The Study Of Rhizobia-Host Interaction And Horizontal Transfer Of Symbiotic Island

Mutualisms are common in nature, though these symbioses can be quite permeable to cheaters in situations where one individual parasitizes the other by discontinuing cooperation yet still exploiting the benefits of the partnership. In the Rhizobiwm-legume system, there are two separate contexts, namely nodulation and nitrogen fixation processes, by which resident Rhizobium individuals can benefit by cheating. Here, we constructed reversible and non-reversible mutations in key nodulation and nitrogen-fixation pathways of R. etli and compared their interaction with plant hosts Phaseolus vulgaris with that of wild type. We show that R. etli mutants deficient in nodulation factor production are capable of intra-specific cheating, wherein mutants exploits other Rhizobium individuals capable of producing these factors. Similarly, we show that R. etli mutants are also capable of cheating inter-specifically, colonizing the host legume Phaseolus vulgaris yet contributing nothing to the partnership in terms of nitrogen fixation. Our findings indicate that cheating is possible in both of these frameworks, seemingly without damaging the stability of the mutualism itself. These results may potentially help explain observations suggesting that legume plants are commonly infected by multiple bacterial lineages during the nodulation process.Due to the host pressure and specificity, many rhizobium can not nodulate with a variety of legumes and establish of stable symbiotic relationship. Symbiosis island in A. caulinodans ORS571 genome, which contains all nodulation genes, can horizontal transfer to two different genera recipient strain and expand the host range. Symbiosis island, located between two Gly-tRNA, could be cut, cyclized and then transferred to the recipient strain, and integrated into the tRNA in recipient cell. Meanwhile, the integrase which located in the edge of symbiosis island plays a key role for the transfer of the symbiotic island.Mesorhizobium tianshanense formed nodules on licorice (Glycyrrhiza uralensis), survival in legume rhizosphere, surrounding rich canavanine secreted by legume as antimetabolites. It is known that rhizobium employed MsiA as a canavanine exporter, which is up-regulated by MsiR, in order to successfully form symbiont with legume. Here, we found that MsiR could spontaneously form dimers and bind the promoter of msiA without additional canavanine through gel shift and bacteria two hybrid examination. Then we made six truncated forms of MsiR and the results showed that the conserved HTH domain, the substrate binding domain and surface-loop domain were necessary for the function of MsiR. Random mutagenesis method was used to study functional sites of MsiR. Seven point mutants were picked out in which three of them were constitutively induced msiA expression without additional canavanine. And two of the rest four mutants were partial changed substrate specificity and other two were nearly null mutants. Taken together, it is suggested that the functional subunits and point sites play important roles in the conformation and function of MsiR regulators.