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Title: Biogeography of a novel clade of Ensifer meliloti associated with the Australian legume Trigonella suavissima

item EARDLY, BERTRAND - Pennsylvania State University
item Elia, Patrick
item BROCKWELL, JOHN - Commonwealth Scientific And Industrial Research Organisation (CSIRO)
item GOLEMBOSKI, DANIEL - Kentucky State University
item VAN BERKUM, PETER - Retired ARS Employee

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2017
Publication Date: 3/10/2017
Citation: Eardly, B., Elia, P.E., Brockwell, J., Golemboski, D., Van Berkum, P.B. 2017. Biogeography of a novel clade of Ensifer meliloti associated with the Australian legume Trigonella suavissima. Applied and Environmental Microbiology. 83(10):e03446-16. doi: 10.1128/AEM.03446-16.

Interpretive Summary: Certain crops and trees that are legumes have importance to agriculture because specific bacteria can be associated with them to produce nitrogen. This symbiosis allows the legumes to live without added fertilizer. These legumes include three species within two closely related genera Melilotus and Trigonella. These legumes are important to agriculture as forage and cover crops. The most well-known example of these plants is alfalfa, which is the second most important legume crop produced in the U.S. The natural habitat of these three legumes extends from islands in the Atlantic through the Mediterranean Basin into Western Asia. However, the species Trigonella suavissima is unusual because it grows only in Australia outside the natural range of these legumes. Therefore, the goal of this investigation was to determine whether the bacteria associated with this Australian legume species also were unusual. The conclusion drawn from a comparison of parts of the genomes of bacteria associated with the Australian legume with a reference collection was that they belonged to the same species as the bacteria that normally associate with alfalfa. Although, they were found to be the same species, there were several characteristics regarding energy metabolism that distinguished them. This investigation contributes to the understanding of the association of bacteria with leguminous crops, how these species evolved and how they can be exploited for agricultural production and land conservation. Consequently, the results are useful to companies and other research establishments that have the mission to improve legumes and the associations with these specific bacteria.

Technical Abstract: Here we describe a novel clade within Ensifer meliloti and consider how geographic and ecological isolation contributed to the limited distribution of this group. Members of the genus Ensifer are best known for their ability to form nitrogen-fixing symbioses with forage legumes of three related genera, Medicago L., Melilotus Mill., and Trigonella L., which are members of the tribe Trifoleae. They have a natural distribution extending from the Mediterranean basin through west Asia, where there is an unsurpassed number of species belonging to these genera. Trigonella suavissima L. is unusual in that it is the only species in the tribe Trifolieae that is native to Australia. We compare the genetic diversity and taxonomic placement of rhizobia nodulating T. suavissima to members of an Ensifer reference collection. Our goal was to determine if the T. suavissima strains, like their plant host, are naturally limited to the Australian continent. We used multilocus sequence analysis to estimate the genetic relatedness of 56 T. suavissima symbionts to 28 Ensifer reference strains. Sequence data was partitioned according to the replicons upon which the loci are located. Results were used to construct replicon-specific phylogenetic trees. In both the chromosomal and chromid trees the Australian strains formed a distinct clade within E. meliloti. The strains also shared few alleles with Ensifer reference strains from other continents. Carbon source utilization assays revealed that the strains are also unusual in their ability to utilize 2- oxoglutarate as a sole carbon source. A strategy was outlined for locating similar strains elsewhere.