Cell Wall Polysaccharide Fractions Research Articles

Atomic Force Microscopy in the Characterization of the Structure of Cell Wall Components.

Atomic force microscopy (AFM) has broken boundaries in the characterization of the supramolecular architecture of cell wall assemblies and single cell wall polysaccharides at the nanoscale level. Moreover, AFM provides an opportunity to evaluate the mechanical properties of cell wall material which is not possible with any other method. However, in the case of plant tissue, the critical step is a smart sample preparation that should not affect the polysaccharide structure or assembly and on the other hand should consider device limitations, especially scanner ranges. In this chapter, the protocols from the sample preparation, including isolation of cell wall material and extraction of cell wall polysaccharide fractions, through AFM imaging of polysaccharide assemblies and single molecules until an image analysis to obtain quantitative data characterizing the biopolymers are presented.

Cell wall polysaccharides from Annona squamosa: Chemical and functional characterization

Several extraction methods were used to find out what the custard apple cell wall polysaccharides were made of and how they were structured. These included water (WSP), chelator (CSP), sodium carbonate (NSP), potassium hydroxide at 1 mol/L (KSH-1), and potassium hydroxide at 4 mol/L (KSH-4). The five fractions had different structures and make-ups, as shown by FT-IR, molecular weight measurements, sugar composition analysis, and methylation experiments. The results indicated that NSP made up the majority of the fruit's cell wall polysaccharide fraction. As for apparent viscosity, the viscosity and thermal stability of WSP were highest. In terms of apparent viscosity and thermal stability, WSP ranked highest. The stability of NSP and KSH-1 was better. Methylation analysis showed that CSP had galactose and galacturonic acid as the main chains. KSH-4 had a xyloglucan backbone. Compared to the other three fractions, NSP and CSP had the highest antioxidant capacity. Through this study, there is potential for custard apple polysaccharides to be used in the food and biological industries.

Cellulose and JbKOBITO 1 mediate the resistance of NaHCO3-tolerant chlorella to saline-alkali stress.

Carbonate stress has profound impacts on both agricultural and industrial production. Although a number of salinity-tolerant genes have been reported and applied in plants, there is a lack of research on the role of cell wall-related genes in resistance to carbonate. Likewise, in industry, current strategies have not been able to more effectively address the conflict between stress-induced microalgal biofuel accumulation and microalgal growth inhibition. It is of great significance to study the adaptation mechanism of carbonate-tolerant organisms and to explore related genes for future genetic modification. In this study, the role of the cell wall in the NaHCO3-tolerant chlorella JB17 was investigated. We found that JB17 possesses a relatively thick cell wall with a thickness of 300-600 nm, which is much higher than that of the control chlorella with a thickness of about 100 nm. Determination of the cell wall polysaccharide fractions showed that the cellulose content in the JB17 cell wall increased by 10.48% after NaHCO3 treatment, and the decrease in cellulose levels by cellulase digestion inhibited its resistance to NaHCO3. Moreover, the saccharide metabolome revealed that glucose, rhamnose, and trehalose levels were higher in JB17, especially rhamnose and trehalose, which were almost 40 times higher than in control chlorella. Gene expression detection identified an up-regulated expressed gene after NaHCO3 treatment, JbKOBITO1, overexpression of which could improve the NaHCO3 tolerance of Chlamydomonas reinhardtii. As it encodes a glycosyltransferase-like protein that is involved in cellulose synthesis, the strong tolerance of JB17 to NaHCO3 may be partly due to the up-regulated expression of JbKOBITO 1 and JbKOBITO 1-mediated cellulose accumulation. The above results revealed a critical role of cellulose in the NaHCO3 resistance of JB17, and the identified NaHCO3-tolerance gene will provide genetic resources for crop breeding in saline-alkali soils and for genetic modification of microalgae for biofuel production.

Mechanisms of texture and cell microstructure changes during post-ripening of ‘Cuixiang’ kiwifruit

The lack of ability to control post-ripening softening is a bottleneck that affects the economic efficiency of ‘Cuixiang’ kiwifruit. This study investigated the softening mechanisms of fruit texture and microstructural changes during post-ripening at room temperature and established a kinetic model to predict and validate shelf life. The starch content of ‘Cuixiang’ decreased, total soluble solid (TSS) and sugar–acid ratio (SAR) increased, and the texture gradually became softer during post-ripening. Microstructural changes reflected starch fragmentation and disintegration, cell structure collapse, plasmolysis, and intensified mitochondrial vacuolization and cell-wall degradation during post-ripening. Changes in cell wall polysaccharide fractions and the evolution of cell membrane lipid metabolism indicated that post-ripening was accompanied by cellulose and hemicellulose degradation. In addition, pectin components transformed into water-soluble pectin, and oxidative stress across the cell membrane caused membrane damage and rupture, increasing membrane permeability. A zero-order kinetic model of firmness was selected to accurately and reliably predict the shelf life of ‘Cuixiang’ kiwifruit.

