The Patterned Structure of Galactoglucomannan Suggests It May Bind to Cellulose in Seed Mucilage

Electrospinning of cellulose nanocrystals; procedure and optimization

Cellulose nanocrystals (CNCs) and cellulose microfibrils (CMFs) are promising materials with the potential to significantly enhance the mechanical properties of electrospun nanofibers. However, the crucial aspect of optimizing their integration into these nanofibers remains a challenge. In this work, we present a method to prepare and electrospin a cellulosic solution, aiming to overcome the existing challenges and realize the optimized incorporation of CNCs into nanofibers. The solution parameters of electrospinning were explored using a combined experimental and simulation (molecular dynamics) approach. Experimental results emphasize the impact of polymer solution concentration on fiber morphology, reinforcing the need for further optimization. Simulations highlight the intricate factors, including the molecular weight of cellulose acetate (CA) polymer chains, electrostatic fields, and humidity, that impact the alignment of CNCs and CMFs. Furthermore, efforts were made to study CNCs/CMFs alignment rate and quality optimization. It is predicted that pure CNCs benefit more from electrostatic alignment, while lower molecular weight CA enables better CNC/CMF alignment.

Advancing plant cell wall modelling: Atomistic insights into cellulose, disordered cellulose, and hemicelluloses - A review

The complexity of plant cell walls on different hierarchical levels still impedes the detailed understanding of biosynthetic pathways, interferes with processing in industry and finally limits applicability of cellulose materials. While there exist many challenges to readily accessing these hierarchies at (sub-) angström resolution, the development of advanced computational methods has the potential to unravel important questions in this field. Here, we summarize the contributions of molecular dynamics simulations in advancing the understanding of the physico-chemical properties of natural fibres. We aim to present a comprehensive view of the advancements and insights gained from molecular dynamics simulations in the field of carbohydrate polymers research. The review holds immense value as a vital reference for researchers seeking to undertake atomistic simulations of plant cell wall constituents. Its significance extends beyond the realm of molecular modeling and chemistry, as it offers a pathway to develop a more profound comprehension of plant cell wall chemistry, interactions, and behavior. By delving into these fundamental aspects, the review provides invaluable insights into future perspectives for exploration. Researchers within the molecular modeling and carbohydrates community can greatly benefit from this resource, enabling them to make significant strides in unraveling the intricacies of plant cell wall dynamics.

An essential role for mannan degradation in both cell growth and secondary cell wall formation

Coordination of secondary cell wall deposition and cell expansion during plant growth is required for cell development, particularly in vascular tissues. Yet the fundamental coordination process has received little attention. We observed that the Arabidopsis endo-1,4-mannanase gene, AtMAN6, is involved in the formation of cell walls in vascular tissues. In the inflorescence stem, the man6 mutant had smaller vessel cells with thicker secondary cell walls and shorter fiber cells. Elongation growth was reduced in the root, and secondary cell wall deposition in vessel cells occurred early. Overexpression of AtMAN6 resulted in the inverse phenotypes of the man6 mutant. AtMAN6 was discovered on the plasma membrane and was specifically expressed in vessel cells during its early development. The AtMAN6 protein degraded galactoglucomannan to produce oligosaccharides, which caused secondary cell wall deposition in vessel and fiber cells to be suppressed. Transcriptome analysis revealed that the expression of genes involved in the regulation of secondary cell wall synthesis was changed in both man6 mutant and AtMAN6 overexpression plants. AtMAN6's C-terminal cysteine repeat motif (CCRM) was found to facilitate homodimerization and is required for its activity. According to the findings, the oligosaccharides produced by AtMAN6 hydrolysis may act as a signal to mediate this coordination between cell growth and secondary cell wall deposition.

Extraction, structural characterization and immunoactivity of glucomannan type polysaccahrides from Lilium brownii var. viridulum Baker

Homogeneous polysaccharide (LBP) was extracted and purified from the bulblets of Lilium brownii var. viridulum Baker with a molecular weight of 312 kDa. The monosaccharides are composed of mannose and glucose, and the corresponding molar ratios are 0.582 and 0.418, respectively. FT-IR, LC-MS, NMR, GC-MS and HPAEC were used to analyze the functional groups, glycosidic linkages and chemical structure of LBP, which was a 1-4-linked glucomannan and contained a dodecasaccharide repeating units of →4)-β-D-Manp-(1 → 4)-β-D-Manp-(1 → 4)-β-D-Manp-(1 → 4)-β-D-Glcp-(1 → 4)-β-D-Manp-(1 → 4)-β-D-Manp-(1 → 4)-β-D-Glcp-(1 → 4)-α-D-Glcp-(1 → 4)-β-D-Glcp-(1 → 4)-β-D-Glcp-(1 → 4)-β-D-Manp-(1 → 4)-β-D-Manp-(1 → . In vitro experimental results showed that LBP had noble biocompatibility, and a low dose of 5 μg/mL LBP significantly up-regulated the mRNA expression of TNF-α, iNOS, IL-6, IL-1β and Toll-like receptors family (TLRs) in RAW 264.7 cells. In conclusion, LBP played an important role in immunomodulation, and further studies on the specific immunomodulatory mechanisms of LBP on RAW 264.7 cells are still needed.

