The effect of environment on endosperm cell-wall development in Triticum aestivum during grain filling: an infrared spectroscopic imaging study

Exploiting Rye in Wheat Quality Breeding: The Case of Arabinoxylan Content

Rye (Secale cereale subsp. cereale L.) has long been exploited as a valuable alternative genetic resource in wheat (Triticum aestivum L.) breeding. Indeed, the introgression of rye genetic material led to significant breakthroughs in the improvement of disease and pest resistance of wheat, as well as a few agronomic traits. While such traits remain a high priority in cereal breeding, nutritional aspects of grain crops are coming under the spotlight as consumers become more conscious about their dietary choices and the food industry strives to offer food options that meet their demands. To address this new challenge, wheat breeding can once again turn to rye to look for additional genetic variation. A nutritional aspect that can potentially greatly benefit from the introgression of rye genetic material is the dietary fibre content of flour. In fact, rye is richer in dietary fibre than wheat, especially in terms of arabinoxylan content. Arabinoxylan is a major dietary fibre component in wheat and rye endosperm flours, and it is associated with a variety of health benefits, including normalisation of glycaemic levels and promotion of the gut microbiota. Thus, it is a valuable addition to the human diet, and it can represent a novel target for wheat-rye introgression breeding.

Influence of Drought and Salt Stress on Durum Wheat Grain Quality and Composition: A Review

Durum wheat is a staple crop for the Mediterranean diet because of its adaptability to environmental pressure and for its large use in cereal-based food products, such as pasta and bread, as a source of calories and proteins. Durum wheat whole grains are also highly valued for their peculiar amount of dietary fiber and minerals, as well as bioactive compounds of particular interest for their putative health-beneficial properties, including polyphenols, carotenoids, tocopherols, tocotrienols, and phytosterols. In Mediterranean environments, durum wheat is mostly grown under rainfed conditions, where the crop often experiences environmental stresses, especially water deficit and soil salinity that may induce a hyperosmotic stress. In particular, changes in C and N accumulation due to these abiotic conditions, during grain filling, can influence starch and storage protein amount and composition in durum wheat caryopsis, thus influencing yield and quality traits. Recent advancements regarding the influence of water deficit and salinity stress on durum wheat are critically discussed. In particular, a focus on stress-induced changes in (a) grain protein content and composition in relation to technological and health quality; (b) starch and dietary fiber accumulation and composition; (c) phytochemical composition; (d) health-related grain micronutrient accumulation, such as Fe and Zn.

Exploration of the functionality of sugars in cake-baking, and effects on cake quality

This review paper describes our exploratory experimental studies on the functionality of sucrose and other sugars in cake-baking, and effects on cake quality. We have used the American Association of Cereal Chemists Method 10-90.01 as a base cake-baking method, and have applied Differential Scanning Calorimetry, Rapid Visco-Analyzer, and time-lapse photography analyses in experimental design studies of the effects of the following ingredient and formulation variables on cake quality (e.g. texture, color, moisture content) and other finished-product properties (e.g. shape, dimensions): (a) cake formula levels of sucrose and water, in terms of %Sucrose and Total Solvent; (b) concentration of sucrose or other sugars (e.g. xylose, ribose, fructose, glucose, maltose, polydextrose) vs. wheat flour starch gelatinization temperature and starch pasting during baking and gluten development during mixing; (c) unchlorinated flour vs. chlorinated flours (of varying pH); (d) cake formula %Sucrose and TS vs. cake color, shape, and dimensions; (e) cakes formulated with sucrose or other sugars (i.e. xylose, fructose, glucose), and variable %S and TS, and unchlorinated or chlorinated flour (pH 4.6), vs. cake color, shape, and dimensions.

Spatial distribution of functional components in the starchy endosperm of wheat grains

The starchy endosperm of the mature wheat grain comprises three major cell types, namely sub-aleurone cells, prismatic cells and central cells, which differ in their contents of functional components: gluten proteins, starch, cell wall polysaccharides (dietary fibre) and lipids. Gradients are established during grain development but may be modified during grain maturation and are affected by plant nutrition, particularly nitrogen application, and environmental factors. Although the molecular controls of their formation are unknown, the high content of protein and low content of starch of sub-aleurone cells, compared to the other starchy endosperm cells types, may result from differences in developmental programming related to the cells having a separate origin (from anticlinal division of the aleurone cells). The gradients within the grain may be reflected in differences in the compositions of mill streams, particularly those streams enriched in the central and outer cells of the starchy endosperm, respectively, allowing the production of specialist flours for specific end uses.

