Dissertation
Structural and Functional Aspects of Protophloem Sieve Element Unloading in the Root Tips of Arabidopsis thaliana
Doctor of Philosophy (PhD), Washington State University
01/2016
Handle:
https://hdl.handle.net/2376/107874
Abstract
The phloem is network of symplastically connected sieve elements. Void of nuceli and many organelles, this system is vitally important for the rapid, long distance transport of growth dependent molecules from the source of assimilation to developing sinks. Presented in this dissertation is a diverse toolkit of phloem mobile probes and cell specific markers used for the observation and analysis of structural features and functional characteristics of the phloem. When used in conjunction with in-vivo imaging techniques, we were able to observe and define a section of protophloem which facilitates unloading in the roots of Arabidopsis thaliana and is appropriately termed the unloading zone. Using Fluorescence Recovery After Photo-bleaching (FRAP) we measured the flow velocity in the unloading zone and show preliminary data regarding the effect of soil water potential on root growth and protophloem flow velocity in the unloading zone. Additionally, with transmission electron microscopy (TEM) we elucidate the regulatory mechanism that defines the boundaries of the unloading zone: callose deposition on plasmodesmata connecting sieve elements to adjacent cells. Finally, using a combination of TEM and confocal imaging of green fluorescent proteins fused to proteins of various sizes we identified a new type of plasmodesmata, funnel plasmodesmata, between protophloem sieve elements and phloem pole pericycle cells that facilitates size dependent batch unloading of macromolecules. The various types of plasmodesmata in the unloading zone filter small solutes and macromolecules between protophloem sieve elements and adjacent cells. With the data gathered we put forth a new hypothesis on phloem unloading: Convective Phloem Unloading (CPU). CPU is a combination of batch unloading of macromolecules through funnel plasmodesmata and diffusion of small solutes such as sucrose out of the sieve elements via all laterally connected cells. Using the relative density of each plasmodesmata type found in the unloading zone, we calculate that a pressure of only 0.05 MPa is needed to facilitate this unloading by division of labor between batch unloading and diffusion.
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Details
- Title
- Structural and Functional Aspects of Protophloem Sieve Element Unloading in the Root Tips of Arabidopsis thaliana
- Creators
- Timothy Ross-Elliott
- Contributors
- Michael Knoblauch (Advisor)Andrew McCubbin (Committee Member)Hanjo Hellmann (Committee Member)Hanu Pappu (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Program in Molecular Plant Sciences
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 140
- Identifiers
- 99900581520301842
- Language
- English
- Resource Type
- Dissertation