Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/26/2007
Publication Date: 10/5/2007
Citation: Van Berkum, P.B., Badri, Y., Elia, P.E., Aouani, M.E., Eardly, B.D. 2007. Chromosomal and symbiotic relationships of rhizobia nodulating Medicago truncatula and M. laciniata. Applied and Environmental Microbiology. 73(23)7597-7604. Interpretive Summary: Soil bacteria known as rhizobia form a symbiosis with legume crops such as soybean and alfalfa. In symbiosis, these rhizobia extract nitrogen gas from the atmosphere and convert it into a form that the plant uses for growth. This results in efficient crop production since growers need not apply fertilizer, which saves money and prevents pollution of the environment. Alfalfa and its relatives, growing naturally in the Mediterranean basin, have been developed as important crops and are legumes that benefit from the symbiosis with rhizobia. Among these, the species Medicago laciniata (cutleaf medick) is unusual. This species is unique because it forms a symbiosis with rhizobia that do not interact with the other Medicago species as for example Medicago truncatula (barrelclover). Since both these legume species grow naturally together in the Tunisian desert, the objective was to determine whether the rhizobia forming symbioses with both cutleaf medick and barrelclover belonged to the same bacterial population inhabiting the soil. A method was used to accurately describe the chromosomes of rhizobia that were isolated from both species growing in a Tunisian desert soil. This was coupled with an analysis of one of the genes necessary for the formation of the symbiosis. From the results it was demonstrated that the rhizobia obtained from both plant species were of the same soil population but differed in their genes for symbiosis. The inference made was that the genes for symbiosis are naturally exchanged among the different strains of rhizobia. These results are important to ecologists, evolutionary biologists and those concerned with the exchange of genes among bacteria in the environment.
Technical Abstract: MultiLocus Sequence Typing (MLST) is a sequence-based method used to characterize bacterial genomes. This method was used to examine the genetic structure of Medicago-nodulating rhizobia at the Amra site, which is located in an arid region of Tunisia. Here the annual medics M. laciniata and M. truncatula are part of the natural flora. The goal of this study was to identify whether distinct chromosomal groups of rhizobia nodulate M. laciniata because of its restricted requirement for specific rhizobia. The MLST analysis involved the determination of sequence variation in 10 chromosomal loci of 74 isolates each from M. laciniata and M. truncatula. Medicago truncatula was used as a control trap host, because unlike M. laciniata, it is relatively unrestricted in its rhizobial requirements. Allelic diversity among the plasmid-encoded nodC alleles in the isolates was also determined. The 148 isolates were placed into 26 chromosomal Sequence Types (STs), of which only 3 had been identified previously. The rhizobia of M. laciniata were shown to be part of the general Medicago-nodulating population in this soil because 99.95% of the isolates had chromosomal genotypes similar to those recovered from M. truncatula. However, the isolates recovered from M. laciniata were less diverse than those from M. truncatula and they also harbored an unusual nodC allele. This could perhaps be best explained by horizontal transfer of the different nodC alleles among members of the Medicago-nodulating rhizobial population at the field site.