Agriculture Reference
In-Depth Information
To summarise, an increasing body of evidence supports the hypothesis that AGPs
function in intercellular communication. The patterns of AGP expression, the ob-
served outcome of exogenous AGPs in various systems and the novel linkages
that can now be made with data from research on chitinases and oligosaccharide
signalling all provide strong indications of a functional role for AGPs in intercellu-
lar signalling. However, a significant amount of data have been obtained from the
analysis of
in vitro
cultured tissue (somatic embryos, suspension cultures,
Zinnia
mesophyll cells), raising the question as to what extent AGPs function in the intact
plant as signalling molecules. The analysis of AGP function using molecular genetic
approaches has begun to shed light on this important issue and it can be expected
that definitive evidence on the physiological role of these fascinating molecules will
be described in the near future.
4.3.3
Cutin and signalling
Aerial organs of plants are normally formed as discrete primordia from meristems
distributed throughout the plant. However, many plants at points in their development
generate structures that are the result of the fusion of initially independently formed
organs. Such post-genital fusion is especially common in the generation of floral
structures (Verbeke, 1992). The observation that only some plant organs undergo
fusion whereas the majority do not suggests that a signalling process is involved.
Forexample, experiments in which young primordia were physically forced into
close contact did not lead to fusion; i.e., the process is not just a consequence of
spatially restricted growth of neighbouring organs. Initial work on carpel fusion
in
Catheranthus roseus
indicated that the organs involved secreted a substance that
promoted subsequent fusion (Siegel & Verbeke, 1989). Thus, insertion of thin agar
slices between the presumptively fused adaxial sides of carpels led to the accumu-
lation within the agar of a water-soluble substance that, when applied to the abaxial
side of carpels, induced elements of epidermal cell de-differentiation that were nor-
mally observed only during carpel fusion. A range of experiments using membranes
of varying porosity and placed at different regions of the developing carpels indi-
cated that the signalling process was a two-way affair, i.e., that both carpels involved
in a fusion event needed to secrete a substance if successful fusion were to occur.
The nature of this substance has not yet been characterised.
More recently, a number of mutants of
Arabidopsis
have been identified that
display abnormal organ fusion, e.g. post-genital fusion of leaves and stem. Molec-
ular genetic analysis of these mutants has revealed that the disrupted genes encode
enzymes that potentially play a role in production of the plant cuticle.
The cuticle is a complex matrix consisting mainly of lipid-based cutin and wax
(Kolattukudy, 1980). These components are secreted onto the epidermal surface
where they play an essentially protective function, e.g. to limit loss of water from
the plant and to reflect irradiance. However, mutants in which the cuticle appears
to be compromised frequently display morphogenic abnormalities, most notably
in the occurrence of abnormal organ fusions. For example, the
LACERATA
gene
