Utilizing radiotracer and genetic approaches to determine the regulation of sucrose export in maize leaves
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI SYSTEM AT AUTHOR'S REQUEST.] To sustain plant growth, development, and ultimately crop yield, carbohydrates must be transported from their site of synthesis in leaves to distant parts of the plant, such as seeds or roots. Yet little is known about the genes controlling carbohydrate distribution in plants. The main goal of this dissertation is to understand an aspect of the genetic control of carbohydrate partitioning in maize. The first chapter summarizes three mechanisms by which sugars are loaded into phloem for long-distance transport and discusses the regulation of cell-to-cell transport through plasmodesmata. I also discuss the main objectives of my dissertation research in the first chapter. Chapter 2 describes my work on developing a new tool using radioactive Fluorine-18 ([18]F) labeled sucrose to visualize sugar transport in maize plants. Physiological, genetic, and biochemical evidence showed that maize SUCROSE TRANSPORTER1 (ZmSUT1) is responsible for transporting sucrose into phloem; however, mechanistic aspects of sucrose binding by maize SUT1 are not well resolved. Specific hydroxyl groups in sucrose have been showed to participate in hydrogen bonding with SUT proteins. Hence, the main question that I asked was whether 18F substitution of hydroxyl groups at various positions within sucrose impacts the binding and the transport ability of ZmSUT1 for [18]F labeled sucrose. In chapter 3, I describes a simple and inexpensive method for constructing a fully automatic controlled growth chamber that can be easily adapted in plant biology laboratories as well as classrooms. The system can be used to study plant responses to numerous abiotic and biotic stress conditions and to grow and characterize large plants, such as maize and soybean. Chapter 4 details my characterization and cloning of carbohydrate partitioning defective33 (cpd33), a recessive mutant that accumulates excess starch and soluble sugars in the mature leaves. Based on the phenotype of cpd33 mutants, and the subcellular localization of the protein, I proposed that CPD33 functions to promote sucrose export into the phloem. Chapter 5 summarizes the findings of this dissertation and presents future research directions. Appendix A is an excerpt from a published review that I coauthored on how heat stress impacts carbohydrate partitioning and phloem transport. Appendix B is taken from a paper that I coauthored, and details my work to apply the 18F labelled sucrose transport assay to study the function of another maize sucrose transporter, Sucrose transporter2. Appendix C is taken from a paper that I coauthored to examine transcriptional responses to reactive oxygen species (ROS) accumulation in maize. Appendix D summarizes my characterization and mapping of Carbohydrate partitioning defective26 and Carbohydrate partitioning defective29.
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