Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 84

Transmission electron microscopy
(TEM) has identifi ed six different types of
cuticle fi ne structures, but how these relate
to the molecular structure of the cutin
matrix is not yet known (Riederer and
Muller, 2006). This lack of knowledge of
the polymer structure makes the decipher-
ing of cutin biosynthesis an interesting but
complex task. The specifi c monomer
composition of numerous species and
organs is, however, well described. The
fatty acid monomers of cutin are thought to
be partially polymerized into di- or tri-
acylglycerols, which are then transported
to the exterior of the cell where further
polymerization may occur (Panikashvili
and Aharoni, 2008). Polymerization of the
Z -hydroxy fatty acids then results in a
linear polymer. However, depending on
species and organ, a large percentage (up
to 90% in tomato) of the Z -hydroxy fatty
acid monomers may contain mid-chain
hydroxyls (Mintz-Oron et al. , 2008), which
allow branching of the polymer via esterifi -
cation of the hydroxyls to Z -hydroxy fatty
acids. Whilst the involvement of glycerol
in the biosynthesis of the initial cutin
oligomers has been reported (Li et al. ,
2007; Chen et al. , 2011), its contribution to
cutin polymer assembly is poorly under-
stood. It has been suggested that it may
lead to further branch points in the
polymer via increased cross-linking and
subsequently a larger cutin matrix.
Additionally, phenolics (such as ferulates,
which are found in low quantities in the
cutin matrix) may be able to act as
branching points for fatty acids, or to allow
cross-linking within the cutin polymer or
to the polysaccharide and lignin com-
ponents of the cell wall (Li et al. , 2007;
Pollard et al. , 2008).

withstand both biotic and abiotic stresses
(Domínguez et al. , 2012). The function
played by the cuticle to reduce water loss
and maintain structural support for the
fruit is illustrated in the tomato delayed
fruit deterioration ( dfd ) mutant (Saladie et
al. , 2005). Cutin deposition in this mutant
continues throughout fruit development, in
contrast to the typical deposition profi le,
which shows signifi cant reduction at the
'breaker' stage of fruit development. Fruit
of dfd mutants therefore have signifi cantly
more cutin than normal fruit at the
ripening stage. These fruit have a
dramatically longer shelf-life than wild-
type fruit and show a reduction in fruit
softening. Saladie et al. (2005) showed that
transpirational water loss was reduced and
that there was elevated cellular turgor in
the fruit of dfd mutants. A combination of
increased physical support provided by the
thicker cuticle and reduced water loss
probably contributed to the reduction in
fruit softening (Saladie et al. , 2005).
Not only is the cuticle able to maintain
its integrity and cope with the increase in
turgor pressure during fruit development
and ripening, it also plays a central role in
protection of the fruit from external factors.
Both biotic stresses (e.g. insects and fungi)
and abiotic stresses (e.g. UV radiation)
must be coped with. There are a number of
specialized (i.e. secondary) metabolites
found in the cuticle that contribute to
protection from environmental conditions
and deter potential pathogens (Mintz-Oron
et al. , 2008). High light levels and tem-
perature cause an increase in the con-
centration of reactive oxygen species in
exposed tissues and these may result in
oxidative damage to the fruit. Antioxidants
are therefore important metabolites in the
outer layer of the fruit to moderate the
potential damage. The yellow fl avonoid
naringenin chalcone accumulates in the
epidermal cells as well as in the cuticular
membrane of tomato fruit (Adato et al. ,
2009; Domínguez et al. , 2009), where it
probably protects against excessive UV
radiation whilst also serving to attract
agents of seed dispersal due to their
intense pigmentation. Another group of

6.2.2 The cuticle of developing fl eshy fruit

The cuticle of developing fl eshy fruit is
adapted to maintain its integrity while the
fruit rapidly expands, as well as to attract
agents of seed dispersal. The structure of
the fruit cuticle has therefore evolved to
perform these roles and to be able to