Sucrose and Mannans Affect Arabidopsis Shoot Gravitropism at the Cell Wall Level

Gravitropism is the plant organ bending in response to gravity. Gravitropism, phototropism and sufficient mechanical strength define the optimal position of young shoots for photosynthesis. Etiolated wild-type Arabidopsis seedlings grown horizontally in the presence of sucrose had a lot more upright hypocotyls than seedlings grown without sucrose. We studied the mechanism of this effect at the level of cell wall biomechanics and biochemistry. Sucrose strengthened the bases of hypocotyls and decreased the content of mannans in their cell walls. As sucrose is known to increase the gravitropic bending of hypocotyls, and mannans have recently been shown to interfere with this process, we examined if the effect of sucrose on shoot gravitropism could be partially mediated by mannans. We compared cell wall biomechanics and metabolomics of hypocotyls at the early steps of gravitropic bending in Col-0 plants grown with sucrose and mannan-deficient mutant seedlings. Sucrose and mannans affected gravitropic bending via different mechanisms. Sucrose exerted its effect through cell wall-loosening proteins, while mannans changed the walls' viscoelasticity. Our data highlight the complexity of shoot gravitropism control at the cell wall level.

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.

Variety-dependent accumulation of glucomannan in the starchy endosperm and aleurone cell walls of rice grains and its possible genetic basis

Plant cell wall plays important roles in the regulation of plant growth/development and affects the quality of plant-derived food and industrial materials. On the other hand, genetic variability of cell wall structure within a plant species has not been well understood. Here we show that the endosperm cell walls, including both starchy endosperm and aleurone layer, of rice grains with various genetic backgrounds are clearly classified into two groups depending on the presence/absence of β-1,4-linked glucomannan. All-or-none distribution of the glucomannan accumulation among rice varieties is very different from the varietal differences of arabinoxylan content in wheat and barley, which showed continuous distributions. Immunoelectron microscopic observation suggested that the glucomannan was synthesized in the early stage of endosperm development, but the synthesis was down-regulated during the secondary thickening process associated with the differentiation of aleurone layer. Significant amount of glucomannan in the cell walls of the glucomannan-positive varieties, i.e., 10% or more of the starchy endosperm cell walls, and its close association with the cellulose microfibril suggested possible effects on the physicochemical/biochemical properties of these cell walls. Comparative genomic analysis indicated the presence of striking differences between OsCslA12 genes of glucomannan-positive and negative rice varieties, Kitaake and Nipponbare, which seems to explain the all-or-none glucomannan cell wall trait in the rice varieties. Identification of the gene responsible for the glucomannan accumulation could lead the way to clarify the effect of the accumulation of glucomannan on the agronomic traits of rice by using genetic approaches.

Differing structures of galactoglucomannan in eudicots and non-eudicot angiosperms

The structures of cell wall mannan hemicelluloses have changed during plant evolution. Recently, a new structure called β-galactoglucomannan (β-GGM) was discovered in eudicot plants. This galactoglucomannan has β-(1,2)-Gal-α-(1,6)-Gal disaccharide branches on some mannosyl residues of the strictly alternating Glc-Man backbone. Studies in Arabidopsis revealed β-GGM is related in structure, biosynthesis and function to xyloglucan. However, when and how plants acquired β-GGM remains elusive. Here, we studied mannan structures in many sister groups of eudicots. All glucomannan structures were distinct from β-GGM. In addition, we searched for candidate mannan β-galactosyltransferases (MBGT) in non-eudicot angiosperms. Candidate AtMBGT1 orthologues from rice (OsGT47A-VII) and Amborella (AtrGT47A-VII) did not show MBGT activity in vivo. However, the AtMBGT1 orthologue from rice showed MUR3-like xyloglucan galactosyltransferase activity in complementation analysis using Arabidopsis. Further, reverse genetic analysis revealed that the enzyme (OsGT47A-VII) contributes to proper root growth in rice. Together, gene duplication and diversification of GT47A-VII in eudicot evolution may have been involved in the acquisition of mannan β-galactosyltransferase activity. Our results indicate that β-GGM is likely to be a eudicot-specific mannan.

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.

Towards an understanding of the enzymatic degradation of complex plant mannan structures

Plant cell walls are composed of a heterogeneous mixture of polysaccharides that require several different enzymes to degrade. These enzymes are important for a variety of biotechnological processes, from biofuel production to food processing. Several classical mannanolytic enzyme functions of glycoside hydrolases (GH), such as β-mannanase, β-mannosidase and α-galactosidase activities, are helpful for efficient mannan hydrolysis. In this light, we bring three enzymes into the model of mannan degradation that have received little or no attention. By linking their three-dimensional structures and substrate specificities, we have predicted the interactions and cooperativity of these novel enzymes with classical mannanolytic enzymes for efficient mannan hydrolysis. The novel exo-β-1,4-mannobiohydrolases are indispensable for the production of mannobiose from the terminal ends of mannans, this product being the preferred product for short-chain mannooligosaccharides (MOS)-specific β-mannosidases. Second, the side-chain cleaving enzymes, acetyl mannan esterases (AcME), remove acetyl decorations on mannan that would have hindered backbone cleaving enzymes, while the backbone cleaving enzymes liberate MOS, which are preferred substrates of the debranching and sidechain cleaving enzymes. The nonhydrolytic expansins and swollenins disrupt the crystalline regions of the biomass, improving their accessibility for AcME and GH activities. Finally, lytic polysaccharide monooxygenases have also been implicated in promoting the degradation of lignocellulosic biomass or mannan degradation by classical mannanolytic enzymes, possibly by disrupting adsorbed mannan residues. Modelling effective enzymatic mannan degradation has implications for improving the saccharification of biomass for the synthesis of value-added and upcycling of lignocellulosic wastes.

