Detailed analysis of the expression of an alpha-gliadin promoter and the deposition of alpha-gliadin protein during wheat grain development

Identifying cis-Acting Elements Associated with the High Activity and Endosperm Specificity of the Promoters of Genes Encoding Low-Molecular-Weight Glutenin Subunits in Common Wheat (Triticum aestivum)

Low-molecular-weight glutenin subunits (LMW-GSs) associated with bread-baking quality and flour nutrient quality accumulate in endosperms of common wheat and related species. However, the mechanism underlying the expression regulation of genes encoding LMW-GSs has not been fully elucidated. In this study, we identified LMW-D2 and LMW-D7, which are highly and weakly expressed, respectively, via the analysis of RNA-sequencing data of Chinese Spring wheat and wheat transgenic lines transformed with 5' deletion promoter fragments and GUS fusion constructs. The 605-bp fragment upstream of the LMW-D2 start codon could drive high levels of GUS expression in the endosperm. The truncated endosperm box located at the -300 site resulted in the loss of LMW-D2 promoter activity, and a single-nucleotide polymorphism on the GCN4 motif was closely related to the expression of LMW-GSs. TCT and TGACG motifs, as well as the others located on the 5' distal end, might also be involved in the transcription regulation of LMW-GSs. In transgenic lines, fusion proteins of LMW-GS and GUS were deposited into protein bodies. Our findings provide new insights into the mechanism underlying the transcription regulation of LMW-GSs and will contribute to the development of wheat endosperm as a bioreactor for the production of nutraceuticals, antibodies, vaccines, and medicinal proteins.

Wheat DOF transcription factors TaSAD and WPBF regulate glutenin gene expression in cooperation with SPA

Grain storage proteins (GSPs) quantity and composition determine the end-use value of wheat flour. GSPs consists of low-molecular-weight glutenins (LMW-GS), high-molecular-weight glutenins (HMW-GS) and gliadins. GSP gene expression is controlled by a complex network of DNA-protein and protein-protein interactions, which coordinate the tissue-specific protein expression during grain development. The regulatory network has been most extensively studied in barley, particularly the two transcription factors (TFs) of the DNA binding with One Finger (DOF) family, barley Prolamin-box Binding Factor (BPBF) and Scutellum and Aleurone-expressed DOF (SAD). They activate hordein synthesis by binding to the Prolamin box, a motif in the hordein promoter. The BPBF ortholog previously identified in wheat, WPBF, has a transcriptional activity in expression of some GSP genes. Here, the wheat ortholog of SAD, named TaSAD, was identified. The binding of TaSAD to GSP gene promoter sequences in vitro and its transcriptional activity in vivo were investigated. In electrophoretic mobility shift assays, recombinant TaSAD and WPBF proteins bound to cis-motifs like those located on HMW-GS and LMW-GS gene promoters known to bind DOF TFs. We showed by transient expression assays in wheat endosperms that TaSAD and WPBF activate GSP gene expression. Moreover, co-bombardment of Storage Protein Activator (SPA) with WPBF or TaSAD had an additive effect on the expression of GSP genes, possibly through conserved cooperative protein-protein interactions.

The α-Gliadins in Bread Wheat: Effect of Nitrogen Treatment on the Expression of the Major Celiac Disease Immunogenic Complex in Two RNAi Low-Gliadin Lines

