An even pattern of xylan substitution is critical for interaction with cellulose in plant cell walls

Material-specific binding peptides empower sustainable innovations in plant health, biocatalysis, medicine and microplastic quantification

Material-binding peptides (MBPs) have emerged as a diverse and innovation-enabling class of peptides in applications such as plant-/human health, immobilization of catalysts, bioactive coatings, accelerated polymer degradation and analytics for micro-/nanoplastics quantification. Progress has been fuelled by recent advancements in protein engineering methodologies and advances in computational and analytical methodologies, which allow the design of, for instance, material-specific MBPs with fine-tuned binding strength for numerous demands in material science applications. A genetic or chemical conjugation of second (biological, chemical or physical property-changing) functionality to MBPs empowers the design of advanced (hybrid) materials, bioactive coatings and analytical tools. In this review, we provide a comprehensive overview comprising naturally occurring MBPs and their function in nature, binding properties of short man-made MBPs (<20 amino acids) mainly obtained from phage-display libraries, and medium-sized binding peptides (20-100 amino acids) that have been reported to bind to metals, polymers or other industrially produced materials. The goal of this review is to provide an in-depth understanding of molecular interactions between materials and material-specific binding peptides, and thereby empower the use of MBPs in material science applications. Protein engineering methodologies and selected examples to tailor MBPs toward applications in agriculture with a focus on plant health, biocatalysis, medicine and environmental monitoring serve as examples of the transformative power of MBPs for various industrial applications. An emphasis will be given to MBPs' role in detecting and quantifying microplastics in high throughput, distinguishing microplastics from other environmental particles, and thereby assisting to close an analytical gap in food safety and monitoring of environmental plastic pollution. In essence, this review aims to provide an overview among researchers from diverse disciplines in respect to material-(specific) binding of MBPs, protein engineering methodologies to tailor their properties to application demands, re-engineering for material science applications using MBPs, and thereby inspire researchers to employ MBPs in their research.

Modifying lignin composition and xylan O-acetylation induces changes in cell wall composition, extractability, and digestibility

BACKGROUND

Lignin and xylan are important determinants of cell wall structure and lignocellulosic biomass digestibility. Genetic manipulations that individually modify either lignin or xylan structure improve polysaccharide digestibility. However, the effects of their simultaneous modifications have not been explored in a similar context. Here, both individual and combinatorial modification in xylan and lignin was studied by analysing the effect on plant cell wall properties, biotic stress responses and integrity sensing.

RESULTS

Arabidopsis plant co-harbouring mutation in FERULATE 5-HYDROXYLASE (F5H) and overexpressing Aspergillus niger acetyl xylan esterase (35S:AnAXE1) were generated and displayed normal growth attributes with intact xylem architecture. This fah1-2/35S:AnAXE1 cross was named as hyper G lignin and hypoacetylated (HrGHypAc) line. The HrGHypAc plants showed increased crystalline cellulose content with enhanced digestibility after chemical and enzymatic pre-treatment. Moreover, both parents and HrGHypAc without and after pre-treating with glucuronyl esterase and alpha glucuronidase exhibited an increase in xylose release after xylanase digestion as compared to wild type. The de-pectinated fraction in HrGHypAc displayed elevated levels of xylan and cellulose. Furthermore, the transcriptomic analysis revealed differential expression in cell wall biosynthetic, transcription factors and wall-associated kinases genes implying the role of lignin and xylan modification on cellular regulatory processes.

CONCLUSIONS

Simultaneous modification in xylan and lignin enhances cellulose content with improved saccharification efficiency. These modifications loosen cell wall complexity and hence resulted in enhanced xylose and xylobiose release with or without pretreatment after xylanase digestion in both parent and HrGHypAc. This study also revealed that the disruption of xylan and lignin structure is possible without compromising either growth and development or defense responses against Pseudomonas syringae infection.

Nano cells from fruit bunch residue: Nestling nanotechnology within the circular oil palm milling residue management

Nano-structured materials gain a vast market acceptance mainly due to their overarching endurance. Nanofibrillar cellulose (NFC) is one example of an augmenting agent unviable for production by small and medium enterprises (SMEs) due to the underlying process complexity. This study aims to characterise the NFC-alternative cells denoted as TRX-cellsⓇ, which is a mix of cellulose and non-cellulose components, ruling out its status as 'cellulose nanofibers, CNF'. The aim to test-fit the TRX-cells® production process into the circularity model was executed by deliberating on the usability of the byproduct. In doing so, fibrous oil palm empty fruit bunch (EFB) was treated with dioxydanyl radicals (DIOR) and homogenised. The rapid EFB-DIOR reaction at 70°C targeting dearomatisation reaction in a 10%-solid open system was performed before refining the DIOR-treated EFB to micro-scale fibres. Subjecting the micro-fibres to 17 kWh/mt PFI-milling yielded 85-95% of nano-scale fibrous mass. Relative to the stiff micro-fibres, the nano-scale cells web exhibit 34-41% softness enhancement judged from the web tear resistance profile associated with inter-fibre space reduction. Advanced chromatographic evidence for 27% xylan amongst TRX-cells®' total aldo-sugars was one form of the non-cellulose nano-component. High-resolution Transmission Electron Microscopy hyphenated to Energy Dispersive Analysis of X-ray (HRTEM-EDX) elemental mapping showed a 0.4 atomic percentage of nano-biominerals, confirming the presence of the redistributed dearomatised cells adjacent to cellulose held in the web of the hemicellulose. Shearing at the dearomatised inter-cell wall layers by PFI mill peeled 5 nm-100 nm thickness laminae. The smorgasbord of cellulose and non-celluloses resulted in crystallinity comparable to softwood NFC of approximately 60%, with unique preservation and precision-printing enabling properties. Given the non-recyclability of the DIOR-treated EFB microfibres, nestling the rapid waste transformation process into the circularity model shed light on circular bio-nanotechnology to the spectrum of opportunity for zero-waste, reduced emission and net zero carbon practices on top of an added value from waste transformation to a product.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

Permethylation as a strategy for high-molecular-weight polysaccharide structure analysis by nuclear magnetic resonance-Case study of Xylella fastidiosa extracellular polysaccharide

Current practices for structural analysis of extremely large-molecular-weight polysaccharides via solution-state nuclear magnetic resonance (NMR) spectroscopy incorporate partial depolymerization protocols that enable polysaccharide solubilization in suitable solvents. Non-specific depolymerization techniques utilized for glycosidic bond cleavage, such as chemical degradation or ultrasonication, potentially generate structural fragments that can complicate complete and accurate characterization of polysaccharide structures. Utilization of appropriate enzymes for polysaccharide degradation, on the other hand, requires prior structural knowledge and optimal enzyme activity conditions that are not available to an analyst working with novel or unknown compounds. Herein, we describe an application of a permethylation strategy that allows the complete dissolution of intact polysaccharides for NMR structural characterization. This approach is utilized for NMR analysis of Xylella fastidiosa extracellular polysaccharide (EPS), which is essential for the virulence of the plant pathogen that affects multiple commercial crops and is responsible for multibillion dollar losses each year.