Galactoglucomannan oligosaccharides alleviate cadmium toxicity by improving physiological processes in maize

Phosphate fertilisers and past mining activity are significant source of cadmium (Cd) pollution; thus, the concentration of Cd in agricultural soils has been substantially rising. Various substances have been tested for their potential to alleviate the toxicity of Cd and stimulate the accumulation of Cd in plant organs. This study brought new insight of the impact of galactoglucomannan oligosaccharides (GGMOs) on the maize plants grown under/in Cd stress. The application of GGMOs reduced concentration of Cd in the maize leaves and thus GGMOs increased their growth (by 24%), concentration of photosynthetic pigments (up to 39.4%), effective quantum yield of photosystem II (up to 29.6%), and net photosynthetic rate (up to 19.6%). The concentrations of stress markers increased in the Cd and Cd + GGMOs treatment; however, significantly lower concentration was detected in the Cd + GGMOs treatment (malondialdehyde by 21.7%, hydrogen peroxide by 13%). The concentration of auxin increased almost by two-fold in the Cd + GGMOs treatment compared to the Cd treatment. The recovered auxin level and enhanced nutrient uptake are proposed mechanisms of GGMOs' action during stress. GGMOs are molecules with biostimulant potential that could support vitality of maize plants in Cd stress.

Regulation of seed coat mucilage production and modification in Arabidopsis

The Arabidopsis seed coat mucilage is a polysaccharide-rich matrix synthesized by the seed coat epidermal cells. It is a specialized cell wall mainly composed of three types of polysaccharides (i. e. pectin, hemicellulose, and cellulose), and represents as an ideal model system for plant cell wall research. A large number of genes responsible for the synthesis and modification of cell wall polysaccharides have been identified using this model system. Moreover, a subset of regulators controlling mucilage production and modification have been characterized, and the underlying transcriptional regulatory mechanisms have been elucidated. This substantially contributes to the understanding of the molecular mechanisms underlying mucilage synthesis and modification. In this review, we concisely summarize the various genes and regulators involved in seed coat cell differentiation, mucilage biosynthesis and modification, and secondary cell wall formation. In particular, we put emphasis on the latest knowledge gained regarding the transcriptional regulation of mucilage production, which is composed of a hierarchal cascade with three-layer transcriptional regulators. Collectively, we propose an updated schematic framework of the genetic regulatory network controlling mucilage production and modification in the Arabidopsis mucilage secretory cells.

Bricks out of the wall: polysaccharide extramural functions

Plant polysaccharides are components of plant cell walls and/or store energy. However, this oversimplified classification neglects the fact that some cell wall polysaccharides and glycoproteins can localize outside the relatively sharp boundaries of the apoplastic moiety, where they adopt functions not directly related to the cell wall. Such polysaccharide multifunctionality (or 'moonlighting') is overlooked in current research, and in most cases the underlying mechanisms that give rise to unconventional ex muro trafficking, targeting, and functions of polysaccharides and glycoproteins remain elusive. This review highlights major examples of the extramural occurrence of various glycan cell wall components, discusses the possible significance and implications of these phenomena for plant physiology, and lists exciting open questions to be addressed by future research.

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.

Modern mannan: a hemicellulose's journey

Hemicellulosic polysaccharides built of β-1,4-linked mannose units have been found throughout the plant kingdom and have numerous industrial applications. Here, I review recent advances in the biosynthesis and modification of plant β-mannans. These matrix polymers can associate with cellulose bundles to impact the mechanical properties of plant fibers or biocomposites. In certain algae, mannan microfibrils even replace cellulose as the dominant structural component of the cell wall. Conversely, patterned galactoglucomannan found in Arabidopsis thaliana seed mucilage significantly modulates cell wall architecture and abiotic stress tolerance despite its relatively low content. I also discuss the subcellular requirements for β-mannan biosynthesis, the increasing number of carbohydrate-active enzymes involved in this process, and the players that continue to be puzzling. I discuss how cellulose synthase-like enzymes elongate (gluco)mannans in orthogonal hosts and highlight the discoveries of plant enzymes that add specific galactosyl or acetyl decorations. Hydrolytic enzymes such as endo-β-1,4-mannanases have recently been involved in a wide range of biological contexts including seed germination, wood formation, heavy metal tolerance, and defense responses. Synthetic biology tools now provide faster tracks to modulate the increasingly-relevant mannan structures for improved plant traits and bioproducts.

Comparison of six lytic polysaccharide monooxygenases from Thermothielavioides terrestris shows that functional variation underlies the multiplicity of LPMO genes in filamentous fungi

Lytic polysaccharide monooxygenases (LPMOs) are mono-copper enzymes that oxidatively degrade various polysaccharides. Genes encoding LPMOs in the AA9 family are abundant in filamentous fungi while their multiplicity remains elusive. We describe a detailed functional characterization of six AA9 LPMOs from the ascomycetous fungus Thermothielavioides terrestris LPH172 (syn. Thielavia terrestris). These six LPMOs were shown to be upregulated during growth on different lignocellulosic substrates in our previous study. Here, we produced them heterologously in Pichia pastoris and tested their activity on various model and native plant cell wall substrates. All six T. terrestris AA9 (TtAA9) LPMOs produced hydrogen peroxide in the absence of polysaccharide substrate and displayed peroxidase-like activity on a model substrate, yet only five of them were active on selected cellulosic substrates. TtLPMO9A and TtLPMO9E were also active on birch acetylated glucuronoxylan, but only when the xylan was combined with phosphoric acid-swollen cellulose (PASC). Another of the six AA9s, TtLPMO9G, was active on spruce arabinoglucuronoxylan mixed with PASC. TtLPMO9A, TtLPMO9E, TtLPMO9G, and TtLPMO9T could degrade tamarind xyloglucan and, with the exception of TtLPMO9T, beechwood xylan when combined with PASC. Interestingly, none of the tested enzymes were active on wheat arabinoxylan, konjac glucomannan, acetylated spruce galactoglucomannan, or cellopentaose. Overall, these functional analyses support the hypothesis that the multiplicity of the fungal LPMO genes assessed in this study relates to the complex and recalcitrant structure of lignocellulosic biomass. Our study also highlights the importance of using native substrates in functional characterization of LPMOs, as we were able to demonstrate distinct, previously unreported xylan-degrading activities of AA9 LPMOs using such substrates.

