Cell Walls Of Vascular Plants Research Articles

Putative rhamnogalacturonan-II glycosyltransferase identified through callus gene editing bypasses embryo lethality.

Rhamnogalacturonan II (RG-II) is a structurally complex and conserved domain of the pectin present in the primary cell walls of vascular plants. Borate crosslinking of RG-II is required for plants to grow and develop normally. Mutations that alter RG-II structure also affect crosslinking and are lethal or severely impair growth. Thus, few genes involved in RG-II synthesis have been identified. Here we developed a method to generate viable loss-of-function Arabidopsis (Arabidopsis thaliana) mutants in callus tissue via CRISPR/Cas9-mediated gene editing. We combined this with a candidate gene approach to characterize the male gametophyte defective 2 (MPG2) gene that encodes a putative family GT29 glycosyltransferase. Plants homozygous for this mutation do not survive. We showed that in the callus mutant cell walls, RG-II does not crosslink normally because it lacks 3-deoxy-D-manno-octulosonic acid (Kdo) and thus cannot form the α-L-Rhap-(1→5)-α-D-kdop-(1→ sidechain. We suggest that MGP2 encodes an inverting RG-II CMP-β-Kdo transferase (RCKT1). Our discovery provides further insight into the role of sidechains in RG-II dimerization. Our method also provides a viable strategy for further identifying proteins involved in the biosynthesis of RG-II.

Discovery, structural characterization, and functional insights into a novel apiosidase from the GH140 family, isolated from a lignocellulolytic-enriched mangrove microbial community.

Apiosidases are enzymes that cleave the glycosidic bond between the monosaccharides linked to apiose, a branched chain furanose found in the cell walls of vascular plants and aquatic monocots. There is biotechnological interest in this enzyme group because apiose is the flavor-active compound of grapes, fruit juice, and wine, and the monosaccharide is found to be a plant secondary metabolite with pharmaceutical properties. However, functional and structural studies of this enzyme family are scarce. Recently, a glycoside hydrolase family member GH140 was isolated fromBacteroides thetaiotaomicron and identified as an endo-apiosidase. The structural characterization and functional identification of a second GH140 family enzyme, termed MmApi, discovered through mangrove soil metagenomic approach, are described. Among the various substrates tested, MmApi exhibited activity on an apiose-containing oligosaccharide derived from the pectic polysaccharide rhamnogalacturonan-II. While the crystallographic model of MmApi was similar to the endo-apiosidase fromBacteroides thetaiotaomicron, differences in the shape of the binding sites indicated that MmApi could cleave apioses within oligosaccharides of different compositions. This enzyme represents a novel tool for researchers interested in studying the physiology and structure of plant cell walls and developing biocatalytic strategies for drug and flavor production.

Insights from molecular dynamics simulations for interaction between cellulose microfibrils and hemicellulose

The interaction between cellulose microfibrils and xylan is crucial for the strength of the secondary cell wall of vascular plants.

Deficiency in flavonoid biosynthesis genes CHS, CHI, and CHIL alters rice flavonoid and lignin profiles.

Lignin is a complex phenylpropanoid polymer deposited in the secondary cell walls of vascular plants. Unlike most gymnosperm and eudicot lignins that are generated via the polymerization of monolignols, grass lignins additionally incorporate the flavonoid tricin as a natural lignin monomer. The biosynthesis and functions of tricin-integrated lignin (tricin-lignin) in grass cell walls and its effects on the utility of grass biomass remain largely unknown. We herein report a comparative analysis of rice (Oryza sativa) mutants deficient in the early flavonoid biosynthetic genes encoding CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), and CHI-LIKE (CHIL), with an emphasis on the analyses of disrupted tricin-lignin formation and the concurrent changes in lignin profiles and cell wall digestibility. All examined CHS-, CHI-, and CHIL-deficient rice mutants were largely depleted of extractable flavones, including tricin, and nearly devoid of tricin-lignin in the cell walls, supporting the crucial roles of CHS and CHI as committed enzymes and CHIL as a noncatalytic enhancer in the conserved biosynthetic pathway leading to flavone and tricin-lignin formation. In-depth cell wall structural analyses further indicated that lignin content and composition, including the monolignol-derived units, were differentially altered in the mutants. However, regardless of the extent of the lignin alterations, cell wall saccharification efficiencies of all tested rice mutants were similar to that of the wild-type controls. Together with earlier studies on other tricin-depleted grass mutant and transgenic plants, our results reflect the complexity in the metabolic consequences of tricin pathway perturbations and the relationships between lignin profiles and cell wall properties.