Altering the substitution and cross-linking of glucuronoarabinoxylans affects cell wall architecture in Brachypodium distachyon

The Poaceae family of plants provides cereal crops that are critical for human and animal nutrition, and also, they are an important source of biomass. Interacting plant cell wall components give rise to recalcitrance to digestion; thus, understanding the wall molecular architecture is important to improve biomass properties. Xylan is the main hemicellulose in grass cell walls. Recently, we reported structural variation in grass xylans, suggesting functional specialisation and distinct interactions with cellulose and lignin. Here, we investigated the functions of these xylans by perturbing the biosynthesis of specific xylan types. We generated CRISPR/Cas9 knockout mutants in Brachypodium distachyon XAX1 and GUX2 genes involved in xylan substitution. Using carbohydrate gel electrophoresis, we identified biochemical changes in different xylan types. Saccharification, cryo-SEM, subcritical water extraction and ssNMR were used to study wall architecture. BdXAX1A and BdGUX2 enzymes modify different types of grass xylan. Brachypodium mutant walls are likely more porous, suggesting the xylan substitutions directed by both BdXAX1A and GUX2 enzymes influence xylan-xylan and/or xylan-lignin interactions. Since xylan substitutions influence wall architecture and digestibility, our findings open new avenues to improve cereals for food and to use grass biomass for feed and the production of bioenergy and biomaterials.

Xylan-directed cell wall assembly in grasses

Xylan is the most abundant hemicellulosic polysaccharide in the cell walls of grasses and is pivotal for the assembly of distinct cell wall structures that govern various cellular functions. Xylan also plays a crucial role in regulating biomass recalcitrance, ultimately affecting the utilization potential of lignocellulosic materials. Over the past decades, our understanding of the xylan biosynthetic machinery and cell wall organization has substantially improved due to the innovative application of multiple state-of-the-art techniques. Notably, novel xylan-based nanostructures have been revealed in the cell walls of xylem vessels, promoting a more extensive exploration of the role of xylan in the formation of cell wall structures. This Update summarizes recent achievements in understanding xylan biosynthesis, modification, modeling, and compartmentalization in grasses, providing a brief overview of cell wall assembly regarding xylan. We also discuss the potential for tailoring xylan to facilitate the breeding of elite energy and feed crops.

Hydration and mechanical properties of arabinoxylan, (1,3;1,4)-β-glucan, and cellulose multilayer films simulating the cell wall of wheat endosperm

The cell walls of wheat endosperm, which play a pivotal role in seed germination, exhibit a laminated structure primarily composed of polysaccharides. In this study, composite multilayer films were prepared using arabinoxylan (AX), (1,3;1,4)-β-D-glucan (MLG), and cellulose nanofibers (CNFs), and the effect of polymer blend structure on cell wall hydration and mechanical properties was investigated. Atomic force microscopy and X-ray diffraction indicated that the network structure of MLG/CNF exhibits a higher degree of continuity and uniformity compared to that of AX/CNF. Mechanically, the extensive linkages between MLG and CNFs chains enhance the mechanical properties of the films. Moreover, water diffusion experiments and TD-NMR analysis revealed that water molecules diffuse faster in the network structure formed by AX. We propose a structural model of the endosperm cell wall, in which the CNFs polymer blend coated with MLG serves as the framework, and the AX network fills the gaps between them, providing diffusion channels for water molecules.

The role of CE16 exo-deacetylases in hemicellulolytic enzyme mixtures revealed by the biochemical and structural study of the novel TtCE16B esterase

Acetyl esterases belonging to the carbohydrate esterase family 16 (CE16) is a growing group of enzymes, with exceptional diversity regarding substrate specificity and regioselectivity. However, further insight into the CE16 specificity is required for their efficient biotechnological exploitation. In this work, exo-deacetylase TtCE16B from Thermothelomyces thermophila was heterologously expressed and biochemically characterized. The esterase targets positions O-3 and O-4 of singly and doubly acetylated non-reducing-end xylopyranosyl residues, provided the presence of a free vicinal hydroxyl group at position O-4 and O-3, respectively. Crystal structure of TtCE16B, the first representative among the CE16 enzymes, in apo- and product-bound form, allowed the identification of residues forming the catalytic triad and oxyanion hole, as well as the structural elements related to the enzyme preference for oligomers. The role of TtCE16B in hemicellulose degradation was investigated on acetylated xylan from birchwood and pre-treated beechwood biomass. TtCE16B exhibited complementary activity to commercially available OCE6 acetylxylan esterase. Moreover, it showed synergistic effects with SrXyl43 β-xylosidase. Overall, supplementation of xylan-targeting enzymatic mixtures with both TtCE16B and OCE6 esterases led to a 3-fold or 4-fold increase in xylose release, when using TmXyn10 and TtXyn30A xylanases respectively.

Silencing ScGUX2 reduces xylan glucuronidation and improves biomass saccharification in sugarcane

There is an increasing need for renewable energy sources to replace part of our fossil fuel-based economy and reduce greenhouse gas emission. Sugarcane bagasse is a prominent feedstock to produce cellulosic bioethanol, but strategies are still needed to improve the cost-effective exploitation of this potential energy source. In model plants, it has been shown that GUX genes are involved in cell wall hemicellulose decoration, adding glucuronic acid substitutions on the xylan backbone. Mutation of GUX genes increases enzyme access to cell wall polysaccharides, reducing biomass recalcitrance in Arabidopsis thaliana. Here, we characterized the sugarcane GUX genes and silenced GUX2 in commercial hybrid sugarcane. The transgenic lines had no penalty in development under greenhouse conditions. The sugarcane GUX1 and GUX2 enzymes generated different patterns of xylan glucuronidation, suggesting they may differently influence the molecular interaction of xylan with cellulose and lignin. Studies using biomass without chemical or steam pretreatment showed that the cell wall polysaccharides, particularly xylan, were less recalcitrant in sugarcane with GUX2 silenced than in WT plants. Our findings suggest that manipulation of GUX in sugarcane can reduce the costs of second-generation ethanol production and enhance the contribution of biofuels to lowering the emission of greenhouse gases.

Atomistic, macromolecular model of the Populus secondary cell wall informed by solid-state NMR

Plant secondary cell walls (SCWs) are composed of a heterogeneous interplay of three major biopolymers: cellulose, hemicelluloses, and lignin. Details regarding specific intermolecular interactions and higher-order architecture of the SCW superstructure remain ambiguous. Here, we use solid-state nuclear magnetic resonance (ssNMR) measurements to infer refined details about the structural configuration, intermolecular interactions, and relative proximity of all three major biopolymers within air-dried Populus wood. To enhance the utility of these findings and enable evaluation of hypotheses in a physics-based environment in silico, the NMR observables are articulated into an atomistic, macromolecular model for biopolymer assemblies within the plant SCW. Through molecular dynamics simulation, we quantitatively evaluate several variations of atomistic models to determine structural details that are corroborated by ssNMR measurements.

Xylan derived fluorescence carbon dots composite with cotton cellulose paper as 'turn-off' fluorescence platform for sensitive and selective detection Cu2+ in real samples

The pollution of heavy metals such as Cu2+ is still serious and the discharge of sewage of Cu2+ will cause damage to soil environment and human health. Herein, a biomass-based solid-state fluorescence detection platform (CPU-CDs) was developed as fluorescent sensor for detection Cu2+ via fluorescence and colorimetric dual-model methods in real time. CPU-CDs was composed of xylan-derived CDs (U-CDs) and cotton cellulose paper, which exhibiting good reusability, non-toxicity, excellent fluorescence characteristics and high biocompatibility. Further, CPU-CDs displayed high effectiveness and sensitivity for Cu2+ with the detection limit as low as 0.14 μM, which was well below U.S. EPA safety levels (20 μM). Practical application indicated that CPU-CDs could achieve precision response of Cu2+ change in real environment water samples with good recovery range of 90 %-119 %. This strategy demonstrated a promising biomass solid-state fluorescence sensor for Cu2+ detection for water treatment research, which is of great significance in dealing with water pollution caused by heavy metal ions.