IMPORTANCE The discovery of LPMOs in 2010 has revolutionized the industrial biotechnology field, mainly by increasing the efficiency of cellulolytic enzyme cocktails. Nonetheless, the biological purpose of the multiplicity of LPMO-encoding genes in filamentous fungi has remained an open question. Here, we address this point by showing that six AA9 LPMOs from a single fungal strain have various substrate preferences and activities on tested cellulosic and hemicellulosic substrates, including several native xylan substrates. Importantly, several of these activities could only be detected when using copolymeric substrates that likely resemble plant cell walls more than single fractionated polysaccharides do. Our results suggest that LPMOs have evolved to contribute to the degradation of different complex structures in plant cell walls where different biomass polymers are closely associated. This knowledge together with the elucidated novel xylanolytic activities could aid in further optimization of enzymatic cocktails for efficient degradation of lignocellulosic substrates and more.

Importance of Water in Maintaining Softwood Secondary Cell Wall Nanostructure

Water is one of the principal constituents by mass of living plant cell walls. However, its role and interactions with secondary cell wall polysaccharides and the impact of dehydration and subsequent rehydration on the molecular architecture are still to be elucidated. This work combines multidimensional solid-state ¹³C magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) with molecular dynamics modeling to decipher the role of water in the molecular architecture of softwood secondary cell walls. The proximities between all main polymers, their molecular conformations, and interaction energies are compared in never-dried, oven-dried, and rehydrated states. Water is shown to play a critical role at the hemicellulose–cellulose interface. After significant molecular shrinkage caused by dehydration, the original molecular conformation is not fully recovered after rehydration. The changes include xylan becoming more closely and irreversibly associated with cellulose and some mannan becoming more mobile and changing conformation. These irreversible nanostructural changes provide a basis for explaining and improving the properties of wood-based materials.

Galactoglucomannan structure of Arabidopsis seed-coat mucilage in GDP-mannose synthesis impaired mutants

Cell-wall polysaccharides are synthesized from nucleotide sugars by glycosyltransferases. However, in what way the level of nucleotide sugars affects the structure of the polysaccharides is not entirely clear. guanosine diphosphate (GDP)-mannose (GDP-Man) is one of the major nucleotide sugars in plants and serves as a substrate in the synthesis of mannan polysaccharides. GDP-Man is synthesized from mannose 1-phosphate and GTP by a GDP-Man pyrophosphorylase, VITAMIN C DEFECTIVE1 (VTC1), which is positively regulated by the interacting protein KONJAC1 (KJC1) in Arabidopsis. Since seed-coat mucilage can serve as a model of the plant cell wall, we examined the influence of vtc1 and kjc1 mutations on the synthesis of mucilage galactoglucomannan. Sugar composition analysis showed that mannose content in adherent mucilage of kjc1 and vtc1 mutants was only 42% and 11% of the wild-type, respectively, indicating a drastic decrease of galactoglucomannan. On the other hand, structural analysis based on specific oligosaccharides released by endo-β-1,4-mannanase indicated that galactoglucomannan had a patterned glucomannan backbone consisting of alternating residues of glucose and mannose and the frequency of α-galactosyl branches was also similar to the wild type structure. These results suggest that the structure of mucilage galactoglucomannan is mainly determined by properties of glycosyltransferases rather than the availability of nucleotide sugars.

Nanoscale Wetting of Crystalline Cellulose

Cellulose possesses considerable potential for a wide range of sustainable applications. Nanocellulose-based material properties are primarily dependent on the structural surface characteristics of its crystalline planes. Experimental measurements of the affinity of crystalline nanocellulose surfaces with water are scarce and challenging to obtain. Therefore, the relative hydrophilicity of different cellulose allomorphs crystalline planes is often inferred from qualitative assessments of their surface and the exposition of polar groups to the solvent. This work investigates the relative hydrophilicity of cellulose surfaces using molecular dynamics simulations. The behavior of a water droplet laid on different crystal planes was used to determine their relative hydrophilicity. The water molecules fully spread onto highly hydrophilic surfaces. However, a water droplet placed on less hydrophilic surfaces equilibrates as an oblate spheroidal cap allowing the measurement of a contact angle. The results indicate that the Iα (010), Iα (11̅0), Iβ (010), and Iβ (110) faces, as well as the faces of human-made celluloses II and III_I (100), (11̅0), (010), and (110) are all highly hydrophilic. They all have a contact angle value inferior to 11°. Not unexpectedly, the Iα (001) and Iβ (100) surfaces are less hydrophilic with contact angles of 48 and 34°, respectively. However, the Iβ (11̅0) plane, often referred to as a hydrophilic surface, forms a contact angle of about 32°. The results are rationalized in terms of structure, exposure of hydroxyl groups to the solvent, and degree of cellulose-cellulose versus cellulose-water hydrogen bonds on each face. The simulations also show that the surface oxidation degree tunes the surface hydrophilicity in a nonlinear manner due to cooperative effects involving water-cellulose interactions. Our study helps us to understand how the degree of hydrophilicity of cellulose emerges from specific structural features of each crystalline surface.

