Detection of single nucleotide polymorphisms in 24 kDa dimeric α-amylase inhibitors from cultivated wheat and its diploid putative progenitors

Allergen relative abundance in several wheat varieties as revealed via a targeted quantitative approach using MS

Food allergy has become a major health issue in developed countries, therefore there is an urgent need to develop analytical methods able to detect and quantify with a good sensitivity and reliability some specific allergens in complex food matrices. In this paper, we present a targeted MS/MS approach to compare the relative abundance of the major recognized wheat allergens in the salt-soluble (albumin/globulin) fraction of wheat grains. Twelve allergens were quantified in seven wheat varieties, selected from three Triticum species: T. aestivum (bread wheat), T. durum (durum wheat), and T. monococcum. The allergens were monitored from one or two proteotypic peptides and their relative abundance was deduced from the intensity of one fragment measured in MS/MS. Whereas the abundance of some of the targeted allergens was quite stable across the genotypes, others like alpha-amylase inhibitors showed clear differences according to the wheat species, in accordance with the results of earlier functional studies. This study enriches the scarce knowledge available on allergens content in wheat genotypes, and brings new perspectives for food safety and plant breeding.

Targeted re-sequencing of the allohexaploid wheat exome

Bread wheat, Triticum aestivum, is an allohexaploid composed of the three distinct ancestral genomes, A, B and D. The polyploid nature of the wheat genome together with its large size has limited our ability to generate the significant amount of sequence data required for whole genome studies. Even with the advent of next-generation sequencing technology, it is still relatively expensive to generate whole genome sequences for more than a few wheat genomes at any one time. To overcome this problem, we have developed a targeted-capture re-sequencing protocol based upon NimbleGen array technology to capture and characterize 56.5 Mb of genomic DNA with sequence similarity to over 100 000 transcripts from eight different UK allohexaploid wheat varieties. Using this procedure in conjunction with a carefully designed bioinformatic procedure, we have identified more than 500 000 putative single-nucleotide polymorphisms (SNPs). While 80% of these were variants between the homoeologous genomes, A, B and D, a significant number (20%) were putative varietal SNPs between the eight varieties studied. A small number of these latter polymorphisms were experimentally validated using KASPar technology and 94% proved to be genuine. The procedures described here to sequence a large proportion of the wheat genome, and the various SNPs identified should be of considerable use to the wider wheat community.

Expression of α-amylase inhibitors in diploid Triticum species

The aim of the work was to characterize the expression of various α-amylase inhibitors (αAIs), well known anti-nutritional compounds, for the development of healthier diploid wheat-based functional foods. The salt-soluble protein fractions from the seeds of 53 accessions among Triticum monococcum subsp. monococcum (T.m.), T. monococcum subsp. boeoticum (T.b.) and Triticum urartu (T.u.) were analyzed by immunoblotting after SDS-PAGE and Urea-PAGE using polyclonal antibodies (PABs) raised against 0.19 and 0.28 αAIs expressed in bread-wheat. Reverse zymography with human saliva and Tenebrio molitor α-amylases was used to assay inhibition activity. A great variability of the expression of αAI-related proteins was observed among T.b. and T.u. PABs, and reverse zymography revealed different bands, often not correlating with those present in bread-wheat. Two-dimensional electrophoresis followed by immunoblotting and mass spectrometric analysis identified these proteins as αAIs. Interestingly, no signal was observed within T.m. accessions. This makes T.m. an important candidate for the production of novel functional foods.

The spectrum of low molecular weight alpha-amylase/protease inhibitor genes expressed in the US bread wheat cultivar Butte 86

BACKGROUND

Wheat grains accumulate a variety of low molecular weight proteins that are inhibitors of alpha-amylases and proteases and play an important protective role in the grain. These proteins have more balanced amino acid compositions than the major wheat gluten proteins and contribute important reserves for both seedling growth and human nutrition. The alpha-amylase/protease inhibitors also are of interest because they cause IgE-mediated occupational and food allergies and thereby impact human health.

RESULTS

The complement of genes encoding alpha-amylase/protease inhibitors expressed in the US bread wheat Butte 86 was characterized by analysis of expressed sequence tags (ESTs). Coding sequences for 19 distinct proteins were identified. These included two monomeric (WMAI), four dimeric (WDAI), and six tetrameric (WTAI) inhibitors of exogenous alpha-amylases, two inhibitors of endogenous alpha-amylases (WASI), four putative trypsin inhibitors (CMx and WTI), and one putative chymotrypsin inhibitor (WCI). A number of the encoded proteins were identical or very similar to proteins in the NCBI database. Sequences not reported previously included variants of WTAI-CM3, three CMx inhibitors and WTI. Within the WDAI group, two different genes encoded the same mature protein. Based on numbers of ESTs, transcripts for WTAI-CM3 Bu-1, WMAI Bu-1 and WTAI-CM16 Bu-1 were most abundant in Butte 86 developing grain. Coding sequences for 16 of the inhibitors were unequivocally associated with specific proteins identified by tandem mass spectrometry (MS/MS) in a previous proteomic analysis of milled white flour from Butte 86. Proteins corresponding to WDAI Bu-1/Bu-2, WMAI Bu-1 and the WTAI subunits CM2 Bu-1, CM3 Bu-1 and CM16 Bu-1 were accumulated to the highest levels in flour.

