Plant development requires cell differentiation throughout the plant life cycle because plants rely upon the initiation and growth of new organs to reach reproductive maturity. Developmental programs specifying cell pigmentation, cell shape, and specification of cell type have been explored in Arabidopsis. Transcription factors are key components of these developmental programs and work in Arabidopsis and other plant systems have been essential in defining the roles that these factors play during development. A prime example of this in Arabidopsis is the trichome patterning program.
The function and structural diversity of trichomes are intimately related, a relationship that this is explored in this thesis. What types of regulatory networks are involved in defining the form of a trichome is visited as well, setting the stage for deeper studies into Arabidopsis trichome development. Use of the glabra 3 shapeshifter (gl3-sst) allele as a proxy for early stages of trichome development in transcriptional profiling reveals the developmental activities of early stage trichomes. Candidate genes from these experiments were then used in a reverse genetics screen to find other genes with trichome phenotypes. Through this method, an R2R3-MYB transcription factor was discovered to play a role in determining cell shape.
R2R3-MYB domain transcription factors constitute a major class of transcriptional regulators in plants. The Arabidopsis genome encodes an estimated 125 functional R2R3-MYB proteins. Additionally, R3-MYBs, R1R2R3-MYBs, R-R MYBs, and a single four-repeat MYB protein are encoded by the Arabidopsis genome. Animal genomes only contain a handful of MYB genes. Clearly plants have expanded and utilized this lineage in their evolutionary history, and not surprisingly, many of the regulatory programs these plant genes function in are prominent or specific to plants. As a group, the R2R3-MYB family has been studied previously and authorities have defined various subgroups to which members of this gene class are assigned.
This thesis focuses upon members of subgroup 9, defined by the presence of the AQWESA amino acid motif. Seminal work describing the function of this group began with the Antirrhinum majus gene MIXTA. This gene is required for the proper differentiation of conical cells in the floral epidermis. AmMYB MIXTA-like 1, AmMYB MIXTA-like 2, AmMYB MIXTA-like 3, have since been described in Antirrhinum and all have been shown to be functionally similar to MIXTA by heterologus expression in tobacco in the control of cell shape, albeit to varying degrees. Collectively, the available Antirrhinum gene data supports the notion that subgroup 9 R2R3-MYBs are determinants of cell shape be it floral trichomes or conical cells.
Here the technical capabilities we possess in Arabidopsis are used to define the function of the subgroup 9 R2R3-MYB NOECK, (NOK, AT3G01140). NOK functions as a negative regulator of trichome branching, leading to trichome cells with increased volume in the mutant line. This phenotype is opposite that of the reduction in cell volume that might occur in mixta Antirrhinum floral epidermal cells that do not become conical by growing out of the epidermal plane. Expression profiling of trichome cells of various mutants including nok revealed coordinately regulated genes that are extracellular matrix components. These findings coupled with the published data indicates that NOK, and perhaps all other subgroup 9 R2R3-MYBs, control cell shape by altering properties of the extracellular matrix.
Preliminary data testing the functional equivalence of selected MIXTA-like genes from Antirrhinum majus, Arabidopsis, Dendrobium crumenatum, and Medicago truncatula are given. These data support the portability of the NOK functional characterization data to other plant species. Furthermore, these findings illustrate that subgroup 9 R2R3-MYBs alter cell shape regardless of phylogenic origin.
University of Minnesota Ph.D. dissertation. June 2009. Major: Plant Biological Sciences. Advisor: M. David Marks, Ph.D. 1 computer file (PDF); xx, 282 pages.
Gilding, Edward Kalani.
An exposition on trichome development and cell shape with a focus on the function of MIXTA-like R2R3-MYBs..
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