulosomes, which efficiently deconstruct arabinoxylan into fermentable monosaccharides.ResultsComplete extracellular arabinoxylan degradation was long thought to be required to fuel the bacterium during this plant cell wall deconstruction stage. We discovered and characterized a second system of “arabinoxylan” degradation in R. cellulolyticum, which challenged this paradigm. This “selfish” system is composed of an ABC transporter dedicated to the import of large and possibly acetylated arabinoxylodextrins, and a set of four glycoside hydrolases and two esterases. These enzymes show complementary action modes on arabinoxylo-dextrins. Two α-L-arabinofuranosidases target the diverse arabinosyl side chains, and two exo-xylanases target the xylo-oligosaccharides backbone either at the reducing or the non-reducing end. Together, with the help of two different esterases removing acetyl decorations, they achieve the depolymerization of arabinoxylo-dextrins in arabinose, xylose and xylobiose. The in vivo study showed that this new system is strongly beneficial for the fitness of the bacterium when grown on arabinoxylan, leading to the conclusion that a part of arabinoxylan degradation is achieved in the cytosol, even if monosaccharides are efficiently provided by the cellulosomes in the extracellular space. These results shed new light on the strategies used by anaerobic primary degrader bacteria to metabolize highly decorated arabinoxylan in competitive environments.ConclusionThe primary degrader model Ruminiclostridium cellulolyticum has developed a “selfish” strategy consisting of importing into the bacterium, large arabinoxylan–dextrin fractions released from a partial extracellular deconstruction of arabinoxylan, thus complementing its efficient extracellular arabinoxylan degradation system. Genetic studies suggest that this system is important to support fitness and survival in a competitive biotope. These results provide a better understanding of arabinoxylan catabolism in the primary degrader, with biotechnological application for synthetic microbial community engineering for the production of commodity chemicals from lignocellulosic biomass.

Depolymerization behaviors of naked oat stem during autohydrolysis in mild subcritical water Ⅱ: Correlative changes in tissue structure, cell morphology, and chemical composition.

Green, efficient depolymerization of lignocellulose is a prerequisite and the key in high-value utilization of its components. Understanding of the changes in morphological chemistry of the cell wall, related to the biomass recalcitrance, is critical for optimizing the processing conditions of cell wall deconstruction. Thus, herein, based on our previous studies, subcritical water autohydrolysis of naked oat stems was conducted at 190 °C and 1.18 MPa for 0–60 min. The depolymerization behaviors of tissues and cell walls were monitored dynamically at 10 min intervals. The micro-and ultrastructural changes of autohydrolyzed residue sections were analyzed. Parenchyma tissue commenced dissociation after 10 min at 190 ℃, whereas mechanical tissue dissociation occurred after 40 min at 190 ℃, along with interlayer separation of the secondary walls of sclerenchyma fibers. The autofluorescence changes of lignin showed that the order of regional lignin removal in tissues was from sieve tubes and companion cells to parenchyma cells, and finally to sclerenchyma fibers and vessel elements. The turning point of tissue and cell wall structural depolymerization of naked oat stems under subcritical water autohydrolysis conditions occurred after 40 min at 190 ℃ and 1.18 MPa. Under this condition, the degree of tissue dissociation was as high as 80%, while the fiber cell morphology remained intact. Dynamic changes in the tissue and cell wall structures and cell morphologies of naked oat stems maintained at 190 ℃ for various durations were captured by differentiating the use of the optical microscope, fluorescence microscope, and scanning electron microscope. The extensive, detailed assay data yield novel insights into the dynamic depolymerization mechanism of lignocellulosic composite cell walls under subcritical water autohydrolysis conditions.

