Synthesis and Secretion of Isoflavones by Field-Grown Soybean

Abstract

Isoflavones play important roles in rhizosphere plant-microbe interactions. Daidzein and genistein secreted by soybean roots induce the symbiotic interaction with rhizobia and may modulate rhizosphere interactions with microbes. Yet despite their important roles, little is known about the biosynthesis, secretion and fate of isoflavones in field-grown soybeans. Here, we analyzed isoflavone contents and the expression of isoflavone biosynthesis genes in field-grown soybeans. In roots, isoflavone contents and composition did not change with crop growth, but the expression of UGT4, an isoflavone-specific 7-O-glucosyltransferase, and of ICHG (isoflavone conjugates hydrolyzing beta-glucosidase) was decreased during the reproductive stages. Isoflavone contents were higher in rhizosphere soil than in bulk soil during both vegetative and reproductive stages, and were comparable in the rhizosphere soil between these two stages. We analyzed the degradation dynamics of daidzein and its glucosides to develop a model for predicting rhizosphere isoflavone contents from the amount of isoflavones secreted in hydroponic culture. Conjugates of daidzein were degraded much faster than daidzein, with degradation rate constants of 8.51 d-1 for malonyldaidzin and 11.6 d-1 for daidzin, vs. 9.15 × 10-2 d-1 for daidzein. The model suggested that secretion of isoflavones into the rhizosphere is higher during vegetative stages than during reproductive stages in field-grown soybean.

Rhizosphere modelling reveals spatiotemporal distribution of daidzein shaping soybean rhizosphere bacterial community

“…Daidzein is known to induce the expression of nod genes in rhizobia to initiate symbiosis, but our previous observation that the daidzein concentration remained consistent throughout growth stages in fieldgrown soybean (Sugiyama et al, 2017) suggests that daidzein also performs other functions. To determine the effects of daidzein on soil bacteria, a steady daidzein concentration was maintained in a test tube containing soil collected from the soybean field.…”

“…The adsorption of daidzein on soils and minerals was caluculated by subtracting the amount of daidzein in the aqueous phase from the added amount in the initial solution (Liang et al, ). The degradation of daidzein was negligible during adsorption experiments, because half‐life of daidzein was calculated to be more than 7 days (Sugiyama et al, ).…”

“…In contrast to primary metabolites, the stability of plant specialized metabolites varies depending on their chemical structure and biological activity (Sugiyama & Yazaki, ). For example, strigolactones are unstable in soil, making them a signal of living plant roots (Ruyter‐Spira, Al‐Babili, van der Krol, & Bouwmeester, ; Seto, Kameoka, Yamaguchi, & Kyozuka, ), whereas flavonoids are relatively stable in soil (Sugiyama, Yamazaki, Hamamoto, Takase, & Yazaki, ).…”

“…, whereas flavonoids are relatively stable in soil (Sugiyama, Yamazaki, Hamamoto, Takase, & Yazaki, 2017).…”

“…Soybeans secrete daidzein into the rhizosphere to initiate symbiosis with rhizobia (Kosslak, Bookland, Barkei, Paaren, & Appelbaum, 1987). Although nodulation occurs predominantly during early vegetative stages (Calvert, Pence, Pierce, Malik, & Bauer, 1984), we previously found that daidzein was present in the rhizosphere throughout all growth stages (Sugiyama et al, 2017). These results suggest that daidzein plays additional roles in shaping the rhizosphere microbial communities.…”