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Title: Plant-mycorrhizal interactions and the relative abundance of limiting resources
Creator: Grman, Emily L.
Date: 2011
Collection: Electronic Theses & Dissertations
Description: The relative abundance of limiting resources, particularly light and soil nutrients, may be a key predictor of plant interactions with arbuscular mycorrhizal fungi (AMF). AMF are typically plant mutualists, increasing access to limiting soil nutrients though their extensive network of soil hyphae in exchange for plant carbon. However, when soil nutrients are abundant, relative to light, AMF are less beneficial to plants. In those situations, as plants shift towards light limitation,...
Show moreThe relative abundance of limiting resources, particularly light and soil nutrients, may be a key predictor of plant interactions with arbuscular mycorrhizal fungi (AMF). AMF are typically plant mutualists, increasing access to limiting soil nutrients though their extensive network of soil hyphae in exchange for plant carbon. However, when soil nutrients are abundant, relative to light, AMF are less beneficial to plants. In those situations, as plants shift towards light limitation, stoichiometric theory predicts decreases in four metrics of plant-mycorrhizal interactions: plant benefit, fungal benefit, plant root colonization by AMF, and plant carbon allocation to AMF. Indeed, fertilization with soil nutrients does decrease at least some of those metrics of plant-mycorrhizal interactions, but many questions remain. First, do conceptual models of stoichiometry adequately capture negotiation between plants and AMF? With Chris Klausmeier and Todd Robinson, I developed a mathematical model that points to two key features of trade between mutualists that have previously been ignored: the negotiated exchange ratio of one resource for another, and allocation to self-provisioning of those resources by each partner. Second, why do AMF sometimes parasitize plants in high nutrient environments? In theory, plants should be able to impose "sanctions" to avoid parasitic carbon drains by "cheating" AMF. In a greenhouse experiment, I show that two C3 grasses (quackgrass, Elymus repens, and smooth brome, Bromus inermis) avoided parasitism by effectively reducing carbon allocation to AMF in high phosphorus environments while one C4 grass (big bluestem, Andropogon gerardii) did not. Third, why do plant allocation to AMF and AMF abundance not always decrease with increases in nutrient availability? Some field studies have shown no change or even increases in those metrics of plant-mycorrhizal interactions with nitrogen or phosphorus fertilization. In a field fertilization experiment with Todd Robinson, I found that AMF increased in response to nitrogen addition in very nitrogen-poor soils, consistent with AMF nitrogen limitation. In an additional field experiment across a natural productivity gradient, I showed that increases in productivity do not necessarily lead to increases in plant light limitation, calling into question the expectation that increases in fertility should change plant-mycorrhizal interactions. Finally, do differences among plant species affect how shifts in stoichiometry alter plant-mycorrhizal interactions? In nutrient poor soils, AMF benefit different plant species differentially, but how those species differences affect mycorrhizal response to fertilization is unclear. In a greenhouse experiment, I found that two C3 grasses did differ from two C4 grasses in terms of how plant benefit, fungal benefit, and plant root colonization responded to increases in phosphorus availability. In a field fertilization experiment, I again found that a C3 grass (B. inermis) differed consistently from a C4 grass (A. gerardii) in how strongly nitrogen and phosphorus fertilization affected plant-mycorrhizal interactions. Taken together, these studies show that stoichiometric theory is a powerful tool for understanding plant-mycorrhizal interactions. However, relationships are complex, and differences among species as well as aspects of negotiation and trade also play important roles.
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Title: Stoichiometric and catalytic carbon-hydrogen borylations of arenes
Creator: Preshlock, Sean Michael
Date: 2013
Collection: Electronic Theses & Dissertations
Description: Regioselective catalytic transformation of carbon-hydrogen bonds to other functional groups represents a long-standing challenge in homogeneous and heterogeneous catalysis. The Ir-catalyzed C-H activation/borylation has emerged as a useful method for synthesizing various aryl and heteroaryl boronic esters with regiochemistry complimentary to traditional methods and tolerant of various functional groups. The steric dominance of C-H activation/borylation has allowed for the synthesis of new...
