PLASMID CONTROL OF SYMBIOTIC PROPERTIES IN Rhizobium fredii A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN MICROBIOLOGY

Large indigenous plasmids are a common feature in members of the genus Rhizobium, and their involvement in the control of symbiotic functions has been established. Studies on these rhizobial plasmids, called pSym plasmids, make up much of what is understood about the molecular and genetic basis of the N2-fixing symbiosis. The soybeans, generally nodulated by slow-growing rhizobia, are an economically important crop but genetic studies about this group of rhizobia have made little progress because of their slow growth rate and the absence of identifiable plasmid-associated symbiotic functions. An approach to understanding the symbiosis in the soybean system is to use as a genetic model, R. fredii, the fast-growing rhizobia that nodulate soybeans. In this study, five strains of R. fredii were examined for the presence of indigenous plasmids. To determine if symbiotic functions are controlled by genes on these plasmids, the strains were subjected to plasmid-curing treatments. The effect of a pSym plasmid from a heterologous species on R. fredii gene function was studied by the introduction of the R. leguminosarum pSym plasmid, pJB5JI. The results of this study show that high molecular weight plasmids that are involved in determining symbiotic functions, as well as cryptic plasmids, are an integral part of the genetic make-up of R. fredii. These strains can receive and maintain p7B5JI, a pSym plasmid from another Rhizobium species. However, the pJB5JI plasmid genes are not expressed in the R. fredii genetic background. The introduction of the plasmid did not enable any of the R. fredii transconjugants to nodulate peas, nor did it restore the ability of the plasmid-cured transconjugants to nodulate soybeans. The presence of the plasmid affected the expression of the R. fredii symbiotic genes, resulting in different levels of symbiotic effectiveness.