Cellular, Metabolic and Molecular Aspects of Chromoplast Differentiation in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 40

tetrapyrrole biosynthesis in a tomato
GLK2-like mutant can therefore be con-
sidered a plastid signal that will regulate
nuclear gene expression, which will alter
the fruit ripening phenotype. Evidence
has been provided for the participation
of an upstream metabolite of the tetra-
pyrrole pathway, 5-aminolevulinic acid
(ALA), in retrograde signalling in Arab-
idopsis chloroplasts (Czarnecki et al. ,
2012). Although the amount of glutamate
1-semialdehyde aminotransferase protein
involved in the synthesis of ALA decreases
during chloroplast-to-chromoplast tran-
sition (Barsan et al. , 2012), a role for ALA
in retrograde signalling in chromoplasts
remains possible.
The potential participation of com-
ponents of the carotenoid biosynthetic path-
way has been mentioned in Arabidopsis
where a retrograde signalling mutant, ceh1,
has been identifi ed. This mutant is caused
by a mutation in the 1-hydroxy-2-methyl-2-
(E)-butenyl 4-diphosphate synthase (HDS)
gene of the plastidial methylerythritol
phosphate (MEP) pathway (Xiao et al. ,
2012). The mutation results in the accumu-
lation of high levels of the intermediate
metabolite MEcPP, which, under stress
conditions, stimulates the expression of
nuclear genes, particularly those encoding
the plastid-localized proteins hydroper-
oxide lyase involved in the production of
aldehydes in the LOX pathway, and
isochorismate synthase 1 involved in
salicylic acid biosynthesis. The MEP
pathway is active in ripening tomato fruits
with high expression of the 1-deoxy- D -
xylulose 5-phosphate synthase gene, the
fi rst step in the pathway (Lois et al. ,
2000). The role of MEcPP signalling in
chromoplast development deserves special
attention inasmuch as hydroperoxide lyase
has been encountered at substantial levels
in chromoplasts (Barsan et al. , 2012) and
has been involved in the synthesis of LOX-
derived aroma volatiles.
Another argument for plastid-to-nucleus
signalling can be found in the tomato fruit
lutescent1 ( l1 ) and lutescent2 ( l2 ) mutants.
These mutants are affected in terms of
chloroplast development and possess

chlorophyll-defi cient phenotypes. The
onset of fruit ripening is delayed by
approximately 1 week, although, once
ripening is initiated, the mutant fruit ripen
at a normal rate and accumulation of
carotenoids is not impaired (Barry et al. ,
2012). The gene responsible for the l2
mutation is the chloroplast-targeted zinc
metalloprotease of the M50 family, which
is homologous to the Arabidopsis gene
ETHYLENE-DEPENDENT GRAVITROPISM
DEFICIENT AND YELLOW-GREEN1 ( EGY1 )
(Chen et al. , 2005). The EGY1 protein has
been detected at the mature green stage of
tomato only (Barsan et al. , 2012). These
data suggest a role for the chloroplast in
mediating the onset of fruit ripening in
tomato and indicate that chromoplast
development in fruit does not depend on
functional chloroplasts. A schematic
representation of the possible signalling
routes operating in plastids during fruit
ripening is presented in Fig. 3.2. All these
data strongly suggest the presence of
plastid-to-nucleus signalling in ripening
fruit, but we are still far from identifying
the molecular events governing the
interactions between plastid and nucleus
and controlling the expression of ripening-
related genes.

3.9 Conclusions and Perspectives

The processes leading to the formation
of chromoplasts represent a major event
in fruit ripening in many fruiting species.
It corresponds to the conversion of
chloroplast-like green plastids into the
non-green plastids named chromoplasts
associated with the accumulation of
carotenoids. The process involves pro-
found structural changes including the
lysis of the thylakoid membrane system. In
parallel, carotenoid-bearing bodies emerge
surrounded by newly synthesized mem-
branes derived from the inner membrane of
the plastid (Fig. 3.3). These carotenoid-
bearing bodies accumulate carotenoids in
the form of fi brillar or crystalloid structures
in combination with polar lipids and
plastid- or carotenoid-associated proteins