Agriculture Reference
In-Depth Information
2002). However, even the novel mechanisms and classical plant hormones that are
the focus of several of the following chapters do not encompass all the likely players
involved in intercellular communication. Indeed, the cast of characters is likely to
increase as research in this highly active area progresses. Recent examples include
the implication of novel carotenoid derivatives in a root-sourced long-distance sig-
nalling mechanism that mediates the classically described repression of axillary bud
growth by auxin (Beveridge
et al.
, 2000, Sorefan
et al.
, 2003) and the identification
of novel peptide phytosulphokine growth factors (Yang
et al.
, 1999).
However, by concentrating on 'success' stories and novel signalling molecules,
there is the danger that we forget alternative mechanisms of intercellular communi-
cation for which there is, as yet, incomplete evidence as to their significance. Some
of these alternative mechanisms were regarded as novel in their time and it is in-
structive to view the progress that has (or has not) been made in consolidating their
potential role in the complex procedure by which plant cells communicate with each
other. In this context, much hard work has been invested in the potential role that the
plant cell wall might play in intercellular communication and the remainder of this
chapter will focus on examining the evidence that the cell wall functions as a key
component in plant signalling. As will be seen, although the evidence is sometimes
not totally conclusive, the various strands of data from the wide ranging experi-
ments into the roles that the cell wall could play in cell-to-cell signalling provide
sufficient indication that it does indeed play an important role in plant intercellular
communication. However, it will also be seen that there is still much work to be done.
4.2
The cell wall and signalling
Plant cells are surrounded by a relatively rigid cellulose-based cell wall. A number
of authors have suggested that this complex structure of polysaccharides, proteins
and lipids be more precisely termed the plant extracellular matrix (ECM) (Roberts,
1994). This provides both a more vivid vision of the complex macromolecular in-
terconnections surrounding and joining plant cells, and draws attention to a possible
comparison with the animal ECM. Since a large and convincing body of data in-
dicates that the ECM plays a key role in intercellular communication in animals,
the ECM could also, by analogy, be a key element in plant cell signalling. In this
chapter, the terms
cell wall
and
ECM
will be used interchangeably simply because
the majority of researchers will be more familiar with the former term.
This chapter will also tend to concentrate on the potential role of the cell wall in
communication events associated with endogenous processes of growth and devel-
opment, rather than on plant response to pathogens and herbivores. However, very
often the analysis of signalling associated with plant-microbe interactions provides
insights into endogenous developmental signalling events and these aspects will be
referred to when necessary.
Some of the initial interest in the cell wall as a source of signals arose on con-
ceptual grounds. It is clear that the plant cell wall is an intricate composite of a large