Celiac Disease (CD) is an autoimmune disorder that affects approximately 1% of the worldwide population. The α-gliadins of wheat contain the 33-mer peptide, the most active peptide in CD both in adults and pediatric patients. In this study, we have characterized the variants and expression profile of an α-gliadins amplicon, harboring the 33-mer peptide, in two low-gliadin RNAi wheat lines, under two different Nitrogen (N) treatments. We estimated that the amplicon expands 45 different α-gliadin variants with high variability due to length, randomly distributed SNPs, and the presence of encoded CD epitopes. Expression of this amplicon is reduced in both RNAi lines in comparison to the wild type. High N treatment significantly increases transcripts of the amplicon in the wild type, but not in the transgenic lines. Classification of α-gliadin variants, considering the number of epitopes, revealed that amplicon variants containing the full complement of 33-mer peptide were affected by N treatment, increasing their expression when N was increased. Line D793 provided higher and more stable silencing through different N fertilization regimes, expressing fewer CD epitopes than D783. Results of this study are important for better understanding of RNAi α-gliadin silencing in response to N treatments, and for undertaking new strategies by RNAi or CRISPR/Cas toward obtaining new varieties suitable for people suffering gluten intolerances.

Wheat amino acid transporters highly expressed in grain cells regulate amino acid accumulation in grain

Amino acids are delivered into developing wheat grains to support the accumulation of storage proteins in the starchy endosperm, and transporters play important roles in regulating this process. RNA-seq, RT-qPCR, and promoter-GUS assays showed that three amino acid transporters are differentially expressed in the endosperm transfer cells (TaAAP2), starchy endosperm cells (TaAAP13), and aleurone cells and embryo of the developing grain (TaAAP21), respectively. Yeast complementation revealed that all three transporters can transport a broad spectrum of amino acids. RNAi-mediated suppression of TaAAP13 expression in the starchy endosperm did not reduce the total nitrogen content of the whole grain, but significantly altered the composition and distribution of metabolites in the starchy endosperm, with increasing concentrations of some amino acids (notably glutamine and glycine) from the outer to inner starchy endosperm cells compared with wild type. Overexpression of TaAAP13 under the endosperm-specific HMW-GS (high molecular weight glutenin subunit) promoter significantly increased grain size, grain nitrogen concentration, and thousand grain weight, indicating that the sink strength for nitrogen transport was increased by manipulation of amino acid transporters. However, the total grain number was reduced, suggesting that source nitrogen remobilized from leaves is a limiting factor for productivity. Therefore, simultaneously increasing loading of amino acids into the phloem and delivery to the spike would be required to increase protein content while maintaining grain yield.

Over-Expressing TaSPA-B Reduces Prolamin and Starch Accumulation in Wheat (Triticum aestivum L.) Grains

Starch and prolamin composition and content are important indexes for determining the processing and nutritional quality of wheat (Triticum aestivum L.) grains. Several transcription factors (TFs) regulate gene expression during starch and protein biosynthesis in wheat. Storage protein activator (TaSPA), a member of the basic leucine zipper (bZIP) family, has been reported to activate glutenin genes and is correlated to starch synthesis related genes. In this study, we generated TaSPA-B overexpressing (OE) transgenic wheat lines. Compared with wild-type (WT) plants, the starch content was slightly reduced and starch granules exhibited a more polarized distribution in the TaSPA-B OE lines. Moreover, glutenin and ω- gliadin contents were significantly reduced, with lower expression levels of related genes (e.g., By15, Dx2, and ω-1,2 gliadin gene). RNA-seq analysis identified 2023 differentially expressed genes (DEGs). The low expression of some DEGs (e.g., SUSase, ADPase, Pho1, Waxy, SBE, SSI, and SS II a) might explain the reduction of starch contents. Some TFs involved in glutenin and starch synthesis might be regulated by TaSPA-B, for example, TaPBF was reduced in TaSPA-B OE-3 lines. In addition, dual-luciferase reporter assay indicated that both TaSPA-B and TaPBF could transactivate the promoter of ω-1,2 gliadin gene. These results suggest that TaSPA-B regulates a complex gene network and plays an important role in starch and protein biosynthesis in wheat.

