Bacterial morphology in the Sinorhizobium/Medicago nitrogen-fixing symbiosis
Abstract/Contents
- Abstract
- In this thesis, I explore how bacteria change shape. I used the model bacterial symbiont Sinorhizobium meliloti, which undergoes significant changes in cell shape during its nitrogen-fixing symbiosis with legume plants. Nitrogen is a limiting nutrient in the environment, and nitrogen-containing fertilizers are heavily used in agricultural systems. Organically available (fixed) nitrogen is energetically expensive to produce. Legume plants, in symbiosis with rhizobia soil bacteria, are able to fix nitrogen and do not require nitrogen-containing fertilizers. The model system of Medicago sativa (alfalfa) and the soil bacterium S. meliloti permit laboratory study of this process. As the human population continues to grow, it is essential to understand the conditions that are required for biological nitrogen fixation. M. sativa and S. meliloti initiate symbiosis by exchanging chemical calling cards. S. meliloti cells then invade plant roots. M. sativa forms nodules -- specialized root organs -- to house the bacteria. S. meliloti cells within the nodules differentiate to form nitrogen-fixing bacteroids. These bacteroid cells can be ten times larger and contain twenty times more genomic DNA than free-living cells, and they are often branched in contrast to the rod shaped bacteria found in the soil. The purpose of this dramatic morphological switch is not well understood but appears to be essential for nitrogen fixation. I used different culture media and environmental conditions and examined bacterial mutants to investigate what external factors are involved in S. meliloti free-living cell shape. I tested multiple conditions including microaerobic versus aerobic, salt concentrations in culture media, a range of temperatures, and different carbon sources. Of all these conditions, I discovered that only the presence of phosphate in growth media was strongly correlated to branching and an increase in cell size. This morphology is specifically related to the activity of a phosphate transporter-encoding gene, pstC. This result was confirmed through whole genome sequencing and epistatic expression of mutated and full-length pstC in different laboratory strains of S. meliloti. I hypothesize that an increase in extracellular phosphate concentration may be present within root nodules and may contribute to bacteroid differentiation. More work remains to be done to determine the role of phosphate in legume/rhizobia symbiosis and uncover the mechanism by which phosphate affects cell morphology.
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2017 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Quinn, Jillynne |
|---|---|
| Associated with | Stanford University, Department of Biophysics. |
| Primary advisor | Long, Sharon |
| Thesis advisor | Long, Sharon |
| Thesis advisor | Huang, Kerwyn Casey, 1979- |
| Thesis advisor | Mudgett, Mary Beth, 1967- |
| Thesis advisor | Theriot, Julie |
| Advisor | Huang, Kerwyn Casey, 1979- |
| Advisor | Mudgett, Mary Beth, 1967- |
| Advisor | Theriot, Julie |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Jillynne Quinn. |
|---|---|
| Note | Submitted to the Department of Biophysics. |
| Thesis | Thesis (Ph.D.)--Stanford University, 2017. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2017 by Jillynne Nicole Quinn
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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