Arabinoxylan (AX) is the dominant component within wheat (Triticum aestivum L.) endosperm cell walls, accounting for 70% of the polysaccharide. The viscosity of aqueous extracts from wheat grain is a ...key trait influencing the processing for various end uses, and this is largely determined by the properties of endosperm AX. We have previously shown dramatic effects on endosperm AX in transgenic wheat by down‐regulating either TaGT43_2 or TaGT47_2 genes (orthologues to IRX9 and IRX10 in Arabidopsis, respectively) implicated in AX chain extension and the TaXAT1 gene responsible for monosubstitution by 3‐linked arabinose. Here, we use these transgenic lines to investigate the relationship between amounts of AX in soluble and insoluble fractions, the chain‐length distribution of these measured by intrinsic viscosity and the overall effect on extract viscosity. In transgenic lines expressing either the TaGT43_2 or TaGT47_2 RNAi transgenes, the intrinsic viscosities of water‐extractable (WE‐AX) and of a water‐insoluble alkaline‐extracted fraction (AE‐AX) were decreased by between 10% and 50% compared to control lines. In TaXAT1 RNAi lines, there was a 15% decrease in intrinsic viscosity of WE‐AX but no consistent effect on that of AE‐AX. All transgenic lines showed decreases in extract viscosity with larger effects in TaGT43_2 and TaGT47_2 RNAi lines (by up to sixfold) than in TaXAT1 RNAi lines (by twofold). These effects were explained by the decreases in amount and chain length of WE‐AX, with decreases in amount having the greater influence. Extract viscosity from wheat grain can therefore be greatly decreased by suppression of single gene targets.
•Microstructure of composite films influences the average water mobility.•Film properties differed in composite films depending on the A/X ratio of AXs.•Composite film characterization agreed with ...the changes observed in wheat grain.
Composite films made with Arabinoxylans (AXs) (with high, middle and low level of substitution by arabinose) and (1→3)(1→4)-β-d-glucans (BGs) extracted from cereal cell walls have been prepared and analyzed using microscopy (SEM and LSCFM), DSC, mechanical tests and TD-NMR spectroscopy. The objectives were to correlate molecular and physico-chemical properties of films with mechanical and hydration properties of wheat cell walls. A phase separation phenomenon was observed for films made with highly substituted AXs and BGs at a ratio AX/BG of 60/40. This phase separation was correlated with lower dipolar interactions between polysaccharide chains and a decrease of ultimate strain and stress of films. Highly substituted AX and BG composite films exhibited very weak mechanical properties in agreement with weaker interactions between the polymer chains. This effect was supported by NMR results showing that interactions between AXs and BGs decreased with increased substitution of AXs in composite films. Lower dipolar interactions between polysaccharides favored the water mobility in relation with a higher specific surface area of polysaccharides in films but also higher distances between polysaccharide chains so larger nanopores in composite films made within highly substituted AXs. These multiscale characterizations agreed with the structural changes observed in wheat grain during its development.
A detailed and comprehensive understanding of seed reserve accumulation is of great importance for agriculture and crop improvement strategies. This work is part of a research programme aimed at ...using Brachypodium distachyon as a model plant for cereal grain development and filling. The focus was on the Bd21-3 accession, gathering morphological, cytological, and biochemical data, including protein, lipid, sugars, starch, and cell-wall analyses during grain development. This study highlighted the existence of three main developmental phases in Brachypodium caryopsis and provided an extensive description of Brachypodium grain development. In the first phase, namely morphogenesis, the embryo developed rapidly reaching its final morphology about 18 d after fertilization (DAF). Over the same period the endosperm enlarged, finally to occupy 80% of the grain volume. During the maturation phase, carbohydrates were continuously stored, mainly in the endosperm, switching from sucrose to starch accumulation. Large quantities of β-glucans accumulated in the endosperm with local variations in the deposition pattern. Interestingly, new β-glucans were found in Brachypodium compared with other cereals. Proteins (i.e. globulins and prolamins) were found in large quantities from 15 DAF onwards. These proteins were stored in two different sub-cellular structures which are also found in rice, but are unusual for the Pooideae. During the late stage of development, the grain desiccated while the dry matter remained fairly constant. Brachypodium exhibits some significant differences with domesticated cereals. Beta-glucan accumulates during grain development and this cell wall polysaccharide is the main storage carbohydrate at the expense of starch.
