Synthesis, processing and export of cytoplasmic endo-beta-1,4-xylanase from barley aleurone during germination

Function Analysis of a Maize Endo-1,4-β-xylanase Gene ZmHSL in Response to High-Temperature Stress

Rising temperature is a major threat to the normal growth and development of maize, resulting in low yield production and quality. The mechanism of maize in response to heat stress remains uncertain. In this study, a maize mutant Zmhsl-1 (heat sensitive leaves) with wilting and curling leaves under high temperatures was identified from maize Zheng 58 (Z58) mutant lines generated by ethyl methanesulfonate (EMS) mutagenesis. The Zmhsl-1 plants were more sensitive to increased temperature than Z58 in the field during growth season. The Zmhsl-1 plants had lower plant height, lower yield, and lower content of photosynthetic pigments. A bulked segregant analysis coupled with whole-genome sequencing (BSA-seq) enabled the identification of the corresponding gene, named ZmHSL, which encodes an endo-β-1,4-xylanase from the GH10 family. The loss-of-function of ZmHSL resulted in reduced lignin content in Zmhsl-1 plants, leading to defects in water transport and more severe leaf wilting with the increase in temperature. RNA-seq analysis revealed that the differentially expressed genes identified between Z58 and Zmhsl-1 plants are mainly related to heat stress-responsive genes and unfolded protein response genes. All these data indicated that ZmHSL plays a key role in lignin synthesis, and its defective mutation causes changes in the cell wall structure and gene expression patterns, which impedes water transport and confers higher sensitivity to high-temperature stress.

Post-sprouting thermal treatment of green barley malt to produce functional clean-label ingredients: Impact on fermentation, bread-making properties and bread quality

Malt flour represents a potential clean label bread improver, but a high enzymatic activity can lead to some bread defects. Thus, this study was focused on applying different thermal treatments (10 and 40 min; 70-90 °C) to green barley malt in order to promote a partial enzyme inactivation. The addition of 1.5 g of thermally treated malt (TTM) per 100 g of flour in wheat bread formulation was evaluated regarding the resulting bread-making properties, dough fermentation and overall bread quality. Activity of starch-degrading enzymes was not detectable above 80 °C/10 min. TTM incorporation improved the gas production by up to 60% during fermentation, mainly in formulations to which malts thermally treated under mild conditions have been added. Compared to untreated malt, thermal treatment reduced dough thermal weakening, improved gel strength during gelatinization and maintained low setback values. Bread collapse observed by baking follow-up was related to gas inflation and low mechanical resistance. Formulations with the addition of malts thermally treated at 70 °C for 40 min resulted in breads with higher specific volume, improved coloration and a crumb with slightly smaller pores than control and untreated malts. Thus, thermal treatment can be used as a technique to produce standardized malted flour to be used as clean label bread improvers.

Barley metallothionein isoforms, MT2b2 and MT4, differentially respond to photohormones in barley aleurone layer and their recombinant forms show different affinity for binding to zinc and cadmium

Metallothioneins (MTs) are metal-binding proteins that have important roles in the homeostasis of heavy metals. In this study, the two MT genes was studied in response to phytohormones using the barley aleurone layer as a kind of model system. The aleurone layer was isolated from barley embryo-less half grains and was incubated for 24 h with different phytohormones. Based on the results the genes encoding HvMT2b2 and HvMT4 were down-regulated through gibberellic acid (GA), while they were and up-regulated through salicylic acid (SA). Despite this, these two genes were differentially expressed to other hormones. Furthermore, the proteins HvMT2b2 and HvMT4 were heterologous expressed as GST-fusion proteins in E. coli. The HvMT4 and HvMT2b2 heterologous expression in E. coli gives rise to 10- and 3-fold improvements in the accumulation capacity for Zn2+, respectively. Whereas the transgenic E. coli strain that expresses HvMT2b2 could accumulate Cd2+ three-fold higher than control. The expression of HvMT4 did not affect the accumulation of Cd2+. HvMT4 which is known as seed-specific isoform seems to be able to bind to Zn2+ with good affinity and cannot bind Cd2+. In comparison, HvMT2b2 was able to bind both Zn2+ and Cd2+. Therefore HvMT4 could serve a noteworthy role in zinc storage in barley seeds. The expression of HvMT4 is induced by SA 30-fold, concerning the untreated aleurone layer. Such results could provide good insights for the assessment of the effects of phytohormones in the molecular mechanism involved in essential metal storage in cereal seeds.

Characterization of Two Wheat-Derived Glycoside Hydrolase Family-10 Xylanases Resistant to Xylanase Inhibitors

Xylanase inhibitors inhibit the activities of microbial xylanases and seriously compromise the efficacy of microbial xylanases added to modify cereals. Cereal endogenous xylanases are unaffected by these xylanase inhibitors, but little information is available regarding their effects in improving cereal quality, a neglected potential application. As a strategy for circumventing the negative effects of xylanase inhibitors, the objective of this study was to use genetic engineering to obtain sufficient amounts of active endo-1,4-β-D-xylanase from wheat to analyze the characteristics of its structure. The endo-1,4-β-D-xylanase from wheat was heterologously expressed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), western blotting, MALDI-TOF/TOF (MS) analyses, and enzyme activity determination confirmed 2 active endo-1,4-β-D-xylanases (EXY3 and EXY4) were successfully obtained. The molecular weights (MW) and isoelectric point (pI) of EXY3 were 36.108 kDa and 5.491, while those of the EXY4 protein were 41.933 kDa and 5.726. They both contained the same catalytic domain of GH10 xylanases from G266 to V276 and have the same catalytic site, Glu273. They shared the same putative N-glycosylation sites (N62-T63-S64 and N280–V281–S282) and 3 putative O-glycosylation sites (Ser8, Ser9, and Thr21), but EXY4 had an additional O-glycosylation site (Thr358). EXY3 was smaller than EXY4 by 51 amino acids because of a nonsense mutation and premature termination. They both had the 8-fold beta/alpha-barrel (TIM-barrel) fold. The specific activities of EXY3 and EXY4 were 152.0891 and 67.2928 U/mg, respectively. This work demonstrates a promising way to obtain wheat xylanases by genetic engineering; the properties of the enzymes indicate their potential application in cereal-based industries.

