Agriculture Reference
In-Depth Information
extracellular ligand. This extracellular domain is linked across the plasma mem-
brane to a cytosolic domain with similarity to serine/threonine kinases implicated
in signal transduction. Biochemical analysis indicates that the extracellular domain
is closely associated or linked with pectin material in the cell wall and that at least
some WAKs can interact with glycine-rich proteins in the cell wall, as well as AGPs
(He
et al.
, 1996; Gens
et al.
, 2000; Wagner & Kohorn, 2001). Thus, WAKs have
the structure and location to act as transducers of information from the ECM to the
cytosol (Anderson
et al.
, 2001). Initial data indicated that WAKs might play a role in
plant response to pathogen attack (He
et al.
, 1998), but recent results suggest a more
basic involvement of WAKs in the regulation of plant growth. For example, use of
an inducible antisense strategy to suppress WAK expression indicated that impaired
WA K function led to a dramatic suppression of plant growth (Lally
et al.
, 2001;
Wagner & Kohorn, 2001). In particular, although germination appeared to proceed
normally, subsequent elongation of hypocotyls and root and expansion of leaves
was blocked. How WAKs might impinge on cell growth processes is still unclear,
but one interpretation of these experiments is that WAKs interact with extracellular
ligands that play a role in stimulating plant growth. Reduction of WAK gene ex-
pression would limit the potential binding sites for these ligands, thus leading to the
phenotypes observed. That this ligand might be a component of the ECM is intrigu-
ing. Mutation of a gene encoding a chimeric leucine-rich repeat/extensin protein has
been shown to lead to abnormal root hair growth, indicating that hydroxyproline-
rich extracellular proteins can be involved in the maintenance of appropriate cell
shape (Baumberger
et al.
, 2001). Whether specific glycine-rich proteins might fulfil
a similar function via interaction with WAKs is open to speculation. A fuller charac-
terisation of the extracellular ligand(s) for WAKs promises to provide an important
insight into how components of the cell wall might transmit information to the cell.
To summarise, a major prerequisite for physical signalling within or from the
cell wall is that there must be a mechanism by which information in the ECM is
transmitted across the plasma membrane to induce a signalling cascade within the
cell. Although a number of receptor-like molecules have begun to be identified in
plant systems (some of which are described in other chapters of this topic), a major
gap in our knowledge lies in our ignorance of the nature of the bridge between the
ECM, the plasma membrane and components of the cytoskeleton. Advances in plant
cell biology, coupled with the identification of gene sequences encoding proteins
that might play a role in this connection, should lead to significant advances in this
area in the near future. Such advances are essential and overdue.
4.5
Conclusions
From this summary of possible mechanisms of intercellular communication in
plants, it is clear that in addition to the classical and well-investigated and accepted
systems described elsewhere in this topic, a number of other mechanisms have been
postulated and investigated. In particular, the idea that the cell wall can be both a
