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Research in the Schiefelbein Lab
- An Introduction
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This page provides some background information about our
research. It's meant to help you begin to understand our experimental system
and why we find it so attractive. If you want to skip this part and find out
what we have discovered or what we are currently doing in the lab, visit the
What We Have Learned or Where
We Are Going sections.
A fundamental feature of development in multicellular organisms is the specification
and patterning of distinct cell types. Our lab studies the formation of the hair and
non-hair cell types in the root epidermis of Arabidopsis thaliana as an elegant and
powerful model system for uncovering the mechanisms of cell specification.
Why is root epidermis development in Arabidopsis such a wonderful experimental
system?
Here are some of the reasons:
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A scanning electron micrograph image of
a hair and non-hair cell.
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The root epidermis is composed of only two cell types. Although most tissues
contain many different cell types, the root epidermis consists of only two: hair
cells and non-hair cells. A hair cell differs from the non-hair cell in the long
extension it forms (the root hair), which is thought to be important in water and
nutrient uptake.The presence of only two cell types in the root epidermis means that
each newly-formed cell makes a simple "either-or" choice: it must decide whether to
develop into a hair cell or a non-hair cell.
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left: Drawing of epidermal cell files in an Arabidopsis root tip. right: Photograph of an Arabidopsis root tip, showing the progresive development of hair cells along its
length.
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The development of root epidermal cells is easy to observe. The epidermal
cells are on the outer surface of the root, making them easy to see and study. Also,
they are continuously produced as the root grows, due to cell divisions in the
meristematic region at the root tip. As they age, the cells become progressively
more differentiated. Notably, root epidermal cells are organized into columns (or
files) along the length of the root, with the youngest (recently-formed) cells
nearest the bottom of a file and the oldest (most differentiated) cells near the top
of a file. Thus, it is easy to study any particular stage of epidermis development,
from beginning to end, by simply examining cells at a specific location along the
length of the files. Furthermore, we can distinguish the developing hair cells from
developing non-hair cells by several criteria, including differences in cell length,
cytoplasmic density, vacuolation, and marker gene expression.
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Drawing of a cross-section of an Arabidopsis root.
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A simple position-dependent pattern of hair and non-hair cell types is generated
in the Arabidopsis root. Like other members of the plant family Brassicaceae, Arabidopsis produces its hair and non-hair cells in a particular pattern. Root hair
cells arise over the intercellular space between underlying cortical cells
(designated the "H" cell position) and non-hair cells develop over a single cortical
cell (designated the "N" cell position). (Note the drawing to the left.) This simple
correlation between cell position and cell differentiation implies that cell-cell
communication is important in the establishment of this cell-type pattern.
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Photograph of a cross
section of an Arabidopsis root treated with a flourescent cell wall
stain.
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The Arabidopsis root has a simple structure. There are an unusually small
number of tissue and cell types in the Arabidopsis root (see the photo on the right).
It possesses a single layer of cells within its epidermal, cortical, endodermal, and
pericycle tissues, as well as a simple set of cells in its vascular tissue. In
addition, the number of cells within each layer is fairly constant. For example, the
Arabidopsis primary root always possesses eight files of cortex cells, eight files
of root-hair cells, and approximately 10-14 files of non-hair cells in the epidermis.
Furthermore, the developmental origin of each of the tissue types within the root
meristem has been defined. For detailed information about the anatomy of the
Arabidopsis root, visit the Scheres
lab homepage.
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Photograph of five day
old Arabidopsis seedlings on nutrient agar in a Petri plate. The
plate was incubated in a vertical orientation, causing the roots to
grow on the agar surface. Click on the image to see a larger picture.
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Root epidermis development can be efficiently analyzed in seedlings. Epidermal
cells develop on the primary root immediately after germination of the Arabidopsis
seed. Therefore, it is possible to analyze root epidermis development at an early
stage in plant life (after only 3 or 4 days of incubation) when plants are small
(about 1 cm long). These features enable large numbers of individuals to be grown on
defined media and studied with ease. For example, we typically examine 200-400
individuals on a single 10 x 10 cm Petri dish (see photograph at the right).
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Root hair cells and non-hair cells are dispensable. We have isolated mutants
that entirely lack root hair cells, as well as mutants that entirely lack non-hair
cells, and these mutants are perfectly healthy and fertile. Thus, root hair cells
and non-hair cells are not essential to the Arabidopsis plant, which means that we
can genetically manipulate the production of hair and non-hair cells in any way we
wish without altering plant viability.
Arabidopsis thaliana is an excellent organism for molecular genetic studies.
Although it has no horticultural value, Arabidopsis has many features that make it
the most popular model organism in plant biology. It grows rapidly (life cycle of
6-8 weeks), is naturally self-fertile (and can be cross-pollinated by hand), produces
a large number of offspring (typically >1000 seeds per plant), is readily mutagenized,
and has a small genome (120 Mb) that has been completely sequenced. These attributes are
extremely helpful for the kind of molecular genetic studies we do in our lab. For
more detailed information about the virtues of Arabidopsis as a model organism, visit
the The Arabidopsis Information Resource
(TAIR) website.
For more information about our research please go to the What We Have Learned or Where We Are Going sections.
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Last Updated: 1/18/2012 |