Structural changes of hemicellulose during pulping process and its interaction with nanocellulose

It is believed that hemicellulose plays a crucial role in binding cellulose and lignin in plant cells. It may provide significant implications through figuring out the interaction between hemicellulose and microfibers and gaining insights how the structure of hemicellulose affects its association with cellulose nanofibers. Herein, the hemicellulose and nanocellulose fractions from pulps obtained by controlling the H-factors of kraft pulping process were quantitatively evaluated for their adsorption behavior using QCM-D. The results showed that harsher cooking (corresponding to high H-factor) significantly affected the chemical composition of hemicellulose, leading to a decrease of its molecular weight and gradually turning it into a linear structure. Hemicellulose possesses a strong natural affinity for CNC-coated sensors. The hemicellulose from the pulp cooked by high H-factor process decreases its ability to adsorb onto nanocellulose, the adsorption rate also slows down, and the conformation of the adsorbed layer changes which makes the binding weak and reversible. In conclusion, the pulping process in high H-factor significantly changed the structure of hemicellulose, leading to a variation in the strength of its interaction with nanocellulose.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

Xylan clustering on the pollen surface is required for exine patterning

Xylan is a crosslinking polymer that plays an important role in the assembly of heterogeneous cell wall structures in plants. The pollen wall, a specialized cell wall matrix, exhibits diverse sculpted patterns that serve to protect male gametophytes and facilitate pollination during plant reproduction. However, whether xylan is precisely anchored into clusters and its influence on pollen wall patterning remain unclear. Here, we report xylan clustering on the mature pollen surface in different plant species that is indispensable for the formation of sculpted exine patterns in dicot and monocot plants. Chemical composition analyses revealed that xylan is generally present at low abundance in the mature pollen of flowering plants and shows plentiful variations in terms of substitutions and modifications. Consistent with the expression profiles of their encoding genes, genetic characterization revealed IRREGULAR XYLEM10-LIKE (IRX10L) and its homologous proteins in the GT47 family of glycosyltransferases as key players in the formation of these xylan micro-/nano-compartments on the pollen surface in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa). A deficiency in xylan biosynthesis abolished exine patterning on pollen and compromised male fertility. Therefore, our study outlines a mechanism of exine patterning and provides a tool for manipulating male fertility in crop breeding.

Structural differences of cell walls in earlywood and latewood of Pinus sylvestris and their contribution to biomass recalcitrance

Scots pine (Pinus sylvestris L.) is an evergreen coniferous tree with wide distribution and good growth performance in a range of habitats. Therefore, wood from P. sylvestris is produced in many managed forests and is frequently used in industry. Despite the importance of pine wood, we still do not fully understand its molecular structure what limits improvements in its processing. One of the basic features leading to variation in wood properties is the presence of earlywood and latewood which form annual growth rings. Here, we characterise biochemical traits that differentiate cell walls of earlywood and latewood in Scots pine. We discover that latewood is less recalcitrant to enzymatic digestion, with galactoglucomannan showing particularly pronounced difference in accessibility. Interestingly, characterisation of lignin reveals a higher proportion of coniferaldehydes in pine latewood and suggests the presence of a different linkage landscape in this wood type. With complementary analysis of wood polysaccharides this enabled us to propose the first detailed molecular model of earlywood and latewood and to conclude that the variation in lignin structure is likely the main determinant of differences in recalcitrance observed between the two wood types in pine. Our discoveries lay the foundation for improvements in industrial processes that use pine wood since we show clear pathways for increasing the efficiency of enzymatic processing of this renewable material. Our work will help guide future breeding of pine trees with desired timber properties and can help link molecular structure of softwood cell walls to function of the different types of xylem in conifers.

Flexure wood formation via growth reprogramming in hybrid aspen involves jasmonates and polyamines and transcriptional changes resembling tension wood development

Stem bending in trees induces flexure wood but its properties and development are poorly understood. Here, we investigated the effects of low-intensity multidirectional stem flexing on growth and wood properties of hybrid aspen, and on its transcriptomic and hormonal responses. Glasshouse-grown trees were either kept stationary or subjected to several daily shakes for 5 wk, after which the transcriptomes and hormones were analyzed in the cambial region and developing wood tissues, and the wood properties were analyzed by physical, chemical and microscopy techniques. Shaking increased primary and secondary growth and altered wood differentiation by stimulating gelatinous-fiber formation, reducing secondary wall thickness, changing matrix polysaccharides and increasing cellulose, G- and H-lignin contents, cell wall porosity and saccharification yields. Wood-forming tissues exhibited elevated jasmonate, polyamine, ethylene and brassinosteroids and reduced abscisic acid and gibberellin signaling. Transcriptional responses resembled those during tension wood formation but not opposite wood formation and revealed several thigmomorphogenesis-related genes as well as novel gene networks including FLA and XTH genes encoding plasma membrane-bound proteins. Low-intensity stem flexing stimulates growth and induces wood having improved biorefinery properties through molecular and hormonal pathways similar to thigmomorphogenesis in herbaceous plants and largely overlapping with the tension wood program of hardwoods.

Xylan derived carbon dots composite with PCL/PLA for construction biomass nanofiber membrane used as fluorescence sensor for detection Cu2+ in real samples

Water and soil pollution caused by Cu2+ is not conducive to sustainable development of environment and could cause damage to environment and even human body. Currently, fluorescent sensor solutions analysis method has been used for Cu2+ detection, but they also suffer from drawbacks including easy leakage, difficult storage, and inaccurate. Herein, a green solid-state biomass fluorescence platform (NBU-CDs) consisting of xylan-derived carbon dots (U-CDs) and polylactic acid/polycaprolactone (PLA/PCL) was designed by using in situ electrospinning technology. The prepared NBU-CDs fluorescence platform showed good fluorescence effect and can be served as fluorescence sensor for detecting Cu2+ with high sensitively, selectively and low detection limit (LOD = 0.83 μM). The practical applications of NBU-CDs exhibited high specificity for Cu2+ detection in zebrafish, water samples (school lake, Xuanwu Lake and Yangtze River) with high recovery rates of 97 %-104 % and soil (pond soil, grassland soil and bamboo soil) samples, respectively. The developed fluorescence platform was utilized to predict water and soil safety by monitoring Cu2+ concentration and provides a new strategy for Cu2+ detection.

Polysaccharide assemblies in fungal and plant cell walls explored by solid-state NMR

Structural analysis of macromolecular complexes within their natural cellular environment presents a significant challenge. Recent applications of solid-state NMR (ssNMR) techniques on living fungal cells and intact plant tissues have greatly enhanced our understanding of the structure of extracellular matrices. Here, we selectively highlight the most recent progress in this field. Specifically, we discuss how ssNMR can provide detailed insights into the chemical composition and conformational structure of pectin, and the consequential impact on polysaccharide interactions and cell wall organization. We elaborate on the use of ssNMR data to uncover the arrangement of the lignin-polysaccharide interface and the macrofibrillar structure in native plant stems or during degradation processes. We also comprehend the dynamic structure of fungal cell walls under various morphotypes and stress conditions. Finally, we assess how the combination of NMR with other techniques can enhance our capacity to address unresolved structural questions concerning these complex macromolecular assemblies.