Xyloglucan processing machinery in Xanthomonas pathogens and its role in the transcriptional activation of virulence factors

Xyloglucans are highly substituted and recalcitrant polysaccharides found in the primary cell walls of vascular plants, acting as a barrier against pathogens. Here, we reveal that the diverse and economically relevant Xanthomonas bacteria are endowed with a xyloglucan depolymerization machinery that is linked to pathogenesis. Using the citrus canker pathogen as a model organism, we show that this system encompasses distinctive glycoside hydrolases, a modular xyloglucan acetylesterase and specific membrane transporters, demonstrating that plant-associated bacteria employ distinct molecular strategies from commensal gut bacteria to cope with xyloglucans. Notably, the sugars released by this system elicit the expression of several key virulence factors, including the type III secretion system, a membrane-embedded apparatus to deliver effector proteins into the host cells. Together, these findings shed light on the molecular mechanisms underpinning the intricate enzymatic machinery of Xanthomonas to depolymerize xyloglucans and uncover a role for this system in signaling pathways driving pathogenesis.

LACCASE2 Negatively Regulates Lignin Deposition of Arabidopsis Roots.

Lignin is a complex aromatic biopolymer in the secondary cell wall of vascular plants. It is crucial for providing physical strength and pathogen resistance to the plant. Lignified xylem vessels are also necessary for efficient water transport. Lignin is formed by oxidative polymerization of

Atypical lignification in eastern leatherwood (Dirca palustris).

Summary Lignin is a complex phenolic biopolymer found mainly in the secondary cell walls of vascular plants, where it contributes to mechanical strength, water conduction, and plant defence. We studied the lignin of eastern leatherwood (Dirca palustris) because this slow‐growing woody shrub is known for its flexible stems.Various analytical techniques and microscopy methods were employed to examine the composition and distribution of lignin and structural polysaccharides in leatherwood xylem in comparison with trembling aspen (Populus tremuloides) and white spruce (Picea glauca).We found that leatherwood has low overall levels of lignin, a high syringyl lignin content, and a unique distribution of lignin. Most remarkably, the cell corners and middle lamellae remain unlignified in mature xylem.These findings help explain the flexibility of leatherwood and also call into question the classical model of lignification, which purports that lignin polymerization begins in the cell corners and middle lamellae. This atypical lignification regime vividly illustrates the diversity in plant secondary cell wall formation that abounds in nature and casts leatherwood as a new model for the study of lignin biogenesis.

OsCAldOMT1 is a bifunctional O-methyltransferase involved in the biosynthesis of tricin-lignins in rice cell walls

Lignin is a phenylpropanoid polymer produced in the secondary cell walls of vascular plants. Although most eudicot and gymnosperm species generate lignins solely via polymerization of p-hydroxycinnamyl alcohols (monolignols), grasses additionally use a flavone, tricin, as a natural lignin monomer to generate tricin-incorporated lignin polymers in cell walls. We previously found that disruption of a rice 5-HYDROXYCONIFERALDEHYDE O-METHYLTRANSFERASE (OsCAldOMT1) reduced extractable tricin-type metabolites in rice vegetative tissues. This same enzyme has also been implicated in the biosynthesis of sinapyl alcohol, a monolignol that constitutes syringyl lignin polymer units. Here, we further demonstrate through in-depth cell wall structural analyses that OsCAldOMT1-deficient rice plants produce altered lignins largely depleted in both syringyl and tricin units. We also show that recombinant OsCAldOMT1 displayed comparable substrate specificities towards both 5-hydroxyconiferaldehyde and selgin intermediates in the monolignol and tricin biosynthetic pathways, respectively. These data establish OsCAldOMT1 as a bifunctional O-methyltransferase predominantly involved in the two parallel metabolic pathways both dedicated to the biosynthesis of tricin-lignins in rice cell walls. Given that cell wall digestibility was greatly enhanced in the OsCAldOMT1-deficient rice plants, genetic manipulation of CAldOMTs conserved in grasses may serve as a potent strategy to improve biorefinery applications of grass biomass.

Host lignin composition affects haustorium induction in the parasitic plants Phtheirospermum japonicum and Striga hermonthica.

