Agriculture Reference
In-Depth Information
processes of growth control. In particular, the vast majority of experiments have
been performed with oligosaccharides derived from chemical or enzymatic prepa-
rations. It is often unclear how these preparations relate to endogenously generated
oligosaccharides. In addition, the mobility of polysaccharide fragments appears to
be rather limited in intact tissue, suggesting that any intercellular signalling role is
likely to be of a paracrine nature. Furthermore, receptors for these ligands have not
yet been identified.
An initial impulse for the interest in the potential role of cell wall polysaccharides
in signalling came from the chemical complexity (thus high information content)
of this matrix. However, this very complexity provides a major challenge to future
work in this area. Our ignorance on basic elements of cell wall biosynthesis and
organisation (as reflected in the continuing evolution of models of cell wall structure
and biosynthesis (e.g. Vincken
et al.
, 2003; Dhugga
et al.
, 2004)) represents a
significant block to progress in this area. Novel techniques for the
in vivo
analysis
of the plant cell wall are being developed and have already led to re-assessments of
current models of cell wall structure (e.g. Wilson
et al.
, 2000; Lerouxel
et al.
, 2002;
Moille
et al.
, 2003), but further progress in this direction is essential. However,
even if advances are made in this area, the metabolic flexibility exhibited by plants,
which allows mutation in one particular polysaccharide biosynthetic pathway to be
compensated for by the generation of novel cell wall components, which can (more
or less) functionally compensate for any deleterious outcome of a single genetic
lesion (e.g. Zablackis
et al.
, 1996), makes the genetic and biochemical analysis of
polysaccharide-derived compounds extremely challenging.
4.3.2 Arabinogalactan proteins as signals
In addition to pure polysaccharides, the plant cell wall contains proteins and lipids
that may contain carbohydrate modifications. With respect to intercellular signalling,
one family of proteoglycan-based molecules has aroused intense interest - the ara-
binogalactan proteins (AGPs) (Majewska-Sawska & Nothnagel, 2000).
AGPs are highly complex molecules that contain a protein core modified to gen-
erate a final structure that may consist of greater than 90% carbohydrate (Fig. 4.2A).
According to cDNA sequences, AGPs generally possess a C-terminal hydrophobic
domain and biochemical analysis has revealed that this can be processed to gener-
ate a glycerolphospatidylinositol (GPI) lipid anchor (Oxley & Bacic, 1999; Svetek
et al.
, 1999) by which some AGPs could be tethered to a membrane. The site and
mechanism of this processing in plants is unclear, but the presence of such a lipid
anchor adds complexity to an already complex molecule and provides intriguing
possibilities for the proposed functions of AGPs in intercellular signalling, as will
be discussed below.
Initial interest in the potential signalling function of AGPs was aroused by the
finding that epitopes of specific AGP-associated carbohydrates occurred in tissue-
specific patterns. For example, Roberts and colleagues demonstrated that an AGP
recognised by the antibody JIM13 is initially expressed exclusively in metaxylem