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.

Maize WI5 encodes an endo-1,4-β-xylanase required for secondary cell wall synthesis and water transport in xylem

Water transport from roots to leaves through xylem is important for plant growth and development. Defects in water transport can cause drought stress, even when there is adequate water in the soil. Here, we identified the maize (Zea mays) wilty5 (wi5) mutant, which exhibits marked dwarfing and leaf wilting throughout most of its life cycle under normal growth conditions. wilty5 seedlings exhibited lower xylem conductivity and wilted more rapidly under drought, NaCl, and high temperature treatments than wild-type plants. Map-based cloning revealed that WI5 encodes an active endo-1,4-β-xylanase from glycosyl dehydration family 10, which mainly functions in degrading and reorganizing cell wall xylan. Reverse-transcription polymerase chain reaction and β-glucuronidase assays revealed that WI5 is highly expressed in stems, especially in internodes undergoing secondary wall assembly. RNA sequencing suggested that WI5 plays a unique role in internode growth. Immunohistochemistry and electron microscopy confirmed that wi5 is defective in xylan deposition and secondary cell wall thickening. Lignin deposition and xylan content were markedly reduced in wi5 compared to the wild-type plants. Our results suggest that WI5 functions in xylem cell wall thickening through its xylanase activity and thereby regulates xylem water transport, the drought stress response, and plant growth in maize.

Genetic variation of HvXYN1 associated with endoxylanase activity and TAX content in barley (Hordeum vulgare L.)

BACKGROUND

Endo-β-1,4-xylanase1 (EA), the key endoxylanase in plants, is involved in the degradation of arabinoxylan during grain germination. In barley (Hordeum vulgare L.), one gene (HvXYN-1) that encode a endo-beta-1,4-xylanase, has been cloned. However, the single nucleotide polymorphisms (SNPs) that affect the endoxylanase activity and total arabinoxylan (TAX) content have yet to be characterized. The investigation of genetic variation in HvXYN1 may facilitate a better understanding of the relationship between TAX content and EA activity in barley.

RESULTS

In the current study, 56 polymorphisms were detected in HvXYN1 among 210 barley accessions collected from 34 countries, with 10 distinct haplotypes identified. The SNPs at positions 110, 305, 1045, 1417, 1504, 1597, 1880 bp in the genomic region of HvXYN1 were significantly associated with EA activity (P < 0.0001), and the sites 110, 305, and 1045 were highly significantly associated with TAX content. The amount of phenotypic variation in a given trait explained by each associated polymorphism ranged from 6.96 to 9.85%. Most notably, we found two variants at positions 1504 bp and 1880 bp in the second exon that significantly (P < 0.0001) affected EA activity; this result could be used in breeding programs to improve beer quality. In addition, African accessions had the highest EA activity and TAX content, and the richest germplasm resources were from Asia, indicating the high potential value of Asian barley.

CONCLUSION

This study provided insight into understanding the relationship, EA activity, TAX content with the SNPs of HvXYN1 in barley. These SNPs can be applied as DNA markers in breeding programs to improve the quality of barley for beer brewing after further validation.

Impact of Preharvest Sprouting on Endogenous Hydrolases and Technological Quality of Wheat and Bread: A Review

The cereal-based food industry faces the challenge to produce food of high and uniform quality to meet consumer demands. However, adverse weather conditions, including prolonged and repeated rainfall, before harvest time evoke germination of the kernels in the ear of the parent plant, which is known as preharvest sprouting (PHS). PHS results in the production of several hydrolytic enzymes in the kernel, which decreases the technological quality of wheat and causes problems during processing of the flour into cereal-based products. Therefore, wheat that is severely sprouted in the field is less suitable for products for human consumption, and is often discounted to animal feed. Up till now, most knowledge on PHS is obtained by research on laboratory-sprouted wheat as a proxy for field-sprouted wheat. Knowledge on PHS in the field itself is more scarce. This review gives a comprehensive overview of the recent findings on PHS of wheat in the field, compared to knowledge on controlled sprouting. The physiological and functional changes occurring in wheat during PHS and their impact on wheat and bread quality are discussed. This review provides a useful background for further research concerning the potential of field-sprouted wheat to be used as raw material in the food industry.

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.

Wheat dough syruping in cold storage is related to structural changes of starch and non-starch polysaccharides

Even though the refrigerated dough industry is growing quickly due to the convenience and freshness of refrigerated dough over a prolonged storage period, dough syruping, which is a brownish liquid that leaches out from dough during the storage, is a quality-diminishing factor that needs to be resolved. The objectives of this study were to understand dough syruping and how it is related to structural changes in water-soluble arabinoxylan (WS-AX) and starch in wheat flours during refrigeration as well as to prevent syruping by applying exogenous cell wall polysaccharides. Dough syruping increased to 6.5, 6.9, and 17.2% in weak, strong, and jopoom wheat flours, respectively, after a 35-day storage period. The endoxylanase activity of jopoom wheat flour was substantially greater compared to other commercial flours, but the activity of this flour did not change over the whole cold storage period. The molecular size reduction of WS-AX was inversely related to the degree of dough syruping. The addition of β-glucan, carboxymethylcellulose, and xylan effectively reduced syrup formation in jopoom wheat flour dough.

