Biotechnology of Fruit Quality - Fruit Ripening: Physiology, Signalling and Genomics - page 281

al. , 2007), rice (Shin et al. , 2006; Furukawa
et al. , 2007), Medicago truncatula (Pang et
al. , 2007), citrus (Moriguchi et al. , 2001;
Frydman et al. , 2004; Koca et al. , 2009),
Brassica (Hüsken et al. , 2005; Auger et al. ,
2009; Nesi et al. , 2009; Wei et al. , 2009),
fl ax seed (Lorenc-Kukula et al. , 2005; Zuk
et al. , 2011), apple (Rühmann et al. , 2006;
Ban et al. , 2007; Flachowsky et al. , 2010;
Flachowsky et al. , 2012; Han et al. , 2012),
soybean (Nagamatsu et al. , 2007) and
tobacco (Aharoni et al. , 2001).

2-methylbutanal, 3-methylbutanal, trans -2-
hexenal, isobutylthiozole and trans -2-
heptenal (Goff and Klee, 2006; Zeigler,
2007; Mathieu et al. , 2009; Klee, 2010).
Fruit breeding programmes that have
focused on developing larger and fi rmer
fruits with an extended shelf-life have
largely ignored organoleptic attributes with
the unintended consequence of loss of
fl avour components (Mathieu et al. , 2009).
The manipulation of fl avour components
in fruits via biotechnology has been
limited, particularly because biosynthetic
pathways are complex and are known only
for a limited number of volatile com-
pounds. Thus, the nature and biosynthetic
pathways of many volatile compounds
remain to be discovered (Tieman et al. ,
2006a). The availability of new molecular
genetics tools has begun to change this
inactivity, and efforts to improve fruit
fl avour components by genetic engineering
have a good future. QTLs regulating the
production and accumulation of several
volatiles compounds in tomato have been
identifi ed, and functional characterization
of genes present at these loci has begun
(Tieman et al. , 2006a; Mathieu et al. , 2009).
Transgenic studies on tomato engineered to
alter the fruit fl avour volatiles are listed in
Table 16.3.
Most of the fl avour volatiles are
synthesized during fruit ripening,
reaching a maximum at or before full
ripening (Klee and Giovannoni, 2011).
This temporal regulation of volatile
compounds is maintained through the
production of their precursors including
lipids, carotenoids, amino acids and
keto acids (Iijima et al. , 2004; Kochevenko
et al. , 2012). Aromatic volatiles,
2-phenylacetaldehdye and 2-phenylethanol
are derived from phenylalanine and
contribute signifi cantly to tomato fruit
fl avour. A family of aromatic amino acid
decarboxylases ( SlAADC1A , SlAADC1B and
SlAADC2 ) has been characterized (Tieman
et al. , 2006b). Constitutive overexpression
of either SlAADC1A or SlAADC2 increased
the emission of 2-phenylacetaldehyde,
2-phenylethanol and 1-nitro-2-phenylethane
more than tenfold in transgenic tomato

16.6 Molecular Engineering of Flavour
Volatiles

Flavour, an important quality attribute of a
fruit, is the sum of specifi c interactions of
fruit constituents among which sugars,
acids and a number of volatile molecules
are signifi cant components (Mathieu et al. ,
2009). Preference for a specifi c fl avour
(sugar:acid ratio) and perception of
volatiles by olfactory receptors in the
human nose are partly a social/cultural
science that vary with diversity in
ethnicity, age, and personal likes and
dislikes. In general, components con-
centration and odour threshold are
important variables in determining the
contribution of various volatiles to fruit
fl avour (Baldwin et al. , 2000). Most fruits
and vegetables produce aromatic volatiles,
as has been revealed by studies on mango
(MacLeod and Snyder, 1985; MacLeod et
al. , 1988; Andrade et al. , 2000), guava
(Wilson et al. , 1982; Porat et al. , 2011),
watermelon (Lewinsohn et al. , 2005),
apple (Dixon and Hewett, 2000),
strawberry (Song et al. , 1998) and tomato
(Buttery et al. , 1988; Buttery and Ling,
1993; Maul et al. , 1997; Krumbein and
Auerswald, 1998; Markovic´ et al. , 2007;
Mayer et al. , 2008; Christiansen et al. ,
2011). Over 400 aroma volatiles have been
detected in tomato, but fewer than 30 have
been proposed to impact on organoleptic
properties (Baldwin et al. , 2000; Tieman et
al. , 2006a). These aroma and fl avour
volatiles include cis -3-hexenal, E -ionone,
hexanal, E -damascenone, 1-penten-3-one,