Other Hormonal Signals during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 214

Auxin application to pre-ripening fruits
produces varying effects. In some fruits,
auxin treatments have resulted in a delay
in ripening, while in others auxin
treatment advanced or enhanced ripening.
The enhancement of ripening by auxin
analogues occurs most frequently in
climacteric fruits, as reported in apple,
loquat, nectarine, peach, pear, persimmon
and plum (Agustí et al. , 1999, 2003, 2004;
Ohmiya, 2000; Kondo et al. , 2004; Yuan
and Carbaugh, 2007; Li and Yuan, 2008).
This effect may be due to increased
ethylene evolution resulting from a
stimulation of ethylene biosynthesis
(Kondo et al. , 2004; Trainotti et al. , 2007;
Li and Yuan, 2008). There are numerous
reports where the application of auxins
during the pre-ripening stage delayed
ripening in both climacteric and non-
climacteric fruits; examples include
avocado (Tingwa and Young, 1975), banana
(Purgatto et al. , 2002), grape (Böttcher et
al. , 2011a,b), kiwifruit (Fabbroni et al. ,
2006), strawberry (Villarreal et al. , 2009),
tomato (Cohen, 1996), loquat (Amorós et
al. , 2004), peach (Ohmiya and Haji, 2002)
and pear (Frenkel and Haard, 1973). These
reports detail some of the specifi c effects of
ripening-delaying auxin treatments such as
delayed reduction of chlorophyll levels,
delayed changes to cell-wall components
that normally result in fruit softening,
delayed accumulation of sugars (or other
forms of carbon storage) and delayed
accumulation of anthocyanins. These
changes are also refl ected by changes in
gene transcription, and in some cases the
corresponding enzyme activities. For
example, in grapes, auxin treatment of pre-
ripening berries maintained the expression
of genes normally expressed during the
pre-ripening stage and delayed the
expression of genes normally associated
with ripening and the ripening-associated
increase in ABA levels (Davies et al. ,
1997). In strawberry, 1-naphthaleneacetic
acid (NAA) treatment reduced the
expression (and in some cases enzyme
activity) of ripening-associated proteins
(Manning, 1994; Villarreal et al. , 2009).
The transcript levels of many ripening-

related genes, including some thought to be
involved in ripening-related cell-wall
changes, were downregulated by NAA
treatments of immature fruit (Harpster et
al. , 1998; Aharoni et al. , 2002; Bustamante
et al. , 2009; Villarreal et al. , 2009), and the
expression of two auxin ( AUX )/ IAA genes
possibly involved in pre-ripening fruit
development was upregulated (Liu et al. ,
2011).
The results of high-throughput transcript
analyses also suggest the importance of IAA
in the control of fruit development,
including ripening. In grapes, expression of
the majority of auxin-related genes (includ-
ing those encoding auxin-responsive factors
(ARFs)) were downregulated at the
initiation of ripening in line with the pro-
posal that auxins inhibit ripening (Deluc et
al. , 2007; Fortes et al. , 2011). Some AUX /
IAA genes, which are repressors of auxin-
regulated transcription, were down-
regulated during ripening but others were
upregulated. An upregulation of AUX / IAA
genes at ripening was also observed in
apricot (Manganaris et al. , 2011) and
peaches (Trainotti et al. , 2007). In tomato,
the expression of certain AUX / IAA genes
has been correlated with the levels of par-
ticular metabolites (Rohrmann et al. , 2011).
The profound effect that changes in
auxin signalling can have on fruit
development and fruit ripening is ex-
emplifi ed by experiments in tomato. The
downregulation, through transgenesis, of
the expression of an ARF gene normally
expressed throughout fruit development,
and most highly in early red fruit, led to a
pleiotropic phenotype (Jones et al. , 2002).
The fruit contained elevated levels of
chlorophyll, had a blotchy appearance
during ripening and were fi rmer, but other
measures of ripening such as total soluble
solids and acid levels were similar to those
of wild-type fruit.
The above discussion demonstrates the
importance of IAA during fruit develop-
ment and ripening but also shows that
there is still much to be understood about
all aspects of auxin action including
interactions with other hormone signalling
pathways.