Cellular, Metabolic and Molecular Aspects of Chromoplast Differentiation in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 37

increase in phospholipids observed in
plastids during fruit ripening originates
mostly from in situ biosynthesis through
plastid phosphatidic acid phosphatase
and lysophospatidic acid:acyl-ACP acyl-
transferase (Wang and Benning, 2012).

Valdivieso and Mullineaux, 2010). The role
of ROS as second messengers has been
demonstrated for the biosynthesis of carot-
enoids. Thus, ROS generated chemically in
green pericarp discs of pepper fruits rapidly
and simultaneously induced the expression
of multiple carotenogenic genes responsible
for the accumulation of capsanthin
(Bouvier et al. , 1998). In addition, the
application of ROS-generating compounds
to pepper protoplasts transiently trans-
formed with the CCS promoter coupled
with the E -glucuronidase ( GUS ) reporter
gene resulted in strong activation of the
GUS gene (Bouvier et al. , 1998).

3.5 Development of an Active
Antioxidant System

Reactive oxygen species (ROS) synthesized
during fruit ripening contribute to the
peroxidation of lipids and deterioration of
membranes and are therefore deleterious for
cell metabolism (Jimenez et al. , 2002). In
order to counteract the action of ROS, fruit
synthesize a number of molecules having
antioxidant capacity. The antioxidant
system includes reactive oxygen scavenging
enzymes, superoxide dismutases, catalase
and enzymes of the ascorbate glutathione
cycle, which is a series of coupled redox
reactions involving four enzymes: ascorbate
peroxidase, monodehydroascorbate reduct-
ase, dehydroascorbate reductase and
glutathione reductase, as well as NADP +
dehydrogenases and the thiol-specifi c
antioxidant proteins including per-
oxiredoxins (Bernier-Villamor et al. , 2004;
Finkemeier et al. , 2005). The main non-
enzymatic antioxidant molecules are
ascorbate and glutathione, which are also
components of the above-mentioned cycle,
and D -tocopherol, D -carotene and fl avonoids
(Noctor and Foyer, 1998). The activity of
antioxidant enzymes (superoxide dismutase
and enzymes of the ascorbate-glutathione
cycle) increases signifi cantly in pepper
plastids during the differentiation of
chloroplasts into chromoplasts, as well as
the content of ascorbate and glutathione
(Marti et al. , 2009). Lipids, rather than
proteins, seem to be an oxidation target in
chromoplasts. The role of a high anti-
oxidant system in the chromoplast could
be to protect plastid components such as
carotenoids against oxidation but also to
mediate signalling between the chromo-
plast and the nucleus. ROS have been
implicated in plastid-to-nucleus com-
munication (Kleine et al. , 2009; Galvez-

3.6 Import of Proteins and Metabolites
and Provision of Energy Accompanying
the Metabolic Changes

3.6.1 Import of proteins

A Toc/Tic (translocon at the outer/inner
envelope membrane of chloroplast) import
machinery performs the translocation of
proteins carrying a transit peptide (Jarvis,
2008). In tomato (Barsan et al. , 2010) and
sweet orange (Zeng et al. , 2011) fruit
chromoplasts, Toc/Tic proteins have been
encountered together with chaperonin-
associated proteins, indicating that the
system is probably still active in chromo-
plasts. In contrast, proteins involved in
internal traffi cking through the thylakoid to
the lumen are absent as a result of the loss
of thylakoid structure. However, intra-
cellular vesicular transport as described in
chloroplasts (Westphal et al. , 2001) seems
to persist in chromoplasts with the
presence of several proteins homologous to
yeast vesicular traffi cking components
(Andersson and Sandelius, 2004; Barsan et
al. , 2010).

3.6.2 Provision of energy, and import
of precursors and metabolites from the
cytosol

The synthesis of metabolites such as lipids
and sugars depends greatly upon the