UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item THE PHYTOHORMONE ETHYLENE: (I) INVESTIGATING THE MOLECULAR FUNCTION OF RTE1 AND (II) INSIGHTS ON THE EVOLUTION OF THE ETHYLENE BIOSYNTHESIS AND SIGNALING PATHWAYS(2017) Clay, John; Chang, Caren; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ethylene is an important phytohormone that regulates growth, development and stress responses in land plants and charophycean green algae. In Arabidopsis thaliana, ethylene is perceived by a family of five receptors. One of these five receptors, ETR1, is dependent on REVERSION-TO-ETHYLENE1 (RTE1) and Cytochrome B5 (Cb5) while the other four receptors are not. We found that RTE1 and Cb5 interact in planta and used genetic analyses to place Cb5 upstream of RTE1 in the ethylene signaling pathway. After comparing different ethylene receptors we identified an N-terminally localized proline that is important in determining whether a receptor is RTE1-dependent. Our results suggest that Cb5 receives electrons from upstream redox molecules, passes these electrons to RTE1; RTE1 is then able to activate the ETR1 receptor possibly by acting a molecular chaperone that refolds the ETR1 receptor into an active conformation. The ethylene signal transduction pathway is functionally conserved in the charophycean green algae such as Spirogyra pratensis, suggesting that this signaling pathway was present in the common ancestor of charophytes and land plants over 450 million years ago. However, it is unclear whether the central regulator of ethylene response, EIN2, was conserved in charophytes. Furthermore, the details of ethylene biosynthesis in charophytes were unresolved. After examining the genomes and transcriptomes of many green algae we are able to report that EIN2 is conserved in most charophytes and even some of the more distantly related chlorophycean green algae. Moreover, the Spirogyra EIN2 is functionally conserved and able to activate ethylene responses in Arabidopsis. Ethylene is synthesized via a two-step reaction involving the conversion of S-adenosyl-L-methionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS), followed by oxidation of ACC to ethylene gas by the enzyme ACC oxidase (ACO). We identified S. pratensis ACS homologs and demonstrated that S. pratensis can synthesize ACC. S. pratensis lacks ACO homologs but we find it is still capable of producing low levels of ethylene. From our results we conclude that the ethylene biosynthesis and signaling pathways were established in early charophytes allowing these algae to establish ethylene as an important signalling molecule.Item High Pressure Emulsion and Miniemulsion Copolymerization of Vinyl Acetate and Ethylene(2017) Narayanan, Manu; Choi, Kyu Y; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Miniemulsion copolymerization in gas-liquid systems such as vinyl acetate-ethylene (VAE) differs from that of conventional VAE emulsion copolymerization. Particle nucleation in miniemulsions is mainly due to the radical entry into the monomer droplets. The objective of this thesis is to study the reaction mechanism of VAE miniemulsion copolymerization, characterize the copolymer formed and compare it to the conventionally and commercially used VAE emulsion copolymer, and highlight any advantages of VAE miniemulsions over normal VAE emulsions. Miniemulsions have shown to be more stable and long-lasting than emulsions in terms of particle size distributions. The amount of ethylene incorporated in the copolymer is higher in the case of miniemulsions as compared to emulsions due to decreased mass transfer limitations of ethylene in the aqueous phase and subsequent incorporation into the VAE copolymer.Item HETEROGENOUS ETHYLENE POLYMERIZATION IN A MICRO REACTOR SYSTEM(2013) Mahadevan, Meera; Choi, Kyu Yong; Ohadi, Michael M; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Micro reactors provide enhanced mass and heat transfer owing to their high surface area to volume ratio. These reactors offer precise control and selectivity and can be used for synthesizing specifically engineered and technically sophisticated olefin polymers. An unsteady state reactor model (using coordination reaction kinetics) was developed to study the concentration profiles of monomer, catalyst, polymer and its molecular weight distribution along the length of the reactor with time. Nano silica particles of diameter 400nm were synthesized as a support for the metallocene catalyst. Heterogeneous ethylene polymerization was carried out in tubular reactors of diameters 800 µm, 1 mm, and 2.37 mm under 2-phase flow conditions. This thesis investigates the effect of operating conditions in a micro reactor on the qualitative and quantitative properties of the polymer. The results can be extended to propose applications for synthesis of polymers with unique morphology using the inherent advantages of these reactors.Item Evaluation of Raspberry (Rubus sp.) Genotypes for Postharvest Quality and Resistance to Botrytis cinerea(2012) Harshman, Julia Mae; Walsh, Chris S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Raspberries are a delicate, high value specialty crop with an extremely short shelf life. This is exacerbated by their susceptibility to postharvest decay caused by Botrytis cinerea. Of the three commercially available species, red raspberry (Rubus idaeus L.) is the most widely grown. Yellow (R. idaeus L.), black (R. occidentalis L.) and purple raspberries (R. × neglectus Peck.) are mainly available from direct marketers. The quality and storageability of 17 cultivars was examined weekly from June to September during two growing seasons. Storage life was assessed weekly, while firmness, color, respiration and ethylene evolution rates were measured in select harvests. Black and purple raspberries outperformed red and yellow cultivars in their ability to resist B. cinerea colonization. Black raspberries also had the lowest ethylene evolution rates and incidence of decay. This information will be useful to raspberry breeding programs by identifying physiological characteristics that are correlated with greater resistance to B. cinerea.Item Physiological and Molecular Studies of Ethylene Effects on Soybean Root Infection by Soybean Cyst Nematodes(2007-12-10) Xue, Ping; Solomos, Theophanes; Tucker, Mark L.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Soybean cyst nematode (SCN), Heterodera glycines, is one of the most devastating pests of soybean in the world. Several earlier reports demonstrated that ethylene is involved in nematode feeding cell formation in Arabidopsis and tomato. I investigated whether or not ethylene is involved in SCN feeding cell formation in soybean. My results show that SCN parasitism was increased by treatment of roots with ethylene and inhibited by suppressors of ethylene action or in an ethylene resistant soybean mutant. My results also indicate that excised soybean roots colonized by SCN produced ethylene at 1.5-3 times the rate of non-infected roots between 14 and 22 days post inoculation. To determine if ethylene was being synthesized in feeding cells, an ethylene-responsive promoter fused to a GUS reporter gene was constructed and transformed into soybean roots with Agrobacterium rhizogenes. Overall, the results suggest that ethylene plays an important role in SCN infection in soybean