Defense Date

2009

Document Type

Thesis

Degree Name

Master of Science

Department

Biology

First Advisor

Wan-ling Chiu

Second Advisor

Jennifer Stewart

Third Advisor

John Ryan

Fourth Advisor

Tracey Dawson Cruz

Abstract

Mitogen-activated protein kinases are part of an evolutionarily conserved protein phosphorylation cascade which serves essential regulatory functions in eukaryotic organisms. Although the role of MAPKs in the regulation of a plant’s response to environmental stress and plant defense has been well established, very little is known about their role in the regulation of plant developmental processes. In order to examine the role of MAPKs in plant growth and development, a strong mammalian MAPK phosphatase (MKP-1), which is known to inactivate MAPKs in plants, was introduced into tobacco plants. In tobacco plants, MKP-1 overexpression altered plant responses to the phytohormones, ethylene and cytokinin. Tobacco plants expressing MKP-1 flowered earlier and senesced later than wild-type. Additionally, these plants exhibited similar floral morphology as flowers from ethylene-insensitive tobacco plants. These observed phenotypes seem to depend on the protein phosphatase activity, as transgenic lines expressing an inactive form of MKP-1 (MKPCS) did not show the same phenotypes. Furthermore, both tobacco and Arabidopsis MKP-1 transgenic plants exhibited increased shoot regeneration when compared to wild-type plants, suggesting increased cytokinin sensitivity. In an attempt to elucidate the mechanism by which MKP-1 affects plant growth and development, expression of selected genes were analyzed using RT-PCR. MKP-1 transformed tobacco plants exhibited downregulated expression of an ethylene biosynthesis gene (NtACO) and upregulated expression of a pathogenesis-related gene (PR-1b), similar to gene expression studies previously conducted in plants with increased production of cytokinin. The same MKP-1 transgenic plants also exhibited upregulated expression of the flowering time gene, FT. Results from this study indicate that constitutive expression of MKP-1 may interfere with ethylene-related MAPK pathways, which normally serves to restrict plant growth during times of environmental stress. The reduced responses to ethylene resulted in elevated sensitivity to cytokinin, promoting an enhanced shoot regeneration phenotype.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

May 2009

Included in

Biology Commons

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