Call number: LD2668 .T4 1979 M35 / Master of Science

An effective, fast-growing strain of Rhizobium was isolated from a species of Lupinus native to the Sonoran desert near San Felipe, Baja, Mexico (generation time, 3.6 h). Bacteria isolated from the roots of lupines are normally slow growing, however, Lupine 43, is a fast grower, possesses multiple flagella and produces acid in a defined medium. In comparison to a slow-growing lupine strain, Nitragin 96A11, Lupine 43 has a low intrinsic resistance to antibiotics and is able to utilize a wide range of carbon and nitrogen sources. Field and incubator studies were conducted to determine if the adaptation of the characteristics of fast-growing strains enables this strain to survive under the desert conditions of the southwestern United States. In the field, where no moisture was added after initial inoculation, Lupine 43 survived in significantly higher numbers than 96A11 for the first two weeks of the low (19C) and the first month of the high (35C) temperature study. In a laboratory study, at a constant moisture level of 1/3 bar, differences in survival between the two strains were dependent on temperature, pH and soil texture.

No description available.

No description available.

In agriculture nitrogen fixation by rhizobial inocula is an environmentally and economically beneficial alternative to synthetic fertilization. The effectiveness of rhizobial inocula can be limited by the inability of inoculum strains to compete with indigenous strains for nodule occupancy. Sinorhizobium meliloti fixes nitrogen in a complex symbiotic relationship with legume hosts including the agriculturally important forage Medicago sativa and the model legume Medicago truncatula. The ability to utilize organic compounds has emerged as an important trait for competitiveness for nodule occupancy in S. meliloti and other rhizobia. This thesis describes the use of bacterial genetics to characterize two carbon metabolism loci in S. meliloti. A genetic locus for erythritol catabolism was characterized and shown to encode an ABC transporter that is required for the catabolism of erythritol, adonitol and L-arabitol, as well as the genes for the catabolism of these three polyols. The ability to utilize erythritol was not necessary for the ability to compete for nodule occupancy in S. meliloti, in contrast to Rhizobium leguminosarum. A genetic locus that encodes components of the De Ley-Doudoroff pathway of galactose catabolism was identified and also characterized. The inability to catabolize galactose resulted in an increased ability to compete for nodule occupancy in S. meliloti. Evidence is presented that is consistent with the hypothesis that increased competitiveness resulted from enhanced production of the symbiotic exopolysaccharide succinoglycan. Inferences are drawn that contribute to the broader understanding of rhizobium-legume symbiosis. / October 2014

Corrigendum attached to back cover. Includes bibliographical references (leaves 160-190). Investigates the role of rhizopines in rhizobial competition for nodulation, and to isolate the rhizopine synthesis genes in Rhizobium leguminosarum bv. viciae.

Thesis (M.S.)--Virginia Polytechnic Institute, 1953. / Typewritten. Vita. References: leaves 88-89. Also available via the Internet.

In a series of greenhouse experiments, isolates representing serotypically distinct subpopulations of Rhizobium leguminosarum bv. trifolii recovered from root nodules of field-grown subclover Trifolium subterraneum cv. "Nangeela"), were evaluated for their symbiotic effectiveness potential, their effect on regrowth potential of subclover, and for their competitive abilities to form nodules. Isolates from the nodule dominant serotypes AS6, AS6-21, and AS21 were shown to be suboptimally effective at fixing nitrogen gas (N₂) relative to RT162X95, a highly effective commercial inoculant strain. Although the majority of isolates representing the minor nodule occupying serotypes AS27, AG4, and AP17 were also shown to be of moderate effectiveness, isolates ADS14 and RW11 of serotype AS27 were equally as effective as RT162X95. Subclover regrowth was influenced by the severity of the first harvest. Plant-isolate combinations possessing superior N₂-fixing capabilities were more adversely affected by severe clipping than were the less effective combinations. Isolate 88FL3,5, a mediocre N₂-fixing member of the nodule dominant serotype AS6, was not universally more competitive at forming nodules than other isolates. Although 88FL3,5 formed more nodules than many of the other isolates when mixed in equal numbers with each of them separately, isolates ADS14 and ADS16 significantly outcompeted 88FL3,5 even though they represented a minor nodule occupying serotype, AS27. The nodule occupying successes of 88FL3,5 or ADS16 could be overcome if their respective competitor had a numerical advantage ranging between 10:1 and 25:1. Since the soil-borne population of 88FL3,5 has been shown to outnumber other subpopulations of rhizobia in the same soil by at least 10:1, it is not surprising that it can occupy the majority of nodules on field-grown plants even in the presence of more competitive types. Why the highly effective and highly competitive members of serotype AS27 are rarely recovered from nodules of field-grown Trifolium subterraneum L. is a question that merits further study. / Graduation date: 1992

