The plant extracellular matrix and signalling - Intercellular Communication in Plants - page 96

or influence the diffusion of a CO 2 -associated extracellular signal. The identity of
this signal is unknown.
An interesting extension of the work on the HIC mutant is the finding that the
exposure of a relatively mature leaf to a high CO 2 concentration leads to a lower
stomatal density in younger primordia that are not exposed to elevated CO 2 levels
(Lake et al. , 2001). These data indicate that leaves exposed to altered CO 2 levels
generate a signal that acts at a distance to regulate stomatal density in developing leaf
primordia. As with the HIC mutant, the nature of the intercellular signal is unknown
and there is no reason to implicate the cell wall in this instance. Nevertheless,
it provides another example of the increasing spectrum of intercellular signalling
pathways that are being uncovered in plants. The application of molecular genetic
tools to these problems promises to reveal the nature of these novel signalling
entities.
To summarise, a number of lines of evidence indicate that the structure of the
cuticle can influence intercellular communication in plants. However, the majority of
these data suggest that this action is essentially permissive. An interesting question
for the future is to investigate whether there is an endogenous regulation of cuticle
permeability with respect to the extracellular signals that are hypothesised to flow
through this component of the ECM; i.e., do the observed outcomes of disruption of
cuticle synthesis and architecture on organ fusion and epidermal patterning reflect
an endogenous mechanism by which intercellular signalling is normally modulated
by the plant? This seems to be especially pertinent to the developmentally regulated
process of organ fusion observed in many floral structures. With respect to actual
signals derived from the cuticle, the evidence is weaker. As with other components
of the ECM, the chemical complexity of the cuticle, coupled with the limits of
our understanding of its synthesis and breakdown, represents significant hurdles to
further progress in this area. Novel approaches to the identification of mutants in
cuticle structure promise to provide important advances in this area (Tanaka et al. ,
2004).

4.3.4 Uncharacterised cell wall determinants involved in signalling

The survey of ECM-associated intercellular signalling mechanisms described above
has focused on chemical components that, although sometimes not fully charac-
terised, at least can be placed into reasonably specific chemical groupings (e.g.
OGAs, xyloglucans, AGPs). However, in addition to these 'known' signals, there
are many data in the literature that indicate or imply the presence of an intercellular
signalling process but do not enable any precise identification of the actual signalling
component itself. For example, sequence analysis has revealed that plants express
proteins with similarity to receptors for tumour-necrosis factor peptide hormones
that influence a variety of developmental processes in animals (Becraft et al. , 1996;
Tanaka et al. , 2002; Gifford et al. , 2003). Moreover, mutations in at least some of
these putative receptors lead to phenotypes consistent with disruption of an intercel-
lular signalling mechanism. Thus mutation of the CR4 gene in maize leads to altered