Biological nitrogen fixation is both ecologically and agriculturally important. Rhizobia are soil bacteria which form a symbiosis with legumes, fixing nitrogen in exchange for carbon. The goal of this research was to identify and determine the function of genes which are not required for the establishment of symbiosis, but have an effect on the efficacy of the symbiotic interaction. In Chapter 1, I demonstrate the type IV secretion system has an effect on symbiosis. Bacterial proteins were tested for their ability to translocate through the type IV secretion system into the host cell. One protein, TfeA, was transported into the host cell. Deleting tfeA resulted in decrease nodule number and strain competitiveness for nodule occupancy. TfeA-like proteins were also identified in 12 different rhizobia species. In Chapter 2, I showed TfeA binds to Medicago truncatula ARF2 in vivo and Glycine max ARF27. Constitutively expressed tfeA was unable to be recovered in Medicago sativa (alfalfa) or M. truncatula. Inducible tfeA was successfully transformed in M. truncatula, but expression is inconsistent. TfeA binding to ARF2 in planta offers additional insights into the role of auxin during nodule formation. In Chapter 3, I use PacBio sequencing technology to assemble 10 strains of Sinorhizobium. Analysis of 16 Sinorhizobium complete genomes showed that most strains have small accessory plasmids. These plasmids carry genes which are not required for symbiosis, but have an effect on the symbiotic process. Accessory plasmids combine with pSymA to allow for the gain and loss of symbiosis genes. The type IV secretion system and tfeA was found on pSymA or an accessory plasmid depending on the strain. Overall, this research identified important bacterial genes which modulate symbiosis.
University of Minnesota Ph.D. dissertation.September 2017. Major: Plant Biological Sciences. Advisor: Deborah Samac. 1 computer file (PDF); viii, 149 pages.
Identification and Characterization of Bacterial Genes Involved in the Medicago-Sinorhizobium symbiosis.
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