Xylanase inhibitors bind to nonstarch polysaccharides

Influence of Particle Size and Xylanase Pretreatment of Proso Millet Bran on Physical, Sensory and Nutritive Features of Gluten-Free Bread

Research background

Millet bran is a by-product rich in dietary fibre, micronutrients and bioactive compounds which are often deficient in a gluten-free diet. Previously, cryogenic grinding has been shown to improve the functionality of bran to some extent, although it offered limited benefits for bread making. This study aims to investigate the effects of adding proso millet bran depending on its particle size and xylanase pretreatment on the physicochemical, sensory and nutritional properties of gluten-free pan bread.

Experimental approach

Coarse bran (d₅₀=223 μm) was ground to medium size (d₅₀=157 μm) using an ultracentrifugal mill or to superfine particles (d₅₀=8 μm) using a cryomill. Millet bran presoaked in water (for 16 h at 55 °C) with or without the addition of fungal xylanase (10 U/g) replaced 10% of the rice flour in the control bread. Bread specific volume, crumb texture, colour and viscosity were measured instrumentally. Along with proximate composition, the content of soluble and insoluble fibre, total phenolic compounds (TPC) and phenolic acids as well as total and bioaccessible minerals of bread were assessed. Sensory analysis of the bread samples included a descriptive, hedonic and ranking test.

Results and conclusions

Dietary fibre content (7.3-8.6 g/100 g) and TPC (42-57 mg/100 g) on dry mass basis of the bread loaves depended on bran particle size and xylanase pretreatment. The effect of xylanase pretreatment was most evident in the loaves with medium bran size in terms of higher content of fibre soluble in ethanol (45%) and free ferulic acid content (5%), improved bread volume (6%), crumb softness (16%) and elasticity (7%), but lower chewiness (15%) and viscosity (20-32%). Bread bitterness and dark colour were increased after adding medium-sized bran but its bitter aftertaste, crust crookedness, crumb hardness and graininess were reduced with xylanase pretreatment. Although bran addition impaired protein digestibility, it enriched the bread with iron (341%), magnesium (74%), copper (56%) and zinc (7.5%). Xylanase pretreatment of the bran resulted in the improved bioaccessibility of zinc and copper of the enriched bread compared to the control and bread without xylanase.

Novelty and scientific contribution

Application of xylanase to medium sized bran obtained by ultracentrifugal grinding was more successful than its application to superfine bran obtained by the multistage cryogrinding as it resulted in more soluble fibre in gluten-free bread. Moreover, xylanase was proven beneficial in maintaining desirable bread sensory properties and mineral bioaccessibility.

Genome-wide identification and characterization of thaumatin-like protein family genes in wheat and analysis of their responses to Fusarium head blight infection

Thaumatin-like proteins (TLPs) play potential roles in plant resistance to various diseases. Identifying TLPs is necessary to determine their function and apply them to plant disease resistance. However, limited information is available about TLP-family genes in wheat, especially regarding their responses to Fusarium species, which cause Fusarium head blight in wheat. In this study, we conducted a comprehensive genome-wide survey of TLP genes in wheat and identified 129 TLP genes in the wheat genome, which were unevenly distributed on 21 wheat chromosomes, with 5A containing the highest number. Phylogenetic analysis showed that these 129 wheat TLP genes together with 24 Arabidopsis TLPs were classified into 7 groups based on the protein sequences. We systematically analyzed the genes in terms of their sequence characterization, chromosomal locations, exon–intron distribution, duplication (tandem and segmental) events and expression profiles in response to Fusarium infection. Furthermore, we analyzed differentially expressed TLP genes based on publicly available RNA-seq data obtained from a resistant near isogenic wheat line at different time points after Fusarium graminearum inoculation. Then, the expression of 9 differentially expressed TLP genes was confirmed by real-time PCR, and these 9 genes were all upregulated in the resistant Sumai 3 variety, which was generally consistent with the RNA-seq data. Our results provide a basis for selecting candidate wheat TLP genes for further studies to determine the biological functions of the TLP genes in wheat.

