Genetic diversity of Lotus tenuis rizobial symbionts is mediated by horizontal transfer of a symbiotic island

ESTRELLA, MARIA JULIA; TONUCCI, FACUNDO; SANNAZZARO, ANALIA I

Santander

Conferencia; International Plasmid Biology Conference 2012; 2012

International Society for Plasmid Biology and other Mobile Genetic Elements

Background and aims: The utilization of Lotus tenuis to obtain a high productivity of pastures in soils of the Flooding Pampa (Buenos Aires, Argentina) is a useful agronomic strategy for cattle production. Mesorhizobium loti was traditionally assumed as the common N-fixing symbiont of L. tenuis. In a previous study we have taxonomically characterized native rhizobia isolated from L. tenuis in constrained environments of the Flooding Pampa. A majority of these isolates were related with different species of Mesorhizobium genera but, surprisingly, some of L. tenuis symbionts were closely related to rhizobial species that have been shown to be common symbionts of Phaseolus vulgaris. This was the first time that L. tenuis was identified as the primary host of a Rhizobium sp. strain. Aditionally, one of the isolates was closely related to Aminobacter, an organism that so far has never been found in bacterial-plant interactions. In Rhizobium species, symbiotic genes are found in plasmids, while in Mesorhizobium species they are located in cromosomal islands. There are few reports in which it has been shown that these islands could be transferred to other Mesorhizobium strains. The aim of this work was to study the horizontal gene transfer of the Mesorhizobium loti symbiosis island to other soil bacteria. Methods: Rhizobial strains analyzed belong to a previously characterized collection from the Flooding Pampa (Estrella et al. 2009). Genomic DNA was isolated and digested with EcoRI, plasmids of bacterial strains were separated according to Eckhardt method. Southern blot was performed by standard methods. Amplification of symbiosis island insertion site in L. tenuis isolates was performed according with the methodology described by Nandasena et al. (2006). Nodulation assays were carried on a growth plant chamber under controlled conditions. Results: Bacteria from genera other than Mesorhizobium were capable of nodulating L. tenuis with the same efficiency than the usual symbiont, but none of them was found to nodulate Phaseolus vulgaris. NodC and nifH genes were found in Rhizobium and Aminobacter isolates, and they weren?t located on plasmids, but on the chromosome. A symbiosis island related to L. tenuis-nodulating mesorhizobia was found next to the chromosomal Phe tRNA insertion in the new isolates. Conclusions: The set of genes that confer the capacity to nodulate L. tenius were transferred from its usual symbiont to diverse genera of soil bacteria. Thus, non-symbiotic bacteria as well as non-specific rhizobia gained the ability to nodulate this legume. This is the first report where the transfer of a symbiotic island from Mesorhizobium to other genera of bacteria was shown. In this way, soil bacteria better adapted to certain environmental conditions could be converted into L. tenuis efficient symbionts.