Researchers from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences (CAS) have uncovered the long-elusive biosynthetic pathway of glycitein, a key soybean isoflavonoid. They also revealed how its production determines the plant's resistance to Phytophthora sojae.
Their findings were published in
PNAS on December 11.
Soybeans are one of the world's most important food and economic crops, valued for their rich isoflavonoid content. These compounds promote human health and play diverse roles in plant immune responses. Despite decades of research, the biosynthesis of glycitein-type isoflavonoids has remained controversial, and the enzyme catalyzing the key reaction step had remained unidentified. Earlier studies proposed that glycitein might be synthesized via a flavanone 6-hydroxylation pathway; however, direct experimental evidence and its physiological relevance were lacking.
To address this issue, the researchers led by WANG Guodong employed metabolome-based genome-wide association studies (mGWAS) in a natural soybean population. Their analysis pinpointed a cytochrome P450 gene, GmIF6H1 (formerly GmCYP76F17). Through heterologous expression in yeast, enzymatic assays, and stable isotope labeling experiments, the researchers demonstrated for the first time that GmIF6H1 specifically catalyzes the A-ring 6-hydroxylation of daidzein, producing 6-hydroxydaidzein, which is subsequently methylated to form glycitein. This discovery overturns the long-standing flavanone-centered biosynthetic model and establishes a new glycitein biosynthetic pathway with daidzein as the central precursor.
The researchers also found that a threonine-to-alanine substitution at amino acid position 248 (T248A) in GmIF6H1 markedly affects enzymatic activity. Interestingly, this residue was subject to selection during soybean domestication, which resulted in reduced glycitein accumulation in cultivated varieties.
Mechanistic studies showed that glycitein-type isoflavonoids and glyceollins—the latter of which are synthesized via the 2′-hydroxylation of daidzein—act synergistically to defend soybeans against infection by P. sojae. Under normal growth conditions, glycitein accumulates in advance as a phytoanticipin, providing a preformed chemical defense. Upon pathogen invasion, glyceollins are rapidly induced as phytoalexins.
Intriguingly, both knockout and overexpression of GmIF6H1 rendered soybean plants more susceptible to P. sojae: Knockout mutants exhibited compromised defense due to the absence of glycitein, whereas overexpression lines disrupted metabolic flux distribution, leading to impaired glyceollin production. These findings reveal a finely tuned metabolic balance between isoflavonoid branches that optimizes disease resistance in soybeans.
This study elucidates the biosynthetic origin of glycitein and its functional role in plant immunity and identifies GmIF6H1 as a promising molecular target for disease-resistant soybean breeding. Precise modulation of GmIF6H1 expression could enable the development of soybean varieties with optimized isoflavonoid composition and enhanced resistance to pathogens, thereby improving crop sustainability.
Building on these findings, the researchers aim to integrate synthetic biology strategies to design "smart soybean" varieties that combine improved health benefits with enhanced stress tolerance.
This work was supported by the National Major Scientific and Technological Project for Biological Breeding, the National Key R&D Program of China, and the Strategic Priority Research Program of CAS.
Mechanistic analysis of GmIF6H1, a newly identified component of the soybean isoflavone biosynthetic pathway, in regulating resistance to Phytophthora sojae (Image by IGDB)
