Date of Award:

5-2012

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Edmund D. Brodie, Jr.

Committee

Edmund D. Brodie, Jr.

Committee

Jon Y. Takemoto

Committee

Daryll B. DeWald

Committee

Dennis L. Welker

Committee

Joan M. Hevel

Committee

Nabil N. Youssef

Abstract

Since the early colonization of land by fungi and plants some 700 million years ago, plants have been continuously faced with changes in their environment. Unlike animals, plants are not free to move about, and can therefore not evade many stress factors. How plants sense and respond to their environment has been of interest not only to scientific research but also in more practical applications such as agriculture.

Signals (such as light or salinity) from the outside of plant cells trigger a flow of information to the inside of the cell. The final target for most of the information is thought to be the nucleus, the structure that contains the cell’s hereditary information. Depending on the signal, DNA replication and RNA transcription in the nucleus may lead to the production of molecules which allow the plant to respond in a biologically relevant way. To get signals from the outside of the cell to the nucleus, both animals and plants can use the phosphoinositide (PI) signaling pathway.

In my research, I was interested in how plants use the PI pathway and what happens to plants and their cells when this pathway is disturbed. I focused on the model plant Arabidopsis thaliana. This is a small, weedy plant with a short life cycle. It has become a model plant because it can be grown easily in the laboratory and is amenable to genetic manipulation. Its genome is completely sequenced, which makes it easier to define segments of DNA as genes and to add explanatory notes to their possible function.

Central to my research was an enzyme called SAC9 that was speculated to modify one of the main components of the PI pathway. The mechanism(s) by which SAC9 acts in the PI pathway is poorly understood. Plants with a defective copy of the gene that codes for SAC9 show signs of being permanently stressed even though they are not exposed to any stress factor. They are slow growing with small leaves and short roots. In signaling terms, it looks like somebody forgot to turn off the light switch. Understanding the molecular and cytological effects of a dysfunctional SAC9 protein in plant cells is the basis for genetically improving crop species, so they can better withstand stresses from the environment.

My work, which was funded by the National Foundation of Science (NSF), provides insights into the function of the protein SAC9 and its substrate, phosphatidylinositol 4,5- bisphosphate, PI(4,5)P2. Using different types of microscopes (light microscope, confocal microscope, transmission electron microscope), I discovered that in plants that were defective for the SAC9 gene, some root cells had unique cell wall abnormalities, which might be the reason for why the roots are shorter. I also found that a network of fibers throughout the cell’s cytoplasm, called the cytoskeleton, had some subtle but important changes.

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Comments

This work made publicly available electronically on April 12, 2012.

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