Browsing by Subject "regeneration"
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Item Biolistic and agrobacterium-mediated genetic transformation of immature and mature embryos of spring wheat cultivar Saratovskaya-29(Texas A&M University, 2005-08-29) Kopbayev, Arman A.Plant transformation provides a promising methodology of introducing new genes that encode desirable traits to a wide range of crop plants. Success in genetic transformation has been achieved in many of the important crop species, such as soybean, cotton, rice, corn. However, wheat, one of the major crops of the world, has been considered to be difficult to transform via either Agrobacterium or biolistic bombardment (Rakszegi et al., 2001). There have been limited studies on A. tumefaciens-mediated transformation of cereals, including wheat, because of the overall refractory character of host-pathogen interactions between Agrobacterium and the cereal plants (Gould et al., 1991; Hiei et al., 1994; Cheng et al., 1997). While the genetic transformation of rice using Agrobacterium has become routine, only a few successful studies of Agrobacterium- mediated transformation of wheat have been reported, and these involved a model spring wheat, Triticum aestivum cultivar Bobwhite (Cheng et al., 1997). Model genotypes are developed for ease of plant regeneration in tissue culture and both Agrobacterium and biolistic mediated transformation methods require regeneration of plants in tissue culture. More success has been achieved in obtaining fertile transgenic wheat plants by particle bombardment, or biolistics method (Vasil et al., 1992; Weeks et al., 1993; Becker et al., 1994; Zhou et al., 1995; Altpeter et al., 1996). Wheat plants of the model system cultivar Bobwhite were used in most of these studies as well. The primary objective of this study was to use the callus-based transformation procedures mentioned above with a non-model cultivar of hexaploid spring wheat Saratovskaya-29, widely grown in Kazakhstan, to test the genotype dependence of the previously developed transformation protocols with respect to stable transfer of DNA and regeneration of transgenic plants. The spring wheat cultivar Saratovskaya-29 (Albidum-24/ Lutescens-55-11) was chosen for the study as being one of the most widely grown wheat cultivars both in Russia and Kazakhstan. It was bred in early 50??s in the Research Institute of the South-East, Saratov. Because of its drought resistance and good baking quality traits, Saratovskaya-29 reached a peak of nearly 21.2 mln ha in the former USSR in 1996 (Martynov and Dobrotvorskaya, 1996). Economical importance of this cultivar makes it an appropriate candidate for further improvement of economically significant traits. Another objective of the study described was to compare the transformation efficiencies and inheritance in the transgenic plants produced.Item Cellular and molecular correlates of neural morphallaxis in Lumbriculus variegatus(Texas A&M University, 2006-08-16) Martinez, Veronica GiselleTissue regeneration has intrigued biologists since the eighteenth century. While regeneration has been studied in many species, the cellular and molecular mechanisms governing successful compensation for lost body parts are poorly defined. This dissertation examines the cellular and molecular correlates of a form of regeneration defined as morphallaxis. Morphallaxis does not involve cell proliferation, but instead relies on the reorganization of existing tissues to recover body structure and function. Morphallaxis is a mechanism used during segmental regeneration (i.e., head or tail replacement) by the aquatic oligochaete, Lumbriculus variegatus. Here, morphallaxis of the nervous system is documented during segmental regeneration of Lumbriculus and during asexual reproduction. The morphallactic processes, which underlie changes in the neural anatomy and physiology of these worms, are reminiscent of mechanisms utilized by other neural plasticity events, including learning and memory. Proteomic and biochemical studies focus on a molecular marker of neural morphallaxis. The expression patterns of morphallaxis-associated-protein 66, MP66, are differentially regulated during both regeneration and asexual reproduction. This expression patterncorrelates with time-points of major cellular changes associated with neural morphallaxis. Thus, cellular and molecular events, demonstrated as part of neural morphallaxis in Lumbriculus, are recruited in two life-history contexts. Chemical disruption experiments, where either segmental regeneration or asexual fission are blocked, reveal that morphallaxis can be mechanistically dissociated from regeneration and reproduction. These results set a foundation for future investigations of specific mechanisms that mediate this novel form of neural plasticity.Item Differential mechanisms by which the aryl hydrocarbon receptor attenuates liver regeneration(2007-10-11) Courtney Alicia Lockhart; Kathleen O'Connor, Ph.D.; Wendy Mars, Ph.D.; Steve Weinman, M.D., Ph.D.; Randall Urban, M.D.; Jingwu Xie, Ph.D.; Chunming Liu, Ph.D.Liver regeneration is orchestrated by a series of autocrine and paracrine cues that function to restore hepatic tissue, however the precise cellular and molecular mechanisms that regulate these signaling events are poorly understood. Recent evidence demonstrates that hepatocyte proliferation following partial hepatectomy (PH) can be attenuated by the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that is involved in hepatic organogenesis and cell cycle control. This growth suppression suggests