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nature of the carotenoid molecules seems
to play a role in determining the type of
structure. In red papayas, where lycopene
is abundant, crystalloid structures are
dominant, whilst in yellow papaya, rich in
E
-carotene and
E
-cryptoxanthin esters,
tubulo-globular chromoplasts are pre-
dominant (Schweiggert
et al.
, 2011).
Interestingly, the sequestration of carote-
noids into crystals can be driven by the
functional overexpression of phytoene
synthase, a gene crucial for the bio-
synthesis of carotenoids. This has been
observed in both non-green plastids of
Arabidopsis
seedlings not requiring a
chromoplast developmental programme
and in white carrots (Maas
et al.
, 2009).
Therefore, the stimulation of the carot-
enoid biosynthesis pathway appears to
play a major role in the development of
carotenoid storage structures.
chromoplasts, PGs acquire specifi c func-
tions in the conversion of carotenoids
through the presence of
]
-carotene de-
saturase (ZDS), lycopene
E
-cyclase (CYC-
E
)
and two
E
-carotene
E
-hydroxylases (CrtR-
E
)
operating in series in bicyclic carotenoid
biosynthesis (Ytterberg
et al.
, 2006).
Stromules represent another structural
element of plastids participating in the
interconnection of plastids. They are
motile protrusions of the plastid membrane
into the cytoplasm that are involved in the
traffi cking of proteins between plastids
(Köhler
et al.
, 1997; Kwok and Hanson,
2004). During chromoplast differentiation,
they increase in length and number, at least
in the inner mesocarp of tomato, suggesting
increased interactions with the cytosol
(Waters
et al.
, 2004).
3.2.3 Internal membrane remodelling
3.2.2 Plastoglobules and stromules
Early electron microscopy observations
described a remodelling of the internal
membrane system during chromoplast
formation in pepper (Spurr and Harris,
1968) in which thylakoid grana and
intergrana, characteristic of chloroplasts,
undergo lysis while new membrane
systems are formed. It has now been
demonstrated in tomato fruit that the
newly synthesized membranes are the site
of formation of carotenoid crystals and that
they are not related to the disassembly of
thylakoids. Rather, they are derived from
vesicles generated from the inner mem-
brane of the plastid (Simkin
et al.
, 2007).
Plastoglobules (PGs) are oval or tubular
lipid-rich subcompartments present in all
plastid types, but their number and size is
increased in ripening fruit in parallel with
chromoplast differentiation (Harris and
Spurr, 1969). PGs contain quinones,
D
-tocopherol and lipids and, in chromo-
plasts, carotenoids as well. They contain
several enzymes involved in the synthesis
of tocopherol and carotenoids (Austin
et
al.
, 2006). Plastoglobules comprise proteins
named plastoglubulins such as fi brillin,
plastid lipid-associated proteins and
carotenoid-associated proteins that form
supramolecular lipoprotein structures with
carotenoids and galacto- and phospholipids
(Deruere
et al.
, 1994). PGs arise from the
stroma-side layer of the thylakoid mem-
brane. In chloroplasts, they form a
functional metabolic link between the inner
envelope and thylakoid membranes and
play a role in the breakdown of carotenoids
and oxidative stress defence (Ytterberg
et
al.
, 2006). In chromoplasts, the metabolic
link with thylakoids is lost and the
production of plasto/phylloquinones dis-
appears. Instead, as shown in sweet pepper
3.2.4 Loss of capacity for division
According to Cookson
et al.
(2003), the
number of plastids remains constant
during the tomato ripening process. This
has been confi rmed by confocal micro-
scopy observations showing that all pre-
existing chloroplasts differentiate into
chromoplasts (Egea
et al.
, 2011). The
cessation of plastid division in ripening
tomato is accompanied by the dis-
appearance after the mature green stage of
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