ABUNDANCE, MOLECULAR CHARACTERIZATION AND SYMBIOTIC POTENTIAL OF ENDOPHYTIC ROOT NODULATING RHIZOBIA COLONISING SOYBEAN IN SOILS OF KAKAMEGA COUNTY

Abstract

Root nodulating bacteria are a diverse group of bacteria that may coexist with legumes and non-legumes in a symbiotic relationship. These bacteria have a wide range of symbiotic flexibility. Rhizobia inoculants have contributed to increase nitrogen fixation. The number of rhizobia in the soil impacts whether soybeans will respond to inoculation. This study aimed at identifying and selecting a potential root nodulating rhizobia of soybean plant which were tested in vermiculite in the greenhouse in the modified Leonard jars. The goal of the study was to characterize symbiotic rhizobia using 16S rRNA as a molecular tool, determine the abundance and symbiotic potential of indigenous rhizobia strains in soil samples collected from six locations in the three agro-ecological zones (Lower midland 1, Lower midland 2 and upper midland). Soil samples were obtained from 24 farms in six different sub-Counties: Kakamega South, Kakamega East in Isukha South, Kakamega Central, Kakamega North, Mumias East and Butere which has been used for soybean growing for more than two seasons. In order to determine the population size of rhizobia, the most probable number technique was applied. Quantitative data on concentration of discrete positive items was obtained through quantification of the concentration of viable rhizobia. The indigenous rhizobia counts were estimated using the most probable number enumeration approach, which revealed that 51.8 percent of the studied sites had high abundance. Each of the six regions had a different number of Rhizobia in the composite soil ranging from 7.3 x 101 to 1.2 x 105 cells g- soil with confidence factor of 4.674404 at p<0.95. Deoxyribonucleic acid (DNA) was extracted from cultured bacteria using Qiagen DNA extraction kit and the 16S rDNA was amplified by 1492R and 27F primers followed by sequencing to establish their genetic diversity. Phylogenetic relationships of eight Kakamega isolates were analysed using their 16S rRNA gene sequences reflecting their heterogeneity as follows: Rhizobium nepotum, Rhizobium skierniewisence, Rhizobium pisi, Rhizobium fabae, Rhizobium phaseoli, Rhizobium etli, Rhizobium azibense, and Pseudomonas glyciae. The ANOVA method was used to determine the association between rhizobial abundance and symbiotic potential in relation to soil chemical conditions, and the means were compared using Turkey's honest significant difference test. Inoculation with bacterial isolates increased the number of nodules and the dry weight of the shoots significantly (P<0.05), however the weight of the seeds varied between groups. Inoculation by rhizobia promotes symbiotic competency for the generation of plant-growth-promoting rhizobacteria inoculants in Kakamega County soils, according to this study. Existence of varying population levels of rhizobia was demonstrated in soils from the region which were also influenced by different agro-ecological zones coupled by micro and macro nutrients. Based on this research, Mumias and Butere had a higher diversity of Rhizobium fabae, Rhizobium phaseoli, Rhizobium pusence and Pseudomonas glyciae per gram of soil, resulting in high shoot biomass and high yields.