Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 93

The gibberellins probably increase cuticle
deposition in the apple peel, thus pre-
venting mechanical failure of the cuticle.
Further analysis of epidermal gene expres-
sion after gibberellin treatment will pro-
vide a better understanding of the role
played by gibberellins in cuticle bio-
synthesis.
Whilst a relationship between ABA
biosynthesis and cuticle biosynthesis is
expected due to the involvement of both
elements in the response to osmotic stress,
there is relatively little understanding of
the extent of this relationship. Analysis of
Arabidopsis mutants that fail to express
ABA biosynthetic genes in response to
osmotic stress have highlighted this con-
nection, as mutant lines were shown to be
defi cient for an allele of the cutin
polymerase gene, BODYGUARD (discussed
earlier), rather than for ABA biosynthesis
genes (Kurdyukov et al. , 2006). How the
cuticle, and in particular cuticle bio-
synthesis, regulates ABA biosynthesis is
not known, but analysis of additional
cuticle biosynthesis mutants has shown
that a functioning cuticle is required for
proper induction of ABA biosynthetic
genes in response to osmotic stress
(Cominelli et al. , 2008; Curvers et al. ,
2010). It would appear that a properly
functioning cuticle mediates a stress signal
that seems to infl uence ABA regulation in
response to osmotic stress.

of the Arabidopsis ANTHOCYANINLESS2
(ANL2), which has been associated with
anthocyanin distribution in epidermal cells
(Kubo et al. , 1999). The CD2 protein
contains a DNA-binding homeodomain and
a steroidogenic acute regulatory lipid-
transfer (START) domain. The function of
the START domain in plants has not been
shown but is thought to facilitate the
binding of regulatory lipids (Schrick et al. ,
2004). The cd2 mutant shows a dramatic
reduction in cutin content in the tomato
fruit peel but an insignifi cant change in
cuticular waxes (Isaacson et al. , 2009). The
fact that cd2 mutants are not severely
affected in terms of water retention
illustrates that the cuticular waxes and the
remaining cutin are suffi cient for water-
proofi ng. Fruit peels of the cd2 mutants
did, however, display an increase in
stiffness and susceptibility to microbial
infection (Isaacson et al. , 2009). This sug-
gests that a correctly formed cutin matrix is
necessary for the cuticle to maintain its
elasticity and to protect the tissues from
microbial infection.
A grape ( Vitis vinifera ) R2R3-MYB
transcription factor (VvMYB5b) has been
implicated in the regulation of cuticular
wax accumulation (Mahjoub et al. , 2009).
When VvMYB5b was overexpressed in
tomato plants, a variety of pleiotropic
phenotypes were observed. These included
modifi ed leaf structure, alterations of fl oral
morphology and glossy fruit appearance.
Chemical analysis of the fruit cuticular
waxes revealed a decrease in total amyrin
content. Oleanolic acid is the major com-
ponent of grape waxes and its precursor,
E -amyrin, showed the strongest decrease in
the transgenic tomato lines (Mahjoub et al. ,
2009). This modifi cation of cuticular wax
was confi rmed by scanning electron
microscopy. One must be cautious when
inferring the action of transcription factors
from overexpression in a non-native
species, but, whilst the native targets of
VvMYB5b are not yet determined, it is
possible that VvMYB5b controls similar
pathways in grapevine. More research is,
however, required to determine the grape-
vine VvMYB5b downstream target genes.

6.5.2 Transcriptional regulation of
biosynthesis pathways

Only a handful of genes have been
implicated in the regulation of fruit cuticle
biosynthesis. These genes typically encode
transcription factors. Regulation of the
biosynthesis of cuticular lipids by these
transcription factors typically leads to
wide-ranging effects on the permeability of
the cuticle and epidermal morphology.
A cuticle-related homeodomain tran-
scription factor was identifi ed in tomato
by characterization and mapping of the
cuticle defi cient 2 ( cd2 ) mutant (Isaacson
et al. , 2009). The protein is a homologue