Comparative in situ analyses of cell wall matrix polysaccharide dynamics in developing rice and wheat grain

Challenges to Increasing Dietary Fiber in White Flour and Bread

Increasing the intake of dietary fiber from staple foods is a key strategy to improve the health of consumers. White bread is an attractive vehicle to deliver increased fiber as it is widely consumed and available to all socio-economic groups. However, fiber only accounts for about 4% of the dry weight of white flour and bread compared to 10-15% in whole grain bread and flour. We therefore discuss the challenges and barriers to developing and exploiting new types of wheat with high fiber content in white flour. These include defining and quantifying individual fiber components and understanding how they are affected by genetic and environmental factors. Rapid high throughput assays suitable for determining fiber content during plant breeding and in grain-utilizing industries are urgently required, while the impact of fiber amount and composition on flour processing quality needs to be understood. Overcoming these challenges should have significant effects on human health.

Utilizing transcriptomics and proteomics to unravel key genes and proteins of Oryza sativa seedlings mediated by selenium in response to cadmium stress

BACKGROUND

Cadmium (Cd) pollution has declined crop yields and quality. Selenium (Se) is a beneficial mineral element that protects plants from oxidative damage, thereby improving crop tolerance to heavy metals. The molecular mechanism of Se-induced Cd tolerance in rice (Oryza sativa) is not yet understood. This study aimed to elucidate the beneficial mechanism of Se (1 mg/kg) in alleviating Cd toxicity in rice seedlings.

RESULTS

Exogenous selenium addition significantly improved the toxic effect of cadmium stress on rice seedlings, increasing plant height and fresh weight by 20.53% and 34.48%, respectively, and increasing chlorophyll and carotenoid content by 16.68% and 15.26%, respectively. Moreover, the MDA, ·OH, and protein carbonyl levels induced by cadmium stress were reduced by 47.65%, 67.57%, and 56.43%, respectively. Cell wall metabolism, energy cycling, and enzymatic and non-enzymatic antioxidant systems in rice seedlings were significantly enhanced. Transcriptome analysis showed that the expressions of key functional genes psbQ, psbO, psaG, psaD, atpG, and PetH were significantly up-regulated under low-concentration Se treatment, which enhanced the energy metabolism process of photosystem I and photosystem II in rice seedlings. At the same time, the up-regulation of LHCA, LHCB family, and C4H1, PRX, and atp6 functional genes improved the ability of photon capture and heavy metal ion binding in plants. Combined with proteome analysis, the expression of functional proteins OsGSTF1, OsGSTU11, OsG6PDH4, OsDHAB1, CP29, and CabE was significantly up-regulated under Se, which enhanced photosynthesis and anti-oxidative stress mechanism in rice seedlings. At the same time, it regulates the plant hormone signal transduction pathway. It up-regulates the expression response process of IAA, ABA, and JAZ to activate the synergistic effect between each cell rapidly and jointly maintain the homeostasis balance.

CONCLUSION

Our results revealed the regulation process of Se-mediated critical metabolic pathways, functional genes, and proteins in rice under cadmium stress. They provided insights into the expression rules and dynamic response process of the Se-mediated plant resistance mechanism. This study provided the theoretical basis and technical support for crop safety in cropland ecosystems and cadmium-contaminated areas.

Specific protein interactions between rice members of the GT43 and GT47 families form various central cores of putative xylan synthase complexes

Members of the glycosyltransferase (GT)43 and GT47 families have been associated with heteroxylan synthesis in both dicots and monocots and are thought to assemble into central cores of putative xylan synthase complexes (XSCs). Currently, it is unknown whether protein-protein interactions within these central cores are specific, how many such complexes exist, and whether these complexes are functionally redundant. Here, we used gene association network and co-expression approaches in rice to identify four OsGT43s and four OsGT47s that assemble into different GT43/GT47 complexes. Using two independent methods, we showed that (i) these GTs assemble into at least six unique complexes through specific protein-protein interactions and (ii) the proteins interact directly in vitro. Confocal microscopy showed that, when alone, all OsGT43s were retained in the endoplasmic reticulum (ER), while all OsGT47s were localized in the Golgi. co-expression of OsGT43s and OsGT47s displayed complexes that form in the ER but accumulate in Golgi. ER-to-Golgi trafficking appears to require interactions between OsGT43s and OsGT47s. Comparison of the central cores of the three putative rice OsXSCs to wheat, asparagus, and Arabidopsis XSCs, showed great variation in GT43/GT47 combinations, which makes the identification of orthologous central cores between grasses and dicots challenging. However, the emerging picture is that all central cores from these species seem to have at least one member of the IRX10/IRX10-L clade in the GT47 family in common, suggesting greater functional importance for this family in xylan synthesis. Our findings provide a new framework for future investigation of heteroxylan biosynthesis and function in monocots.

