Characterization of nitrogen-fixing bacteria from Phaseolus vulgaris L. in Kenya

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Phaseolus vulgaris (common bean) is an important food crop in Sub-Saharan Africa. Low soil nitrogen limits the productivity of P. vulgaris in Kenya and a greater exploitation of symbiotic nitrogen fixation, resulting from interactions with rhizobia, has the potential to improve yields. To enable the increased use of the symbiosis in Kenyan agriculture in the future, studies in this thesis examined the genetic diversity of rhizobia that nodulate P. vulgaris in the central and western parts of Kenya, their nitrogen-fixing capabilities, and their competitiveness against Rhizobium tropici CIAT 899, a leading commercial inoculant strain for P. vulgaris. Lastly, studies investigated the relative importance of the genotypes of resident soil rhizobia, soil rhizobial population densities, inocula densities, and levels of soil nitrogen, in determining nodule occupancy by R. tropici CIAT 899 inoculated onto P. vulgaris.

Phylogenetic studies using 16S rRNA and recA genes indicated at least five species of Rhizobium viz., R. sophoriradicis, R. phaseoli, R. leucaenae, R. paranaense and R. etli nodulate P. vulgaris in Kenya. In addition to the five species, strains that likely belong to new species in the genus Rhizobium also widely nodulate P. vulgaris in Kenyan soils. In glasshouse studies, recovered strains were variably effective on Kenyan cultivars of P. vulgaris and 11 fixed as much nitrogen as R. tropici CIAT 899. From the 11, strains such as NAK 227, NAK 288, NAK 214 and NAK 157 were also highly competitive in liquid co-inoculation assays, carried out with the aid of gusA and celB marker genes, and are potential future inoculants for P. vulgaris in Kenya. The genotype of the rhizobia in the soil was found to be the primary determinant of the nodule occupancy achieved by the inoculant strain, a finding that conflicts previous reports that indicated nodule occupancy was mainly determined by soil rhizobial densities. The rhizobial genotypes varied in their rhizosphere competence and in their ability to preferentially nodulate the host, suggesting these two traits are important for the successful colonization of P. vulgaris nodules by rhizobia.

It is anticipated that future studies will leverage on the results in this thesis, to develop locally-targeted inoculation solutions that optimize nitrogen fixation in P. vulgaris in Kenya, and to elucidate the molecular basis for preferential nodulation in P. vulgaris.

Mwenda, G.
Murdoch University