Parasitic plants in the family Orobanchaceae are destructive weeds of agriculture worldwide. The haustorium, an essential parasitic organ used by these plants to penetrate host tissues, is induced by host-derived phenolic compounds called haustorium-inducing factors (HIFs). The origin of HIFs remains unknown, although the structures of lignin monomers resemble that of HIFs. Lignin is a natural phenylpropanoid polymer, commonly found in secondary cell walls of vascular plants. We therefore investigated the possibility that HIFs are derived from host lignin. Various lignin-related phenolics, quinones and lignin polymers, together with nonhost and host plants that have different lignin compositions, were tested for their haustorium-inducing activity in two Orobanchaceae species, a facultative parasite, Phtheirospermum japonicum, and an obligate parasite, Striga hermonthica. Lignin-related compounds induced haustoria in P.japonicum and S.hermonthica with different specificities. High concentrations of lignin polymers induced haustorium formation. Treatment with laccase, a lignin degradation enzyme, promoted haustorium formation at low concentrations. The distinct lignin compositions of the host and nonhost plants affected haustorium induction, correlating with the response of the different parasitic plants to specific types of lignin-related compounds. Our study provides valuable insights into the important roles of lignin biosynthesis and degradation in the production of HIFs.

Disrupting Flavone Synthase II Alters Lignin and Improves Biomass Digestibility.

Lignin, a ubiquitous phenylpropanoid polymer in vascular plant cell walls, is derived primarily from oxidative couplings of monolignols (p-hydroxycinnamyl alcohols). It was discovered recently that a wide range of grasses, including cereals, utilize a member of the flavonoids, tricin (3',5'-dimethoxyflavone), as a natural comonomer with monolignols for cell wall lignification. Previously, we established that cytochrome P450 93G1 is a flavone synthase II (OsFNSII) indispensable for the biosynthesis of soluble tricin-derived metabolites in rice (Oryza sativa). Here, our tricin-deficient fnsII mutant was analyzed further with an emphasis on its cell wall structure and properties. The mutant is similar in growth to wild-type control plants with normal vascular morphology. Chemical and nuclear magnetic resonance structural analyses demonstrated that the mutant lignin is completely devoid of tricin, indicating that FNSII activity is essential for the deposition of tricin-bound lignin in rice cell walls. The mutant also showed substantially reduced lignin content with decreased syringyl/guaiacyl lignin unit composition. Interestingly, the loss of tricin in the mutant lignin appears to be partially compensated by incorporating naringenin, which is a preferred substrate of OsFNSII. The fnsII mutant was further revealed to have enhanced enzymatic saccharification efficiency, suggesting that the cell wall recalcitrance of grass biomass may be reduced through the manipulation of the flavonoid monomer supply for lignification.

Understanding of plant cell wall biosynthesis for utilization of lignocellulosic biomass resources

How do plants make cell walls? How can the cell wall biomass be turned into fuel energy? These questions are still unsolved puzzles that have attracted a great deal of interest in scientific community as well as in industrial and political worlds. Different from animal cells, plant cells develop characteristic structures of cell walls, which play crucial roles in planting growth and response to environments. As cell walls accumulate approximately seventy percent of the land pant photosynthesized products on the globe, cell wall biomass is the most abundant renewable resource for production of bioenergy and biomaterials. However, only a small portion (~2%) of the plant cell wall resource has been used by human society so far. More efficient utilization of lignocellulosic cell wall biomass is highly desired. Great effort has been invested in understanding of how cell walls are formed in plant in recent years. Plants cell walls contain cellulose, hemicelluloses, pectic polysaccharides and lignin. These polysaccharides and phenolic polymers are constructed in concrete cell walls for supporting plant growth and development. Cellulose is composed of β-1,4-linked glucose units and a bundle of multiple -1,4-glucan chains together form microfibrils, providing a skeleton of cell wall structures. Cellulose is synthesized in plants by cellulose synthase (CesA) complexes which are integrated and localized in plasma membrane. There are a family of CesA genes in plants which direct cellulose synthesis in different layer of cell walls or regulate cellulose microfibril structures. Although cellulose has been used in many aspects of human life, such as fiber cloth, paper, and modified cellulosic products, Cellulose, the most abundant biopolymer on the Earth, promises more utilization in supporting for sustainability of human society development. Hemicellulose is a group of diverse polysaccharides, such as xyloglucan, heteromannans, heteroxylans and mixed- linkage glucan. Their diverse structure and sugar composition varies in plant species, tissues, even cell types. Hemicellulose polysaccharides, amorphous and soluble in aqueous solutions, are composed of heterogeneous monosaccharide units or linkages in their backbone and branched side chains. Hemicelluloses are usually covalently linked to cellulose and lignin in cell wall matrix. Hemicellulose has been used in many applications such as food products and nutritional supplements. How is hemicellulose synthesized? How do the diverse hemicelluloses function in plant development? How can the abundant hemicellulose sugars be used? Many questions remain to be investigated. Lignin is polymerized from phenolic monolignols and mainly deposited in the secondary cell walls of vascular plants. Evolved in higher plants, lignin is essential for long distance water transport, mechanical support of up-growth, pathogen defense and for other stress responses. In the cell walls, lignin is usually cross-linked with cellulose and hemicellulose. Although biosynthesis of monolignols including p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) monolignols has been elucidated genetically and biochemically in various plant species, regulation of monolignol formation in different species, tissues and cell types is still unclear. As the second most abundant plant biopolymer after cellulose on the globe, utilization of lignin is highly attended. Lignin is considered as a main recalcitrance during cell wall biomass conversion, therefore modification of lignin biosynthesis is of great interest in engineering plant cell walls for efficient biomass utilization. Laboratory research has showed potential benefits of lignin engineering for lignocellulosic biomass conversion, while application of the technology in the field requires more evaluation. This review summarizes recent studies of the biosynthesis of plant cell wall components such as cellulose, hemicelluloses and lignin as well as the modification of cell walls for the improvement of lignocellulosic biomass utilization.