Two conifer GUX clades are responsible for distinct glucuronic acid patterns on xylan

Wood of coniferous trees (softwood), is a globally significant carbon sink and an important source of biomass. Despite that, little is known about the genetic basis of softwood cell wall biosynthesis. Branching of xylan, one of the main hemicelluloses in softwood secondary cell walls, with glucuronic acid (GlcA) is critical for biomass recalcitrance. Here, we investigate the decoration patterns of xylan by conifer GlucUronic acid substitution of Xylan (GUX) enzymes. Through molecular phylogenetics we identify two distinct conifer GUX clades. Using transcriptional profiling we show that the genes are preferentially expressed in secondary cell wall forming tissues. With in vitro and in planta assays we demonstrate that conifer GUX enzymes from both clades are active glucuronyltransferases. Conifer GUX enzymes from each clade have different specific activities. While members of clade one add evenly spaced GlcA branches, the members of clade two are also capable of glucuronidating two consecutive xyloses. Importantly, these types of xylan patterning are present in softwood. As xylan patterning might modulate xylan-cellulose and xylan-lignin interactions, our results further the understanding of softwood cell wall biosynthesis and provide breeding or genetic engineering targets that can be used to modify softwood properties.

Tailoring renewable materials via plant biotechnology

Plants inherently display a rich diversity in cell wall chemistry, as they synthesize an array of polysaccharides along with lignin, a polyphenolic that can vary dramatically in subunit composition and interunit linkage complexity. These same cell wall chemical constituents play essential roles in our society, having been isolated by a variety of evolving industrial processes and employed in the production of an array of commodity products to which humans are reliant. However, these polymers are inherently synthesized and intricately packaged into complex structures that facilitate plant survival and adaptation to local biogeoclimatic regions and stresses, not for ease of deconstruction and commercial product development. Herein, we describe evolving techniques and strategies for altering the metabolic pathways related to plant cell wall biosynthesis, and highlight the resulting impact on chemistry, architecture, and polymer interactions. Furthermore, this review illustrates how these unique targeted cell wall modifications could significantly extend the number, diversity, and value of products generated in existing and emerging biorefineries. These modifications can further target the ability for processing of engineered wood into advanced high performance materials. In doing so, we attempt to illuminate the complex connection on how polymer chemistry and structure can be tailored to advance renewable material applications, using all the chemical constituents of plant-derived biopolymers, including pectins, hemicelluloses, cellulose, and lignins.

Wood transcriptome analysis of Pinus densiflora identifies genes critical for secondary cell wall formation and NAC transcription factors involved in tracheid formation

Although conifers have significant ecological and economic value, information on transcriptional regulation of wood formation in conifers is still limited. Here, to gain insight into secondary cell wall (SCW) biosynthesis and tracheid formation in conifers, we performed wood tissue-specific transcriptome analyses of Pinus densiflora (Korean red pine) using RNA sequencing. In addition, to obtain full-length transcriptome information, PacBio single molecule real-time iso-sequencing was carried out using RNAs from 28 tissues of P. densiflora. Subsequent comparative tissue-specific transcriptome analysis successfully pinpointed critical genes encoding key proteins involved in biosynthesis of the major secondary wall components (cellulose, galactoglucomannan, xylan and lignin). Furthermore, we predicted a total of 62 NAC (NAM, ATAF1/2 and CUC2) family transcription factor members and identified seven PdeNAC genes preferentially expressed in developing xylem tissues in P. densiflora. Protoplast-based transcriptional activation analysis found that four PdeNAC genes, homologous to VND, NST and SND/ANAC075, upregulated GUS activity driven by an SCW-specific cellulose synthase promoter. Consistently, transient overexpression of the four PdeNACs induced xylem vessel cell-like SCW deposition in both tobacco (Nicotiana benthamiana) and Arabidopsis leaves. Taken together, our data provide a foundation for further research to unravel transcriptional regulation of wood formation in conifers, especially SCW formation and tracheid differentiation.

Cellulose-hemicellulose interactions - A nanoscale view

In this work, we study interactions of five different hemicellulose models, i.e. Galactoglucomannan, O-Acetyl-Galactoglucomannan, Fuco-Galacto-Xyloglucan, 4-O-Methylglucuronoxylan, and 4-O-Methylglucuronoarabinoxylan, and their respective binding strength to cellulose nanocrystals by molecular dynamics simulations. Glucuronoarabinoxylan showed the highest free energy of binding, whereas Xyloglucan had the lowest interaction energies amongst the five models. We further performed simulated shear tests and concluded that failure mostly happens at the inter-molecular interaction level within the hemicellulose fraction, rather than at the interface with cellulose. The presence of water molecules seems to have a weakening effect on the interactions of hemicellulose and cellulose, taking up the available hydroxyl groups on the surface of the cellulose for hydrogen bonding. We believe that these studies can shed light on better understanding of plant cell walls, as well as providing evidence on variability of the structures of different plant sources for extractions, purification, and operation of biorefineries.

Modular biosynthesis of plant hemicellulose and its impact on yeast cells

BACKGROUND

The carbohydrate polymers that encapsulate plants cells have benefited humans for centuries and have valuable biotechnological uses. In the past 5 years, exciting possibilities have emerged in the engineering of polysaccharide-based biomaterials. Despite impressive advances on bacterial cellulose-based hydrogels, comparatively little is known about how plant hemicelluloses can be reconstituted and modulated in cells suitable for biotechnological purposes.

