Other Hormonal Signals during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 212

biosynthesis have been shown to delay
climacteric and non-climacteric fruit
ripening. For example, in tomato, while
ABA application was shown to enhance
ripening, including inducing ethylene
biosynthesis gene expression, treatment
with fl uridone and nordihydroguaiaretic
acid delayed ripening (Zhang et al. ,
2009a). Fluridone treatment also delayed
strawberry fruit ripening (Jia et al. , 2011).
The most convincing evidence for ABA
being involved in the ripening of both
climacteric and non-climacteric fruits
comes from recent experiments using
transient or stably transformed strawberry
and tomato fruit. Bastías et al. (2011)
overexpressed and downregulated an ABA
response element-binding factor in tomato.
They found higher levels of the sugars
glucose and fructose in fruit from the
overexpression lines and elevated expres-
sion of genes encoding a vacuolar invertase
and sucrose synthase. ABA levels were
increased in immature green and red-ripe
fruit, while ethylene levels were elevated
in red-ripe fruit alone. In strawberries
( Fragaria × ananassa ), Chai et al. (2011)
used virus-induced gene silencing to
downregulate the expression of a putative
ABA receptor, FaPYR1. This delayed fruit
ripening and increased ABA content by
more than twofold. ABA application did
not overcome the ripening inhibition, and
the expression of some ABA-responsive
genes was downregulated. These data
suggest that the FaPYR1 gene encodes an
ABA receptor and that ABA positively
regulates strawberry ripening. In further
experiments on strawberry using virus-
induced gene silencing, Jia et al. (2011)
downregulated NCED expression, which
resulted in uncoloured fruit with lower
ABA levels. Coloration could be restored
by exogenous ABA. They also down-
regulated the expression of a putative ABA
receptor gene, FaCHLH/ABAR , and again
produced uncoloured fruit that did not
colour with applied ABA. These fruit also
had elevated ABA levels and reduced sugar
content, and there were alterations in the
expression of ABA-related and sugar-

responsive genes. These results indicated
that FaCHLH/ABAR acts as an ABA
receptor and that ABA can promote
ripening. The downregulation of NCED in
tomato fruit provides support for ABA
being involved in another important aspect
of fruit ripening - changes in cell walls
that relate to fruit texture. An RNA
interference construct with the fruit-
specifi c E8 gene promoter was used to
specifi cally downregulate NCED expres-
sion (Sun et al. , 2012). This resulted in a
signifi cant reduction in ABA levels and
a reduction in the transcript levels of a
number of genes encoding proteins
involved in cell-wall metabolism, includ-
ing polygalaturonase, pectin methyl
esterase, E -galactosidase, xyloglucan
endotransglycosylase, endo-1,4- E -cellulase
and an expansin. This resulted in fi rmer
fruit with higher pectin levels (Sun et al. ,
2012). In summary, ABA appears to act as a
positive regulator of ripening, or some
aspects of it, in a range of fruits.

12.2.2 Brassinosteroids (BRs)

The role of BRs in plant growth is well
established, but more recently many other
functions have been defi ned (Gudesblat
and Russinova, 2011). Among these
functions, and like ABA, BRs have been
shown to be involved in the response of
plants to various stresses. Interestingly,
studies in a small range of fruit species
indicate that BRs may also play a role in
the control of fruit ripening, another
property shared with ABA. Indications that
BRs could be involved in the control of
fruit ripening fi rst came from tomato.
Tomato pericarp discs treated with BRs
exhibited an increase in ripening-related
parameters, such as increased lycopene
accumulation, a reduction in chlorophyll
levels, a decrease in ascorbic acid levels
and an increase in sugar levels (Vardhini
and Rao, 2002) . As seen with the
application of some other hormones, such
as auxin, to climacteric fruits, advance-
ment of ripening by BRs may operate