Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Theses & dissertations (Interdisciplinary)


Genetics; Molecular, Cellular, and Developmental Biology;


Soybean-Rhizobium combinations with high rates of nitrogen fixation are useful in soybean breeding programs. Thirteen soybean (Glycine max L. Merr.) cultivars and wild soybean (G. soja Sieb. and Zucc.) plant introductions were inoculated with Bradyrhizobium japonicum strain 61A76 and 4 fast-growing strains of Rhizobium fredii. All fast-growing strains formed effective nodules on cultivars 'Peking', 'Virginia', 'Hardee', and the G. soja plant introductions. Three of the fast-growing strains formed ineffective nodules on the other cultivars. USDA191 formed effective nodules on all cultivars tested. No nodules were formed on 'Harosoy rj1 rj1'. Combinations that produced significant acetylene reduction, nodule number, nodule fresh weight, and plant-top dry weight compared to uninoculated and ineffectively nodulated plants were B. japonicum strain 61A76 with 'Evans' and 'Hill' and the fast-growing R. fredii strains with 'Peking' and 'Virginia';A hydroponic growth system was used to study host control of nodulation by fast-growing R. fredii and slow-growing B. japonicum. Nodules were not formed on cultivars with R. fredii strains that produced ineffective nodules when soybean plants were grown in vermiculite. Nodules were formed on compatible cultivars by mutant strains that fixed little or no nitrogen. The inability to form nodules in hydroponics may be related to delays in nodule initiation;The 192 Mdal. plasmid containing symbiotic genes from R. fredii strain USDA193 (pSym193) was transferred to other species of Rhizobium, B. japonicum, Escherichia coli, and Agrobacterium tumefaciens. Host specificity genes for nodulation of soybean cultivar 'Peking' were not expressed in E. coli, but were expressed in the other genetic backgrounds. Similar results were obtained by transferring cloned DNA fragments containing host-specificity genes for Peking and early nodulation genes, demonstrating that the nod genes are unlinked. Site-specific Tn5 mutagenesis was used to locate genes on DNA fragments encoding symbiotic functions. A recombination mutant of R. fredii strain USDA193 was generated to allow complementation studies and the development of a maxi-cell system.



Digital Repository @ Iowa State University,

Copyright Owner

Nancy Marie DuTeau



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195 pages

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Genetics Commons