Spatiotemporal deposition of cell wall polysaccharides in oat endosperm during grain development

Oat (Avena sativa) is a cereal crop whose grains are rich in (1,3;1,4)-β-D-glucan (mixed-linkage glucan or MLG), a soluble dietary fiber. In our study, we analyzed oat endosperm development in 2 Canadian varieties with differing MLG content and nutritional value. We confirmed that oat undergoes a nuclear type of endosperm development but with a shorter cellularization phase than barley (Hordeum vulgare). Callose and cellulose were the first polysaccharides to be detected in the early anticlinal cell walls at 11 days postemergence (DPE) of the panicle. Other polysaccharides such as heteromannan and homogalacturonan were deposited early in cellularization around 12 DPE after the first periclinal walls are laid down. In contrast to barley, heteroxylan deposition coincided with completion of cellularization and was detected from 14 DPE but was only detectable after demasking. Notably, MLG was the last polysaccharide to be laid down at 18 DPE within the differentiation phase, rather than during cellularization. In addition, differences in the spatiotemporal patterning of MLG were also observed between the 2 varieties. The lower MLG-containing cultivar AC Morgan (3.5% w/w groats) was marked by the presence of a discontinuous pattern of MLG labeling, while labeling in the same walls in CDC Morrison (5.6% w/w groats) was mostly even and continuous. RNA-sequencing analysis revealed higher transcript levels of multiple MLG biosynthetic cellulose synthase-like F (CSLF) and CSLH genes during grain development in CDC Morrison compared with AC Morgan that likely contributes to the increased abundance of MLG at maturity in CDC Morrison. CDC Morrison was also observed to have smaller endosperm cells with thicker walls than AC Morgan from cellularization onwards, suggesting the processes controlling cell size and shape are established early in development. This study has highlighted that the molecular processes influencing MLG content and deposition are more complex than previously imagined.

Concluding Embryogenesis After Diaspora: Seed Germination in Illicium Parviflorum

Albuminous seeds, dispersed with a minimally developed embryo surrounded by nutrient storage tissue, are pervasive across extinct and extant early diverging angiosperm lineages. Typically, seed ontogenic studies have focused on the time between fertilization and seed release, but in albuminous seeds, embryogenesis is incomplete at the time of seed dispersal. Here, I studied the morphological and nutritional relationships between the embryo and the endosperm after seed dispersal in Illicium parviflorum (Austrobaileyales). Seeds of I. parviflorum germinate over a period of three months. Different stages during the germination process were anatomically evaluated using a combination of histochemistry and immunocytochemistry. At dispersal, the seeds of Illicium contain a tiny achlorophyllous embryo with minimal histological differentiation, surrounded by copious amounts of lipo-protein globules stored in the endosperm within cell walls rich in un-esterified pectins. Six weeks later, the embryo expanded and differentiated the vascular tissues before the emergence of the radicle through the seed coat, as the stored lipids and proteins coalesced within cells. Six weeks later, the cotyledons contained starch and complex lipids intracellularly, and accumulated low-esterified pectins in their cell walls. The proteolipid-rich albuminous seeds of Illicium exemplify how woody angiosperms of the Austrobaileyales, Amborellales, and many magnoliids release seeds with high-energy storage compounds that are reprocessed by embryos that complete development during germination. Seedlings of these lineages thrive in the understory of tropical environments, which match with the predicted habitats where angiosperms evolved.

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

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

Genetic dissection of monosaccharides contents in rice whole grain using genome-wide association study

The simplest form of carbohydrates are monosaccharides which are the building blocks for the synthesis of polymers or complex carbohydrates. Monosaccharide contents of 197 rice accessions were quantified by HPAEC-PAD in rice (Oryza sativa L.) whole grain (RWG). A genome-wide association study (GWAS) was carried out using 33,812 single nucleotide polymorphisms (SNPs) to identify corresponding genomic regions influencing neutral monosaccharides contents. In total, 49 GWAS signals contained in 17 genomic regions (quantitative trait loci [QTLs]) on seven chromosomes of rice were determined to be associated with monosaccharides contents of whole grain. The QTLs were found for fucose (1), mannose (1), xylose (2), arabinose (2), galactose (4), and rhamnose (7) contents, all of which are novel. Based on co-location of annotated rice genes in the vicinity of GWAS signals, the constituents of the whole grain were associated with the following candidate genes: arabinose content with α-N-arabinofuranosidase, pectinesterase inhibitor, and glucosamine-fructose-6-phosphate aminotransferase 1; xylose content with ZOS1-10 (a C2H2 zinc finger transcription factor [TF]); mannose content with aldose 1-epimerase-like protein and a MYB family TF; galactose content with a GT8 family member (galacturonosyltransferase-like 3), a GRAS family TF, and a GH16 family member (xyloglucan endotransglucosylase/hydrolase xyloglucan 23); fucose content with gibberellin 20 oxidase and a lysine-rich arabinogalactan protein 19, and finally rhamnose content with myo-inositol-1-phosphate synthase, UDP-arabinopyranose mutase, and COBRA-like protein precursor. The results of this study should improve our understanding of the genetic basis of the factors that might be involved in the biosynthesis, regulation, and turnover of monosaccharides in RWG, aiming to enhance the nutritional value of rice grain and impact the related industries.