Drought stress affects the protein and dietary fiber content of wholemeal wheat flour in wheat/Aegilops addition lines

Wild relatives of wheat, such as Aegilops spp. are potential sources of genes conferring tolerance to drought stress. As drought stress affects seed composition, the main goal of the present study was to determine the effects of drought stress on the content and composition of the grain storage protein (gliadin (Gli), glutenin (Glu), unextractable polymeric proteins (UPP%) and dietary fiber (arabinoxylan, β-glucan) components of hexaploid bread wheat (T. aestivum) lines containing added chromosomes from Ae. biuncialis or Ae. geniculata. Both Aegilops parents have higher contents of protein and β-glucan and higher proportions of water-soluble arabinoxylans (determined as pentosans) than wheat when grown under both well-watered and drought stress conditions. In general, drought stress resulted in increased contents of protein and total pentosans in the addition lines, while the β-glucan content decreased in many of the addition lines. The differences found between the wheat/Aegilops addition lines and wheat parents under well-watered conditions were also manifested under drought stress conditions: Namely, elevated β-glucan content was found in addition lines containing chromosomes 5U^g, 7U^g and 7M^b, while chromosomes 1U^b and 1M^g affected the proportion of polymeric proteins (determined as Glu/Gli and UPP%, respectively) under both well-watered and drought stress conditions. Furthermore, the addition of chromosome 6M^g decreased the WE-pentosan content under both conditions. The grain composition of the Aegilops accessions was more stable under drought stress than that of wheat, and wheat lines with the added Aegilops chromosomes 2M^g and 5M^g also had more stable grain protein and pentosan contents. The negative effects of drought stress on both the physical and compositional properties of wheat were also reduced by the addition of these. These results suggest that the stability of the grain composition could be improved under drought stress conditions by the intraspecific hybridization of wheat with its wild relatives.

Development of an oligosaccharide library to characterise the structural variation in glucuronoarabinoxylan in the cell walls of vegetative tissues in grasses

BACKGROUND

Grass glucuronoarabinoxylan (GAX) substitutions can inhibit enzymatic degradation and are involved in the interaction of xylan with cell wall cellulose and lignin, factors which contribute to the recalcitrance of biomass to saccharification. Therefore, identification of xylan characteristics central to biomass biorefining improvement is essential. However, the task of assessing biomass quality is complicated and is often hindered by the lack of a reference for a given crop.

RESULTS

In this study, we created a reference library, expressed in glucose units, of Miscanthus sinensis GAX stem and leaf oligosaccharides, using DNA sequencer-Assisted Saccharide analysis in high throughput (DASH), supported by liquid chromatography (LC), nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Our analysis of a number of grass species highlighted variations in substitution type and frequency of stem and leaf GAX. In miscanthus, for example, the β-Xylp-(1 → 2)-α-Araf-(1 → 3) side chain is more abundant in leaf than stem.

CONCLUSIONS

The reference library allows fast identification and comparison of GAX structures from different plants and tissues. Ultimately, this reference library can be used in directing biomass selection and improving biorefining.

Infrared microspectroscopic imaging of plant tissues: spectral visualization of Triticum aestivum kernel and Arabidopsis leaf microstructure

Infrared microspectroscopy is a tool with potential for studies of the microstructure, chemical composition and functionality of plants at a subcellular level. Here we present the use of high-resolution bench top-based infrared microspectroscopy to investigate the microstructure of Triticum aestivum L. (wheat) kernels and Arabidopsis leaves. Images of isolated wheat kernel tissues and whole wheat kernels following hydrothermal processing and simulated gastric and duodenal digestion were generated, as well as images of Arabidopsis leaves at different points during a diurnal cycle. Individual cells and cell walls were resolved, and large structures within cells, such as starch granules and protein bodies, were clearly identified. Contrast was provided by converting the hyperspectral image cubes into false-colour images using either principal component analysis (PCA) overlays or by correlation analysis. The unsupervised PCA approach provided a clear view of the sample microstructure, whereas the correlation analysis was used to confirm the identity of different anatomical structures using the spectra from isolated components. It was then demonstrated that gelatinized and native starch within cells could be distinguished, and that the loss of starch during wheat digestion could be observed, as well as the accumulation of starch in leaves during a diurnal period.

