Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Biology

First Advisor

David S. Pederson

Second Advisor

Jeanne M. Harris


Nitrogen is an essential element for life on earth for all organisms. Sitting at the bottom of the food chain, plants provide nutrition for most of life. Although plants are surrounded by the nitrogen in the atmosphere, they can only utilize the inorganic soil nitrogen. To fulfill the world’s nitrogen need, approximately 150 million tonnes of ammonia is produced annually via the Haber-Bosch process, which accounts for approximately 1-2% of the world’s entire energy usage. Leguminous plants such as soybean and alfalfa have evolved a symbiotic relationship with a group of nitrogen-fixing bacteria, collectively called Rhizobia, to receive nitrogen that they can utilize. Understanding how biological nitrogen fixation works is crucial for agriculture and climate change. Legumes form specialized root organs called nodules where symbiotic nitrogen fixation happens. In the model legume Medicago truncatula, the LATD/NIP gene is required for proper root and nodule development. LATD/NIP encodes a nitrate transporter with a proposed secondary function. The root defect of the latd mutant can be rescued by exogenous application of the plant hormone Abscisic Acid (ABA), suggesting an intimate relationship between LATD/NIP and ABA signaling. To understand the evolution of LATD/NIP’s function and its role in the ABA response, we tested LATD/NIP orthologs for their ability to functionally complement latd mutants and analyzed the effect of latd mutants on ABA localization within the root. We find that LATD/NIP originated at the base of eudicots, predating the evolution of nodulation, and acquired its root and nodule functions later. We show that ABA is primarily localized to the endodermis of young M. truncatula roots and ABA accumulation increases with exogenous application of nitrate. When young, latd mutants show similar patterns of ABA accumulation to wild-type plants. However, in older latd plants when the mutant phenotype is more profound, we observed altered ABA accumulation. Together, our results provide insight into the evolution of nodulation and LATD/NIP’s role in root development.



Number of Pages

155 p.

Available for download on Monday, February 10, 2025