Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 137

Ascorbate in plants has many cellular
functions, linked mostly to the molecule's
capacity to donate electrons. Ascorbate acts
as a scavenger of the free radicals generated
by photosynthesis, cellular respiration and
abiotic stresses such as ozone and UV
radiation (Conklin et al. , 1996; Noctor and
Foyer, 1998; Smirnoff and Wheeler, 2000).
This molecule is a cofactor for numerous
enzymatic reactions such as those
catalysed by oxygenases involved in the
synthesis of fl avonoids and alkaloids and
hormones, such as ACC oxidase catalysing
the last step of ethylene biosynthesis (see
Part III) (Prescott and John, 1996).
Ascorbate also has a role in plant growth
and development (Arrigoni and De Tullio,
2002; Pastori et al. , 2003), including
aspects such as fl owering (Pavet et al. ,
2005; Dowdle et al. , 2007). Finally,
numerous studies have linked ascorbate to
both biotic and abiotic stress tolerance in
various species (Muckenschnabel et al. ,
2002; Huang et al. , 2005; Kuzniak and
Skłodowska, 2005; Yamamoto et al. , 2005;
Stevens et al. , 2008). Besides the ascorbate
biosynthetic pathway, the ascorbate re-
cycling pathway also plays a crucial role,
as both MDHA reductase (MDHAR) and
DHA reductase (DHAR) have roles in stress
tolerance (Eltayeb et al. , 2007; Stevens et
al. , 2008; Gest et al. , 2010).
In fruit, in addition to its effect on stress
tolerance, ascorbate may contribute to
postharvest fruit quality (Davey and
Keulemans, 2004; Malacrida et al. , 2006).
Ascorbic acid levels have been linked to
fl esh browning in pear (Veltman et al. ,
1999) and a quantitative trait locus (QTL)
for fl esh browning colocalizes with a QTL
for oxidized ascorbate content in apple
(Davey et al. , 2006). In tomato fruit,
ascorbic acid content is linked to MDHAR
activity and tolerance to chilling stress
(Stevens et al. , 2008; Gest et al. , 2010).

(Smirnoff, 2000; Zechmann et al. , 2011)
and may vary within organs of the same
plant. Photosynthetic tissues, in particular
meristems, are often higher in ascorbate
than roots, fl owers and fruits (Loewus and
Loewus, 1987). Fruit ascorbate levels are
highly variable in diverse species (Table
8.1). For instance, citrus fruits are known
for being vitamin C rich, their content
being about 50 mg per 100 g of fresh
weight (FW). Nevertheless, the highest
vitamin C levels in fruits are found in two
berries from South America, the camu-
camu (2-3 g per 100 g FW) and acerola
(1-2 g per 100 g FW). In addition, within
the same genus, large disparities may be
observed, as for example in tomato. Wild
tomato species such as Solanum pennellii
generally accumulate more ascorbate
(50 mg per 100 g FW) than the large-fruited
domesticated cultivars (10-20 mg per 100 g
FW) of Solanum lycopersicum (Stevens et
al. , 2007; Roselló et al. , 2011). Other major
sources of natural variability are the
cultural and environmental conditions,
especially light (Massot et al. , 2012), and
the postharvest losses during storage and
processing, which can considerably reduce
the vitamin C content (Lee and Kader,
2000; Franck et al. , 2003).

8.4.3 Vitamin C biosynthetic pathways

Whilst the biosynthesis of vitamin C was
elucidated in the early 1960s in the animal
kingdom (Chatterjee et al. , 1960), it was not
until 1998 following work by the Smirnoff
group that the fi rst biosynthetic pathway in
higher plants was proposed, known as the
'Smirnoff-Wheeler' pathway. It is now
accepted that this pathway is the major de
novo synthesis route for ascorbate (Wheeler
et al. , 1998). Smirnoff and coworkers
established that the direct precursor of
ascorbate was L -galacto-1-4 lactone itself
produced from GDP- D -mannose via oxid-
ation of the comparatively rare sugar
L -galactose (Wheeler et al. , 1998). This
pathway was supported by feeding studies
and by partial purifi cation of a new
enzyme, L -galactose dehydrogenase, which
catalyses the NAD-dependent oxidation

8.4.2 Vitamin C in fl eshy fruits

Ascorbate is present in all plant organs and
in all cell compartments, including the
nucleus. The concentrations of this
molecule are highly variable in plants