Absence of the A4 peptide in the G4 glycinin subunit of soybean cultivar Enrei is caused by a point mutation in the Gy4 gene

Genetic mapping and validation of the loci controlling 7S α' and 11S A-type storage protein subunits in soybean [Glycine max (L.) Merr.]

KEY MESSAGE

Four soybean storage protein subunit QTLs were mapped using bulked segregant analysis and an F2 population, which were validated with an F5 RIL population. The storage protein globulins β-conglycinin (7S subunit) and glycinin (11S subunits) can affect the quantity and quality of proteins found in soybean seeds and account for more than 70% of the total soybean protein. Manipulating the storage protein subunits to enhance soymeal nutrition and for desirable tofu manufacturing characteristics are two end-use quality goals in soybean breeding programs. To aid in developing soybean cultivars with desired seed composition, an F2 mapping population (n = 448) and an F5 RIL population (n = 180) were developed by crossing high protein cultivar 'Harovinton' with the breeding line SQ97-0263_3-1a, which lacks the 7S α', 11S A1, 11S A2, 11S A3 and 11S A4 subunits. The storage protein composition of each individual in the F2 and F5 populations were profiled using SDS-PAGE. Based on the presence/absence of the subunits, genomic DNA bulks were formed among the F2 plants to identify genomic regions controlling the 7S α' and 11S protein subunits. By utilizing polymorphic SNPs between the bulks characterized with Illumina SoySNP50K iSelect BeadChips at targeted genomic regions, KASP assays were designed and used to map QTLs causing the loss of the subunits. Soybean storage protein QTLs were identified on Chromosome 3 (11S A1), Chromosome 10 (7S α' and 11S A4), and Chromosome 13 (11S A3), which were also validated in the F5 RIL population. The results of this research could allow for the deployment of marker-assisted selection for desired storage protein subunits by screening breeding populations using the SNPs linked with the subunits of interest.

Molecular analysis of glycinin genes in soybean mutants for development of gene-specific markers

Soybean mutant lines that differ in 11S glycinin and 7S β-conglycinin seed storage protein subunit compositions were developed. These proteins have significant influence on tofu quality. The molecular mechanisms underlying the mutant lines are unknown. In this study, gene-specific markers for five of the glycinin genes (Gy1 to Gy5) were developed using three 11S null lines, two A(4) null Japanese cultivars, Enrei and Raiden, and a control cultivar, Harovinton. Whereas gene-specific primers produced the appropriate products in the control cultivar for the Gy1, Gy2, Gy3 and Gy5 genes, they did not amplify in mutants missing the A(1a)B(2), A(2)B(1a), A(1b) B(1b), and A(3)B(4) subunits. However, ecotype targeting induced local lesions in genomes (EcoTILLING) and sequencing analysis revealed that the absence of the A(4) peptide in the mutants is due to the same point mutation as that in Enrei and Raiden. Selection efficiency of the gene-specific primer pairs was tested using a number of breeding lines segregating for the different subunits. Primer pairs specific to each of the Gy1, Gy2, Gy3, and Gy5 genes can be used to detect the presence or absence of amplification in normal or mutant lines. The Gy4 null allele can be selected for by temperature-switch PCR (TS-PCR) for identification of the A(4) (G4) null genotypes. In comparison to protein analysis by SDS-PAGE, gene-specific markers are easier, faster and more accurate for analysis, they do not have to use seed, and can be analyzed at any plant growth stage for marker-assisted selection.

Identification of several gy4 nulls from the USDA soybean germplasm collection provides new genetic resources for the development of high-quality tofu cultivars

Tofu, a cheese-like food made by curdling soy milk, is a major dietary staple of Asian countries. Consumption of tofu and other soy products is steadily increasing in North America due to its well-known health benefits. Soybean A(5), A(4), and B(3) peptide null lines 'Enrei' and 'Raiden' are commonly utilized in breeding programs to develop high-quality tofu cultivars. To expand the genetic diversity it is desirable to identify and utilize other A(5), A(4), and B(3) null genotypes in the development of improved tofu cultivars that are adapted to North American conditions. In this study were screened diverse soybean accessions from the USDA Soybean Germplasm Collection to identify Gy4 mutants, the locus that controls A(5), A(4), and B(3) peptide production. Analysis of total seed proteins from 485 soybean lines by SDS-PAGE enabled the identification of 38 accessions that lacked the A(5), A(4), and B(3) peptides. These accessions showed marked differences in seed size and seed coat color and represented different maturity groups ranging from 0 to IX. To ascertain the molecular basis for the lack of A(5), A(4), and B(3) peptides in the newly identified Gy4 mutants, the nucleotide sequence of a portion of the Gy4 gene was determined from eight soybean accessions representing different maturity groups. These eight Gy4 mutants revealed a single point mutation that changed the translation initiation codon ATG to ATA, resulting in the A(5), A(4), and B(3) null phenotype. The newly identified Gy4 mutants from this study will enable plant breeders to expand the genetic diversity of North American food-quality soybeans and also aid in the development of hypoallergenic soybeans.

Proteomic and genetic analysis of glycinin subunits of sixteen soybean genotypes

We investigated proteomic and genomic profiles of glycinin, a family of major storage proteins in 16 different soybean genotypes consisting of four groups including wild soybean (Glycine soja), unimproved cultivated soybean landraces from Asia (G. max), ancestors of N. American soybean (G. max), and modern soybean (G. max) genotypes. We observed considerable variation in all five glycinin subunits, G1, G2 G3, G4 and G5 using proteomics and genetic analysis. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass spectrometry (MS) analysis showed that the wild genotypes had a range of 25-29 glycinin protein spots that included both acidic and basic polypeptides followed by the ancestors with 24-28, modern cultivars with 24-25, and landraces with 17-23 protein spots. Overall, the wild genotypes have a higher number of protein spots when compared to the other three genotypes. Major variation was observed in acidic polypeptides of G3, G4 and G5 compared to G1 and G2, and minor variation was observed in basic polypeptides of all subunits. Our data indicated that there are major variations of glycinin subunits between wild and cultivated genotypes rather than within the same groups. Based on Southern blot DNA analysis, we observed genetic polymorphisms in group I genes (G1, G2, and G3) between and within the four genotype groups, but not in group II genes (G4 and G5). This is the first study reporting the comparative analysis of glycinin in a diverse set of soybean genotypes using combined proteomic and genetic analysis.