Low-abundance populations distinguish microbiome performance in plant cell wall deconstruction

BackgroundPlant cell walls are interwoven structures recalcitrant to degradation. Native and adapted microbiomes can be particularly effective at plant cell wall deconstruction. Although most understanding of biological cell wall deconstruction has been obtained from isolates, cultivated microbiomes that break down cell walls have emerged as new sources for biotechnologically relevant microbes and enzymes. These microbiomes provide a unique resource to identify key interacting functional microbial groups and to guide the design of specialized synthetic microbial communities.ResultsTo establish a system assessing comparative microbiome performance, parallel microbiomes were cultivated on sorghum (Sorghum bicolor L. Moench) from compost inocula. Biomass loss and biochemical assays indicated that these microbiomes diverged in their ability to deconstruct biomass. Network reconstructions from gene expression dynamics identified key groups and potential interactions within the adapted sorghum-degrading communities, including Actinotalea, Filomicrobium, and Gemmatimonadetes populations. Functional analysis demonstrated that the microbiomes proceeded through successive stages that are linked to enzymes that deconstruct plant cell wall polymers. The combination of network and functional analysis highlighted the importance of cellulose-degrading Actinobacteria in differentiating the performance of these microbiomes.ConclusionsThe two-tier cultivation of compost-derived microbiomes on sorghum led to the establishment of microbiomes for which community structure and performance could be assessed. The work reinforces the observation that subtle differences in community composition and the genomic content of strains may lead to significant differences in community performance.3Ck-Q-KzsX1uPWsLmUjZUvVideo

Biodegradation of Gramineous Lignocellulose by Locusta migratoria manilensis (Orthoptera: Acridoidea)

Exploring an efficient and green pretreatment method is an important prerequisite for the development of biorefinery. It is well known that locusts can degrade gramineous lignocellulose efficiently. Locusts can be used as a potential resource for studying plant cell wall degradation, but there are few relative studies about locusts so far. Herein, some new discoveries were revealed about elucidating the process of biodegradation of gramineous lignocellulose in Locusta migratoria manilensis. The enzyme activity related to lignocellulose degradation and the content of cellulose, hemicellulose, and lignin in the different gut segments of locusts fed corn leaves were measured in this study. A series of characterization analyses were conducted on corn leaves and locust feces, which included field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction pattern (XRD), and thermogravimetric (TG) analysis. These results showed that the highest activities of carboxymethyl cellulase (CMCase), filter paper cellulase (FPA), and xylanase were obtained in the foregut of locusts, which strongly indicated that the foregut was the main lignocellulose degradation segment in locusts; furthermore, the majority of nutritional components were absorbed in the midgut of locusts. The activity of CMCase was significantly higher than that of xylanase, and manganese peroxidase (MnPase) activity was lowest, which might be due to the basic nutrition of locusts being cellulose and hemicellulose and not lignin based on the results of FE-SEM, FTIR, XRD, and TG analysis. Overall, these results provided a valuable insight into lignocellulosic degradation mechanisms for understanding gramineous plant cell wall deconstruction and recalcitrance in locusts, which could be useful in the development of new enzymatic pretreatment processes mimicking the locust digestive system for the biochemical conversion of lignocellulosic biomass to fuels and chemicals.

The Potential for Cellulose Deconstruction in Fungal Genomes

Fungal cellulolytic enzymes are carbohydrate active enzymes (CAzymes) essential for the deconstruction of the plant cell wall. Cellulolytic activity is described in some glycoside hydrolases (GH-cellulases) and in auxiliary activities (AA-cellulases) families. Across environments, these enzymes are mostly produced by some fungi and some bacteria. Cellulolytic fungi secrete these enzymes to deconstruct polysaccharides into simple and easy to metabolize oligo- and mono-saccharides. The fungal ability to degrade cellulose result from their repertoire of CAZymes-encoding genes targeting many substrates (e.g., xylan, arabinose). Over the past decade, the increased number of sequenced fungal genomes allowed the sequence-based identification of many new CAZyme-encoding genes. Together, the predicted cellulolytic enzymes constitute the fungal potential for cellulose deconstruction. As not all fungi have the same genetic makeup, identifying the potential for cellulose deconstruction across different lineages can help identify the various fungal strategies to access and degrade cellulose (conserved vs. variable genomic features) and highlight the evolution of cellulase-encoding genes. Here, the potential for cellulose deconstruction identified across publicly accessible, and published, fungal genomes is discussed.

