Agriculture Reference
In-Depth Information
chromoplasts (Table 3.1). They participate
in processes such as the formation of
thylakoid membranes, vesicular traffi cking,
protein import, provision of precursors,
and assembly or repair of photosystems.
The increase in Stay-GReen (SGR) protein
is interesting. A mutation of the SGR gene
prevents chlorophyll degradation (Thomas
et al. , 1999). SGR interacts with fi ve
chlorophyll catabolic enzymes at the light-
harvesting complex II to ensure chlorophyll
degradation during leaf senescence in
Arabidopsis (Sakuraba et al. , 2012).
out development as granules in plastids and
then undergoes complete degradation at
maturity. In growing tomato and kiwi fruit,
starch can reach up to 20 and 50% of the
fruit dry weight, respectively. Starch pattern
tests are sometimes used as maturity
indices for some fruit such as apples. Starch
accumulation results from an imbalance
between synthesis and degradation. Indeed,
tomato fruit can synthesize starch during
the period of net starch breakdown,
illustrating that these two mechanisms
coexist (Luengwilai and Beckles, 2009).
Proteins of both starch synthesis and starch
degradation have been encountered in the
plastids of ripening tomato fruit, including
in chromoplasts (Bian et al. , 2011), sup-
porting the persistence of intense starch
turnover during chloroplast-to-chromoplast
conversion. Also supporting starch turnover
is the presence in chromoplasts of a glucose
translocator for the export of sugars
generated by starch degradation (Bian et al. ,
2011). In olive fruit, active expression of a
glucose transporter gene was observed at
full maturity when the chromoplasts were
devoid of starch (Butowt et al. , 2003). The
absence of starch accumulation in plastids
and the prevalence of degradation can be
related to the action of proteins that
facilitate the action of E -amylases, named
starch-excess proteins (SEX1, correspond-
ing to glucan water dikinase, and SEX4,
corresponding to phosphoglucan phos-
phatase), which have been encountered in
tomato fruit plastids, although they decrease
in abundance during chromoplastogenesis
(Barsan et al. , 2012). Plants harbouring
mutants of these proteins accumulate large
amounts of starch (Zeeman et al. , 2010).
Interestingly, a E -amylase protein
(Solyc01g067660) increases in abundance
during the differentiation of chromoplasts
(Barsan et al. , 2012). Another important
regulatory mechanism is related to the
presence in the tomato chromoplast
proteome (Barsan et al. , 2010, 2012) of
orthologues of the 14-3-3 proteins of the
H -subgroup family, which have been shown
to be involved in the regulation of starch
accumulation in Arabidopsis (Sehnke et
al. , 2001). The 14-3-3 proteins participate
3.4 Metabolic Re-orientations:
Carbohydrate and Lipid Metabolisms
3.4.1 The Calvin cycle and oxidative pentose
phosphate pathway (OxPPP)
During the chloroplast-to-chromoplast
transition of sweet pepper fruit, the activity
of transaldolase, which participates in the
regenerative phase of the Calvin cycle,
increases considerably (Thom et al. , 1998).
In ripening tomato fruits, several enzymes
of the Calvin cycle remain active
(Obiadalla-Ali et al. , 2004). However,
interference of the Calvin cycle with photo-
synthesis disappears due to degradation of
the photosynthetic apparatus so that the
Calvin cycle operates only for the recycling
of carbon within the OxPPP. The OxPPP
remains active during fruit ripening as
indicated by a high activity of glucose-6-
phosphate dehydrogenase (G6PDH), a key
component of the OxPPP. G6PDH levels are
even higher in fully ripe tomato fruit
chromoplasts than in leaves or green fruits
(Aoki et al. , 1998). A functional OxPPP has
also been encountered in isolated
buttercup chromoplasts (Tetlow et al. ,
2003). The role of OxPPP in the chromo-
plasts is probably to support metabolic
activities within the organelle.
3.4.2 Starch biosynthesis
In many fruits, including apples, bananas
and kiwifruit, starch accumulates through-
 
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