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

Cell-type proteomic and metabolomic resolution of early and late grain filling stages of wheat endosperm

The nutritional value of wheat grains, particularly their protein and metabolite composition, is a result of the grain-filling process, especially in the endosperm. Here, we employ laser microdissection (LMD) combined with shotgun proteomics and metabolomics to generate a cell type-specific proteome and metabolome inventory of developing wheat endosperm at the early (15 DAA) and late (26 DAA) grain-filling stages. We identified 1803 proteins and 41 metabolites from four different cell types (aleurone (AL), sub-aleurone (SA), starchy endosperm (SE) and endosperm transfer cells (ETCs). Differentially expressed proteins were detected, 67 in the AL, 31 in the SA, 27 in the SE and 50 in the ETCs between these two-time points. Cell-type accumulation of specific SUT and GLUT transporters, sucrose converting and starch biosynthesis enzymes correlate well with the respective sugar metabolites, suggesting sugar upload and starch accumulation via nucellar projection and ETC at 15 DAA in contrast to the later stage at 26 DAA. Changes in various protein levels between AL, SA and ETC support this metabolic switch from 15 to 26 DAA. The distinct spatial and temporal abundances of proteins and metabolites revealed a contrasting activity of nitrogen assimilation pathways, e.g. for GOGAT, GDH and glutamic acid, in the different cell types from 15 to 26 DAA, which can be correlated with specific protein accumulation in the endosperm. The integration of cell-type specific proteome and metabolome data revealed a complex metabolic interplay of the different cell types and a functional switch during grain development and grain-filling processes.

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.

Proteolytic activities and profiles as useful traits to select barley cultivars for beer production

Barley malting depends on hydrolytic enzymes that degrade storage macromolecules. Identifying barley cultivars with proteolytic activity that guarantees appropriate foaming, flavor, and aroma in the beer is of great importance. In this work, the proteolytic activity and profiles of brewing malt from Mexican barley cultivars were analyzed. Data showed that Cys- (at 50°C) and Ser-proteases (at 70°C) are the major contributors to proteolytic activity during mashing. Essential amino acids, necessary for fermentation and production of good flavor and aroma in beer, were detected at the end of mashing. According to our results, Mexican cultivar HV2005-19 exhibits similar proteolytic activities as those from cultivar Metcalfe, which is one of the most utilized for the brewing industry. Moreover, we propose Cys- and Ser-proteases as biochemical markers during mashing at 50 and 70°C, respectively, to select barley cultivars for beer production. PRACTICAL APPLICATIONS: Proteolytic activity, which depends on activation and de novo synthesis of proteases in the aleurone layer of barley seeds, is crucial in beer production. Identifying new barley varieties that have optimal proteolytic activities is of great interest for genetic improvement programs. In this study, we propose the variety HV2005-19 as a genotype with Cys- and Ser-proteases activity similar to that from Metcalfe, which is a top variety in the brewing industry.

Identification of candidate MYB transcription factors that influence CslF6 expression in barley grain

(1,3;1,4)-β-Glucan is a non-cellulosic polysaccharide required for correct barley grain fill and plant development, with industrial relevance in the brewing and the functional food sector. Barley grains contain higher levels of (1,3;1,4)-β-glucan compared to other small grain cereals and this influences their end use, having undesirable effects on brewing and distilling and beneficial effects linked to human health. HvCslF6 is the main gene contributing to (1,3;1,4)-β-glucan biosynthesis in the grain. Here, the transcriptional regulation of HvCslF6 was investigated using an in-silico analysis of transcription factor binding sites (TFBS) in its putative promoter, and functional characterization in a barley protoplast transient expression system. Based on TFBS predictions, TF classes AP2/ERF, MYB, and basic helix-loop-helix (bHLH) were over-represented within a 1,000 bp proximal HvCslF6 promoter region. Dual luciferase assays based on multiple HvCslF6 deletion constructs revealed the promoter fragment driving HvCslF6 expression. Highest HvCslF6 promoter activity was narrowed down to a 51 bp region located -331 bp to -382 bp upstream of the start codon. We combined this with TFBS predictions to identify two MYB TFs: HvMYB61 and HvMYB46/83 as putative activators of HvCslF6 expression. Gene network analyses assigned HvMYB61 to the same co-expression module as HvCslF6 and other primary cellulose synthases (HvCesA1, HvCesA2, and HvCesA6), whereas HvMYB46/83 was assigned to a different module. Based on RNA-seq expression during grain development, HvMYB61 was cloned and tested in the protoplast system. The transient over-expression of HvMYB61 in barley protoplasts suggested a positive regulatory effect on HvCslF6 expression.

