Abundance, genetic diversity and symbiotic potential of common bean (phaseolus vulgaris l.) nodule associated bacteria in Western Kenya soils
Simiyu, Wekesa Clabe
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Plant growth-promoting rhizobacteria (PGPR) are beneficial native soil bacteria that colonize plant roots and result in increased plant growth. Those that colonise the nodules of legumes are known as nodule associated bacteria (NAB). The aim of this study was to determine the distribution and genetic diversity of NAB that colonize Phaseolus vulgaris, their abundance, and symbiotic efficiency when coinoculated with Phaseolus vulgaris in Western Kenya soils. The soil samples were collected from cultivated lands in Kisumu near Lake Victoria, slopes of Mt. Elgon and Kakamega. In each of these regions, the soil samples were collected from four regions. 1ml of soil solution at 10 fold dilution for seven dilution steps (10-1 to 10-7) and three replications for each dilution was used to inoculate common bean seedling in Leonard jars. They were harvested after four weeks to determine abundance of NAB using most probable number method. Common bean nodules were also collected directly from the farmers’ farms in the above three regions. Harvested nodules and those collected from the field were cleaned and surface sterilized, crushed and exudates streaked on YEM agar growth media. Pure colonies were further cultured in YEM broth at 280C for three days and the genomic DNA isolated from the bacteria using Qiagen DNA extraction kit. 16SrRNA gene was amplified by 27F and 1492R primers and PCR products resolved by agarose gel electrophoresis and sequenced. 16SrRNA gene analysis revealed that NAB that nodulate with common beans are genetically diverse as they formed clusters on the phylogenetic tree and their distribution depends on chemical characteristics of the soil. BLASTn showered that isolated strains belonged to the genus Pseudomonas, Providencia, Rhizobia, Klebsiella, Sphingobacterium, Enterobacter, Delfitia, Acinetobacter and one strain did not have sequence homology at the GenBank. Mt. Elgon region had the highest population of NAB (120000 cells per gram of the soil), followed by Kisumu (1290 cells per gram of the soil) and Kakamega region had the lowest (17 cells per gram of the soil). The effect of PGPR on the yield of common beans was significantly higher (p < 0.001) when co-inoculated with Rhizobia compared to the yield of Rhizobia inoculated alone or control (not inoculated) (p < 0.05). This study therefore provides knowledge on the type of NAB that nodulates with common beans and factors that favour their distribution necessary for production of PGPR inoculants suitable to the soils of Western Kenya.