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" A phylogenetic approach to examining symbiotic specificity and evolution in the legume-rhizobia nitrogen-fixing symbiosis "
/Jamie Howard
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سازمان تحقیقات، آموزش و جهاد کشاورزی
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| Document Type
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Latin Dissertation
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| Language of Document
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English
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| Record Number
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18577776
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| Call number
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TL1321
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| Main Entry
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Howard, Jamie
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| Title & Author
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A phylogenetic approach to examining symbiotic specificity and evolution in the legume-rhizobia nitrogen-fixing symbiosis/Jamie Howard
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| College
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Arizona State University
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| Date
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, 2010
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| Degree
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Ph.D.
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| Page No
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260 pages
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| Abstract
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The legume-rhizobia nitrogen-fixing symbiosis is characterized by a wide range of specificities shown by both symbionts in choosing suitable partners. Rhizobial specificity relies primarily on signaling molecules called "Nod factors' (NF), which vary in chemistry and structure among different bacterial strains. Legume specificity is controlled primary through Nod factor receptor proteins (NFRs), which `sense' specific types of NF and initiate the nodulation process in response. This dissertation focused on investigating, using primarily molecular phylogenetic analyses as well as other methods, one type of NFR, nfr5 , and its possible interactions with NF within specific legume groups that might explain variation in specificity. Sequences of nfr5 genes from over 110 legume species were analyzed and compared to sequences of rhizobial genes used to produce NF ( nodA, nodB/C and nodE ). Additionally, the diversity of bacterial symbionts isolated from legumes collected primarily in Arizona was assessed using both culture-dependent and non-culture methods including denaturing gradient gel electrophoresis and cloning of 16S rRNA genes.Phylogenetic analyses of nfr5 sequences suggest (i) that a gene duplication event, leading to paralogous copies of nfr5 , occurred early in legume evolution and may explain more expansive symbiotic ranges of legumes possessing multiple gene copies; (ii) that the inability of some legumes to nodulate is not due to the absence of a nfr5 gene; (iii) that nfr5 gene sequences are useful for resolving legume systematics; (iv) that differences in amino acids where NF binding occurs likely account for differences in symbiont specificity; and, (v) that coevolution has occurred between certain clades of legume and rhizobia symbionts. Moreover, it was found that nodules of many legumes collected from natural habitats, primarily in Arizona, harbor unusual 'symbionts,' including strains not typically categorized as 'rhizobia' but which possess at least some nod genes. This suggests that diverse bacterial strains are exchanging genetic material with more typical legume symbionts, and that, contrary to common assumptions, most nodules contain more than one strain of rhizobium. These findings contribute to an understanding of the molecular bases of specificity within the legume-rhizobia symbiosis and raise intriguing questions about symbiotic specificity of wild legumes in desert environments.
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| Descriptor
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Symbiosis
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Nitrogen fixation
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Legumes
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Rhizobia
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| Added Entry
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Wojciechowski, Martin F.
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| electronic file name
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TL1321.pdf
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