Award Date

1-1-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Life Sciences

First Committee Member

Jeffery Qingxi Shen

Second Committee Member

Dawn Neuman

Number of Pages

192

Abstract

Global climate change due to elevated atmospheric [CO2] and abiotic stresses have a strong impact on the establishment, survival and reproduction of plants. The goal of my research is to understand the molecular mechanism by which hormones regulate plant responses to elevated CO2 and abiotic stresses. Abscisic acid (ABA) is well known as a stress hormone, which also promotes seed formation and dormancy, and inhibits seed germination. In contrast, another hormone, gibberellin (GA) breaks seed dormancy and promotes seed germination and post-germination growth including flowering. The focus of my research is to study the functions of a group of transcription factors (called WRKY) from creosote bush (Larrea tridentata) and barley (Hordeum vulgare) in ABA- and GA-regulated developmental and physiological processes; This study sheds light on the mechanism controlling seed dormancy and germination, as well as plant responses to elevated [CO2] and environmental stresses. This information will eventually help us develop a strategy to improve crop yields and to combat against global warming resulted from elevated atmospheric [CO2] by engineering plants that can consume [CO2] at a massively enhanced rate, thereby reducing the elevated atmospheric concentration of CO2; In Chapter 2, I show for the first time that like abiotic stresses, elevated [CO2] treatments increased ABA concentrations in the leaves of Larrea tridentata which is an extremely drought-tolerant evergreen C3 shrub dominating the North American warm desert. I then show that a regulatory gene from Larrea tridentata, LtWRKY21, is responsive to elevated [CO2], water deficit, high salinity, and wounDing However, cold and heat treatments decreased the wounding-induced LtWRKY21 mRNA level. In addition, ABA, jasmonic acid (JA, another hormone), and glucose induced the expression of LtWRKY21. Transient expression of LtWRKY21 suggests that this transcription factor acts as an activator of ABA signaling and as a repressor of GA signaling. These results suggest that LtWRKY21 might function as a key regulator of signaling networks in Larrea tridentata; The function of LtWRKY21 in ABA signaling is further studied in Chapter 3. Our data demonstrate that LtWRKY21 interacts synergistically with ABA and transcriptional activators, VP1 and ABI5, to control the expression of the ABA-inducible HVA22 promoter. The transactivating activity of LtWRKY21 relies on the C-terminal sequence containing the WRKY domain and a N-terminal motif that is essential for the repression activity of some regulators in ethylene signaling. In contrast, the LtWRKY2I-mediated transactivation is inhibited by 1-butanol, an inhibitor of phospholipase D, and abi1-1, a dominant-negative mutant protein phosphatase. However, abi1-1 does not block the synergistic effect of LtWRKY21, VPI, and ABI5 co-expression. Taken together, these data support a novel model: ABA signaling is negatively regulated by a protein complex including protein phosphatases (ABI1 or ABI2) and positively regulated by transcription factors such as VPI/ABI3, ABI4, and ABI5. LtWRKY21, VPI, ABI4, and ABI5 may form a complex that functions downstream of ABI1 to control ABA-regulated gene expression; Chapter 4 presents the role of HvWRKY38, an LtWRKY21 homologous gene, in seeds that are tolerant to dessication in the ABA-promoted dormant form, but sensitive to stresses in the GA-promoted germination form. HvWRKY38 physically and functionally interacts with other transcription factors BPBF, DOF, and GAMYB to repress transcription of the GA-inducible Amy32b promoter. Our data suggest that the expression of Amy32b is modulated by protein complexes containing activators and repressors, respectively. The ratio of activators and repressors and their affinities to individual cis-acting elements determine the expression level of Amy32b and hence dormancy or germination of seeds.

Keywords

Abiotic Stresses; Abscisic Acid; Atmospheric Carbon Dioxide; Carbon; Carbon Dioxide; CO2; Controlling; Dioxide; Elevated; Gibberellin; Hormonal Regulation; Mechanisms; Molecular; Plant Responses; Regulation; Stress

Controlled Subject

Molecular biology; Botany

File Format

pdf

File Size

5713.92 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

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