Agriculture Reference
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growing tissue and to what extent they actually move from cell to cell (i.e. whether
they function in communicating or coordinating growth) (Aldington & Fry, 1993).
A recent paper from Takeda et al. (2002) may shed an important light on this issue.
These authors provide data demonstrating that small xyloglucan polymers supplied
exogenously to plant tissue can become incorporated into the cell wall matrix via
the activity of the enzyme xyloglucan transglycosylase (XET). This incorporation
leads to an alteration in the growth characteristics of the wall such that it becomes
more extensible. This would tend to increase the potential for extension growth. It
may be that incorporation of xyloglucan polymers into the cell wall matrix reflects
the normal mechanism by which XET activity synthesises xyloglucan. This would
certainly influence plant cell growth characteristics but would not necessarily imply
that small xyloglucan polymers normally act as intercellular communicators to regu-
late the growth characteristics of neighbouring cells. In this scenario, the modulation
of extension by the exogenous supply of xyloglucan polymers to in vitro cultured
tissue might mimic or disrupt the endogenous process of xyloglucan biosynthesis,
but might not accurately reflect an endogenous mechanism for the coordination of
tissue growth.
Data also exists that fragments from the pectin matrix can act in intercellular sig-
nalling. As with xyloglucans, most of these experiments have been performed with
preparations of cell walls obtained by enzymatic or chemical treatments that lead to
the release of fragments of differing polymer size. The best characterised of these
putative signals are oligomers of galacturonic acid (OGAs), which can be defined
by their degree of polymerisation (DP). For example, fragments of polygalaturonic
acid generated by pectinase have been reported to block the growth-promoting ac-
tivity of auxin in excised pea segments (Branca et al. , 1988), with OGAs of DP
10-17 being most effective. These experiments indicated a requirement for an OGA
concentration in the micromolar range. However, as with xyloglucans, it is still un-
clear as to whether such free OGAs occur to a significant level in unwounded tissue
and whether their mobility in the apoplast is sufficient to allow them to function in
intercellular communication to coordinate growth in vivo .
An added interest in OGAs and other pectin-derived fragments arose from reports
that they influence developmental processes, most notably flowering and morpho-
genesis. This was most dramatically demonstrated in experiments in which pectin-
derived material was shown to induce floral morphogenesis from cultured strips of
tobacco tissue (Tran Thanh Van et al. , 1985). Further investigation of these initial
observations indicated a very complex situation in which the influence of pectin-
derived fragments on morphogenesis was heavily dependent on the hormonal regime
required to maintain the plant explants used in these experiments (Eberhard et al. ,
1989). As a consequence, research and interest in the potential role of pectin-derived
fragments as developmental signals appears to have dwindled.
To summarise, the physiological role of oligosaccharides in controlling plant
growth processes (as well as their role in defence responses) remains debatable
(Ryan & Farmer, 1991; Aldington & Fry, 1993). It is clear that responses can be ob-
served under in vitro conditions, but it is unclear to what extent this reflects normal
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