Date of Graduation

Spring 2023


Master of Science in Biology



Committee Chair

Babur Mirza


Soybean plants fulfill most of their nitrogen requirement by developing symbiotic associations with four different rhizobial genera, including Bradyrhizobium, Sinorhizobium, Mesorhizobium, and genus Rhizobium. In general, members of Bradyrhizobium and Sinorhizobium have been reported as dominant root nodule endophytes under acidic soils and alkaline soil conditions, respectively. So far, it is unknown whether the selection of rhizobial endophytes is regulated by their ability to survive under different soil pH or driven by host plants regardless of their relative abundance in soil. This study was focused on the assessment of the potential role of soil pH in selecting rhizobial endophytes and determining whether the selection of rhizobial endophytes is controlled by their relative abundance in rhizosphere soil. In a greenhouse study, I inoculated soybean plants with different cell densities of Bradyrhizobium japonicum and Sinorhizobium fredii cultures. Plants were grown under three soil pH conditions. I assessed the distribution of rhizobial endophytes within root nodules and rhizosphere using high-throughput DNA sequencing of 16S rRNA gene amplicons. I observed a high dominance of Bradyrhizobium related sequences in the root nodules of all plants in the greenhouse study. I also observed high abundance of Bradyrhizobium in the root nodules of uninoculated plants suggesting the presence of native Bradyrhizobium population in all three soils. The root nodules of the plants grown under two field conditions (acidic and neutral sites) also showed a high abundance, suggesting the preferred selection of Bradyrhizobium as a soybean endophyte regardless of soil pH or their relative abundance in rhizosphere soil. I detected the presence of DNA sequences related to all rhizobial genera in the rhizosphere soil. I also observed statistically significant differences in plant growth and physiological characteristics. These results helped in identifying that Bradyrhizobium may be a better inoculant for soybean crops under various soil conditions or host genotypes


soybean endophytes, soil pH and rhizobial endophytes, abundance of Bradyrhizobium and Sinorhizobium in rhizosphere soil, rhizobial inoculation and soybean plant growth

Subject Categories

Environmental Microbiology and Microbial Ecology | Plant Biology


© Brianne Danielle Edwards

Available for download on Thursday, May 01, 2025

Open Access