Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item LEUNIG, LEUNIG HOMOLOG, AND SEUSS ARE TRANSCRIPTIONAL CO-REPRESSORS THAT REGULATE FLOWER DEVELOPMENT, MUCILAGE SECRETION, AND PATHOGEN RESISTANCE(2009) Bui, Minh; Higgins, William J; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Transcriptional repression is an important regulatory mechanism for development. My thesis focuses on dissecting the function of Groucho (Gro)/Transducin-Like Enhancer of split (TLE) family of transcriptional repressors in plant development. My work characterizes two Arabidopsis thaliana genes, LEUNIG (LUG), first discovered to repress transcription of the floral homeotic gene AGAMOUS (AG), and LEUNIG_HOMOLOG (LUH), a gene with the highest sequence similarity to LUG. To investigate the functional redundancy between LUG and LUH, I constructed and analyzed lug; luh double mutants, and concluded that both LUG and LUH repress AG expression in the flower, with LUG playing a more prominent role than LUH. The double mutant also revealed a previously unknown function of LUG and LUH in embryogenesis because lug-3; luh-1 double mutants are embryo lethal, while the single mutants develop normal embryos. During the course of this study, I developed a new genotyping method called Simple Allele-discriminating PCR (SAP), which is cost-effective, quick, and easy to perform. This method has greatly facilitated my research as well as others in the lab. A second part of my thesis addresses the role of LUG and LUH in other developmental processes besides flower development. My data indicate that these two genes, like their counter parts in fungi and animals, act as "global co-repressors" in various developmental and physiological processes. My thesis work revealed that both co-repressors, together with its interacting protein SEUSS (SEU), repress the Salicylic Acid (SA) pathogen defense pathway. Although lug-3, luh-1, and seu-1> mutants induced PR1 expression at higher levels than wild-type, only lug-3 and seu-1 mutants were pathogen resistant. Furthermore, LUH functions as a positive regulator in seed mucilage secretion, a process important for proper seed germination, hydration, and dispersal. I propose a possible connection between the defect in mucilage secretion and pathogen defense in luh-1 mutant plants and seeds, which places the foundation for further investigation and may uncover mucilage secretion as a major defense mechanism. My thesis has provided important insights into how transcriptional co-repressors regulate diverse developmental and physiological pathways in higher plants.Item Acclimation of marine macrophytes (Saccharina latissima and Zostera marina) to water flow(2008-05-12) Jordan, Terry Lynn; Koch, Evamaria; Davison, Ian; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I examined the physiological response of two marine macrophytes, the brown alga Saccharina latissima and the angiosperm Zostera marina, to water flow in nature and in controlled experiments. Limitation of photosynthesis of both species by the availability of dissolved inorganic carbon (DIC) was increased under low current velocities. Physiological acclimation to low water flow occurred via upregulation of DIC uptake mechanisms in both S. latissima and Z. marina. Both species increased their ability to generate CO2 in the boundary layer by increasing external carbonic anhydrase and in Z. marina by also increasing proton extrusion and photosynthetic capacity. Changes in the xanthophyll-cycle in low-flow grown S. latissima increased non-photochemical quenching, thus reducing photodamage when photosynthesis was limited by DIC uptake. Water flow also affected root length in Z. marina but root length and below ground biomass were also significantly affected by sediment type, an indirect effect of water flow.Item COMPARATIVE ECOLOGY OF THE INVASIVE RUBUS PHOENICOLASIUS AND THE NATIVE RUBUS ARGUTUS(2005-05-26) Innis, Anne Foss; Forseth , Irwin N; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Invasive species are one of the most significant factors in human influenced global change. Management actions that prevent the spread and impacts of invasive species require knowledge of their ecological characteristics. The characteristics of the invasive wine raspberry (Rubus phoenicolasius Maxim.) and the native sawtooth blackberry (Rubus argutus Link) were examined in two forest habitats on the Maryland Coastal Plain. The invasive had greater negative effects on a common herbaceous plant (Duchesnea indica Andr. Focke) than the native. The invasive, R. phoenicolasius had higher leaf nitrogen concentrations (Nleaf), greater specific leaf areas (SLA) and higher maximal rates of photosynthesis (Amax) for a given dark respiration rate (Rd) than R. argutus. R. phoenicolasius depended less upon pollinators for fruit development and had higher fruiting rates with more seeds per fruit than the native species. In addition, seeds of R. phoenicolasius had higher germination rates. Survival of invasive seedlings was negatively affected by leaf litter additions, but seedling growth was not negatively influenced by shading. R. phoenicolasius seedlings grown in a greenhouse and inoculated with arbuscular mycorrhizal fungi produced less biomass than seedlings that were not inoculated. The distribution of R. phoenicolasius may be affected by leaf litter, but presence of AMF is probably not necessary for seedling success. A three year demographic study showed that both species were negatively impacted by drought, but the invasive recovered faster than the native species in the higher light forest. Overall, results of these studies indicate that the invasive R. phoenicolasius was more likely to competitively exclude understory herbs which can coexist with the native R. argutus. The ability of R. phoenicolasius to adapt physiologically to different light levels helps explain its ability to form dense populations under varying environmental conditions. The combination of high fruit production, plasticity and lower mortality in high light habitats is in agreement with previous studies on invasive species. The combined characteristics demonstrated by R. phoenicolasius allow this invader to spread, expand and persist in mid-successional forests in the Coastal Plain of Maryland.