Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 69

2009), olive (Padilla et al. , 2009), peach
(Zhang et al. , 2010) and strawberry (Leone
et al. , 2006). Particular evidence for the
involvement of LOX in the generation of
fruit aroma volatiles came from transgenic
work in tomato, in which the specifi c
downregulation of TomLOXC resulted in a
signifi cant reduction of grassy C6
aldehydes (Chen et al. , 2004). A recent
report showed that the expression of a
plastid-located TomLOXC plays a key role
in determining the volatile composition of
tomato ripening mutations ( hp1 , rin , Cnr ,
nor and Nr ) and wild-type controls ('Ailsa
Craig') (Kovács et al. , 2009).
HPL belongs to a novel family of
cytochrome P450s (CYP74) and can be
divided into four subfamilies based on
sequence identity or grouped into three
clades according to preferred substrates
(Padilla et al. , 2010). Defi lippi et al. (2009b)
outlined the progress in cloning HPL genes
from tomato, guava, cucumber and melon
fruit. Characterization of HPL related to the
biosynthesis of aroma aldehydes has
recently been reported in olive and peach
fruit. The expression of peach ( Prunus
persica ) PpHPL was at high levels on the
harvest day and decreased progressively
with ripening, which is consistent with
changes in n -hexanal and ( E )-2-hexenal,
which are major contributors to peach fruit
aroma at harvest (Zhang et al. , 2010).
Recombinant olive ( Olea europaea )
OepHPL1 is specifi c for 13-hydroperoxide
derivatives of linolenic acid, producing ( E )-
2-hexenal and ( Z )-3-hexenal (Padilla et al. ,
2010). Moreover, the expression profi le of
OepHPL1 has been shown to be spatially
and temporally regulated during olive fruit
development and ripening.
ADH is a Zn-binding enzyme that acts
as a dimer and is involved in the reversible
conversion of aldehydes to their cor-
responding alcohols. The structure, bio-
chemistry, function and evaluation of the
ADH gene family in plants have been
reviewed recently by Strommer (2011).
Multiple ADH genes have been char-
acterized in a number of fruits, including
apricot (González-Agüero et al. , 2009),
banana (Yang et al. , 2011), melon

(Manríquez et al. , 2006), peach (Zhang et
al. , 2010) and tomato (Longhurst et al. ,
1994). In apple and peach fruit, ADH
expression and alcohol levels are the
highest on the initial ripening day, followed
by a progressive decline with further
ripening and senescence (Schaffer et al. ,
2007; Zhang et al. , 2010). Ripening-
dependent changes in ADH activity and
alcohols have also been observed in tomato
(Longhurst et al. , 1994) and melon
(Manríquez et al. , 2006) fruit. These results
indicate that the expression of ADH is
under tight developmental regulation in
ripening fruit. In tomato fruit, over-
expression of ADH2 resulted in sig-
nifi cantly increased hexanol and
( Z )-3-hexenol, producing more of a 'ripe
fruit' fl avour (Speirs et al. , 1998).
Identifi cation of AAT genes and enzymes
responsible for ester biosynthesis has been
reported in a number of ripening fruits,
including apple (Souleyre et al. , 2005; Li et
al. , 2006), apricot (González-Agüero et al. ,
2009), banana (Yang et al. , 2011), melon
(Yahyaoui et al. , 2002; El-Sharkawy et al. ,
2005), strawberry (Aharoni et al. , 2000) and
peach (Zhang et al. , 2010). Acyltransferases
are a large family of proteins that are
widely distributed in plants and consists of
fi ve major clades according to preferred
substrate or to the condition under which
genes and enzymes are active (D'Auria,
2006). There is considerable divergence
among AATs, not only among species but
also within species. The maximum se-
quence identity between Prunus armeniaca
( PaAAT1 ), Pyrus communis ( PAAT1 ) and
Malus domestica ( MdAAT2 ) is only 58%,
although all belong to the Rosaceae family.
In Cucumis melo , the sequence identity
between CmAAT1 and CmAAT4 is only
22%, whereas CmAAT1 , CmAAT2 and
CmAAT3 are more closely identical (58-
84%) (El-Sharkawy et al. , 2005). In the case
of the substrate utilized and the product
generated, melon CmAAT4 has a strong
preference for producing cinnamoyl
acetate, strawberry SAAT prefers to yield
methyl hexanoate and hexyl butyrate, and
apple MdAAT2 has a preference for the
formation of pentyl acetate and hexyl