Integrated genome-wide differentiation and association analyses identify causal genes underlying breeding-selected grain quality traits in japonica rice

Improving grain quality is a primary objective in contemporary rice breeding. Japanese modern rice breeding has developed two different types of rice, eating and sake-brewing rice, with different grain characteristics, indicating the selection of variant gene alleles during the breeding process. Given the critical importance of promptly and efficiently identifying genes selected in past breeding for future molecular breeding, we conducted genome scans for divergence, genome-wide association studies, and map-based cloning. Consequently, we successfully identified two genes, OsMnS and OsWOX9D, both contributing to rice grain traits. OsMnS encodes a mannan synthase that increases the white core frequency in the endosperm, a desirable trait for sake brewing but decreases the grain appearance quality. OsWOX9D encodes a grass-specific homeobox-containing transcription factor, which enhances grain width for better sake brewing. Furthermore, haplotype analysis revealed that their defective alleles were selected in East Asia, but not Europe, during modern improvement. In addition, our analyses indicate that a reduction in grain mannan content during African rice domestication may also be caused a defective OsMnS allele due to breeding selection. This study not only reveals the delicate balance between grain appearance quality and nutrition in rice but also provides a new strategy for isolating causal genes underlying complex traits, based on the concept of "breeding-assisted genomics" in plants.

Analysis of glycans in a Burnt-on/Baked-on (BoBo) model food soil using Microarray Polymer Profiling (MAPP) and immunofluorescence microscopy

Burning of food materials during cooking can increase the difficulty in removal from solid surfaces, forming residual food soils. Using molecular probe-based technologies, the aim of this work was to elucidate the composition and relative abundance of glycans within a Burnt-On/Baked-On (BoBo) model food soil and investigate enzyme systems that may facilitate soil breakdown. Microarray Polymer Profiling identified xylan, arabinoxylan, mixed-linkage glucan and mannan as target substrates for the enzymatic cleaning of BoBo residues from surfaces. Indirect immunofluorescence microscopy revealed that burning resulted in extensive structural modifications and degradation of the three-dimensional architecture of constituent polysaccharide matrices. Results from high-throughput enzyme screening indicate that inclusion of xylan depolymerising enzymes in automatic dishwashing detergents may improve cleaning of recalcitrant, plant glycan-rich BoBo soils. Collectively, this study provides new insight into the composition and removal chemistry of complex, multi-component food soils.

Ovule cell wall composition is a maternal determinant of grain size in barley

In cereal species, grain size is influenced by growth of the ovule integuments (seed coat), the spikelet hull (lemma and palea) and the filial endosperm. Whether a highly conserved ovule tissue, the nucellus, has any impact on grain size has remained unclear. Immunolabelling revealed that the barley nucellus comprises two distinct cell types that differ in terms of cell wall homogalacturonan (HG) accumulation. Transcriptional profiling of the nucellus identified two pectin methylesterase (PME) genes, OVULE PECTIN MODIFIER 1 (OPM1) and OPM2, which are expressed in the unfertilized ovule but absent from the seed. Ovules from an opm1 opm2 mutant and plants expressing an ovule-specific pectin methylesterase inhibitor (PMEI), exhibit reduced HG accumulation. This results in changes to ovule cell size and shape and ovules that are longer than wild-type (WT) controls. At grain maturity, this is manifested as significantly longer grain. These findings indicate that cell wall composition during ovule development acts to limit ovule and seed growth. The investigation of ovule PME and PMEI activity reveals an unexpected role of maternal tissues in controlling grain growth before fertilization, one that has been lacking from models exploring improvements in grain size.

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.

Systems seed biology to understand and manipulate rice grain quality and nutrition

Rice is one of the most essential crops since it meets the calorific needs of 3 billion people around the world. Rice seed development initiates upon fertilization, leading to the establishment of two distinct filial tissues, the endosperm and embryo, which accumulate distinct seed storage products, such as starch, storage proteins, and lipids. A range of systems biology tools deployed in dissecting the spatiotemporal dynamics of transcriptome data, methylation, and small RNA based regulation operative during seed development, influencing the accumulation of storage products was reviewed. Studies of other model systems are also considered due to the limited information on the rice transcriptome. This review highlights key genes identified through a holistic view of systems biology targeted to modify biochemical composition and influence rice grain quality and nutritional value with the target of improving rice as a functional food.

The role and its transcriptome mechanisms of cell wall polysaccharides in vanadium detoxication of rice

Cell wall-polysaccharides play a crucial role in heavy metals binding, and hence, contribute to heavy metal detoxication in plants. However, there is no data regarding the molecular mechanisms of vanadium (V) binding to root cell walls in plants, especially in rice (Oryza sativa L.). Taking two rice cultivars with various V tolerance as the research material, the present study investigated the effect of various V concentrations on subcellular distribution of V and revealed the regulatory mechanism of cell wall polysaccharides to V exposure. The results showed that rice roots inhibited the upward movement of V, and root cell walls accumulated 69.85-82.71% of V in roots. Furthermore, hemicellulose-1 (HC-1) in cell walls shared up to 67.72% and 66.95% of the cell-wall-bound V in tolerant and sensitive cultivars, respectively. FTIR spectroscopy demonstrated that V stress induced the remolding of cell wall polysaccharides. Under V stress, V-tolerant rice generated up to 19.3% pectin, 40.9% HC-1, and 49.34% HC-2, which were higher than V-sensitive cultivar. The genes encoding UGDH, UGE, and AXS for polysaccharide biosynthesis were higher expressed in V-tolerant rice than V-sensitive rice when exposed to V. The results could provide novel insight for phytoremediation and food security guarantees.