Show moreRegioselective catalytic transformation of carbon-hydrogen bonds to other functional groups represents a long-standing challenge in homogeneous and heterogeneous catalysis. The Ir-catalyzed C-H activation/borylation has emerged as a useful method for synthesizing various aryl and heteroaryl boronic esters with regiochemistry complimentary to traditional methods and tolerant of various functional groups. The steric dominance of C-H activation/borylation has allowed for the synthesis of new aromatic building blocks which were previously unaccessible or hard to synthesize.With the aid of high throughput experimentation (HTE), we designed reaction screens that would not only optimize Ir-catalyzed C-H borylation reactions for more challenging substrates, but also broaden the scope of this chemistry by assessing the efficiency and compatibility of the reactions as functions of precatayst, boron reagent, ligand, order of addition, temperature, solvent and substrate.We then sought to apply these results to unprotected anilines which were inert under conventional Ir-catalyzed C-H borylation conditions. Building off a recent report from our research group that utilized a unique outer sphere directing effect to obtain ¬ortho¬ functionalized C-H borylation products from NBoc protected anilines, we investigated whether we could use HBPin as a traceless protecting/directing group for the borylation of primary anilines under more forcing conditions. Our attempts were successful and provided for a one-pot protection/deprotection procedure that used a lower catalyst loading and gave products in better to comparable yields than the NBoc protected borylation reaction. Using an (η6-C6Me3H3)Ir(BPin)3 as starting material, five-coordinate bisphosphine complexes can be synthesized by displacement of the η6-C6Me3H3 with the incoming phosphine ligands. Using this method we have isolated the trans-Ir(PAr3F)2BPin3 complex which is thought to be the active catalyst for the ortho¬-C-H directed borylation of benzoate esters first discovered by Miyaura. The trans-Ir(PAr3F)2BPin3 complex is not catalytically competent with HBPin as borylating reagent. We have since been able to develop silylphosphine chelates (Figure 8) that are compatible with HBPin while retaining the unique selectivity shown with the Ir(PAr3F)2BPin3 complex. Studies to further improve on the silylphosphine ligand design and extend the substrate scope are ongoing.
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Title: Experimental evolution and ecological consequences : new niches and changing stoichiometry
Creator: Turner, Caroline B.
Date: 2015
Collection: Electronic Theses & Dissertations
Description: Evolutionary change can alter the ecological conditions in which organisms live and continue to evolve. My dissertation research used experimental evolution to study two aspects of evolutionary change with ecological consequences: the generation of new ecological niches and evolution of the elemental composition of biomass. I worked with the long-term evolution experiment (LTEE), which is an ongoing experiment in which E. coli have evolved under laboratory conditions for more than 60,000...
Show moreEvolutionary change can alter the ecological conditions in which organisms live and continue to evolve. My dissertation research used experimental evolution to study two aspects of evolutionary change with ecological consequences: the generation of new ecological niches and evolution of the elemental composition of biomass. I worked with the long-term evolution experiment (LTEE), which is an ongoing experiment in which E. coli have evolved under laboratory conditions for more than 60,000 generations. The LTEE began with extremely simple ecological conditions. Twelve populations were founded from a single bacterial genotype and growth was limited by glucose availability. In Chapter 1, I focused on a population within the LTEE in which some of the bacteria evolved the ability to consume a novel resource, citrate. Citrate was present in the growth media throughout the experiment, but E. coli is normally unable to consume it under aerobic conditions. The citrate consumers (Cit+) coexisted with a clade of bacteria which were unable to consume citrate (Cit-). Specialization on glucose, the standard carbon source in the LTEE, was insufficient to explain the frequency-dependent coexistence of Cit- with Cit+. Instead Cit– evolved to cross-feed on molecules released by Cit+. The evolutionary innovation of citrate consumption led to a more complex ecosystem in which two co-existing ecotypes made use of five different carbon sources.After 10,000 generations of coexistence, Cit- went extinct from the population (Chapter 2). I conducted replay experiments, re-evolving for 500 generations 20 replicate populations from prior to extinction. Cit- was retained in all populations, indicating that the extinction was not deterministic. Furthermore, when I added small numbers of Cit- to the population after extinction, Cit- was able to reinvade. It therefore appears that the Cit- extinction was not due to exclusion by Cit+, but rather to unknown laboratory variation.Chapter 3 shifts focus to studying evolutionary changes in stoichiometry, the ratio of different elements within organisms’ biomass. Variation in stoichiometry between organisms has important ecological consequences, but the evolutionary origin of that variation had not previously been studied experimentally. Growth in the LTEE is carbon limited and nitrogen and phosphorus are abundant. Additionally, daily transfer to fresh media selects for increased growth rate, which other research has suggested correlates to higher phosphorus content. Consistent with our predictions based on this environment, clones isolated after 50,000 generations of evolution had significantly higher nitrogen and phosphorus content than ancestral clones. There was no change in the proportion of carbon in biomass, but the total amount of carbon retained in biomass increased, indicating that the bacteria also evolved higher carbon use efficiency.To test whether the increases in nitrogen and phosphorus observed in the LTEE were a result of carbon limitation or were side effects of other selective factors in the experiment, I evolved clones from the LTEE for 1000 generations under nitrogen rather than carbon limitation (Chapter 4). The stoichiometry of the bacteria did change over the course of 1000 generations, indicating that evolution of stoichiometry can occur over relatively short time frames. Unexpectedly however, the evolved bacteria had higher nitrogen and phosphorus content. It appears that the bacteria were initially poor at incorporating nitrogen into biomass, but evolved improved nitrogen uptake.
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