Graphical Abstract

EMS Derived Wheat Mutant BIG8-1 (Triticum aestivum L.)-A New Drought Tolerant Mutant Wheat Line

Drought response in wheat is considered a highly complex process, since it is a multigenic trait; nevertheless, breeding programs are continuously searching for new wheat varieties with characteristics for drought tolerance. In a previous study, we demonstrated the effectiveness of a mutant known as RYNO3936 that could survive 14 days without water. In this study, we reveal another mutant known as BIG8-1 that can endure severe water deficit stress (21 days without water) with superior drought response characteristics. Phenotypically, the mutant plants had broader leaves, including a densely packed fibrous root architecture that was not visible in the WT parent plants. During mild (day 7) drought stress, the mutant could maintain its relative water content, chlorophyll content, maximum quantum yield of PSII (Fv/Fm) and stomatal conductance, with no phenotypic symptoms such as wilting or senescence despite a decrease in soil moisture content. It was only during moderate (day 14) and severe (day 21) water deficit stress that a decline in those variables was evident. Furthermore, the mutant plants also displayed a unique preservation of metabolic activity, which was confirmed by assessing the accumulation of free amino acids and increase of antioxidative enzymes (peroxidases and glutathione S-transferase). Proteome reshuffling was also observed, allowing slow degradation of essential proteins such as RuBisCO during water deficit stress. The LC-MS/MS data revealed a high abundance of proteins involved in energy and photosynthesis under well-watered conditions, particularly Serpin-Z2A and Z2B, SGT1 and Calnexin-like protein. However, after 21 days of water stress, the mutants expressed ABC transporter permeases and xylanase inhibitor protein, which are involved in the transport of amino acids and protecting cells, respectively. This study characterizes a new mutant BIG8-1 with drought-tolerant characteristics suited for breeding programs.

Sensitivity of the Bacillus subtilis Xyn A Xylanase and Its Mutants to Different Xylanase Inhibitors Determines Their Activity Profile and Functionality during Bread Making

The importance of inhibition sensitivity for xylanase functionality in bread making was investigated using mutants of the wild-type Bacillus subtilis xylanase (XBS_TAXI), sensitive to Triticum aestivum xylanase inhibitor (TAXI). XBS_NI, a mutant with reduced sensitivity to TAXI, and XBS_TI, a mutant sensitive to all wheat endogenous proteinaceous inhibitors (TAXI, Xylanase Inhibiting Protein and Thaumatin-like Xylanase Inhibitor) were used. The higher inhibition sensitivity of XBS_TAXI and XBS_TI compared to XBS_NI was associated with a respective 7- and 53-fold increase in enzyme dosage required for a maximal increase in bread loaf volume. XBS_TI and XBS_TAXI were only active during the mixing phase and the beginning of fermentation, while XBS_NI was able to hydrolyze arabinoxylan until the end of fermentation. In spite of this difference in activity profile, no differences in loaf volume were observed for the different xylanases at optimal concentrations. Dough extensional viscosity analysis suggests that increased water availability as a result of xylanase activity favors starch-starch and starch-gluten interactions and drives the improvement in bread loaf volume.

Superior Growth Rates in Broilers Fed Wheat with Low In Vitro Feed-Xylanase Inhibition

Grain-batch variation in xylanase-inhibitor levels may account for variations in the efficacy of feed xylanase supplementation. This would make inhibition an important quality parameter in the routine analysis of feedstuffs. Two analytical procedures for testing feedstuffs against specific xylanases were researched: the high-throughput viscosity-pressure assay (ViPr) and the extraction-free remazol-brilliant-blue-beechwood-xylan (RBBX) assay. Thirty-two wheat cultivars were analyzed for inhibition of a commercial xylanase, Ronozyme WX. Four cultivars were selected for a feeding experiment in which the growth of 1440 broilers from ages 7-33 days was monitored. The treatments resulted up to 7 % difference (day 14) in broiler weight . The cultivar choice had an effect throughout the experiment ( p < 0.05). The performance ranking of the treatments corresponded better to xylanase inhibition than to crude-protein content or nonstarch-polysaccharide content. Wheat-grain xylanase-inhibitor content is therefore a highly relevant quality parameter when broiler diets are supplemented with feed xylanase.