Spatiotemporal Distribution of Homogalacturonans and Hemicelluloses in the Placentas, Ovules and Female Gametophytes of Utricularia nelumbifolia during Pollination

Utricularia nelumbifolia is a large carnivorous plant that is endemic to Brazil. It forms an extra-ovular female gametophyte, which surpasses the entire micropylar canal and extends beyond the limit of the integument and invades the placenta tissues. Due to the atypical behavior of the female gametophyte, it is interesting to determine the interaction between the gametophyte and sporophytic tissue. Therefore, the aim of this study was to evaluate the role of the placenta, the ovular tissues, the hypertrophied central cell and the integument in guiding the pollen tube in Utricularia nelumbifolia Gardner by studying the distribution of homogalacturonans and hemicelluloses. It was also determined whether the distribution of the homogalacturonans (HG) and hemicelluloses in Utricularia are dependent on pollination. The antibodies directed against the wall components (anti-pectin: JIM5, JIM7, LM19, LM20 and the anti-hemicelluloses: LM25, LM11, LM15, LM20, LM21) were used. Because both low- and high-esterified HG and xyloglucan were observed in the placenta, ovule (integument, chalaza) and female gametophyte of both pollinated and unpollinated flowers, the occurrence of these cell-wall components was not dependent on pollination. After fertilization, low methyl-esterified HGs were still observed in the cell walls of somatic cells and female gametophyte. However, in the case of high-esterified HG, the signal was weak and occurred only in the cell walls of the somatic cells. Because xyloglucans were observed in the cell walls of the synergids and egg cells, this suggests that they play a role in sexual reproduction. Utricularia nelumbifolia with an extra ovule-female gametophyte is presented as an attractive model for studying the male-female dialogue in plants.

Comparative compositions of metabolites and dietary fibre components in doughs and breads produced from bread wheat, emmer and spelt and using yeast and sourdough processes

Wholemeal flours from blends of bread wheat, emmer and spelt were processed into bread using yeast-based and sourdough fermentation. The bread wheat flour contained significantly higher concentrations of total dietary fibre and fructans than the spelt and emmer flours, the latter having the lowest contents. Breadmaking using sourdough and yeast systems resulted in changes in composition from flour to dough to bread including increases in organic acids and mannitol in the sourdough system and increases in amino acids and sugars (released by hydrolysis of proteins and starch, respectively) in both processing systems. The concentrations of fructans and raffinose (the major endogenous FODMAPs) were reduced by yeast and sourdough fermentation, with yeast having the greater effect. Both systems resulted in greater increases in sugars and glycerol in emmer than in bread wheat and spelt, but the significance of these differences for human health has not been established.

Comparison of β-Glucan Content in Milled Rice, Rice Husk and Rice Bran from Rice Cultivars Grown in Different Locations of Thailand and the Relationship between β-Glucan and Amylose Contents

β-glucan is a dietary fiber that is beneficial to human health, and its content varies according to its different parts, type of cereal grain, and growing environment. In this study, the β-glucan of milled rice, rice husk, and rice bran fractions, as well as the amylose content of milled rice fraction, from 38 selected rice-paddy grains from six regions of Thailand were quantitatively determined. The milled rice of the Sakon Nakhon (SN) cultivar grown in the northeast contained the highest β-glucan content (0.88 ± 0.03%), followed by the milled rice of the Jow Khao Chiangmai (JKC) cultivar (0.71 ± 0.03%) and rice bran of the Sew Mae Jan (SMJ) cultivar (0.67 ± 0.03%) grown in the north. The results reveal that the rice cultivars from each region showing variation in the β-glucan level in each fraction, which is mainly found in milled rice and rice bran, are similar to those found in other cereal grains, although low amounts are found in the husk. The amylose and β-glucan contents in the milled rice fraction showed a strong negative correlation (r = −0.805; p < 0.0001). This new information about the β-glucan content of Thai rice cultivars could be used for the development of cereal-based functional food.

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

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

Spatio-temporal immunolocalization of extensin protein and hemicellulose polysaccharides during olive fruit abscission

Immunocytochemical and molecular analyses reveal that the disassembly of the cell wall may be mediated by changes in the level and subcellular location of extensin protein and hemicelluloses during olive-fruit abscission. Although cell-wall modification is believed to underlie the changes in organ abscission, information concerning the changes in cell-wall proteins and hemicellulose polysaccharides is still limited. The aim of this work was to analyze the spatio-temporal patterns of the distribution of different extensin proteins and hemicelluloses in the abscission zone (AZ) during natural ripe-fruit abscission in olive (Olea europaea L.). In this study, we employed immunogold labeling in the ripe-fruit AZ during olive AZ cell separation, using an expanded set of monoclonal antibodies that recognize different types of hemicelluloses (LM11, LM15, and LM21), callose (anti-(1,3)-β-D-glucan) and extensin (JIM19) epitopes, and transmission electron microscopy imaging. Our data demonstrate that AZ cell separation was accompanied by a loss of the JIM19 extensin epitopes and a reduction in the detection of the LM15 xyloglucan epitopes in AZ cell walls, whereas AZ cells were found to be enriched with respect to the xylan and callose levels of the cell wall during olive ripe-fruit abscission. By contrast, AZ cell-wall polysaccharide remodeling did not involve mannans. Moreover, in ripe-fruit AZ, quantitative RT-PCR analysis revealed that OeEXT1, OeEXT2, OeXTH9, and OeXTH13 genes were downregulated during abscission, whereas the expression of OeXTH1, OeXTH5, and OeXTH14 genes increased during abscission. Taken together, the results indicate that AZ cell-wall dynamics during olive ripe-fruit abscission involves extensin protein and hemicellulose modifications, as well as related expressed genes. This is the first study available demonstrating temporal degradation of extensin protein and hemicelluloses in the AZ at the subcellular level.