Regulatory controls of duplicated gene expression during fiber development in allotetraploid cotton

Polyploidy complicates transcriptional regulation and increases phenotypic diversity in organisms. The dynamics of genetic regulation of gene expression between coresident subgenomes in polyploids remains to be understood. Here we document the genetic regulation of fiber development in allotetraploid cotton Gossypium hirsutum by sequencing 376 genomes and 2,215 time-series transcriptomes. We characterize 1,258 genes comprising 36 genetic modules that control staged fiber development and uncover genetic components governing their partitioned expression relative to subgenomic duplicated genes (homoeologs). Only about 30% of fiber quality-related homoeologs show phenotypically favorable allele aggregation in cultivars, highlighting the potential for subgenome additivity in fiber improvement. We envision a genome-enabled breeding strategy, with particular attention to 48 favorable alleles related to fiber phenotypes that have been subjected to purifying selection during domestication. Our work delineates the dynamics of gene regulation during fiber development and highlights the potential of subgenomic coordination underpinning phenotypes in polyploid plants.

Cell-type-specific control of secondary cell wall formation by Musashi-type translational regulators in Arabidopsis

Deciphering the mechanism of secondary cell wall/SCW formation in plants is key to understanding their development and the molecular basis of biomass recalcitrance. Although transcriptional regulation is essential for SCW formation, little is known about the implication of post-transcriptional mechanisms in this process. Here we report that two bonafide RNA-binding proteins homologous to the animal translational regulator Musashi, MSIL2 and MSIL4, function redundantly to control SCW formation in Arabidopsis. MSIL2/4 interactomes are similar and enriched in proteins involved in mRNA binding and translational regulation. MSIL2/4 mutations alter SCW formation in the fibers, leading to a reduction in lignin deposition, and an increase of 4-O-glucuronoxylan methylation. In accordance, quantitative proteomics of stems reveal an overaccumulation of glucuronoxylan biosynthetic machinery, including GXM3, in the msil2/4 mutant stem. We showed that MSIL4 immunoprecipitates GXM mRNAs, suggesting a novel aspect of SCW regulation, linking post-transcriptional control to the regulation of SCW biosynthesis genes.

Arabinoxylan in Water through SANS: Single-Chain Conformation, Chain Overlap, and Clustering

Using small-angle neutron scattering (SANS), we examine the structure and conformational behavior of wheat arabinoxylan (AX) prepared at various concentrations in a sodium phosphate aqueous buffer. As for another major hemicellulose, xyloglucan, we observe a small number of large clusters surrounded by AX chains that behave exactly as a polymer in good solvent with a Flory exponent ν = 0.588. The fit of the data at high q-values to a standard worm-like chain model gives the persistence length lp = 45 Å and cross section of the chains 2Rc = 11-12 Å. In addition, using a dedicated modeling approach, we extract from the SANS data at the intermediate q-range the correlation length ξ of the solutions in the semidilute regime. The decay of ξ with concentration follows a scaling law that further confirms the self-avoiding statistical behavior of the AX chains. This first comprehensive study about the properties of water-soluble AX at different length scales may help in the development of products and processes involving AX as a substitute for fossil carbon molecules.

Impact of xylan on field productivity and wood saccharification properties in aspen

Xylan that comprises roughly 25% of hardwood biomass is undesirable in biorefinery applications involving saccharification and fermentation. Efforts to reduce xylan levels have therefore been made in many species, usually resulting in improved saccharification. However, such modified plants have not yet been tested under field conditions. Here we evaluate the field performance of transgenic hybrid aspen lines with reduced xylan levels and assess their usefulness as short-rotation feedstocks for biorefineries. Three types of transgenic lines were tested in four-year field tests with RNAi constructs targeting either Populus GT43 clades B and C (GT43BC) corresponding to Arabidopsis clades IRX9 and IRX14, respectively, involved in xylan backbone biosynthesis, GATL1.1 corresponding to AtGALT1 involved in xylan reducing end sequence biosynthesis, or ASPR1 encoding an atypical aspartate protease. Their productivity, wood quality traits, and saccharification efficiency were analyzed. The only lines differing significantly from the wild type with respect to growth and biotic stress resistance were the ASPR1 lines, whose stems were roughly 10% shorter and narrower and leaves showed increased arthropod damage. GT43BC lines exhibited no growth advantage in the field despite their superior growth in greenhouse experiments. Wood from the ASPR1 and GT43BC lines had slightly reduced density due to thinner cell walls and, in the case of ASPR1, larger cell diameters. The xylan was less extractable by alkali but more hydrolysable by acid, had increased glucuronosylation, and its content was reduced in all three types of transgenic lines. The hemicellulose size distribution in the GALT1.1 and ASPR1 lines was skewed towards higher molecular mass compared to the wild type. These results provide experimental evidence that GATL1.1 functions in xylan biosynthesis and suggest that ASPR1 may regulate this process. In saccharification without pretreatment, lines of all three constructs provided 8-11% higher average glucose yields than wild-type plants. In saccharification with acid pretreatment, the GT43BC construct provided a 10% yield increase on average. The best transgenic lines of each construct are thus predicted to modestly outperform the wild type in terms of glucose yields per hectare. The field evaluation of transgenic xylan-reduced aspen represents an important step towards more productive feedstocks for biorefineries.

Cuticle architecture and mechanical properties: a functional relationship delineated through correlated multimodal imaging

Cuticles are multifunctional hydrophobic biocomposites that protect the aerial organs of plants. During plant development, plant cuticles must accommodate different mechanical constraints combining extensibility and stiffness, and the corresponding relationships with their architecture are unknown. Recent data showed a fine-tuning of cuticle architecture during fruit development, with several chemical clusters which raise the question of how they impact the mechanical properties of cuticles. We investigated the in-depth nanomechanical properties of tomato (Solanum lycopersicum) fruit cuticle from early development to ripening, in relation to chemical and structural heterogeneities by developing a correlative multimodal imaging approach. Unprecedented sharps heterogeneities were evidenced including an in-depth mechanical gradient and a 'soft' central furrow that were maintained throughout the plant development despite the overall increase in elastic modulus. In addition, we demonstrated that these local mechanical areas are correlated to chemical and structural gradients. This study shed light on fine-tuning of mechanical properties of cuticles through the modulation of their architecture, providing new insight for our understanding of structure-function relationships of plant cuticles and for the design of bioinspired material.

Transcriptome, Ectopic Expression and Genetic Population Analysis Identify Candidate Genes for Fiber Quality Improvement in Cotton

Comparative transcriptome analysis of fiber tissues between Gossypium barbadense and Gossypium hirsutum could reveal the molecular mechanisms underlying high-quality fiber formation and identify candidate genes for fiber quality improvement. In this study, 759 genes were found to be strongly upregulated at the elongation stage in G. barbadense, which showed four distinct expression patterns (I-IV). Among them, the 346 genes of group IV stood out in terms of the potential to promote fiber elongation, in which we finally identified 42 elongation-related candidate genes by comparative transcriptome analysis between G. barbadense and G. hirsutum. Subsequently, we overexpressed GbAAR3 and GbTWS1, two of the 42 candidate genes, in Arabidopsis plants and validated their roles in promoting cell elongation. At the secondary cell wall (SCW) biosynthesis stage, 2275 genes were upregulated and exhibited five different expression profiles (I-V) in G. barbadense. We highlighted the critical roles of the 647 genes of group IV in SCW biosynthesis and further picked out 48 SCW biosynthesis-related candidate genes by comparative transcriptome analysis. SNP molecular markers were then successfully developed to distinguish the SCW biosynthesis-related candidate genes from their G. hirsutum orthologs, and the genotyping and phenotyping of a BC₃F₅ population proved their potential in improving fiber strength and micronaire. Our results contribute to the better understanding of the fiber quality differences between G. barbadense and G. hirsutum and provide novel alternative genes for fiber quality improvement.

