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softer and associated with precocious and
extensive depolymerization of structural
hemicelluloses without altering poly-
uronide depolymerization (Brummell
et
al.
, 1999b). It was proposed that
Exp1
modulates relaxation of the cell walls and
regulates polyuronide depolymerization by
controlling access of a pectinase to its
substrate, whereas the depolymerization of
hemicellulose occurs independently or
requires only very small amounts of Exp1
protein (Brummell
et al.
, 1999b). A fi rmer
fruit texture and higher cellular integrity
during longer storage was observed in the
fruit in which
Exp1
and
PG
were
simultaneously downregulated (Powell
et
al.
, 2003).
After the initial demonstration that a
protein glycosylation inhibitor, tuni-
camycin, impaired fruit ripening (Handa
et al.
, 1985), the role of protein glyco-
sylation in fruit ripening and textural
changes has begun to emerge using trans-
genic technologies. Tunicamycin inhibits
the UDP-HexNAc:polyprenol-P HexNAc-
1-P family of enzymes and blocks the
synthesis of all
N
-linked glycoproteins
(
N
-glycans). Suppression by antisense
RNA technology of two
N
-glycosylating
enzymes,
E
-mannosidase and
E
-
D
-
N
-
acetylhexosaminidase, led to reduced
ripening-associated softening and improved
fruit shelf-life (Meli
et al.
, 2010), whereas
their ectopic expression caused excessive
softening of the transgenic fruit. These
studies provided a novel way to alter fruit
ripening and extend their shelf-life.
quenching, lipid peroxidation and as a
substrate for the phytohormone abscisic
acid (ABA) (Namitha and Negi, 2010).
However, it is the human health benefi ts of
carotenoids that have attracted signifi cant
attention in recent years (Dixon, 2005;
Mattoo
et al.
, 2010). The role of vitamin A
(retinal) in preserving eyesight, especially
in preventing night blindness, is one of the
best-known functions of carotenoids in
human health (Cook, 2010). Due to their
high antioxidant activity, carotenoids are
implicated in protection against cataract
and macular degeneration of the eye, and
against cervical, lung, prostate, colorectal,
stomach, pancreatic and oesophagus
cancers. Carotenoids may also reduce low-
density lipoproteins, implicated in cardio-
vascular disease, and boost the immune
system to provide protection against many
other diseases such as osteoporosis, hyper-
tension and neurodegenerative diseases
like Alzheimer's, Parkinson's and vascular
dementia (Mattoo
et al.
, 2010; Namitha and
Negi, 2010). The emerging consensus in
favour of the benefi cial role of carotenoids
has led to signifi cant research activity to
raise their cellular levels in fruit and
vegetable crops using novel approaches.
Genes encoding carotenoid biosynthetic
pathway enzymes have been identifi ed and
cloned from several species, but regulation
of their accumulation in plants is com-
plicated and poorly understood (Klee and
Giovannoni, 2011). A detailed carotenoid
biosynthesis pathway is illustrated in Plate
7. A series of addition and condensation
reactions convert isopentenyl diphosphate
to form geranylgeranyl diphosphate. Two
different pathways, mevalonate-dependent
(cytosolic) and mevalonate-independent
(plastid), generate isopentenyl diphosphate
(Rodríguez-Concepción, 2010). Phytoene
synthase (PSY) is the fi rst committed step
in carotenoid biosynthesis and catalyses
the condensation of two geranylgeranyl
diphosphates to form phytoene, which is
converted into
]
-carotene by phytoene
desaturase (PDS).
]
-Carotene desaturase
(ZDS) converts
]
-carotene to lycopene,
which in turn is converted into either
16.4 Molecular Engineering of
Carotenoids
Fruits are naturally rich in carotenoids, one
of the most abundant groups of plant
pigments. Over 600 carotenoids have been
structurally identifi ed, and this list
continues to increase as new compounds
are added. Carotenoids play several roles
in plants including photosystem assembly,
light harvesting, free-radical detoxifi cation,
photomorphogenesis, non-photochemical
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