Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 138

of L -galactose to L -galactono-1,4-lactone.
Furthermore, clear evidence for this
pathway was given by characterization of
the ozone-sensitive and ascorbate-defi cient
Arabidopsis vtc1 mutant that was later
revealed to be defective in GDP- D -mannose
pyrophosphorylase activity (Conklin et al. ,
1996, 1999). The 'Smirnoff-Wheeler'
pathway involves the conversion of
D -mannose into ascorbate via a succession
of L -galactose-containing intermediates,
namely GDP- D -mannose, GDP- L -galactose,
L -galactose-1-phosphate, L -galactose and
L -galactono-1,4-lactone (Fig. 8.3) (Wheeler
et al. , 1998; Conklin et al. , 1999, 2006;
Bartoli et al. , 2000; Wolucka and Van
Montagu, 2003; Dowdle et al. , 2007; Laing
et al. , 2007). Based on radiotracer, bio-
chemical and expression studies, three
alternative routes to ascorbate have been
proposed. The 'Smirnoff-Wheeler' pathway
can be augmented through a 'pectin-
scavenging' pathway. This pathway,
initially described in strawberry fruit,
initiates from D -galacturonic acid to
produce an L -galactonic acid derivative via
D -galacturonate reductase (Agius et al. ,
2003). Alternatively, L -gulose (Wolucka and
van Montagu, 2003) and myo -inositol
(Lorence et al. , 2004) have also been
proposed as intermediates of ascorbate
biosynthesis pathways, which partially
overlap with the animal pathway (Fig. 8.3).
Although the main plant pathway is the
'Smirnoff-Wheeler' pathway, the preva-
lence of this pathway over the others is
dependent on the fl eshy fruit species and
developmental stage (Lorence et al. , 2004;
Laing et al. , 2007; Cruz-Rus et al. , 2010,
2011).

during development, ripening and post-
harvest storage. Ascorbate can accumulate
to very high levels in fruits (e.g. mango,
kiwifruit, citrus fruits, strawberry, tomato),
which are sink organs. In addition, fruits
from the same genus display a large scale
of ascorbate content, for example in tomato
from 10 to more than 500 mg per 100 g FW
(Galiana-Balaguer et al. , 2006), as well as
in kiwifruit and related species where
vitamin C content may range from 80 to
800 mg per 100 g FW (Bulley et al. , 2009).
Many recent studies on fruits like tomato
(Gautier et al. , 2009), kiwifruit (Bulley et
al. , 2009), strawberry (Agius et al. , 2003),
apple (Razavi et al. , 2005) and blackcurrant
(Hancock et al. , 2007) have revealed that
fruits are able to synthesize their own
ascorbate. However, its translocation from
source tissues to fruits has been described
in kiwifruit (Bulley et al. , 2009) and tomato
(Gautier et al. , 2009). Moreover, the
recycling of ascorbate from MDHA and
DHA signifi cantly contributes to the
regulation of ascorbate content (Stevens et
al , 2008; Yin et al. , 2010). As in the leaf,
light intensity infl uences ascorbate levels
in fruits (Massot et al. , 2012).
However, little is known about the
mechanisms underlying light regulation of
ascorbate synthesis in fruits, even though
this has been described since 1945
(Hamner et al. , 1945; McCollum, 1946;
Madamba et al. , 1974; El-Gizawi et al. ,
1993; Ioannidi et al. , 2009). Light
regulation of the expression of ascorbate
biosynthesis-related genes has been
demonstrated in apple (Li et al. , 2009) and
tomato (Massot et al. , 2012). In whole
plants and leaves, recent studies have
demonstrated that the GDP- L -galactose
phosphorylase gene (Fig. 8.3) exerts most
of the control of the fl ux through the
synthesis pathway (Bulley et al. , 2009),
and expression of this gene as well as the
protein activity are often correlated with
ascorbate content (reviewed by Linster and
Clarke, 2008). The presence in strawberry
fruit of the 'pectin-scavenging' pathway,
which uses D -galacturonic acid to produce
an L -galactonic acid derivative via
D -galacturonate reductase, and ultimately

8.4.4 Regulation of vitamin C content
in fruit

Unlike foliar tissues, few studies on
ascorbate synthesis, recycling and
regulation have been carried out in fruit.
Nevertheless, fruits represent pertinent
models to unravel this aspect of the
ascorbate metabolism as they experience
physiological and biochemical changes