Discovery of novel secretome CAZymes from Penicillium sclerotigenum by bioinformatics and explorative proteomics analyses during sweet potato pectin digestion.

Novel selective enzymatic refining of sweet potato processing residues requires judicious enzyme selection and enzyme discovery. We prepared a pectinaceous cell wall polysaccharide fraction from sweet potato using an enzymatic a treatment to preserve the natural linkages and substitutions. Polysaccharide composition and linkage analysis data confirmed the pectinaceous polysaccharide fraction to be a rhamnogalacturonan I-rich fraction with a high content of arabinogalactan Type I. We hypothesized that the post-harvest tuber pathogenic fungus Penicillium sclerotigenum would harbor novel enzymes targeting selective sweet potato pectin modification. As part of the study, we also report the first genome sequence of P. sclerotigenum. We incubated the sweet potato pectinaceous fraction with P. sclerotigenum. Using proteomics accompanied by CUPP-bioinformatics analysis, we observed induced expression of 23 pectin-associated degradative enzymes. We also identified six abundantly secreted, induced proteins that do not correspond to known CAZymes, but which we suggest as novel enzymes involved in pectin degradation. For validation, the predicted CUPP grouping of putative CAZymes and the exo-proteome data obtained for P. sclerotigenum during growth on sweet potato pectin were compared with proteomics and transcriptomics data reported previously for pectin-associated CAZymes from Aspergillus niger strain NRRL3. The data infer that P. sclerotigenum has the capacity to express several novel enzymes that may provide novel opportunities for sweet potato pectin modification and valorization of sweet potato starch processing residues. In addition, the methodological approach employed represents an integrative systematic strategy for enzyme discovery.

Sequential extraction, preliminary characterization and functional properties of sesame (Sesamum indicum L.) hull polysaccharides

To expand the applications of sesame hull waste produced from industrial processing, five cell wall polysaccharide fractions of sesame hulls were sequentially extracted with water (WSP), chelator (CSP), sodium carbonate (NSP), 1 mol/L potassium hydroxide (KS1), and 4 mol/L potassium hydroxide (KS4). The preliminary structural characteristics of the five polysaccharide fractions were investigated by analytical techniques, such as sugar composition analysis, UV–vis, FT-IR, molecular weight measurement, and HSQC. The results showed that CSP had the main chain of xylogalacturonans and the content of galacturonic acid reached 70.07 g/100 g. Both KS1 and KS4 had xyloglucans backbones. NSP had the lowest molecular weight and the best thermal stability, while KS4 had the highest molecular weight. Rheological measurement indicated KS4 had the highest viscosity. The antioxidant abilities of NSP and KS1 were stronger than the other three fractions. This study provides valuable new information about the structural and functional properties of cell wall polysaccharides from sesame hulls.

Occurrence of early softening in papaya due to specific cell wall-related changes

This study aims to find characteristics that might explain the rapid softening disorder in papaya. At the time of harvest, it was observed that papaya with early softening disorder (ESD) and those without the disorder could not be distinguished exclusively on the basis of their skin color and pulp firmness. Fruit with ESD and without ESD were harvested at maturity stage 1 and evaluated for 10 d at 10 °C, according to the softening index (SI) methodology. The symptoms of early softening disorder were clearly observed between 2 d and 8 d of storage, with firmness 71% lower than the initial firmness, presenting atypical behavior for fruit stored under refrigeration. There was no difference between the fruit with and without ESD for soluble solids contents, but titratable acidity and ascorbic acid were significantly different between both stages of fruit that could be associated with the ripening patterns. Furthermore, there were no differences between fruit with ESD and those without ESD in respiratory rate and ethylene production under refrigerated storage, what suggested that the occurrence of disorder was not related to an increase in ethylene post-harvest period. This work provided evidence to discriminate fruit susceptible to suffer from ESD from those not susceptible to this disorder. The cell wall polysaccharide fractions indicated significant differences between fruit with ESD and without ESD at the moment of harvest suggesting modifications in the cellulose-hemicellulose domain. Moreover, the ethylene production did not follow cell wall modification, suggesting that changes in the cell wall structure could have happened during fruit growth. In this way, depending on the intensity of the cell wall modification, the fruit will exhibit the flesh early softening after harvest.

