|Van Berkum, Peter|
Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2001
Publication Date: N/A
Interpretive Summary: Globally, soybean is the most important grain legume. Because soybean is a legume it benefits from biological nitrogen fixation through a symbiosis with soil bacteria of two different genera. In agriculture, the benefit is in the form of 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 by industry. The problem is that many different bacteria are available, but comprehensive investigations for evolutionary differences have not been done. Here we report that the bacterial symbionts vary genetically and we have distinguished them by tracing their evolution. We report the analysis of a novel section of the bacterial chromosome for determining the evolutionary relationships among strains that have been poorly characterized. We concluded that these strains are as diverse as strains for which more information exists Our results will be useful to scientists who are interested in evolutionary biology, taxonomy and strain identification.
Technical Abstract: The USDA, ARS National Rhizobium Germplasm Collection contains 143 accessions of slow-growing soybean strains among which there are 17 distinct serological groups. However, eleven strains appear to have no serological affinity with the 17 serogroups. Therefore, we determined whether these strains were diverse and examined their phylogenetic placement. Nine strains formed nitrogen-fixing symbioses with soybean indicating that these accessions were not contaminants. We concluded from results of Amplified Fragment Length Polymorphism (AFLP) analysis, using three selective primers with 8 strains, that they were genetically dissimilar. Nine strains were examined for their fatty acid composition using Fatty Acid Methyl Ester derivatives (FAME). The FAME results with 5 strains and serotype strains of B. elkanii were similar, while results with each of the remaining two pairs were either similar to the type strain of B. japonicum (USDA 6) or to USDA 110. Evolutionary history of 9 strains was reconstructed from sequence divergence of a combination of the complete 16S rRNA gene, the Internally Transcribed Space region and about 400 bases of the 5' end of the 23S rRNA gene. Placement of 5 strains was nested within B. elkanii, of 2 was with USDA 110, and with the other 2 was with USDA 6. We concluded that soybean isolates that cannot be placed within one of the 17 established serogroups are phenotypically and genetically as diverse as the serotype strains.