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expression of genes encoding specific cell wall hydrolases, leading to abscission zone cell
separation and to fruitlet shedding.
The avocado (
Persea americana
)
PA-ERS1
mRNA increased gradually from the day of
harvest, and did not change significantly until the climacteric peak when it was hyperin-
duced. 1-MCP however suppressed the accumulation of
PA-ERS1
to basal levels suggesting
that the stimulated induction of
PA-ERS1
at the climacteric peak maybe a mechanism by
the avocado fruit to dissipate the high levels autocatalytic ethylene (Owino et al., 2002).
In peach (
Prunus persica
), the expression of
Pp-ETR1
appeared to be constitutive and
ethylene independent during fruit development and ripening, while
Pp-ERS1
transcripts
increased during fruit ripening and its expression appeared to be upregulated by propylene
treatment (Rasori et al., 2002). Application of the ethylene antagonist, 1-MCP, delayed fruit
ripening, ethylene evolution, and concurrently downregulated
Pp-ERS1
, while
Pp-ETR1
transcription was unaffected. 1-MCP action was rapidly abolished after moving fruits to
air, when a rapid stimulation of ethylene evolution and a concurrent increase of
Pp-ERS1
mRNAs were observed.
Cold treatment of late-season pear (
Pyrus communis
cv. Passe-Crassane) fruit leads
to a gradual increase in ethylene production and a commensurate increase in ethylene
receptor mRNA expression (El-Sharkawy et al., 2003). The
Pc-ETR1a
mRNA accumulation
was upregulated by cold and during ripening, whereas
Pc-ERS1a
and
Pc-ETR5
were less
affected by cold treatment, but all increased during postcold treatment, ethylene-dependent
ripening. A sharp peak of
Pc-ETR1a
and
Pc-ERS1a
mRNA accumulation was observed
during ripening in the early-season pear cultivars, in contrast to the gradual increase seen
in late-season pear cultivar, Passe-Crassane (PC). A more pronounced difference between
early-season cultivars and late-season cultivar PC was seen in the behavior of
Pc-ETR5
transcript accumulation. Transcript levels for
Pc-ETR5
diminish sharply before and during
the ethylene climacteric and ripening of early-season pear fruit, whereas in late-season
cultivar they increase sharply. This suggests that a decrease in the expression of a negative
regulator could result in an increase in ethylene sensitivity early in the ripening phase
of early fruit development. However, given the potential for redundancy in the ethylene
receptor family, it remains to be determined whether reduced levels of
Pc-ETR5
affect
the overall ethylene sensitivity of early-season pear fruit.
Three ethylene receptors—
DkERS1
,
DkETR1
, and
DkETR2
—have been isolated and
their expression determined during ripening of persimmon (
Diospyroskaki
) fruit (Pang et al.,
2006). The
DkETR1
mRNA is constitutively expressed during all stages of fruit ripening and
is ethylene-independent. Conversely,
DkERS1
and
DkETR1
mRNA levels correlated with
ethylene production during fruit development and ripening and were induced by ethylene.
The DkERS1 protein decreased gradually prior to fruit maturation and reached its lowest
level at the ripening stage when ripening-related ethylene was produced, suggesting the
involvement of DkERS1 in ethylene perception during fruit ripening.
In contrast to the great deal of information available regarding the ethylene receptors
in climacteric fruits, much less is known about nonclimacteric fruits. At present, no single
growth regulator appears to play a positive role analogous to the role played by ethylene in
the ripening of climacteric fruits. Nonclimacteric fruits are also able to synthesize ethylene,
and in some cases, it has been shown that ethylene can hasten the postharvest deterioration.
However, in spite of many efforts, no results have been obtained that can demonstrate a
clear relation between ethylene and the ripening of these fruits. Three ethylene receptors
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