Researchers led by Dr. WANG Guodong from the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences (CAS) have systematically investigated the physiological functions of type-A soyasaponin acetylation.
Soyasaponins, oleanane-type triterpenoid saponins abundant in soybean (Glycine max L.) seeds, especially in hypocotyls, are classified into four groups based on moieties at C21 and C22 positions of the soyasapogenol (aglycone) carbon backbone. Among them, type-A soyasaponins have hydroxyl groups at both C21 and C22, with sugar chains typically at C3 (usually three sugar units) and C22 (usually two sugar units).
Previous studies show that acetylated sugars in type-A soyasaponins contribute to the undesirable bitter taste in soybean-derived foods, while null-acetyl type-A soyasaponins lack this bitterness. However, the specific acyltransferase(s) responsible for acetylation during type-A soyasaponin biosynthesis remains unclear.
Although targeting the type-A soyasaponin acetyltransferase could improve soybean flavor characteristics, no soybean accession or artificial mutant, isolated through extensive screening, has been characterized for high accumulation of partial-/null-acetyl type-A soyasaponins in the past three decades.
In this study, WANG's team identified a candidate BAHD acyltransferase gene, GmSSAcT1, through gene coexpression analysis and the soybean hairy root system, and biochemically demonstrated that GmSSAcT1 catalyzes sequential acetylation reactions on type-A soyasaponins. Disruption of GmSSAcT1 via CRISPR/Cas9 genome editing resulted in the complete loss of type-A soyasaponins, leading to an accumulation of null-acetyl type-A soyasaponins and a seed germination defect.
While the precise mechanism of phytotoxicity in null-acetyl soyasaponin A requires further investigation, this study highlights the dual role of GmSSAcT1 as a detoxification enzyme that reduces toxic compounds in soybean seeds and as a potential target for breeding less bitter soybean varieties.
Despite the germination issue in ssact1 mutant seeds, the researchers suggested the possibility of selecting soybean varieties with reduced soyasaponin A and increased null-acetyl soyasaponin A levels from a large natural soybean population.
This work was supported by the National Key R&D Program of China, the "Priority Research Program" of CAS, and the State Key Laboratory of Plant Genomics of China.
A challenging dilemma in improving soybean seed quality. The BAHD acetyltransferase GmSSAcT1 plays a crucial role in soybean seed germination by catalyzing the conversion of null-acetyl type-A soyasaponins (which are non-bitter but toxic) to type-A soyasaponins (which are non-toxic but bitter). This enzymatic activity is essential for the successful germination of soybean seeds. Image by YUAN Jia and WANG Guodong)
