Cellular, Metabolic and Molecular Aspects of Chromoplast Differentiation in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 36

in phosphorylation-mediated regulatory
functions in plants and are involved in the
post-translational regulation of enzymes of
starch metabolism (Sehnke et al. , 2001;
Tetlow et al. , 2004).

lipids upon uptake of glucose, pyruvate
and malate precursors (Angaman et al. ,
2012). The fi rst step corresponding to the
activation of acetate into acetyl CoA
through the action of a pyruvate de-
hydrogenase has been characterized in
daffodil chromoplasts (Kleinig and
Lievogel, 1978). A chromoplastic pyruvate
dehydrogenase has been encountered in
tomato (Barsan et al. , 2010) and in sweet
orange (Zeng et al. , 2011) chromoplast
proteomes. Interestingly the amount of
protein remains stable during chloroplast-
to-chromoplast differentiation in tomato
(Barsan et al. , 2012). All the subunits of
acetyl CoA carboxylase involved in the
formation of malonyl CoA from acetyl CoA,
including the ACCD plastid-encoded
protein, are present, generally at stable
levels during chromoplast differentiation.
The ACCD protein is the major product of
the transcription and translation machinery
of the chromoplast (Kahlau and Bock,
2008). Sustained biosynthesis of lipids
therefore occurs in chromoplasts, probably
for providing a lipid storage matrix for the
accumulation of carotenoids. Among the
proteins involved in lipid metabolism, of
special interest is the presence in the
tomato and sweet orange chromoplast
proteomes of several proteins of the
lipoxygenase (LOX) pathway leading to the
generation of aroma volatiles (Barsan et al. ,
2010; Zeng et al. , 2011). Fatty acids
synthesized in the plastids can have several
destinations: either they remain in the
plastid or they are exported to the
endoplasmic reticulum where they are
glycosylated and phosphorylated (Wang
and Benning, 2012). Analysis of the
changes in the plastid proteome during
tomato fruit ripening (Barsan et al. , 2012)
indicates that proteins involved in the
export pathway (acyl carrier proteins
(ACPs)) remain abundant, whilst proteins
of the endoplasmic reticulum pathway
(trigalatosyldiacetylglycerol protein), which
act as import proteins (Xu et al. , 2010),
decrease in abundance. As the phos-
pholipid content of plastids is increased
signifi cantly during fruit ripening (Lenucci
et al. , 2012), it can be concluded that the

3.4.3 Lipid synthesis and metabolism

Lipids fulfi l a specifi c storage/sequestering
function in chromoplasts. The amount of
phospholipids increases markedly during
the chloroplast-to-chromoplast transition in
relation to the de novo synthesis of new
membranes required for the sequestration
and growth of lycopene crystals (Lenucci et
al. , 2012). Plastid lipids are located in both
plastoglobules and membranes. In tomato,
the total amount of diacylglycerides
remains almost unchanged, whilst the
triacyl glyceride content markedly de-
creases and the amount of phytosterol and
phospholipids increases during plastid
transition from chloroplasts to chromo-
plasts (Lenucci et al. , 2012). The lipid
composition of chromoplast membranes
has scarcely been evaluated specifi cally.
However, some data are available for
daffodil fl owers, showing that phospho-
glycerolipids constitute a minor proportion
of the lipid content, whilst galacto-
glycerolipids are abundant (Liedvogel
and Kleinig, 1977), as in chloroplasts
(Andersson and Dörmann, 2009). In
chloroplasts, the envelope contains very
low amounts of sterols, and the thylakoid
membrane is devoid of sterols (Hartmann,
1998). In daffodil chromoplasts, membrane-
free sterols, steryl glycosides and acetylated
steryl glycosides have been found
associated with the activities of glyco-
sylation and acylation of sterols (Liedvogel
and Kleinig, 1977). During tomato fruit
ripening, plastids undergo a four- to fi ve-
fold increase in phytosterols and phos-
pholipids (Lenucci et al. , 2012), suggesting
that the newly synthesized membranes of
chromoplasts have a specifi c composition
in terms of sterols. Chromoplasts possess
the entire metabolic equipment for the
synthesis of fatty acids. Isolated tomato
chromoplasts are capable of synthesizing