Investigating the molecular mechanisms affecting legume symbioses : from signaling to nutrient transport
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Legumes establish symbiotic relationships with nitrogen-fixing rhizobia and arbuscular mycorrhizal (AM) fungi, enhancing nutrient acquisition and reducing reliance on costly, environmentally damaging fertilizers. These interactions are tightly regulated and require the formation of specialized structures that facilitate nutrient exchange. The establishment and maintenance of these symbioses requires extensive transcriptional reprogramming and cell-to-cell communication, involving both plant-microbe signaling and intercellular signaling within the plant. In this work, I investigate transcriptional changes in soybean roots during interaction with Bradyrhizobium japonicum. I describe transcriptional programs in the root cortex following rhizobial inoculation and identify the gibberellin biosynthetic gene GmGA3ox1 as an important regulator of nodule formation. I also investigate the dynamic regulation of small secreted peptides in the phloem and xylem during nodulation, uncovering a novel role for Inflorescence Deficient in Abscission (IDA) peptides. Specifically, I characterize GmIDA1 as a negative regulator of nodulation in soybean. Since legumes often interact with rhizobia and AM fungi simultaneously in natural ecosystems, I investigate how nutrient acquisition and carbon allocation are affected in Medicago truncatula plants inoculated with AM fungi and strains of rhizobia with different nitrogen (N) fixation capacities. I also examine how AM fungimediated nutrient transport affects plant nutrition, N fixation, and carbon allocation in tripartite interactions. I show that Sinorhizobium medicae WSM419 is a more effective symbiont for M. truncatula than Sinorhizobium meliloti Rm1021, promoting higher plant biomass and nitrogen content. Furthermore, I demonstrate that AM fungi enhance phosphorus (P) uptake and that high rates of symbiotic N fixation can deplete P and potassium (K) concentrations, emphasizing the potential role of AM fungi in supporting nutrient demands associated with N fixation.
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Ph. D.