Composition, variation, expression and evolution of low-molecular-weight glutenin subunit genes in Triticum urartu

Abstract

Background: Wheat (AABBDD, 2n = 6x = 42) is a major dietary component for many populations across the world. Bread-making quality of wheat is mainly determined by glutenin subunits, but it remains challenging to elucidate the composition and variation of low-molecular-weight glutenin subunits (LMW-GS) genes, the major components for glutenin subunits in hexaploid wheat. This problem, however, can be greatly simplified by characterizing the LMW-GS genes in Triticum urartu, the A-genome donor of hexaploid wheat. In the present study, we exploited the high-throughput molecular marker system, gene cloning, proteomic methods and molecular evolutionary genetic analysis to reveal the composition, variation, expression and evolution of LMW-GS genes in a T. urartu population from the Fertile Crescent region.

Results: Eight LMW-GS genes, including four m-type, one s-type and three i-type, were characterized in the T. urartu population. Six or seven genes, the highest number at the Glu-A3 locus, were detected in each accession. Three i-type genes, each containing more than six allelic variants, were tightly linked because of their co-segregation in every accession. Only 2-3 allelic variants were detected for each m- and s-type gene. The m-type gene, TuA3-385, for which homologs were previously characterized only at Glu-D3 locus in common wheat and Aegilops tauschii, was detected at Glu-A3 locus in T. urartu. TuA3-460 was the first s-type gene identified at Glu-A3 locus. Proteomic analysis showed 1-4 genes, mainly i-type, expressed in individual accessions. About 62% accessions had three active i-type genes, rather than one or two in common wheat. Southeastern Turkey might be the center of origin and diversity for T. urartu due to its abundance of LMW-GS genes/genotypes. Phylogenetic reconstruction demonstrated that the characterized T. urartu might be the direct donor of the Glu-A3 locus in common wheat varieties.

Conclusions: Compared with the Glu-A3 locus in common wheat, a large number of highly diverse LMW-GS genes and active genes were characterized in T. urartu, demonstrating that this progenitor might provide valuable genetic resources for LMW-GS genes to improve the quality of common wheat. The phylogenetic analysis provided molecular evidence and confirmed that T. urartu was the A-genome donor of hexaploid wheat.

Figure 1

Electropherograms of DNA fragments detected in accession PI428198 using the LMW-GS gene molecular marker system. The horizontal axis shows the detected fragment sizes, and the vertical axis displays the signal intensities during the capillary electrophoresis. The orange peaks were size standard DNA fragments in the GeneScan 1200 LIZ and each blue peak represents a LMW-GS gene.

Figure 2

Separation and identification of LMW-GS proteins in T. urartu using 2-DE and SDS-PAGE. (A-D) are 2-DE (left) and SDS-PAGE (right) gels of PI428270 (U10), PI428335 (U8), PI428202 (U2), and PI428255 (U9); LMW-GS protein spots are circled in 2-DE gels. (E) is the SDS-PAGE banding pattern of LMW-GS proteins in 15 genotypes of T. urartu and two representatives of common wheat: Chinese spring (CS) and Xiaoyan54 (XY54). *, #, + and $ denote protein bands of active allelic variants of TuA3-397/TuA3-400, TuA3-502, TuA3-538 and TuA3-576 genes, respectively.

Figure 3

Phylogenetic reconstruction of all the LMW-GS genes and their allelic variants identified in the T. urartu population. All LMW-GS genes were divided into three groups, consistent with the i-, s- and m-type genes.

Figure 4

Geographic distribution of LMW-GS genes/allelic variants and genotypes in T. urartu . (A) Geographic distribution of LMW-GS genes/allelic variants. (B) Geographic distribution of LMW-GS genotypes. Iraq and Iran were not considered for only one accession was collected in each. SE Turkey stands for southeastern Turkey, and NE Lebanon for northeastern Lebanon.

Figure 5

Phylogenetic analysis of deduced protein sequences of LMW-GS genes in T. urartu and common wheat. (A) Phylogenetic analysis of m- and s-type genes. (B) Phylogenetic analysis of i-type genes. Triangle represents sequences from common wheat. The gliadin protein sequence (AFF27498) was used as the out-group.