Date of Award

Spring 1999

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

Noel, Dale

Second Advisor

Courtright, James

Third Advisor

Fredricks, Walter


The symbiosis of the soil microbe Rhizobium with leguminous plants is a model system of bacterial interaction with higher eukaryotes. In nature, symbiotic rhizobia contribute to the nitrogen cycle. Through nitrogen fixation, they allow legumes to thrive in nitrogen-poor soil. Symbiosis begins by Rhizobium eliciting root nodule development on the roots of its eukaryotic host. At the same time, the bacteria infect the nodule through an infection thread. The most abundant surface molecule of Gram-negative bacteria, lipopolysaccharide, is essential for this process. In R.etli, the most distal portion of the LPS, the 0-antigen (0-Ag) is necessary for successful infection. The infection process and various environmental cues such as low pH or oxygen tension, or the presence of seed exudate induce antigenic modifications of the 0-Ag. In addition, growth at low pH induces a shift in the electrophoretic LPS profile. R.etli's proficiency in sensing environmental changes, and modulating its surface structure in response may be important for persistence in nature or in symbiosis. This study characterizes the antigenic and electrophoretic LPS modification. The plant-released inducers of the LPS changes, the biochemical basis of the LPS modifications, bacterial genetics pertaining to this change and the physiological consequences of the LPS modifications were assessed.



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