Cell wall degradation is required for normal starch mobilisation in barley endosperm

Starch degradation in barley endosperm provides carbon for early seedling growth, but the control of this process is poorly understood. We investigated whether endosperm cell wall degradation is an important determinant of the rate of starch degradation. We identified iminosugar inhibitors of enzymes that degrade the cell wall component arabinoxylan. The iminosugar 1,4-dideoxy-1, 4-imino-l-arabinitol (LAB) inhibits arabinoxylan arabinofuranohydrolase (AXAH) but does not inhibit the main starch-degrading enzymes α- and β-amylase and limit dextrinase. AXAH activity in the endosperm appears soon after the onset of germination and resides in dimers putatively containing two isoforms, AXAH1 and AXAH2. Upon grain imbibition, mobilisation of arabinoxylan and starch spreads across the endosperm from the aleurone towards the crease. The front of arabinoxylan degradation precedes that of starch degradation. Incubation of grains with LAB decreases the rate of loss of both arabinoxylan and starch, and retards the spread of both degradation processes across the endosperm. We propose that starch degradation in the endosperm is dependent on cell wall degradation, which permeabilises the walls and thus permits rapid diffusion of amylolytic enzymes. AXAH may be of particular importance in this respect. These results provide new insights into the mobilization of endosperm reserves to support early seedling growth.

Suppression of xylan endotransglycosylase PtxtXyn10A affects cellulose microfibril angle in secondary wall in aspen wood

Certain xylanases from family GH10 are highly expressed during secondary wall deposition, but their function is unknown. We carried out functional analyses of the secondary-wall specific PtxtXyn10A in hybrid aspen (Populus tremula × tremuloides). PtxtXyn10A function was analysed by expression studies, overexpression in Arabidopsis protoplasts and by downregulation in aspen. PtxtXyn10A overexpression in Arabidopsis protoplasts resulted in increased xylan endotransglycosylation rather than hydrolysis. In aspen, the enzyme was found to be proteolytically processed to a 68 kDa peptide and residing in cell walls. Its downregulation resulted in a corresponding decrease in xylan endotransglycosylase activity and no change in xylanase activity. This did not alter xylan molecular weight or its branching pattern but affected the cellulose-microfibril angle in wood fibres, increased primary growth (stem elongation, leaf formation and enlargement) and reduced the tendency to form tension wood. Transcriptomes of transgenic plants showed downregulation of tension wood related genes and changes in stress-responsive genes. The data indicate that PtxtXyn10A acts as a xylan endotransglycosylase and its main function is to release tensional stresses arising during secondary wall deposition. Furthermore, they suggest that regulation of stresses in secondary walls plays a vital role in plant development.

Spatio-temporal appearance of α-amylase and limit dextrinase in barley aleurone layer in response to gibberellic acid, abscisic acid and salicylic acid

BACKGROUND

Cereal seed germination involves mobilization of storage reserves in the starchy endosperm to support seedling growth. In response to gibberellin produced by the embryo the aleurone layer synthesizes hydrolases that are secreted to the endosperm for degradation of storage products. In this study analysis of intracellular protein accumulation and release from barley aleurone layers is presented for the important enzymes in starch degradation: α-amylase and limit dextrinase (LD).

RESULTS

Proteins were visualized by immunoblotting in aleurone layers and culture supernatants from dissected aleurone layers incubated up to 72 h with either gibberellic acid (GA), abscisic acid (ABA) or salicylic acid (SA). The results show that α-amylase is secreted from aleurone layer treated with GA soon after synthesis but the release of LD to culture supernatants was significantly delayed and coincided with a general loss of proteins from aleurone layers.

CONCLUSIONS

Release of LD was found to differ from that of amylase and was suggested to depend on programmed cell death (PCD). Despite detection of intracellular amylase in untreated aleurone layers or aleurone layers treated with ABA or SA, α-amylase was not released from these samples. Nevertheless, the release of α-amylase was observed from aleurone layers treated with GA+ABA or GA+SA.

Transgenic barley: a prospective tool for biotechnology and agriculture

Barley (Hordeum vulgare L.) is one of the founder crops of agriculture, and today it is the fourth most important cereal grain worldwide. Barley is used as malt in brewing and distilling industry, as an additive for animal feed, and as a component of various food and bread for human consumption. Progress in stable genetic transformation of barley ensures a potential for improvement of its agronomic performance or use of barley in various biotechnological and industrial applications. Recently, barley grain has been successfully used in molecular farming as a promising bioreactor adapted for production of human therapeutic proteins or animal vaccines. In addition to development of reliable transformation technologies, an extensive amount of various barley genetic resources and tools such as sequence data, microarrays, genetic maps, and databases has been generated. Current status on barley transformation technologies including gene transfer techniques, targets, and progeny stabilization, recent trials for improvement of agricultural traits and performance of barley, especially in relation to increased biotic and abiotic stress tolerance, and potential use of barley grain as a protein production platform have been reviewed in this study. Overall, barley represents a promising tool for both agricultural and biotechnological transgenic approaches, and is considered an ancient but rediscovered crop as a model industrial platform for molecular farming.