Evidence of intermolecular associations of β-glucan and high-molar mass xylan in a hot water extract of raw oat groat

High molar mass (HM) β-glucan present in oat products can reduce the risk of diet-related diseases, mainly owing to significantly increased digesta viscosity in human small intestine. To verify a research hypothesis that arabinoxylan (AX) present in oat water extract is associated with HM-β-glucan and thus may influence its functionality, multi-detection HPSEC coupled with enzymatic hydrolysis, sugar and 1H NMR analyses were performed. Isolated cell wall polysaccharide fraction comprised branched AX (arabinose-to-xylose ratio, Ara/Xyl ~ 0.8) and arabinogalactan-protein (AG-P). Nevertheless, it contained (10%) unbranched HM-xylan subfraction (weight-average molar mass, Mw ~ 3153 kg/mol), which was aggregated with β-glucan (Mw ~ 1029–1589 kg/mol) through its HM cellulose-like region. Of the two low molar mass-AXs (LM-AXs) isolated, the first having C(O)-3-monosubstituted and C(O)-2,3-disubstituted β-d-xylopyranosyl residues was covalently interlinked to HM-xylan. The second highly feruloylated, containing C(O)-3- and C(O)-2-monosubstituted units, was tightly bound to AG-P with terminal and 5-linked α-L-arabinofuranosyl residues, and non-covalently associated with LM-β-glucan subfraction.

Methanol bioeconomy: promotion of rice crop yield in paddy fields with microbial cells prepared from natural gas-derived C1 compound.

Methylotrophs, which can utilize methanol as a sole carbon source, are promising microorganisms to be exploited in a methanol-based bioeconomy, in which a variety of useful compounds are biotechnologically produced from natural gas-derived methanol. Pink-pigmented facultative methylotrophs (PPFMs) are common plant phyllospheric bacteria and are known to enhance seedling growth and total biomass of various plants. However, improvement of crop yield by inoculation of PPFMs at the field level has not been well investigated. We herein describe improvement of crop yield of several rice cultivars by foliar spraying of PPFMs. After selection of PPFM strains and rice cultivars by the invitro seedling growth test, we further conducted paddy field experiments. The crop yield of the sake-brewing rice Oryza sativa cultivar Hakutsurunishiki was reproducibly improved in a commercial paddy field for over a 5-year period. A one-time foliar spray of PPFM cells (living or killed) or a cell wall polysaccharide fraction, after the heading date, acted in the phyllosphere and effectively improved crop yield. Our results show that the established process with PPFMs is feasible for improvement of food production in the methanol bioeconomy.

The effects of foliar-applied manganese on major cell wall polysaccharide fractions and soluble sugar composition of cucumber leaves

Increasing evidence shows that micronutrients can induce changes in cell wall (CW) properties in plants. A detailed knowledge of these changes will contribute to understand the beneficial and detrimental impacts of trace elements on plant development and defence. In the present work, the influence of foliar spray of manganese (Mn) on the composition of CW polysaccharides and soluble sugars was investigated in cucumber leaves using spectrophotometric and chromatographic techniques. Cucumber seedlings were hydroponically cultured under controlled situations for 21 days. At the two-leaf stage, the above facade of cucumber leaves was fertilized with MnSO4 solutions at various dosages (1.8, 4.5 and 7.2 mM). Results showed that the foliar Mn spray decreased the galacturonic acid (GalA) content, while it increased the degree-of-methyl-esterification (DM) of pectin. The amounts of fucose, arabinose and galactose diminished upon Mn application. The contents of rhamnose and xylose decreased or did not change by Mn treatment. The glucose level enhanced at low concentration of Mn (1.8 mM), but it decreased at medium and high concentrations of Mn (4.5 and 7.2 mM). The increase of mannose content was significant only at medium concentration of Mn. Overall, the amount of total hemicellulose increased at 1.8 mM concentration of Mn, while a decrease was observed at 4.5 and 7.2 mM concentrations of Mn. The cellulose content decreased at low and high concentrations of Mn, although it remained unchanged at medium concentration of Mn. Generally, the total contents of CW polysaccharides increased at low concentration of Mn, but they decreased at the higher concentrations of Mn. Such changes can be important in plant resistance to environmental stresses via reduction of permeability against fungal pathogens and decrease of water loss from tissues under stresses such as drought and salinity.

