Characterization of a new sg-5 variant with reduced biosynthesis of group A saponins in soybean (Glycine max (L.) Merr.)

Triterpenoid saponins are major secondary metabolites in soybean; they are classified into group A and DDMP saponins. Group A saponins cause bitterness and an astringent aftertaste, whereas DDMP saponins are less bitter and more beneficial to human health. Therefore, a need exists for genetic improvement to develop bitter-free soybeans without losing other beneficial saponins. In this study, an ethyl methanesulfonate (EMS)–induced mutant, PE1327, was isolated and characterized as having a reduced level of group A saponins by chromatography. The PE1327 phenotype, which exhibits no accumulation of group A saponins, resembles that of a previously reported sg-5 wild soybean mutant whose biosynthesis is controlled by the Sg-5 (Glyma.15G243300) gene. The Sg-5 gene sequence in PE1327 revealed a single-nucleotide polymorphism (G1036A) that causes an amino acid change from Glu346 to Lys346 in the sg-5 protein. Multiple alignment analysis of cytochrome P450 enzymes from numerous organisms revealed that Glu346 is highly conserved in eukaryotes, including plants and animals, but not in prokaryotes. The predicted 3D model of the Sg-5 protein showed that the Glu346 residue is located in the J helix and is likely involved in the linkage between the J and K helices and the stabilization of the J-K loop, suggesting that the amino acid substitution from Glu346 to Lys346 in the PE1327 mutant results in hypofunction of Sg-5. Co-segregation analysis revealed that the Sg-5 locus is tightly linked to reduced group A saponin biosynthesis in PE1327 and that the novel sg-5 variant is recessive to Sg-5.