•Lignins were detected earlier than previously reported in developing wheat grain.•Alkali-releasable ferulate increased during development of grain outer layers.•Ferulate and lignin monomers ...releasable from pericarp decreased at grain maturity.•Resistant bonds between cell wall polymers probably form at maturity.
Important biological, nutritional and technological roles are attributed to cell wall polymers from cereal grains. The composition of cell walls in dry wheat grain has been well studied, however less is known about cell wall deposition and modification in the grain outer layers during grain development.
In this study, the composition of cell walls in the outer layers of the wheat grain (Triticum aestivum Recital cultivar) was investigated during grain development, with a focus on cell wall phenolics. We discovered that lignification of outer layers begins earlier than previously reported and long before the grain reaches its final size. Cell wall feruloylation increased in development. However, in the late stages, the amount of ferulate releasable by mild alkaline hydrolysis was reduced as well as the yield of lignin-derived thioacidolysis monomers. These reductions indicate that new ferulate-mediated cross-linkages of cell wall polymers appeared as well as new resistant interunit bonds in lignins. The formation of these additional linkages more specifically occurred in the outer pericarp.
Our results raised the possibility that stiffening of cell walls occur at late development stages in the outer pericarp and might contribute to the restriction of the grain radial growth.
Arabinoxylans (AX) and (1to3),(1to4)-β-glucans are major components of wheat endosperm cell walls. Their chemical heterogeneity has been described but little is known about the sequence of their ...deposition in cell walls during endosperm development. The time course and pattern of deposition of the (1to3) and (1to3),(1to4)-β-glucans and AX in the endosperm cell walls of wheat (Triticum aestivum L. cv. Recital) during grain development was studied using specific antibodies. At approximately 45°D (degree-days) after anthesis the developing walls contained (1to3)-β-glucans but not (1to3),(1to4)-β-glucans. In contrast, (1to3),(1to4)-β-glucans occurred widely in the walls of maternal tissues. At the end of the cellularization stage (72°D), (1to3)-β-glucan epitopes disappeared and (1to3),(1to4)-β-glucans were found equally distributed in all thin walls of wheat endosperm. The AX were detected at the beginning of differentiation (245°D) in wheat endosperm, but were missing in previous stages. However, epitopes related to AX were present in nucellar epidermis and cross cells surrounding endosperm at all stages but not detected in the maternal outer tissues. As soon as the differentiation was apparent, the cell walls exhibited a strong heterogeneity in the distribution of polysaccharides within the endosperm.
The wild grass Brachypodium distachyon has been proposed as an alternative model species for temperate cereals. The present paper reports on the characterization of B. distachyon grain, placing ...emphasis on endosperm cell walls. Brachypodium distachyon is notable for its high cell wall polysaccharide content that accounts for ~52% (w/w) of the endosperm in comparison with 2-7% (w/w) in other cereals. Starch, the typical storage polysaccharide, is low <10% (w/w) in the endosperm where the main polysaccharide is (1-3) (1-4)-β-glucan 40% (w/w) of the endosperm, which in all likelihood plays a role as a storage compound. In addition to (1-3) (1-4)-β-glucan, endosperm cells contain cellulose and xylan in significant amounts. Interestingly, the ratio of ferulic acid to arabinoxylan is higher in B. distachyon grain than in other investigated cereals. Feruloylated arabinoxylan is mainly found in the middle lamella and cell junction zones of the storage endosperm, suggesting a potential role in cell-cell adhesion. The present results indicate that B. distachyon grains contain all the cell wall polysaccharides encountered in other cereal grains. Thus, due to its fully sequenced genome, its short life cycle, and the genetic tools available for mutagenesis/transformation, B. distachyon is a good model to investigate cell wall polysaccharide synthesis and function in cereal grains.
