Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 132

species are sources of provitamin A in the
human diet. They include widespread
species such as papaya, apricot, pepper
and tomato and more local species
produced in South-east Asia such as gac
fruit ( Momordica cochinchinensis ) (Hyun
et al. , 2012) and other species found in
Latin America (Rodriguez-Amaya, 1999).
Provitamin A rich fruits also contain
usually other phytonutrients with potential
health benefi ts, such as the red-coloured
lycopene found in papaya (Schweiggert et
al. , 2012) and in tomato (Fraser et al. ,
2009). Tomato has been studied ex-
tensively because of the striking increase in
carotenoids that takes place during fruit
ripening and the many colour mutants
available (Hirschberg, 2001). Thanks to
the genomics tools currently available,
carotenoid biosynthesis and regulation are
also beginning to be well studied in less
well-known species such as papaya (Devitt
et al. , 2010), palm fruit (Tranbarger et al. ,
2011) and gac (Hyun et al. , 2012).

the carotenoid pathway is the con-
densation head to tail of two GGDP (C20)
molecules, which produces the non-
coloured 15- cis -phytoene and is catalysed
by phytoene synthase (PSY). The coloured
carotenoid chromophore is then generated
by a series of desaturation reactions, which
extend the number of conjugated double
bonds. The fi rst desaturase enzyme is
phytoene desaturase (PDS), which intro-
duces double bonds in the molecule to
form 9,15,9 c -tri- cis - ] -carotene, a yellow-
green molecule with seven conjugated
double bonds. This step is followed by
an isomerization producing 9,9 c -di- cis - ] -
carotene. A further desaturation catalysed
by ] -carotene desaturase (ZDS) forms
7,9,7 c ,9 c -tetra- cis -lycopene (prolycopene),
which is converted in the fruit by carotene
isomerase (CRTISO) to all - trans -lycopene,
a molecule with 11 conjugated double
bonds. Lycopene is responsible for the red
coloration of fruits such as tomato (Fraser
et al. , 2009), red-fl eshed papaya (Devitt et
al. , 2010; Schweiggert et al. , 2012) and
orange (Alquezar et al. , 2008). Cyclization
of lycopene by E -cyclase (LCYB) and
H -cyclase (LCYE) forms the orange-
coloured provitamin A cyclic carotenoids
E -carotene (found for example in mango)
and D -carotene. Further introduction of
hydroxyl groups into E -carotene by
E -hydroxylase results in the formation of
the orange-coloured E -cryptoxanthin found,
for example, in orange-fl eshed papaya
(Schweiggert et al. , 2012) and, ultimately,
to the phytohormone ABA. Other pre-
cursors are shared between the carotenoid
biosynthetic pathway and the phyto-
hormones gibberellins (GAs) (Fig. 8.1) and
cytokinins.

8.2.3 Carotenoid biosynthetic pathway

Carotenoids are synthesized by nuclear-
encoded enzymes within the plastids of the
fruit, chloroplasts in photosynthetic tissues
and chromoplasts in the ripening fruit
(Hirschberg 2001; Fraser et al. , 2009). The
sequence of reactions involved in
carotenoid biosynthesis that lead to the
formation of the major provitamin A
carotenoids, D - and E -carotene and
E -cryptoxanthin, is presented in Fig. 8.1.
In the well-studied tomato fruit (Fraser
et al. , 2009), isopentenyl diphosphate (IPP),
a C5 precursor from which the carotenoids
and tocochromanols are derived, is
synthesized mostly by the plastidial
methylerythritol 4-phosphate (MEP) path-
way (Figs 8.1 and 8.2). IPP is isomerized
to dimethylallyl diphosphate by IPP
isomerase (IPI). Three IPP molecules are
then sequentially added to dimethylallyl
diphosphate by geranylgeranyl diphosphate
(GGDP) synthase (GGPS), producing GGDP,
which constitutes the immediate precursor
of carotenoids. The fi rst committed step in

8.2.4 Regulation of carotenoids in fruits

Tomato has also been the model fruit for
studying the regulation of carotenoid
biosynthesis in fl eshy fruits. Like many
carotenoid-coloured fl eshy fruits, during
fruit ripening tomato undergoes a massive
accumulation of carotenoids coordinated
with other ripening-associated changes