Mashing performance as a function of malt particle size in beer production

Significant innovations have occurred over the past 50 years in the malting and brewing industries, focused on optimization of the beer mashing, boiling and fermentation processes. One of the challenges faced in beer brewing has been in the malting process to obtain the desired malt and wort quality to produce high-quality beer products. The hydrolytic enzymes produced during grain germination are mostly entrapped inside the cellular matrices of the grain. The intra-grain diffusion of enzymes for in-situ hydrolysis, as well as diffusion of enzymes to wort, depends upon the malt size and malt size fractions obtained after milling. This review investigates the relationship between varying barley grain particle size distribution and the efficiency of the malting and mashing processes. Recommended ideal particle size of barley grain before and after milling are proposed based on the review of existing literature. Each brewing batch of grains with a proportion of >80% plump grains (>2.5 mm in size) is suggested to be the optimal size before milling, whereas the optimum grain particle size after milling ranged between 0.25 and 0.5 mm. The current review will summarize the theoretical aspects for malt milling and the particle size characteristics for optimizing the brewing process.

Identification and spatio-temporal expression analysis of barley genes that encode putative modular xylanolytic enzymes

Arabinoxylans are cell wall polysaccharides whose re-modelling and degradation during plant development are mediated by several classes of xylanolytic enzymes. Here, we present the identification and new annotation of twelve putative (1,4)-β-xylanase and six β-xylosidase genes, and their spatio-temporal expression patterns during vegetative and reproductive growth of barley (Hordeum vulgare cv. Navigator). The encoded xylanase proteins are all predicted to contain a conserved carbohydrate-binding module (CBM) and a catalytic glycoside hydrolase (GH) 10 domain. Additional domains in some xylanases define three discrete phylogenetic clades: one clade contains proteins with an additional N-terminal signal sequence, while another clade contains proteins with multiple CBMs. Homology modelling revealed that all fifteen xylanases likely contain a third domain, a β-sandwich folded from two non-contiguous sequence segments that bracket the catalytic GH domain, which may explain why the full length protein is required for correct folding of the active enzyme. Similarly, predicted xylosidase proteins share a highly conserved domain structure, each with an N-terminal signal peptide, a split GH 3 domain, and a C-terminal fibronectin-like domain. Several genes appear to be ubiquitously expressed during barley growth and development, while four newly annotated xylanase and xylosidase genes are expressed at extremely high levels, which may be of broader interest for industrial applications where cell wall degradation is necessary.

Genes That Mediate Starch Metabolism in Developing and Germinated Barley Grain

Starch is synthesized in the endosperm of developing barley grain, where it functions as the primary source of stored carbohydrate. In germinated grain these starch reserves are hydrolyzed to small oligosaccharides and glucose, which are transported to the embryo to support the growth of the developing seedling. Some of the mobilized glucose is transiently stored as starch in the scutellum of germinated grain. These processes are crucial for early seedling vigor, which is a key determinant of crop productivity and global food security. Several starch synthases (SS), starch-branching enzymes (SBEs), and starch debranching enzymes (isoamylases, ISA), together with a limit dextrinase (LD), have been implicated in starch synthesis from nucleotide-sugar precursors. Starch synthesis occurs both in the developing endosperm and in the scutellum of germinated grain. For the complete depolymerization of starch to glucose, α-amylase (Amy), β-amylase (Bmy), isoamylase (ISA), limit dextrinase (LD), and α-glucosidase (AGL) are required. Most of these enzymes are encoded by gene families of up to 10 or more members. Here RNA-seq transcription data from isolated tissues of intact developing and germinated barley grain have allowed us to identify the most important, specific gene family members for each of these processes in vivo and, at the same time, we have defined in detail the spatio-temporal coordination of gene expression in different tissues of the grain. A transcript dataset for 81,280 genes is publicly available as a resource for investigations into other cellular and biochemical processes that occur in the developing grain from 6 days after pollination.

Comparative de novo transcriptome analysis of barley varieties with different malting qualities

Barley is one of the most important crops in the world. Barley is used as both food and feed and is important for malt production. Demands for malting quality differ among countries and customs. Malting quality is a complex characteristic involving barley genetics, the environmental conditions during barley growth, and the technological parameters of the malting process. In this study, the hypothesis was that there were no differences between two groups of barley varieties with different but defined malting qualities, which was tested using RNA sequencing during selected stages of malting. In total, 919 differentially transcribed genes between the two barley groups were identified and annotated. Differentially expressed genes (DEGs) were primarily assigned to gene ontology (GO) terms of oxidation-reduction process - oxidoreductase activity, response to stress, carbohydrate metabolic process, and proteolysis - hydrolase activity, and metal ion binding. Genes connected with the plasma membrane and its integral components also play important roles in malting quality. DEG profiles of selected genes in the three malting stages indicate a complex character of malting quality. Many single-nucleotide polymorphisms (SNPs) and insertions and deletions (indels) were identified. SNPs and indels with the best quality were used for primer design. After optimization and validation, five molecular markers were developed for use in barley breeding.