Computational identification and characterization of vascular wilt pathogen (Fusarium oxysporum f. sp. lycopersici) CAZymes in tomato xylem sap

Fusarium oxysporum f. sp. lycopersici is a devastating plant pathogenic fungi known for wilt disease in the tomato plant and secrete cell wall degrading enzymes. These enzymes are collectively known as carbohydrate-active enzymes (CAZymes), crucial for growth, colonization and pathogenesis. Therefore, the present study was aimed to identify and annotate pathogen CAZymes in the xylem sap of a susceptible tomato variety using downstream proteomics and meta servers. Further, structural elucidation and conformational stability analysis of the selected CAZyme families were done through homology modeling and molecular dynamics simulation. Among all the fungal proteins identified, the carbohydrate metabolic process was found to be enriched. Most of the annotated CAZymes belonged to the hydrolase and oxidoreductase families, and 90% were soluble and extracellular. Moreover, using a publically available interactome database, interactions were observed between the families acting on chitin, hemicellulose and pectin. Subsequently, important catalytic residues were identified in the candidate CAZymes belonging to carbohydrate esterase (CE8) and glycosyl hydrolase (GH18 and GH28). Further, essential dynamics after molecular simulation of 100 ns revealed the overall behavior of these CAZymes with distinct global minima and transition states in CE8. Thus, our study identified some of the CAZyme families that assist in pathogenesis and growth through host cell wall deconstruction with further structural insight into the selected CAZyme families. Communicated by Ramaswamy H. Sarma

Mechanical processing effects on the secondary plant cell wall arrangement in Sorghum bicolor

Unlocking one third of plant biomass as a renewable feedstock for fuels and materials depends on the effective deconstruction of the secondary plant cell wall. Over 90% of the native secondary plant cell wall is composed of cellulose and hemicellulose polysaccharides and the lignin aromatic polymers. Deconstruction refers to the processes which digest polymers into desired subunits. Present deconstruction processes are centered on lignin-first extraction to overcome the recalcitrance of biomass: the accumulation of indigestible plant polymers during deconstruction.

Xylan Deconstruction by Thermophilic Thermoanaerobacterium bryantii Hemicellulases Is Stimulated by Two Oxidoreductases

Thermoanaerobacterium bryantii strain mel9T is a thermophilic bacterium isolated from a waste pile of a corn-canning factory. The genome of T. bryantii mel9T was sequenced and a hemicellulase gene cluster was identified. The cluster encodes seven putative enzymes, which are likely an endoxylanase, an α-glucuronidase, two oxidoreductases, two β-xylosidases, and one acetyl xylan esterase. These genes were designated tbxyn10A, tbagu67A, tbheoA, tbheoB, tbxyl52A, tbxyl39A, and tbaxe1A, respectively. Only TbXyn10A released reducing sugars from birchwood xylan, as shown by thin-layer chromatography analysis. The five components of the hemicellulase cluster (TbXyn10A, TbXyl39A, TbXyl52A, TbAgu67A, and TbAxe1A) functioned in synergy to hydrolyze birchwood xylan. Surprisingly, the two putative oxidoreductases increased the enzymatic activities of the gene products from the xylanolytic gene cluster in the presence of NADH and manganese ions. The two oxidoreductases were therefore named Hemicellulase-Enhancing Oxidoreductases (HEOs). All seven enzymes were thermophilic and acted in synergy to degrade xylans at 60 °C. Except for TbXyn10A, the other enzymes encoded by the gene cluster were conserved with high amino acid identities (85–100%) in three other Thermoanaerobacterium species. The conservation of the gene cluster is, therefore, suggestive of an important role of these enzymes in xylan degradation by these bacteria. The mechanism for enhancement of hemicellulose degradation by the HEOs is under investigation. It is anticipated, however, that the discovery of these new actors in hemicellulose deconstruction will have a significant impact on plant cell wall deconstruction in the biofuel industry.

