Agriculture Reference
In-Depth Information
of
Arabidopisis
encodes a cytochrome P450 monooxygenase which catalyses fatty
acid hydroxylation (Wellesen
et al.
, 2001) and the
FIDDLEHEAD
gene encodes
aketoacyl synthase (Pruitt
et al.
, 2000). Both enzymes could potentially play a
role in cutin synthesis and both mutants are characterised by organ fusions. The
potential importance of cutin in organ fusion was further substantiated by the work
of Sieber
et al.
(2000). These authors over-expressed a gene encoding a cutinase in
Arabidopsis
and observed that this led to a number of organ fusion events and an
abnormal structure of the plant cuticle.
Although these data indicate the importance of cutin in influencing organ fusion,
they do not necessarily imply that cutin (or cutin-derivatives) acts as a signal. It
could be that the role of the cuticle is permissive rather than instructive. In this inter-
pretation, fusion of primary cell walls would be a default pathway that is normally
prevented only by the presence of an intact cuticle. Any factor leading to loss of cu-
ticle structure would thus lead to organ fusion. How earlier data indicating a positive
factor inducing organ fusion fit into this interpretation is unclear. One possibility is
that during normal post-genital fusion events a positive diffusible signal is involved,
which leads to either the induction of cutin breakdown in the target tissue or to the
suppression of cutin biosynthesis. In such a scenario, the mutants in which uncon-
trolled post-genital fusion events are observed would mimic the normal process but
might not necessarily identify the normal control mechanism.
Further evidence pointing to an important (but possibly permissive) role of cutin
and wax in intercellular communication comes from the analysis of patterning events
in the leaf epidermis, in particular of stomata and trichome formation. Stomatal den-
sity is normally negatively correlated with the CO
2
level to which a plant is exposed
(Gray
et al.
, 2002). In the
HIC
mutant of
Arabidopsis
, this linkage is broken and
a decrease in stomatal density does not occur in response to elevated CO
2
concen-
tration. At ambient CO
2
levels, stomatal density in the
HIC
genetic background is
similar to that observed in wild-type plants. Molecular genetic analysis revealed that
the
HIC
gene encodes a ketoacyl synthase similar to that identified by analysis of the
FIDDLEHEAD
mutation, i.e., an enzyme potentially involved in cutin biosynthesis.
Organ fusion events are not observed in the
HIC
mutant background and the cuticle
appears normal. However, it is interesting that a component of the
FIDDLEHEAD
mutant phenotype is an altered density of trichomes on the leaves (Pruitt
et al.
, 2000).
Both trichomes and stomata are epidermal-derived structures that are generated in
specific patterns (see Chapter 9). Moreover, analysis of
Arabidopsis
mutants has in-
dicated that stomatal patterning is disrupted by the altered expression of a secreted
protease, giving rise to the idea that a diffusible peptide generated with the cell
wall might act as intercellular signal influencing stomatal differentiation (Berger &
Altmann, 2000). Viewing the analysis of the
HIC
mutant in the context of these data,
it is possible that (again) the cuticle is influencing intercellular signalling in a per-
missive fashion rather than an instructive one, i.e., that cutin structure influences the
diffusion of an extracellular signal. This idea is strengthened by the observation that
at least some mutants with altered wax biosynthesis also show altered stomatal den-
sity (Gray
et al.
, 2002). In this scenario, the general cuticle structure would restrict