Genomic and functional genomics analyses of gluten proteins and prospect for simultaneous improvement of end-use and health-related traits in wheat

KEY MESSAGE

Recent genomic and functional genomics analyses have substantially improved the understanding on gluten proteins, which are important determinants of wheat grain quality traits. The new insights obtained and the availability of precise, versatile and high-throughput genome editing technologies will accelerate simultaneous improvement of wheat end-use and health-related traits. Being a major staple food crop in the world, wheat provides an indispensable source of dietary energy and nutrients to the human population. As worldwide population grows and living standards rise in both developed and developing countries, the demand for wheat with high quality attributes increases globally. However, efficient breeding of high-quality wheat depends on critically the knowledge on gluten proteins, which mainly include several families of prolamin proteins specifically accumulated in the endospermic tissues of grains. Although gluten proteins have been studied for many decades, efficient manipulation of these proteins for simultaneous enhancement of end-use and health-related traits has been difficult because of high complexities in their expression, function and genetic variation. However, recent genomic and functional genomics analyses have substantially improved the understanding on gluten proteins. Therefore, the main objective of this review is to summarize the genomic and functional genomics information obtained in the last 10 years on gluten protein chromosome loci and genes and the cis- and trans-factors regulating their expression in the grains, as well as the efforts in elucidating the involvement of gluten proteins in several wheat sensitivities affecting genetically susceptible human individuals. The new insights gathered, plus the availability of precise, versatile and high-throughput genome editing technologies, promise to speed up the concurrent improvement of wheat end-use and health-related traits and the development of high-quality cultivars for different consumption needs.

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

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

The bZIP transcription factor SPA Heterodimerizing Protein represses glutenin synthesis in Triticum aestivum

The quality of wheat grain is mainly determined by the quantity and composition of its grain storage proteins (GSPs). Grain storage proteins consist of low- and high-molecular-weight glutenins (LMW-GS and HMW-GS, respectively) and gliadins. The synthesis of these proteins is essentially regulated at the transcriptional level and by the availability of nitrogen and sulfur. The regulation network has been extensively studied in barley where BLZ1 and BLZ2, members of the basic leucine zipper (bZIP) family, activate the synthesis of hordeins. To date, in wheat, only the ortholog of BLZ2, Storage Protein Activator (SPA), has been identified as playing a major role in the regulation of GSP synthesis. Here, the ortholog of BLZ1, named SPA Heterodimerizing Protein (SHP), was identified and its involvement in the transcriptional regulation of the genes coding for GSPs was analyzed. In gel mobility shift assays, SHP binds cis-motifs known to bind to bZIP family transcription factors in HMW-GS and LMW-GS promoters. Moreover, we showed by transient expression assays in wheat endosperm that SHP acts as a repressor of the activity of these gene promoters. This result was confirmed in transgenic lines overexpressing SHP, which were grown with low and high nitrogen supply. The phenotype of SHP-overexpressing lines showed a lower quantity of both LMW-GS and HMW-GS, while the quantity of gliadin was unchanged, whatever the nitrogen availability. Thus, the gliadin/glutenin ratio was increased, which suggests that gliadin and glutenin genes may be differently regulated.

Active DNA demethylation: mechanism and role in plant development

Active DNA demethylation (enzymatic removal of methylated cytosine) regulates many plant developmental processes. In Arabidopsis, active DNA demethylation entails the base excision repair pathway initiated by the Repressor of silencing 1/Demeter family of bifunctional DNA glycosylases. In this review, we first present an introduction to the recent advances in our understanding about the mechanisms of active DNA demethylation. We then focus on the role of active DNA demethylation in diverse developmental processes in various plant species, including the regulation of seed development, pollen tube formation, stomatal development, fruit ripening, and nodule development. Finally, we discuss future directions of research in the area of active DNA demethylation.

Identification and molecular characterization of a trans-acting small interfering RNA producing locus regulating leaf rust responsive gene expression in wheat (Triticum aestivum L.)