Multiscale characterization of arabinoxylan and β-glucan composite films

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.

Effect of heat and drought stress on the structure and composition of arabinoxylan and β-glucan in wheat grain

The effects of heat (H), drought (D) and H+D (from 12th day after heading for 15 days) on the dietary fiber content and composition (arabinoxylan (AX) and β-glucan) of three winter wheat varieties (Plainsman V, Mv Magma and Fatima 2) were determined. Results showed that H and D stress decreased the TKW, the β-glucan contents of the seeds and the quantity of the DP3+DP4 units, while the protein and AX contents increased. The highest amounts of AX and proteins were in the H+D stressed samples with heat stress also increasing the water extractability (WE) of the AX. However, while the content of AX content was generally increased by all stresses, drought stress had negative effect on the AX content of the drought tolerant Plainsman V. Fatima 2 behaved similarly to Plainsman V as regards to its drought tolerance, but was very sensitive to heat stress, while Mv Magma was the most resistant to heat stress.

Effect of dough mixing on wheat endosperm cell walls

Dough-derived cell wall fragments isolated by ultracentrifugation were largely derived from the starchy endosperm, with some fragments deriving from the aleurone and outer layers, as indicated by fluorescence microscopy. Dough mixing had little effect on the structure and composition of cell wall fragments compared to thin grain sections, as determined by Fourier transform infrared (FTIR) and (1)H nuclear magnetic resonance (NMR) spectroscopy. These analyses confirmed that the fragments largely comprised water-unextractable arabinoxylan and β-glucan. FTIR microspectroscopy of dough-derived cell wall fragments prepared from five bread wheat cultivars showed that two largely comprised highly substituted arabinoxylan (cv. Manital and San Pastore), one comprised a mixture of low, medium, and highly substituted arabinoxylan (cv. Hereward), and the remaining two comprised a greater proportion of low substituted arabinoxylan (cv. Claire and Yumai 34). Yumai 34 yielded a greater mass of cell wall material, and its cell walls comprised a high proportion of medium substituted arabinoxylan. Such methods will allow for the impact of bakery ingredients and processing on endosperm cells, including the addition of xylanases, to be investigated in the future to ensure any potential health benefits arising from wheat breeding are realized in the food that reaches the consumer.

Arabinoxylans and arabinogalactans: a comprehensive treatise

The functional and nutraceutical importance of various foods is often attributed to the bioactive molecules present in them. A number of components have been studied but dietary fiber and its different constituents are of prime consideration. Among these, arabinoxylan (AX) and arabinogalactan (AG) are of significant importance in that they hold potential in improving the quality of the baked products along with providing health benefits against various ailments. However, the improvements are dependent on their molecular weights, cross linkages, and solubility. Water-Extractable Arabinoxylan (WEAX) is more effective as compared to Water-Unextractable Arabinoxylan (WUEAX). In this review article, efforts were directed to describe the structural and molecular conformations of these functional ingredients. The discussion has been made regarding the functional properties of AX and AG, for example, improvements in water absorption capacities, dough stability time, and viscosity. They also improve the baking absorption of flour that is positively correlated with bread volume, the most important criterion for stakeholders. The arguments are also provided on the detrimental effects on gluten quality with some possible solutions. Their role in improving the quality and extending the shelf life of bread by reducing the process of staling and retrogradation is the main idea presented in the article. The nutraceutical perspectives were also highlighted as they are helpful in regulating blood cholesterol which thereby protect the body from cardiovascular disorders like atherosclerosis. Additionally, they act as prebiotics for microorganisms residing in the gastrointestinal tract.

