Agriculture Reference
In-Depth Information
fruit. Much of the knowledge on ethylene
perception and ethylene signalling derives
from research work on
Arabidopsis
. In
Arabidopsis
, ethylene is perceived by a
family of fi ve ethylene receptors (ETR1,
ETR2, ERS1, ERS2 and EIN4), which are
similar to bacterial two-component histi-
dine kinase receptors. Recent experiments
have linked ethylene phenotypes to the
unregulated activity of EIN2 (Hall and
Bleecker, 2003).
Research studies on ethylene and light
signal transduction pathways, performed
mainly in
Arabidopsis
, have advanced
ripening research in fl eshy fruit species
such as tomato. Tomato has emerged as a
model for an understanding of fl eshy fruit
development and ripening due to
important features such as the availability
of mutants, a rapid life cycle, routine
transformation, and numerous molecular
and genomics tools (
http://solgenomics.
net/).
Characterization of gene regulatory
networks during tomato fruit ripening have
clarifi ed understanding of molecular
mechanisms of the ripening process.
(Adams-Phillips
et al.
, 2004). Several
ethylene signal transduction components
homologous to those identifi ed in
Arabidopsis
have been isolated from
various plant species. Although the
sequences of genes involved in ethylene-
related pathways are conserved, the
regulation and the number of genes vary
among fl eshy fruit species. Six ethylene
receptors have been isolated in tomato, fi ve
of which bind ethylene.
Ripe fruits come in diverse forms,
colours, textures, aromas, fl avours and
nutrient compositions. Severe cell-wall
modifi cations occur during fruit ripening.
Among the different processes, those that
are noteworthy are the conversion of starch
to sugars, the modifi cation of pigment
biosynthesis/accumulation, and increased
synthesis of fl avour and aromatic volatiles.
Several ripening features can be
problematic, decreasing shelf-life and
needing highly expensive harvest and post-
harvest procedures. Particularly important
in this respect are the changes in fi rmness
and susceptibility to microbial and fungal
infection caused by tissue breakdown
associated with ripening.
Much progress over the last 15 years has
allowed us to gain insight into the
molecular regulation of specifi c ripening
processes, especially those involved in
cell-wall metabolism and ethylene bio-
synthesis (reviewed by Giovannoni, 2001).
The molecular understanding of ripening
allowed the development of novel bio-
technological approaches to improve
quanti-qualitative aspects of fruits.
Ripening impacts on important com-
ponents of the human diet such as fi bre
abundance and composition, lipid meta-
bolism and the concentrations of vitamins
and various antioxidants (Ronen
et al.
,
1999). The ability to understand and
manipulate, through breeding or bio-
technology, key control points of ripening
or to regulate the synthesis of carotenoids,
fl avonoids, vitamins and fl avour volatiles,
could improve the control of nutrition and
quality changes associated with ripening.
Currently unpopular genetic engineering
techniques might be viewed more
favourably by the public if they were used
to improve the quality and nutrition of
food.
18.2 Tomato as a Model Organism for
Fruit Ripening
Tomato has been a key plant model for
molecular fruit ripening studies over the
past two decades for several reasons.
Tomatoes are easily cultivated and have a
short life cycle. Unlike rice or the classic
molecular model organism
Arabidopsis
,
tomato has fl eshy fruit. Because tomato and
Arabidopsis
diverged from their common
ancestor early in dicot radiation, the
similarities and differences between the
two model organisms are particularly
informative. The tomato genome is
moderately sized (950 Mb) and was recently
sequenced through an international
initiative entitled the 'International
Solanaceae Genome Project'. Homozygous
inbred lines and other well-characterized
genetic and genomic resources are available.
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