A recently evolved BAHD acetyltransferase, responsible for bitter soyasaponin A production, is indispensable for soybean seed germination

ABSTRACT Soyasaponins are major small molecules that accumulate in soybean (Glycine max) seeds. Among them, type‐A soyasaponins, fully acetylated at the terminal sugar of their C22 sugar chain, are responsible for the bitter taste of soybean‐derived foods. However, the molecular basis for the acetylation of type‐A soyasaponins remains unclear. Here, we identify and characterize GmSSAcT1, encoding a BADH‐type soyasaponin acetyltransferase that catalyzes three or four consecutive acetylations on type‐A soyasaponins in vitro and in planta. Phylogenetic analysis and biochemical assays suggest that GmSSAcT1 likely evolved from acyltransferases present in leguminous plants involved in isoflavonoid acylation. Loss‐of‐function mutants of GmSSAcT1 exhibited impaired seed germination, which attribute to the excessive accumulation of null‐acetylated type‐A soyasaponins. We conclude that GmSSAcT1 not only functions as a detoxification gene for high accumulation of type‐A soyasaponins in soybean seeds but is also a promising target for breeding new soybean varieties with lower bitter soyasaponin content. Type‐A soyasaponins that are fully acetylated at the terminal sugar of their C22 sugar chain are responsible for the bitter taste of soybean‐derived foods. GmSSAcT1 catalyzes consecutive acetylations of type‐A soyasaponins. GmSSAcT1 likely evolved from isoflavonoid acyltransferase genes in leguminous plants and loss‐of‐function mutants of GmSSAcT1 exhibited impaired seed germination.