An enzyme activity capable of endotransglycosylation of heteroxylan polysaccharides is present in plant primary cell walls

Heteroxylans in the plant cell wall have been proposed to have a role analogous to that of xyloglucans or heteromannans, forming growth-restraining networks by interlocking cellulose microfibrils. A xylan endotransglycosylase has been identified that can transglycosylate heteroxylan polysaccharides in the presence of xylan-derived oligosaccharides. High activity was detected in ripe fruit of papaya (Carica papaya), but activity was also found in a range of other fruits, imbibed seeds and rapidly growing seedlings of cereals. Xylan endotransglycosylase from ripe papaya fruit used a range of heteroxylans, such as wheat arabinoxylan, birchwood glucuronoxylan and various heteroxylans from dicotyledonous primary cell walls purified from tomato and papaya fruit, as donor molecules. As acceptor molecules, the enzyme preferentially used xylopentaitol over xylohexaitol or shorter-length acceptors. Xylan endotransglycosylase was active over a broad pH range and could perform transglycosylation reactions up to 55 °C. Xylan endotransglycosylase activity was purified from ripe papaya fruit by ultrafiltration and cation exchange chromatography. Highest endotransglycosylase activity was identified in fractions that also contained high xylan hydrolase activity and correlated with the presence of the endoxylanase CpaEXY1. Recombinant CpaEXY1 protein transiently over-expressed in Nicotiana benthamiana leaves showed both endoxylanase and xylan endotransglycosylase activities in vitro, suggesting that CpaEXY1 is a single enzyme with dual activity in planta. Purified native CpaEXY1 showed two- to fourfold higher endoxylanase than endotransglycosylase activity, suggesting that CpaEXY1 may act primarily as a hydrolase. We propose that xylan endotransglycosylase activity (like xyloglucan and mannan endotransglycosylase activities) could be involved in remodelling or re-arrangement of heteroxylans of the cellulose-non-cellulosic cell wall framework.

Molecular population genetics of elicitor-induced resistance genes in European aspen (Populus tremula L., Salicaceae)

Owing to their long life span and ecological dominance in many communities, forest trees are subject to attack from a diverse array of herbivores throughout their range, and have therefore developed a large number of both constitutive and inducible defenses. We used molecular population genetics methods to examine the evolution of eight genes in European aspen, Populus tremula, that are all associated with defensive responses against pests and/or pathogens, and have earlier been shown to become strongly up-regulated in poplars as a response to wounding and insect herbivory. Our results show that the majority of these defense genes show patterns of intraspecific polymorphism and site-frequency spectra that are consistent with a neutral model of evolution. However, two of the genes, both belonging to a small gene family of polyphenol oxidases, show multiple deviations from the neutral model. The gene PPO1 has a 600 bp region with a highly elevated K(A)/K(S) ratio and reduced synonymous diversity. PPO1 also shows a skew toward intermediate frequency variants in the SFS, and a pronounced fixation of non-synonymous mutations, all pointing to the fact that PPO1 has been subjected to recurrent selective sweeps. The gene PPO2 shows a marked excess of high frequency, derived variants and shows many of the same trends as PPO1 does, even though the pattern is less pronounced, suggesting that PPO2 might have been the target of a recent selective sweep. Our results supports data from both Populus and other species which have found that the the majority of defense-associated genes show few signs of selection but that a number of genes involved in mediating defense against herbivores show signs of adaptive evolution.

Transgene expression systems in the Triticeae cereals

The control of transgene expression is vital both for the elucidation of gene function and for the engineering of transgenic crops. Given the dominance of the Triticeae cereals in the agricultural economy of the temperate world, the development of well-performing transgene expression systems of known functionality is of primary importance. Transgenes can be expressed either transiently or stably. Transient expression systems based on direct or virus-mediated gene transfer are particularly useful in situations where the need is to rapidly screen large numbers of genes. However, an unequivocal understanding of gene function generally requires that a transgene functions throughout the plant's life and is transmitted through the sexual cycle, since this alone allows its effect to be decoupled from the plant's response to the generally stressful gene transfer event. Temporal, spatial and quantitative control of a transgene's expression depends on its regulatory environment, which includes both its promoter and certain associated untranslated region sequences. While many transgenic approaches aim to manipulate plant phenotype via ectopic gene expression, a transgene sequence can be also configured to down-regulate the expression of its endogenous counterpart, a strategy which exploits the natural gene silencing machinery of plants. In this review, current technical opportunities for controlling transgene expression in the Triticeae species are described. Apart from protocols for transient and stable gene transfer, the choice of promoters and other untranslated regulatory elements, we also consider signal peptides, as they too govern the abundance and particularly the sub-cellular localization of transgene products.

Post-translational processing of beta-d-xylanases and changes in extractability of arabinoxylans during wheat germination

Endo-1,4-beta-d-xylanase (EC 3.2.1.8, beta-d-xylanase) activity, and arabinoxylan (AX) level and extractability were monitored for the first time simultaneously in wheat kernels (Triticum aestivum cv. Glasgow) up to 24 days post-imbibition (DPI), both in the absence and presence of added gibberellic acid (GA). Roughly three different stages (early, intermediate and late) can be discriminated. Addition of GA resulted in a faster increase of water extractable arabinoxylan (WEAX) level in the early stage (up to 3-4 DPI). This increase was not accompanied by the discernible presence of homologues of the barley X-I beta-d-xylanase as established by immunodetection. This suggests that other, yet unidentified beta-d-xylanases operate in this early time window. The intermediate stage (up to 13 DPI) was characterized by the presence of unprocessed 67 kDa X-I like beta-d-xylanase, which was much more abundant in the presence of GA. The occurrence of higher levels of the unprocessed enzyme was related with higher beta-d-xylanase activities and a further increase in WEAX level, pointing to in vivo activity of the unprocessed 67 kDa beta-d-xylanase. During the late stage (up to 24 DPI) gradual processing of the 67 kDa beta-d-xylanase occurred and was associated with a drastic increase in beta-d-xylanase activity. Up to 120-fold higher activity was recorded at 24 DPI, with approx. 85% thereof originating from the kernel remnants. The WEAX level decreased during the late stage, suggesting that the beta-d-xylanase is processed into more active forms to achieve extensive AX breakdown.