Soluble Dietary Fiber Fractions in Wheat Bran and Their Interactions with Wheat Gluten Have Impacts on Dough Properties

Six soluble dietary fiber (SDF) fractions were prepared via stepwise ethanol precipitation from natural and fermented wheat bran. The chemical composition, molecular weight distribution, and glycosidic linkage and substitution pattern of each SDF fraction were elucidated by sugar analysis, periodate oxidation and Smith degradation, molecular determination, and ¹H nuclear magnetic resonance (NMR) analysis. The impacts of SDF fractions on the rheological properties and morphologies of doughs were investigated by farinography, rheometry, and scanning electron microscopy (SEM) to clarify the relationship between the microstructural features of SDF fractions and the macroscopic properties of SDF-containing doughs. The interactions between SDF fractions and wheat glutens in doughs were further studied by confocal laser scanning microscopy (CLSM). The experimental results indicated that the SDF fraction with an intermediate molecular weight but a higher substitution degree and a larger disubstitution ratio was most compatible with the dough network and beneficial to dough quality.

Pyramiding PvPGIP2 and TAXI-III But Not PvPGIP2 and PMEI Enhances Resistance Against Fusarium graminearum

Plant protein inhibitors counteract the activity of cell wall-degrading enzymes (CWDEs) secreted by pathogens to breach the plant cell-wall barrier. Transgenic plants expressing a single protein inhibitor restrict pathogen infections. However, since pathogens secrete a number of CWDEs at the onset of infection, we combined more inhibitors in a single wheat genotype to reinforce further the cell-wall barrier. We combined polygalacturonase (PG) inhibiting protein (PGIP) and pectin methyl esterase inhibitor (PMEI), both controlling the activity of PG, one of the first CWDEs secreted during infection. We also pyramided PGIP and TAXI-III, a xylanase inhibitor that controls the activity of xylanases, key factors for the degradation of xylan, a main component of cereal cell wall. We demonstrated that the pyramiding of PGIP and PMEI did not contribute to any further improvement of disease resistance. However, the presence of both pectinase inhibitors ensured a broader spectrum of disease resistance. Conversely, the PGIP and TAXI-III combination contributed to further improvement of Fusarium head blight (FHB) resistance, probably because these inhibitors target the activity of different types of CWDEs, i.e., PGs and xylanases. Worth mentioning, the reduction of FHB symptoms is accompanied by a reduction of deoxynivalenol accumulation with a foreseen great benefit to human and animal health.

Modification of the Secondary Binding Site of Xylanases Illustrates the Impact of Substrate Selectivity on Bread Making

To investigate the importance of substrate selectivity for xylanase functionality in bread making, the secondary binding site (SBS) of xylanases from Bacillus subtilis (XBS) and Pseudoalteromonas haloplanktis was modified. This resulted in two xylanases with increased relative activity toward water-unextractable wheat arabinoxylan (WU-AX) compared to water-extractable wheat arabinoxylan, i.e., an increased substrate selectivity, without changing other biochemical properties. Addition of both modified xylanases in bread making resulted in increased loaf volumes compared to the wild types when using weak flour. Moreover, maximal volume increase was reached at a lower dosage of the mutant compared to wild-type XBS. The modified xylanases were able to solubilize more WU-AX and decreased the average degree of polymerization of soluble arabinoxylan in dough more during fermentation. This possibly allowed for additional water release, which might be responsible for increased loaf volumes. Altered SBS functionality and, as a result, enhanced substrate selectivity most probably caused these differences.

Genomic analyses and expression evaluation of thaumatin-like gene family in the cacao fungal pathogen Moniliophthora perniciosa

Thaumatin-like proteins (TLPs) are found in diverse eukaryotes. Plant TLPs, known as Pathogenicity Related Protein (PR-5), are considered fungal inhibitors. However, genes encoding TLPs are frequently found in fungal genomes. In this work, we have identified that Moniliophthora perniciosa, a basidiomycete pathogen that causes the Witches' Broom Disease (WBD) of cacao, presents thirteen putative TLPs from which four are expressed during WBD progression. One of them is similar to small TLPs, which are present in phytopathogenic basidiomycete, such as wheat stem rust fungus Puccinia graminis. Fungi genomes annotation and phylogenetic data revealed a larger number of TLPs in basidiomycetes when comparing with ascomycetes, suggesting that these proteins could be involved in specific traits of mushroom-forming species. Based on the present data, we discuss the contribution of TLPs in the combat against fungal competitors and hypothesize a role of these proteins in M. perniciosa pathogenicity.