Permethylation as a Strategy for High Molecular Weight Polysaccharide Structure Analysis by NMR - Case Study of Xylella fastidiosa EPS

Current practices for structure analysis of extremely large molecular weight polysaccharides via solution-state NMR spectroscopy incorporate partial depolymerization protocols that enable polysaccharide solubilization in suitable solvents. Non-specific depolymerization techniques utilized for glycosidic bond cleavage, such as chemical degradation or ultrasonication, potentially generate structure fragments that can complicate the complete characterization of polysaccharide structures. Utilization of appropriate enzymes for polysaccharide degradation, on the other hand, requires prior structure information and optimal enzyme activity conditions that are not available to the analyst working with novel or unknown compounds. Herein, we describe the application of a permethylation strategy that allows the complete dissolution of the intact polysaccharides for NMR structure characterization. This approach is utilized for NMR analysis of Xylella fastidiosa EPS, which is essential for the virulence the plant pathogen that affects multiple commercial crops and is responsible for multibillion dollar losses each year.

The Colloidal Properties of Nanocellulose

Nanocelluloses are anisotropic nanoparticles of semicrystalline assemblies of glucan polymers. They have great potential as renewable building blocks in the materials platform of a more sustainable society. As a result, the research on nanocellulose has grown exponentially over the last decades. To fully utilize the properties of nanocelluloses, a fundamental understanding of their colloidal behavior is necessary. As elongated particles with dimensions in a critical nanosize range, their colloidal properties are complex, with several behaviors not covered by classical theories. In this comprehensive Review, we describe the most prominent colloidal behaviors of nanocellulose by combining experimental data and theoretical descriptions. We discuss the preparation and characterization of nanocellulose dispersions, how they form networks at low concentrations, how classical theories cannot describe their behavior, and how they interact with other colloids. We then show examples of how scientists can use this fundamental knowledge to control the assembly of nanocellulose into new materials with exceptional properties. We hope aspiring and established researchers will use this Review as a guide.

Hemicellulolytic enzymes in lignocellulose processing

Lignocellulosic biomass is the most abundant source of carbon-based material on a global basis, serving as a raw material for cellulosic fibers, hemicellulosic polymers, platform sugars, and lignin resins or monomers. In nature, the various components of lignocellulose (primarily cellulose, hemicellulose, and lignin) are decomposed by saprophytic fungi and bacteria utilizing specialized enzymes. Enzymes are specific catalysts and can, in many cases, be produced on-site at lignocellulose biorefineries. In addition to reducing the use of often less environmentally friendly chemical processes, the application of such enzymes in lignocellulose processing to obtain a range of specialty products can maximize the use of the feedstock and valorize many of the traditionally underutilized components of lignocellulose, while increasing the economic viability of the biorefinery. While cellulose has a rich history of use in the pulp and paper industries, the hemicellulosic fraction of lignocellulose remains relatively underutilized in modern biorefineries, among other reasons due to the heterogeneous chemical structure of hemicellulose polysaccharides, the composition of which varies significantly according to the feedstock and the choice of pretreatment method and extraction solvent. This paper reviews the potential of hemicellulose in lignocellulose processing with focus on what can be achieved using enzymatic means. In particular, we discuss the various enzyme activities required for complete depolymerization of the primary hemicellulose types found in plant cell walls and for the upgrading of hemicellulosic polymers, oligosaccharides, and pentose sugars derived from hemicellulose depolymerization into a broad spectrum of value-added products.

Enzyme synergy for plant cell wall polysaccharide degradation

Valorizing plant cell wall, marine and algal polysaccharides is of utmost importance for the development of the circular bioeconomy. This is because polysaccharides are by far the most abundant organic molecules found in nature with complex chemical structures that require a large set of enzymes for their degradation. Microorganisms produce polysaccharide-specific enzymes that act in synergy when performing hydrolysis. Although discovered since decades enzyme synergy is still poorly understood at the molecular level and thus it is difficult to harness and optimize. In the last few years, more attention has been given to improve and characterize enzyme synergy for polysaccharide valorization. In this review, we summarize literature to provide an overview of the different type of synergy involving carbohydrate modifying enzymes and the recent advances in the field exemplified by plant cell-wall degradation.

Elongated galactan side chains mediate cellulose-pectin interactions in engineered Arabidopsis secondary cell walls

The plant secondary cell wall is a thickened matrix of polysaccharides and lignin deposited at the cessation of growth in some cells. It forms the majority of carbon in lignocellulosic biomass, and it is an abundant and renewable source for forage, fiber, materials, fuels, and bioproducts. The complex structure and arrangement of the cell wall polymers mean that the carbon is difficult to access in an economical and sustainable way. One solution is to alter the cell wall polymer structure so that it is more suited to downstream processing. However, it remains difficult to predict what the effects of this engineering will be on the assembly, architecture, and properties of the cell wall. Here, we make use of Arabidopsis plants expressing a suite of genes to increase pectic galactan chain length in the secondary cell wall. Using multi-dimensional solid-state nuclear magnetic resonance, we show that increasing galactan chain length enhances pectin-cellulose spatial contacts and increases cellulose crystallinity. We also found that the increased galactan content leads to fewer spatial contacts of cellulose with xyloglucan and the backbone of pectin. Hence, we propose that the elongated galactan side chains compete with xyloglucan and the pectic backbone for cellulose interactions. Due to the galactan topology, this may result in comparatively weak interactions and disrupt the cell wall architecture. Therefore, introduction of this strategy into trees or other bioenergy crops would benefit from cell-specific expression strategies to avoid negative effects on plant growth.

The xylobiohydrolase activity of a GH30 xylanase on natively acetylated xylan may hold the key for the degradation of recalcitrant xylan

Acetyl substitutions are common on the hemicellulosic structures of lignocellulose, which up until recently were known to inhibit xylanase activity. Emerging data, however, suggest that xylanases are able to accommodate acetyl side-groups within their catalytic site. In the present work, a fungal GH30 xylanase from Thermothelomyces thermophila, namely TtXyn30A, was shown to release acetylated xylobiose when acting on pretreated lignocellulosic substrate. The released disaccharides could be acetylated at the 2-OH, 3-OH or both positions of the non-reducing end xylose, but the existence of the acetylation on the reducing end cannot be excluded. The synergy of TtXyn30A with acetyl esterases indicates that particular subsites within its active site cannot tolerate acetylated xylopyranose residues. Molecular docking showed that acetyl group can be accommodated on the 2- or 3-OH position of the non-reducing end xylose, unlike the reducing-end xylose (subsite -1), where only 3-OH decoration can be accommodated. Such insight into the catalytic activity of TtXyn30A could contribute to a better understanding of its biological role and thus lead to a more sufficient biotechnological utilization.

