Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 90

Mintz-Oron et al. , 2008). Expression of a
reporter gene driven by the upstream
promoter region of SlCER6 was localized to
both the exocarp and endocarp of tomato
fruit (see Plate 3) (Mintz-Oron et al. , 2008).
Analysis of slcer6 tomato lines revealed
that, starting from the mature green stage of
fruit development, there is a reduction of
n -alkanes with a chain length greater than
C28 when compared with wild-type tomato
(Vogg et al. , 2004). Substrates for SlCER6
are therefore likely to include n -alkanes
with chain lengths longer than C28. This
decrease in n -alkanes ( n >C28) is coupled
with an increase in cyclic triterpenoids and
sterols, as well as an increase in the weight
of the cuticle, considered to be a com-
pensating mechanism. The triterpenoids,
however, do not suffi ciently compensate
for the permeability of the cuticle as the
mutants display a three- to eightfold
increase in water loss when compared with
wild-type tomato. The effect of the absence
of E -ketoacyl-CoA synthase is not limited
to n -alkanes, as slcer6 lines also show a
complete lack of cuticular alkene (C33,
C35), aldehyde (C24, C26, C32), alkenol
(C24, C26) and alkadienol (C22, C24, C26)
formation, probably caused by the absence
of a common precursor (Vogg et al. , 2004).
Another tomato mutant that has shed
light on the biosynthesis of fruit cuticular
wax is the recessive positional sterile ( ps )
mutant (Leide et al. , 2011). The ps pheno-
type is characterized by fl oral organ fusions
(a commonly observed phenotype in plants
with depleted cuticles), positional sterility
and the formation of wrinkled ripe and
over-ripe fruit. The surface area of the ps
fruit at the fully ripe stage was approxi-
mately 33% of that seen in the wild type
(Leide et al. , 2011). Chemical analysis of
the cuticular wax composition of the ps
mutants revealed a severe depletion of
alkanes and aldehydes. As with the slcer6
mutant, an increase in triterpenoids and
sterol derivatives was observed as well as a
fi ve- to eightfold increase in water
permeability. There was, however, almost
no effect on the cutin composition of the ps
mutant cuticle. The observed modifi cation
of the cuticular waxes coupled with the

lack of change seen in the cutin com-
position indicate that the ps mutation
causes disabling of the decarbonylation
pathway of wax biosynthesis in the fruit
epidermal cells (Leide et al. , 2011).

6.4.3 Triterpenoid-related genes in the
fl eshy fruit

In a number of fruits, including tomato,
triterpenoids are found in the cuticle layer
and in particular as constituents of the
intracuticular waxes (Leide et al. , 2007;
Mintz-Oron et al. , 2008). Typically, they
are more abundant in fruit than leaf
cuticles and are suggested to be a con-
tributing factor to the increased per-
meability of fruit cuticles, as they have
been shown to provide a less effective
barrier to water transport than long-chain
hydrocarbons (Leide et al. , 2007). A number
of genes that code for triterpenoid synthases
(TTSs) have been characterized in tomato
(Wang et al. , 2011) and apple (Brendolise et
al. , 2011) fruit. Characterization of these
enzymes has focused on the oxidosqualene
cyclases (OSCs), which catalyse the fi rst
committed step of triterpenoid biosynthesis,
the cyclization of epoxysqualene into
various triterpene alcohol isomers. Whilst
the expression of OSC genes contributes
signifi cantly to the distribution of triter-
penoids in plants, there are additional
points of control. This can be observed by
the only partial correlation between expres-
sion levels and cuticular triterpenoids
across tomato cultivars. Within a cultivar,
however, OSC expression between organs
correlated well with triterpenoid accumu-
lation (Wang et al. , 2011).
The product specifi city (or lack thereof)
categorizes OSCs and determines the
triterpenoid profi le found in fruit waxes.
Tomato SlTTS1 is a monofunctional
E -amyrin synthase, whilst SlTTS2 is a
multifunctional amyrin synthase pro-
ducing predominantly G -amyrin and six
other terpenoid products (Wang et al. ,
2011). The major role of the genes as the
producers of cuticular triterpenoids in the
fruit is illustrated in their exclusively fruit