Genetic analysis of variations in the sugar chain composition at the C-3 position of soybean seed saponins

Saponins are sterols or triterpene glycosides that are widely distributed in plants. The biosynthesis of soybean saponins is thought to involve many kinds of glycosyltransferases, which is reflected in their structural diversity. Here, we performed linkage analyses of the Sg-3 and Sg-4 loci, which may control the sugar chain composition at the C-3 sugar moieties of the soybean saponin aglycones soyasapogenols A and B. The Sg-3 locus, which controls the production of group A saponin Af, was mapped to chromosome (Chr-) 10. The Sg-4 locus, which controls the production of DDMP saponin βa, was mapped to Chr-1. To elucidate the preference of sugar chain formation at the C-3 and C-22 positions, we analyzed the F(2) population derived from a cross between a mutant variety, Kinusayaka (sg-1(0)), for the sugar chain structure at C-22 position, and Mikuriya-ao (sg-3), with respect to the segregation of the composition of the group A saponins, and found that the formation of these sugar chains was independently regulated. Furthermore, a novel saponin, predicted to be A0-γg, 3-O-[β-d-galactopyranosyl (1→2)-β-d-glucuronopyranosyl]-22-O-α-l-arabinopyranosyl-soyasapogenol A, appeared in the hypocotyl of F(2) individuals with genotype sg-1(0)/sg-1(0)sg-3/sg-3.