that AhR modulates critical signaling processes of the regenerative program. In particular, the regeneration process is initiated by both cytokines and matrix enzymes and propagated by the potent mitogenic activity of two proteins, the c-Met transmembrane receptor and urokinase plasminogen activator (uPA). However, this growth response is limited by the expression of plasminogen activator inhibitor-1 (PAI-1) and TGF-?, which terminates hepatocyte proliferation. The goal of these studies was to determine the influence of AhR on these moieties in the context of the regenerative program. The hypothesis that AhR modulates these signaling molecules in a mito-inhibitory manner was tested using an in vivo model system of 70% PH in mice pre-treated with 2,3,7,8-tertachlorodibenzo-p-dioxin (TCDD), a potent, prototypical, and persistent AhR agonist. We demonstrate that AhR did not alter cytokine or matrix enzyme expression during the regenerative process, but markedly upregulated PAI-1 and TGF-? protein levels post-PH. As a consequence, both c-Met and uPA activation were greatly suppressed in an AhR-dependent fashion during liver regeneration as well. Conclusion: These observations suggest a novel mechanism of AhR-mediated attenuation of the regenerative response and identify a possible physiologic function of AhR in vivo.Item Plasticity in the Rapid Escape Reflex of the Annelid Worm, Lumbriculus variegatus(2012-10-19) Lybrand, ZaneNeural plasticity is the process by which anatomical (structural) and physiological (functional) changes in the nervous system of an organism lead to alterations in behavior. This dissertation examines the structural and functional changes that occur during neural morphallaxis, a rare form of neural plasticity, in the annelid worm, Lumbriculus variegatus. Neural morphallaxis involves the reorganization of the animal's nervous system during segmental regeneration following injury. Here, I have examined neural morphallaxis of the giant fiber pathway, which mediates rapid escape reflex behaviors in Lumbriculus. Electrophysiological recording techniques, immunohistochemistry, and transmission electron microscopy were used to demonstrate that prior to injury and neural morphallactic regeneration, activation of the escape reflex neural circuitry is nonfunctional in specific regions of the worm's nervous system. Following body fragmentation, neural circuits underlying specific escape responses rapidly become functional. The speed of functional changes in sensory-to-giant interneuron physiology, less than 24 hours, did not coincide with significant anatomical changes to sensory afferent synapses, suggesting a role for the unsilencing of existing sensory synapses. Furthermore, I have discovered and described a sensory interneuron system that mediates sensory inputs via electrical synapses onto the giant interneuron pathway. This finding led to my hypothesis that the site of sensory plasticity during neural morphallaxis is not at the giant axon, but rather at the glutamatergic synapses between sensory neurons and their sensory interneuron targets. Results from this dissertation demonstrate that sensory inputs onto the giant interneuron pathway are functionally silent prior to neural morphallaxis and the awakening of ineffective synapses occurred rapidly, within hours, following injury. Neural morphallactic plasticity was determined to occur at glutamatergic synapses onto bilaterally paired sensory interneurons that were coupled to the giant interneuronal pathway. The early phase of morphallaxis is then followed by gradual structural and functional changes to enhance aspects of the escape response network. This research provides a foundation for future studies of the mechanisms underlying neural morphallactic regeneration in Lumbriculus variegatus and provides comparative insight into the evolution and plasticity of neural circuit underlying discrete animal behavior.Item Visualization of cellular mechanisms regulating differential neuronal synapse formation(Texas A&M University, 2005-11-01) Neunuebel, Joshua PaulOver thirty years ago electrical coupling was observed in embryonic cells prior to chemical communication. This temporal relationship of electrical coupling preceding functional chemical neurotransmission occurs throughout neurogenesis, prompting the idea that gap junctional coupling synchronizes the synaptogenic establishment of functional neural networks. Helisoma neuronal pairs treated with trophic factors exhibit increased electrical coupling and subsequently delay the formation of inhibitory chemical connections. Studies in this thesis addressed the mechanism regulating this inverse relationship between electrotonic and chemical communication. Synaptogenesis between two neurons from the Helisoma buccal ganglia, B110 and B19, were examined using alternative culturing conditions that were either exposed to or deprived of trophic factors. Incubating neuronal pairs in trophic factors induced transient electrical synapses and postponed the formation of chemical connections. In electrically coupled neuronal pairs, presynaptic secretory vesicles were recruited to the sites of presynaptic contact, but did not respond to calcium elevation (i.e., photolytic release of calcium from NP-EGTA) with neurotransmitter release. These and other studies demonstrated that transient electrical coupling does not disrupt calcium handling or postsynaptic responsiveness. Rather, electrotonic coupling delays chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and the synchronized vesicular release of neurotransmitter.