Absorption, transport, content, and subcellular distribution of vanadium in the polysaccharide fraction of cell wall in corn seedlings

This study investigated the tolerance of plants to vanadium (Ⅴ). The hydroponic method was employed to evaluate the absorption, transport, content, and subcellular distribution of vanadium in the polysaccharide fraction of corn seedlings cell wall under different concentrations of vanadium stress. Results showed that: (a) vanadium was mainly concentrated in the roots of the corn seedlings, and only trace amounts were transported to the leaves; (b) in terms of its subcellular distribution, vanadium was mainly enriched in cell wall regions followed by soluble fraction; (c) the content of vanadium in polysaccharide fraction was highest in alkali-soluble pectin, followed by chelated pectin (P < 0.05).

Structure, rheological, thermal and antioxidant properties of cell wall polysaccharides from Chinese quince fruits

To investigate the composition and structural characteristics of cell wall polysaccharides, three pectic fractions and two hemicellulose fractions, namely water-soluble pectin (WSP), chelator-soluble pectin (CSP), sodium carbonate-soluble pectin (NSP), 1 mol/L KOH soluble hemicellulose (KSH-1) and 4 mol/L KOH soluble hemicellulose (KSH-2), were isolated from Chinese quince fruits. The five fractions exhibited structural and compositional variation. The results showed NSP was the predominant cell wall polysaccharide fraction in the fruit. All pectic fractions had a low degree of esterification (31.7–42.4%). WSP fraction had the highest thermal stability among the five fractions. The polysaccharide chain lengths ranged from 19.4 nm to 121.4 nm. CSP had the highest molecular weight, giving it also the highest solution viscosity. NMR spectra revealed that NSP was composed of RG-I and galacturonic acid main chains, KSH-1 was composed of 1,4-β-D-Xylp backbone attached to 1,5-α-L-Araf units. Among the five fractions, CSP has the highest DPPH radical scavenging activity while KSH-1 has the highest reducing power. This study can contribute to the applications of Chinese quince fruit polysaccharides in food and pharmaceutical industries.

Contributions of root cell wall polysaccharides to Cu sequestration in castor (Ricinus communis L.) exposed to different Cu stresses

Cell wall polysaccharides play a vital role in binding with toxic metals such as copper (Cu) ions. However, it is still unclear whether the major binding site of Cu in the cell wall varies with different degrees of Cu stresses. Moreover, the contribution of each cell wall polysaccharide fraction to Cu sequestration with different degrees of Cu stresses also remains to be verified. The distribution of Cu in cell wall polysaccharide fractions of castor (Ricinus communis L.) root was investigated with various Cu concentrations in the hydroponic experiment. The results showed that the hemicellulose1 (HC1) fraction fixed 44.9%–67.8% of the total cell wall Cu under Cu stress. In addition, the pectin fraction and hemicelluloses2 (HC2) fraction also contributed to the Cu binding in root cell wall, accounting for 11.0%–25.9% and 14.1%–26.6% of the total cell wall Cu under Cu treatments, respectively. When the Cu levels were ≤25 μmol/L, pectin and HC2 contributed equally to Cu storage in root cell wall. However, when the Cu level was higher than 25 μmol/L, the ability of the pectin to bind Cu was easy to reach saturation. Much more Cu ions were bound on HC1 and HC2 fractions, and the HC2 played a much more important role in Cu binding than pectin. Combining fourier transform infrared (FT-IR) and two-dimensional correlation analysis (2D-COS) techniques, the hemicellulose components were showed not only to accumulate most of Cu in cell wall, but also respond fastest to Cu stress.

Composition and biosynthetic machinery of the Blumeria graminis f. sp. hordei conidia cell wall

Infection of barley with the powdery mildew causal agent, Blumeria graminis f. sp. hordei (Bgh), can lead to devastating damage to barley crops. The recent emergence of fungicide resistance imposes a need to develop new antifungal strategies. The enzymes involved in cell wall biosynthesis are ideal targets for the development of fungicides. However, in order to narrow down any target proteins involved in cell wall formation, a greater understanding of the cell wall structure and composition is required. Here, we present a detailed carbohydrate analysis of the Bgh conidial cell wall, a full annotation of Carbohydrate Active enZymes (CAZy) in the Bgh genome, and a comprehensive expression profile of the genes involved in cell wall metabolism. Glycosidic linkage analysis has revealed that the cell wall polysaccharide fraction of Bgh conidia predominantly consists of glucosyl residues (63.1%) and has a greater proportion of galactopyranosyl residues compared to other species (8.5%). Trace amounts of xylosyl residues were also detected, which is unusual in ascomycetes. Transcripts of the genes involved in cell wall metabolism show high expression of chitin deacetylases, which assist fungi in evading the host defence system by deacetylating chitin to chitosan. The data presented suggest that the cell wall components of the conidia and the corresponding obligate biotrophic CAZy gene profile play a key role in the infection process.

