Agriculture Reference
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pathway and its regulation have been
studied extensively (Ruiz-Sola and
Rodriguez-Concepción, 2012). Nevertheless
chromoplasts carry out many other func-
tions such as synthesis of sugars, starch,
lipids, aromatic compounds, vitamins
(ribofl avine, folate, tocopherols) and hor-
mones (Neuhaus and Emes, 2000; Bouvier
and Camara, 2007).
In recent years, transcriptomic and
proteomic approaches have generated
novel and extensive information on the
specifi city of the chromoplast proteome of
the bell pepper (Siddique
et al.
, 2006),
tomato (Barsan
et al.
, 2010) and sweet
orange (Zeng
et al.
, 2011), and on the
molecular and biochemical events occur-
ring during the chloroplast-to-chromoplast
transition in tomato (Kahlau and Bock,
2008; Egea
et al.
, 2010). In this chapter, we
review the most recent fi ndings on the
metabolic shifts occurring during the bio-
genesis of chromoplasts and the accom-
panying structural and regulatory events.
Most of the data comes from the study of
the chloroplast-to-chromoplast transition in
tomato, but reference will also be made to
other fruit species.
outer mesocarp having an oblong, needle-
like appearance, and chromoplasts in the
inner mesocarp being much larger and
ovoid (Waters
et al.
, 2004). At the breaker
stage, plastids show considerable intra-
cellular variability in size and dif-
ferentiation status, with the chromoplast
being smaller than the chloroplasts. The
chloroplast-to-chromoplast transition events
are presumably not simultaneous through-
out the fruit tissues, leading to a hetero-
geneous population of plastids within a
whole fruit. However, an
in situ
real-time
recording showed that the chloroplast-to-
chromoplast transition was synchronous
for all plastids of a single cell (Egea
et al.
,
2011). In addition, all chromoplasts are
derived from pre-existing chloroplasts,
thus confi rming that plastid division ceases
during chromoplast differentiation (Pyke
and Howells, 2002; Waters
et al.
, 2004).
This is further supported by the loss of
several proteins involved in the plastid
division machinery during the chloroplast-
to-chromoplast transition in tomato fruit
(Barsan
et al.
, 2012).
Fruit chromoplasts have been cat-
egorized according to the predominant
structure of the carotenoid-bearing bodies,
into globular (e.g. yellow and orange
pepper, orange, yellow kiwi fruit), crystal-
line (e.g. tomato) and fi brillar (e.g. red
pepper). However, it is rare that only
one kind of substructure exists alone in
a chromoplast. Different structures
accumulating carotenoids have been
described for a number of fruit by Jeffery
et al.
(2012). For instance, in mango
chromoplasts, most of the carotenoids
occur as globules, but different tubular
membrane structures are also present
(Vasquez-Caicedo
et al.
, 2006). In tomato,
besides the dominant crystalloid bodies
of lycopene, globuli and membranous
structures are also encountered (Harris and
Spurr, 1969). The structure of the
carotenoid-containing bodies is of great
importance for the bioavailability of
carotenoids during human digestion. A
decreasing order of bioavailability has been
established from plastoglobules to crystals
and membranes (Jeffery
et al.
, 2012). The
3.2 Changes in Plastid Morphology and
Structure during Chromoplast
Differentiation
3.2.1 Changes in structure and morphology
Morphological changes in plastids during
chromoplast differentiation have been
investigated by confocal microscopy
coupled with the plastid-located green
fl uorescent protein (GFP) (Köhler and
Hanson, 2000; Waters
et al.
, 2004; Forth
and Pyke, 2006). As fruit ripens, the red
autofl uorescence conferred by chlorophyll
decreases concomitant with chlorophyll
degradation, so fully ripe pericarp cells
possess a large population of chromoplasts,
appearing green due to the exclusive
fl uorescence of GFP (Forth and Pyke, 2006).
Differences in plastid size and appearance
among various tomato fruit tissues have
been reported, with chromoplasts of the
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