Phenotypic and genotypic characterization of soybean and cowpea bradyrhizobia isolated from United States, Thai, and Chinese soils

Diversity of indigenous soybean and cowpea bradyrhizobia isolated from the United States and Thailand and soybean bradyrhizobia isolated from China was examined using phenotypic analyses: serological reactions (ELISA) with antisera produced against reference soybean and cowpea-miscellany strains, carbon substrate utilization (BIOLOG GN system), and fatty acid methyl esters (FAME) profiles (MIDI-Microbial Identification System), and genetic analyses: restriction fragment length polymorphism of PCR-amplified 16S rRNA gene (PCR-RFLP) with endonuclease DdeI and BOX-PCR fingerprinting. All phenotypic groupings revealed considerable diversity among the strains and confirmed the similarity between soybean and cowpea bradyrhizobia. Bradyrhizobia contained in one phenotypic group were diverse in other corresponding characteristics. Serogroupings were primarily associated with geographical locations. Soybean isolates were often similar to Bradyrhizobium japonicum, whereas cowpea isolates were similar to Bradyrhizobium elkanii with respect to serological and FAME phenotypes. Groupings based on carbon substrate utilization pattern were in some agreement with serogroupings. PCR-RFLP analysis of the 16S rRNA gene yielded groupings corresponding well with geographical and host origins of the strains. Most of soybean and cowpea bradyrhizobia from the United States were related to B. japonicum, whereas those from Thailand were related to B. elkanii based on PCR-RFLP analysis. The BOX-PCR fingerprinting further confirmed similarity between soybean and cowpea bradyrhizobia from the United States and Thailand. The plant nodulation results indicated that many of cowpea bradyrhizobia were not typical cowpea-miscellany strains and could be very effective in symbiosis with soybean. Overall, most experimental groupings indicated that many of soybean and cowpea bradyrhizobia were similar, although the diversity revealed by the different analyses were not necessarily in good agreement with one another.