Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Biochemistry and Cellular and Molecular Biology

Major Professor

Gladys Alexandre

Committee Members

Brad Binder, Elizabeth Fozo, Elizabeth Howell, Jenny Morrell-Falvey


Chemotaxis is the biased movement of bacteria in response to environmental stimuli. Chemotaxis is initiated with dedicated receptors that sense environmental signals, and these signals are transduced through a phosphorylation cascade that can influence flagellar rotation and swimming direction. Most knowledge of chemotaxis signal transduction comes from studies of Escherichia coli, which possesses a simple system consisting of five chemotaxis receptors and a single chemotactic signaling pathway. Unfortunately, E. coli is not representative of the diversity of chemotaxis signal transduction systems revealed by the analysis of completely sequenced bacterial genomes. Most sequenced bacterial genomes to date suggest most motile bacteria able of chemotaxis rely on at least two distinct chemotaxis signaling systems and a large number (>20) chemotaxis receptors. This dissertation focuses on the plant growth promoting alphaproteobacterium Azospirillum brasilense as a representative model for chemotaxis. The genome of A. brasilense encodes four chemotaxis operons and fifty-one chemotaxis receptors. Here we show that chemotaxis paralogs encoded in two of these chemotaxis systems, Che1 and Che4, interact to produce two functional chemotaxis signaling arrays segregated by chemotaxis receptor lengths. One of these arrays contains a subclass of receptors that possess an additional Cterminal domain and comprise the PilZ-containing chemotaxis receptors. In bacteria, the Cterminal PilZ domain was shown to bind the secondary messenger cyclic diguanylate monophosphate which affects enzyme activity, gene expression and protein-protein interactions. Here we develop a novel real time tool (slide-in-chamber) and assay (root-in-pool) for monitoring the role of A. brasilense chemotaxis and PilZ-containing chemotaxis receptors (Tlp1 and Aer) in bacterial behavior and colonization of wheat rhizospheres. We found that chemotaxis is essential for A. brasilense to sense and subsequently colonize specific zones of wheat roots. A. brasilense exhibits an attractant response to the root hair and elongation zones of wheat and is repelled from the wheat root tip, and this response is dependent upon functional PilZ receptors Tlp1 and Aer. Furthermore, cyclic diguanylate monophosphate binding to Tlp1 and Aer is essential for bacterial response to roots. Taken together, these data reveal a new role for cyclic diguanylate monophosphate in modulating sensitivity to complex gradients.

Files over 3MB may be slow to open. For best results, right-click and select "save as..."