Lignin properties and cell wall response to deconstruction by alkaline pretreatment and enzymatic hydrolysis in brown midrib sorghums

Lignin has an adverse impact on the deconstruction of plant cell wall biopolymers in biorefining processes and its reduction and/or alteration during biosynthesis is one target for decreasing plant cell wall recalcitrance. In this work, the impact of two brown midrib mutations (bmr6 and bmr12) in two sorghum background lines (the commercial hybrid Atlas and near-isogenic BTx623) on lignin properties and the plants’ response to cell wall deconstruction to monomeric sugars via alkaline pretreatment and enzymatic hydrolysis is investigated with the goal of assessing how differences in lignin content and properties impact the plant’s response to pretreatment. We identify that both bmr sorghum lines show significantly lower abundance of water-extractable sugars (glucose, sucrose, and fructose) and alkali-saponifiable p-coumarate. Furthermore, both these properties exhibited identical trends across both background lines. Next, both untreated and mild alkali-pretreated bmr sorghums were shown to exhibit higher glucose hydrolysis yields following enzymatic hydrolysis than the control lines. Following pretreatment, the Atlas bmr sorghums exhibited more lignin solubilization and the solubilized lignin was of lower molar mass than the background control line suggesting that differences in the lignin response to pretreatment resulted these differences. Finally, significant differences were observed in the lignin content, lignin monomer distribution, and inter-unit linkages in the Atlas bmr line relative to the control line with key differences including lower syringyl monomer content in both bmr lines, higher relative abundance of β-O-4 linkages in the bmr6 line, and the presence of 5-hydroxy guaiacyl monomers and benzodioxane (α-O-5/β-O-4) linkages in the bmr12 line.

Novel, recyclable Brønsted acidic deep eutectic solvent for mild fractionation of hemicelluloses

Acidic deep eutectic solvents (DESs) are promising media for lignin valorization and cellulose conversion due to their good ability in efficient deconstruction of plant cell wall. However, hemicellulose extraction from lignocellulose using acidic DESs remains a challenge. Herein, novel and green Brønsted acidic DESs (BDESs) were synthesized from natural organic acids and common polyols and successively adopted to deconstruct corncob for mild fractionation of hemicelluloses. Oxalic acid (OA)-based BDESs were preferred for corncob processing due to the high solubility of xylan. The results revealed that the suitable acidity of DESs and mild temperature effectively avoided the over-degradation of hemicelluloses. The chemical composition and structural features of the recovered hemicelluloses were investigated systematically. Moreover, after ethylene glycol (EG)-OA BDES was recycled and reused three times, the extraction still resulted in a satisfactory hemicellulose yield. The novel and eco-friendly processing offers a practical and sustainable route for hemicellulose extraction in acidic condition.

Untangling the impact of red wine maceration times on wine ageing. A multidisciplinary approach focusing on extended maceration in Shiraz wines

Phenolic composition of young red wines has been shown to play an important role in their ageing potential. Therefore, the modulation of phenolic extraction during maceration may influence the subsequent phenolic evolution of these wines. The present work aimed to evaluate the impact of three different maceration times on the phenolic levels and evolution observed over time, using spectrophotometric and chromatography methods, and the effect on the aroma, taste, and mouthfeel sensory properties using Projective Mapping. Additionally, grape cell wall deconstruction was monitored during the extended maceration phase by GC-MS and Comprehensive Comprehensive Microarray Polymer Profiling (CoMPP). Our findings demonstrated that longer maceration times did not always correspond to an increase in wine phenolic concentration, although the level of complexity of these molecules seemed to be higher. Additionally, continuous depectination and possible solubilisation of the pectin is observed during the extended maceration which may be influencing the sensory perception of these wines. Maceration time was also shown to influence the evolution of the polymeric fraction and sensory perception of the wines.

Tandem Character of Liquid Hot Water and Deep Eutectic Solvent to Enhance Lignocellulose Deconstruction.

Pretreatment with efficient fractionation, eco-friendliness, and low-cost brings high security to future biorefinery systems. Synergistic pretreatment is a compelling blueprint to tackle the compact structure of lignocellulose towards a high-level valorization. Here, a stepwise approach was designed using hydrothermal and deep eutectic solvent (DES) pretreatments to hierarchically extract hemicelluloses and lignin from poplar, while delivering a cellulose-rich substrate that could easily undergo enzymatic hydrolysis to obtain fermentable glucose and residual lignin. The lifetime of recyclable DES showed that the pretreatment efficiency was still largely maintained after the fourth recycling. An enhancement of enzymatic digestibility from 13.9 to 90.4 % was initiated by the deconstruction of amorphous portions and robust cell wall. 23.7 % Xylooligosaccharides (degree of polymerization 2-6), 47.5 % DES-isolated lignin, and 19.2 % cellulose enzymatic lignin were harvested via this coupled process. This study could promote the precise design of sustainable tandem pretreatment that can boost the frontier of highly available biorefinery.

