Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 82

caused by microorganisms and damaging
UV light (Bargel et al. , 2006). Additionally,
the cuticle provides mechanical support to
the plant organ and acts as a division to
prevent, or in some cases promote, the
fusion of plant organs during development.
Being the outermost part of the plant, the
cuticle can be regarded as a detector for
environmental changes and plays a role in
transmitting signals to the interior of the
plant (Curvers et al. , 2010).
The cuticles of the various organs of
higher plants have been adapted to allow
optimal functionality of the organ. This is
refl ected in the diversity of compounds
found in plant cuticle membranes.
Analysis of a variety of cutin polymers has
found the following compounds to be
most abundant: unsubstituted fatty acids
(1-25% of the cutin matrix), Z -hydroxy
fatty acids (1-32%), dicarboxylic acids
(typically less than 5%), mid-chain poly-
hydroxy fatty acids (16-92%), fatty
alcohols (0-8%), glycerol (1-14%) and
phenolics (0-1%) (Pollard et al. , 2008). As
in many fi elds of plant research, much
work on cuticle biology has been per-
formed in Arabidopsis . In retrospect, this
may not have been the most representative
model, as the cutin in green tissue of
Arabidopsis comprises in excess of 50%
dicarboxylic acids, whilst the majority of
other plants contain less than 5%
dicarboxylic acids in the cutin. However, it
has been shown that Arabidopsis petals
have a very similar cutin composition
when compared with the cutin found in
fl eshy fruits. Specifi cally, both fl eshy fruits
and Arabidopsis fl owers contain high
levels of 10,16-dihydroxypalmitic acid
(10,16-DHPA) (Li-Beisson et al. , 2009).
Other models commonly used include
maize, which has been the model of choice
in the study of wax composition during
phase transition (Sturaro et al. , 2005), and
more recently tomato, which is used as a
model for the cuticle of fl eshy fruit (Mintz-
Oron et al. , 2008; Isaacson et al. , 2009;
Matas et al ., 2011).
The leaf, being the primary photo-
synthetic organ of the plant, has asym-
metric cuticle deposition, with more

deposition occurring on the adaxial than
the abaxial side (Jetter and Schäffer, 2001).
This extra protection for the sun-exposed
side of the leaf probably protects the
underlying cells from excessive radiation
damage. The cuticle covers not only the
leaf's external epidermal cells but also the
epidermal cells lining the substomatal
cavity (Roth-Nebelsick et al. , 2013). This
suggests that the inner epidermal cells
have signifi cantly reduced transpiration. In
seeds, the outer coat comprises a cuticular
membrane that isolates the seed from the
plant with the exception of a vascular
bundle, which provides nutrients (Van
Dongen et al. , 2003). Once the seed has
reached maturity, this cutin membrane
extends to surround the entire seed. This
protective barrier limits the rate of water
uptake and is therefore the primary control
of germination. The cuticle of the fl ower is
adapted to serve the role of this organ,
which is the attraction of pollinators. This
is illustrated when examining Arabidopsis
thaliana mutants that are defi cient in cutin
biosynthesis (Li-Beisson et al. , 2009).
These mutants display an absence of petal
nanoridges (cuticular ridges), which are
distinctive to fl ower organs and have been
suggested to attract pollinators in a number
of ways, including trapping refl ective dew
drops, refl ecting light in a specifi c pattern,
creating a surface that provides perceptible
stimulation and providing a desirable
surface on which to walk (Li-Beisson et al. ,
2009).
Whilst in the past the majority of
studies have focused on the cuticle of
vegetative tissues, in recent years there has
been a growing interest in understanding
the biology of the specialized fruit cuticle
(Leide et al. , 2007; Isaacson et al. , 2009;
Shi et al. , 2011). During fruit development,
the surface area of the fruit may increase
rapidly in size, whilst the biosynthesis of
the cuticle is seldom able to match the fruit
expansion (Mintz-Oron et al. , 2008;
Domínguez et al. , 2012). The cuticle must
therefore be elastic enough to tolerate the
high tensions created by the expanding
fruit. As the primary means of protection of
the fruit, the cuticle has signifi cant