Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

130 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
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 hyperinduced.
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 (Diospyros kaki) 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