CHARACTERIZATION OF GENES THAT REGULATE CARBOHYDRATE PARTITIONING IN MAIZE
Open Access
- Author:
- Slewinski, Thomas Louis
- Graduate Program:
- Plant Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 22, 2010
- Committee Members:
- David Braun, Dissertation Advisor/Co-Advisor
David M Braun, Committee Chair/Co-Chair
Paula M Mc Steen, Committee Member
Kathleen Marie Brown, Committee Member
Richard Cyr, Committee Member
Mark Guiltinan, Committee Member - Keywords:
- Carbohydrate partitioning
Maize
Sucrose Transporter
Genetics - Abstract:
- Carbohydrate partitioning is the process by which carbon is fixed in photosynthetic “source” tissues of a plant, in most cases leaves, and then translocated to carbon consuming “sink” tissues, usually roots, stems, inflorescences, and seeds. Although carbohydrate partitioning is critical for the proper growth and development of plants, little is known about the genes that regulate carbon flux. The main goals of this thesis were to identify and characterize these genetic factors and to determine their function within the context of whole-plant carbohydrate partitioning. The first two chapters of this thesis provide a comprehensive review of the previously identified mechanisms and molecular components that function in carbohydrate transport in plants. Much of the research presented in this thesis focuses on the structure and function of the phloem, a vascular tissue that transports sugars and other compounds throughout the plant. Chapter 1 summarizes the three main mechanisms of phloem loading of sugars and discuses our current state of knowledge of how they are regulated. Chapter 2 reviews the functions of sucrose transporters, proteins that are responsible for moving sugars across membranes in cells. This chapter mainly summarizes the roles of sucrose transporters in monocot species of plants, which includes maize (Zea mays), the model system used in all of the research presented in this thesis. The maize genome encodes seven putative sucrose transporters (SUTs). When my research began, it was unclear which of these genes functioned in phloem loading in maize. Previous research suggested that the SUT1 protein would be the most likely candidate, although the orthologous proteins in rice and sugarcane, two closely related monocot species, do not appear to function in phloem loading. Chapter 3 details my characterization of the sut1 mutant which shows that the SUT1 protein has an essential function in phloem loading in maize. To identify additional components involved in carbohydrate partitioning, a forward genetic approach was taken to identify mutants defective in carbohydrate export from leaves. tie-dyed1 (tdy1) was the first of these mutants to be identified and characterized. Tdy1 was previously cloned and found to encode a novel grass-specific phloem expressed protein. Chapter 4 details my efforts to elucidate the function of TDY1 using molecular studies. I found is a phloem-specific transmembrane protein that localized to the membranes of the endoplasmic reticulum. In chapter 5, I used a genetic analysis to demonstrate that TDY1 does not participate in starch synthesis or breakdown, providing support for our hypothesis that TDY1 functions in carbon export from maize leaves. Chapter 7 describes my efforts to further understand the function of the Tdy pathway by cloning the Tdy2 locus. Tdy2 encodes a callose synthase1 catalytic subunit that is expressed in the cells that will give rise to the vasculature. Based on the predicted function of TDY2 and phenotypic analysis of the tdy2 mutant, I proposed a new hypothesis that the Tdy pathway functions in plasmodesmata development in the cell wall interface between the companion cells and sieve elements, the cells that principally comprise the phloem tissue. Forward genetic screening also led to the identification of another mutant with a similar carbohydrate export defective phenotype to that of both tdy1 and tdy2, named psychedelic. In chapter 6, genetic analyses reveal that the psychedelic mutant displays duplicate factor inheritance and functions independently of the known pathways controlling carbohydrate partitioning. Collectively, the research conducted in this thesis has led to the identification and functional characterization of multiple genes and genetic pathways that regulate carbohydrate partitioning in maize. These studies establish the foundation for future experiments to determine the genetic pathways controlling carbohydrate partitioning in plants. Chapter 8 discusses the broader impacts of these findings and future directions for carbohydrate partitioning research.