Functional Genomics for the Study of Fruit Ripening and Quality: Towards an Integrative Approach - Fruit Ripening: Physiology, Signalling and Genomics - page 314

limited. Only a few genes involved in fatty
acid metabolism have been functionally
studied. Among these, a sugar transporter
( OeMST2 ), expressed during maturation,
has been cloned (Conde et al. , 2007). A
gene encoding a geranylgeranyl reductase
( OeCHLP ) has been identifi ed and its
involvement in organ development and
stress response was shown (Bruno et al. ,
2009). Knowledge regarding molecular
regulation mechanisms in important
pathways such as polyphenol and tri-
terpenoid metabolism is scarce, as well as
the mechanisms involved in olive fruit
development and ripening.
The elucidation of gene regulatory
networks based on the regulation of key
metabolic pathways during fruit growth
and development is essential for improving
olive oil quality and nutritional value. One
olive transcriptomic analysis used the
cultivar 'Leccino', a popular Italian variety
with a short, highly synchronized fruit
developmental cycle (Galla et al. , 2009).
The suppression subtractive hybridization
approach identifi ed 1132 differentially
expressed gene sequences at three stages:
initial fruit set (30 days after fl owering
(DAF)), completed pit hardening (90 DAF)
and veraison (130 DAF). The analysis
identifi ed 642 differentially regulated
sequences. Among these, 89 (14%) cor-
responding to 61 key genes were further
analysed by real-time PCR, which con-
fi rmed expression patterns for up to 69%
of the results. The bioinformatic annotation
of all gene sequences allowed insight into
the metabolic pathways and elucidated
specifi c regulatory networks.
These data are a signifi cant contribution
to the elucidation of control of carbo-
hydrates, fatty acids, transcription factors,
secondary metabolites, hormones and
responses to environmental stress at the
transcript level. Of particular interest are
data showing the complexity of the role
played by hormones in olive fruit
development and ripening. These molecular
and bioinformatic data represent a fi rst step
towards the elucidation of gene functions
and regulatory networks active in olive fruit
biochemical and morphological processes.

18.8 Tomato Mutants with
Modifi ed Light Signal
Perception

While the hormone ethylene is required to
complete ripening in climacteric fruit, the
impact of light is specifi c to the regulation
of pigment accumulation (Alba et al. ,
2000). Tomato high-pigment mutations
( hp1 and hp2 ) accumulate more carot-
enoids and fl avonoids due to greater light
sensitivity without changing other
ripening processes (Peters et al. , 1989).
Ripe fruit pigments like carotenoids and
fl avonoids have antioxidant properties that
neutralize the effects of photo-oxidation
and are important human nutrients.
Because mutations in the light signalling
pathway increase pigmentation of ripe
fruit, the light signalling pathway is a
potential target for efforts to engineer
increased fruit nutrition. Although
carotenoid content in edible parts has been
changed by altering the content of
biosynthetic enzymes (e.g. Golden Rice),
the results of such approaches did not
follow expectations due to mis-
understandings of the molecular mech-
anisms and/or undesirable side effects on
non-target metabolites of the modifi ed
pathway (Beyer et al. , 2002). Engineering
an existing signal transduction network
that regulates fl ux through the carotenoid
synthesis pathway in a biologically viable
way could represent an alternative to
enhance carotenoids in fruit.
Regulating expression of HY5 and COP1
involved in signal transduction using
transgenic approaches, it is possible to
modify fruit carotenoid content. MADS-
box genes are present in eukaryotes and are
linked to fl oral determination and develop-
ment in plants. MADS-box proteins form
heterodimers and higher-order multimers,
implying that MADS-box genes might play
a key role in ripening. Indeed, several
MADS-box genes expressed in ripening
tomato fruit could be good candidates for
functional analysis of fruit ripening.
Orthologous genes from agriculturally
important fruit species are being targeted
to enhance fruit quality and shelf-life.