Plant cell wall proteomics: mass spectrometry data, a trove for research on protein structure/function relationships

Proteomics allows the large-scale study of protein expression either in whole organisms or in purified organelles. In particular, mass spectrometry (MS) analysis of gel-separated proteins produces data not only for protein identification, but for protein structure, location, and processing as well. An in-depth analysis was performed on MS data from etiolated hypocotyl cell wall proteomics of Arabidopsis thaliana. These analyses show that highly homologous members of multigene families can be differentiated. Two lectins presenting 93% amino acid identity were identified using peptide mass fingerprinting. Although the identification of structural proteins such as extensins or hydroxyproline/proline-rich proteins (H/PRPs) is arduous, different types of MS spectra were exploited to identify and characterize an H/PRP. Maturation events in a couple of cell wall proteins (CWPs) were analyzed using site mapping. N-glycosylation of CWPs as well as the hydroxylation or oxidation of amino acids were also explored, adding information to improve our understanding of CWP structure/function relationships. A bioinformatic tool was developed to locate by means of MS the N-terminus of mature secreted proteins and N-glycosylation.

Cloning and expression analysis of a wood-associated xylosidase gene (PtaBXL1) in poplar tension wood

In stems of woody angiosperms responding to mechanical stress, imposed for instance by tilting the stem or formation of a branch, tension wood (TW) forms above the affected part, while anatomically distinct opposite wood (OW) forms below it. In poplar TW the S3 layer of the secondary walls is substituted by a "gelatinous layer" that is almost entirely composed of cellulose and has much lower hemicellulose contents than unstressed wood. However, changes in xylan contents (the predominant hemicelluloses), their interactions with other wall components and the mechanisms involved in TW formation have been little studied. Therefore, in the study reported here we determined the structure and distribution of xylans, cloned the genes encoding the xylan remodeling enzymes beta-xylosidases (PtaBXLi), and examined their expression patterns during tension wood, normal wood and opposite wood xylogenesis in poplar. We confirm that poplar wood xylans are substituted solely by 4-O-methylglucuronic acid in both TW and OW. However, although glucuronoxylans are strongly represented in both primary and secondary layers of OW, no 4-O-methylGlcA xylan was found in G-layers of TW. Four full-length BXL cDNAs encoding putative beta-xylosidases were cloned. One, PtaBXL1, for which xylosidase activity was confirmed by heterologous expression in Escherichia coli, exhibited a wood-specific expression pattern in TW. In conclusion, xylan as PtaBXL1, encoding beta4-xylosidase activity, are down-regulated in TW.

RNAi-mediated knockdown of the XIP-type endoxylanase inhibitor gene, OsXIP, has no effect on grain development and germination in rice

OsXIP (Oryza sativa xylanase inhibitor protein) is a XIP-type xylanase inhibitor which was identified as a protein encoded by a wound stress-responsive gene in rice. Although the OsXIP gene was specifically expressed in mature grains under basal conditions, recombinant OsXIP had no effect on rice endogenous xylanases, and OsXIP-suppressed transgenic rice plants did not exhibit any change in grain development and germination, suggesting that rice development may be independent of OsXIP. Analysis using an OsXIP-specific antibody revealed that OsXIP is markedly accumulated in apoplast in rice root cells by wounding. These results reinforced the possibility that OsXIP is involved in plant defense mechanisms against phytopathogens.

Papaya fruit softening, endoxylanase gene expression, protein and activity

Papaya (Carica papaya L.) cell wall matrix polysaccharides are modified as the fruit starts to soften during ripening and an endoxylanase is expressed that may play a role in the softening process. Endoxylanase gene expression, protein amount and activity were determined in papaya cultivars that differ in softening pattern and in one cultivar where softening was modified by the ethylene receptor inhibitor 1-methylcyclopropene (1-MCP). Antibodies to the endoxylanase catalytic domain were used to determine protein accumulation. The three papaya varieties used in the study, 'Line 8', 'Sunset', and 'Line 4-16', differed in softening pattern, respiration rate, ethylene production and showed similar parallel relationships during ripening and softening in endoxylanase expression, protein level and activity. When fruit of the three papaya varieties showed the respiratory climacteric and started to soften, the level of endoxylanase gene expression increased and this increase was related to the amount of endoxylanase protein at 32 kDa and its activity. Fruit when treated at less than 10% skin yellow stage with 1-MCP showed a significant delay in the respiratory climacteric and softening, and reduced ethylene production, and when ripe was firmer and had a 'rubbery' texture. The 1-MCP-treated fruit that had the 'rubbery' texture showed suppressed endoxylanase gene expression, protein and enzymatic activity. Little or no delay occurred between endoxylanase gene expression and the appearance of activity during posttranslational processing from 65 to 32 kDa. The close relationship between endoxylanase gene expression, protein accumulation and activity in different varieties and the failure of the 1-MCP-treated fruit to fully soften, supported de novo synthesis of endoxylanase, rapid posttranslation processing and a role in papaya fruit softening.

Mutational analysis of endoxylanases XylA and XylB from the phytopathogen Fusarium graminearum reveals comprehensive insights into their inhibitor insensitivity

Endo-beta-1,4-xylanases (EC 3.2.1.8; endoxylanases), key enzymes in the degradation of xylan, are considered to play an important role in phytopathogenesis, as they occupy a prominent position in the arsenal of hydrolytic enzymes secreted by phytopathogens to breach the cell wall and invade the plant tissue. Plant endoxylanase inhibitors are increasingly being pinpointed as part of a counterattack mechanism. To understand the surprising XIP-type endoxylanase inhibitor insensitivity of endoxylanases XylA and XylB from the phytopathogen Fusarium graminearum, an extensive mutational study of these enzymes was performed. Using combinatorial and site-directed mutagenesis, the XIP insensitivity of XylA as well as XylB was proven to be solely due to amino acid sequence adaptations in the "thumb" structural region. While XylB residues Cys141, Asp148, and Cys149 were shown to prevent XIP interaction, the XIP insensitivity of XylA could be ascribed to the occurrence of only one aberrant residue, i.e., Val151. This study, in addition to providing a thorough explanation for the XIP insensitivity of both F. graminearum endoxylanases at the molecular level, generated XylA and XylB mutants with altered inhibition specificities and pH optima. As this is the first experimental elucidation of the molecular determinants dictating the specificity of the interaction between endoxylanases of phytopathogenic origin and a plant inhibitor, this work sheds more light on the ongoing evolutionary arms race between plants and phytopathogenic fungi involving recognition of endoxylanases.

