Browsing by Subject "homeostasis"
Item Arabidopsis Thaliana CARBOXYL-TERMINAL DOMAIN PHOSPHATASE-Like1 (CPL1) Mediates Responses to Iron Deficiency and Cadmium Toxicity(2014-04-24) Aksoy, EmreThe expression of genes that control iron (Fe) uptake and distribution (i.e., Fe utilization- related genes) is under a strict regulation. Fe deficiency strongly induces Fe utilization- related gene expression; however, little is known about the mechanisms that regulate this response in plants. In this dissertation, a RNA metabolism factor, RNA POLYMERASE II CTD-PHOSPHATASE-LIKE1 (CPL1) was shown to localize to the root stele, and to be involved in the regulation of Fe deficiency responses in Arabidopsis thaliana. An analysis of multiple cpl1 alleles established that cpl1 mutations enhanced transcriptional responses of Fe utilization-related genes, e.g. IRON-REGULATED TRANSPORTER1 (IRT1), to low Fe availability. In addition to the lower Fe content in the roots, but higher Fe content in the shoots of cpl1-2 plants, the root growth of cpl1-2 showed improved tolerance to Fe deficiency. Genetic data indicated that cpl1-2 likely activates Fe deficiency responses upstream of both FE?DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT)- dependent and -independent signaling pathways. Interestingly, various osmotic stress/ABA-inducible genes were up-regulated in cpl1-2, and the expression of some ABA-inducible genes was controlled by Fe availability. Unlike Fe, accumulation of the heavy-metal cadmium (Cd) in plants is toxic and it is absorbed by the roots due to the low selectivity of metal transporters such as AtIRT1. In this dissertation, CPL1 was also shown to regulate the transcriptional responses to Cd toxicity. cpl1-2 showed higher tolerance to the Cd toxicity by enhancing the root-to-shoot translocation of Cd by an unknown mechanism. A knowledge-based screening resulted in identification of a putative metal transporter, OLIGOPEPTIDE TRANSPORTER (OPT), which was highly induced in cpl1-2 upon exposure to Cd. OPT was localized to the plastids, indicating a role of plastids in Cd transport and accumulation. The root growth of opt mutants showed higher tolerance to the Cd toxicity, and the mutants accumulated less Cd, Fe and Zn, indicating the involvement of OPT in the transport of these metals. This presented dissertation suggests that 1) CPL1 functions as a negative regulator of the Fe deficiency signaling at the crosstalk with a branch of the osmotic stress/ABA signaling pathway, and 2) CPL1 regulates the Cd distribution in plants by repressing the expression of OPT.Item Effects of Sterol Structure on Insect Herbivore Physiology, Biochemistry and Molecular Biology(2012-02-14) Jing, XiangfengSterols serve two important biological functions in animals - they act as cellular membrane components, and as the precursor to steroid hormones. Insects require a dietary source of sterol because they cannot synthesize sterols de novo. Cholesterol is the most common sterol in plant-feeding insects, but because plants contain very little cholesterol, plant-feeding insects must convert plant sterols into cholesterol. In this dissertation I investigate the effect of common and novel plant sterols and steroids found in a transgenic tobacco line on several caterpillar species. I also explore the metabolism of these sterols and steroids, and use a microarray approach to identify genes involved in sterol use and metabolism in plant-feeding insects. I also study cholesterol homeostasis using a grasshopper species. Modified tobacco plants containing a novel sterol profile negatively affected performance three different caterpillar species, especially in the second generation. Insects reared on modified plants contained less total sterols and cholesterol than those on control plants having normal sterol profile. Similar results were found using artificial diets containing atypical steroids, e.g., cholestanol and cholestan-3-one, identified in the tobacco plants that were fed to my experimental caterpillars. More importantly, the sterol/steroid ratio, but not their absolute amount in the diets, determined the negative effects. Caterpillar species could convert stigmasterol, a common plant sterol, into cholesterol. They could also convert cholestan-3-one into cholestanol and epicholestanol, although this ability varied among different species. A microarray study, that focused on gene expression in midgut tissue, indicated that stigmasterol, cholestanol and cholestan-3-one could induce different gene expression level, and that cholestan-3-one caused a the largest pool of genes to be regulated. The genes possibly involved in the metabolism of stigmasterol and cholestan-3-one were reported. These findings are important in directing further research on the potential application of plant sterol modification to control pests in agricultural systems. Insect herbivores could behaviorally regulate the intake of several nutrients, but they could not regulate their sterol intake. They did, however, practice cholesterol homeostasis, by postingestively regulating tissue sterol levels, even when feeding on diets with high cholesterol content. Collectively, the results from this dissertation provide unique insight into cholesterol regulation, which is difficult to achieve in mammals that are capable of synthesizing their own sterols.