Identification of Mutants in the Soyasaponin Biosynthetic Pathway Using a High-Density Soybean Mutant Library: An Indispensable Resource for Functional Genomics

The availability of whole-genome sequence accelerates functional genomics research in soybean. However, many genes involved in secondary metabolite biosynthesis are unknown. Soyasaponins are secondary metabolites with health benefits, whose biosynthetic genes have been isolated using natural mutants and data mining. Natural and induced mutants are powerful resources that help in characterizing biosynthetic pathways and breeding new varieties. We developed a high-density soybean mutant library (one mutation/74 kb) by repeated ethyl methanesulfonate treatments and screened 909 mutant lines for seed saponin composition (SSC) using thin-layer chromatography and LC-PDA/MS/MS. Of the many lines with novel SSC variations (n=33), 11 lacked or were deficient in total saponins; 13 were devoid of DDMP saponins; 6 were devoid or deficient of total saponins, but accumulated a unique saponin; 2 accumulated group A saponins without C-22 sugar chain; and 1 lacked the galactose sugar of C-3 sugar chain in both group A and DDMP saponins. Furthermore, 6 lines displayed a known SSC mutant phenotype, in which, the majority of accumulated saponin was A0-αg. It was observed that disabled function of Sg-1 results in A0-αg accumulation; however, no Sg-1 mutation was observed following sequence analysis. We speculate that mutation in other genes such as those of transcription factor(s) may affect the function of Sg-1. The objective of our continued work is to characterize all saponin mutant lines and describe the complete saponin biosynthetic pathway in the near future. Our findings highlight the significance of a high-density soybean mutant library in functional genomics research and molecular breeding.