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.
•Evaluation of a pilot scale arabinoxylan production process.•Influence of different alkaline extraction conditions.•Study of different combinations of operation stages for recovery of arabinoxylans.
...A process for hemicellulose fractionation and purification from wheat straw and bran has been investigated. The aim was to define the efficiency of a pilot scale process combining twin-screw extrusion and refining steps such as ultrafiltration and chromatography, to replace alcoholic precipitation.
Extraction by twin-screw extrusion gave a complex extract containing only 22.7% carbohydrates. Evaporation (EV) followed by ethanol precipitation (P) and freeze-drying (FD), gave a 25.7% arabinoxylan yield with 24.1% arabinoxylan content. The other purification process studied was based on a combination of ultrafiltration (UF), anion exchange chromatography (CHR) and freeze-drying (FD), without precipitation. It gave a 24.3% yield of arabinoxylans into the hemicellulosic powders and 13.9% arabinoxylan content. Despite the obvious interest of using ultrafiltration for demineralization and anion exchange chromatography for decoloration, the purity of the final powder is still too low, because of difficulties in achieving separation of proteins and hemicelluloses and concentration of the solution, in the same step.
•Hydration properties of AX films are affected by arabinose substitution.•Mechanical properties of AX films are affected by arabinose substitution.•BG films exhibit higher extensibility than AX ...films.•The extensibility of BG layer may favor cell wall extension during grain development.•The diversity of AX possibly modulates the hydration properties of cell walls.
Arabinoxylans (AX) and (1→3)(1→4)-β-d-glucans (BG) are the main components of the cell walls in the endosperm of wheat grain. The relative occurrence of these two polysaccharides and the fine structure of the AX are highly variable within the endosperm. Films of AX and BG were used as models of the cell wall to study the impact of polymer structure on the hydration and mechanical properties of the cell walls. Effective moisture diffusivities (Deff) of AX and BG films were determined from 0 to 95% relative humidity (RH) at 20°C. Deff was influenced by the water content, and the structure of polysaccharides. Higher Deff was obtained for films made with highly substituted AX compared to values obtained for films made with BG or lowly substituted AX. Proton dipolar second moments M2 and water T2 relaxation times measured by TD-NMR, indicated that the highly branched AX films exhibited a higher nano-porosity, favoring water motions within films. Results from traction tests showed significant different mechanical properties between the AX and BG films. BG films exhibited much higher extensibility than AX films. Strength and extensibility of AX films decreased with increasing arabinose to xylose ratio. Our results show that the water motions and the mechanical properties of AX and BG films can be linked to the polysaccharide chains interactions that modulate the nanostructure of films.
Development of microbiota-directed foods (MDFs) that selectively increase the abundance of beneficial human gut microbes, and their expressed functions, requires knowledge of both the bioactive ...components of MDFs and the mechanisms underlying microbe-microbe interactions. Here, gnotobiotic mice were colonized with a defined consortium of human-gut-derived bacterial strains and fed different combinations of 34 food-grade fibers added to a representative low-fiber diet consumed in the United States. Bioactive carbohydrates in fiber preparations targeting particular Bacteroides species were identified using community-wide quantitative proteomic analyses of bacterial gene expression coupled with forward genetic screens. Deliberate manipulation of community membership combined with administration of retrievable artificial food particles, consisting of paramagnetic microscopic beads coated with dietary polysaccharides, disclosed the contributions of targeted species to fiber degradation. Here, our approach, including the use of bead-based biosensors, defines nutrient-harvesting strategies that underlie, as well as alleviate, competition between Bacteroides and control the selectivity of MDF components.
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