Other Hormonal Signals during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 211

must be viewed carefully due to the
complexity of these experiments. The
complexity is due to there being a wide
range of fruit species with a range of
growth and ripening modes, variable
sensitivity to hormonal treatments depend-
ing on growth conditions and stage,
considerable variation in experimental
design and procedures (e.g. depending on
the species, the response of the fruit to
hormone application can differ depending
on whether the fruit is on or off the plant)
and the number of candidate hormones to
be tested, which is further complicated by
interactions between them.

signalling or putatively involved in ABA
responses have been shown to change
during fruit development in response to
ABA application and in association with
ripening. Increases in the expression of
9- cis -epoxycarotenoid dioxygenase (NCED),
which catalyses the key step in ABA
synthesis, concomitant with an increase in
ABA levels have been reported in both
climacteric and non-climacteric fruits (e.g.
Deluc et al. , 2007; Ren et al. , 2010; Sun et
al. , 2012). Microarray experiments, par-
ticularly in grape, have been used to study
changes in ABA-related gene expression in
fruits during ripening and in response to
stress (Deluc et al. , 2007, 2009; Grimplet et
al. , 2007; Pilati et al. , 2007; Koyama et al. ,
2010; Fortes et al. , 2011). Changes in
protein levels following ABA treatment
have also been observed in grapes
(Giribaldi et al. , 2010).
The list of genes involved in the above
studies and their putative functions is too
long to discuss in detail here, but a few
examples, which tie in with the effects of
ABA application to developing fruits,
deserve particular mention. In many cases,
the application of ABA to unripe fruit has
promoted ripening, or at least some aspect
of it. The timing of ABA application is
crucial for effect, as it is with other
hormones that can alter fruit ripening.
ABA application has frequently been
reported to increase the activity of the
fl avonoid pathway, leading to increased
levels of anthocyanins and other fl avonoids
related to fruit quality. This increase has
been associated with ABA-induced
increases in the transcript levels of genes
encoding biosynthetic enzymes and
transcription factors (Ban et al. , 2003;
Jeong et al. , 2004). ABA application has
also resulted in increased sugar uptake and
accumulation and increased expression of
enzymes thought to be associated with
these processes such as invertases (Pan
et al. , 2005). Interactions between ABA and
sugars have been reported (Rook et al. ,
2006) and an example is given in another
part of this chapter.
In addition to the positive effects of
ABA on fruit ripening, inhibitors of ABA

12.2 Regulators of Ripening, Both
Positive and Negative and Others of
Uncertain Status

12.2.1 Abscisic acid (ABA)

ABA is most commonly thought of as being
associated with embryogenesis, seed
maturation and abiotic stress responses
(Nambara and Marion-Poll, 2005), but a
role in the control of fruit ripening can be
added to this list. ABA levels in a range
of fruits, both climacteric and non-
climacteric, appear to follow the same
basic pattern: they are low prior to ripening
and then increase at about the time
ripening commences (e.g. strawberry, Chen
et al. , 2011; avacado, Chernys and
Zeevaart, 2000; grape, Davies et al. , 1997;
cherry, Kondo and Gemma, 1993; orange,
Rodrigo et al. , 2006; tomato, Sun et al. ,
2012). In a number of these examples, ABA
levels were high at fl owering and during
early fruit development and then decreased
prior to the increase coincident with
ripening. This pattern of accumulation is
consistent with a role for ABA as a positive
regulator of ripening. Interestingly, in
tomato, the increase in ABA levels peaks
before the peak in ethylene levels (Sun
et al. , 2012), suggesting that ABA may
have some role in modulating ethylene
accumulation.
The transcript levels of a number of
genes either involved in ABA synthesis or