Agriculture Reference
In-Depth Information
In this scenario, all epidermal cells are initially equivalent and the position of indi-
vidual trichome cells is determined by intercellular communication. How could the
trichome patterning genes described above explain such a patterning mechanism?
In particular, how is it possible that not only the positively acting patterning genes
but also the negatively acting genes are expressed in trichomes?
A model that explains
de novo
patterning and accommodates the above-
mentioned observations is the reaction-diffusion model by Meinhardt and Gierer
(Meinhardt & Gierer, 1974; Meinhardt, 1982, 1994). It is a theoretical model
that attempts to define the minimal requirements to create a spacing pattern in a
two-dimensional field of cells (Fig. 9.3). According to this model, the activator
component activates its own inhibitor whereas the inhibitor inhibits the activator.
Both the activator and the inhibitor may move, but the movement of the inhibitor
(+)
Activator
(--)
(+)
Activator
Inhibitor
Inhibitor
Time
Figure 9.3
Models and expression patterns. The Meinhardt and Gierer model can explain the creation
of a two-dimensional pattern by a two-component system (top-left scheme). The activator activates the
production of an inhibitor, which in turn can suppress the production of the activator. In addition, a
self-activation of the activator is postulated to theoretically allow the rapid amplification of small
differences. Simulations are shown below. Small fluctuations result in the local amplification of the
activator (solid line) and as a consequence also of that of the inhibitor (dotted line). In the end, a
common peak of the activator and the inhibitor is formed. The expression of the inhibitors and the
activators in the trichome system show this predicted co-expression (top right). As an example for a
trichome activator the expression of
GL1
as revealed by a GL1:GUS construct is shown. As an example
for an inhibitor, the expression of a CPC:GUS construct is shown. Note that in both lines the leaf base
shows general expression (arrows) and that in more mature parts of the leaf only trichome cells show
expression (arrow heads).
