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

• Published in: The Plant journal : for cell and molecular biology 2017-02, Vol.89 (3), p.527-539
• Main Authors: Yano, Ryoichi, Takagi, Kyoko, Takada, Yoshitake, Mukaiyama, Kyosuke, Tsukamoto, Chigen, Sayama, Takashi, Kaga, Akito, Anai, Toyoaki, Sawai, Satoru, Ohyama, Kiyoshi, Saito, Kazuki, Ishimoto, Masao
• Format: Article
• Language: English
• Subjects: Base Sequence; Biosynthesis; Cloning; Cytochrome; Cytochrome P-450 Enzyme System - classification; Cytochrome P-450 Enzyme System - genetics; Cytochrome P-450 Enzyme System - metabolism; cytochrome P450; Enzymatic activity; Enzymes; functionalization; gene evolution; Gene Expression Regulation, Enzymologic; Gene Expression Regulation, Plant; Genetic Variation; Glycine max - genetics; Glycine max - metabolism; Health promotion; Metabolism; Metabolites; Mixed Function Oxygenases - genetics; Mixed Function Oxygenases - metabolism; Molecular Structure; Mutants; Mutation; Oleanolic Acid - analogs & derivatives; Oleanolic Acid - chemistry; Oleanolic Acid - metabolism; Phaseolus vulgaris; Phylogeny; Plant Proteins - genetics; Plant Proteins - metabolism; Saponins - chemistry; Saponins - metabolism; secondary metabolism; Secondary metabolites; soybean; Soybeans; Triterpenes - chemistry; Triterpenes - metabolism; triterpenoid saponins
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.13403

Summary 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 gene Sg‐5, which is responsible for SA biosynthesis. The naturally occurring sg‐5 mutant lacks group A saponins and has a loss‐of‐function mutation (L164*) in Glyma15g39090, 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, induced Glyma15g39090 mutants (R44* or S348P) lacked group A saponins similar to the sg‐5 mutant, indicating that Glyma15g39090 corresponds to Sg‐5. Endogenous levels of DDMP saponins were higher in the sg‐5 mutant 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 multiple Sg‐5 paralogs in a genomic region syntenic to the soybean Sg‐5 region. 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 disabling Sg‐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.