A novel leaf rust responsive ta-siRNA-producing locus was identified in wheat showing similarity to 28S rRNA and generated four differentially expressing ta-siRNAs by phasing which targeted stress responsive genes. Trans-acting-small interfering RNAs (Ta-siRNAs) are plant specific molecules generally involved in development and are also stress responsive. Ta-siRNAs identified in wheat till date are all responsive to abiotic stress only. Wheat cultivation is severely affected by rusts and leaf rust particularly affects grain filling. This study reports a novel ta-siRNA producing locus (TAS) in wheat which is a segment of 28S ribosomal RNA but shows differential expression during leaf rust infestation. Four small RNA libraries prepared from wheat Near Isogenic Lines were treated with leaf rust pathogen and compared with untreated controls. A TAS with the ability to generate four ta-siRNAs by phasing events was identified along with the microRNA TamiR16 as the phase initiator. The targets of the ta-siRNAs included α-gliadin, leucine rich repeat, trans-membrane proteins, glutathione-S-transferase, and fatty acid desaturase among others, which are either stress responsive genes or are essential for normal growth and development of plants. Expression of the TAS, its generated ta-siRNAs, and their target genes were profiled at five different time points after pathogen inoculation of susceptible and resistant wheat isolines and compared with mock-inoculated controls. Comparative analysis of expression unveiled differential and reciprocal relationship as well as discrete patterns between susceptible and resistant isolines. The expression profiles of the target genes of the identified ta-siRNAs advocate more towards effector triggered susceptibility favouring pathogenesis. The study helps in discerning the functions of wheat genes regulated by ta-siRNAs in response to leaf rust.

Food processing and breeding strategies for coeliac-safe and healthy wheat products

A strict gluten-free diet is currently the only treatment for the 1-2% of the world population who suffer from coeliac disease (CD). However, due to the presence of wheat and wheat derivatives in many food products, avoiding gluten consumption is difficult. Gluten-free products, made without wheat, barley or rye, typically require the inclusion of numerous additives, resulting in products that are often less healthy than gluten-based equivalents. Here, we present and discuss two broad approaches to decrease wheat gluten immunogenicity for CD patients. The first approach is based on food processing strategies, which aim to remove gliadins or all gluten from edible products. We find that several of the candidate food processing techniques to produce low gluten-immunogenic products from wheat already exist. The second approach focuses on wheat breeding strategies to remove immunogenic epitopes from the gluten proteins, while maintaining their food-processing properties. A combination of breeding strategies, including mutation breeding and possibly genome editing, will be necessary to produce coeliac-safe wheat. Individuals suffering from CD and people genetically susceptible who may develop CD after prolonged gluten consumption would benefit from reduced CD-immunogenic wheat. Although the production of healthy and less CD-toxic wheat varieties and food products will be challenging, increasing global demand may require these issues to be addressed in the near future by food processing and cereal breeding companies.

Conserved cis-regulatory modules in promoters of genes encoding wheat high-molecular-weight glutenin subunits

The concentration and composition of the gliadin and glutenin seed storage proteins (SSPs) in wheat flour are the most important determinants of its end-use value. In cereals, the synthesis of SSPs is predominantly regulated at the transcriptional level by a complex network involving at least five cis-elements in gene promoters. The high-molecular-weight glutenin subunits (HMW-GS) are encoded by two tightly linked genes located on the long arms of group 1 chromosomes. Here, we sequenced and annotated the HMW-GS gene promoters of 22 electrophoretic wheat alleles to identify putative cis-regulatory motifs. We focused on 24 motifs known to be involved in SSP gene regulation. Most of them were identified in at least one HMW-GS gene promoter sequence. A common regulatory framework was observed in all the HMW-GS gene promoters, as they shared conserved cis-regulatory modules (CCRMs) including all the five motifs known to regulate the transcription of SSP genes. This common regulatory framework comprises a composite box made of the GATA motifs and GCN4-like Motifs (GLMs) and was shown to be functional as the GLMs are able to bind a bZIP transcriptional factor SPA (Storage Protein Activator). In addition to this regulatory framework, each HMW-GS gene promoter had additional motifs organized differently. The promoters of most highly expressed x-type HMW-GS genes contain an additional box predicted to bind R2R3-MYB transcriptional factors. However, the differences in annotation between promoter alleles could not be related to their level of expression. In summary, we identified a common modular organization of HMW-GS gene promoters but the lack of correlation between the cis-motifs of each HMW-GS gene promoter and their level of expression suggests that other cis-elements or other mechanisms regulate HMW-GS gene expression.

