The genetic control of carbohydrate partitioning and nodal root growth during water deficit in Zea mays
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The growth and development of all plants, including crops, depends on the coordinated partitioning of carbohydrate resources between photosynthetic source tissues, such as leaves, and heterotrophic sink tissues, such as roots. This process is essential for plant survival, and a major determinant of crop yield. When growing in water-limited conditions, such as a drought, reallocating carbohydrate resources to root systems allows continued root growth in order to reach water deep beneath the soil surface. The work described in this dissertation aims to further describe the genetic control of carbohydrate partitioning in Zea mays through the use of genetic mutants, and to understand the genetics and physiology of maize root growth during water deficit stress. The first chapter describes the anatomy and physiology of phloem tissue and the function of the cell wall in establishing and maintaining long-distance phloem transport. The second chapter describes the maize gene Brittle Stalk2-Like3 (Bk2L3, which encodes a COBRA protein that functions in primary cell wall synthesis. Using two bk2l3 mutants, it is demonstrated that unlike previously described COBRA genes, BK2L3 functions in carbohydrate partitioning between source and sink tissues. The third chapter describes the maize GT14 family of glycosyltransferases, and establishes a role for GT14 family members in sucrose export from mature leaves. I identify three mutants of a GT14 gene that exhibit altered cell wall composition, and an additional mutant of another closely related family member that exhibits similar phenotypes. I propose that GT14 family members function in the modification of cell surface proteins which may be involved in cell wall formation. Chapter four summarizes the mechanisms governing carbon partitioning to root systems and the effect of water deficit stress on root growth. Chapter five builds on these ideas, describing the physiology and transcriptomic response of maize nodal roots experiencing precisely-controlled water deficit stresses using a split chamber growth apparatus. Chapter six summarizes the research findings of this dissertation and presents future research directions.
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Ph. D.