Immunolocalization of cell wall carbohydrate epitopes in seaweeds: presence of land plant epitopes in Fucus vesiculosus L. (Phaeophyceae).

Land plant cell wall glycan epitopes are present in Fucus vesiculosus. RG-I/AG mAbs recognize distinct glycan epitopes in structurally different galactans, and 3-linked glucans are also present in the cell walls. Cell wall-directed monoclonal antibodies (mAbs) have given increased knowledge of fundamental land plant processes but are not extensively used to study seaweeds. We profiled the brown seaweed Fucus vesiculosus glycome employing 155 mAbs that recognize predominantly vascular plant cell wall glycan components. The resulting profile was used to inform in situ labeling studies. Several of the mAbs recognized and bound to epitopes present in different thallus parts of Fucus vesiculosus. Antibodies recognizing arabinogalactan epitopes were divided into four groups based on their immunolocalization patterns. Group 1 bound to the stipe, blade, and receptacles. Group 2 bound to the antheridia, oogonia and paraphyses. Group 3 recognized antheridia cell walls and Group 4 localized on the antheridia inner wall and oogonia mesochite. This study reveals that epitopes present in vascular plant cell walls are also present in brown seaweeds. Furthermore, the diverse in situ localization patterns of the RG-I/AG clade mAbs suggest that these mAbs likely detect distinct epitopes present in structurally different galactans. In addition, 3-linked glucans were also detected throughout the cell walls of the algal tissues, using the β-glucan-directed LAMP mAb. Our results give insights into cell wall evolution, and diversify the available tools for the study of brown seaweed cell walls.

Vibrational fingerprint mapping reveals spatial distribution of functional groups of lignin in plant cell wall.

Highly lignified vascular plant cell walls represent the majority of cellulosic biomass. Complete release of the biomass to deliver renewable energy by physical, chemical, and biological pretreatments is challenging due to the "protection" provided by polymerized lignin, and as such, additional tools to monitor lignin deposition and removal during plant growth and biomass deconstruction would be of great value. We developed a hyperspectral stimulated Raman scattering microscope with 9 cm(-1) spectral resolution and submicrometer spatial resolution. Using this platform, we mapped the aromatic ring of lignin, aldehyde, and alcohol groups in lignified plant cell walls. By multivariate curve resolution of the hyperspectral images, we uncovered a spatially distinct distribution of aldehyde and alcohol groups in the thickened secondary cell wall. These results collectively contribute to a deeper understanding of lignin chemical composition in the plant cell wall.