Quantitative and qualitative differences in celiac disease epitopes among durum wheat varieties identified through deep RNA-amplicon sequencing

Background

Wheat gluten is important for the industrial quality of bread wheat (Triticum aestivum L.) and durum wheat (T. turgidum L.). Gluten proteins are also the source of immunogenic peptides that can trigger a T cell reaction in celiac disease (CD) patients, leading to inflammatory responses in the small intestine. Various peptides with three major T cell epitopes involved in CD are derived from alpha-gliadin fraction of gluten. Alpha-gliadins are encoded by a large multigene family and amino acid variation in the CD epitopes is known to influence the immunogenicity of individual gene family members. Current commercial methods of gluten detection are unable to distinguish between immunogenic and non-immunogenic CD epitope variants and thus to accurately quantify the overall CD epitope load of a given wheat variety. Such quantification is indispensable for correct selection of wheat varieties with low potential to cause CD.

Results

A 454 RNA-amplicon sequencing method was developed for alpha-gliadin transcripts encompassing the three major CD epitopes and their variants. The method was used to screen developing grains on plants of 61 different durum wheat cultivars and accessions. A dedicated sequence analysis pipeline returned a total of 304 unique alpha-gliadin transcripts, corresponding to a total of 171 ‘unique deduced protein fragments’ of alpha-gliadins. The numbers of these fragments obtained in each plant were used to calculate quantitative and quantitative differences between the CD epitopes expressed in the endosperm of these wheat plants. A few plants showed a lower fraction of CD epitope-encoding alpha-gliadin transcripts, but none were free of CD epitopes.

Conclusions

The dedicated 454 RNA-amplicon sequencing method enables 1) the grouping of wheat plants according to the genetic variation in alpha-gliadin transcripts, and 2) the screening for plants which are potentially less CD-immunogenic. The resulting alpha-gliadin sequence database will be useful as a reference in proteomics analysis regarding the immunogenic potential of mature wheat grains.

Association study of wheat grain protein composition reveals that gliadin and glutenin composition are trans-regulated by different chromosome regions

Wheat grain storage protein (GSP) content and composition are the main determinants of the end-use value of bread wheat (Triticum aestivum L.) grain. The accumulation of glutenins and gliadins, the two main classes of GSP in wheat, is believed to be mainly controlled at the transcriptional level through a network of transcription factors. This regulation network could lead to stable cross-environment allometric scaling relationships between the quantity of GSP classes/subunits and the total quantity of nitrogen per grain. This work conducted a genetic mapping study of GSP content and composition and allometric scaling parameters of grain N allocation using a bread wheat worldwide core collection grown in three environments. The core collection was genotyped with 873 markers for genome-wide association and 167 single nucleotide polymorphism markers in 51 candidate genes for candidate association. The candidate genes included 35 transcription factors (TFs) expressed in grain. This work identified 74 loci associated with 38 variables, of which 19 were candidate genes or were tightly linked with candidate genes. Besides structural GSP genes, several loci putatively trans-regulating GSP accumulation were identified. Seven candidate TFs, including four wheat orthologues of barley TFs that control hordein gene expression, were associated or in strong linkage disequilibrium with markers associated with the composition or quantity of glutenin or gliadin, or allometric grain N allocation parameters, confirming the importance of the transcriptional control of GSP accumulation. Genome-wide association results suggest that the genes regulating glutenin and gliadin compositions are mostly distinct from each other and operate differently.

