Agriculture Reference
In-Depth Information
number of extremely complex molecules (McNeil
et al.
, 1984). Such complexity
could engender a large supply of information. Such information might, for instance,
be used to distinguish one cell from another and to inform neighbouring cells of
their tissue context. Since virtually all cells in the plant are surrounded by a cell
wall, it could act as a universal mediator of signalling events. Thus, the idea that cell
wall polymers (or breakdown products) play a role in intercellular communication is
intellectually enticing. Certainly, a significant body of data from research on animal
systems has revealed how the extracellular matrix can influence signalling, acting
as a source and modulator of signalling compounds (e.g. Perrimon & Bernfield,
2001). It seems reasonable to suppose that the plant cell wall could function in an
analogous way. However, in addition to the conceptual similarity of the plant and
animal ECM, it is also clear that the plant cell wall has some unique characteristics.
Chief among these is its rigidity. This biophysical specialisation of the plant cell
wall has encouraged some researchers to explore the possibility that the mechanical
properties of the plant ECM might allow it to be part of a biophysical-based system
of intercellular signalling, with the cell wall acting as the conduit (and modulator)
of the forces involved. This concept is discussed further towards the end of this
chapter. To start with, however, let us examine the biochemical structure of the cell
wall and consider the evidence that at least some of these chemical moieties act as
intercellular signals.
4.3
The cell wall as a potential source of chemical signals
Central to wall structure is a polymer of (1-4)-b -D-glucan (cellulose) that is coated
with a heteropolymer of xyloglucan. This is structurally similar to cellulose but is
substituted at positions along the backbone with mono-, di- and trisaccharide side
chains. These side chains always contain xylose and may contain galactose and fu-
cose residues (Fig. 4.1). Cellulose and xyloglucan are thought to constitute a matrix
that provides the cell wall with resistance to tensile stress. A second matrix within
the cell wall is based on pectin. This is an extraordinarily complex matrix whose
essential structure is based on polygalacturonic acid. This may be a homopoly-
mer (homogalacturonan) or can be substituted at particular sites with complex side
chains to form rhamnogalacturonan II. Other pectins include rhamnogalacturonan I
(a polymer containing a backbone of alternating galacturonan and rhamnose units)
and arabinogalactan (containing a backbone polymer of arabinose). This complex
pectin matrix is thought to function biophysically as a gel to contain compressive
forces that build up within the cell wall.
In addition to these purely polysaccharide components, the cell wall also contains
protein and lipids. Thus, many cell walls contain an extensin-based protein matrix
and also an array of proteins and enzymes thought to function in cell wall assembly.
In addition, most epidermal cells possess a coating of lipid-derived cutins and waxes
that perform primarily a protective role to restrict water loss and to defend the plant
against attack. Finally, there is a spectrum of complex molecules that may consist of