RESULTS

Here, we assembled cellulose synthase-like A (CSLA) enzymes using an optimized Pichia pastoris platform to produce tunable heteromannan (HM) polysaccharides in yeast. By swapping the domains of plant mannan and glucomannan synthases, we engineered chimeric CSLA proteins that made β-1,4-linked mannan in quantities surpassing those of the native enzymes while minimizing the burden on yeast growth. Prolonged expression of a glucomannan synthase from Amorphophallus konjac was toxic to yeast cells: reducing biomass accumulation and ultimately leading to compromised cell viability. However, an engineered glucomannan synthase as well as CSLA pure mannan synthases and a CSLC glucan synthase did not inhibit growth. Interestingly, Pichia cell size could be increased or decreased depending on the composition of the CSLA protein sequence. HM yield and glucose incorporation could be further increased by co-expressing chimeric CSLA proteins with a MANNAN-SYNTHESIS-RELATED (MSR) co-factor from Arabidopsis thaliana.

CONCLUSION

The results provide novel routes for the engineering of polysaccharide-based biomaterials that are needed for a sustainable bioeconomy. The characterization of chimeric cellulose synthase-like enzymes in yeast offers an exciting avenue to produce plant polysaccharides in a tunable manner. Furthermore, cells modified with non-toxic plant polysaccharides such as β-mannan offer a modular chassis to produce and encapsulate sensitive cargo such as therapeutic proteins.

The goo-d stuff: Plantago as a myxospermous model with modern utility

Mucilage, a gel-like layer formed around wetted seeds in a process called myxospermy, has importance as a proxy for studying cell wall polysaccharide biosynthesis and interactions and as a source of valuable health supplements and hydrocolloids. Arabidopsis thaliana has provided unrivalled insight into mucilage/cell wall synthesis, but its lack of commercial utility presents an opportunity to develop an alternative myxospermous model linking genetics, chemistry and functionality. Here, we discuss recent advances in the understanding of mucilage production, composition and properties of Plantago, a promising candidate as an alternative model with economic relevance. We outline how genomic/transcriptomic and chemical analysis advances could be made to strengthen Plantago's use as a model system, through challenging but achievable approaches.

Seed hemicelluloses tailor mucilage properties and salt tolerance

While Arabidopsis seed coat epidermal cells have become an excellent genetic system to study the biosynthesis and structural roles of various cell wall polymers, the physiological function of the secreted mucilaginous polysaccharides remains ambiguous. Seed mucilage is shaped by two distinct classes of highly substituted hemicelluloses along with cellulose and structural proteins, but their interplay has not been explored. We deciphered the functions of four distinct classes of cell wall polymers by generating a series of double mutants with defects in heteromannan, xylan, cellulose, or the arabinogalactan protein SALT-OVERLY SENSITIVE 5 (SOS5), and evaluating their impact on mucilage architecture and seed germination during salt stress. We discovered that muci10 seeds, lacking heteromannan branches, had elevated tolerance to salt stress, while heteromannan elongation mutants exhibited reduced germination in calcium chloride (CaCl₂ ). By contrast, xylan made by MUCILAGE-RELATED21 (MUCI21) was found to be required for the adherence of mucilage pectin to microfibrils made by CELLULOSE SYNTHASE5 (CESA5) as well as to a SOS5-mediated network. Our results indicate that the substitution of xylan and glucomannan in seeds can fine-tune mucilage adherence and salt tolerance, respectively. The study of germinating seeds can thus provide insights into the synthesis, modification and function of complex glycans.

The TaCslA12 gene expressed in the wheat grain endosperm synthesizes wheat-like mannan when expressed in yeast and Arabidopsis

Mannan is a class of cell wall polysaccharides widespread in the plant kingdom. Mannan structure and properties vary according to species and organ. The cell walls of cereal grains have been extensively studied due to their role in cereal processing and to their beneficial effect on human health as dietary fiber. Recently, we showed that mannan in wheat (Triticum aestivum) grain endosperm has a linear structure of β-1,4-linked mannose residues. The aim of this work was to study the biosynthesis and function of wheat grain mannan. We showed that mannan is deposited in the endosperm early during grain development, and we identified candidate mannan biosynthetic genes expressed in the endosperm. The functional study in wheat was unsuccessful therefore our best candidate genes were expressed in heterologous systems. The endosperm-specificTaCslA12 gene expressed in Pichia pastoris and in an Arabidopsis thaliana mutant depleted in glucomannan led to the production of wheat-like linear mannan lacking glucose residues and with moderate acetylation. Therefore, this gene encodes a mannan synthase and is likely responsible for the synthesis of wheat endosperm mannan.

Seed mucilage evolution: Diverse molecular mechanisms generate versatile ecological functions for particular environments

Plant myxodiasporous species have the ability to release a polysaccharidic mucilage upon imbibition of the seed (myxospermy) or the fruit (myxocarpy). This is a widespread capacity in angiosperms providing multiple ecological functions including higher germination efficiency under environmental stresses. It is unclear whether myxodiaspory has one or multiple evolutionary origins and why it was supposedly lost in several species. Here, we summarize recent advances on three main aspects of myxodiaspory. (a) It represents a combination of highly diverse traits at different levels of observation, ranging from the dual tissular origin of mucilage secretory cells to diverse mucilage polysaccharidic composition and ultrastructural organization. (b) An asymmetrical selection pressure is exerted on myxospermy-related genes that were first identified in Arabidopsis thaliana. The A. thaliana and the flax intra-species mucilage variants show that myxospermy is a fast-evolving trait due to high polymorphism in a few genes directly acting on mucilage establishment. In A. thaliana, these actors are downstream of a master regulatory complex and an original phylogenetic overview provided here illustrates that this complex has sequentially evolved after the common ancestor of seed plants and was fully established in the common ancestor of the rosid clade. (c) Newly identified myxodiaspory ecological functions indicate new perspectives such as soil microorganism control and plant establishment support.