Spectroscopic analysis of diversity of Arabinoxylan structures in endosperm cell walls of wheat cultivars (Triticum aestivum) in the HEALTHGRAIN diversity collection

Fifty bread wheat (Triticum aestivum L.) cultivars were selected from the HEALTHGRAIN germplasm collection based on variation in their contents of total and water-extractable arabinoxylan. FT-IR spectroscopic mapping of thin transverse sections of grain showed variation in cell wall arabinoxylan composition between the cultivars, from consisting almost entirely of low-substituted arabinoxylan (e.g., T.aestivum 'Claire') to almost entirely of highly substituted arabinoxylan (e.g., T.aestivum 'Manital') and a mixture of the two forms (e.g., T.aestivum 'Hereward'). Complementary data were obtained using endoxylanase digestion of flour followed by HP-AEC analysis of the arabinoxylan oligosaccharides. This allowed the selection of six cultivars for more detailed analysis using FT-IR and (1)H NMR spectroscopy to determine the proportions of mono-, di-, and unsubstituted xylose residues. The results of the two analyses were consistent, showing that variation in the composition and structure of the endosperm cell wall arabinoxylan is present between bread wheat cultivars. The heterogeneity and spatial distribution of the arabinoxylan in endosperm cell walls may be exploited in wheat processing as it may allow the production of mill streams enriched in various arabinoxylan fractions which have beneficial effects on health.

Endosperm development in Brachypodium distachyon

Grain development and its evolution in grasses remains poorly understood, despite cereals being our most important source of food. The grain, for which many grass species have been domesticated, is a single-seeded fruit with prominent and persistent endosperm. Brachypodium distachyon, a small wild grass, is being posited as a new model system for the temperate small grain cereals, but little is known about its endosperm development and how this compares with that of the domesticated cereals. A cellular and molecular map of domains within the developing Brachypodium endosperm is constructed. This provides the first detailed description of grain development in Brachypodium for the reference strain, Bd21, that will be useful for future genetic and comparative studies. Development of Brachypodium grains is compared with that of wheat. Notably, the aleurone is not regionally differentiated as in wheat, suggesting that the modified aleurone region may be a feature of only a subset of cereals. Also, the central endosperm and the nucellar epidermis contain unusually prominent cell walls that may act as a storage material. The composition of these cell walls is more closely related to those of barley and oats than to those of wheat. Therefore, although endosperm development is broadly similar to that of temperate small grain cereals, there are significant differences that may reflect its phylogenetic position between the Triticeae and rice.

Temporal and spatial changes in cell wall composition in developing grains of wheat cv. Hereward

A combination of enzyme mapping, FT-IR microscopy and NMR spectroscopy was used to study temporal and spatial aspects of endosperm cell wall synthesis and deposition in developing grain of bread wheat cv. Hereward. This confirmed previous reports that changes in the proportions of the two major groups of cell wall polysaccharides occur, with beta-glucan accumulating earlier in development than arabinoxylan. Changes in the structure of the arabinoxylan occurred, with decreased proportions of disubstituted xylose residues and increased proportions of monosubstituted xylose residues. These are likely to result, at least in part, from arabinoxylan restructuring catalysed by enzymes such as arabinoxylan arabinofurano hydrolase and lead to changes in cell wall mechanical properties which may be required to withstand stresses during grain maturation and desiccation.

Remodelling of arabinoxylan in wheat (Triticum aestivum) endosperm cell walls during grain filling

