Agriculture Reference
In-Depth Information
ECM
CYT
Figure 4.3
Mechanical signalling within the ECM. Components of the ECM form physical bridges
between adjacent cells. Because of their rigidity, altered physical stress in one region of the ECM will
be transmitted to the adjacent cells via these bridges. If these ECM components are connected to the
cytosol (CYT) of the respective cells, the possibility exists of a direct mechanical interaction of
neighbouring cells that might initiate intracellular signal transduction pathways.
Eventually, depending on the attributes of the material and the growth rates and
boundaries set, the material may buckle to take up an energetically more stable
configuration. Since the process of force redistribution within the material will
occur very rapidly, the buckling process can lead to the simultaneous generation of
a pattern of spatially distributed humps apparently growing out of an initially planar
surface. These patterns of hump formation are reminiscent of the process of organ
initiation observed in, for example, many flowers. In these models, the physical
stresses within the tissue (primarily the cell wall) act as a type of signal. The pattern
generated is dependent on the cellular growth characteristics of the tissue (local or
global) and the extensibility of the cell wall (local or global).
Although the mathematical modelling approaches and arguments are convincing,
experimental data to support the idea that physical forces in plant tissue act over
a distance to coordinate morphogenesis are limited. For example, it can be shown
that constraining the growth of an apex by physical means leads to alterations in
morphogenesis that are consistent with the models put forward, but the question re-
mains open as to whether these manipulations mimic the processes that occur during
normal development (Hernandez & Green, 1993). One prediction of the biophysical
models is that local alteration of cell wall extensibility should influence morpho-
genesis. Experiments in which the local activity of a cell wall protein (expansin)
was manipulated on the apical meristem indicated that local alteration of cell wall
characteristics could induce morphogenesis and, indeed, leaf formation (Fleming
et al.
, 1997; Pien
et al.
, 2001). However, although ectopic leaf formation was as-
sociated with loss (or delay) of the expected leaf morphogenesis on the opposite
side of the meristem, it is unclear if altered patterns of mechanical stress across the
meristem were instrumental in this phenotype. At present, it seems that although
local discontinuities in cell wall tension allow the local coordinated outgrowth of
tissue during organogenesis, a causal role in patterning needs to be substantiated.
To summarise, outgrowth of a tissue at a particular area requires an imbalance
of forces to allow such outgrowth to occur. Cessation of growth must require the