Synergistic Improvement of Carbohydrate and Lignin Processability by Biomimicking Biomass Processing

The sustainability and economic feasibility of modern biorefinery depend on the efficient processing of both carbohydrate and lignin fractions for value-added products. By mimicking the biomass degradation process in white-rote fungi, a tailored two-step fractionation process was developed to maximize the sugar release from switchgrass biomass and to optimize the lignin processability for bioconversion. Biomimicking biomass processing using Formic Acid: Fenton: Organosolv (F2O) and achieved high processability for both carbohydrate and lignin. Specifically, switchgrass pretreated by the F2O process had 99.6% of the theoretical yield for glucose release. The fractionated lignin was also readily processable by fermentation via Rhodococcus opacus PD630 with a lipid yield of 1.16 g/L. Scanning electron microscope analysis confirmed the fragmentation of switchgrass fiber and the cell wall deconstruction by the F2O process. 2D-HSQC NMR further revealed the cleavage of aryl ether linkages (β-O-4) in lignin components. These results revealed the mechanisms for efficient sugar release and lignin bioconversion. The F2O process demonstrated effective mimicking of natural biomass utilization system and paved a new path for improving the lignin and carbohydrate processability in next generation lignocellulosic biorefinery.

Breeding Targets to Improve Biomass Quality in Miscanthus.

Lignocellulosic crops are attractive bioresources for energy and chemicals production within a sustainable, carbon circular society. Miscanthus is one of the perennial grasses that exhibits great potential as a dedicated feedstock for conversion to biobased products in integrated biorefineries. The current biorefinery strategies are primarily focused on polysaccharide valorization and require severe pretreatments to overcome the lignin barrier. The need for such pretreatments represents an economic burden and impacts the overall sustainability of the biorefinery. Hence, increasing its efficiency has been a topic of great interest. Inversely, though pretreatment will remain an essential step, there is room to reduce its severity by optimizing the biomass composition rendering it more exploitable. Extensive studies have examined the miscanthus cell wall structures in great detail, and pinpointed those components that affect biomass digestibility under various pretreatments. Although lignin content has been identified as the most important factor limiting cell wall deconstruction, the effect of polysaccharides and interaction between the different constituents play an important role as well. The natural variation that is available within different miscanthus species and increased understanding of biosynthetic cell wall pathways have specified the potential to create novel accessions with improved digestibility through breeding or genetic modification. This review discusses the contribution of the main cell wall components on biomass degradation in relation to hydrothermal, dilute acid and alkaline pretreatments. Furthermore, traits worth advancing through breeding will be discussed in light of past, present and future breeding efforts.

Pectin biosynthesis pathways are adapted to higher rhamnogalacturonan formation in lignocellulosic jute (Corchorus spp.)

Pectin and lignin are two enigmatic macromolecular components of cell wall, which are spatially and temporally deposited during plant growth. While the former is important for primary growth, the latter accumulates during secondary growth. With the evolution of land plants, the structural complexity of pectin increased to protect cell walls against the intrusion of pectinolytic pathogens besides offering multiple choices for its interaction with lignin. We reconstructed pectin biosynthesis pathways in an annual lignocellulosic bast fibre-producing crop jute (Corchorus spp.) from hypocotyl transcriptomes and identified 27 isoforms of 17 genes and 12 isoforms of galacturonosyltransferase involved in nucleotide-sugar interconversion and pectin polymerization, respectively. Salvage pathways were found to be functional for replenishing nucleotide sugars in jute hypocotyls. Phylogenetic analyses revealed that the genes of pectin biosynthesis pathways are well conserved across taxa. For significant upregulation of the rhamnose biosynthesis gene (RHM1), we traced its evolution, identified its two-domain protein structure proposed to have been evolved from charophycean green algae and generated a structural model explaining its functional conservation. Retting of jute stem with pectinolytic bacteria showed that pectin complexity and its interaction with lignin play crucial roles in resisting cell wall deconstruction that may have accrued from increased rhamnogalacturonan biosynthesis.