Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 141

8.5.1 Enhancing fruit vitamin content by
genetic engineering

prephenate dehydrogenase ( tyrA ), a feed-
back insensitive enzyme, and the plant
HPPD genes. This led to up to 1.8-2.6-fold
increases in tocochomanol levels in the
seeds (Karunanandaa et al. , 2005). Likewise,
expression of the yeast ( Saccharomyces
cerevisiae ) prephenate dehydrogenase gene
( PDH ) in tobacco overexpressing the
Arabidopsis HPPD gene resulted in a
massive accumulation of tocotrienols in
the leaves (Rippert et al. , 2004). Using a
similar strategy, based on the stable
coexpression in tomato of the yeast PDH
and Arabidopsis HPPD genes under the
control of the SlPPC2 fruit-specifi c pro-
moter (Fernandez et al. , 2009; Guillet
et al. , 2012), our group recently obtained a
threefold increase in tocotrienol content in
tomato fruit (unpublished results). In
parallel, also in tomato, overexpression of
homogentisate phytyl transferase (HPT; Fig.
8.2) isolated from apple fruit led to
approximately 1.8-3.6-fold and approxi-
mately 1.6-2.9-fold increases in tomato leaf
of D -tocopherol and J -tocopherol, re-
spectively, whilst the levels of D -tocopherol
and J -tocopherol in the fruit increased up
to 1.7- and 3.1-fold, respectively (Seo et al. ,
2011).

Provitamin A
Tomato fruit has been the model of choice
for the discovery of important steps in the
carotenoid biosynthetic pathway and for
the genetic engineering of carotenoids in
plants. More than 20 studies aimed at
enhancing fruit carotenoid levels have
been conducted in this species in recent
years. Most of these studies have been
thoroughly presented and discussed in a
recent excellent review on genetic
engineering of carotenoid formation in
tomato fruit (Fraser et al. , 2009). A two- to
fourfold increase in total carotenoids or in
E -carotene was generally obtained by
ectopic expression, overexpression or
silencing of genes encoding various
enzymes from the carotenoid pathway or
proteins involved in light signalling,
originating from tomato, other plants and
bacteria. Notable achievements were a
greater than 30-fold increase in E -carotene
triggered by overexpression of E -lycopene
cyclase (Fig. 8.1) from tomato, although
there was a corresponding reduction in
lycopene (D'Ambrosio et al. , 2004), the
production of E -cryptoxanthin and
zeaxanthin by coexpressing E -lycopene
cyclase from Arabidopsis and E -carotene
hydroxylase from pepper (Dharmapuri et
al. , 2002), and concomitant and strong
increases in E -carotene (tenfold), lycopene
(fourfold) and fl avonoids obtained by
specifi cally silencing the DET-1 gene
involved in light signalling using fruit-
specifi c promoters (Davuluri et al. , 2005).

Vitamin C
Success has been achieved by the
expression of plant vitamin C biosynthesis
or regulatory genes in Arabidopsis and
crop species (Ishikawa et al. , 2006; Bulley
et al. , 2009; Zhang et al. , 2009). Increases
in vitamin content were usually two- to
threefold, reaching 12-fold when the newly
discovered GDP- L -galactose phosphorylase
(VTC2/VTC5) enzyme, which catalyses the
fi rst committed step in the L -galactose
ascorbate biosynthetic pathway (Fig. 8.3),
was transiently coexpressed with GDP- D -
mannose epimerase (GME) in tobacco
(Bulley et al. , 2009). In fruit, the stable
overexpression of GME led to a slight
increase in ascorbate content in tomato
(1.6-fold increase in red fruit; Zhang et al. ,
2011). The most promising results
were obtained recently through the stable
overexpression of VTC2 in tomato and

Vitamin E
In recent years, many biotechnological
attempts to increase vitamin E have been
carried out successfully in plants (Mène-
Saffrané and DellaPenna, 2009). In oilseed
crops, which are the main source of
vitamin E in the human diet, metabolic
engineering of vitamin E in Arabidopsis ,
canola and soybean has been done by
expressing both Synechocystis bifunctional