Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology



First Advisor

Gwyn A. Beattie


Knowledge of the chemicals available during plant-microbe interactions is useful for promoting the growth of beneficial bacteria and predicting pathogen populations and thus the probability of disease. Pseudomonas syringae B728a is able to utilize plant-derived quaternary ammonium compounds (QACs), such as L-carnitine, choline, and choline derivatives, as osmoprotectants and nutrient sources. We explored the biological roles of carnitine and choline-O-sulfate in P. syringae. Although genes involved in carnitine catabolism were identified in other organisms, the roles of these genes have not been fully elucidated. We analyzed conflicting reports of the carnitine degradation pathway and provided evidence for a modified pathway based on the growth and accumulation of metabolic intermediates in catabolic mutants. We discovered that carnitine is released during bean seed germination, specifically during radicle emergence and elongation, and strongly contributes to the growth of B728a on germinating seeds. In exploring the genes involved in the catabolism of choline-O-sulfate (COS), we confirmed that a choline sulfatase was required for catabolizing COS as a nutrient and osmoprotectant, but did not impact P. vulgaris leaf or seed colonization. However, we found evidence that a sulfate exporter is critical to the use of COS; this supports a model in which this exporter removes sulfate released from the cleavage of COS and prevents accumulation to toxic levels. Lastly, we systematically evaluated the roles of a broad set of QACs, including choline, phosphorylcholine, glycine betaine, carnitine, and COS, in the colonization of leaves and germinating seeds by B728a using a collection of metabolic and transporter mutants. P. syringae derives osmoprotection from choline while colonizing the leaf surface and our studies demonstrated that choline was the most abundant and influential QAC present on P. vulgaris leaves. During colonization of the spermosphere, P. syringae derives nutrients from choline released throughout seed development, as demonstrated by a compositional analysis of seed exudates. Collectively, these results demonstrate a key role for QACs as a driver of bacterial population dynamics on seeds and leaves, and thus support the possibility of exploiting QAC availability for the establishment of targeted beneficial microbes on plants.


Copyright Owner

Michael David Millican



File Format


File Size

160 pages