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Symbiotic Effectiveness, Phylogeny and Genetic Stability of Biserrula pelecinus-nodulating Mesorhizobium sp. isolated from Eritrea and Ethiopia

Bekuma, Amanuel (2017) Symbiotic Effectiveness, Phylogeny and Genetic Stability of Biserrula pelecinus-nodulating Mesorhizobium sp. isolated from Eritrea and Ethiopia. PhD thesis, Murdoch University.

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Abstract

Biserrula pelecinus is a productive pasture legume with potential for replenishing soil fertility and providing quality livestock feed in Southern Australia. The experience with growing B. pelecinus in Australia suggests an opportunity to evaluate this legume in Ethiopia, due to its relevance to low-input farming systems such as those practiced in Ethiopia. However, the success of B. pelecinus is dependent upon using effective, competitive, and genetically stable inoculum strains of root nodule bacteria (mesorhizobia). Mesorhizobium strains isolated from the Mediterranean region were previously reported to be effective on B. pelecinus in Australian soils. Subsequently, it was discovered that these strains transferred genes required for symbiosis with B. pelecinus (contained on a “symbiosis island’ in the chromosome) to non-symbiotic soil bacteria. This transfer converted the recipient soil bacteria into symbionts that were less effective in N2-fixation than the original inoculant. This study investigated selection of effective, stable inoculum strains for use with B. pelecinus in Ethiopian soils. Genetically diverse and effective mesorhizobial strains of B. pelecinus were shown to be present in Ethiopian and Eritrean soils. These strains were shown to belong to the genus Mesorhizobium and carry highly mobile symbiosis islands, with a novel integration hotspot (ser-tRNA). In vitro, the transfer of the symbiosis island from these strains to a non-symbiotic recipient strain resulted in novel bacteria with a poorly effective phenotype, except for one highly effective strain. By deleting a relaxase gene, which is involved in the conjugative transfer of the symbiosis island, a more stable strain was created containing an immobile symbiosis island. The study highlights the presence of taxonomically and symbiotically distinct B. pelecinus-nodulating Mesorhizobium strains in East African soils. In these Mesorhizobium strains, the rate of symbiosis island transfer was as high as 3x10-3 in vitro. It is suggested that island transfer has a significant role in the rapid evolution of poorly effective strains. Further, it is likely that this transfer contributes to one of the most intractable problems compromising N2-fixation in agricultural systems - that of poorly effective but competitive background rhizobia. In this study, the management of symbiosis island transfer through inactivation of the relaxase gene without affecting the symbiotic phenotype was found to be a viable approach for tackling this problem.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Howieson, John, O'Hara, Graham, Terpolilli, Jason, Tiwari, Ravi, Mengistu, Solomon and Ramsay, J.
URI: http://researchrepository.murdoch.edu.au/id/eprint/37308
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