Cellular, Metabolic and Molecular Aspects of Chromoplast Differentiation in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 41

Mutants:
Golden 2-like

2

1

PLASTID PHENOTYPE RIPENING PHENOTYPE

CHLORO/CHROMOPLAST NUCLEUS

EGY1 protein
Alteration of plastid

1

EGY1

MEP
pathway

Tetrapyrrole
pathway
ALA

2

3

Redox
system

GLK-2

4

5

Mg-Proto.IX

EXPRESSION OF
RIPENING-RELATED GENES

RO

MEcPP

Fig. 3.2. Schematic representation of the possible signalling routes operating during the chloroplast-to-
chromoplast transition between the plastids and the nucleus for regulating the expression of nuclear-encoded
ripening-related genes. Experimental support for a role in plastid-to-nucleus signalling arises from the results
of: (1) Barry et al. (2012) for the EGY1 protein of the lutescent-2 mutant; (2) Powell et al. (2012) for the
Golden 2-like protein (GLK-2) via the tetrapyrrole pathway; (3) Czarnecki et al. (2011) for 5-aminolevulinic
acid (ALA); (4) Bouvier et al. (1998) and Marti et al. (2009) for reactive oxygen species (ROS); and (5) Xiao et
al. (2012) for the methylerythritol phosphate (MEP) pathway via methylerythritol cyclodiphosphate (MEcPP).

that play a role in the sequestration of the
carotenoids. The chromoplast acts as a
metabolic sink requiring the presence of
active systems for provision of energy
(ATPases) for import of proteins and lipid
precursors (Fig. 3.3). Traffi cking between
the plastid and the cytosol is also
stimulated by the increase in size and
length of stromules. The carotenoid
biosynthesis pathway located in the plastid
membrane is strongly upregulated. Part of
the carotenoid biosynthesis pathway
remains active in plastoglobules, which
persist from green plastids and undergo
enlargement during the ripening process.
Recent progress has been made into
understanding the regulatory mechanisms
governing the differentiation of chromo-
plasts. The discovery of the Or gene,

which controls differentiation of non-
coloured plastids into chromoplasts, has
been an important step. Other regulatory
genes, such as specifi c heat-shock proteins
and ATP casein lytic proteinases, are also
candidates for regulating chromoplast
differentiation. Plastid-to-nucleus com-
munication is another new fi eld of in-
vestigation. Signifi cant progress has been
made and numerous reports published on
the discovery of signalling events arising
from chloroplasts to control the
expression of nuclear-encoded
photosynthesis genes. Research is much
less advanced for chromoplasts. However,
experimental data are starting to emerge
that support the presence of plastid-to-
nucleus signalling in ripening fruit. These
discoveries open new perspectives