Agriculture Reference
In-Depth Information
with growing cells to create defined mechanical landscapes (Tan
et al.
, 2003). As
cell growth occurred over the surface of the spots, the individual elements of the
array were deflected and the degree of deflection could be measured. By integrating
the degree of spot deflection over time and space, estimates of traction forces at the
subcellular level were made and, in conjunction with specific staining techniques,
correlated with the behaviour of contact points between the ECM and the plasma
membrane. This novel strategy allowed the simultaneous manipulation of the ECM,
estimation of the traction forces in the system and observation of cell behaviour as
a consequence of the manipulations performed.
Akey element underpinning the advances in the animal field in this area has been
the characterisation of the membrane-spanning molecular machinery that connects
the ECM and the cytoskeleton. However, before considering the evidence in plants
for such ECM/cytoskeletal adhesion points, let us first examine the evidence that
physical force is involved at all in plant intercellular signalling.
First, it is clear that plant form is responsive to the mechanical stresses imposed
on the organism. In the wild, the growth of plants grown in an environment with a
constant wind direction is different from that of plants grown under still conditions.
Experimental manipulations indicate that the plane of cell division in callus cultures
can be manipulated by controlling the vector of external force applied (Yeoman &
Brown, 1971; Linthilac & Vesecky, 1984) and that the growth form of regenerating
protoplasts is influenced by the force applied on them (Wymer
et al.
, 1996). More-
over, molecule evidence has revealed that plants may be exquisitely sensitive to
mechanical stimulation, with signal transduction pathways being induced by even
the slightest transient touch (Braam & Davies, 1990; Knight
et al.
, 1991, 1992).
Thus, it is apparent that plant cells possess the machinery to respond rapidly to
external changes in pressure. The question is: do plant cells use this machinery
to coordinate responses during the normal developmental program of growth and
differentiation?
The observation that plants show responses to changes in physical stress, coupled
with the view of plant tissue as a material through which force can easily and rapidly
be transmitted, has led to the proposition that patterns of physical stress play a
causal role in the coordination of morphogenic events. Essentially, because of the
contiguous nature of the cell wall, a change in tension around one group of cells will
inevitably influence the tensile forces in neighbouring tissue (Fig. 4.3). If cells can
sense these changes and provide an output in terms of altered growth, then a simple,
direct and rapid system would be generated for the control of form (Trewavas &
Knight, 1994). These ideas are most closely associated with a progression of ideas
from Green and colleagues (Green, 1992, 1994, 1999), and have been formulated into
a number of theories based on minimal energy configurations and the mechanical
process of buckling. In these models, plant tissue (in particular the outer epidermal
layer) is thought of as a growing composite material whose physical attributes can
be altered and whose growth is spatially constricted by set boundaries. In such
a system, modelling methods demonstrate that growth may continue in a planar
fashion but accompanied by a build-up of tensional forces within the material.
