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provision of energy and import of pre-
cursors into the plastid. An adenylate
(ATP/ADP) translocator has been char-
acterized in Narcissus chromoplasts that is
suggested to provide ATP for supporting
biosynthetic activity in the plastid, as ATP
external to chromoplasts stimulates fatty
acid biosynthesis (Liedvogel and Kleinig,
1980). During the chromoplast dif-
ferentiation process in tomato, ATP
synthase subunits (nuclear and plastid
encoded), as well as an ADP/ATP carrier
and transporters of glucose-6-phosphate,
phosphoenolpyruvate and triosephosphate,
remain abundant (Barsan et al. , 2012).
Isolated tomato fruit chromoplasts are
capable of de novo ATP production
through a respiratory pathway using
NADPH as the electron donor. ATP
synthesis involves an ATP synthase
harbouring an atypical J -subunit, which is
induced during ripening and replaces the
J -subunit present in tomato leaf and green
fruit chloroplasts (Pateraki et al. , 2013).
These data indicate that the machinery for
the provision of energy and precursors
keeps very active to allow the synthesis of
fatty acids and sucrose within the
chromoplasts. It has been suggested that
lipids or lipid precursors can be imported
into the plastids via vesicles derived from
the endoplasmic reticulum membrane and
fused with Golgi membranes (Andersson
and Dörmann, 2009; Benning, 2009).
Interestingly, two proteins of the SEC
translocase system, homologues of a yeast
phosphatidylinositol transfer protein (Yakir-
Tamang and Gerst, 2009), are highly
expressed and increase continuously in
abundance during the biogenesis of chromo-
plasts in tomato (Barsan et al. , 2012).
regulation of their expression during the
developmental shift corresponding to the
onset of fruit ripening. Among these, the
early light-inducible protein ( ELIP ) gene,
showing homology with light-harvesting
complex proteins, displays elevated expres-
sion during the breaker/turning stages of
fruit ripening in tomato. Yet direct evidence
for the role of ELIP in chromoplast dif-
ferentiation is still lacking. New prospects
of uncovering the mechanisms regulating
chromoplast differentiation have been
provided by the discovery that a mutated
version of the caulifl ower Or gene results in
the accumulation of large amounts of
E -carotene in tissues typically devoid of
carotenoids (Li et al. , 2001; Lu et al. , 2006).
The Or gene encodes a plastid-associated
protein containing a cysteine-rich domain
present in DnaJ-like chaperones and
expression of the Or mutated form of the
gene confers an orange pigmentation
without signifi cantly affecting the expres-
sion of carotenoid biosynthetic genes (Lu et
al. , 2006; Lopez et al. , 2008). The Or
mutation consists of the insertion of a
retrotransposon in the Or gene that leads to
the generation of multiple splicing variants.
Strikingly, only ectopic expression of the
Or mutated form is able to induce
carotenoid accumulation in different plant
species, whilst neither the wild-type gene
nor the mutated version yielding the
various splicing forms can reproduce the
carotenoid-accumulating phenotype when
expressed in the plant (Lu et al. , 2006).
Moreover, downregulation of the Or gene
also fails to produce the Or -associated
phenotype, which, taken together with the
absence of phenotypes in the over-
expressing lines, suggest that carotenoid
accumulation in the Or mutants results
from a dominant-negative mutation (Lu et
al. , 2006; Giuliano and Diretto, 2007). Even
though the mechanisms by which the Or
mutation works remain obscure, the
functional role of the Or protein appears to
be essential for the differentiation of
uncoloured plastids into chromoplasts,
which creates a deposition sink for
carotenoid accumulation (Li and van
Eck, 2007). Another remarkable feature
3.7 Regulatory Events Controlling
Chromoplast Differentiation and
Development of Carotenoid Storage
Structures
Some genes have been described as
potential players in regulating the process
leading to the transition from choloroplast
to chromoplast based mainly on the up-
 
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