Changes of cell wall components during embryogenesis of Castanea mollissima

The Chinese chestnut (Castanea mollissima Blume) 'Huaihuang' was chosen as the experimental material to observe embryogenesis and the dynamic changes of cell wall components during this process. Various developmental stages of embryos, including globular embryos, heart embryos, torpedo embryos and cotyledon embryos, were observed. The results showed that during embryogenesis, cellulose increased, and callose rapidly degraded. In the cell walls of developing embryos, pectic homogalacturonan (HG), especially low-esterified HG, was abundant, suggesting rapid synthesis and de-methyl-esterification of HG. Extensin and galactan increased with the development of the embryos. In contrast, the arabinan epitopes decreased in developing embryos but were more abundant than galactan epitopes at all stages. Xylan epitopes showed explicit boundaries between the outer epidermal wall and the rest of the inner tissues, and the fluorescence intensity of the outer epidermal wall was significantly higher than that of the inner tissues. Furthermore, the results indicated that the outer epidermal wall contained high amounts of cellulose, HG pectin and hemicellulose, especially arabinan and xylan. These results suggested the presence of rapid pectin metabolism, cellulose synthesis, rapid degradation of callose, different distributive patterns and dynamic changes of hemicellulose (galactan, arabinan and xylan) and extensin during embryogenesis. Various cell wall components exist in different tissues of the embryo, and dynamic changes in cell wall components are involved in the embryonic development process.

Comprehensive comparison and applications of different sections in investigating the microstructure and histochemistry of cereal kernels

This review summarizes the main applications of different sections and some improved sectioning methods in investigating the microstructure and histochemistry of cereal kernels. Thick sections of developing kernels prepared by free-hand and sliding microtome-aided sectioning method can be employed to elucidate tissue anatomy and histochemistry. The thin sections of mature kernels prepared by ultramicrotome-aided sectioning method can exhibit the micromorphology of starch granules when stained with iodine solution. The paraffin sections of developing kernels can exhibit the tissue anatomy of kernel, the accumulation of storage substances, and the location of protein and gene transcripts with immunohistochemistry and in situ hybridization techniques. The semithin resin sections can clearly exhibit the morphology of cells, starch granules, and protein bodies in kernel, but the sections prepared with different resins have various advantages and disadvantages for research investigating the morphology and histochemistry of cereal kernels. The improved methods of free-hand sectioning and ultramicrotome-aided sectioning of mature kernels are suitable for investigating the morphology of starch granules in a large number of samples in a short time. The modified method for preparing resin sections of whole kernels can be employed to determine the morphology and distribution of cells, starch granules, and storage protein in mature, developing, germinated, and cooked kernels in situ. This review could help researchers choose appropriate sections for investigating the microstructure and histochemistry of cereal kernels according to their study objectives.

Cell Wall Proteome of Wheat Grain Endosperm and Outer Layers at Two Key Stages of Early Development

The cell wall is an important compartment in grain cells that fulfills both structural and functional roles. It has a dynamic structure that is constantly modified during development and in response to biotic and abiotic stresses. Non-structural cell wall proteins (CWPs) are key players in the remodeling of the cell wall during events that punctuate the plant life. Here, a subcellular and quantitative proteomic approach was carried out to identify CWPs possibly involved in changes in cell wall metabolism at two key stages of wheat grain development: the end of the cellularization step and the beginning of storage accumulation. Endosperm and outer layers of wheat grain were analyzed separately as they have different origins (maternal and seed) and functions in grains. Altogether, 734 proteins with predicted signal peptides were identified (CWPs). Functional annotation of CWPs pointed out a large number of proteins potentially involved in cell wall polysaccharide remodeling. In the grain outer layers, numerous proteins involved in cutin formation or lignin polymerization were found, while an unexpected abundance of proteins annotated as plant invertase/pectin methyl esterase inhibitors were identified in the endosperm. In addition, numerous CWPs were accumulating in the endosperm at the grain filling stage, thus revealing strong metabolic activities in the cell wall during endosperm cell differentiation, while protein accumulation was more intense at the earlier stage of development in outer layers. Altogether, our work gives important information on cell wall metabolism during early grain development in both parts of the grain, namely the endosperm and outer layers. The wheat cell wall proteome is the largest cell wall proteome of a monocot species found so far.

High-throughput method for preliminary screening of high dietary fiber rice

Dietary fiber has several benefits for humans, and the development of healthier rice with an improved dietary fiber composition has attracted increasing amounts of attention. Based on the method of AOAC 2002.02, we developed a simplified method to screen polished rice containing high total dietary fiber (TDF). Mutant cw with a high TDF content could be distinguished easily from R7954 (indica) and Nipponbare (japonica) by the digestion-resistant phenotype, which is characterized as an almost intact grain after hydrolysis by pepsin, pancreatic α-amylase and amyloglucosidase. The individuals identified from the F₂ population showed digestion resistance all had TDF content higher than 5%, while those without a digestion-resistant phenotype had TDF content lower than 5%. The phenotype of digestion resistance could be a valuable index for identifying rice with higher TDF content, and the identification of this phenotype provides a simplified, economical and high throughput method for high TDF rice breeding.

