Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 68

pyrophosphate, farnesyl pyrophosphate and
geranylgeranyl pyrophosphate, respectively.
The TPS family can be separated into
clusters according to the substrate used,
and the current knowledge of this mid-
sized gene family in plants has been
reviewed by Chen et al. (2011). In tomato,
44 TPS genes located on eight chromo-
somes have been cloned, including 29
genes that have open reading frames
(Falara et al. , 2011). In contrast, the
remaining 15 TPS genes appear to have
mutations and deletions. Among these 29
functional or potentially functional genes,
12 tomato TPS genes encode presumed
cytosolic synthases and belong to the
TPS-a clade, which is responsible for
sesquiterpene biosynthesis, eight genes
belong to the TPS-b clade encoding mono-
terpene synthases, two genes belong to the
TPS-c clade, fi ve genes belong to the TPS-
e/f clade and two genes belong to the TPS-g
clade. d-Limonene is the most abundant
volatile compound in orange fruit, and a
gene named CitMTSE1 was confi rmed to be
involved in the biosynthesis of this com-
pound (Rodríguez et al. , 2011). In addition,
Cstps1 has been identifi ed as a key gene
in the production of the sesquiterpene
valencene in citrus fruit (Sharon-Asa et al. ,
2003).
Carotenoids are widely distributed
compounds in plants. In addition to their
involvement in the formation of pigments
and colourants, carotenoids are also pre-
cursors of apocarotenoids (norisoprenes),
which are important aroma volatiles with
low odour thresholds. Apocarotenoids
such as E -ionone and geranylacetone are
formed through oxidative cleavage of
carotenoids, and the cleavage process is
catalysed by carotenoid cleavage di-
oxygenase (CCD). Antisense expression of
LeCCD1A and LeCCD1B in tomato
( Lycopersicon esculentum ) fruit resulted in
greater than a 50% decrease in E -ionone
and 60% in geranylacetone, indicating that
LeCCD1 is involved in the formation of
aroma apocarotenoids (Simkin et al. , 2004).
Yellow-fl eshed peach fruit and its white-
fl eshed mutant were used to study the role
of CCD in the metabolism of carotenoids

and apocarotenoids. Signifi cantly up-
regulated expression of CCD4 at later
ripening stages was observed in the white-
fl eshed peach fruit, which is concomitant
with signifi cantly higher levels of any
identifi ed apocarotenoid volatiles through-
out peach fruit ripening (Brandi et al. ,
2011). In melon ( Cucumis melo ) fruit,
CmCCD1 was suggested to be involved in
the generation of important apocarotenoid
aroma compounds (Ibdah et al. , 2006).

5.3.2 Fatty acid pathway

Saturated and unsaturated fatty acids are
the most important precursors for the
majority of fruit aroma volatiles, including
straight-chain aldehydes, alcohols, esters,
lactones and ketones. These compounds are
biosynthesized mainly through the lipo-
xygenase (LOX) pathway and E -oxidation.
The current knowledge of the fatty acid
pathway involved in the biosynthesis of
volatiles has been reviewed by Schwab et
al. (2008).
In the LOX pathway, linoleic (18:2) and
linolenic acid (18:3) are catalysed into
hydroperoxide isomers, which are further
cleaved by hydroperoxide lyase (HPL) to
form hexanal and hexenal, respectively.
The aldehydes are subsequently reduced to
the corresponding C6 alcohols by alcohol
dehydrogenase (ADH). Alcohol acyl-
transferase (AAT) catalyses the fi nal
linkage of an acyl moiety and an alcohol to
form esters and is thus directly responsible
for the production of esters. LOX is a non-
haem iron-containing dioxygenase and can
be divided into 9- and 13-LOX according to
the oxidation position of fatty acids
(Feussner and Wasternack, 2002; Porta and
Rocha-Sosa, 2002). Generally, there is a
close association of both LOX genes and
enzyme activity with fruit ripening and
associated aroma quality development
(Defi lippi et al. , 2009a). The relationship
between LOX genes and aroma volatiles
has been observed in ripening fruits such
as apple (Schaffer et al. , 2007), apricot
(González-Agüero et al. , 2009), banana
(Yang et al. , 2011), kiwifruit (Zhang et al. ,