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the role of ethylene in certain non-
climacteric fruits and various ripening
processes.
four phenomena: (i) ABA treatment led to
increases in both ABA content and ethyl-
ene production, although ethylene treat-
ment did not produce an increase in ABA
content; (ii) the application of NDGA
suppressed the increase in ABA content at
fruit ripening, although the suppression
was not clearly correlated with ethylene
production; (iii) treatment with a
combination of ABA and 1-MCP inhibited
ethylene production but did not affect
ABA content; and (iv) the application of
NDGA or 1-MCP inhibited fruit ripening
and softening. Sun et al. (2012a) also
demonstrated that expression of genes
encoding cell-wall degradation enzymes
was controlled by both ABA and ethylene.
Taken together, these fi ndings support the
hypothesis that ABA might act as an
upstream regulator prior to ethylene
production in the fruit-ripening process,
and suggest that the presence and
perception of both ABA and ethylene is
important for normal ripening in tomato
fruit (Fig. 1.2). This model is supported by
studies in other non-climacteric (grape)
and climacteric (peach) fruits (Zhang et al. ,
2009a).
Additional molecular mechanisms have
been demonstrated based on evidence that
9- cis -epoxycarotenoid dioxygenase (NCED),
a key enzyme in the biosynthesis of ABA,
is involved in the ripening of several fruits,
including avocado (Chernys and Zeevaart,
2000), orange (Rodrigo et al. , 2003), peach,
grape, tomato (Zhang et al. , 2009a,b) and
strawberry (Jia et al. , 2011). In tomato fruit
( Solanum lycopersicum ), the suppression
of SlNCED1 led to the inhibition of fruit
softening and extended the shelf-life by up
to four times compared with that of the
wild type (Sun et al. , 2012a). Moreover, in
strawberry fruit ( Fragaria × ananassa ), the
downregulation of FaNCED1 resulted in
an uncoloured phenotype, which was
rescued by exogenous ABA (Jia et al. ,
2011). However, the suppression of
SlNCED1 was not suffi cient to inhibit fruit
ripening as measured by coloration and the
ethylene production rate. In fact, coloration
and ethylene production in the SlNCED1-
suppressed fruit were enhanced relative to
1.4 Involvement of Other
Phytohormones in Fruit Ripening
Abscisic acid (ABA) is a plant hormone
that plays roles in the regulation of plant
growth and development, seed dormancy
and the adaptation to various stresses (e.g.
cold and osmotic stress). However, its
involvement in fruit ripening is also
considered possible because an increase in
ABA content is found during and/or
preceding fruit ripening in both climacteric
(Vendrell and Buesa, 1989; Chernys and
Zeevaart, 2000; Zhang et al. , 2009a,b) and
non-climacteric (Inaba et al. , 1976; Kondo
and Tomiyama, 1998; Wheeler et al. , 2009;
Jia et al. , 2011) fruits. In addition, the
application of ABA accelerated fruit
coloration and softening (Inaba et al. 1976;
Jiang and Joyce, 2003; Wheeler et al. , 2009;
Zhang et al. , 2009a,b).
In an ABA-defi cient mutant in orange
fruit, the coloration of the fruit skin was
delayed compared with that of the wild
type during maturation (Rodrigo et al. ,
2003). This phenotype was overcome by
the application of ABA. In contrast, the
response to exogenous ethylene and the
ethylene inhibitor 2,5-norbornadiene was
weaker than that of the wild-type fruit
(Alferez and Zacarias, 1999). These results
suggest that both ethylene and ABA are
involved in the coloration of citrus fruits
and have a complex correlation.
The function of ABA as a ripening factor
has recently been emphasized in several
fruits. Zhang et al. (2009b) showed that
endogenous ABA accumulated prior to the
ethylene burst in tomato fruit, and
exogenous ABA treatment promoted
ethylene synthesis and fruit ripening.
However, the application of fl uridone or
nordihydroguaiaretic acid (NDGA), which
are inhibitors of ABA, suppressed the
ripening processes, including fruit
coloration and softening. In addition to the
aforementioned results, they demonstrated
 
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