Biotechnology of Fruit Quality - Fruit Ripening: Physiology, Signalling and Genomics - page 283

fruit compared with levels in the wild-type
fruit. Antisense inhibition of SlAADC2
also resulted in reduced emission of
these volatiles. Expression of tomato
phenylaldehyde reductase ( SlPAR1 and
SlPAR2 ) in transgenic petunia accelerated
the emission of 2-phenylethanol at the
expense of 2-phenylacetaldehyde (Tieman
et al. , 2007). However, how expression of
this gene affects the quality and quantity of
volatiles in fruits has not yet been
evaluated.
Hexanals and ( Z )-hex-3-enal are derived
from lipoxygenase pathway, and a higher
( Z )-hex-3-enal/hexanal ratio correlates with
a higher consumer appreciation of tomato
varieties (Carbonell-Barrachina et al. ,
2006). The enzyme Z -3 fatty acid
desaturase converts linoleic acid (18:2) to
linolenic acid (18:3), the precursor of
hexanal and its derivatives. Expression of
Z -3 fatty acid desaturase ( BnFAD3 ) from
Brassica napus in transgenic tomato
increased the ratios of 18:3/18:2 and ( Z )-
hex-3-enal/hexanal (Domínguez et al. ,
2010). The constitutive expression of an
antisense gene of chloroplast-targeted
lipoxygenase, TomloxC , greatly reduced
the production of hexanal, hexenal and
hexanol compared with wild-type levels
(Chen et al. , 2004).
Monoterpenes and sesquiterpenes are
other important contributors to fruit aroma
and volatile components, and are con-
nected with the early steps of carotenoid
biosynthesis pathway (see Plate 7).
D -Zingiberene synthase catalyses the
formation of D -zingiberene and other
sesquiterpenes from farnesyl diphosphate,
while geraniol synthase catalyses the
conversion of geranyl diphosphate to
geraniol (Iijima et al. , 2004). Geraniol is an
acyclic monoterpene and the precursor of
geranial, nerol, citronellol, and geraniol
and citronellol acetate esters (Davidovich-
Rikanati et al. , 2007), compounds that are
produced in minute amounts in ripe
tomato fruit (Baldwin et al. , 2000).
Overexpression of lemon basil geraniol
synthase under the control of the tomato
PG promoter resulted in multiple-fold
enrichment of endogenous carotenoid-

derived aroma volatiles at the expense of
phytoene, lycopene and E -carotene, and
induced the biosynthesis of geraniol and
its derivatives and monoterpenes, which
were not detected in wild-type fruit
(Davidovich-Rikanati et al. , 2007).
Linalool, another monoterpene, is synthe-
sized directly from geranyl diphosphate by
linalool synthase. Tomato fruit does not
contain any linalool synthase activity.
However, tomato transformed with the
Clarkia breweri S -linalool synthase gene
under control of the E8 promoter exhibited
greatly induced production of S -linalool
and 8-hydroxylinalool with no alteration in
the phenotype or in the level of terpenoids
(Lewinsohn et al. , 2001). Transgenic
tomato fruit overexpressing lemon basil
D -zingiberene synthase under the fruit
ripening-specifi c PG promoter produced
higher levels of D -zingiberene and other
sesquiterpenes and monoterpenes, which
were otherwise undetectable in the wild-
type fruit (Davidovich-Rikanati et al. ,
2008).
Apocarotenoid volatiles, 6-methyl-5-
hepten-2-one, 6-methyl-5-hepten-2-ol, E -
ionone, E -cyclocitral and geranylacetone,
are derived from carotenoid degradation.
Thus, production of apocarotenoid vola-
tiles depends on the type and amount of
carotenoids being synthesized and the
stage of ripening. Constitutive over-
expression of the native carotenoid cleav-
age dioxygenase antisense gene did not
alter plant morphology or carotenoid
accumulation in fruit tissues, but reduced
E -ionone levels by 50% and geranylacetone
by ≥60% (Simkin et al. , 2004). As
carotenoids are synthesized in plasto-
globules - lipid bodies within plastids -
and none of the carotenoid cleavage
dioxygenase genes is upregulated during
ripening, the ripening-associated increase
in these volatiles was attributed to physical
changes in plastids, such as chromoplast
differentiation (Klee and Giovannoni,
2011). Several studies have suggested
that carotenoid-derived synthesis of
aroma volatiles is ethylene dependent.
However, the pericarp discs of ACC
synthase-suppressed transgenic tomato