Spatial and temporal distribution of cell wall polysaccharides during grain development of Brachypodium distachyon

Brachypodium distachyon (Brachypodium) is now well considered as being a suitable plant model for studying temperate cereal crops. Its cell walls are phylogenetically intermediate between rice and poaceae, with a greater proximity to these latter. By microscopic and biochemical approaches, this work gives an overview of the temporal and spatial distribution of cell wall polysaccharides in the grain of Brachypodium from the end of the cellularization step to the maturation of grain. Variation in arabinoxylan chemical structure and distribution were demonstrated according to development and different grain tissues. In particular, the kinetic of arabinoxylan feruloylation was shown occuring later in the aleurone layers compared to storage endosperm. Mixed linked β-glucan was detected in whole the tissues of Brachypodium grain even at late stage of development. Cellulose was found in both the storage endosperm and the outer layers. Homogalacturonan and rhamnogalacturonan I epitopes were differentially distributed within the grain tissues. LM5 galactan epitope was restricted to the aleurone layers contrary to LM6 arabinan epitope which was detected in the whole endosperm. A massive deposition of highly methylated homogalacturonans in vesicular bodies was observed underneath the cell wall of the testa t2 layer at early stage of development. At maturity, low-methylated homogalacturonans totally fulfilled the lumen of the t2 cell layer, suggesting pectin remodeling during grain development. Xyloglucans were only detected in the cuticle above the testa early in the development of the grain while feruloylated arabinoxylans were preferentially deposited into the cell wall of t1 layer. Indeed, the circumscribed distribution of some of the cell wall polysaccharides raises questions about their role in grain development and physiology.

Gradients in compositions in the starchy endosperm of wheat have implications for milling and processing

BACKGROUND

Wheat is the major food grain consumed in temperate countries. Most wheat is consumed after milling to produce white flour, which corresponds to the endosperm storage tissue of the grain. Because the starchy endosperm accounts for about 80% of the grain dry weight, the miller aims to achieve flour yields approaching this value.

SCOPE AND APPROACH

Bioimaging can be combined with biochemical analysis of fractions produced by sequential pearling of whole grains to determine the distributions of components within the endosperm tissue.

KEY FINDINGS AND CONCLUSIONS

This reveals that endosperm is not homogeneous, but exhibits gradients in composition from the outer to the inner part. These include gradients in both amount and composition. For example, the content of gluten proteins decreases but the proportion of glutenin polymers increases from the outside to the centre of the tissue. However, the content of starch increases with changes in the granule size distribution, the proportions of amylose and amylopectin, and their thermal properties. Hence these parts of the endosperm differ in the functional properties for food processing. Gradients also exist in minor components which may affect health and processing, such as dietary fibre and lipids. The gradients in grain composition are reflected in differences in the compositions of the mill streams which are combined to give white flour (which may number over 20). These differences could therefore be exploited by millers and food processors to develop flours with compositions and properties for specific end uses.

Seed coats as an alternative molecular factory: thinking outside the box

Seed coats as commodities. Seed coats play important roles in the protection of the embryo from biological attack and physical damage by the environment as well as dispersion strategies. A significant part of the energy devoted by the mother plant to seed production is channeled into the production of the cell layers and metabolites that surround the embryo. Nevertheless, in crop species these are often discarded post-harvest and are a wasted resource that could be processed to yield co-products. The production of novel compounds from existing metabolites is also a possibility. A number of macromolecules are already accumulated in these maternal layers that could be exploited in industrial applications either directly or via green chemistry, notably flavonoids, lignin, lignan, polysaccharides, lipid polyesters and waxes. Here, we summarize our knowledge of the in planta biosynthesis pathways of these macromolecules and their molecular regulation as well as potential applications. We also outline recent work aimed at providing further tools for increasing yields of existing molecules or the development of novel biotech approaches, as well as trial studies aimed at exploiting this underused resource.

Tissue-specific changes in apoplastic proteins and cell wall structure during cold acclimation of winter wheat crowns

The wheat (Triticum aestivum L.) crown is the critical organ of low temperature stress survival over winter. In cold-acclimated crowns, ice formation in the apoplast causes severe tissue disruption as it grows at the expense of intracellular water. While previous crown studies have shown the vascular transition zone (VTZ) to have a higher freezing sensitivity than the shoot apical meristem (SAM), the mechanism behind the differential freezing response is not fully understood. Cooling cold-acclimated crowns to -10 °C resulted in an absence of VTZ tetrazolium chloride staining, whereas the temperatures at which 50% of the SAM stained positive and 50% of plants recovered (LT50) were similar after cold acclimation for 21 (-16 °C) and 42 d (-20 °C) at 4 °C. Proteomic analysis of the apoplastic fluids identified dehydrins, vernalization-responsive proteins, and cold shock proteins preferentially accumulated in the SAM. In contrast, modifications to the VTZ centered on increases in pathogenesis-related proteins, anti-freeze proteins, and sugar hydrolyzing enzymes. Fourier transform infrared spectroscopy focal plane array analysis identified the biochemical modification of the cell wall to enhance methyl-esterified cross-linking of glucuronoarabinoxylans in the VTZ. These findings indicate that the SAM and VTZ express two distinct tissue-specific apoplastic responses during cold acclimation.

