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specialized metabolites found in the
tomato cuticle are pentacyclic triter-
penoids. Although suggested to perform a
structural role in the cuticle membrane,
triterpenoids also possess antimicrobial
properties and contribute to defence
against fungal pathogens (Brendolise
et al.
,
2011; Wang
et al.
, 2011).
monomer, the most abundant monomer in
the inner cuticle was 16-hydroxypalmitic
acid (lacking mid-chain hydroxylation).
6.3 Assembly of the Cutin Polymer
The precise mechanism of cutin polymer
assembly and polymerization still remains
largely undescribed, whilst the bio-
synthesis of the monomers/oligomers that
form the building blocks of the polymer is
far better described. Cutin polymer
assembly can be divided into three parts:
(i) monomer/oligomer biosynthesis; (ii)
extracellular transport; and (iii) poly-
merization (Fig. 6.2). The order of these
reactions is not completely clear, and it is
likely that there is some degree of overlap
between the processes.
Biosynthesis of the fatty acid monomers
occurs in the plastids. Free fatty acids
(FFAs) (mostly C16 and C18) undergo
three major reactions on their way to in-
corporation into
Z
-hydroxy acylglycerols,
namely, oxidation, acyl activation and acyl
transfer (Tang
et al.
, 2007; Li-Beisson
et
al.
, 2009; Chen
et al.
, 2011). The sequence
of these reactions is yet to be determined;
however, individual knockouts in
Arabidopsis
of the genes involved in these
processes results in signifi cantly reduced
biosynthesis of acylglycerols. A possible
candidate for the fi rst reaction is acyl
activation of the FFAs through a member
of the long-chain acyl-CoA synthetase
(LACS) family (Schnurr
et al.
, 2004;
Bessire
et al.
, 2007). Metabolism of acyl-
CoAs may then lead to a variety of
pathways such as membrane lipid
biosynthesis, storage lipid biosynthesis,
and cuticular wax and cutin biosynthesis.
Acyl-CoAs destined for cutin biosynthesis
will undergo oxidation by members of the
cytochrome P450 (CYP450) family (the
CYP86A and CYP77A subfamilies) and
transfer of the acyl chains to a glycerol-
based acceptor mediated by glycerol
3-phosphate:acyl-CoA
sn
-1 acyltransferase
(GPAT) (Li
et al.
, 2007; Li-Beisson
et al.
,
2009; Shi
et al.
, 2011). CYP86 family
members and CYP77A6 have been shown
6.2.3 The inner fruit epidermis contains a
cuticular layer
An inner epidermal layer (or endoderm)
containing a cuticle-like structure was fi rst
suggested in the mid-20th century (Kraus,
1949) but was only recently described in
tomato by Mintz-Oron
et al.
(2008) and
further characterized by Matas
et al.
(2011).
This internal cuticle covers the cells of the
endocarp, separating the fl eshy tissue
pericarp from the locular region (i.e. the
placental tissue, locular tissue and seeds).
The cuticle is impermeable to water during
the early stages of development, but as the
fruit matures, the permeability increases
(Matas
et al.
, 2011). The inner epidermal
cuticle possesses similar properties with
regard to the thicker external cuticle and,
according to expression analysis, also
shares common routes of biosynthesis.
Whilst a number of known cuticle bio-
synthesis genes are found to be expressed
in the endodermal cell layer, there are
some noteworthy exceptions such as
CYP77A
, which encodes a protein
catalysing the mid-chain hydroxylation of
fatty acids (Li-Beisson
et al.
, 2009) and has
40-fold lower expression in the endo-
dermal layer (Matas
et al.
, 2011). The
differences observed in expression of
biosynthesis genes are refl ected in the
structure of the inner epidermal cuticle,
which, whilst being structurally similar to
the outer cuticle, does show marked
differences. Chemical analysis of the cutin
monomer composition of the inner and
outer layer of tomato found the same two
dominant monomers (16-hydroxypalmitic
acid and 10,16-DHPA). However, although
in the outer cuticle 10,16-DHPA (a mid-
chain fatty acid) was the most abundant
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