Metabolic switching of astringent and beneficial triterpenoid saponins in soybean is achieved by a loss-of-function mutation in cytochrome P450 72A69

    loading  Checking for direct PDF access through Ovid

Abstract

Triterpenoid saponins are major components of secondary metabolites in soybean seeds and are divided into two groups: group A saponins, and 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) saponins. The aglycone moiety of group A saponins consists of soyasapogenol A (SA), which is an oxidized β-amyrin product, and the aglycone moiety of the DDMP saponins consists of soyasapogenol B (SB). Group A saponins produce a bitter and astringent aftertaste in soy products, whereas DDMP saponins have known health benefits for humans. We completed map-based cloning and characterization of the geneSg-5, which is responsible for SA biosynthesis. The naturally occurringsg-5mutant lacks group A saponins and has a loss-of-function mutation (L164*) inGlyma15g39090, which encodes the cytochrome P450 enzyme, CYP72A69. An enzyme assay indicated the hydroxylase activity of recombinant CYP72A69 against SB, which also suggested the production of SA. Additionally, inducedGlyma15g39090mutants (R44* or S348P) lacked group A saponins similar to thesg-5mutant, indicating thatGlyma15g39090corresponds toSg-5. Endogenous levels of DDMP saponins were higher in thesg-5mutant than in the wild-type lines due to the loss of the enzyme activity that converts SB to SA. Interestingly, the genomes of palaeopolyploid soybean and the closely related common bean carry multipleSg-5paralogs in a genomic region syntenic to the soybeanSg-5region. However, SA did not accumulate in common bean samples, suggesting that Sg-5 activity evolved after gene duplication event(s). Our results demonstrate that metabolic switching of undesirable saponins with beneficial saponins can be achieved in soybean by disablingSg-5.

Significance Statement

Triterpenoid saponins in soybeans are divided into two groups: bitter and astringent group A saponins, and health-promoting DDMP saponins. Disabling a single cytochrome P450 gene (Sg-5) can switch the metabolism from the undesirable group A saponins to the beneficial DDMP saponins. This finding will help soybean breeders improve the quality and consumer acceptance of soy food products.

Related Topics

    loading  Loading Related Articles