Morphology, Carbohydrate Distribution, Gene Expression, and Enzymatic Activities Related to Cell Wall Hydrolysis in Four Barley Varieties during Simulated Malting

Many biological processes, such as cell wall hydrolysis and the mobilisation of nutrient reserves from the starchy endosperm, require stringent regulation to successfully malt barley (Hordeum vulgare) grain in an industrial context. Much of the accumulated knowledge defining these events has been collected from individual, unrelated experiments, and data have often been extrapolated from Petri dish germination, rather than malting, experiments. Here, we present comprehensive morphological, biochemical, and transcript data from a simulated malt batch of the three elite malting cultivars Admiral, Navigator, and Flagship, and the feed cultivar Keel. Activities of lytic enzymes implicated in cell wall and starch depolymerisation in germinated grain have been measured, and transcript data for published cell wall hydrolytic genes have been provided. It was notable that Flagship and Keel exhibited generally similar patterns of enzyme and transcript expression, but exhibited a few key differences that may partially explain Flagship's superior malting qualities. Admiral and Navigator also showed matching expression patterns for these genes and enzymes, but the patterns differed from those of Flagship and Keel, despite Admiral and Navigator having Keel as a common ancestor. Overall (1,3;1,4)-β-glucanase activity differed between cultivars, with lower enzyme levels and concomitantly higher amounts of (1,3;1,4)-β-glucan in the feed variety, Keel, at the end of malting. Transcript levels of the gene encoding (1,3;1,4)-β-glucanase isoenzyme EI were almost three times higher than those encoding isoenzyme EII, suggesting a previously unrecognised importance for isoenzyme EI during malting. Careful morphological examination showed that scutellum epithelial cells in mature dry grain are elongated but expand no further as malting progresses, in contrast to equivalent cells in other cereals, perhaps demonstrating a morphological change in this critical organ over generations of breeding selection. Fluorescent immuno-histochemical labelling revealed the presence of pectin in the nucellus and, for the first time, significant amounts of callose throughout the starchy endosperm of mature grain.

Immunodetection of some pectic, arabinogalactan proteins and hemicellulose epitopes in the micropylar transmitting tissue of apomictic dandelions (Taraxacum, Asteraceae, Lactuceae)

In apomictic Taraxacum species, the development of both the embryo and the endosperm does not require double fertilisation. However, a structural reduction of ovular transmitting tissue was not observed in apomictic dandelions. The aim of this study was to analyse the chemical composition of the cell walls to describe the presence of arabinogalactan proteins (AGPs), hemicellulose and some pectic epitopes in the micropylar transmitting tissue of apomictic Taraxacum. The results point to (1) the similar distribution of AGPs in different developmental stages, (2) the absence of highly methyl-esterified homogalacturonan (HG) in transmitting tissue of ovule containing a mature embryo sac and the appearance of this pectin domain in the young seed containing the embryo and endosperm, (3) the similar pattern of low methyl-esterified pectin occurrence in both an ovule and a young seed with an embryo and endosperm in apomictic Taraxacum and (4) the presence of hemicelluloses recognised by LM25 and LM21 antibodies in the reproductive structure of Taraxacum.

Distribution of some pectic and arabinogalactan protein epitopes during Solanum lycopersicum (L.) adventitious root development

BACKGROUND

The adventitious roots (AR) of plants share the same function as primary and lateral roots (LR), although their development is mainly an adaptive reaction to stress conditions. Regeneration of grafted plants is often accompanied by AR formation thus making the grafting technique a good model for studying AR initiation and development and their means of emergence. Pectins and arabinogalactan proteins (AGP) are helpful markers of particular cellular events, such as programmed cell death (PCD), elongation, proliferation or other differentiation events that accompany AR development. However, little is known about the distribution of pectins and AGPs during AR ontogeny, either in the primordium or stem tissues from which AR arise or their correspondence with these events during LR formation.

RESULTS

AR were developed from different stem tissues such as parenchyma, xylem rays and the cambium, depending on the stem age and treatment (grafting versus cutting) of the parental tissue. Immunochemical analysis of the presence of pectic (LM8, LM19, LM20) and AGP (JIM8, JIM13, JIM16) epitopes in AR and AR-associated tissues showed differential, tissue-specific distributions of these epitopes. Two pectic epitopes (LM19, LM20) were developmentally regulated and the occurrence of the LM8 xylogalacturonan epitope in the root cap of the AR differed from other species described so far. AGP epitopes were abundantly present in the cytoplasmic compartments (mainly the tonoplast) and were correlated with the degree of cell vacuolisation. JIM8 and JIM13 epitopes were detected in the more advanced stages of primordium development, whereas the JIM16 epitope was present from the earliest division events of the initial AR cells. The comparison between AR and LR showed quantitative (AGP,) and qualitative (pectins) differences.

CONCLUSION

The chemical compositions of adventitious and lateral root cells show differences that correlate with the different origins of these cells. In AR, developmental changes in the distribution of pectins and AGP suggest the turnover of wall compounds. Our data extend the knowledge about the distribution of pectin and AGP during non-embryogenic root development in a species that is important from an agronomic point of view.