Impact of cadmium stress on two maize hybrids

Some physiological parameters and composition of the root cell walls of two maize hybrids (monocots), the sensitive Novania and the tolerant Almansa were studied after treatment with cadmium cations. After 10 days of Cd2+ treatment (1 × 10−5 M and 5 × 10−5 M), plant growth inhibition, in the sensitive hybrid in particular, as well as a certain alteration in root structure and pigment content were observed. The Cd2+ accumulation was ten times higher in the roots than in the shoots. Chemical analyses and atomic absorption spectroscopy proved that Cd2+ modified the composition of the root cell walls by a significant increase in the content of alkali-soluble polysaccharide fractions, particularly in the tolerant hybrid. An increase in the content of phenolic compounds, mainly in the tolerant hybrid, and a decrease in protein content were observed in the presence of Cd2+ in the alkali fractions. The results indicate that the changes in the cell wall polysaccharide fractions and their proportion to lignin and cellulose are obviously involved in the tolerance and/or defence against Cd2+ of the maize hybrids studied.

The highly conserved spermatophyte cell wall DUF642 protein family: Phylogeny and first evidence of interaction with cell wall polysaccharides in vitro

The evolution of spermatophyte plants involved fundamental changes in cell wall structure and function which resulted from diversification of carbohydrates and proteins. Cell wall proteomic analyses identified a novel family of proteins of yet unknown function, the DUF642 (Domain of Unknown Function 642) proteins. To investigate the evolution of the DUF642 gene family, 154 gene sequences from 24 plant species were analyzed, and phylogenetic inferences were conducted using the Maximum Likelihood and Bayesian Inference methods. Orthologous genes were detected in spermatophyte species and absent in non-seed known plant genomes. Protein sequences shared conserved motifs that defined the signature of the family. Distribution of conserved motifs indicated an ancestral intragenic duplication event. Gene phylogeny documented paleoduplication events originating three or four clades, depending on root position. When based on mid-point rooting, it retrieved four monophyletic clades: A, B, C, and D. A glycosylphosphatidylinositol (GPI)-anchor site and one or two galactose-binding domains-like (GBDLs) could be predicted for some DUF642 proteins. The B, C, and D clades grouped the predicted GPI-anchored proteins. First evidence of in vitro interaction of a DUF642 protein with a cell wall polysaccharide fraction is provided. A competition assay with cellulose prevented this interaction. The degree of diversification and the conservation of the family suggested that DUF642 proteins are key components in seed plant evolution.

Chitinases Are Essential for Sexual Development but Not Vegetative Growth in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic pathogen that mainly infects immunocompromised individuals. The fungal cell wall of C. neoformans is an excellent target for antifungal therapies since it is an essential organelle that provides cell structure and integrity. Importantly, it is needed for localization or attachment of known virulence factors, including melanin, phospholipase, and the polysaccharide capsule. The polysaccharide fraction of the cryptococcal cell wall is a complex structure composed of chitin, chitosan, and glucans. Chitin is an indispensable component of many fungal cell walls that contributes significantly to cell wall strength and integrity. Fungal cell walls are very dynamic, constantly changing during cell division and morphogenesis. Hydrolytic enzymes, such as chitinases, have been implicated in the maintenance of cell wall plasticity and separation of the mother and daughter cells at the bud neck during vegetative growth in yeast. In C. neoformans we identified four predicted endochitinases, CHI2, CHI21, CHI22, and CHI4, and a predicted exochitinase, hexosaminidase, HEX1. Enzymatic analysis indicated that Chi2, Chi22, and Hex1 actively degraded chitinoligomeric substrates. Chi2 and Hex1 activity was associated mostly with the cellular fraction, and Chi22 activity was more prominent in the supernatant. The enzymatic activity of Hex1 increased when grown in media containing only N-acetylglucosamine as a carbon source, suggesting that its activity may be inducible by chitin degradation products. Using a quadruple endochitinase deletion strain, we determined that the endochitinases do not affect the growth or morphology of C. neoformans during asexual reproduction. However, mating assays indicated that Chi2, Chi21, and Chi4 are each involved in sexual reproduction. In summary, the endochitinases were found to be dispensable for routine vegetative growth but not sexual reproduction.

