Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 166-190).

Starch is the main storage biopolymer in cereal plants. Several enzymes and carrier proteins are involved in the starch biosynthesis process. ADP-glucose pyrophosphorylase (AGPase) has been characterized as a key factor in this process, which catalyzes the conversion of glucose 1-phosphate into ADP-glucose in the cytosol of the endospermic cell. The freshly synthesized ADP-glucose must be transported into amyloplasts by the activity of ADP-glucose transporter. In the current research, we have characterized HvBT1 biochemically in E. coli system. HvBT1 shows high affinity to ADP-glucose as a transport substrate in counter-exchange with ADP with affinities of 614 and 334 µM, respectively. The cellular and subcellular localization of HvBT1 indicated its target the amyloplasts envelopes. The comparison between two barley cultivars; Harrington and Golden Promise shed some light on the impact of HvBT1 on starch accumulation. Higher expression of AGPase and HvBT1 (10 fold) provide an ideal combination for improving starch yield, where starch content was higher by 2.5% in Harrington. Unlike Harrington, the expression of soluble starch synthase encoded genes was higher in Golden Promise which accumulates less starch. This result provided evidence of the importance of HvBT1 in starch synthesis process along with AGPase. Down-regulation of HvBT1 also provided a cement evidence of its effect on the starch accumulation process, where the knock down lines showed 17% lower starch and altered starch composition. Also, as a result of decreasing starch, protein content increased in the transgenic grains by 4-5 % of its content in the wild type, while β-glucan was 37% lower than the wild type control. Down-regulation of HvBT1 led to decrease the grain yield by ~ 30% as a result of increase the grain size. Also, it seems to have pleotropic effects on other starch synthesis genes, where AGPLs was down-regulated while the plastidial SSU genes, AGPS1b and S2 were up-regulated. Soluble starch synthases SS2a and SS3a were down-regulated, while SS2b was up-regulated in the transgenic plants. The accumulated evidences indicated that HvBT1 is a key factor in starch biosynthesis process. / February 2015

Hybrids and polyploids are very common in plants and some animals. Although hybrid vigor or heterosis has been widely adopted in agricultural practices, the underlying mechanisms are poorly understood partly because of their multigenic nature and the lack of a good model system for the study. Allotetraploidy is an emerging model system for investigating molecular mechanisms of hybrid vigor. An allotetraploid is formed by interspecific hybridization followed by chromosome doubling or hybridization between two autotetraploid parents and is genetically stable. A previous study showed nonadditive expression (different from the mid-parent value) of over 5% of genes in the allotetraploids, suggesting altered transcriptional and post-transcriptional regulation. Here oligo-gene microarray analysis of mRNA stability in allotetraploids was carried out to investigate how nonadditive gene regulation upon allopolyploidization is achieved at the posttranscriptional level. Approximately 1% of annotated genes were identified as unstable transcripts, and their estimated half-life is less than 60 minutes. The unstable transcripts in Arabidopsis allotetraploids are associated with nonadditive gene expression and with stress and environmental responses. The nonadditively expressed genes identified in the previous study include those encoding proteins involved in energy and metabolic pathways, which are putative targets of circadian clock regulators. To test how circadian clock genes affect downstream genes and pathways, expression of CIRCADIAN CLOCK ASSOCIATED1 (CCA1) was up- or down-regulated by overexpressing CCA1 or cca1(RNAi) driven by the promoter of TIMING OF CAB EXPRESSION 1 (TOC1). Upregulation of CCA1 was associated with repression of downstream genes in chlorophyll biosynthesis and starch metabolism, whereas down-regulation of CCA1 correlated with upregulation of these downstream genes. As a result, chlorophyll and starch content was ~10% higher in the TOC1::cca1(RNAi) transgenic plants than the controls, while the growth vigor is lower in the TOC1::CCA1 transgenic plants. To further test the effects of clock genes in growth vigor, CCA1 expression was examined in reciprocal hybrids of A. thaliana ecotypes. The maternal effect on starch content was observed in several combinations of hybrids, which was correlated with preferential expression of maternal CCA1 during early stages of seed development. Although the cause of parent-of-origin effects is still unclear, the data have clearly documented parent-of-origin effects on circadian clock gene expression and starch metabolism in hybrids. / text

published_or_final_version / Botany / Master / Master of Philosophy

Ribulose 1,5-biphosphate carboxylase/oxygenase is activated by incubation with CO₂ and Mg²⁺. Several compounds are known to mediate this CO₂ dependent activation, including several chloroplast metabolites. A search, based on structural similarities to known activators, encountered more than twenty previously unreported effectors. Activators appear to share several common characteristics, including two anionic groups which usually occupy terminal positions on the molecule. Terminal groups on activators include carboxylate, phosphate, phosphonate, and sulfonate. Activators which do not have at least one phosphate or phosphonate terminal group have a hydroxyl or carboxyl containing side chain. Positive effectors change the response of the carboxylase by allowing activation at lower levels of CO₂ and/or Mg²⁺, and at lower pH values. Higher CO₂ concentrations also allow activation at lower pH values. The ratio between the carboxylase and oxygenase functions of this enzyme at air levels of CO₂ and O₂ does not change with effector induced changes in enzyme activity, suggesting that they share the same active site. There is an apparent difference in the response of the enzyme before and after purification to incubation in phosphate, and to inhibition by the substrate ribulose 1,5-biphosphate. These observations suggest that the isolated protein is different from the native enzyme. Several carboxylase activators are taken up by isolated chloroplasts in an exchange for phosphate catalyzed by the phosphate translocator. Included in this group is phosphonopropionate, a non-metabolized analogue of phosphoglycerate. Addition of this effector to chloroplasts under conditions of photosynthesis results in increased carboxylase activity, but a decrease in both ribulose 1,5-biphosphate levels and CO₂ fixation. There is a slow leakage of phosphate from isolated chloroplasts at room temperature. This leakage does not occur during illumination, nor at ice temperature, but increases with increasing hydroxide ion over physiological pH values. This slow leakage is probably an artifact of chloroplast isolation procedures. In isolated chloroplasts, phosphate levels appear to control the partition of photosynthate between starch formation and sugar phosphate export. During periods of net accumulation of starch in the chloroplast, there is concomitant degradation occurring, such that the accumulation of label into the starch fraction may not reflect the actual rate of starch synthesis.

Thesis (M.S.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on May 11, 2009) Thesis advisor: Fu-Hung Hsieh. Includes bibliographical references.

Thesis (doctoral)--University of Göteborg, 1993. / Added t.p. with thesis statement inserted. Includes bibliographical references.

Thesis (doctoral)--University of Göteborg, 1993. / Added t.p. with thesis statement inserted. Includes bibliographical references.

Thesis (Ph. D.)--Illinois State University, 1983. / Title from title page screen, viewed May 5, 2005. Dissertation Committee: Mathew J. Nadakavukaren, Derek A. McCracken (co-chairs), Jerome R. Cain, Anthony E. Liberta, David F. Weber. Includes bibliographical references (leaves 108-117) and abstract. Also available in print.

Thesis (Ph. D.)--Illinois State University, 1983. / Title from title page screen, viewed May 18, 2005. Dissertation Committee: Derek A. McCracken (chair), Mathew Nadakavukaren, Jerome R. Cain, Arlan G. Richardson, David F. Weber. Includes bibliographical references (leaves 137-150) and abstract. Also available in print.