Cellular, Metabolic and Molecular Aspects of Chromoplast Differentiation in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 33

fruit. A mutation of the gene results in the
accumulation of lycopene in red papayas,
whilst in yellow papayas, the gene is
thought to be functional allowing the
accumulation of E -carotene (Blas et al. ,
2010). The presence of E -carotene in red
papayas is probably due to the expression
of another gene, a chloroplast-specifi c
CYC-B expressed at low levels in fruit (Blas
et al. , 2010).
Zeaxanthin is converted into anther-
axanthin and violaxanthin present in
yellow peaches and orange fl avedo by
violaxanthin de-epoxidase and zeaxanthin
epoxydase. In red peppers, the ac-
cumulation of red pigments, capsanthin
and capsorubin, is associated with the
expression of the capsanthin-capsorubin
synthase ( CCS ) gene (Lefebvre et al. , 1998;
Ha et al. , 2007). In yellow pepper, deletion
of the CCS gene has been considered as
responsible for the phenotype (Lefebvre et
al. , 1998), although mutations of the gene
rather than complete deletion can also be
responsible for the yellow ripening
phenotype (Ha et al. , 2007). The orange
colour of some peppers can be due to
either a deletion of the N terminus of CCS
(Lang et al. , 2004) or a mutation of the PHY
gene (Huh et al. , 2001). Other processes
controlling the colour in some orange-
pigmented peppers have also been
described, such as post-transcriptional and
post-translational regulations leading to the
inhibition of expression of the CCS gene
and the synthesis of the CCS protein
(Rodriguez-Uribe et al. , 2012).

upstream pathway of tetrapyrroles until the
magnesium-protoporphyrin IX monomethyl
ester (Lützow and Kleinig, 1990). A similar
observation has been made in proteomic
studies of tomato fruit plastids, indicating
that, among proteins of the tetrapyrrole
biosynthetic pathway, proteins involved in
the chlorophyll biosynthesis branch down-
stream of protoporphyrin IX are absent in
red chromoplasts, whilst all proteins up-
stream of the chlorophyll formation branch
remain present in red chromoplasts (Barsan
et al. , 2012). The photosynthetic apparatus
is strongly impaired during chromoplast
differentiation with a decrease in the
amount of proteins and downregulation of
many photosynthetic genes (Piechulla et
al. , 1985). In bell pepper, for instance,
nuclear genes encoding chloroplast pro-
teins such as the major chlorophyll a / b -
binding protein and the small subunit of
ribulose-1,5-bisphosphate carboxylase dis-
appeared in fruit chromoplasts (Kuntz et
al. , 1989). Surprisingly, the expression of
several plastid-encoded photosynthetic
genes remains high in fruit chromoplasts of
tomatoes (Piechulla et al. , 1985), peppers
(Kuntz et al. , 1989) and squash (Obukosia
et al. , 2003). In particular, transcripts of the
plastid genes coding for the rbcL and psbA
proteins were detected in fruit chromo-
plasts sometimes at higher levels than in
chloroplasts (Kuntz et al. , 1989; Obukosia
et al. , 2003). It was also demonstrated that
the rbcL protein remained stable during the
chloroplast-to-chromoplast transition in
tomato (Barsan et al. , 2012). The function
of proteins of the photosynthetic apparatus
in chromoplasts, at a stage where
chlorophylls have totally disappeared, is
unknown. Early observations by electron
microscopy showed that the differentiation
of chromoplasts comprised a lysis of the
grana and thylakoids (Rosso, 1968; Spurr
and Harris, 1968), in agreement with the
loss of photosynthetic activity. Proteomic
studies have revealed that the abundance
of a number of proteins participating in
the build-up of thylakoids strongly
decreases during the chloroplast-to-
chromoplast transition in tomato and that
they ultimately become absent in red

3.3.2 Loss of proteins involved in
photosynthesis and in the machinery for the
biogenesis of thylakoids

The loss of chlorophyll-synthesizing cap-
acity during the chloroplast-to-chromoplast
transition results in a strong decline in
photosynthetic activity (Piechulla et al. ,
1987; Carrara et al. , 2001). Chlorophyll-free
chromoplasts isolated from daffodil fl owers
were unable to synthesize in vitro the
isocyclic chlorophyll ring, whilst they were
capable of generating all compounds of the