|Van Berkum, Peter|
Submitted to: Archives of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2003
Publication Date: 11/21/2003
Citation: Van Berkum, P.B., Beyene, D., Kassa, S., Asseffa, A., Gebremedhin, T. 2003. Ethiopian soils harbor natural populations of rhizobia that form symbioses with common bean (Phaseolus Vulgaris L.).. Archives for Microbiology. 181(2): 129-136.
Interpretive Summary: Common bean is an important grain legume that benefits from biological nitrogen fixation through a symbiosis with soil bacteria collectively known as rhizobia. In agriculture, the benefit is enhanced efficiency of crop production. Management of biological nitrogen fixation involves the inoculation of the appropriate bacterial cultures at the time of sowing. These bacterial cultures are available to the farmer as inoculants manufactured in industry. The problem is that many different bacteria are available, but comprehensive investigations for evolutionary, genetic and symbiotic differences have not been done. Here we report that a bean cultivated in Ethiopia forms a symbiosis with local rhizobia even though the crop was introduced from the Americas. We obtained evidence that the genomes of these rhizobia had characteristics of an indigenous species, but that the ribosomal genes were similar to bean-nodulating rhizobia of American origin. To explain these results we suggested that the ribosomal genes of the Ethiopian rhizobia and those from the Americas were similar because of convergent evolution. Our results will be useful to scientists who are interested in evolutionary biology, taxonomy and strain identification, and the legume symbiosis.
Technical Abstract: The diversity of 83 bean-nodulating rhizobia indigenous to Ethiopian soils was characterized by PCR-RFLP of the Internally Transcribed Space (ITS) region between the 16S and 23S rRNA genes, 16S rRNA gene sequence analysis, Multilocus Enzyme Electrophoresis (MLEE), and Amplified Fragment Length Polymorphism (AFLP). The isolates fell into 13 distinct genotypes according to analysis of the ITS region. The majority of these genotypes (70%) were genetically related to the type strain for R. leguminosarum. However, from analysis of their 16S rRNA genes the majority was placed with R. etli. Therefore, it was concluded that the Ethiopian bean-nodulating R. leguminosarum had divergent 16S rRNA alleles reminiscent of R. etli. Transfer and recombination of the 16S rRNA gene from introduced R. etli to local R. leguminosarum is a possiblity, but little evidence was obtained to support such a conclusion because R. tropici was not present and the single isolate that was characterized as R. etli was ineffective for nitrogen fixation and originated from a remote location. Therefore, convergent 16S rRNA gene nucleotide sequences would seem a more likely explanation for the disparity between the 16S rRNA gene sequencing and isozyme results. The formation of symbioses between Ethiopian R. leguminosarum and common bean possibly may be the result of the permissive nature of this legume host for nodulation rather than transfer of genetic determinants for symbiosis of bean between introduced and local rhizobia.