Agriculture Reference
In-Depth Information
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XG9
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XG10
Figure 4.1 Structure of xyloglucan to demonstrate the generation of potential signalling compounds
(based on Aldington & Fry, 1993). Enzymatic hydrolysis of xyloglucan tends to occur at free positions
along the glucan backbone of the molecule, as indicated in the diagram. Depending on the pattern of
side chains attached to the glucan polymer, such hydrolysis leads to the generation of xyloglucan
fragments (XGs) of various specific sizes (e.g. XG7, XG8, XG9 and XG10, as indicated). Some of
these fragments have been implicated in growth control.
combinations of lipid or carbohydrate-modified proteins. Among the most complex
of these are the arabinogalactan proteins, which will be discussed later.
This brief summary can only provide a glimpse of the complexity and variety of
cell wall chemistry and architecture, elements of which are still being discovered
(e.g. Vincken et al. , 2003). Nevertheless, it forms a basis for examining the idea that
components derived from the cell wall function in signalling.
4.3.1 Polysaccharide signals
Enzymatic breakdown of the xyloglucan matrix leads to the release of fragments
of different sizes. Since hydrolysis tends to occur at the non-substituted glucose
residues along the glucan backbone, these fragments can be characterised by the
number of sugar residues that they contain, e.g. XG7, XG8, XG9 and XG10 (see
Fig. 4.1; Aldington & Fry, 1993). A number of studies have shown that such xyloglu-
can fragments can influence tissue growth and that the effectiveness of the fragment
depends on its size. For example, York et al. (1984) showed that a particular xy-
loglucan derivative (XG9) at a low concentration (1 nM) could block the promotion
of growth induced by auxin in excised pea stem segments, whereas higher concen-
trations (1 mM) of the same fragment induced growth (McDougall & Fry, 1988).
Similar reports have been made by other groups (e.g. Hoson & Masuda, 1991). These
observations led to the proposal that xyloglucan fragments derived from the cell wall
might play a role as an endogenous regulator or mediator of growth. Moreover, since
such fragments would be free within the ECM, it is easy to conceive that they might
move within the apoplast to coordinate growth both at a local level and at a distance.
However, despite the convincing demonstrations that xyloglucan derivatives can in-
fluence tissue growth, a number of questions as to their physiological significance
remain. In particular, it is unclear to what extent such fragments exist in normally
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