Biotechnology of Fruit Quality - Fruit Ripening: Physiology, Signalling and Genomics - page 276

respectively (Mehta et al. , 2002; Nambeesan
et al. , 2010). Transgenic overexpression of
apple spermidine synthase in tomato fruit
not only upregulated PSY and PDS but also
downregulated the catabolic enzymes
lycopene E - and H -cyclases, resulting in an
overall 1.3-2.2-fold increase in lycopene
content (Neily et al. , 2011). These fi ndings
indicate a positive correlation between
polyamines with carotenoids levels during
fruit ripening and microarray-based tran-
scriptional profi ling of E8:ySAMdc tomato
fruit (Kolotilin et al. , 2011).
Defi ciency of vitamin A is a major issue
affecting child health, especially in develop-
ing nations. To increase its synthesis in
staple foods, transgenic technologies have
successfully been used to develop rice
varieties (Golden Rice) engineered to
accumulate higher levels of the provitamin
A ' E -carotene'. Introduction of maize PSY in
combination with carotene desaturase from
Erwinia uredovora resulted in 23-fold
increase in total carotenoids in rice (Paine et
al. , 2005). During rice processing, an
aleurone layer is removed to avoid rancidity
of rice grains during storage, while rice
endosperm lacks E -carotene. Using DNA
recombination technology, three transgene
constructs were co-transformed and trans-
formants containing all three transgenes
were selected and characterized. The three
transgenes introduced were daffodil PSY
under the control of an endosperm-specifi c
gluten promoter, E. uredovora phytoene
desaturase under the control of the CaMV
35S promoter and Narcissus pseudo-
narcissus lycopene E -cyclase under the
control of a rice gluten promoter.
Expression of these transgenes in rice
endosperm led to higher accumulation of
E -carotene (Ye et al. , 2000).
Transgenic approaches have also been
used to enhance levels of carotenoids in
fl axseed (Fujisawa and Misawa, 2010),
maize (Naqvi et al. , 2011), kumquat citrus
(Zhang et al. , 2009), wheat (Cong et al. ,
2009), Brassica (Yu et al. , 2008; Fujisawa et
al. , 2009; Wei et al. , 2009), rice (Burkhardt
et al. , 1997; Rai et al. , 2007), tobacco (Qin
and Zeevaart, 2002; Frey et al. , 2006) and
canola (Ravanello et al. , 2003).

16.5 Molecular Engineering of
Flavonoids

Flavonoids are aromatic, low-molecular-
weight secondary metabolites classifi ed as
plant phenolics (Robards and Antolovich,
1997). Their hydrophilic properties (Rice-
Evans et al. , 1997) complement the hydro-
phobic nature of carotenoids. More than
6000 naturally occurring fl avonoids have
been identifi ed (Harborne and Herbert,
1999) and classifi ed based on the degree of
unsaturation and oxidation of a three-
carbon bridge in the fl avone skeleton
between their phenyl groups.
Antioxidant and free-radical scavenging
properties of fl avonoids have been
associated with reducing the risks of heart
and age-related diseases and cancers (Ross
and Kasum, 2002). Fruit juice is a major
source of fl avonoids in the human diet, and
total fruit juice consumption seems to
account for 20-30% of the dietary intake of
fl avonoids (Robards and Antolovich, 1997).
As well as their emerging therapeutic role
in alternative medicinal science, fl avonoids
are also known to provide protection to
plants against UV-B light and microbial
interactions (Harborne and Williams,
2000). This attribute is important for fruits
to maintain their resistance against fungi
during storage. Flavonoids also contribute
towards various fruit quality attributes
including colour (red, violet, blue), fl avour
and texture. In contrast, undesirable brown
pigmentation (bruises) on the fruit surface
has been attributed to oxidation of phenols
to quinones, which then polymerize into
brown pigments, for example, fl avan-3-
ols in apples (Amiot et al. , 1992; Goupy
et al. , 1995; Robards and Antolovich,
1997). Different classes of fl avonoids also
combine with proteins and cause
sedimentation in fruit juices and wines
(Amiot et al. , 1992). The various fl avonoid
compounds found in different fruits and
vegetables have been summarized
elsewhere (Robards and Antolovich, 1997;
Nicoletti et al. , 2007; USDA, 2007;
Slimestad and Verheul, 2009).
The genetic regulation of the fl avonoid
biosynthesis pathway was initially