Wood hemicelluloses exert distinct biomechanical contributions to cellulose fibrillar networks

Hemicelluloses, a family of heterogeneous polysaccharides with complex molecular structures, constitute a fundamental component of lignocellulosic biomass. However, the contribution of each hemicellulose type to the mechanical properties of secondary plant cell walls remains elusive. Here we homogeneously incorporate different combinations of extracted and purified hemicelluloses (xylans and glucomannans) from softwood and hardwood species into self-assembled networks during cellulose biosynthesis in a bacterial model, without altering the morphology and the crystallinity of the cellulose bundles. These composite hydrogels can be therefore envisioned as models of secondary plant cell walls prior to lignification. The incorporated hemicelluloses exhibit both a rigid phase having close interactions with cellulose, together with a flexible phase contributing to the multiscale architecture of the bacterial cellulose hydrogels. The wood hemicelluloses exhibit distinct biomechanical contributions, with glucomannans increasing the elastic modulus in compression, and xylans contributing to a dramatic increase of the elongation at break under tension. These diverging effects cannot be explained solely from the nature of their direct interactions with cellulose, but can be related to the distinct molecular structure of wood xylans and mannans, the multiphase architecture of the hydrogels and the aggregative effects amongst hemicellulose-coated fibrils. Our study contributes to understanding the specific roles of wood xylans and glucomannans in the biomechanical integrity of secondary cell walls in tension and compression and has significance for the development of lignocellulosic materials with controlled assembly and tailored mechanical properties.

Hemicelluloses are an essential constituent of plant cell walls, but the individual biomechanical roles remain elusive. Here the authors report on the interaction of wood hemicellulose with bacterial cellulose during deposition and explore the resultant fibrillar architecture and mechanical properties.

Enzymatic processing of lignocellulosic biomass: principles, recent advances and perspectives

Efficient saccharification of lignocellulosic biomass requires concerted development of a pretreatment method, an enzyme cocktail and an enzymatic process, all of which are adapted to the feedstock. Recent years have shown great progress in most aspects of the overall process. In particular, increased insights into the contributions of a wide variety of cellulolytic and hemicellulolytic enzymes have improved the enzymatic processing step and brought down costs. Here, we review major pretreatment technologies and different enzyme process setups and present an in-depth discussion of the various enzyme types that are currently in use. We pay ample attention to the role of the recently discovered lytic polysaccharide monooxygenases (LPMOs), which have led to renewed interest in the role of redox enzyme systems in lignocellulose processing. Better understanding of the interplay between the various enzyme types, as they may occur in a commercial enzyme cocktail, is likely key to further process improvements.

Structure and function of Bs164 β-mannosidase from Bacteroides salyersiae the founding member of glycoside hydrolase family GH164

Recent work exploring protein sequence space has revealed a new glycoside hydrolase (GH) family (GH164) of putative mannosidases. GH164 genes are present in several commensal bacteria, implicating these genes in the degradation of dietary glycans. However, little is known about the structure, mechanism of action, and substrate specificity of these enzymes. Herein we report the biochemical characterization and crystal structures of the founding member of this family (Bs164) from the human gut symbiont Bacteroides salyersiae. Previous reports of this enzyme indicated that it has α-mannosidase activity, however, we conclusively show that it cleaves only β-mannose linkages. Using NMR spectroscopy, detailed enzyme kinetics of WT and mutant Bs164, and multiangle light scattering we found that it is a trimeric retaining β-mannosidase, that is susceptible to several known mannosidase inhibitors. X-ray crystallography revealed the structure of Bs164, the first known structure of a GH164, at 1.91 Å resolution. Bs164 is composed of three domains: a (β/α)8 barrel, a trimerization domain, and a β-sandwich domain, representing a previously unobserved structural-fold for β-mannosidases. Structures of Bs164 at 1.80-2.55 Å resolution in complex with the inhibitors noeuromycin, mannoimidazole, or 2,4-dinitrophenol 2-deoxy-2-fluoro-mannoside reveal the residues essential for specificity and catalysis including the catalytic nucleophile (Glu-297) and acid/base residue (Glu-160). These findings further our knowledge of the mechanisms commensal microbes use for nutrient acquisition.

Vascular Plants Are Globally Significant Contributors to Marine Carbon Fluxes and Sinks

More than two-thirds of global biomass consists of vascular plants. A portion of the detritus they generate is carried into the oceans from land and highly productive blue carbon ecosystems-salt marshes, mangrove forests, and seagrass meadows. This large detrital input receives scant attention in current models of the global carbon cycle, though for blue carbon ecosystems, increasingly well-constrained estimates of biomass, productivity, and carbon fluxes, reviewed in this article, are now available. We show that the fate of this detritus differs markedly from that of strictly marine origin, because the former contains lignocellulose-an energy-rich polymer complex of cellulose, hemicelluloses, and lignin that is resistant to enzymatic breakdown. This complex can be depolymerized for nutritional purposes by specialized marine prokaryotes, fungi, protists, and invertebrates using enzymes such as glycoside hydrolases and lytic polysaccharide monooxygenases to release sugar monomers. The lignin component, however, is less readily depolymerized, and detritus therefore becomes lignin enriched, particularly in anoxic sediments, and forms a major carbon sink in blue carbon ecosystems. Eventual lignin breakdown releases a wide variety of small molecules that may contribute significantly to the oceanic pool of recalcitrant dissolved organic carbon. Marine carbon fluxes and sinks dependent on lignocellulosic detritus are important ecosystem services that are vulnerable to human interventions. These services must be considered when protecting blue carbon ecosystems and planning initiatives aimed at mitigating anthropogenic carbon emissions.

