Making the Surface of Fleshy Fruit: Biosynthesis, Assembly and Role of the Cuticular Layer - Fruit Ripening: Physiology, Signalling and Genomics - page 87

Regardless of the order of the reactions,
the resultant Z -hydroxy acylglycerol
molecules are considered the putative
structural element of lipid polymers.
However, the extracellular transportation
of these molecules and their subsequent
polymerization raises further questions.
Analysis of Arabidopsis mutants has
revealed the major role of a BAHD acyl
transferase in cutin formation (Panikashvili
et al. , 2009). DCR (DEFECTIVE IN
CUTICULAR RIDGES) was suggested to be
required for the incorporation of 10,16-
DHPA into cutin. This mid-chain
Z -hydroxy fatty acid is the major monomer
of cutin in both Arabidopsis fl owers and
fruit of species such as tomato (Mintz-Oron
et al. , 2008), cherry (Peschel et al. , 2007)
and gooseberry (Kolattukudy, 2001). DCR
mutants in Arabidopsis displayed almost
undetectable levels of 10,16-DHPA in their
fl owers and leaves, and were susceptible to
salinity, osmotic and water-deprivation
stress (Panikashvili et al. , 2009). DCR
seems likely to carry out partial poly-
merization of the cutin monomers. Some
possible mechanisms of action for DCR
have been suggested - it may catalyse: (i)
the acylation of aromatics or hydroxy fatty
acids with the CoA of 10,16-DHPA; or (ii)
the acylation of cutin dimers or trimers
using aromatic or aliphatic CoAs or the
CoA of 10,16-DHPA. These reactions could
result in linear and branched chains of
10,16-DHPA (Panikashvili et al ., 2009; Rani
et al ., 2010). Panikashvili et al. (2009)
localized the DCR protein to the cytosol,
suggesting that polymerization of cutin
commences inside the cell.
Whilst partial polymerization seems to
occur in the cell, the extent of this
polymerization is yet to be determined.
The degree of intracellular polymerization
will have a direct effect on extracellular
transport and the mechanisms capable of
this process. The waxes embedded in the
cutin matrix have been shown to require
transport to the apoplast by ATP-
dependant ATP-binding cassette (ABC)
transporters (Bird et al ., 2007; Panikashvili
and Aharoni, 2008). However, the strict
substrates of these transporters are yet to be

discovered. Of particular interest is the
level of cutin polymerization that occurs
before transport and therefore the degree of
oligomerized cutin that is accepted by ABC
transporters. If monomers are exclusively
transported out of the cell, the entire cutin
matrix must be polymerized extracellularly.
It is likely that there are multiple routes for
extracellular transport depending on the
specifi c monomer or the degree of intra-
cellular polymerization. Highly poly-
merized molecules would probably require
transport via vesicles. Studies in
Arabidopsis have shown that ABC tran-
sporters are at least partially responsible for
cutin and wax monomer transport (Pighin
et al. , 2004; Bird et al. , 2007; Panikashvili
et al. , 2007, 2011; Panikashvili and
Aharoni, 2008; Bessire et al. , 2011).
One such ABC transporter that was
characterized by Panikashvili et al. (2011)
is ABCG13. Arabidopsis mutants for
abcg13 showed fl ower-specifi c phenotypes
including fusion of fl ower organs and
abnormal epidermal cell development
(Panikashvili et al. , 2011). These pheno-
types are likely to be the result of the
signifi cant reduction in cutin seen in
abcg13 fl owers. The fi rst ABC transporter
characterized for wax transport was CER5
(Pighin et al. , 2004). Arabidopsis cer5
mutants showed reduced deposition of
stem cuticular wax. Electron microscopy
analysis found that wax inclusions had
formed in the cytoplasm of the epidermal
cells indicating that wax biosynthesis was
normal, but the extracellular transport was
defective. The CER5 gene was found to
encode an ABC transporter (ABCG12)
specifi c for wax monomers (Pighin et al. ,
2004).
Arabidopsis ABCG11 and ABCG32
mutants also show reduced deposition of
cutin (Panikashvili et al ., 2007; Bessire et
al ., 2011). Analysis of the permeable
cuticle 1 ( pec1 ) mutant of Arabidopsis
revealed the role of ABCG32 in cuticular
lipid transport (Bessire et al. , 2011). The
pec1 mutation was mapped to the ABCG32
gene, a member of the PLEIOTROPIC
DRUG RESISTANCE gene family. The pec1
m utants displayed phenotypes associated