Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 88

with cuticle permeability, and chemical
analysis of the fl owers revealed a 40%
reduction in both Z -hydroxylated fatty
acids and 10,16DHPA. As expected for an
extracellular transport protein, ABCG32 is
localized to the plasma membrane of the
epidermal cells, but, interestingly,
localization occurs in a polar manner so
that the proteins are found on the surface
side of the epidermal cells (Bessire et al. ,
2011).
Another protein implicated in the extra-
cellular transport of cuticular lipids is the
Arabidopsis glycosylphosphatidylinositol-
anchored lipid-transfer protein (LTPG)
(DeBono et al. , 2009). LTPG is able to bind
lipids in vitro and was localized to the
plasma membrane of epidermal cells in
growth regions of Arabidopsis , whilst
mutants for LTPG showed a reduction in
wax load on the stem surface (DeBono et
al. , 2009).
Once the monomers/oligomers are
outside the cell, polymerization taken place
through the formation of ester bonds
between the monomers or oligomers.
Lipases are likely candidates for driving
these reactions and may be found outside
the cell in the cuticular membrane. The
most characterized enzyme is
BODYGUARD (Kurdyukov et al. , 2006), an
extracellular epidermal protein with lipase
domains, whilst other lipase candidates are
the monoacylglycerol or glycine-aspartic
acid-serine-leucine (GDSL) motif lipases.
Arabidopsis mutants for BODYGUARD in
fact show an increase in cutin and wax
monomers, but this is coupled with
phenotypes characteristic of a plant with a
defi cient cuticle. It is suggested that the
increase in cutin monomers is a response of
the plant after sensing a disrupted cuticle,
but the monomers remain unpolymerized
(Kurdyukov et al. , 2006). Recently, a GDSL
from tomato ( Solanum lycopersicum )
(SlGDSL1) has been shown to be localized
to the exterior of the cell in the cuticular
membrane, where it is able to polymerize
monoacylglycerol cutin monomers (Girard
et al. , 2012; Yeats et al. , 2012).

6.4 Genes Involved in Fleshy Fruit
Cuticle Biosynthesis and Assembly

6.4.1 Building the cutin polymer

Whilst the majority of the fundamental
work to describe cuticle biosynthesis has
been performed in Arabidopsis , a dry fruit-
bearing species, several genes have also
been characterized in various fl eshy fruit
species, and these will be discussed in this
section (see Table 6.1). Mapping of the
cutin defi cient 1 ( cd1 ) mutant gene led to
the identifi cation of SlGDSL1 , a candidate
for the extracellular polymerization of
cutin monomers (Isaacson et al. , 2009). The
cd1 mutants in tomato possessed a
signifi cantly thinner cuticle than that of
wild-type tomato as well as a reduction in
cutin monomers and in the ester bonds
cross-linking the cutin matrix (Isaacson et
al. , 2009; Yeats et al. , 2012). This was
coupled with an increase in cuticle
permeability and postharvest water loss.
SlGDSL1 was characterized as an
acyltransferase that is able to perform the
polymerization of cutin monomers,
specifi cally 2-mono(10,16-
dihydroxyhexadecanoyl)glycerol (Yeats et
al. , 2012). Importantly, this protein has
been localized to the exterior of the cell in
tomato fruit, providing evidence for
polymerization occurring after the
extracellular transport of monomers and/or
oligomers.
A gene involved in the biosynthesis of
fruit cutin monomers was discovered
through the analysis of the tomato cuticle
defi cient 3 ( cd3 ) mutant (Isaacson et al. ,
2009). The mutation was mapped to
SlCYP86A69, a cytochrome P450 oxidase
(Shi et al. , 2013). SlCYP86A69 shares an
amino acid sequence similarity of 96%
with petunia CYP86A22, a fatty acyl-CoA
Z -hydroxylase shown to be required for
the production of Z -hydroxy fatty acids
and the biosynthesis of cutin polymer in
the petunia stigma (Han et al. , 2010).
Whilst cd3 mutants display a strong
reduction in cutin, there is an insignifi cant