Genetic diversity and relationship between Bradyrhizobium strains isolated from blackgram and cowpea

The genetic diversity of bradyrhizobial strains associated with blackgram and cowpea grown in two different agricultural soils (non-saline and saline) along the coastline of Tamil Nadu has been analysed. Phenotypically indistinguishable isolates were analysed for DNA polymorphism using random amplification of polymorphic DNA (RAPD) and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) of 16S rDNA and nifD. Although these bacteria belong to a group with a broad host range, RAPD analysis showed a considerable level of genetic diversity among the strains isolated from different host plants. Soil pH and salinity seem to have an effect on the selection of natural populations as revealed by PCR-RFLP of 16S rDNA. A combination of PCR-RFLP genotyping with nodulation studies indicates that monocropping of blackgram and the salinity of the soil have made ineffective rhizobia the dominant genotype, thereby creating an ecological burden on their other compatible hosts. A group of strains and a type strain sharing three different 16S PCR-RFLP types were shown to have the same set of symbiotic genes as inferred from the PCR-RFLP pattern of nifD. Another group of cowpea rhizobia that were found to be effective nitrogen fixers and sharing distinct 16S profiles were found to have a different set of symbiotic genes.