Characterization of cultivar differences in beta-1,3 glucanase gene expression, glucanase activity and fruit pulp softening rates during fruit ripening in three naturally occurring banana cultivars.

beta-1,3 glucanase (E.C.3.2.1.39) is the key enzyme involved in the hydrolytic cleavage of 1,3 beta-D glucosidic linkages in beta-1,3 glucans. This work describes a comparative analysis of expression patterns of beta-1,3 glucanase gene in relation to changes in fruit pulp softening rates in three banana cultivars, Rasthali (AAB), Kanthali (AB), and Monthan (ABB). Analysis of transcript and protein levels of beta-1,3 glucanase gene during ripening revealed differential timing in expression of the gene which correlated well with the variation in enzymatic activity of glucanase and fruit pulp softening rates in the three cultivars. Exogenously applied ethylene strongly induced beta-1,3 glucanase expression during the early ripening days in Rasthali, while the expression of the gene was marginally stimulated following ethylene treatment in preclimacteric Kanthali fruit. Conversely, in Monthan, beta-1,3 glucanase expression was very low throughout the ripening stages, and ethylene treatment did not induce the expression of the gene in this cultivar. Analysis of glucanase activity using protein extracts from unripe and ripe fruit of Monthan with crude cell wall polysaccharide fractions (used as substrate) indicated that the natural substrate for glucanase remained almost unutilized in this cultivar due to low in vivo glucanase activity. Furthermore, the recombinant beta-1,3 glucanase protein, overexpressed in E. coli, showed requirement for substrates with contiguous beta-1,3 linkages for optimal activity. Overall, our results provide new information on the expression profile of beta-1,3 glucanase gene in connection with the pattern of changes in fruit firmness at the physiological and molecular levels during ripening in three banana cultivars.

Post-harvest heat treatments modify cell wall composition of strawberry (Fragaria×ananassa Duch.) fruit

Abstract ‘Pajaro’ strawberry fruit were treated at 45 °C by hot air or hot water prior to storage at 3 °C for 10 days. Control fruit were stored without treatment. Modifications in cell wall composition and fruit quality caused by heat treatment were assessed after removal of fruit from storage. Treatment with hot water improved fruit resistance to fungal infection, but caused external damage, which rendered fruit commercially unacceptable. Hot air treatment did not affect external appearance, improved resistance to fungal infection and preserved firmness. Heat treatments did not affect cell wall yields obtained, but caused alterations in solubility of the different cell wall polysaccharide fractions. Higher yields for CDTA- and KOH-soluble fractions were observed as a consequence of treatment, suggesting decreased solubilization of cell wall polymers in treated fruit, even though no apparent relationship to fruit firmness was found. Higher degrees of esterification as well as total sugar contents in both CDTA- and KOH-soluble fractions were also found in heated fruit.

Arabidopsis Fragile Fiber8, Which Encodes a Putative Glucuronyltransferase, Is Essential for Normal Secondary Wall Synthesis

Secondary walls in vessels and fibers of dicotyledonous plants are mainly composed of cellulose, xylan, and lignin. Although genes involved in biosynthesis of cellulose and lignin have been intensively studied, little is known about genes participating in xylan synthesis. We found that Arabidopsis thaliana fragile fiber8 (fra8) is defective in xylan synthesis. The fra8 mutation caused a dramatic reduction in fiber wall thickness and a decrease in stem strength. FRA8 was found to encode a member of glycosyltransferase family 47 and exhibits high sequence similarity to tobacco (Nicotiana plumbaginifolia) pectin glucuronyltransferase. FRA8 is expressed specifically in developing vessels and fiber cells, and FRA8 is targeted to Golgi. Comparative analyses of cell wall polysaccharide fractions from fra8 and wild-type stems showed that the xylan and cellulose contents are drastically reduced in fra8, whereas xyloglucan and pectin are elevated. Further structural analysis of cell walls revealed that although wild-type xylans contain both glucuronic acid and 4-O-methylglucuronic acid residues, xylans from fra8 retain only 4-O-methylglucuronic acid, indicating that the fra8 mutation results in a specific defect in the addition of glucuronic acid residues onto xylans. These findings suggest that FRA8 is a glucuronyltransferase involved in the biosynthesis of glucuronoxylan during secondary wall formation.