Arabinoxylan, β-glucan and pectin in barley and malt endosperm cell walls: a microstructure study using CLSM and cryo-SEM

The architecture of endosperm cell walls in Hordeum vulgare (barley) differs remarkably from that of other grass species and is affected by germination or malting. Here, the cell wall microstructure is investigated using (bio)chemical analyses, cryogenic scanning electron microscopy (cryo-SEM) and confocal laser scanning microscopy (CLSM) as the main techniques. The relative proportions of β-glucan, arabinoxylan and pectin in cell walls were 61, 34 and 5%, respectively. The average thickness of a single endosperm cell wall was 0.30 µm, as estimated by the cryo-SEM analysis of barley seeds, which was reduced to 0.16 µm after malting. After fluorescent staining, 3D confocal multiphoton microscopy (multiphoton CLSM) imaging revealed the complex cell wall architecture. The endosperm cell wall is composed of a structure in which arabinoxylan and pectin are colocalized on the outside, with β-glucan depositions on the inside. During germination, arabinoxylan and β-glucan are hydrolysed, but unlike β-glucan, arabinoxylan remains present in defined cell walls in malt. Integrating the results, an enhanced model for the endosperm cell walls in barley is proposed.

Transcriptional and biochemical analyses of gibberellin expression and content in germinated barley grain

Mobilization of reserves in germinated cereal grains is critical for early seedling vigour, global crop productivity, and hence food security. Gibberellins (GAs) are central to this process. We have developed a spatio-temporal model that describes the multifaceted mechanisms of GA regulation in germinated barley grain. The model was generated using RNA sequencing transcript data from tissues dissected from intact, germinated grain, which closely match measurements of GA hormones and their metabolites in those tissues. The data show that successful grain germination is underpinned by high concentrations of GA precursors in ungerminated grain, the use of independent metabolic pathways for the synthesis of several bioactive GAs during germination, and a capacity to abort bioactive GA biosynthesis. The most abundant bioactive form is GA1, which is synthesized in the scutellum as a glycosyl conjugate that diffuses to the aleurone, where it stimulates de novo synthesis of a GA3 conjugate and GA4. Synthesis of bioactive GAs in the aleurone provides a mechanism that ensures the hormonal signal is relayed from the scutellum to the distal tip of the grain. The transcript data set of 33 421 genes used to define GA metabolism is available as a resource to analyse other physiological processes in germinated grain.

Barley grain (1,3;1,4)-β-glucan content: effects of transcript and sequence variation in genes encoding the corresponding synthase and endohydrolase enzymes

The composition of plant cell walls is important in determining cereal end uses. Unlike other widely consumed cereal grains barley is comparatively rich in (1,3;1,4)-β-glucan, a source of dietary fibre. Previous work showed Cellulose synthase-like genes synthesise (1,3;1,4)-β-glucan in several tissues. HvCslF6 encodes a grain (1,3;1,4)-β-glucan synthase, whereas the function of HvCslF9 is unknown. Here, the relationship between mRNA levels of HvCslF6, HvCslF9, HvGlbI (1,3;1,4)-β-glucan endohydrolase, and (1,3;1,4)-β-glucan content was studied in developing grains of four barley cultivars. HvCslF6 was differentially expressed during mid (8-15 DPA) and late (38 DPA) grain development stages while HvCslF9 transcript was only clearly detected at 8-10 DPA. A peak of HvGlbI expression was detected at 15 DPA. Differences in transcript abundance across the three genes could partially explain variation in grain (1,3;1,4)-β-glucan content in these genotypes. Remarkably narrow sequence variation was found within the HvCslF6 promoter and coding sequence and does not explain variation in (1,3;1,4)-β-glucan content. Our data emphasise the genotype-dependent accumulation of (1,3;1,4)-β-glucan during barley grain development and a role for the balance between hydrolysis and synthesis in determining (1,3;1,4)-β-glucan content, and suggests that other regulatory sequences or proteins are likely to be involved in this trait in developing grain.

