Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 142

strawberry, which increased the fruit
vitamin C content by two- to sixfold
(Bulley et al. , 2012).

QTLs have been mapped, candidate gene
approaches, positional cloning and/or
association genetics can be used to identify
the locus responsible for the vitamin
variation. Using a combination of QTL and
candidate gene mapping, components of
the light signal transduction pathway
responsible for variations in provitamin A
levels and in overall fruit nutritional
quality have been identifi ed in tomato
(Lieberman et al. , 2004; Liu et al. , 2004).
Using a similar approach, Almeida et al. ,
(2011) identifi ed 16 candidate genes
putatively affecting the various tocopherol
isoforms found in tomato fruit. Likewise,
Stevens et al. (2007, 2008) identifi ed an
allelic form of MDHAR, which co-
segregated with a major vitamin C QTL
and explained more than 80% of the
variation in reduced ascorbic acid levels
following storage. A candidate gene
approach has also been successful in less-
studied species such as papaya, in which
an allelic form of lycopene E -cyclase was
shown to control fruit colour and
provitamin A content (Devitt et al. , 2010).
Positional cloning of loci responsible for
variations in vitamin levels have been
reported for provitamin A (Ronen et al. ,
2000; Isaacson et al. , 2002) but not yet
for vitamins E and C in fl eshy fruit
species. The rapid acquisition of genome
sequences and high-density-marker genetic
maps in fl eshy fruit species will
considerably improve the speed and
effi ciency of the identifi cation of genes
underlying fruit vitamin QTLs and,
subsequently, the transfer of the alleles of
interest to elite varieties.

8.5.2 Enhancing fruit vitamin content by
breeding

Most fl eshy fruit species present con-
siderable variation in vitamin content.
Fruit ascorbate content of wild tomato
species accessions can be as low as 11 mg
per 100 g FW or reach >500 mg per 100 g
FW (Galiana-Balaguer et al. , 2006), whilst
in kiwifruit and related species, vitamin C
ranges from 80 to 800 mg per 100 g FW
(Bulley et al. , 2009). Similarly wide
variations can be found for carotenoids or
vitamin E in existing cultivars or in wild
related species or accessions of cultivated
fl eshy fruits species such as tomato
(Schauer et al. , 2005; Almeida et al. , 2011),
pepper (Brand et al. , 2012), melon (Cuevas
et al. , 2009), papaya (Schweigert et al. ,
2012) and Citrus species (Fanciullino et al. ,
2006). For several fruit species, the
genomic regions controlling the quan-
titative variations in vitamin content
(QTLs) have been located on genetic maps
using introgression lines, advanced
backcrossing and recombinant inbred line
populations derived from crosses between
genotypes with contrasting vitamin content
(cultivated varieties or wild related
species). To date, the most extensive
studies have been carried out in tomato in
which, for example, metabolite profi ling of
76 introgression lines issued from crosses
between S. esculentum and S. pennellii
allowed the identifi cation of up to 889
QTLs controlling the variations in several
metabolites linked to fruit sensorial and
nutritional quality, including vitamin E
and vitamin C (Schauer et al. , 2006). QTLs
controlling fruit vitamin content have been
detected in a large number of species such
as provitamin A in melon (Cuevas et al. ,
2009) and pepper (Brand et al. , 2012),
vitamin E in tomato (Almeida et al. , 2011),
and vitamin C in tomato (Stevens et al. ,
2007), strawberry (Zorrilla-Fontanesi et al. ,
2011) and apple (Davey et al. , 2006). Once

8.6 Conclusion

In the near future, analysis of cultivated
fruit varieties, introgression lines from
wild related species, mutants and trans-
genic lines with contrasting vitamin
content, using a combination of the cur-
rently available genomic tools (meta-
bolomics, transcriptomics and proteomics),
should allow the discovery of new target
genes for enhancing the fruit levels of