Previous studies using spectroscopic imaging have allowed the spatial distribution of structural components in wheat endosperm cell walls to be determined. FT-IR microspectroscopy showed differing changes in arabinoxylan (AX) structure, during grain development under cool/wet and hot/dry growing conditions, for differing cultivars (Toole et al. in Planta 225:1393-1403, 2007). These studies have been extended using Raman microspectroscopy, providing more details of the impact of environment on the polysaccharide and phenolic components of the cell walls. NMR studies provide complementary information on the types and levels of AX branching both early in development and at maturity. Raman microspectroscopy has allowed the arabinose:xylose (A/X) ratio in the cell wall AX to be determined, and the addition of ferulic acid and related phenolic acids to be followed. The changes in the A/X ratio during grain development were affected by the environmental conditions, with the A/X ratio generally being slightly lower for samples grown under cool/wet conditions than for those from hot/dry conditions. The degree of esterification of the endosperm cell walls with ferulic acid was also affected by the environment, being lower under hot/dry conditions. The results support earlier suggestions that AX is either delivered to the cell wall in a highly substituted form and is remodelled through the action of arabinoxylan arabinofuranohydrolases or arabinofuranosidases, or that low level substituted AX are incorporated into the wall late in cell wall development, reducing the average degree of substitution, and that the rate of this remodelling is influenced by the environment. (1)H NMR provided a unique insight into the chemical structure of intact wheat endosperm cell walls, providing qualitative information on the proportions of mono- and disubstituted AX and the levels of branching of adjacent units. The A/X ratio did not change greatly with either the development stage or the growth conditions, but the ratio of mono- to disubstituted Xylp residues increased markedly (by about fourfold) in the more mature samples, confirming the changes in branching levels determined using FT-IR. To the best of our knowledge, this is the first time that intact endosperm cell walls have been studied by (1)H NMR.

Effect of environmental stress during grain filling on the soluble proteome of wheat (Triticum aestivum) dough liquor

The influence of genotype and environment on a soluble wheat dough liquor proteome was studied for four cultivars grown under field conditions and under hot/dry and cool/wet regimes by two-dimensional electrophoresis followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry or quadrupole time-of-flight mass spectrometry. Although the four cultivars had similar patterns, differences in the relative abundances of some components were observed. Similarly, some differences were observed between the control samples and the samples grown under cool/wet and hot/dry conditions. These included differences in the abundances of storage proteins belonging to the 7S globulin (vicilin-like) and alpha-globulin families and of protective proteins including members of the serpin, described as allergens, and chitinase families. A number of novel annotations were made as compared to previous work on the dough liquor of cv. Hereward, including two 19 kDa alpha-globulins, precursors of endochitinases A and C, and several polypeptides belonging to the 7S globulin (vicilin-like) family.

Sequential solvent extraction and structural characterization of polysaccharides from the endosperm cell walls of barley grown in different environments

The objective of this study was to examine the composition and molecular structure of the endosperm cell walls (CW) derived from barley grain grown in three environments in Canada, and differing in grain hardness, protein, and total β-glucan contents. The endosperm CW were isolated from barley, cv. Metcalfe, grown in Davidson, SK (Sample A), Hythe, AB (sample B), and Hamiota, MB (sample C). The CW were sequentially extracted with water at 65(o)C, saturated Ba(OH)2, again with water at 25(o)C, and 1M NaOH, resulting in fractions designated WE65, BaE, Ba/WE, and NaE, respectively. The monosaccharide analysis indicated the presence of β-glucans, arabinoxylans, and small amounts of arabinogalactans, glucomannans, and xyloglucans. Cellulose was detected in the CW remnants. The CW of sample A, exhibiting a lower grain hardness than sample B, contained the lowest amount of β-glucans, but the highest amount of arabinoxylans and the mannose-containing polysaccharides. The CW of sample C, characterized by very high protein content in the grain, contained the highest amount of β-glucans and the lowest amount of other polysaccharides. Polysaccharides in the CW of sample B, exhibiting the highest grain hardness, were characterized by the highest weight average molecular weights (Mw). β-Glucans in the CW of Sample B showed the highest ratio of DP3/DP4 and the longest cellulosic fragments in the polymeric chains. Of the three barley samples, arabinoxylans in the endosperm CW of sample A exhibited the lowest degree of branching, the highest amount of unsubstituted Xyl residues, and the highest ratio of singly to doubly substituted Xylp. The highest water solubility of the CW of sample C was associated with the highest concentration of β-glucans, the lowest DP3/DP4 ratio, and the lowest Mw of the polymeric constituents. Arabinoxylans with the lowest amount of doubly substituted but the highest amount of unsubstituted xylose residues and long sequences of unsubstituted xylan regions were found in the NaE fractions. The NaE fractions showed a high ratio of →4)-Glcp-(1→ to →3)-Glcp-(1→ linkages and some →4)-Manp-(1→ linkages, indicating a high level of long cellulosic regions in β-glucan chains and the presence of glucomannans.