Abstract
Flavonoids, secondary plant metabolites which mainly have a polyphenolic structure, play an important role in plant-microbe communications for nitrogen-fixing symbiosis. Among 10 polyphenolic compounds isolated from soybean roots in our previous study, coumestrol showed the highest antioxidant activity. In this study, its effect on the soybean nodulation was tested. The soybean symbiont Bradyrhizobium japonicum USDA110 pretreated with 20 μM coumestrol enhanced soybean nodulation by increasing the number of nodules 1.7-fold compared to the control. We also tested the effect of coumestrol on B. japonicum biofilm formation. At a concentration of 2 μM, coumestrol caused a higher degree of biofilm formation than two major soybean isoflavonoids, genistein and daidzein, although no biofilm formation was observed at a concentration of 20 μM each compound. A genome-wide transcriptional analysis was performed to obtain a comprehensive snapshot of the B. japonicum response to coumestrol. When the bacterium was incubated in 20 μM coumestrol for 24 h, a total of 371 genes (139 upregulated and 232 downregulated) were differentially expressed at a 2-fold cutoff with a q value of less than 5%. No common nod gene induction was found in the microarray data. However, quantitative reverse transcription-PCR (qRT-PCR) data showed that incubation for 12 h resulted in a moderate induction (ca. 2-fold) of nodD1 and nodABC, indicating that soybean coumestrol is a weak inducer of common nod genes. In addition, disruption of nfeD (bll4952) affected the soybean nodulation by an approximate 30% reduction in the average number of nodules.