Browsing by Subject "symbiosis"

Now showing 1 - 4 of 4
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    Data and analysis scripts for the paper: Resource acquisition and allocation traits in symbiotic rhizobia with implications for life-history outside of legume hosts
    (2018-10-15) Muller, Katherine E; Denison, R Ford; mulle374@umn.edu; Muller, Katherine E; University of Minnesota Department of Ecology Evolution and Behavior; University of Minnesota Graduate Program in Plant and Microbial Biology; Denison Lab, University of Minnesota
    This repository contains all analysis scripts and data files for Katherine Muller's first dissertation chapter: Resource acquisition and allocation traits in symbiotic rhizobia with implications for life-history outside of legume hosts. The data are being released to meet a publisher's data sharing requirements. The paper addresses two gaps in knowledge needed to understand how polyhydroxybutyrate (PHB), a storage compound accumulated by many nitrogen-fixing rhizobia during interactions with legume hosts, contributes to the in the evolution of the legume rhizobia mutualism. Part 1 provides evidence for wide-ranging, heritable phenotypic variation within natural rhizobia populations in the amount of PHB stored during symbiosis. The conclusions are based on observations of phenotypic variation in rhizobial PHB per cell measured in the field (soybean and Chamaecrista fasciulata) and in greenhouse plants inoculated with field soil, plus experiments with rhizobia isolates showing high heritability in PHB per cell within soybean nodules. Part 2 explores the implications of heritable PHB variation for variation in fitness during the free-living stage using modeling and laboratory experiments. The model shows that the levels of PHB accumulated in nodules could potentially support survival functions for many years after symbiosis, based on published metabolic parameters and soil temperature measurements. The experiments compared survival and PHB use among rhizobia isolates with heritable variation in initial PHB supply, incubated in starvation cultures for over a year. The results on survival were inconclusive (partially due to contamination), but the results on PHB use suggest that variation in PHB accumulation in nodules could contribute to fitness of individual rhizobia long after they are released into the cell.
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    Data from "Detrimental effects of rhizobial inoculum early in the life of the partridge pea Chamaecrista fasciculata"
    (2018-01-19) Pain, Rachel E; Shaw, Ruth G; Sheth, Seema N; repain@umn.edu; Pain, Rachel E
    Premise of the study: Mutualistic relationships with microbes may aid plants in overcoming environmental stressors, and increase the range of abiotic environments where plants can persist. Rhizobia, nitrogen-fixing bacteria associated with legumes, often confer fitness benefits to their host plants by increasing access to nitrogen in nitrogen-limited soils, but effects of rhizobia on host fitness under other stresses, such as drought, remain unclear. Methods: In this greenhouse study, we varied application of rhizobia (Bradyrhizobium sp.) inoculum and drought to examine whether the fitness benefits of rhizobia to their host, the partridge pea (Chamaecrista fasciculata), would differ between drought and well-watered conditions. Plants were harvested nine weeks after seeds were sown. Key results: Young Chamaecrista fasciculata plants that had been inoculated had lower biomass, leaf relative growth rate, and stem relative growth rate compared to young uninoculated plants in both drought and well-watered environments. Conclusions: Under the conditions of this study, the rhizobial inoculation imposed a net cost to their hosts early in development. Potential reasons for this cost include allocating more carbon to nodule and root development than to above-ground growth and a geographic mismatch between the source populations of host plants and rhizobia. If developing plants incur such costs from rhizobia in nature, they may suffer an early disadvantage relative to other plants, whether conspecifics lacking rhizobia or heterospecifics.
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    Evolution of energy storage traits in symbiotic rhizobia
    (2018-01) Muller, Katherine
    The mutually-beneficial symbiosis between legume plants and nitrogen-fixing rhizobia involves an inherent conflict-of-interest over how rhizobia allocate the resources they receive from the host plant. In theory, rhizobia could enhance their future fitness by diverting resources from nitrogen-fixation into storage compounds like polyhydroxybutyrate (PHB). Although the conflict-of-interest between PHB accumulation and nitrogen-fixation has been discussed as a driving factor in the evolution of legume-rhizobia interactions, its role in natural populations is unclear. Therefore, this dissertation fills in key empirical gaps between what we know about the functional role of PHB and hypotheses about how natural selection might act on continuous variation in the amount of PHB that rhizobia acquire during symbiosis. The first chapter assesses the extent of heritable phenotypic variation within natural rhizobia populations interacting with soybean (Glycine max) and partridge pea (Chamaecrista fasciculata) and evaluates implications for fitness in the free-living stage after rhizobia are released from nodules. The results from my first chapter show that 1) natural populations of rhizobia contain heritable, quantitative variation in the amount of PHB they accumulate during symbiosis (a prerequisite for evolution by natural selection) and, 2) natural selection on PHB accumulation may be mediated by how rhizobia allocate PHB over time and among life functions, which could vary independently from traits underlying the amount of PHB acquired during symbiosis. The second chapter assesses phenotypic response to selection based on resident rhizobia populations from long-term agricultural plots varying in host (soybean) or non-host (maize) frequency over years. The mean PHB per cell (measured in nodules) was two times higher in rhizobia populations from plots with 5 or 30 years of continuous maize than from plots where soybean was grown in the previous year. An apparent decrease in mean PHB per cell after the first year of soybean following five years of maize supports the hypothesis that low-PHB rhizobia have higher reproduction in nodules, perhaps due to host sanctions against rhizobia that divert more resources to PHB. A model used to interpret the results suggests that, 1) PHB acquired during symbiosis may contribute to fitness variation for several years after the last host crop, and 2) host sanctions against less-beneficial rhizobia may be stronger in the first soybean crop due to a combination of lower initial rhizobia population size and negative frequency-dependent selection during symbiosis. Collectively, these findings provide empirical support for previously unsubstantiated hypotheses about how conflicts-of-interest over resource allocation contribute to the evolution of the legume-rhizobia mutualism and develop a more nuanced framework for future research.