Genome-wide analysis of complex wheat gliadins, the dominant carriers of celiac disease epitopes

Abstract

Gliadins, specified by six compound chromosomal loci (Gli-A1/B1/D1 and Gli-A2/B2/D2) in hexaploid bread wheat, are the dominant carriers of celiac disease (CD) epitopes. Because of their complexity, genome-wide characterization of gliadins is a strong challenge. Here, we approached this challenge by combining transcriptomic, proteomic and bioinformatic investigations. Through third-generation RNA sequencing, full-length transcripts were identified for 52 gliadin genes in the bread wheat cultivar Xiaoyan 81. Of them, 42 were active and predicted to encode 25 α-, 11 γ-, one δ- and five ω-gliadins. Comparative proteomic analysis between Xiaoyan 81 and six newly-developed mutants each lacking one Gli locus indicated the accumulation of 38 gliadins in the mature grains. A novel group of α-gliadins (the CSTT group) was recognized to contain very few or no CD epitopes. The δ-gliadins identified here or previously did not carry CD epitopes. Finally, the mutant lacking Gli-D2 showed significant reductions in the most celiac-toxic α-gliadins and derivative CD epitopes. The insights and resources generated here should aid further studies on gliadin functions in CD and the breeding of healthier wheat.

Analysis of the Gli‐D2 locus identifies a genetic target for simultaneously improving the breadmaking and health‐related traits of common wheat

“…Nevertheless, there were seven α‐gliadin genes with an intact ORF for both loci (Figure ). The finding of seven active α‐gliadin genes carried by Gli‐D2‐Xy81 was in agreement with the identification of seven distinct α‐gliadin full‐length gene transcripts with an intact ORF for Gli‐D2‐Xy81 in our previous study (Wang et al ., ).…”

“…Reducing gliadin expression can make space for increased accumulation of other seed proteins that are relatively rich in lysine (Gil‐Humanes et al ., , ). Our previous study showed that DLGliD2 had a significantly decreased CD epitope content when assayed with the monoclonal antibody G12 recognizing a hexapeptide (QPQLPY) located in the 33‐mer motif (Morón et al ., ; Wang et al ., ). QPQLPY was present in six of the seven α‐gliadins encoded by Gli‐D2 (Figure S3).…”

“…Increased grain lysine content and reduction of highly toxic CD epitopes are additional benefits conferred by Gli‐D2 deletion in wheat breeding, although the elevated glutenin content due to this deletion is not beneficial for NCGS patients and not safe for CD patients. In practical breeding programs, transfer of the Gli‐D2 deletion may be aided by the use of the microsatellite markers reported previously (Wang et al ., ). Alternatively, as shown in this work (Figure S2), new DNA markers can be developed using the Gli‐D2 sequence information reported for Ae.…”

“…Owing to the involvement of gliadins in multiple types of wheat sensitivities, substantial efforts have been devoted to the development of low-gliadin wheat through various approaches, i.e. silencing gliadin gene expression by RNA interference (RNAi), preparation of mutants lacking one or more gliadin species (loci) and CRISPR/Cas9mediated genome editing of multiple gliadin genes (Gil-Humanes et al, 2010, 2014aBecker et al, 2012;Altenbach et al, 2014;S anchez-Le on et al, 2018;Wang et al, 2017a). The resultant lines showed reduced levels of CD or WDEIA epitopes; some of them also exhibited improved end-use quality owing to an elevated glutenin/ gliadin (Glu/Gli) ratio (Gil-Humanes et al, 2010, 2014aBecker et al, 2012;Altenbach et al, 2014), which is vital for promoting gluten and dough functionalities by forming more functional GMPs (Wrigley et al, 2009;MacRitchie, 2014;Rasheed et al, 2014).…”

“…Although a recent study reported an evolutionary analysis of Gli-A2, -B2 and -D2 in the common wheat landrace Chinese Spring (CS) (Huo et al, 2018), there is still no structural information about these loci in commercial wheat. To improve our understanding of the six Gli loci we previously developed six deletion mutants, each lacking one Gli locus, using Xiaoyan 81 (Xy81), a commercial winter wheat cultivar, as the wild type (WT) progenitor (Wang et al, 2017a). These mutants have been used for identifying of and assigning to specific chromosomes the full-length transcripts of gliadin genes and their corresponding protein products.…”