Elucidation of the genetic control of carbohydrate partitioning in Zea mays
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Carbohydrate partitioning, the process of transporting carbohydrates from sites of synthesis in hotosynthetic tissue to developing sink tissues, is crucial for the growth, development, and yield in plants and crops. Anatomical, physiological, and biochemical studies have shed light on this process; however, there is not much known in regards to the genetic control of carbohydrate partitioning. The work described here uses forward and reverse genetic methods to further elucidate the genetic control of carbohydrate partitioning in Zea mays (maize). Chapter 1 reviews what is currently known regarding the function of sugar transporters in the phloem loading and unloading pathways. Additionally, the importance and role of carbohydrate partitioning during abiotic and biotic stress responses is discussed. Chapters 2-4 describe the characterization and cloning of the allelic recessive mutants carbohydrate partitioning defective28 (cpd28) and carbohydrate partitioning defective47 (cpd47) (Chapter 2) and the semi-dominant gain-of-function mutant Carbohydrate partitioning defective1 (Cpd1) (Chapters 3 and 4). All three mutant's hyper-accumulate carbohydrates in their mature leaves due to decreased sucrose export. The cpd28 and cpd47 mutations decrease the amount of crystalline cellulose and likely impair primary cell wall development. The Cpd1 mutation results in the unregulated deposition of the callose in the phloem of leaf and root tissue. Chapter 5 summarizes the Sugars Will Eventually Exit Tissue (SWEET) transporter family and their function across a number of plant species. In this review I was responsible for the generation of a 15 angiosperm protein phylogenetic analysis describing the relationship and structure found in the transporter family. Chapter 6 builds upon the phylogentic study performed in Chapter 5 and identifies and characterizes the function of the SWEET13a, SWEET13b, and WEET13c transporters in the phloem loading pathway in maize utilizing CRISPR/Cas9 mutagenesis. Chapter 7 summarizes the discoveries made in the previous chapters, their contribution to the field, and discusses future research directions. Appendix A describes a protocol I optimized for the fixation, embedding, sectioning, Laser Capture Microdissection, and RNA-sequencing of desired cell groups in mature maize leaf tissue. Appendix B describes the characterization and cloning of the recessive mutant carbohydrate partitioning defective33 (cpd33) and its role in plasmodesmatal function. I contributed to the cloning of cpd33. Appendix C analyzes the expression levels and potential roles of SWEET and TST transporters in sweet and grain sorghum. I was responsible for the expression analysis of the SWEET genes discussed in this publication.
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