Analysis of symbiotic mineral nutrient transport and mechanisms underlying regulation of the arbuscular mycorrhizal (AM) symbiosis

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Over 80% of the vascular flowering plants are capable of forming mutualistic symbioses with arbuscular mycorrhizal (AM) fungi. These associations develop in the roots, where in exchange for carbon, the fungal symbionts transfer phosphorus and nitrogen from the soil to the plant. In the symbiosis, the AM fungi inhabit the root cortical cells where they form branched hyphae called arbuscules. The arbuscules are responsible for nutrient delivery to the root cells. Previous studies in Medicago truncatula indicated that a symbiosis-specific phosphate transporter, MtPT4, is essential for symbiotic phosphate transport. In this project the investigators will further analyze MtPT4 and in addition, begin to characterize the roles of a second phosphate transporter, MtPT8, and a novel ammonium transporter, MtAMT2. Single, double and triple mutant plants lacking these transporters will be generated and they will be used to determine the extent to which symbiotic phosphate and nitrogen transport regulate the AM symbiosis. Experiments will address the underlying mechanisms and signaling pathways involved. A second aspect of the project focuses on protein targeting in the symbiosis. The MtPT4 protein is located exclusively on a specialized membrane called the peri-arbuscular membrane, which surrounds the arbuscule. Using immunological approaches, and live cell imaging of MtPT4 tagged with fluorescent proteins, the investigators will analyze the mechanisms underlying protein targeting to the peri-arbuscular membrane. The AM symbiosis is formed by almost all vascular flowering plants but the proteins that mediate symbiotic phosphate and ammonium transport, the mechanisms that regulate maintenance of key symbiotic interfaces, and the signaling pathways that integrate regulatory mechanisms with the mineral nutrient status of the plant, are all largely unknown. The data generated in these experiments will advance our understanding of these aspects of the symbiosis. The project includes opportunities for undergraduate students to obtain experience with confocal microscopy.