Phylogenetic diversity and symbiotic effectiveness of root-nodulating bacteria associated with cowpea in the south-west area of Japan

The phylogenetic diversity of cowpea root-nodulating bacteria in the South-West of Japan was investigated using 60 isolates. Seeds of cowpea were aseptically sown in vermiculite and inoculated with a suspension of Cowpea Soil (CS) or Bean Soil (BS) or without a soil suspension as a control. CS and BS were collected from the Kyushu University's farm (Japan) at sites where cowpea and bean, respectively, have been cultivated previously. Based on an analysis of the 16S rRNA gene and the Internal Transcribed Spacer (ITS) sequence between the 16S and 23S rRNA genes, 56 isolates were assigned to the genus Bradyrhizobium, while one isolate was found to be closely related to the genus Ralstonia. The ITS-based phylogeny showed 53 isolates, 2 isolates, and 1 isolate, to be closely related to B. yuanmingense, B. elkanii and B. japonicum, respectively, suggesting that B. yuanmingense strains predominated in the soils. Among the isolates tested, B. yuanmingense TSC10 and TTC9 exhibited a greater symbiotic activity and could be considered efficient inoculants for cowpea.