Genetic Diversity of Tropical Fruit - Fruit Ripening: Physiology, Signalling and Genomics - page 229

Citrus fruits are designated as non-
climacteric, and the fruit type is termed a
hesperidium, basically a leathery rinded
berry. The endocarp is the edible portion,
divided into 10-14 segments separated by
thin septa, each containing up to eight
seeds but usually only one. Placentation is
axile. The central axis may be open as in
tangerines. Each segment is composed of
juice vesicles ('pulp'), with long stalks
attached to the outer wall, containing juice.
The mesocarp is the white tissue usually
adherent to the outer surface of the
endocarp, except for tangerines; it is also
called the albedo. The exocarp, or fl avedo,
is the thin, pigmented outer portion of the
rind, with numerous oil glands.
In citrus fruits, vast diversity in each
group, namely mandarin, sweet orange,
lime and lemon, and pumellos, exists, and
in each group various genotypes with early
to late ripening type, diverse shelf-life,
maturity and diverse fruit characters are
available.

by analysis of ethylene-inducible and
ripening-related gene expression (Lincoln
et al. , 1987; Maunder et al. , 1987). Several
genetic studies have been carried out to
study the molecular mechanisms of fruit
ripening in, for example, tomato, melon,
apple, pear, grape, strawberry, mango and
papaya in different laboratories (Giovan-
noni, 2007; Seymour et al. , 2007; Bouzayen
et al. , 2010; Li et al. , 2010). Among these
crops, tomato and melon have been com-
monly used as model systems for studying
biosynthetic pathways and molecular
mechanisms underlying fruit ripening.
Numerous fruit development and
ripening-related genes have been isolated
and characterized using differential gene
expression patterns and biochemical
functions (Gray et al. , 1992; Bouzayen et
al. , 2010; Li et al. , 2010; Handa et al. ,
2011), including the regulatory functions of
genes involved in ethylene biosynthesis in
climacteric (tomato, melon, apple) and
non-climacteric (strawberry, melon) fruits
(Giovannoni, 2007; Seymour et al. , 2007;
Bouzayen et al. , 2010). Two genes, ripening
inhibitor ( rin ) and Colourless non-ripening
( Cnr ) have been identifi ed in tomato
( Lycopersicon esculentum ) mutants, en-
coding transcription factors, which play
important roles in the regulation of the
ethylene biosynthesis pathway (Vrebalov et
al. , 2002; Manning et al. , 2006; Giovan-
noni, 2007). Similarly, the Gr mutant
( Green-Ripe ) contains a mutation in a gene
encoding a conserved protein that disrupts
ethylene signalling in tomato (Barry and
Giovannoni, 2006). Strawberry (a non-
climacteric fruit) also has a fruit-specifi c
LeMADS-RIN orthologue, suggesting the
presence of common ethylene-independent
regulatory pathway involving MADS-box
genes in both climacteric and non-
climacteric types of fruit ripening
(Vrebalov et al. , 2002; Giovannoni, 2007).

13.4 Genetic Studies on Fruit Ripening

Fruit ripening is a complex process of
molecular, physiological and biochemical
mechanisms leading to changes in colour,
sugar, acidity, aroma volatiles, phyto-
chemicals and texture (Handa et al. , 2011).
Thus, fruit ripening has been considered a
step of a programmed cell death (Mattoo
and Handa, 2004; Bouzayen et al. , 2010).
These changes during ripening are driven
by the coordinated expression of several
ripening-related genes. Ripening in
climacteric fruits is induced by the action
of ethylene and results in the activation of
several cell-wall hydrolases. The action of
these hydrolases on cell walls results in
wall disassembly leading to softening of
fruit. Several studies have been carried out
to investigate fruit-ripening mechanisms in
tomato and have inferred that ethylene
plays a crucial role in ripening in
climacteric fruits (Yang, 1985; Tucker and
Brady, 1987). The main role of ethylene in
climacteric fruit ripening was fi rst
observed in tomato at the molecular level

13.4.1 Molecular mechanism of fruit
softening

Genetic and environmental factors simul-
taneously affect texture changes in