Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 57

fi rmness and storage life. Fruits dipped in
CaCl 2 include but are not limited to apples
(Saftner et al. , 1998), pears (Rosen and
Kader, 1989), tomatoes (Floros et al. , 1992),
blueberries (Camire et al. , 1994) and
strawberries (García et al. , 1996).
Up to this point, we have focused on
degradative processes in the cell wall, but
biosynthesis of cell-wall polysaccharides
continues throughout tomato fruit ripening
(Mitcham et al. , 1989; Greve and Labavitch,
1991). In one study, the researchers injected
radioactive sucrose into the pedicel of
tomato fruit and then collected pericarp
tissue at several stages of fruit development
(Mitcham et al. , 1989). Radioactivity
increased slightly in the pectin and
hemicellulose fractions extracted from the
fruit pericarp early in tomato fruit ripening
and then declined at the red-ripe stage. The
greatest increase in label per gram of
pericarp was in the pectin fraction. It is not
clear, however, whether the polymers
synthesized during ripening were different
from those synthesized earlier in develop-
ment and how the newly synthesized
polymers affected fruit softening.
Of possible importance to fruit fi rmness
and more importantly texture, which
includes juiciness, is that the cell wall is
not the same all around the cell. We have
already mentioned that the middle lamella
is different where intercellular air spaces
form, but the primary cell wall is also non-
uniform. Pitfi elds are one example.
Pitfi elds are regions of the cell wall where
plasmodesmata transverse the walls
between adjacent cells (Burch-Smith et al. ,
2011). The plasmodesmata create channels
between cells that play a role in cell-to-cell
communication (Burch-Smith et al. , 2011).
Immunoanalysis of the cell wall shows that
regions near the plasmodesmata have a
different carbohydrate composition from
other parts of the wall (Orfi la and
Knox, 2000). There was an absence of
1,4- E -galactan near the plasmodesmata and
the pectin material in this region was not
easily extracted with calcium chelators
(e.g. CDTA). It was concluded that the
pectin structure (porosity) around the
plasmodesmata

enzymes from reaching the plasmo-
desmata. Ultrastructural studies with apple
and pear have indicated that the micro-
domain around plasmodesmata is indeed
resistant to decomposition during ripening
(Ben-Arie and Kislev, 1979; Roy et al. ,
1997).

4.4 Conclusions

Ultrastructural (electron microscopy) stud -
ies indicate decomposition and spreading
of the middle lamella during ripening
of apple and pear (Ben-Arie and Kislev,
1979) and tomato (Crookes and Grierson,
1983). Disassembly of the middle lamella
is probably a common theme in fruit
ripening. The same ultrastructural studies
also showed a disintegration of the fi brillar
arrangement in the primary cell wall late
in ripening. The disruption of fi brillar
organization is even more pronounced in
ripening avocado fruit, which, when ripe,
has a MF texture (O'Donoghue et al. ,
1994). The disruption of fi brillar
organization can be accomplished simply
by untethering the cellulose microfi brils
with expansin (Cosgrove, 1999), de-
polymerization of the xyloglucan matrix
that cross-links the microfi brils (Miedes
and Lorences, 2009) or possibly by
modifi cation of peripheral cellulose poly-
mers with cellulase (O'Donoghue et al. ,
1994). However, the porosity of the cell
wall is determined largely by the pectin
matrix that the cellulose microfi brils and
hemicelluloses are embedded in, and
partial disassembly of the pectin matrix
may be necessary to give enzymes access
to the cellulose and hemicellulose poly-
saccharides (Carpita and Gibeaut, 1993).
Extraction of cell-wall fractions at different
stages of ripening supports extensive
depolymerization of pectin but does not
distinguish between the middle lamella
and the pectin matrix in the primary cell
wall (Brummell, 2006). Depolymerization
of the hemicellulose (xyloglucan-rich)
fraction does occur during ripening but is
not nearly as extensive as the frag-
mentation of pectins (Brummell, 2006). might exclude some