Genome-wide analysis of eukaryote thaumatin-like proteins (TLPs) with an emphasis on poplar

BACKGROUND

Plant inducible immunity includes the accumulation of a set of defense proteins during infection called pathogenesis-related (PR) proteins, which are grouped into families termed PR-1 to PR-17. The PR-5 family is composed of thaumatin-like proteins (TLPs), which are responsive to biotic and abiotic stress and are widely studied in plants. TLPs were also recently discovered in fungi and animals. In the poplar genome, TLPs are over-represented compared with annual species and their transcripts strongly accumulate during stress conditions.

RESULTS

Our analysis of the poplar TLP family suggests that the expansion of this gene family was followed by diversification, as differences in expression patterns and predicted properties correlate with phylogeny. In particular, we identified a clade of poplar TLPs that cluster to a single 350 kb locus of chromosome I and that are up-regulated by poplar leaf rust infection. A wider phylogenetic analysis of eukaryote TLPs - including plant, animal and fungi sequences - shows that TLP gene content and diversity increased markedly during land plant evolution. Mapping the reported functions of characterized TLPs to the eukaryote phylogenetic tree showed that antifungal or glycan-lytic properties are widespread across eukaryote phylogeny, suggesting that these properties are shared by most TLPs and are likely associated with the presence of a conserved acidic cleft in their 3D structure. Also, we established an exhaustive catalog of TLPs with atypical architectures such as small-TLPs, TLP-kinases and small-TLP-kinases, which have potentially developed alternative functions (such as putative receptor kinases for pathogen sensing and signaling).

CONCLUSION

Our study, based on the most recent plant genome sequences, provides evidence for TLP gene family diversification during land plant evolution. We have shown that the diverse functions described for TLPs are not restricted to specific clades but seem to be universal among eukaryotes, with some exceptions likely attributable to atypical protein structures. In the perennial plant model Populus, we unravelled the TLPs likely involved in leaf rust resistance, which will provide the foundation for further functional investigations.

Immunoblot quantification of three classes of proteinaceous xylanase inhibitors in different wheat ( Triticum aestivum ) cultivars and milling fractions

In wheat ( Triticum aestivum ) grains, TAXI- (T. aestivum xylanase inhibitor), XIP- (xylanase inhibiting protein), and TLXI-type (thaumatin-like xylanase inhibitor) xylanase inhibitors (XIs) are expressed in considerable levels and under different forms. As these proteins have a significant impact on microbial xylanases frequently used in cereal-based biotechnological processes, knowledge of their quantitative and qualitative variability in wheat is of great interest. This paper reports the successful use of immunoquantification by Western blotting to determine the intercultivar variation in the three structurally different classes of XIs, as well as their distribution among various industrial milling fractions. TAXI and XIP protein levels in eight wheat cultivars ranged from 81 to 190 ppm and from 156 to 371 ppm, with average values of 133 and 235 ppm, respectively. Using immunoblotting, TLXI protein levels could be measured directly for the first time. They ranged from 51 to 150 ppm and amounted to 112 ppm on average. The three classes of XIs were distributed among different wheat milling fractions in a similar way, with 4 and 10 times higher concentrations in the aleurone-enriched fraction than in white flour and pericarp fractions, respectively. Immunoblot patterns suggested that the observed intercultivar and spatial variabilities within the wheat grain are not due to the presence or absence of specific members of the large polymorphic XI families but to differences in the overall level and/or proportions of the specific members.

Enzyme-inhibitor interactions at the plant-pathogen interface

The plant apoplast during plant-pathogen interactions is an ancient battleground that holds an intriguing range of attacking enzymes and counteracting inhibitors. Examples are pathogen xylanases and polygalacturonases that are inhibited by plant proteins like TAXI, XIP, and PGIP; and plant glucanases and proteases, which are targeted by pathogen proteins such as GIP1, EPI1, EPIC2B, and AVR2. These seven well-characterized inhibitors have different modes of action and many probably evolved from inactive enzymes themselves. Detailed studies of the structures, sequence variation, and mutated proteins uncovered molecular struggles between these enzymes and their inhibitors, resulting in positive selection for variant residues at the contact surface, where single residues determine the outcome of the interaction.