- Where We Are Going
Our current research is designed to exploit our recent exciting advances in understanding the molecular basis of cell fate specification. In particular, we are focused on testing and extending our proposed model for cell patterning in the root epidermis (shown below).
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| Model for the molecular control of root epidermal cell fate in Arabidopsis. For detailed information about this model, check out the What We Have Learned section of our research |
Testing the Model
How are bHLH proteins involved in this cell fate mechanism? Although we know that bHLH protein(s) are important for this cell specification process, we don't know what the specific roles are for these. At its simplest, our model predicts that the bHLHs interact with WER to help specify the non-hair cell fate. We are currently analyzing three bHLHs (GL3, EGL3, and MYC1). Each of these appear to affect root epidermis development when mutated. In particular, the GL3 and EGL3 are required for the non-hair fate and seem to be largely redundant in their function. Surprisingly, these genes are preferentially expressed in the H cells, yet the bHLH proteins appear to accumulate in the N cells, which suggests a novel feedback loop. To further examine this, we plan to analyze the localization and potential interactions of each of these bHLHs.
How does TTG work? The precise role of the TTG protein in cell fate
specification is still a mystery to us. To gain a handle on this, we are
studying the expression pattern of the TTG gene to determine whether it is
expressed throughout the epidermis (as predicted by the model) or is expressed
in only one of the cell types. We are also planning to use biochemical and
molecular methods to define the way TTG interacts with and/or controls the
activity of the bHLH protein.
How do WER and CPC exert opposing effects on cell fate? A central aspect of our model is the proposal that WER and CPC have opposite effects on the cell
fate pathway. We are currently employing molecular and biochemical approaches
to test three possible explanations for this: (1) CPC directly binds to and
inhibits the WER protein, (2) CPC competes with WER by binding to the same
promoter element, or (3) CPC competes with WER by binding to the same bHLH
protein.
Extending the Model
What are the downstream targets of GL2? In addition to studying the components that control cell fate, we are also trying to understand how the cell fate decisions are translated into appropriate changes in cell shape and cell characteristics. To accomplish this, we are identifying genes that are regulated by GL2 and studying their role in cell differentiation. We suspect that these genes may encode components of the cytoskeleton, cell wall, and/or secretory pathway, which are all known to influence the morphogenesis of plant cells. We are using genomics approaches, including microarray analyses in various mutant and overexpression backgrounds, to identify these downstream factors.
What molecular components control the cell division branch of the pathway?
Epidermal cells in the N-cell position undergo fewer rounds of cell division
than cells in the H-position. This aspect of differentiation is controlled by
TTG and WER, but not by GL2. We are using genetic approaches to identify new
molecular components that specifically regulate this fascinating process of
differential cell division.
When does this cell fate mechanism begin to act? The mechanism outlined in
our model defines the fate of both root and hypocotyl epidermal cells.
Because the root and hypocotyl epidermis is derived from a common set of
protodermal cells at the globular stage of the Arabidopsis embryo, it is likely
that this cell fate mechanism is established during early embryogenesis. We are
analyzing the expression of the cell fate genes throughout embryonic development
to define the spatial and temporal origin of this patterning mechanism.
What is the relationship between the specification mechanism in the root and
the above-ground organs? It is amazing to us that many of the same molecular
components are employed to specify epidermal cell fate in the root, hypocotyl,
and leaf tissues of the Arabidopsis plant. Because each of the final cell types
in these tissues are distinct, there must be organ-specific elements that
influence the common mechanism. We are interested in defining the similarities,
and especially the differences, between cell fate specification in these tissues
in order to understand how the same basic mechanism may lead to different
developmental outcomes.
What is the evolutionary origin of this patterning mechanism? Arabidopsis
and other members of its family (the Brassicaceae family) generates a
position-dependent pattern of root epidermal cells, but most plant species
outside this family do not display this type of pattern. We are interested in
knowing how this cell fate mechanism arose during the evolution of plants. In
particular, we are keen to understand the relationship between this cell fate
mechanism and alternative mechanisms employed by other plant species.
What are the downstream factors that participate in the larger gene regulatory network?
We are using large-scale transcriptomics to define sets of genes that are affected by various mutations and treatments. These studies enable placement of genes and factors relative to one another in an extensive gene regulatory network, and they provoke hypotheses that are being tested using genetic and molecular genetic methods.
Acknowledgements: Our research has been generously supported over the years by the U.S.
National Science Foundation, the U.S. Department of Agriculture, and the U.S.
Department of Energy.
For more detailed information about our research, please consult our
recent publications or contact us directly.