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be expected. With respect to other data (such as those implicating cell wall epitopes
as determinants of differentiation), progress may be rapid if molecules already
implicated in cell-to-cell signalling turn out to be the causal agent (e.g., AGPs).
However, if these signals are, for example, novel polysaccharide components of the
cell wall, their identification and characterisation may present a major challenge. It
seems safe to state that we have certainly not exhausted the list of chemical signals
involved in plant intercellular communication.
4.4
The cell wall and biophysical signalling
The plant cell wall forms a semi-rigid case around the protoplast and connects cells
together to form a tissue. Because of turgor pressure, the cell wall is generally
under tension, leading to a pattern of physical stress both around individual cells
and connecting neighbouring cells. Altered tissue growth or altered biophysical
characteristics of the cell wall are likely to lead to alteration in this stress pattern.
If individual cells can sense and respond to changes of stress pattern around them,
then the cell wall could act as a conduit for a physical-stressed-based transfer of
information within a tissue. These ideas, and the data to support them, are most
advanced in animal systems.
Animal cells (like plant cells) are also embedded in an ECM. This is generally
not as rigid as a cellulose-based matrix, but it can nevertheless generate a physical
stress on the cell. Moreover, different types of matrixes lead to and are associated
with specific differentiation pathways. Although there are clearly chemical-based
interactions between the cell and the ECM, models have been proposed in which
the mechanical stresses generated around cells are transmitted across the plasma
membrane to the cytoskeleton to generate cell-specific responses (Ingber, 2003).
Akey element in these models is that the tensional forces generated within the
cytoskeleton and the surrounding ECM constitute a balanced system (termed ten-
sional integrity). Such tensegrity systems are characterised by an inherent stability
so that any shift in the stress vector tends to be counterbalanced by the system.
Forexample, magnetic microbeads can be coated with ligands that interact with
cell surface receptor complexes (integrins) that link the ECM with the internal cy-
toskeleton (Wang
et al.
, 1993). By applying magnetic fields across cells coated
with such micro-beads, Wang and colleagues demonstrated that bead twisting was
limited, suggesting that the cytoskeletal/ECM contact points to which the beads
bound could counteract changes in stress applied to them. The idea is that such
mechanical foci at the ECM/cytoskeleton interface sense changes in physical stress
imposed on them and transduce such changes into secondary signals to influence
both cytoskeletal structure and gene expression.
A characteristic of research in this area has been the utilisation of novel and
imaginative approaches to manipulate and measure cellular growth and deformation.
Forexample, Tan and colleagues recently developed a microfabricated post-array
detector system in which spots of specific ECM material were brought into contact
