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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Plant Biology

Year Degree Awarded

Summer 2014

First Advisor

Dr. Samuel P Hazen

Subject Categories

Biology | Biotechnology | Molecular Genetics | Plant Biology

Abstract

Secondary cell wall synthesis occurs in specialized cell types following completion of cell enlargement. By virtue of mechanical strength provided by a wall thickened with cellulose, hemicelluloses, and lignin, these cells can function as water-conducting vessels and provide structural support. Several transcription factor families regulate genes encoding wall synthesis enzymes. Certain NAC and MYB proteins directly bind upstream of structural genes and other transcription factors. The most detailed model of this regulatory network is established predominantly for a eudicot, Arabidopsis thaliana. In grasses, both the patterning and the composition of secondary cell walls are distinct from that of eudicots. These differences suggest transcriptional regulation is similarly distinct. Putative rice and maize orthologs of several eudicot cell wall regulators genetically complement mutants of A. thaliana or result in wall defects when constitutively over-expressed; nevertheless, aside from maize ZmMYB31, switchgrass PvMYB4, and Brachypodium BdSWN5, function has not been tested in a grass. Similar to the seminal work conducted in A. thaliana, gene expression profiling in maize, rice, and other grasses implicates additional genes as regulators. Characterization of these genes in a grass species will continue to elucidate the relationship between the transcription regulatory networks of eudicots and grasses.

In the context of this dissertation two cell wall genes responsible for synthesizing cellulose in the secondary cell walls were characterized. Several MYBs, a NAC and a bZIP protein was found to interact with the cellulose gene promoters. A reverse genetics approach was used to functionally characterize two of those regulators, MYB48 and GNRF. MYB48 is the first grass specific cell wall regulator found to positively regulate cell wall biosynthesis by binding to the cellulose and lignin gene promoters. It regulates above ground biomass in B. distachyon. GNRF, on the hand, unlike the characterized NAC proteins, was shown to repress cell wall biosynthesis. GNRF is also repressing flowering in B. distachyon, a novel regulatory function that has not been associated with the characterized NAC proteins to date. Further characterization of GNRF is likely to provide new insights into the pleiotropic regulatory roles of this protein.

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