Xylan glucuronic acid side chains fix suberin-like aliphatic compounds to wood cell walls

Wood is the most important repository of assimilated carbon in the biosphere, in the form of large polymers (cellulose, hemicelluloses including glucuronoxylan, and lignin) that interactively form a composite, together with soluble extractives including phenolic and aliphatic compounds. Molecular interactions among these compounds are not fully understood. We have targeted the expression of a fungal α-glucuronidase to the wood cell wall of aspen (Populus tremula L. × tremuloides Michx.) and Arabidopsis (Arabidopsis thaliana (L.) Heynh), to decrease contents of the 4-O-methyl glucuronopyranose acid (mGlcA) substituent of xylan, to elucidate mGlcA's functions. The enzyme affected the content of aliphatic insoluble cell wall components having composition similar to suberin, which required mGlcA for binding to cell walls. Such suberin-like compounds have been previously identified in decayed wood, but here, we show their presence in healthy wood of both hardwood and softwood species. By contrast, γ-ester bonds between mGlcA and lignin were insensitive to cell wall-localized α-glucuronidase, supporting the intracellular formation of these bonds. These findings challenge the current view of the wood cell wall composition and reveal a novel function of mGlcA substituent of xylan in fastening of suberin-like compounds to cell wall. They also suggest an intracellular initiation of lignin-carbohydrate complex assembly.

A large-scale forward genetic screen for maize mutants with altered lignocellulosic properties

The development of efficient pipelines for the bioconversion of grass lignocellulosic feedstocks is challenging due to the limited understanding of the molecular mechanisms controlling the synthesis, deposition, and degradation of the varying polymers unique to grass cell walls. Here, we describe a large-scale forward genetic approach resulting in the identification of a collection of chemically mutagenized maize mutants with diverse alterations in their cell wall attributes such as crystalline cellulose content or hemicellulose composition. Saccharification yield, i.e. the amount of lignocellulosic glucose (Glc) released by means of enzymatic hydrolysis, is increased in two of the mutants and decreased in the remaining six. These mutants, termed candy-leaf (cal), show no obvious plant growth or developmental defects despite associated differences in their lignocellulosic composition. The identified cal mutants are a valuable tool not only to understand recalcitrance of grass lignocellulosics to enzymatic deconstruction but also to decipher grass-specific aspects of cell wall biology once the genetic basis, i.e. the location of the mutation, has been identified.

Grass xylan structural variation suggests functional specialization and distinctive interaction with cellulose and lignin

Xylan is the most abundant non-cellulosic polysaccharide in grass cell walls, and it has important structural roles. The name glucuronoarabinoxylan (GAX) is used to describe this variable hemicellulose. It has a linear backbone of β-1,4-xylose (Xyl) residues that may be substituted with α-1,2-linked (4-O-methyl)-glucuronic acid (GlcA), α-1,3-linked arabinofuranose (Araf), and sometimes acetylation at the O-2 and/or O-3 positions. The role of these substitutions remains unclear, although there is increasing evidence that they affect the way xylan interacts with other cell wall components, particularly cellulose and lignin. Here, we used substitution-dependent endo-xylanase enzymes to investigate the variability of xylan substitution in grass culm cell walls. We show that there are at least three different types of xylan: (i) an arabinoxylan with evenly distributed Araf substitutions without GlcA (AXe); (ii) a glucuronoarabinoxylan with clustered GlcA modifications (GAXc); and (iii) a highly substituted glucuronoarabinoxylan (hsGAX). Immunolocalization of AXe and GAXc in Brachypodium distachyon culms revealed that these xylan types are not restricted to a few cell types but are instead widely detected in Brachypodium cell walls. We hypothesize that there are functionally specialized xylan types within the grass cell wall. The even substitutions of AXe may permit folding and binding on the surface of cellulose fibrils, whereas the more complex substitutions of the other xylans may support a role in the matrix and interaction with other cell wall components.

Unraveling the Diverse Roles of Neglected Genes Containing Domains of Unknown Function (DUFs): Progress and Perspective

Domain of unknown function (DUF) is a general term for many uncharacterized domains with two distinct features: relatively conservative amino acid sequence and unknown function of the domain. In the Pfam 35.0 database, 4795 (24%) gene families belong to the DUF type, yet, their functions remain to be explored. This review summarizes the characteristics of the DUF protein families and their functions in regulating plant growth and development, generating responses to biotic and abiotic stress, and other regulatory roles in plant life. Though very limited information is available about these proteins yet, by taking advantage of emerging omics and bioinformatic tools, functional studies of DUF proteins could be utilized in future molecular studies.

Electron tomography unravels new insights into fiber cell wall nanostructure; exploring 3D macromolecular biopolymeric nano-architecture of spruce fiber secondary walls

Lignocellulose biomass has a tremendous potential as renewable biomaterials for fostering the "bio-based society" and circular bioeconomy paradigm. It requires efficient use and breakdown of fiber cell walls containing mainly cellulose, hemicellulose and lignin biopolymers. Despite their great importance, there is an extensive debate on the true structure of fiber walls and knowledge on the macromolecular nano-organization is limited and remains elusive in 3D. We employed dual-axis electron tomography that allows visualization of previously unseen 3D macromolecular organization/biopolymeric nano-architecture of the secondary S2 layer of Norway spruce fiber wall. Unprecedented 3D nano-structural details with novel insights into cellulose microfibrils (~ 2 nm diameter), macrofibrils, nano-pore network and cell wall chemistry (volume %) across the S2 were explored and quantified including simulation of structure related permeability. Matrix polymer association with cellulose varied between microfibrils and macrofibrils with lignin directly associated with MFs. Simulated bio-nano-mechanical properties revealed stress distribution within the S2 and showed similar properties between the idealized 3D model and the native S2 (actual tomogram). Present work has great potential for significant advancements in lignocellulose research on nano-scale understanding of cell wall assembly/disassembly processes leading to more efficient industrial processes of functionalization, valorization and target modification technologies.

Novel constituents of Salvia hispanica L. (chia) nutlet mucilage and the improved in vitro fermentation of nutlets when ground

Upon wetting, chia (Salvia hispanica L.) nutlets produce a gel-like capsule of polysaccharides called mucilage that comprises a significant part of their dietary fibre content. Seed/nutlet mucilage is often used as a texture modifying hydrocolloid and bulking dietary fibre due to its water-binding ability, though the utility of mucilage from different sources is highly structure-function dependent. The composition and structure of chia nutlet mucilage is poorly defined, and a better understanding will aid in exploiting its dietary fibre functionality, particularly if, and how, it is utilised by gut microbiota. In this study, microscopy, chromatography, mass spectrometry and glycome profiling techniques showed that chia nutlet mucilage is highly complex, layered, and contains several polymer types. The mucilage comprises a novel xyloamylose containing both β-linked-xylose and α-linked-glucose, a near-linear xylan that may be sparsely substituted, a modified cellulose domain, and abundant alcohol-soluble oligosaccharides. To assess the dietary fibre functionality of chia nutlet mucilage, an in vitro cumulative gas production technique was used to determine the fermentability of different chia nutlet preparations. The complex nature of chia nutlet mucilage led to poor fermentation where the oligosaccharides appeared to be the only fermentable substrate present in the mucilage. Of note, ground chia nutlets were better fermented than intact whole nutlets, as judged by short chain fatty acid production. Therefore, it is suggested that the benefits of eating chia as a "superfood", could be notably enhanced if the nutlets are ground rather than being consumed whole, improving the bioaccessibility of key nutrients including dietary fibre.

