Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 83

economic importance. It plays a major role
in postharvest shelf-life as well as con-
ferring a number of quality parameters
including colour and texture. Although
research has been performed in other
fl eshy fruit species, including apple
(Brendolise et al. , 2011), grape (Mahjoub et
al. , 2009) and cherry (Peschel et al. , 2007;
Alkio et al. , 2012), the model of choice for
the characterization of genes involved in
fl eshy fruit cuticle is tomato. Tomato is a
valuable model for studying the cuticle for
a number of reasons: (i) it has a relatively
short life cycle; (ii) genetic transformation
of this plant species is relatively simple;
(iii) the fruit possesses a relatively thick
fruit cuticle that is comparatively easy to
isolate and study; (iv) the ripe fruit peel
lacks stomata; and (v) the genome has been
sequenced and is well annotated (The
Tomato Genome Consortium, 2012). An
interesting phenomenon of the tomato fruit
cuticle that is not observed in the cuticle of
leaves is that epidermal cells are often
entirely surrounded by the cuticle (Fig. 6.1)
(Mintz-Oron et al. , 2008).
This chapter will describe the structure
and assembly of the cuticle, focusing on
the genes and enzymes implicated in
constructing the cuticle of fl eshy fruit and
its role in this organ. It is worth noting
that, in the majority of studies, the 'peel'
or 'skin' of the fruit is discussed. This is
not a scientifi c term and describes in
general the fruit outer layer including the
pericarp and epidermal cells as well as
the cuticle.

20 m m

CM

EC

Fig. 6.1. Scanning electron micrograph of tomato
fruit surface. A cross-section of a ripe tomato peel
(cv. Ailsa Craig ) showing the thick cuticular
membrane (CM) surrounding the entire epidermal
cell (EC).

and Muller, 2006). The major components
of the plant cuticle are the cutin matrix and
epicuticular and intracuticular waxes. In
some species, a non-hydrolysable polymer
matrix named cutan may also be present
(Pollard et al. , 2008). The cutin is attached
to the underlying epidermal cells, whilst a
thin layer of wax crystals covers the
outermost surface of the cutinized layer.
This hydrophobic epicuticular wax allows
the plant to repel water and is thus
important as a transpiration barrier. A
cutinized cell wall is formed at the inner
surface of the cutin where it is inter-
connected with the polysaccharides of the
epidermal cell wall (López-Casado et al. ,
2007). The typical fatty acid monomers
found in cutin are C16 and C18 Z -hydroxy
fatty acids and glycerol. The cuticular
waxes are comprised of very-long-
chain saturated non-polar hydrocarbons
(typically C20-C60) and their derivatives
(alcohols, aldehydes and alkanes), as well
as secondary metabolites such as
triterpenoids and phenylpropanoids (e.g.
fl avonoids) (Kunst and Samuels, 2003).
The polysaccharides present in the cutin
matrix are understood to be the major
factor contributing to the elasticity of the
matrix, whilst the cutin provides the
strength (López-Casado et al. , 2007, 2010).

6.2 Composition, Structure and
Function of the Cuticle

6.2.1 General: composition and structure of
the cuticle

The structure and composition of the plant
cuticle can vary widely among plant
species, and among organs and develop-
mental stages within a species. This is
illustrated in the typical range of thickness
(1-10 μm) and quantity (100-1000 μg
cm -2 ) of the deposited cuticle (Riederer