Phosphate Transport in the Arbuscular Mycorrhizal Symbiosis: Functional Analysis of a Medicago Truncatula Mycorrhiza-Specific Phosphate Transporter

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IBN: 0343975AbstractIn natural ecosystems, most vascular flowering plants live in symbiosis with arbuscular mycorrhizal (AM) fungi. These mutually beneficial associations develop in the root system, where the fungus colonizes the root cells to obtain carbon from the plant, while assisting the plant with the acquisition of mineral nutrients, particularly phosphorus, from the soil. Arbuscular mycorrhizal symbioses are formed by almost all vascular flowering plant species and play a significant role in ecosystem biodiversity and functioning. The underlying mechanisms are not yet understood, but the contribution of AM fungi to plant phosphorus nutrition, and the resulting impact on plant health, is clearly a major contributing factor. The overall objectives of this project are to dissect the mechanisms via which the plant obtains phosphate from the fungus in an arbuscular mycorrhizal symbiosis. A model legume, Medicago truncatula, will be used for these analyses. The arbuscular mycorrhizal symbiosis is a highly co-evolved partnership, in which the differentiation of both symbionts results in specialized symbiotic interfaces that enable the exchange of carbon and phosphate. Within the root cells, the fungus forms highly branched hyphae, called arbuscules, which are enveloped in a specialized plant membrane, the peri-arbuscular membrane. Previous studies led to the identification of a unique phosphate transporter (MtPT4) from Medicago truncatula that is located exclusively in the peri-arbuscular membrane. Current objectives are to evaluate motifs implicated in targeting MtPT4 to the peri-arbuscular membrane and to determine the role of MtPT4 in phosphate transport in the symbiosis. Using transgenic lines in which MtPT4 expression is disrupted, or loss-of-function alleles of MtPT4 obtained via TILLING, the hypothesis that MtPT4 is essential for the acquisition of phosphate released from the fungus will be tested. In addition, the mutant MtPT4 lines will be used to explore fundamental questions about phosphate transport and the arbuscular mycorrhizal symbiosis, including the role of the different symbiotic interfaces and the importance of bi-directional nutrient exchange in a mutualistic symbiosis. Currently, the molecular events underlying arbuscule development are unknown. Comparative transcriptional profiling of the MtPT4 mutant lines and a M. truncatula arbuscule development mutant, will provide insights into the transcriptional events associated with structural and functional components of arbuscule development and phosphate transport in the symbiosis. Because phosphate limits plant production, an understanding of the AM/legume symbiosis has significant agricultural and ecological implications. Other Broader Impacts include the training of graduate students and postdoctoral fellows, as well as undergraduates and high school students.