Distribution of lignin and lignin precursors in differentiating xylem of Japanese cypress and poplar

Lignin is an integral component of the cell wall of vascular plants. The mechanism of supply of lignin precursors from the cytosol into the cell wall of differentiating xylem has not yet been elucidated. The present study showed that a certain amount of coniferyl alcohol glucoside (coniferin) occurred in the differentiating xylem of Japanese cypress (Chamaecyparis obtusa), as previously reported in gymnosperms. Coniferin content peaked in the early stages of secondary wall formation and decreased during lignification. In contrast to gymnosperms, coniferin content was limited in the differentiating xylem of poplar (Populus sieboldii × Populus grandidentata). Moreover, coniferyl alcohol was not detected in all specimens. In the differentiating xylem of poplar, a higher amount of sinapyl alcohol occurred than glucoside (syringin). However, the phloem contained syringin and not sinapyl alcohol. The sinapyl alcohol content in the xylem peaked in the cells with ceasing cell wall formation, and decreased gradually towards the boundary of the annual ring, where the lignin content kept increasing. Sinapyl alcohol in the differentiating xylem of poplar may be used for the lignification of the xylem.

Putative Cationic Cell-Wall-Bound Peroxidase Homologues in Arabidopsis, AtPrx2, AtPrx25, and AtPrx71, Are Involved in Lignification

The final step of lignin biosynthesis, which is catalyzed by a plant peroxidase, is the oxidative coupling of the monolignols to growing lignin polymers. Cationic cell-wall-bound peroxidase (CWPO-C) from poplar callus is a unique enzyme that has oxidative activity for both monolignols and synthetic lignin polymers. This study shows that putative CWPO-C homologues in Arabidopsis , AtPrx2, AtPrx25, and AtPrx71, are involved in lignin biosynthesis. Analysis of stem tissue using the acetyl bromide method and derivatization followed by the reductive cleavage method revealed a significant decrease in the total lignin content of ATPRX2 and ATPRX25 deficient mutants and altered lignin structures in ATPRX2, ATPRX25, and ATPRX71 deficient mutants. Among Arabidopsis peroxidases, AtPrx2 and AtPrx25 conserve a tyrosine residue on the protein surface, and this tyrosine may act as a substrate oxidation site as in the case of CWPO-C. AtPrx71 has the highest amino acid identity with CWPO-C. The results suggest a role for CWPO-C and CWPO-C-like peroxidases in the lignification of vascular plant cell walls.

Ceratopteris richardii (C-fern): a model for investigating adaptive modification of vascular plant cell walls.

Plant cell walls are essential for most aspects of plant growth, development, and survival, including cell division, expansive cell growth, cell-cell communication, biomechanical properties, and stress responses. Therefore, characterizing cell wall diversity contributes to our overall understanding of plant evolution and development. Recent biochemical analyses, concomitantly with whole genome sequencing of plants located at pivotal points in plant phylogeny, have helped distinguish between homologous characters and those which might be more derived. Most plant lineages now have at least one fully sequenced representative and although genome sequences for fern species are in progress they are not yet available for this group. Ferns offer key advantages for the study of developmental processes leading to vascularisation and complex organs as well as the specific differences between diploid sporophyte tissues and haploid gametophyte tissues and the interplay between them. Ceratopteris richardii has been well investigated building a body of knowledge which combined with the genomic and biochemical information available for other plants will progress our understanding of wall diversity and its impact on evolution and development.

Role of lignin in a biorefinery: separation characterization and valorization

AbstractLignin, a major component of the cell wall of vascular plants, has long been recognized for its negative impact and treated as a by‐product in a biorefinery. This highly abundant by‐product of the biorefinery is undervalued and underdeveloped due to its complex nature. The development of value‐added products from lignin would greatly improve the economics of the biorefinery. The inherent properties of lignin significantly affect the productivity of the biorefinery processes and its potential applications. Although the structure and biosynthetic pathway of lignin have been studied for more than a century, they have not yet been completely elucidated. In this mini‐review, the primary obstacles to elucidating the structure of native lignin, including separation and characterization, are highlighted. Several classical methods for separation and various NMR techniques, especially 2D HSQC NMR, for characterization of lignin are reviewed. Some potential applications of lignin are introduced. It is believed that a knowledge of the method to separate lignin from the cell wall and structural features of the lignin polymer from lignocellulosic materials will help to maximize the exploitation of lignocelluloses for the biorefinery as well as the utilization of lignin for novel materials and chemicals. © 2012 Society of Chemical Industry

Structures of xyloglucans in primary cell walls of gymnosperms, monilophytes (ferns sensu lato) and lycophytes

