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through induction of ethylene evolution.
High levels of endogenous BRs have been
reported during early tomato development
(Montoya
et al.
, 2005), but their role in the
control of ripening is still unclear and we
could fi nd no information regarding the
effects of BR mutants on tomato ripening.
Exogenous BRs also seem to affect
passionfruit ripening, but the response
varied depending on the frequency and
timing of treatment, and the increase in
fruit number due to treatments at fl owering
was the most dramatic effect (Gomes
et al.
,
2006).
Further evidence of a possible involve-
ment of BRs in ripening comes from gene
expression studies. The expression of a BR-
6-oxidase gene was increased in red-
skinned apples when compared with
green-skinned apples, but there was no
other data to support a role for BRs in
apple ripening (Han
et al.
, 2011).
Bombarely
et al.
(2010) sequenced expres-
sed sequence tags from strawberry fruit and
used quantitative PCR to confi rm that
expression of the BR receptor gene and a
gene encoding a BR signalling component
was upregulated in the receptacle in the
red stage, but there was no statistical
analysis to support this observation.
The most convincing evidence that BRs
play a role in fruit ripening comes from
grape berries. Symons
et al.
(2006) showed
that the levels of castasterone were elevated
in fl owers and young berries, decreased
prior to veraison and then increased
sharply at veraison, consistent with a role
in ripening. After peaking immediately
after veraison, the levels of castasterone
declined (similar to the profi le for ABA).
The enhancement of grape berry ripen-
ing (as measured by colour increase)
through the application of epi-brassinolide
and its delay by the application of
brassinazole, an inhibitor of BR bio-
synthesis, is further evidence for BR
involvement in berry ripening (Symons
et
al.
, 2006).
BRs have been implicated in the control
of ripening of both climacteric and non-
climacteric fruits, which is indicative of a
similarity in ripening between the two fruit
types. As can been seen from the above
discussion, although there is evidence
supporting a role for BRs in fruit ripening,
particularly in grapes, there is still a great
deal of work to be done to convincingly
establish this as a general phenomenon.
12.2.3 Auxins
Auxins are involved in a multitude of
processes during fruit development, but
their role in the control of fruit ripening
has received relatively little attention.
Various patterns of accumulation in fruit
have been reported for indole-3-acetic acid
(IAA), the most abundant auxin found in
plants, but the consensus is that levels are
usually high early in development and
decrease to be low at, or before, the
initiation of ripening (e.g. Buta and
Spaulding, 1994; Abbas
et al.
, 2000;
Böttcher
et al.
, 2010). This pattern of
accumulation is consistent with a possible
role in ripening inhibition.
Many of the hormones described in this
chapter are modifi ed as part of normal
metabolism - for example ABA is
glycosylated and then further modifi ed (for
example by oxidation) - and such path-
ways are an important part of maintaining
hormone levels. The sequestration of
auxins through conjugation to amino acids
seems to be important for ripening, and
changes in the levels of conjugated forms
of IAA occur during fruit development
(Dunlap
et al.
, 1996; Purgatto
et al.
, 2002;
Böttcher
et al.
, 2010). A detailed study in
grape berries showed that IAA-amido
synthetases appear to reduce the levels of
free IAA prior to the initiation of ripening
(and perhaps during ripening; Böttcher
et
al.
, 2010, 2011a). This decrease may be
essential for the initiation of ripening due
to the proposed role of auxins as inhibitors.
There is also some evidence that auxin
oxidation products may accelerate fruit
ripening under some circumstances, which
may suggest an even more complex control
of ripening by IAA and its products
(Frenkel, 1975; Frenkel
et al.
, 1975;
Guttridge
et al.
, 1977).
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