Date of Award

Spring 2009

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Schlappi, Michael

Second Advisor

Dorweiler, Jane

Third Advisor

Yang, Pinfen

Abstract

Plants are immobile and need specific defense strategies to deal with stressful conditions such as extreme temperatures, high salinity, dehydration, or pathogen attack. These environmental stresses lead to cell membrane and/or the cell wall damage and can cause cell death without protective mechanisms. We hypothesize plants have specific proteins that protect these vulnerable barriers against environmental stresses. A genetic screen in Arabidopsis yielded a plant specific candidate gene, EARLll, which encodes a hybrid proline-rich protein (HyPRP) with a three-domain architecture: a putatively cleavable signal peptide for secretion at theN-terminus; a hydrophilic proline-rich domain (PRD) in the middle; and a hydrophobic eight-cysteine (8CM) domain at the C-terminus. Four EARLll paralogs are located at the same chromosome in a tandem array and are co-induced during stress treatment. Overexpression of some of these genes protects cells from freezing induced cellular damage. To further characterize the EARLil family of proteins a heterologous expression system was developed and the following accomplished as part of this Master's thesis. First, while it was previously shown that Saccharomyces cerevisiae expressing separately three of the four EARLl I paralogs showed significant increases in freeze protection, it was newly discovered here that one paralog had a strong inhibitory effect on yeast growth and survival. Second, it was shown here that the three EARLl 1 domains had different functions associated with freeze protection and growth inhibition. That is, inclusion of signal peptides was required for growth inhibition, but not for freeze protection. The PRDs were essential for both effects, while addition of the 8CM domain possibly masked the growth inhibition activities of the PRDs. Finally, biochemical analyses suggested that secreted EARLII proteins containing PRDs appeared to be posttranslationally modified.

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