Dynamics of symbiotic relationship of soybean with Bradyrhizobium diazoefficiens and involvements of root-secreted daidzein behind the continuous cropping

Abstract

Symbiosis with a selectively nodulating rhizobium strain, such as Bradyrhizobium diazoefficiens USDA110, improves soybean productivity. In continuous cropping, however, the concentrations of root exudates – of which daidzein is one of the major secretions – might influence nodulating rhizobium communities and cause ineffective nitrogen fixation in soybean. The dynamics of indigenous rhizobia that form nodules with the soybean cultivars Tamahomare, Enrei and Tamabaguro at three cycles of a continuous cropping plot under different experimental conditions were examined. PCR-RFLP analysis targeted to the 16S-23S rRNA gene internal transcribed spacer (ITS) region of a bacterium, confirmed by sequencing analysis, was used to determine the bacterial type for each root nodule. The results showed that as the number of cycles increased, the proportion of B. diazoefficiens USDA110 of all cultivars decreased. Under all experimental conditions, the average differences between the proportion of B. diazoefficiens USDA110 of the first and third cycles in Tamahomare, Enrei and Tamabaguro were 67%, 70% and 44%, respectively. After the HPLC analysis of the root-secreted isoflavonoids, the results showed that the root-secreted daidzein from Tambaguro was excreted in the highest levels among the three cultivars. Increase of the proportion of B. diazoefficiens USDA110 among seedlings treated with daidzein was also noted. Therefore, the present study reports that the symbiotic relationship of soybean with B. diazoefficiens USDA110 decreases in continuous cropping and suggests that root-secreted daidzein helps to maintain populations of this rhizobial strain in rhizosphere of some cultivars.