Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 136

8.3.4 Regulation of tocochromanols
in fruits

regulation of vitamin E in fl eshy fruits
warrants further investigation, current data
suggest that its accumulation during fruit
ripening is developmentally controlled and
submitted to hormonal regulation by
ethylene in climacteric fruits.

Regulation of tocochromanol biosynthesis
in fruits remains poorly investigated. As
mentioned in Section 8.2.3, tocochromanols
and carotenoids share GGDP as a common
precursor (Figs 8.1 and 8.2). As a con-
sequence, manipulation of carotenoid bio-
synthetic pathways in the fruit will
probably also alter tocochromanol levels in
this organ. This was indeed the case in
tomato fruit overexpressing a fruit PSY,
which accumulated more D -tocopherol
(Fraser et al. , 2007), although the mech-
anisms responsible for the increased
vitamin E content in the fruit were unclear.
Light regulation of tocopherol accumulation
in the fruit through the transcriptional
activation of tocopherol biosynthetic genes
( GGDR and J -TMT ; see Fig. 8.2) is likely, as
shown by DET-1 defective tomato
transgenics that accumulated two- to ten-
fold more tocopherol (Enfi ssi et al. , 2010).
However, the DET-1 mutation, which
disturbs the light signal transduction
pathway, also affects plastid biogenesis,
carotenoid biosynthesis and other second-
ary metabolites, therefore opening up the
possibility that tocopherol alterations are
due to a more general effect on plastid
compartment size or fruit metabolism.
Available data also indicate a develop-
mental regulation of tocopherol bio-
synthesis during fruit ripening. In tomato,
in which the main tocochromanols are D -
and J -tocopherol (Almeida et al. , 2011), as
in most fruits (Chun et al. , 2006), the
tocopherol content increases during fruit
ripening (Enfi ssi et al. , 2010) and silencing
of J -TMT leads to substantial alterations of
the tocopherol profi les of the fruits
(Quadrana et al. , 2011). Likewise, in
mango, tocopherol content increases
during fruit ripening, together with that of
carotenoids (Singh et al. , 2011). Toco-
pherol accumulation in mango ( Mangifera
indica ) is correlated with increased
transcripts for HPPD, which catalyses the
committed step in chromanol head-group
synthesis (Fig. 8.2). Furthermore, MiHPPD
expression is ethylene inducible, as shown
by promoter studies. Thus, although the

8.4 Vitamin C Biosynthesis and
Regulation in Fleshy Fruits

In general terms, vitamin C is used with
reference to its nutritional virtues, whereas
ascorbic acid refers to the purifi ed
compound. At physiological pH, ascorbic
acid exists as monoanion form and is hence
called ascorbate. This di-acid (C 6 H 8 O 6 )
contains an ene-diol group, which confers
its reducing agent or antioxidant properties
(Smirnoff, 2000). In vivo , the oxidation
products of ascorbate are monodehydro-
ascorbate (MDHA) and dehydroascorbate
(DHA). As a whole, these comprise the
ascorbate pool, which can be oxidized more
or less according to the redox state of the
cell. Eventually, if not recycled to
ascorbate, DHA is degraded to produce
intermediates such as oxalate, tartrate, and
threonate (Green and Fry, 2005).

8.4.1 Role of ascorbate (vitamin C) in
humans and plants

Humans, like a small number of mammals,
are unable to synthesize ascorbate due to
a mutation in the L -guluno-1,4-lactone
oxidase gene corresponding to the last step
of the biosynthetic pathway (Linster and
Van Schaftingen, 2007). In humans,
ascorbate is associated mainly with
metabolism related to ageing, free radicals
(mainly reactive oxygen and nitrogen
species), redox homeostasis and carcino-
genesis (Valko et al. , 2007). Numerous
epidemiological studies have established a
positive link between vitamin C content in
food and/or plasma and health benefi ts, for
example in the prevention of cardio-
vascular disease, cancer, infl uenza and
other diseases (Blot et al. , 1993; Steinmetz
and Potter, 1996), or an increase in iron
bioavailability (López and Martos, 2004).