Unprocessed barley aleurone endo-β-1,4-xylanase X-I is an active enzyme

Endo-beta-1,4-xylanase X-I is a major hydrolase produced by the aleurone tissue of germinating barley grain. It was previously reported that this cytosolic enzyme is synthesized as an inactive precursor which is proteolytically processed to active forms upon its programmed cell death dependent release. We here demonstrate, however, that the precursor form of X-I is an active enzyme. Purified recombinant precursor X-I was characterised with respect to its molecular weight, iso-electric point and temperature and pH activity and stability. Analysis of the hydrolysis products showed that it is an endo-acting enzyme which has the striking ability to release xylose from both polymeric xylan as well as from small xylo-oligosaccharides. The implications of these findings in relation to the putative role of the N-terminal propeptide as a carbohydrate binding module and the possible consequences for the way X-I fulfils its role in the germination process, are discussed.

Spatio-temporal profiling and degradation of α-amylase isozymes during barley seed germination

Ten genes from two multigene families encode barley alpha-amylases. To gain insight into the occurrence and fate of individual isoforms during seed germination, the alpha-amylase repertoire was mapped by using a proteomics approach consisting of 2D gel electrophoresis, western blotting, and mass spectrometry. Mass spectrometric analysis confirmed that the 29 alpha-amylase positive 2D gel spots contained products of one (GenBank accession gi|113765) and two (gi|4699831 and gi|166985) genes encoding alpha-amylase 1 and 2, respectively, but lacked products from seven other genes. Eleven spots were identified only by immunostaining. Mass spectrometry identified 12 full-length forms and 12 fragments from the cultivar Barke. Products of both alpha-amylase 2 entries co-migrated in five full-length and one fragment spot. The alpha-amylase abundance and the number of fragments increased during germination. Assessing the fragment minimum chain length by peptide mass fingerprinting suggested that alpha-amylase 2 (gi|4699831) initially was cleaved just prior to domain B that protrudes from the (betaalpha)(8)-barrel between beta-strand 3 and alpha-helix 3, followed by cleavage on the C-terminal side of domain B and near the C-terminus. Only two shorter fragments were identified of the other alpha-amylase 2 (gi|166985). The 2D gels of dissected tissues showed alpha-amylase degradation to be confined to endosperm. In contrast, the aleurone layer contained essentially only full-length alpha-amylase forms. While only products of the above three genes appeared by germination also of 15 other barley cultivars, the cultivars had distinct repertoires of charge and molecular mass variant forms. These patterns appeared not to be correlated with malt quality.

Carbohydrate-binding module of a rice endo-beta-1,4-glycanase, OsCel9A, expressed in auxin-induced lateral root primordia, is post-translationally truncated

We report the cloning of a glycoside hydrolase family (GHF) 9 gene of rice (Oryza sativa L. cv. Sasanishiki), OsCel9A, corresponding to the auxin-induced 51 kDa endo-1,4-beta-glucanase (EGase). This enzyme reveals a broad substrate specificity with respect to sugar backbones (glucose and xylose) in beta-1,4-glycans of type II cell wall. OsCel9A encodes a 640 amino acid polypeptide and is an ortholog of TomCel8, a tomato EGase containing a carbohydrate-binding module (CBM) 2 sequence at its C-terminus. The expression of four rice EGase genes including OsCel9A showed different patterns of organ specificity and responses to auxin. OsCel9A was preferentially expressed during the initiation of lateral roots or subcultured root calli, but was hardly expressed during auxin-induced coleoptile elongation or in seed calli, in contrast to OsCel9D, a KORRIGAN (KOR) homolog. In situ localization of OsCel9A transcripts demonstrated that its expression was specifically up-regulated in lateral root primordia (LRP). Northern blotting analysis showed the presence of a single product of OsCel9A. In contrast, both mass spectrometric analyses of peptide fragments from purified 51 kDa EGase proteins and immunogel blot analysis of EGase proteins in root extracts using two antibodies against internal peptide sequences of OsCel9A revealed that the entire CBM2 region was post-translationally truncated from the 67 kDa nascent protein to generate 51 kDa EGase isoforms. Analyses of auxin concentration and time course dependence of accumulation of two EGase isoforms suggested that the translation and post-translational CBM2 truncation of the OsCel9A gene may participate in lateral root development.

Plant glycoside hydrolases involved in cell wall polysaccharide degradation

The cell wall plays a key role in controlling the size and shape of the plant cell during plant development and in the interactions of the plant with its environment. The cell wall structure is complex and contains various components such as polysaccharides, lignin and proteins whose composition and concentration change during plant development and growth. Many studies have revealed changes in cell walls which occur during cell division, expansion, and differentiation and in response to environmental stresses; i.e. pathogens or mechanical stress. Although many proteins and enzymes are necessary for the control of cell wall organization, little information is available concerning them. An important advance was made recently concerning cell wall organization as plant enzymes that belong to the superfamily of glycoside hydrolases and transglycosidases were identified and characterized; these enzymes are involved in the degradation of cell wall polysaccharides. Glycoside hydrolases have been characterized using molecular, genetic and biochemical approaches. Many genes encoding these enzymes have been identified and functional analysis of some of them has been performed. This review summarizes our current knowledge about plant glycoside hydrolases that participate in the degradation and reorganisation of cell wall polysaccharides in plants focussing particularly on those from Arabidopsis thaliana.

