Complete deficiency of endogenous soyasaponins reduces seed antioxidant capacity and alters specific rhizosphere bacterial interactions in soybean

Soyasaponins, triterpenoid saponins commonly found in soybeans, play a well-established role in chemical defense; however, their necessity for other biological functions remains to be validated. Because isolating individual soyasaponins is technically challenging, GmBAS1-disrupted (bas1) mutants lacking all soyasaponin groups were generated to investigate the distinct biological roles of endogenous soyasaponins. Results revealed that 2,2-diphenyl-1-picrylhydrazyl antioxidant capacity in bas1 seeds was significantly lower than that in controls. Because KEGG enrichment analysis of differentially expressed genes indicated altered flavonoid and phenylpropanoid biosynthesis in bas1 hypocotyls, we quantified total polyphenols. Although total polyphenol levels in bas1 hypocotyls were significantly higher than those in controls, the bas1 mutant exhibited lower radical-scavenging activity. Thus, soyasaponins, rather than polyphenols, primarily determine antioxidant capacity in soybean hypocotyls. Moreover, 16S rRNA gene amplicon sequencing showed that Novosphingobium relative abundance was significantly decreased in the bas1 rhizosphere, despite no detectable changes in the overall bacterial community structure. To our knowledge, this research provides the first direct evidence that endogenous soyasaponins are required to establish a specific association between soybean roots and rhizobacteria. Collectively, these results highlight the diverse and specialized roles of soyasaponins in soybean physiology.