Barley grain (1,3;1,4)-β-glucan content: effects of transcript and sequence variation in genes encoding the corresponding synthase and endohydrolase enzymes

The composition of plant cell walls is important in determining cereal end uses. Unlike other widely consumed cereal grains barley is comparatively rich in (1,3;1,4)-β-glucan, a source of dietary fibre. Previous work showed Cellulose synthase-like genes synthesise (1,3;1,4)-β-glucan in several tissues. HvCslF6 encodes a grain (1,3;1,4)-β-glucan synthase, whereas the function of HvCslF9 is unknown. Here, the relationship between mRNA levels of HvCslF6, HvCslF9, HvGlbI (1,3;1,4)-β-glucan endohydrolase, and (1,3;1,4)-β-glucan content was studied in developing grains of four barley cultivars. HvCslF6 was differentially expressed during mid (8-15 DPA) and late (38 DPA) grain development stages while HvCslF9 transcript was only clearly detected at 8-10 DPA. A peak of HvGlbI expression was detected at 15 DPA. Differences in transcript abundance across the three genes could partially explain variation in grain (1,3;1,4)-β-glucan content in these genotypes. Remarkably narrow sequence variation was found within the HvCslF6 promoter and coding sequence and does not explain variation in (1,3;1,4)-β-glucan content. Our data emphasise the genotype-dependent accumulation of (1,3;1,4)-β-glucan during barley grain development and a role for the balance between hydrolysis and synthesis in determining (1,3;1,4)-β-glucan content, and suggests that other regulatory sequences or proteins are likely to be involved in this trait in developing grain.

Differences in hydrolytic enzyme activity accompany natural variation in mature aleurone morphology in barley (Hordeum vulgare L.)

The aleurone is a critical component of the cereal seed and is located at the periphery of the starchy endosperm. During germination, the aleurone is responsible for releasing hydrolytic enzymes that degrade cell wall polysaccharides and starch granules, which is a key requirement for barley malt production. Inter- and intra-species differences in aleurone layer number have been identified in the cereals but the significance of this variation during seed development and germination remains unclear. In this study, natural variation in mature aleurone features was examined in a panel of 33 Hordeum vulgare (barley) genotypes. Differences were identified in the number of aleurone cell layers, the transverse thickness of the aleurone and the proportion of aleurone relative to starchy endosperm. In addition, variation was identified in the activity of hydrolytic enzymes that are associated with germination. Notably, activity of the free fraction of β-amylase (BMY), but not the bound fraction, was increased at grain maturity in barley varieties possessing more aleurone. Laser capture microdissection (LCM) and transcriptional profiling confirmed that HvBMY1 is the most abundant BMY gene in developing grain and accumulates in the aleurone during early stages of grain fill. The results reveal a link between molecular pathways influencing early aleurone development and increased levels of free β-amylase enzyme, potentially highlighting the aleurone as a repository of free β-amylase at grain maturity.

Differences in hydrolytic enzyme activity accompany natural variation in mature aleurone morphology in barley (Hordeum vulgare L.)

The aleurone is a critical component of the cereal seed and is located at the periphery of the starchy endosperm. During germination, the aleurone is responsible for releasing hydrolytic enzymes that degrade cell wall polysaccharides and starch granules, which is a key requirement for barley malt production. Inter- and intra-species differences in aleurone layer number have been identified in the cereals but the significance of this variation during seed development and germination remains unclear. In this study, natural variation in mature aleurone features was examined in a panel of 33 Hordeum vulgare (barley) genotypes. Differences were identified in the number of aleurone cell layers, the transverse thickness of the aleurone and the proportion of aleurone relative to starchy endosperm. In addition, variation was identified in the activity of hydrolytic enzymes that are associated with germination. Notably, activity of the free fraction of β-amylase (BMY), but not the bound fraction, was increased at grain maturity in barley varieties possessing more aleurone. Laser capture microdissection (LCM) and transcriptional profiling confirmed that HvBMY1 is the most abundant BMY gene in developing grain and accumulates in the aleurone during early stages of grain fill. The results reveal a link between molecular pathways influencing early aleurone development and increased levels of free β-amylase enzyme, potentially highlighting the aleurone as a repository of free β-amylase at grain maturity.

Tracking polysaccharides through the brewing process

Brewing is a highly complex stepwise process that starts with a mashing step during which starch is gelatinized and converted into oligo- and/or monosaccharides by enzymes and heat. The starch is mostly degraded and utilised during the fermentation process, but grains and hops both contain additional soluble and insoluble complex polysaccharides within their cell walls that persist and can have beneficial or detrimental effects on the brewing process. Previous studies have mostly been restricted to analysing the grain and/or malt prior to entering the brewing process, but here we track the fates of polysaccharides during the entire brewing process. To do this, we utilised a novel approach based on carbohydrate microarray technology. We demonstrate the successful application of this technology to brewing science and show how it can be utilised to obtain an unprecedented level of knowledge about the underlying molecular mechanisms at work.