Although much has been learned about the comparative nodulating behavior of simple mixtures of rhizobial strains under non-soil situations, it is unclear how these findings relate to the factors influencing nodulation success by the complex mixtures of strains found within soil-borne rhizobial populations. Information on the structure and physiological behavior of soil populations is almost nonexistent. To achieve a better understanding of the situation in soil, studies were carried out with the following objectives. (i) To delineate by serological analysis the population composition of nodule occupants of Rhizobium leguminosarum bv.trifolii recovered from a variety of annual and perennial clover (Trifolium) species planted into Abiqua soil. (ii) To further the development of an assay to evaluate the substrate responsiveness of specific indigenous serotypes of R. leguminosarum bv. trifolii while they reside within the soil microbial community. Immunodiffusional analysis of isolates recovered from nodules of five annual (T. subterraneum, T. incarnatum, T. vesiculosum, T. parviflorum, T. patens) and three perennial (T. pratense, T. repens, T. hybridum) species of clover revealed that the serotypic composition of the natural population of R. leguminosarum bv. trifolii in Abiqua soil is almost completely known. With antisera to 14 antigenically distinct serotypes at our disposal, only 19 of 272 isolates recovered from these eight clover species were antigenically unknown. While the perennial species showed no pronounced preference for particular serotypes, a substantial proportion (37-75%) of nodule occupants from each of the annual clovers (with the exception of T. vesiculosum) reacted with antiserum AS6. These isolates could be subdivided by their serological reactions of non-identity with either antisera AS6, AS27, or both antisera AS21 and AS27. Using multi-locus allozyme electrophoresis (MLAE) to analyze population structure within serotypes, isolates representing serocluster AS6 were found to be rooted at a similarity of 0.82 and clustered with the other three serotypes (AG4, AS21, and AS27) only at a similarity of 0.37. In contrast to AS6, MLAE analysis revealed that "genotypic distances" between the 7 ETs representing AG4 could be large. The chapter on the nalidixic acid cell-elongation assay only represents the second report of its use on soil microbial populations. Nalidixic acid was found to be the most suitable DNA gyrase inhibitor for rhizobial studies since norfloxacin and ciprofloxacin at extremely low concentrations (2.0 and 0.5 mg/l, respectively) reduced the proportion of elongating cells significantly. In contrast to other indigenous serotypes, the majority of members of serotype AR23 did not elongate in response to yeast extract (YE). Regardless of nutrient type, or concentration, the percentage of elongated cells of AR23 remained low (<16%) even after 24 h of incubation. While the cell elongation response of serotype AS6 occured more rapidly to YE than did AR23, a less vigorous response by AS6 was observed when other nutrient sources were used. The appearance of elongated cells was delayed and the final percentage of elongated cells was reduced. / Graduation date: 1992

A total of 43 Rhizobium leguminosarium bv. trifolii isolates were obtained from soil samples of two ball clover (Trifolium nigrescens) pastures from Iola and Kilgore (Texas) using ball clover as capture plants. The isolates were phenotypically characterized by intrinsic antibiotic resistance (IAR) against a range of concentrations of eight antibiotics, and by the utilization of 95 different carbon sources (BIOLOG system). The rhizobial isolates were also evaluated for their tolerance to salinity, high temperatures and low pH. The isolates showed two different ranges of growth rates: fast-growing (doubling times between 1.4 - 3.7 h) and slow- growing isolates (12.3 - 21.3 h). The numerical analysis of the phenotypic characteristics showed that the 43 isolates could be grouped in 24 different strains. Cluster analysis based on sensitivity responses of IAR, metabolic profiles of BIOLOG and salt, temperature and acidity tolerance levels could distinguish the Rhizobium strains from a Pseudomonas isolate. The analysis also showed that the rhizobial strains isolated from ball clover nodules are different from a commercial R. leguminosarium bv. trifolii strain used as inoculant for this legume.