Structure of a plant β-galactosidase C-terminal domain

Most plant β-galactosidases, which belong to glycoside hydrolase family 35, have a C-terminal domain homologous to animal galactose and rhamnose-binding lectins. To investigate the structure and function of this domain, the C-terminal domain of the rice (Oryza sativa L.) β-galactosidase 1 (OsBGal1 Cter) was expressed in Escherichia coli and purified to homogeneity. The free OsBGal1 Cter is monomeric with a native molecular weight of 15kDa. NMR spectroscopy indicated that OsBGal1 Cter comprises five β-strands and one α-helix. The structure of this domain is similar to lectin domains from animals, but loops A and C of OsBGal1 Cter are longer than the corresponding loops from related animal lectins with known structures. In addition, loop A of OsBGal1 Cter was not well defined, suggesting it is flexible. Although OsBGal1 Cter was predicted to be a galactose/rhamnose-binding domain, binding with rhamnose, galactose, glucose, β-1,4-d-galactobiose and raffinose could not be observed in NMR experiments.

Understanding the Remodelling of Cell Walls during Brachypodium distachyon Grain Development through a Sub-Cellular Quantitative Proteomic Approach

Brachypodiumdistachyon is a suitable plant model for studying temperate cereal crops, such as wheat, barley or rice, and helpful in the study of the grain cell wall. Indeed, the most abundant hemicelluloses that are in the B. distachyon cell wall of grain are (1-3)(1-4)-β-glucans and arabinoxylans, in a ratio similar to those of cereals such as barley or oat. Conversely, these cell walls contain few pectins and xyloglucans. Cell walls play an important role in grain physiology. The modifications of cell wall polysaccharides that occur during grain development and filling are key in the determination of the size and weight of the cereal grains. The mechanisms required for cell wall assembly and remodelling are poorly understood, especially in cereals. To provide a better understanding of these processes, we purified the cell wall at three developmental stages of the B. distachyon grain. The proteins were then extracted, and a quantitative and comparative LC-MS/MS analysis was performed to investigate the protein profile changes during grain development. Over 466 cell wall proteins (CWPs) were identified and classified according to their predicted functions. This work highlights the different proteome profiles that we could relate to the main phases of grain development and to the reorganization of cell wall polysaccharides that occurs during these different developmental stages. These results provide a good springboard to pursue functional validation to better understand the role of CWPs in the assembly and remodelling of the grain cell wall of cereals.

Lytic polysaccharide monooxygenases and other oxidative enzymes are abundantly secreted by Aspergillus nidulans grown on different starches

BACKGROUND

Starch is the second most abundant plant-derived biomass and a major feedstock in non-food industrial applications and first generation biofuel production. In contrast to lignocellulose, detailed insight into fungal degradation of starch is currently lacking. This study explores the secretomes of Aspergillus nidulans grown on cereal starches from wheat and high-amylose (HA) maize, as well as legume starch from pea for 5 days.

RESULTS

Aspergillus nidulans grew efficiently on cereal starches, whereas growth on pea starch was poor. The secretomes at days 3-5 were starch-type dependent as also reflected by amylolytic activity measurements. Nearly half of the 312 proteins in the secretomes were carbohydrate-active enzymes (CAZymes), mostly glycoside hydrolases (GHs) and oxidative auxiliary activities (AAs). The abundance of the GH13 α-amylase (AmyB) decreased with time, as opposed to other starch-degrading enzymes, e.g., the GH13 AmyF, GH15 glucoamylases (GlaA and GlaB), and the GH31 α-glucosidase (AgdE). Two AA13 LPMOs displayed similar secretion patterns as amylolytic hydrolases and were among the most abundant CAZymes. The starch-active AnLPMO13A that possesses a CBM20 carbohydrate-binding module dominated the starch-binding secretome fraction. A striking observation is the co-secretion of several redox-active enzymes with the starch-active AA13 LPMOs and GHs, some at high abundance. Notably nine AA9 LPMOs, six AA3 sub-family 2 (AA_2) oxidoreductases, and ten AA7 glyco-oligosaccharide oxidases were identified in the secretomes in addition to other non-CAZyme oxidoreductases.

CONCLUSIONS

The co-secretion and high abundance of AA13 LPMOs are indicative of a key role in starch granule deconstruction. The increase in AA13 LPMO abundance with culture time may reflect accumulation of a more resistant starch fraction towards the later stages of the culture. The identification of AmyR sites upstream AA13 LPMOs unveils co-regulation of LPMOs featuring in starch utilization. Differential deployment of amylolytic hydrolases and LPMOs over time suggests additional regulatory mechanisms. The abundant co-secretion of distinct AA3 and AA7 oxidoreductases merits further studies into their roles and possible interplay with LPMOs and other enzymes in the deconstruction of starchy substrates. The study reports for the first time the biological significance of LPMOs in starch degradation and the temporal interplay between these and amylolytic hydrolases.

The contribution of wheat to human diet and health

Wheat is the most important staple crop in temperate zones and is in increasing demand in countries undergoing urbanization and industrialization. In addition to being a major source of starch and energy, wheat also provides substantial amounts of a number of components which are essential or beneficial for health, notably protein, vitamins (notably B vitamins), dietary fiber, and phytochemicals. Of these, wheat is a particularly important source of dietary fiber, with bread alone providing 20% of the daily intake in the UK, and well-established relationships between the consumption of cereal dietary fiber and reduced risk of cardio-vascular disease, type 2 diabetes, and forms of cancer (notably colo-rectal cancer). Wheat shows high variability in the contents and compositions of beneficial components, with some (including dietary fiber) showing high heritability. Hence, plant breeders should be able to select for enhanced health benefits in addition to increased crop yield.