Structure, variation and expression analysis of glutenin gene promoters from Triticum aestivum cultivar Chinese Spring shows the distal region of promoter 1Bx7 is key regulatory sequence

In this study, ten glutenin gene promoters were isolated from model wheat (Triticum aestivum L. cv. Chinese Spring) using a genomic PCR strategy with gene-specific primers. Six belonged to high-molecular-weight glutenin subunit (HMW-GS) gene promoters, and four to low-molecular-weight glutenin subunit (LMW-GS). Sequence lengths varied from 1361 to 2,554 bp. We show that the glutenin gene promoter motifs are conserved in diverse sequences in this study, with HMW-GS and LMW-GS gene promoters characterized by distinct conserved motif combinations. Our findings show that HMW-GS promoters contain more functional motifs in the distal region of the glutenin gene promoter (> -700 bp) compared with LMW-GS. The y-type HMW-GS gene promoters possess unique motifs including RY repeat and as-2 box compared to the x-type. We also identified important motifs in the distal region of HMW-GS gene promoters including the 5'-UTR Py-rich stretch motif and the as-2 box motif. We found that cis-acting elements in the distal region of promoter 1Bx7 enhanced the expression of HMW-GS gene 1Bx7. Taken together, these data support efforts in designing molecular breeding strategies aiming to improve wheat quality. Our results offer insight into the regulatory mechanisms of glutenin gene expression.

Biolistic- and Agrobacterium-mediated transformation protocols for wheat

After rice, wheat is considered to be the most important world food crop, and the demand for high-quality wheat flour is increasing. Although there are no GM varieties currently grown, wheat is an important target for biotechnology, and we anticipate that GM wheat will be commercially available in 10-15 years. In this chapter, we summarize the main features and challenges of wheat transformation and then describe detailed protocols for the production of transgenic wheat plants both by biolistic and Agrobacterium-mediated DNA-delivery. Although these methods are used mainly for bread wheat (Triticum aestivum L.), they can also be successfully applied, with slight modifications, to tetraploid durum wheat (T. turgidum L. var. durum). The appropriate size and developmental stage of explants (immature embryo-derived scutella), the conditions to produce embryogenic callus tissues, and the methods to regenerate transgenic plants under increasing selection pressure are provided in the protocol. To illustrate the application of herbicide selection system, we have chosen to describe the use of the plasmid pAHC25 for biolistic transformation, while for Agrobacterium-mediated transformation the binary vector pAL156 (incorporating both the bar gene and the uidA gene) has been chosen. Beside the step-by-step methodology for obtaining stably transformed and normal fertile plants, procedures for screening and testing transgenic wheat plants are also discussed.

Delivery of prolamins to the protein storage vacuole in maize aleurone cells

Zeins, the prolamin storage proteins found in maize (Zea mays), accumulate in accretions called protein bodies inside the endoplasmic reticulum (ER) of starchy endosperm cells. We found that genes encoding zeins, α-globulin, and legumin-1 are transcribed not only in the starchy endosperm but also in aleurone cells. Unlike the starchy endosperm, aleurone cells accumulate these storage proteins inside protein storage vacuoles (PSVs) instead of the ER. Aleurone PSVs contain zein-rich protein inclusions, a matrix, and a large system of intravacuolar membranes. After being assembled in the ER, zeins are delivered to the aleurone PSVs in atypical prevacuolar compartments that seem to arise at least partially by autophagy and consist of multilayered membranes and engulfed cytoplasmic material. The zein-containing prevacuolar compartments are neither surrounded by a double membrane nor decorated by AUTOPHAGY RELATED8 protein, suggesting that they are not typical autophagosomes. The PSV matrix contains glycoproteins that are trafficked through a Golgi-multivesicular body (MVB) pathway. MVBs likely fuse with the multilayered, autophagic compartments before merging with the PSV. The presence of similar PSVs also containing prolamins and large systems of intravacuolar membranes in wheat (Triticum aestivum) and barley (Hordeum vulgare) starchy endosperm suggests that this trafficking mechanism may be common among cereals.

