The Roles of Cytokinins and Cytokinin-N-Glucosides in Development and Gene Expression of Arabidopsis thaliana
Hallmark, Howard Tucker
Type of DegreePhD Dissertation
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The phytohormone cytokinin (CK) was first discovered over sixty years ago, and the decades following have produced an enormous amount of data about the various roles CK plays in intracellular signaling, gene expression, and plant development. An often-overlooked characteristic of this phytohormone is that it is not one single molecule; “cytokinin” is a generalized term for dozens of molecules which occur naturally in plants. Much of CK research has used synthetic CKs or only a single form of CK because it has long been believed that only a small subset of CKs display biological activity. Much of this dissertation focuses on CK-N-glucosides (CKNGs), long believed to be inactive forms of CK. Chapter 1 provides a broad overview of CK biology and their functions in plants, with particular focus on the model plant species Arabidopsis thaliana, which the rest of this dissertation work is completed in. Chapter 2 reviews recent advances in understanding of Cytokinin Response Factors, a family of transcription factors involved in response to a variety of environmental signals. Chapter 3 focuses on trans-Zeatin-N-glucosides, revealing that this subclass of CKNGs are capable of delaying leaf senescence and altering gene expression. Chapter 4 focuses on another subclass of CKNGs, isopentenyladenine-N-glucosides, revealing one of these compounds is capable of delaying leaf senescence and mildly altering gene expression in an overlapping manner with isopentenyladenine, a highly active form of CK. Chapter 5 explores potential mechanisms by which trans-Zeatin-N-glucosides delay leaf senescence and suggests they are capable of regulating levels of an active CK, trans-Zeatin. Chapter 6 combines CK measurement data from twelve studies, revealing CKNGs are the most abundant forms of CK in young Arabidopsis thaliana plants.