CONCLUSIONS

Information on the spectrum of alpha-amylase/protease inhibitor genes and proteins expressed in a single wheat cultivar is central to understanding the importance of these proteins in both plant defense mechanisms and human allergies and facilitates both breeding and biotechnology approaches for manipulating the composition of these proteins in plants.

Molecular characterization and functional analysis of elite genes in wheat and its related species

The tribe Triticeae includes major cereal crops (bread wheat, durum wheat, triticale, barley and rye), as well as abundant forage and lawn grasses. Wheat and its wild related species possess numerous favourable genes for yield improvement, grain quality enhancement, biotic and abiotic stress resistance, and constitute a giant gene pool for wheat improvement. In recent years, significant progress on molecular characterization and functional analysis of elite genes in wheat and its related species have been achieved. In this paper, we review the cloned functional genes correlated with grain quality, biotic and abiotic stress resistance, photosystem and nutrition utilization in wheat and its related species.

Molecular evolution of dimeric alpha-amylase inhibitor genes in wild emmer wheat and its ecological association

BACKGROUND

alpha-Amylase inhibitors are attractive candidates for the control of seed weevils, as these insects are highly dependent on starch as an energy source. In this study, we aimed to reveal the structure and diversity of dimeric alpha-amylase inhibitor genes in wild emmer wheat from Israel and to elucidate the relationship between the emmer wheat genes and ecological factors using single nucleotide polymorphism (SNP) markers. Another objective of this study was to find out whether there were any correlations between SNPs in functional protein-coding genes and the environment.

RESULTS

The influence of ecological factors on the genetic structure of dimeric alpha-amylase inhibitor genes was evaluated by specific SNP markers. A total of 244 dimeric alpha-amylase inhibitor genes were obtained from 13 accessions in 10 populations. Seventy-five polymorphic positions and 74 haplotypes were defined by sequence analysis. Sixteen out of the 75 SNP markers were designed to detect SNP variations in wild emmer wheat accessions from different populations in Israel. The proportion of polymorphic loci P (5%), the expected heterozygosity He, and Shannon's information index in the 16 populations were 0.887, 0.404, and 0.589, respectively. The populations of wild emmer wheat showed great diversity in gene loci both between and within populations. Based on the SNP marker data, the genetic distance of pair-wise comparisons of the 16 populations displayed a sharp genetic differentiation over long geographic distances. The values of P, He, and Shannon's information index were negatively correlated with three climatic moisture factors, whereas the same values were positively correlated by Spearman rank correlation coefficients' analysis with some of the other ecological factors.

CONCLUSION

The populations of wild emmer wheat showed a wide range of diversity in dimeric alpha-amylase inhibitors, both between and within populations. We suggested that SNP markers are useful for the estimation of genetic diversity of functional genes in wild emmer wheat. These results show significant correlations between SNPs in the alpha-amylase inhibitor genes and ecological factors affecting diversity. Ecological factors, singly or in combination, explained a significant proportion of the variations in the SNPs, and the SNPs could be classified into several categories as ecogeographical predictors. It was suggested that the SNPs in the alpha-amylase inhibitor genes have been subjected to natural selection, and ecological factors had an important evolutionary influence on gene differentiation at specific loci.

Dimeric inhibitors of human salivary alpha-amylase from emmer (Triticum dicoccon Schrank) seeds

The proteins belonging to the cereal trypsin/alpha-amylase inhibitor family are abundant water/salt-soluble flour proteins active against alpha-amylases from several seed parasites and pests and inactive against endogenous alpha-amylases. Three alpha-amylase inhibitor families have been described in cereals that vary in size and are differently expressed among Triticeae seeds. The present work investigates the presence of human salivary alpha-amylase inhibitors in emmer (Triticum dicoccon Schrank) flour. The isolation was obtained by a series of chromatography steps, and the purification progress was monitored through the inhibition of human salivary alpha-amylase activity. The purified fraction was subjected to protein sequencing by tandem mass spectrometry (MSMS) of the tryptic digests obtained after the sample separation on 2-DE. MSMS data indicated that the emmer alpha-amylase inhibitory fraction was composed of two newly identified proteins [emmer dimeric inhibitor 1 (EDI-1) and emmer dimeric inhibitor 2 (EDI-2)] sharing very high identity levels with related proteins from Triticum aestivum.

