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.
Highlights
In bread wheat, the genes encoding gliadins are contained mainly in six compound loci, with Gli-A1, -B1 and -D1 located on the short arms of group 1 chromosomes (1AS, 1BS and 1DS) and Gli-A2, -B2 and -D2 on the short arms of group 6 chromosomes (6AS, 6BS and 6DS)[6,7,8]
Efficient identification of gliadin genes expressed in bread wheat
Two outstanding questions in the genetic and genomic studies of gliadins are (1) how many gliadin genes exist in the bread wheat genome and (2) how many of them are expressed during grain development
Summary
The genes encoding gliadins are contained mainly in six compound loci, with Gli-A1, -B1 and -D1 located on the short arms of group 1 chromosomes (1AS, 1BS and 1DS) and Gli-A2, -B2 and -D2 on the short arms of group 6 chromosomes (6AS, 6BS and 6DS)[6,7,8]. CS is unlikely useful for genome-wide analysis of gliadin genes because of lacking Gli-D2 locus due to a natural deletion on chromosome 6D19,20 Gliadins, especially those in bread wheat, are highly heterogeneous and complex. The epitopes are generally rich in proline and glutamine, and the high content of proline renders them resistant to protease digestion[21,22,23] These epitopes bind to specific haplotypes of human leucocyte antigen (HLA) class II proteins, i.e., HLA-DQ2.2, HLA-DQ2.5, HLA-DQ8 and HLA-DQ8.5, expressed on the surface of CD4+ T cells. The data generated allowed us to identify the spectrum of gliadin genes expressed through matching full-length transcripts to their protein products, to assign the expressed gliadin genes to individual Gli loci, and to bioinformatically assess the presence of CD epitopes in all of the gliadin proteins accumulated in the grains of a bread wheat cultivar
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