Evolutionary origin of O-acetyltransferases responsible for glucomannan acetylation in land plants

Mannans are an abundant cell wall polysaccharide in bryophytes, seedless vascular plants and gymnosperms. A previous study has shown that mannan acetylation in Arabidopsis and konjac is mediated by mannan O-acetyltransferases belonging to the Domain of Unknown Function (DUF) 231 family. However, little is known about the acetylation patterns of mannans in bryophytes and seedless vascular plants, and the evolutionary origin of mannan O-acetyltransferases in land plants has not yet been studied. Phylogenetic analysis of the DUF231 family revealed that DUF231 members were present in the charophycean green algae and evolved to form overlapped and divergent phylogenetic groups in different taxa of land plants. Acetyltransferase activity assays of recombinant proteins demonstrated that a number of group II DUF231 members from moss, Selaginella, pine, spruce, rice and poplar were mannan 2-O- and 3-O-acetyltransferases, whereas the two group I DUF231 members from the alga Klebsormidium nitens were not. Structural analysis of mannans from moss and Selaginella showed they were composed of mannosyl and glucosyl residues and the mannosyl residues were acetylated at O-2 and O-3. These findings indicate that although the DUF231 genes originated in algae, their recruitment as mannan O-acetyltransferases probably occurred in bryophytes, and the biochemical functions of these O-acetyltransferases are evolutionarily conserved throughout land plants.

Structural Imaging of Native Cryo-Preserved Secondary Cell Walls Reveals the Presence of Macrofibrils and Their Formation Requires Normal Cellulose, Lignin and Xylan Biosynthesis

The woody secondary cell walls of plants are the largest repository of renewable carbon biopolymers on the planet. These walls are made principally from cellulose and hemicelluloses and are impregnated with lignin. Despite their importance as the main load bearing structure for plant growth, as well as their industrial importance as both a material and energy source, the precise arrangement of these constituents within the cell wall is not yet fully understood. We have adapted low temperature scanning electron microscopy (cryo-SEM) for imaging the nanoscale architecture of angiosperm and gymnosperm cell walls in their native hydrated state. Our work confirms that cell wall macrofibrils, cylindrical structures with a diameter exceeding 10 nm, are a common feature of the native hardwood and softwood samples. We have observed these same structures in Arabidopsis thaliana secondary cell walls, enabling macrofibrils to be compared between mutant lines that are perturbed in cellulose, hemicellulose, and lignin formation. Our analysis indicates that the macrofibrils in Arabidopsis cell walls are dependent upon the proper biosynthesis, or composed, of cellulose, xylan, and lignin. This study establishes that cryo-SEM is a useful additional approach for investigating the native nanoscale architecture and composition of hardwood and softwood secondary cell walls and demonstrates the applicability of Arabidopsis genetic resources to relate fibril structure with wall composition and biosynthesis.

Mapping and Dynamics of Regulatory DNA in Maturing Arabidopsis thaliana Siliques

The genome is reprogrammed during development to produce diverse cell types, largely through altered expression and activity of key transcription factors. The accessibility and critical functions of epidermal cells have made them a model for connecting transcriptional events to development in a range of model systems. In Arabidopsis thaliana and many other plants, fertilization triggers differentiation of specialized epidermal seed coat cells that have a unique morphology caused by large extracellular deposits of polysaccharides. Here, we used DNase I-seq to generate regulatory landscapes of A. thaliana seeds at two critical time points in seed coat maturation (4 and 7 DPA), enriching for seed coat cells with the INTACT method. We found over 3,000 developmentally dynamic regulatory DNA elements and explored their relationship with nearby gene expression. The dynamic regulatory elements were enriched for motifs for several transcription factors families; most notably the TCP family at the earlier time point and the MYB family at the later one. To assess the extent to which the observed regulatory sites in seeds added to previously known regulatory sites in A. thaliana, we compared our data to 11 other data sets generated with 7-day-old seedlings for diverse tissues and conditions. Surprisingly, over a quarter of the regulatory, i.e. accessible, bases observed in seeds were novel. Notably, plant regulatory landscapes from different tissues, cell types, or developmental stages were more dynamic than those generated from bulk tissue in response to environmental perturbations, highlighting the importance of extending studies of regulatory DNA to single tissues and cell types during development.

Emerging Functions for Cell Wall Polysaccharides Accumulated during Eudicot Seed Development

The formation of seeds is a reproductive strategy in higher plants that enables the dispersal of offspring through time and space. Eudicot seeds comprise three main components, the embryo, the endosperm and the seed coat, where the coordinated development of each is important for the correct formation of the mature seed. In addition, the seed coat protects the quiescent progeny and can provide transport mechanisms. A key underlying process in the production of seed tissues is the formation of an extracellular matrix termed the cell wall, which is well known for its essential function in cytokinesis, directional growth and morphogenesis. The cell wall is composed of a macromolecular network of polymers where the major component is polysaccharides. The attributes of polysaccharides differ with their composition and charge, which enables dynamic remodeling of the mechanical and physical properties of the matrix by adjusting their production, modification or turnover. Accordingly, the importance of specific polysaccharides or modifications is increasingly being associated with specialized functions within seed tissues, often through the spatio-temporal accumulation or remodeling of particular polymers. Here, we review the evolution and accumulation of polysaccharides during eudicot seed development, what is known of their impact on wall architecture and the diverse roles associated with these in different seed tissues.