Date of Graduation

Summer 2024

Degree

Master of Science in Biology

Department

Biology

Committee Chair

Babur Mirza

Abstract

Soybeans form symbiotic relationships with nitrogen-fixing bacteria in their root nodules to acquire most of their nitrogen requirement; nitrogen is often one of the limiting nutrients in the soil. Members of the Bradyrhizobium, Sinorhizobium, Mesorhizobium, and Rhizobium genera can nodulate soybeans. Bradyrhizobium is preferentially selected over other types of rhizobia in varying environmental conditions and is often found in acidic soils. Sinorhizobium is often identified in the root nodules of soybeans in neutral and alkaline soil. So far, the potential role of soil pH on the selection of rhizobial endophytes with soybean root nodules is unknown. Likewise, how the relative abundance of rhizobial cells in the rhizosphere influences the selection of rhizobial endophytes is unclear. This study was focused on the potential influence of the sand pH and rhizobial abundance in the rhizosphere in the selection of rhizobial endophytes within soybean root nodules. Soybeans were grown for fifty-four days in sterilized sand and inoculated with Bradyrhizobium japonicum USDA 110 (B. japonicum) and Sinorhizobium fredii USDA 191 (S. fredii) in three different cell densities (1:1, 1:100, 100:1) during different weeks of their growth. I assessed the bacterial composition within soybean root nodules and rhizospheres using Illumina paired-end DNA sequencing of bacterial 16S rRNA genes. I observed a preferred selection of B. japonicum over the S. fredii strain as soybean root nodule endophytes across both treatments including various sand pH and inoculum cell density. In summary, sand pH and the number of rhizobial cells inoculated did not appear to be the driving factors behind the selection of rhizobial endophytes in soybean root nodules. This study can help identify potential rhizobia strains for developing superior bioinoculants under different soil pH conditions.

Keywords

rhizobial endophytes, soybean root nodule, 16S rRNA, inoculum, cell density, sand pH

Subject Categories

Environmental Microbiology and Microbial Ecology | Plant Biology

Copyright

© Cole Alexander Roman

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Available for download on Monday, September 15, 2025

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