Role of Cuticles in Produce Quality and Preservation 31
deterioration of cells in the host plant. Such dormant periods of fungal growth prior
to full infection, which coincided with the climacteric rise in respiration, indicated
an orchestrated plant–pathogen interaction [74].
Mounting evidence indicates that the cuticle provides a store of molecules that
enables the recognition of plant surfaces. As mentioned previously, cutin is a biopoly-
mer in which C 16 and C 18 fatty acids are cross-linked between carboxy and
ω-hydroxy groups. More recent results indicate that the ω-hydroxy fatty acids play
a role in plant–pathogen interactions in addition to their role as a protective cutin
layer [68]. Virulent fungi spores secrete cutinases, which results in the production
of cutin monomers. The cutin monomers are known to enhance the transcription of
cutinase genes in fungi as well as to stimulate the formation of the appressorium
and penetration peg, ultimately leading to infection of the host plant [68,75]. In
addition, the constituents of plant surface waxes can also trigger further fungal
growth [75]. Controversy still exists over the importance of cutinases in fungal
infection, since some fungal mutants that are unable to produce cutinases are still
pathogenic. However, all the evidence taken together indicates that the plant cuticle
constituents act as signals for the recognition of plant surfaces by fungal pathogens.
There is also growing evidence that cutinases elicit a defense response in the
host plant [76]. Parker [76] found that fungal cutinase and other lipid esterases could
protect bean leaves from disease. The lipid esterases seemed to be the most effective
in inhibiting infection. However, the protective mechanism of the esterase activity
was not known. Kim et al. [77] studied the interaction of pepper (Capsicum annuum)
with the anthractnose fungus (Colletotrichum gloeosporioides). They found that a
pepper esterase gene became highly expressed during fungal infection. The authors
cloned the pepper esterase gene, expressed it in E. coli, and then isolated it. They
inoculated the pepper with the athractose fungi amended with the pepper esterase
protein. The results showed no infection from the inoculum. Furthermore, the pepper
esterase was shown to inhibit the formation of the fungal appressorium by modu-
lating the cAMP-dependent signaling pathway. The authors concluded that pepper
esterase activity could provide a defense mechanism against fungal attack. In another
study, Schweizer et al. [78] investigated the chemical synthesis of the major cutin
monomers of barley leaves and tested the molecules to see whether they could induce
defense-related genes. They showed that an mRNA induced by pathogen attack could
also be induced by a particular cutin monomer. In addition, they showed that topical
sprays of cutin monomers on barley and rice leaves provided resistance against
Erysiphe graminis f. sp. Hordei and M. grisea, respectively. Since the cutin mono-
mers exhibited no fungicidal effect, Schweizer et al. [78] concluded that the cutin
monomers acted as signal molecules for the induction of disease resistance in barley.
The production of H 2 O 2 is associated with the pathogen defense system in
plants [79]. Fauth et al. [79] noted that fungi are able to secrete cutinases during
the initial stages of fungal attack that most likely results in the formation of cutin
monomers. To counter these attacks, plants have two early defense mechanisms;
these include accumulation of phenolic compounds in the epidermal cell walls and
apoplastic chitinase [79,80]. Fauth et al. [79] showed that fungal attack on cucumber
hypocotyls treated with cutin monomers induced accumulation of cell wall phenolics
that possibly required H 2 O 2 production for polymerization. Accumulation of H 2 O 2