Sequence variations and haplotype identification of wheat dimeric alpha-amylase inhibitor genes in einkorn wheats

This study characterizes 80 dimeric alpha-amylase inhibitor genes from 68 accessions of the einkorn wheats Triticum urartu, T. boeoticum, and T. monococcum. The mature protein coding sequences of WDAI genes were analyzed. Nucleotide sequence variations in these regions resulted from base substitution and/or indel mutations. Most of the WDAI gene sequences from T. boeoticum and all sequences from T. monococcum had one nucleotide insertion in the coding region, such that these alpha-amylase inhibitor sequences could not encode the correct mature proteins. We identified 21 distinct haplotypes from the diploid wheat WDAI gene sequences. A main haplotype was found in 15 gene samples from the A(u) genome and 35 gene samples from the A(m) genome. The T. monococcum and T. boeoticum accessions shared the same main haplotype, with 25 samples from T. monococcum and 10 from T. boeoticum. The WDAI gene sequences from the A(u) and A(m) genomes could be obviously clustered into two clades, but the sequences from the A(m) genome of T. boeoticum and T. monococcum could not be clearly distinguished. The phylogenetic analysis revealed that the WDAI gene sequences from the A(m) genome had accumulated fewer variations and evolved at a slower rate than the sequences from the A(u) genome. Although some accessions from only one or two areas had unique mutations at the same position, the diversity of WDAI gene sequences in diploid wheat showed little relationship to the origin of the accessions.

Full-fledged proteomic analysis of bioactive wheat amylase inhibitors by a 3-D analytical technique: Identification of new heterodimeric aggregation states

Wheat proteinaceous alpha-amylase inhibitors (alpha-AIs) are increasingly investigated for their agronomical role as natural defence molecules of plants against the attack of insects and pests, but also for their effects on human health. The wheat genomes code for several bioactive alpha-AIs that share sequence homology, but differ in their specificity against alpha-amylases from different species and for their aggregation states. Wheat alpha-AIs are traditionally classified as belonging to the three classes of tetrameric, homodimeric and monomeric forms, each class being constituted by a number of polypeptides that display different electrophoretic mobilities. Here we describe a proteomic approach for the identification of bioactive alpha-AIs from wheat and, in particular, a 3-D technique that allows to best identify and characterize the dimeric fraction. The technique takes advantage of the thermal resistance of alpha-AIs (resistant to T > 70 degrees C) and consists in the separation of protein mixtures by 2-D polyacrylamide/starch electrophoresis under nondissociating PAGE (ND-PAGE, first dimension) and dissociating (urea-PAGE or U-PAGE second dimension) conditions, followed by in-gel spontaneous reaggregation of protein complexes and identification of the alpha-amylase inhibitory activity (antizymogram, third dimension) using enzymes from human salivary glands and from the larvae of Tenebrio molitor coleopter (yellow mealworm). Dimeric alpha-AIs from Triticum aestivum (bread wheat) were observed to exist as heterodimers. The formation of heterodimeric complexes was also confirmed by in vitro reaggregation assays carried out on RP-HPLC purified wheat dimeric alpha-AIs, and their bioactivity assayed by antizymogram analysis. The present 3-D analytical technique can be exploited for fast, full-fledged identification and characterization of wheat alpha-AIs.

Analysis and validation of genome-specific DNA variations in 5' flanking conserved sequences of wheat low-molecular-weight glutenin subunit gene

The thirty-three 5' flanking conserved sequences of the known low-molecular-weight subunit (LMW-GS) genes have been divided into eight clusters, which was in agreement with the classification based on the deduced N-terminal protein sequences. The DNA polymorphism between the eight clusters was obtained by sequence alignment, and a total of 34 polymorphic positions were observed in the approximately 200 bp regions, among which 18 polymorphic positions were candidate SNPs. Seven cluster-specific primer sets were designed for seven out of eight clusters containing cluster-specific bases, with which the genomic DNA of the ditelosomic lines of group 1 chromosomes of a wheat variety 'Chinese Spring' was employed to carry out chromosome assignment. The subsequent cloning and DNA sequencing of PCR fragments validated the sequences specificity of the 5' flanking conserved sequences between LMW-GS gene groups in different genomes. These results suggested that the coding and 5' flanking regions of LMW-GS genes are likely to have evolved in concerted fashion. The seven primer sets developed in this study could be used to isolate the complete ORFs of seven groups of LMW-GS genes, respectively, and therefore possess great value for further research in the contributions of a single LMW-GS gene to wheat quality in the complex genetic background and the efficient selections of quality-related components in breeding programs.