Naturally and chemically acetylated polysaccharides: Structural characteristics, synthesis, activities, and applications in the delivery system: A review

Acetylated polysaccharides refer to polysaccharides containing acetyl groups on sugar units. In the past, the acetylation modification of wall polysaccharides has been a hot research topic for scientists. However, in recent years, many studies have reported that acetylation-modified plant, animal, and microbial polysaccharide show great potential in delivery systems. From the latest perspective, this review systematically presents the different sources of naturally acetylated polysaccharides, the regularity of their modification, the chemical preparation of acetylation modifications, the biological activities and functions of acetylated polysaccharides, and the application in the delivery system. In nature, acetylated polysaccharides are extensively distributed in plants, microorganism, and animals. The level of acetylation modification, the distribution of chains, and the locations of acetylation modification sites differ between species. An increasing number of acetylated polysaccharides were prepared in the aqueous medium, which is safe, environment friendly, and low-cost. In addition to being necessary for plant growth and development, acetylated polysaccharides have immunomodulatory, antioxidant, and anticancer properties. The above-mentioned multiple sources, multifunctional and multi-active acetylated polysaccharides, make them an increasingly important part of delivery systems. We conclude by discussing the future directions for research and development and the potential uses for acetylated polysaccharides.

An update on xylan structure, biosynthesis, and potential commercial applications

•Xylan is an abundant carbohydrate component of plant cell walls that is vital for proper cell wall structure and vascular tissue development.•Xylan structure is known to vary between different tissues and species.•The role of xylan in the plant cell wall is to interact with cellulose, lignin, and hemicelluloses.•Xylan synthesis is directed by several types of Golgi-localized enzymes.•Xylan is being explored as an eco-friendly resource for diverse commercial applications.

β-1,4-Xylan backbone synthesis in higher plants: How complex can it be?

Xylan is a hemicellulose present in the cell walls of all land plants. Glycosyltransferases of the GT43 (IRX9/IRX9L and IRX14/IRX14L) and GT47 (IRX10/IRX10L) families are involved in the biosynthesis of its β-1,4-linked xylose backbone, which can be further modified by acetylation and sugar side chains. However, it remains unclear how the different enzymes work together to synthesize the xylan backbone. A xylan synthesis complex (XSC) has been described in the monocots wheat and asparagus, and co-expression of asparagus AoIRX9, AoIRX10 and AoIRX14A is required to form a catalytically active complex for secondary cell wall xylan biosynthesis. Here, we argue that an equivalent XSC exists for the synthesis of the primary cell wall of the eudicot Arabidopsis thaliana, consisting of IRX9L, IRX10L and IRX14. This would suggest the existence of distinct XSCs for primary and secondary cell wall xylan synthesis, reminiscent of the distinct cellulose synthesis complexes (CSCs) of the primary and secondary cell wall. In contrast to the CSC, in which each CESA protein has catalytic activity, the XSC seems to contain proteins with non-catalytic function with each component bearing potentially unique but crucial roles. Moreover, the core XSC formed by a combination of IRX9/IRX9L, IRX10/IRX10L and IRX14/IRX14L might not be stable in its composition during transit from the endoplasmic reticulum to the Golgi apparatus. Instead, potential dynamic changes of the XSC might be a means of regulating xylan biosynthesis to facilitate coordinated deposition of tailored polysaccharides in the plant cell wall.

Enzymatic debranching is a key determinant of the xylan-degrading activity of family AA9 lytic polysaccharide monooxygenases

BACKGROUND

Previous studies have revealed that some Auxiliary Activity family 9 (AA9) lytic polysaccharide monooxygenases (LPMOs) oxidize and degrade certain types of xylans when incubated with mixtures of xylan and cellulose. Here, we demonstrate that the xylanolytic activities of two xylan-active LPMOs, TtLPMO9E and TtLPMO9G from Thermothielavioides terrestris, strongly depend on the presence of xylan substitutions.

RESULTS

Using mixtures of phosphoric acid-swollen cellulose (PASC) and wheat arabinoxylan (WAX), we show that removal of arabinosyl substitutions with a GH62 arabinofuranosidase resulted in better adsorption of xylan to cellulose, and enabled LPMO-catalyzed cleavage of this xylan. Furthermore, experiments with mixtures of PASC and arabinoglucuronoxylan from spruce showed that debranching of xylan with the GH62 arabinofuranosidase and a GH115 glucuronidase promoted LPMO activity. Analyses of mixtures with PASC and (non-arabinosylated) beechwood glucuronoxylan showed that GH115 action promoted LPMO activity also on this xylan. Remarkably, when WAX was incubated with Avicel instead of PASC in the presence of the GH62, both xylan and cellulose degradation by the LPMO9 were impaired, showing that the formation of cellulose-xylan complexes and their susceptibility to LPMO action also depend on the properties of the cellulose. These debranching effects not only relate to modulation of the cellulose-xylan interaction, which influences the conformation and rigidity of the xylan, but likely also affect the LPMO-xylan interaction, because debranching changes the architecture of the xylan surface.

CONCLUSIONS

Our results shed new light on xylanolytic LPMO9 activity and on the functional interplay and possible synergies between the members of complex lignocellulolytic enzyme cocktails. These findings will be relevant for the development of future lignocellulolytic cocktails and biomaterials.

Cellulose synthesis in land plants

All plant cells are surrounded by a cell wall that provides cohesion, protection, and a means of directional growth to plants. Cellulose microfibrils contribute the main biomechanical scaffold for most of these walls. The biosynthesis of cellulose, which typically is the most prominent constituent of the cell wall and therefore Earth's most abundant biopolymer, is finely attuned to developmental and environmental cues. Our understanding of the machinery that catalyzes and regulates cellulose biosynthesis has substantially improved due to recent technological advances in, for example, structural biology and microscopy. Here, we provide a comprehensive overview of the structure, function, and regulation of the cellulose synthesis machinery and its regulatory interactors. We aim to highlight important knowledge gaps in the field, and outline emerging approaches that promise a means to close those gaps.

Enzyme Discovery in Anaerobic Fungi (Neocallimastigomycetes) Enables Lignocellulosic Biorefinery Innovation

Lignocellulosic biorefineries require innovative solutions to realize their full potential, and the discovery of novel lignocellulose-active enzymes could improve biorefinery deconstruction processes. Enzymatic deconstruction of plant cell walls is challenging, as noncarbohydrate linkages in hemicellulosic sidechains and lignin protect labile carbohydrates from hydrolysis. Highly specialized microbes that degrade plant biomass are attractive sources of enzymes for improving lignocellulose deconstruction, and the anaerobic gut fungi (Neocallimastigomycetes) stand out as having great potential for harboring novel lignocellulose-active enzymes. We discuss the known aspects of Neocallimastigomycetes lignocellulose deconstruction, including their extensive carbohydrate-active enzyme content, proficiency at deconstructing complex lignocellulose, unique physiology, synergistic enzyme complexes, and sizeable uncharacterized gene content. Progress describing Neocallimastigomycetes and their enzymes has been rapid in recent years, and it will only continue to expand. In particular, direct manipulation of anaerobic fungal genomes, effective heterologous expression of anaerobic fungal enzymes, and the ability to directly relate chemical changes in lignocellulose to fungal gene regulation will accelerate the discovery and subsequent deployment of Neocallimastigomycetes lignocellulose-active enzymes.

