Chemotropic and Contact Responses of Phytophthora sojae Hyphae to Soybean Isoflavonoids and Artificial Substrates1

We have investigated the role of the isoflavones daidzein and genistein on the chemotropic behavior of germinating cysts of Phytophthora sojae. Hyphal germlings were shown to respond chemotropically to daidzein and genistein, suggesting that hyphal tips from zoospores that have encysted adjacent to the root may use specific host isoflavones to locate their host. Observations of the contact response of hyphal germlings were made on several different substrates in the presence and absence of isoflavones. Hyphal tips of germlings detected and penetrated pores in membranes and produced multiple appressoria on smooth, impenetrable surfaces. Hyphae that successfully penetrated the synthetic membrane were observed to grow away from the membrane surface. The presence of isoflavones in the medium surrounding the hyphal germlings did not appear to alter any of those habits. Daidzein and genistein did not inhibit germination or initial hyphal growth at concentrations up to 20 μm.

Release of Flavonoids by the Soybean Cultivars McCall and Peking and Their Perception as Signals by the Nitrogen-Fixing Symbiont Sinorhizobium fredii1

Sinorhizobium fredii strain USDA191 forms N-fixing nodules on the soybean (Glycine max L. Merr.) cultivars (cvs) McCall and Peking, but S. fredii strain USDA257 nodulates only cv Peking. We wondered whether specificity in this system is conditioned by the release of unique flavonoid signals from one of the cultivars or by differential perception of signals by the strains. We isolated flavonoids and used nodC and nolX, which are nod-box-dependent and -independent nod genes, respectively, to determine how signals activate genes in the microsymbionts. Seeds of cv McCall and cv Peking contain the isoflavones daidzein, genistein, and glycitein, as well as their glucosyl and malonylglucosyl glycosides. Roots exude picomolar concentrations of daidzein, genistein, glycitein, and coumestrol. Amounts are generally higher in cv Peking than in cv McCall, and the presence of rhizobia markedly influences the level of specific signals. Nanomolar concentrations of daidzein, genistein, and coumestrol induce expression of nodC and nolX in strain USDA257, but the relative nolX-inducing activities of these signals differ in strain USDA191. Glycitein and the conjugates are inactive. Strain USDA257 deglycosylates daidzin and genistin into daidzein and genistein, respectively, thereby converting inactive precursors into active inducers. Although neither soybean cultivar contains unique nod-gene-inducing flavonoids, strain- and cultivar-specific interactions are characterized by distinct patterns of signal release and response.

Potentiation of the bioavailability of daidzin by an extract of Radix puerariae.

The dose effect of pure daidzin on the suppression of ethanol intake in Syrian golden hamsters was compared with that of crude daidzin contained in a methanol extract of Radix puerariae (RP). EC50 values estimated from the graded dose-response curves for pure daidzin and RP extract daidzin are 23 and 2.3 mg per hamster per day, respectively. Apparently the antidipsotropic activity of the RP extract cannot be accounted for solely by its daidzin content (22 mg/g). In addition to daidzin, six other isoflavones were identified in the RP extract and quantified--namely, puerarin (160 mg per g of extract), genistin (3.7 mg/g), daidzein (2.6 mg/g), daidzein-4',7-diglucoside (1.2 mg/g), genistein (0.2 mg/g), and formononetin (0.16 mg/g). None of these, administered either alone or combined, contributes in any significant way to the antidipsotropic activity of the extract. Plasma daidzin concentration-time curves determined in hamsters administered various doses of pure daidzin or RP extract by i.p.injection indicate that the crude extract daidzin has approximately 10 times greater bioavailability than the pure compound. Reconstruction of the dose-response effects for pure and crude daidzin using bioavailable daidzin rather than administered dose gives a single curve. Synthetic daidzin added to the RP extract acquires the bioavailability of the endogenous daidzin that exists naturally in the extract. These results show that (i) daidzin is the major active principle in methanol extracts of RP, and (ii) additional constituents in the methanol extract of RP assist uptake of daidzin in golden hamsters.