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.

Identification of three wheat globulin genes by screening a Triticum aestivum BAC genomic library with cDNA from a diabetes-associated globulin

BACKGROUND

Exposure to dietary wheat proteins in genetically susceptible individuals has been associated with increased risk for the development of Type 1 diabetes (T1D). Recently, a wheat protein encoded by cDNA WP5212 has been shown to be antigenic in mice, rats and humans with autoimmune T1D. To investigate the genomic origin of the identified wheat protein cDNA, a hexaploid wheat genomic library from Glenlea cultivar was screened.

RESULTS

Three unique wheat globulin genes, Glo-3A, Glo3-B and Glo-3C, were identified. We describe the genomic structure of these genes and their expression pattern in wheat seeds. The Glo-3A gene shared 99% identity with the cDNA of WP5212 at the nucleotide and deduced amino acid level, indicating that we have identified the gene(s) encoding wheat protein WP5212. Southern analysis revealed the presence of multiple copies of Glo-3-like sequences in all wheat samples, including hexaploid, tetraploid and diploid species wheat seed. Aleurone and embryo tissue specificity of WP5212 gene expression, suggested by promoter region analysis, which demonstrated an absence of endosperm specific cis elements, was confirmed by immunofluorescence microscopy using anti-WP5212 antibodies.

CONCLUSION

Taken together, the results indicate that a diverse group of globulins exists in wheat, some of which could be associated with the pathogenesis of T1D in some susceptible individuals. These data expand our knowledge of specific wheat globulins and will enable further elucidation of their role in wheat biology and human health.

Tetraploid and hexaploid wheat varieties reveal large differences in expression of alpha-gliadins from homoeologous Gli-2 loci

BACKGROUND

Alpha-gliadins form a multigene protein family encoded by multiple alpha-gliadin (Gli-2) genes at three genomic loci, Gli-A2, Gli-B2 and Gli-D2, respectively located on the homoeologous wheat chromosomes 6AS, 6BS, and 6DS. These proteins contain a number of important celiac disease (CD)-immunogenic domains. The alpha-gliadins expressed from the Gli-B2 locus harbour fewer conserved CD-epitopes than those from Gli-A2, whereas the Gli-D2 gliadins have the highest CD-immunogenic potential. In order to detect differences in the highly CD-immunogenic alpha-gliadin fraction we determined the relative expression level from the homoeologous Gli-2 loci in various tetraploid and hexaploid wheat genotypes by using a quantitative pyrosequencing method and by analyzing expressed sequence tag (EST) sequences.

RESULTS

We detected large differences in relative expression levels of alpha-gliadin genes from the three homoeologous loci among wheat genotypes, both as relative numbers of expressed sequence tag (EST) sequences from specific varieties and when using a quantitative pyrosequencing assay specific for Gli-A2 genes. The relative Gli-A2 expression level in a tetraploid durum wheat cultivar ('Probstdorfer Pandur') was 41%. In genotypes derived from landraces, the Gli-A2 frequency varied between 12% and 58%. In some advanced hexaploid bread wheat cultivars the genes from locus Gli-B2 were hardly expressed (e.g., less than 5% in 'Lavett') but in others they made up more than 40% (e.g., in 'Baldus').

CONCLUSION

Here, we have shown that large differences exist in relative expression levels of alpha-gliadins from the homoeologous Gli-2 loci among wheat genotypes. Since the homoelogous genes differ in the amount of conserved CD-epitopes, screening for differential expression from the homoeologous Gli-2 loci can be employed for the pre-selection of wheat varieties in the search for varieties with very low CD-immunogenic potential. Pyrosequencing is a method that can be employed for such a 'gene family-specific quantitative transcriptome profiling'.