Agriculture Reference
In-Depth Information
These results confi rm the pivotal role of
ethylene in regulating the ripening of
climacteric fruits.
To determine whether ethylene is
necessary only for the initiation of
ripening, the physiological role of ethylene
in the initiation and subsequent progres-
sion of ripening has been investigated.
When 1-methylcyclopropene (1-MCP), a
gaseous inhibitor of ethylene action, was
applied after the initiation of ethylene
production in pear fruit, the subsequent
fl esh softening and ethylene production
was suppressed (Hiwasa
et al.
, 2003). A
similar suppression has been observed in
melon fruit treated with 1-MCP during
ripening (Nishiyama
et al.
, 2007). In the
Charentais melon cultivar, ethylene pro-
duction was inhibited by the introduction
of the antisense gene of ACO, which
encodes the ethylene synthesis enzyme. In
this transgenic melon, the ripening para-
meters, including the degreening of the
rind, fl esh softening, detachment of the
peduncle (Guis
et al.
, 1997) and aroma
production (Bauchot
et al.
, 1998), were
signifi cantly suppressed by a reduction in
ethylene production, and treatment with
exogenous ethylene accelerated the ripen-
ing process (Flores
et al.
, 2001). These
results indicate that ethylene is directly
involved not only in the initiation of
ripening but also in maintenance of the
ripening process.
carotenoids, are completely regulated by
ethylene (Guis
et al.
, 1997; Flores
et al.
,
2001). In mountain papayas, the de-
greening of the skin and fl esh softening are
only partially dependent on ethylene, and
the production of volatile aromatic com-
pounds is strictly dependent on ethylene
for certain esters, including ethyl acetate,
and partially dependent on other esters,
including butyl acetate (Moya-León
et al.
,
2004; Balbontín
et al.
, 2007). Moreover,
pear fruit softening is completely sup-
pressed by the absence of ethylene, where-
as the softening process in melon and
papaya shows only a partial dependence
on ethylene (Hiwasa
et al.
, 2003). Thus, the
ripening of climacteric fruits proceeds
under both developmental and ethylene-
related regulation, and the degree of
dependence on ethylene varies with
respect to the individual ripening para-
meter and the diversity of the species and
cultivars.
The threshold values for the required
ethylene levels are known to be different
for each ripening parameter. For example,
ethylene-dependent fl esh softening is not
induced by exogenous treatment with less
than 1 ppm. of ethylene in an ethylene-
suppressed transgenic melon but is
induced by 2.5 ppm. of ethylene, which is
suffi cient to produce softening similar to
that observed in wild-type melons (Flores
et al.
, 2001). The abscission zone detach-
ment and rind degreening processes are
induced by 2.5 and 1 ppm. of ethylene,
respectively, and the extent of change is
concentration dependent up to 5 ppm.
Furthermore, at the molecular level, the
expression level of each ethylene-inducible
gene, such as
E4
,
E8
,
E17
and
J49
, has a
unique dose-response curve that is de-
pendent on the exogenous ethylene con-
centration from 0.11 to 72 ppm. (Lincoln
and Fischer, 1988). The expression peak
varies between 0.75 and 23 ppm. These
data show that individual ethylene-
dependent processes have distinct sensitiv-
ities to ethylene, and that the complicated
ripening processes of climacteric fruits are
intricately controlled by these differences
in ethylene sensitivity.
1.3.2 Ethylene-dependent and -independent
ripening processes
The ripening processes in climacteric fruits
are not necessarily regulated in an
ethylene-dependent manner, although
continuous ethylene action is necessary for
adequate ripening. In melon fruit, fl esh
softening and membrane deterioration are
not strictly ethylene-dependent processes
and are regulated by both ethylene-
dependent and -independent processes. In
contrast, the detachment of the abscission
zone and colour change of the rind, which
includes chlorophyll degradation and an
increase in yellow components such as
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