Symbiotic and saprophytic characteristics of a soil population of Rhizobium leguminosarum bv. trifolii Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x633f362x

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  • 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.
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