New TAXI-type Xylanase Inhibitor Genes are Inducible by Pathogens and Wounding in Hexaploid Wheat

TAXI-I (Triticum aestivum xylanase inhibitor I) is a wheat grain protein that inhibits arabinoxylan fragmentation by microbial endo-beta-1,4-xylanases used in the food industry. Although TAXI was speculated to be involved in counterattack against pathogens, there is actually no evidence to support this hypothesis. We have now demonstrated the presence of TAXI family members with isolation of two mRNA species, Taxi-III and Taxi-IV. At the nucleotide sequence level, Taxi-III and Taxi-IV were 91.7% and 92.0% identical, respectively, to Taxi-I, and Taxi-III and Taxi-IV were 96.8% identical. Accumulation of Taxi-III/IV transcripts was most evident in roots and older leaves where transcripts of Taxi-I were negligible. When challenged by fungal pathogens Fusarium graminearum and Erysiphe graminis, the concentrations of Taxi-III/IV transcripts increased significantly. In contrast, the increases in Taxi-I transcripts in response to these pathogens were rather limited. Both Taxi-I and Taxi-III/IV were strongly expressed in wounded leaves. The upstream region of Taxi-III contained W boxes and GCC boxes, which are sufficient to confer pathogen and wound inducibility on promoters. Recombinant TAXI-III protein inhibited Aspergillus niger and Trichoderma sp. xylanases: it was also active against some spelt xylan-induced xylanases of F. graminearum. These features suggest that some, but not all, TAXI-type xylanase inhibitors have a role in plant defense.

Environmental and transgene expression effects on the barley seed proteome

The barley (Hordeum vulgare) cultivar Golden Promise is no longer widely used for malting, but is amenable to transformation and is therefore a valuable experimental cultivar. Its characteristics include high salt tolerance, however it is also susceptible to several fungal pathogens. Proteome analysis was used to describe the water-soluble protein fraction of Golden Promise seeds in comparison with the modern malting cultivar Barke. Using 2D-gel electrophoresis to visualise several hundred proteins in the pH ranges 4-7 and 6-11, 16 protein spots were found to differ between the two cultivars. Eleven of these were identified by mass spectrometric peptide mass mapping, including an abundant chitinase implicated in defence against fungal pathogens and a small heat-shock protein. To enable a comparison with transgenic seed protein patterns, differences in spot patterns between field and greenhouse-grown seeds were analysed. Four spots were observed to be increased in intensity in the proteome of greenhouse-grown seeds, three of which may be related to nitrogen availability during grain filling and total protein content of the seeds, since they also increased in field grown seeds supplied with extra nitrogen. Finally, the fate of transgene products in barley seeds was followed. Spots containing two green fluorescent protein constructs and the herbicide resistance marker phosphinothricin acetyltransferase were observed in 2D-gel patterns of transgenic seeds and identified by mass spectrometry. Phosphinothricin acetyltransferase was observed in three spots differing in pI suggesting that post-translational modification of the transgene product had occurred.

Purification and characterization of enzymes exhibiting beta-D-xylosidase activities in stem tissues of Arabidopsis

This work describes the purification and characterization of enzymes that exhibit beta-d-xylosidase activity in stem tissues of Arabidopsis. This is the first detailed investigation that concerns the characterization of catalytic properties and sequence identity of enzymes with beta-D-xylosidase activities in a dicotyledonous plant. Three different enzymes, ARAf, XYL4, and XYL1 with apparent molecular masses of 75, 67, and 64 kD, respectively, were purified to homogeneity. ARAf was identified as a putative alpha-L-arabinofuranosidase, and XYL4 and XYL1 as putative beta-D-xylosidases using matrix-assisted laser-desorption ionization time of flight. ARAf belongs to family 51 and XYL4 and XYL1 to family 3 of glycoside hydrolases. ARAf and XYL1 have highest specificity for p-nitrophenyl-alpha-L-arabinofuranoside and XYL4 for p-nitrophenyl-beta-D-xylopyranoside and natural substrates such as xylobiose and xylotetraose. XYL4 was shown to release mainly D-Xyl from oat spelt xylan, rye arabinoxylan, wheat arabinoxylan, and oligoarabinoxylans. ARAf and XYL1 can also release D-Xyl from these substrates but less efficiently than XYL4. Moreover, they can also release L-Ara from arabinoxylans and arabinan. Overall, the results indicate that XYL4 possesses enzymatic specificity characteristic for a beta-D-xylosidase, while ARAf and XYL1 act as bifunctional alpha-L-arabinofuranosidase/beta-D-xylosidases. Analysis of the activity of these three enzymes in stem tissues at different stages of development has shown that young stems possess the highest activities for all three enzymes in comparison to the activities of the enzymes present in stems at older stages of development. High enzyme activities are most likely related to the necessary modifications of cell wall structure occurring during plant growth.

Occurrence of proteinaceous endoxylanase inhibitors in cereals

Cereals contain proteinaceous inhibitors of endoxylanases, which affect the efficiency and functionality of these enzymes in cereal processing. This review relates their first discovery in wheat and the subsequent purification of two distinct classes of endoxylanase inhibitors, namely Triticum aestivum xylanase inhibitor (TAXI)-type and xylanase inhibitor protein (XIP)-type inhibitors in cereals. Both inhibitor classes occur in monocots as multi-isoform families. The reported data provide an overview of the relative quantitative and qualitative variation of these inhibitors in cereals. Wheat and rye are particularly rich in TAXI-type and XIP-type inhibitors with the latter inhibitors being more abundant. Lower inhibitor levels are present in durum wheat and barley, while maize contains solely XIP-type inhibitors. No inhibitors have been isolated from rice, oats and buckwheat.

Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells

Numerous examples have been presented of enzyme activities, assayed in vitro, that appear relevant to the synthesis of structural polysaccharides, and to their assembly and subsequent degradation in the primary cell walls (PCWs) of higher plants. The accumulation of the corresponding mRNAs, and of the (immunologically recognized) proteins, has often also (or instead) been reported. However, the presence of these mRNAs, antigens and enzymic activities has rarely been shown to correspond to enzyme action in the living plant cell. In some cases, apparent enzymic action is observed in vivo for which no enzyme activity can be detected in in-vitro assays; the converse also occurs. Methods are reviewed by which reactions involving structural wall polysaccharides can be tracked in vivo. Special attention is given to xyloglucan endotransglucosylase (XET), one of the two enzymic activities exhibited in vitro by xyloglucan endotransglucosylase/hydrolase (XTH) proteins, because of its probable importance in the construction and restructuring of the PCW's major hemicellulose. Attention is also given to the possibility that some reactions observed in the PCW in vivo are not directly enzymic, possibly involving the action of hydroxyl radicals. It is concluded that some proposed wall enzymes, for example XTHs, do act in vivo, but that for other enzymes this is not proven. Contents I. Primary cell walls: composition, deposition and roles 642 II. Reactions that have been proposed to occur in primary cell walls 645 III. Tracking the careers of wall components in vivo: evidence for action of enzymes in the walls of living plant cells 656 IV. Evidence for the occurrence of nonenzymic polymer scission in vivo? 666 VI. Conclusion 667 References 667.

Properties of TAXI-type endoxylanase inhibitors

Two types of proteinaceous endoxylanase inhibitors occur in different cereals, i.e. the TAXI [Triticum aestivum endoxylanase inhibitor]-type and XIP [endoxylanase inhibiting protein]-type inhibitors. The present paper focuses on the TAXI-type proteins and deals with their structural characteristics and the identification, characterisation and heterologous expression of a TAXI gene from wheat. In addition, to shed light on the mechanism by which TAXI-type endoxylanase inhibitors work, the enzyme specificity, the optimal conditions for maximal inhibition activity, the molar complexation ratio and the inhibition kinetics of the inhibitors are explained and the effect of mutations of an endoxylanase on the inhibition by TAXIs is discussed.

Bifunctional family 3 glycoside hydrolases from barley with alpha -L-arabinofuranosidase and beta -D-xylosidase activity. Characterization, primary structures, and COOH-terminal processing

An alpha-l-arabinofuranosidase and a beta-d-xylosidase, designated ARA-I and XYL, respectively, have been purified about 1,000-fold from extracts of 5-day-old barley (Hordeum vulgare L.) seedlings using ammonium sulfate fractional precipitation, ion exchange chromatography, chromatofocusing, and size-exclusion chromatography. The ARA-I has an apparent molecular mass of 67 kDa and an isoelectric point of 5.5, and its catalytic efficiency during hydrolysis of 4'-nitrophenyl alpha-l-arabinofuranoside is only slightly higher than during hydrolysis of 4'-nitrophenyl beta-d-xyloside. Thus, the enzyme is actually a bifunctional alpha-l-arabinofuranosidase/beta-d-xylosidase. In contrast, the XYL enzyme, which also has an apparent molecular mass of 67 kDa and an isoelectric point of 6.7, preferentially hydrolyzes 4'-nitrophenyl beta-d-xyloside, with a catalytic efficiency approximately 30-fold higher than with 4'-nitrophenyl alpha-l-arabinofuranoside. The enzymes hydrolyze wheat flour arabinoxylan slowly but rapidly hydrolyze oligosaccharide products released from this polysaccharide by (1 --> 4)-beta-d-xylan endohydrolase. Both enzymes hydrolyze (1 --> 4)-beta-d-xylopentaose, and ARA-I can also degrade (1 --> 5)-alpha-l-arabinofuranohexaose. ARA-I and XYL cDNAs encode mature proteins of 748 amino acid residues which have calculated molecular masses of 79.2 and 80.5 kDa, respectively. Both are family 3 glycoside hydrolases. The discrepancies between the apparent molecular masses obtained for the purified enzymes and those predicted from the cDNAs are attributable to COOH-terminal processing, through which about 130 amino acid residues are removed from the primary translation product. The genes encoding the ARA-I and XYL have been mapped to chromosomes 2H and 6H, respectively. ARA-I transcripts are most abundant in young roots, young leaves, and developing grain, whereas XYL mRNA is detected in most barley tissues.

AtBXL1, a novel higher plant (Arabidopsis thaliana) putative beta-xylosidase gene, is involved in secondary cell wall metabolism and plant development

To investigate mechanisms involved in cell wall development, an Arabidopsis T-DNA insertion mutant collection was screened to identify mutants with beta-glucuronidase fusion gene expression in tissues undergoing secondary cell wall thickening. This promoter-trapping strategy allowed the isolation of a transformant containing the GUS coding sequence inserted 700 bp upstream of the ATG of a putative beta-xylosidase gene. The transformant has no phenotype as the expression of the gene was not disrupted by the insertion. The analysis of the predicted protein, AtBXL1, suggests its targeting to the extracellular matrix and its involvement in cell wall metabolism through a putative activity towards xylans. The 2-kb promoter sequence of AtBXL1 was fused to the GUS coding sequence and introduced into wild-type Arabidopsis thaliana. GUS expression was shown to be restricted to tissues undergoing secondary cell wall formation. Beta-xylosidase activity was associated with the cell wall-enriched fraction of different organs of wild-type plants. The level of activity correlates with transcript accumulation of AtBXL1 and other AtBXL1-related genes. Transgenic plants expressing the AtBXL1 cDNA in antisense orientation were generated. Lines exhibiting the highest decrease in AtBXL1 transcript accumulation and beta-xylosidase activity had phenotypic alterations. This newly identified gene is proposed to be involved in secondary cell wall hemicellulose metabolism and plant development.