Little is known about the structures of the xyloglucans in the primary cell walls of vascular plants (tracheophytes) other than angiosperms. Xyloglucan structures were examined in 13 species of gymnosperms, 13 species of monilophytes (ferns sensu lato), and two species of lycophytes. Wall preparations were obtained, extracted with 6M sodium hydroxide, and the extracts treated with a xyloglucan-specific endo-(1→4)-β-glucanase preparation. The oligosaccharides released were analysed by matrix-assisted laser-desorption ionisation time-of-flight mass spectrometry and by high-performance anion-exchange chromatography. The xyloglucan oligosaccharide profiles from the gymnosperm walls were similar to those from the walls of most eudicotyledons and non-commelinid monocotyledons, indicating that the xyloglucans were fucogalactoxyloglucans, containing the fucosylated units XXFG and XLFG. The xyloglucan oligosaccharide profiles for six of the monilophyte species were similar to those of the gymnosperms, indicating they were also fucogalactoxyloglucans. Phylogenetically, these monilophyte species were from both basal and more derived orders. However, the profiles for the other monilophyte species showed various significant differences, including additional oligosaccharides. In three of the species, these additional oligosaccharides contained arabinosyl residues which were most abundant in the profile of Equisetum hyemale. The two species of lycophytes examined, Selaginella kraussiana and Lycopodium cernuum, had quite different xyloglucan oligosaccharide profiles, but neither were fucogalactoxyloglucans. The S. kraussiana profile had abundant oligosaccharides containing arabinosyl residues. The L. cernuum profile indicated the xyloglucan had a very complex structure.

Lignin modification improves the biofuel production potential in transgenic Populus tomentosa

Lignin has been recognized for its negative impact on forage digestibility, tree pulping properties, and cellulosic biofuel production, although it is the major structural component of the secondarily thickened cell walls of vascular plants. Earlier studies have demonstrated that lignin modification improves forage digestibility and poplar pulping properties. To determine whether lignin modification has beneficial effect on saccharification of lignocellulosic biomass, we pretreated and then enzymatically hydrolyzed the mature wood from transgenic poplar plants that expressed the antisense transgenes of monolignol biosynthesis genes 4-coumarate: CoA ligase (4CL) or caffeoyl CoA 3-O-methyltransferase (CCoAOMT). Firstly, a long-term field trial was set up for the transgenic plants. Over five years, the reduced trend of lignin content remained stable in all transgenic lines. And a total lignin reduction of up to 10% did not alter the growth rate or biomass yield of the transgenic poplars. In the mature wood, suppression of CCoAOMT increased saccharification potential, but 4CL down-regulation had no significantly positive effect on saccharification. Sugar yield were negatively correlated with soluble lignin content of dried, extractive-free stem biomass. These results imply that lignin modification can facilitate the process of saccharification for biofuel production in tree crops.

The ability of land plants to synthesize glucuronoxylans predates the evolution of tracheophytes

Glucuronoxylans with a backbone of 1,4-linked β-D-xylosyl residues are ubiquitous in the secondary walls of gymnosperms and angiosperms. Xylans have been reported to be present in hornwort cell walls, but their structures have not been determined. In contrast, the presence of xylans in the cell walls of mosses and liverworts remains a subject of debate. Here we present data that unequivocally establishes that the cell walls of leafy tissue and axillary hair cells of the moss Physcomitrella patens contain a glucuronoxylan that is structurally similar to glucuronoxylans in the secondary cell walls of vascular plants. Some of the 1,4-linked β-D-xylopyranosyl residues in the backbone of this glucuronoxylan bear an α-D-glucosyluronic acid (GlcpA) sidechain at O-2. In contrast, the lycopodiophyte Selaginella kraussiana synthesizes a glucuronoxylan substituted with 4-O-Me-α-D-GlcpA sidechains, as do many hardwood species. The monilophyte Equisetum hyemale produces a glucuronoxylan with both 4-O-Me-α-D-GlcpA and α-D-GlcpA sidechains, as does Arabidopsis. The seedless plant glucuronoxylans contain no discernible amounts of the reducing-end sequence that is characteristic of gymnosperm and eudicot xylans. Phylogenetic studies showed that the P. patens genome contains genes with high sequence similarity to Arabidopsis CAZy family GT8, GT43 and GT47 glycosyltransferases that are likely involved in xylan synthesis. We conclude that mosses synthesize glucuronoxylan that is structurally similar to the glucuronoxylans present in the secondary cell walls of lycopodiophytes, monilophytes, and many seed-bearing plants, and that several of the glycosyltransferases required for glucuronoxylan synthesis evolved before the evolution of tracheophytes.