Agriculture Reference
In-Depth Information
information from the ECM (Anderson
et al.
, 2001). The possibility of a complex of
WAKs and AGPs at the plasma membrane/cell wall interface is intriguing.
A further unexpected link between AGPs and intercellular signalling has come
from the finding that they can act as substrates for chitinases (Van Hengel
et al.
,
2001). Chitinases are enzymes that catalyse the hydrolysis of linkages between
b -(1-4) linked polymers of
N
-acetylglucosamine. Such polymers are found in the
ECM of most fungi and the generally accepted view is that chitinases act as part
of a defence mechanism for the plant (Mauch
et al.
, 1988). Surprisingly, investiga-
tion of a temperature-sensitive mutant line of a carrot embryogenic culture revealed
that the mutation (which led to decreased embryogenic potential) could be res-
cued by the addition of chitinase to the medium (De Jong
et al.
, 1992). Moreover,
embryogenic cultures themselves secreted a chitinase that could be purified from
the medium and which could rescue mutant cultures in cross-feeding experiments.
Further experiments also indicated that chitinases influenced somatic embryogen-
esis (Van Hengel
et al.
, 1998). A puzzle at the time of these experiments was that
there was no known endogenous substrate for chitinases in plant tissue. However,
recent data show that at least some AGPs contain
N
-acetylglucosamine and that
these AGPs can act as a substrate for cleavage by chitinases, presumably leading to
the release of
N
-acetylglucosamine-containing oligosaccharides (Van Hengel
et al.
,
2001). Moreover, the pretreatment of AGPs with chitinase influenced the potency
of the AGPs in an assay designed to test their efficacy in promoting embryoge-
nesis. Interestingly, it has been shown that a class of lipooligosaccharides (nod
factors) can also rescue defects in somatic embryogenesis (De Jong
et al.
, 1993).
These molecules contain an oligosaccharide backbone of four to five b -(1-4)-linked
residues of
N
-acetylglucosamine attached to a C16 or C18 fatty acid and play a key
role in signalling between
Rhizobium
species and plant roots destined for nodulation
(Lerouge
et al.
, 1990).
Taken together, these data provide a tantalising link between three previously
disparate lines of investigation, i.e. AGPs, chitinases and oligosaccharides. Thus,
AGPs could act as the endogenous substrates for developmentally regulated chiti-
nases whose action leads to the generation of modified AGPs and oligosaccharides,
both of which might act as signals involved in growth and development.
A final piece to the puzzle of AGPs is the discovery that the C-terminal part of
the some AGPs can undergo modification to generate a lipid anchor (Youl
et al.
,
1998). Such lipid anchors would attach the AGP to the plasma membrane but could
be susceptible to attack by phospolipases to generate a free, presumably diffusible
AGP. Phospholipases have been shown to be involved in many signalling path-
ways, thus (theoretically) providing a link between classically defined signalling
mechanisms and a novel AGP-based mechanism of intercellular communication. In
such a speculative pathway, specific AGPs might be initially linked to the plasma
membrane until cleavage by phospolipase to release the proteoglycan element of
the AGP. This moiety might itself be subject to further processing by chitinases to
generate signals, as described above (Figs. 4.2B and 4.2C). It should be stressed that
such a model is speculative, but it provides a model for testing in future work.
