Extracellular adenosine triphosphate (eATP) functions as a signaling molecule in plants, regulating processes such as root hair growth, wound response, and stomata behavior. Although the eATP receptors in animals have been studied extensively, the equivalent receptors in plants are just beginning to be investigated. Recently, a transmembrane receptor kinase called "Does Not Respond to Nucleotides" (DORN1) was identified as the first eATP receptor in plants. This study was aimed at testing the

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... pothesis that the DORN1 receptor is the receptor for the eATP signaling pathway regulating stomatal opening and closing. Previous results in Arabidopsis thaliana wildtype plants indicate a bi-phasic response to applied eATP and eADP, with low concentrations inducing stomatal opening and high concentrations inducing stomatal closing. The effects of applied ATP were tested on leaves of Arabidopsis thaliana wild type (Col-0) and the loss-of-function mutant dorn1-3. In addition, ATPγS, the poorly hydrolyzable version of ATP, was tested on wild type and mutant plants. If the dorn1-3 mutant lacks the wild type's responses to applied eATP, then DORN1 is the likely receptor for the guard cells' responses to eATP. Another hypothesis tested in this study is whether extracellular adenosine diphosphate (eADP), the hydrolysis product of eATP, controls stomata behavior via DORN1. If dorn1-3 responds like the wild type to eADP, then DORN1 may not be the receptor for the guard cells' responses to eADP. Using scanning electron microscopy, differences were documented in dorn1-3 and wild type Arabidopsis thaliana leaf epidermal morphology. Altered guard cell patterning with slight clustering of guard cells was observed in the dorn1-3 mutant leaves. Preliminary results support the hypothesis that DORN1 is part of the eATP-induced changes in stomatal aperture, suggesting that DORN1 is the receptor for the eATP signaling pathway in guard cells. In contrast, the preliminary results support the hypothesis that DORN1 is not involved in the eADP signaling pathway in guard cells. ! iv! Acknowledgements This research would not have possible without the support of Dr. Stanley Roux and Dr. Greg Clark. Through his skill as a professor, Dr. Roux has inspired me to transition from thinking like a student to thinking like a scientist. I am sincerely grateful to Dr. Roux for the plethora of scientific wisdom he has imparted through his lectures and our personal conversations. Since welcoming me to the Roux Lab as a volunteer in 2011, Dr. Greg Clark has been a loyal mentor who has encouraged me to take on new research questions while adding to my laboratory skill set. Dr. Clark has helped me gain confidence and competence as an independent researcher through his unfailing attention, care, and respect towards me as a young scientist. Both these gentlemen have provided me with opportunities to advance in research and achieve much in my time as an undergraduate researcher at UT Austin. I also want to thank Shane Merrell, the chief horticulturist and manager of Welch greenhouse for his tireless commitment to maintaining a healthy reliable supply of plants for my experiments. Working with a person who shares my enthusiasm for botany has been a true pleasure. Special thanks are given to Dr. Dwight Romanovicz for his patience and guidance in training me to use the scanning electron microscope at the Institute for Cellular and Molecular Biosciences facilities at UT Austin. Finally, I want to thank my colleagues, including graduate students and fellow undergraduates, at the Roux Lab for their camaraderie, helpful advice, and support over the years.