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Doctor of Philosophy in Quantitative Biology



First Advisor

Woo-Suk Chang


Rhizobia are well known for their ability to fix atmospheric nitrogen (N) into ammonia, so called biological nitrogen fixation (BNF), either as free-living bacteria in soil or as symbiotic partners of legume plants. Particularly, symbiotic nitrogen fixation (SNF) gains much attention due to its compatibility as a source of N fertilizer in sustainable agriculture. Symbiotic association between Bradyrhizobium japonicum and soybean (Glycine max) has been used not only as a model system to study SNF, but also as a means of biological soil fertilization to improve crop yield. However, failure to perform optimal BNF in the field is the major limitation associated with rhizobial inoculants. This is mainly due to the sudden death of bacterial cells caused by abiotic stressors upon introduction into soil. Heat stress and soil acidity are the main causes of death of the inoculants. Therefore, gene expression studies were carried out using microarray technology to identify important genes that are responsible for the survival of the bacterium under heat stress and acid shock. Results revealed that a number of small heat shock proteins are induced under heat stress, presumably to protect cells from accumulation of denatured proteins, while cellular mechanisms such as mobility and cell division are shut off, presumably to save energy to overcome the stressful condition. A similar response was triggered under acid shock to reserve cellular energy to fight against the low pH, while several multi-drug resistance efflux pumps played a key role in maintaining neutral pH within cells. Mutagenesis studies proved the importance of multi-drug resistance efflux pump coding gene blr7593 under acid shock. There were several small heat shock proteins that were induced under both heat stress and acid shock. Interestingly, some of the up-regulated genes (i.e., blr7740, blr2203 and blr2694) are not vital for the survival of the bacterium under the stressful conditions. The second objective of the study was to investigate the possibility of employing B. japonicum as a phosphate biofertilizer. Studies showed that B. japonicum is capable of solubilizing inorganic phosphate. It was found that B. japonicum produces pyrroloquinoline quinone (PQQ) which is the prosthetic group of glucose dehydrogenase (GDH) which may have an indirect effect on phosphate solubilizing. The findings of this work encourage the use of a biofertilizer with dual functions of BNF and phosphate solubilization to create a green economy by mitigating the adverse effects of synthetic fertilizers on environments and human health.


Nitrogen fixation, Phosphate solubilization, Bradyrhizobium japonicum


Biology | Life Sciences


Degree granted by The University of Texas at Arlington

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