Cellular, Metabolic and Molecular Aspects of Chromoplast Differentiation in Ripening Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 39

displayed by the Or mutant is that chromo-
plast differentiation occurs mostly in the
infl orescence tissues but not in the leaves,
with the development of carotenoid mem-
branous inclusions reminiscent of those
found in carrot roots. Moreover, plastid
divisions are arrested in the Or mutant,
which may explain why the number of
chromoplasts in affected cells is limited to
one or two (Paolillo et al. , 2004). The Or
mutant supports the idea that carotenoid
sequestration capacity is an essential
mechanism by which carotenoid stability
is regulated and that creating a sink
structure may confer the ability to
accumulate provitamin A compounds in
crops naturally devoid of carotenoids. In
validating this hypothesis, tuber-specifi c
expression of the Or gene in transgenic
potato was found to result in enhanced
accumulation of carotenoids without
altering the expression of endogenous
carotenoid biosynthetic genes, thus
demonstrating that Or can provide an
effi cient genetic tool for improving the
nutritional value of food crops (Lu et al. ,
2006; Lopez et al. , 2008; Li et al. , 2012).
In addition to Or, other proteins may
also participate in the differentiation of
chromoplasts, such the plastidial heat-
shock protein HSP21, which has been
implicated in the promotion of colour
changes during tomato fruit maturation
(Neta-Sharir et al. , 2005). Some ATP-
dependent casein lytic proteinases located
in the stroma and known to participate in
chloroplast development (Lee et al. , 2006)
remain abundant during chloroplast-to-
chromoplast transition, suggesting a
sustained function during chromoplast dif-
ferentiation (Barsan et al. , 2012).

2012) classify the plastid signals into fi ve
types originating from: (i) tetrapyrrole
biosynthesis (Mg-protoporphyrin IX); (ii)
carotenoid biosynthesis (methylerythritol
cyclodiphosphate, MEcPP); (iii) ROS- and
redox-related processes; (iv) metabolite
pool changes; and (v) plastidial gene
expression. As carotenoid-, ROS- and
redox-related metabolisms are enhanced in
ripening fruit, it may be assumed that
signals originating from these metabolisms
play a role in chromoplast formation.
However, our knowledge of plastid-to-
nucleus communication is restricted to the
development of chloroplasts, studied by
characterizing mutants impaired for
chloroplast development such as the gun
mutants. Several of the gun mutants are
affected in tetrapyrrole biosynthesis,
resulting in alteration of the expression of
photosynthesis-associated genes (Woodson
et al. , 2011).
The fact that several fruit ripening
mutants are altered in terms of plastid
development (see Chapter 15, this volume)
supports the presence of plastid-to-nucleus
signalling pathways in ripening fruit.
Compounds of the tetrapyrrole pathway
have long been identifi ed as possible
messengers involved in retrograde sig-
nalling in chloroplasts. In chromoplasts,
proteins of the upper part of the pathway
up to the synthesis of Mg-protoporphyrin
are present. Only the route to chlorophyll
biosynthesis has disappeared (Barsan et al. ,
2012). A possible role of tetrapyrrole
signalling has been found from a study of
the Golden 2-like 2 ( GLK2-like ) mutation in
tomato. An allele of the GLK2-like gene
encoding a truncated loss of function of the
GLK protein is responsible for the
uniformly light green colour and the
absence of a green shoulder in many
modern varieties of tomatoes (Powell et al. ,
2012). In Arabidopsis , Golden 2-like 1 and
2 (GLK1/2) transcription factors are
involved in the regulation of genes of the
tetrapyrrole biosynthesis pathway and are
required for chloroplast development
(Waters et al. , 2009). In addition, the
expression of GLKs responds to retrograde
signals from the plastid. An alteration of

3.8 Plastid-to-nucleus Communication

It is acknowledged that nuclear and plastid
genes require tight coordination for
expression. This involves an intracellular
signalling network where plastids emit
signals that regulate nuclear gene
expression. Recent reviews on the topic
(Waters et al. , 2009; Barajas-Lopez et al. ,