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Origin of Gene Specificity in the Nitrogen-Fixing Symbiosis

Many legumes form a symbiosis with nitrogen-fixing bacteria found in the soil. This relationship is beneficial to both the plant and the bacteria; the plant receives nitrogen that is otherwise limited, and the bacteria receive fixed carbon. Upon sensing the bacteria, the plant forms a new organ (the nodule) where the bacteria are housed within the cells. Many genes are required for the proper formation and function of nodules; this dissertation is broadly focused on how genes required for nitrogen-fixing symbiosis are co-opted from other cellular processes and how they are specialized for symbiosis. Protein trafficking from the plant to the intracellular bacteria is critical to the success of the symbiosis. This protein trafficking is the cellular anterograde secretory pathway repurposed toward a new intracellular compartment. In the model legume Medicago truncatula, a deletion in DNF1, which encodes the nodule-specific 22kDa signal peptidase complex (SPC) subunit, causes the nodules to be unable to fix nitrogen because the nodule-specific protein trafficking machinery disrupted. Here, we have shown that DNF1 became specialized in symbiosis through nodule-specific expression, and we identify cis-elements that are crucial for that transcriptional control in the DNF1 promoter and other SPC subunit genes. Furthermore, we have found that another protein trafficking protein, SYP132A, was co-opted from arbuscular mycorrhizal symbiosis for its role in nitrogen-fixing symbiosis. This t-SNARE is localized to the symbiotic membranes in both symbioses and is required for proper arbuscule and bacteroid development within host cells. One class of proteins that are specifically targeted to the symbiosome are nodule-specific cysteine-rich (NCR) peptides, which are involved in the differentiation of the intracellular bacteria. In general, NCR peptides are required for nitrogen fixation in Medicago, but their specific modes of action are largely unknown. Here, we describe and characterize two NCR motifs that are found in many different peptides, pointing to a common, conserved amino acid sequence, possibly contributing to the efficacy of these peptides. Finally, we show that the receptor, DMI2, which is required for rhizobia infection, is regulated at the protein level in Medicago. DMI2 is constitutively expressed, but in the absence of rhizobia infection, it is degraded by the proteasome. During infection, however, it is protected from degradation, and the protein accumulates.
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