Outstanding questions on xylan biosynthesis

Xylan is the second most abundant polysaccharide in plant biomass. It is a crucial component of cell wall structure as well as a significant factor contributing to biomass recalcitrance. Xylan consists of a linear chain of β-1,4-linked xylosyl residues that are often substituted with glycosyl side chains, such as glucuronosyl/methylglucuronosyl and arabinofuranosyl residues, and acetylated at O-2 and/or O-3. Xylan from gymnosperms and dicots contains a unique reducing end tetrasaccharide sequence that is not detected in xylan from grasses, bryophytes and seedless vascular plants. Grass xylan is heavily decorated at O-3 with arabinofuranosyl residues that are frequently esterified with hydroxycinnamates. Genetic and biochemical studies have uncovered a number of genes involved in xylan backbone elongation and acetylation, xylan glycosyl substitutions and their modifications, and the synthesis of the unique xylan reducing end tetrasaccharide sequence, but some outstanding issues on the biosynthesis of xylan still remain unanswered. Here, we provide a brief overview of xylan structure and focus on discussion of the current understanding and open questions on xylan biosynthesis. Further elucidation of the biochemical mechanisms underlying xylan biosynthesis will not only shed new insights into cell wall biology but also provide molecular tools for genetic modification of biomass composition tailored for diverse end uses.

Acetyl Group Migration in Xylan and Glucan Model Compounds as Studied by Experimental and Computational Methods

It was recently demonstrated by us that acetyl groups in oligosaccharides can migrate not only within one saccharide unit but also between two different saccharide units. Kinetics of this phenomenon were previously investigated in both mannan model compounds and a naturally occurring polysaccharide. In addition to mannans, there are also several other naturally acetylated polysaccharides, such as xyloglucans and xylans. Both xyloglucans and xylans are some of the most common acetylated polysaccharides in nature, displaying important roles in the plant cells. Considering the various biological roles of natural polysaccharides, it could be hypothesized that the intramolecular migration of acetyl groups might also be associated with regulation of the biological activity of polysaccharides in nature. Consequently, a better understanding of the overall migration phenomenon across the glycosidic bonds could help to understand the potential role of such migrations in the context of the biological activity of polysaccharides. Here, we present a detailed investigation on acetyl group migration in the synthesized xylan and glucan trisaccharide model compounds by a combination of experimental and computational methods, showing that the migration between the saccharide units proceeds from a secondary hydroxyl group of one saccharide unit toward a primary hydroxyl group of the other unit.

The cutin polymer matrix undergoes a fine architectural tuning from early tomato fruit development to ripening

The cuticle is a complex polymer matrix that protects all aerial organs of plants, fulfills multiple roles in plant-environment interactions, and is critical for plant development. These functions are associated with the structural features of cuticles, and the architectural modeling of cuticles during plant development is crucial for understanding their physical properties and biological functions. In this work, the in-depth architecture of the cutin polymer matrix during fruit development was investigated. Using cherry tomato fruit (Solanum lycopersicum) as a model from the beginning of the cell expansion phase to the red ripe stage, we designed an experimental scheme combining sample pretreatment, Raman mapping, multivariate data analyses, and biochemical analyses. These approaches revealed clear chemical areas with different contributions of cutin, polysaccharides, and phenolics within the cutin polymer matrix. Besides, we demonstrated that these areas are finely tuned during fruit development, including compositional and macromolecular rearrangements. The specific spatiotemporal accumulation of phenolic compounds (p-coumaric acid and flavonoids) suggests that they fulfill distinct functions during fruit development. In addition, we highlighted an unexpected dynamic remodeling of the cutin-embedded polysaccharides pectin, cellulose, and hemicellulose. Such structural tuning enables consistent adaption of the cutin-polysaccharide continuum and the functional performance of the fruit cuticle at the different developmental stages. This study provides insights into the plant cuticle architecture and in particular into the organization of the epidermal cell wall-cuticle.

Eudicot primary cell wall glucomannan is related in synthesis, structure, and function to xyloglucan

Hemicellulose polysaccharides influence assembly and properties of the plant primary cell wall (PCW), perhaps by interacting with cellulose to affect the deposition and bundling of cellulose fibrils. However, the functional differences between plant cell wall hemicelluloses such as glucomannan, xylan, and xyloglucan (XyG) remain unclear. As the most abundant hemicellulose, XyG is considered important in eudicot PCWs, but plants devoid of XyG show relatively mild phenotypes. We report here that a patterned β-galactoglucomannan (β-GGM) is widespread in eudicot PCWs and shows remarkable similarities to XyG. The sugar linkages forming the backbone and side chains of β-GGM are analogous to those that make up XyG, and moreover, these linkages are formed by glycosyltransferases from the same CAZy families. Solid-state nuclear magnetic resonance indicated that β-GGM shows low mobility in the cell wall, consistent with interaction with cellulose. Although Arabidopsis β-GGM synthesis mutants show no obvious growth defects, genetic crosses between β-GGM and XyG mutants produce exacerbated phenotypes compared with XyG mutants. These findings demonstrate a related role of these two similar but distinct classes of hemicelluloses in PCWs. This work opens avenues to study the roles of β-GGM and XyG in PCWs.

Phenotyping and modeling of root hydraulic architecture reveal critical determinants of axial water transport

Water uptake by roots is a key adaptation of plants to aerial life. Water uptake depends on root system architecture (RSA) and tissue hydraulic properties that, together, shape the root hydraulic architecture. This work investigates how the interplay between conductivities along radial (e.g. aquaporins) and axial (e.g. xylem vessels) pathways determines the water transport properties of highly branched RSAs as found in adult Arabidopsis (Arabidopsis thaliana) plants. A hydraulic model named HydroRoot was developed, based on multi-scale tree graph representations of RSAs. Root water flow was measured by the pressure chamber technique after successive cuts of a same root system from the tip toward the base. HydroRoot model inversion in corresponding RSAs allowed us to concomitantly determine radial and axial conductivities, providing evidence that the latter is often overestimated by classical evaluation based on the Hagen-Poiseuille law. Organizing principles of Arabidopsis primary and lateral root growth and branching were determined and used to apply the HydroRoot model to an extended set of simulated RSAs. Sensitivity analyses revealed that water transport can be co-limited by radial and axial conductances throughout the whole RSA. The number of roots that can be sectioned (intercepted) at a given distance from the base was defined as an accessible and informative indicator of RSA. The overall set of experimental and theoretical procedures was applied to plants mutated in ESKIMO1 and previously shown to have xylem collapse. This approach will be instrumental to dissect the root water transport phenotype of plants with intricate alterations in root growth or transport functions.

Arabidopsis GELP7 functions as a plasma membrane-localized acetyl xylan esterase, and its overexpression improves saccharification efficiency

Acetyl substitution on the xylan chain is critical for stable interaction with cellulose and other cell wall polymers in the secondary cell wall. Xylan acetylation pattern is governed by Golgi and extracellular localized acetyl xylan esterase (AXE). We investigated the role of Arabidopsis clade Id from the GDSL esterase/lipase or GELP family in polysaccharide deacetylation. The investigation of the AtGELP7 T-DNA mutant line showed a decrease in stem esterase activity and an increase in stem acetyl content. We further generated overexpressor AtGELP7 transgenic lines, and these lines showed an increase in AXE activity and a decrease in xylan acetylation compared to wild-type plants. Therefore, we have named this enzyme as AtAXE1. The subcellular localization and immunoblot studies showed that the AtAXE1 enzyme is secreted out, associated with the plasma membrane and involved in xylan de-esterification post-synthesis. The cellulose digestibility was improved in AtAXE1 overexpressor lines without pre-treatment, after alkali and xylanases pre-treatment. Furthermore, we have also established that the AtGELP7 gene is upregulated in the overexpressor line of AtMYB46, a secondary cell wall specific transcription factor. This transcriptional regulation can drive AtGELP7 or AtAXE1 to perform de-esterification of xylan in a tissue-specific manner. Overall, these data suggest that AtGELP7 overexpression in Arabidopsis reduces xylan acetylation and improves digestibility properties of polysaccharides of stem lignocellulosic biomass.