Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 169
glycoproteins and in the side chains of xyloglucan, glucuronoarabinoxylan, and RG-II
(Carpita and Gibeaut, 1993). Debranching of RG-I side chains appear to be an important
component of changes that alter fruit firmness. During ripening, loss of polymeric Gal and
Ara from the cell wall takes place, the extent of which is dependent on species (Gross and
Sams, 1984; Redgwell et al., 1997b). The appearance or disappearance of the 1,4-β-Dgalactan
was correlated with wall firmness in pea cotyledons (McCartney et al., 2000) and
potato (Ulvskov et al., 2005).
Cell wall arabinan is necessary for maintaining cell wall flexibility, as its degradation
prevented the cell wall movements necessary for stomatal opening and closing (Jones et al.,
2003). Arabinan is also essential for normal intercellular adhesion, as a mutant in Nicotiana
lacking firmly bound arabinan had loosely attached cells (Iwai et al., 2001). Reduced
deposition of arabinan in the regions surrounding intercellular spaces was correlated with
weakened cell-to-cell contacts in Cnr mutant of tomato, which does not ripen properly
(Orfila et al., 2001). Retention of cell wall Ara correlated with the altered intercellular
adhesion and increased wall strength in mealy peaches (Brummell et al.,
2004b).
Depolymerization of matrix glycans from cell wall is closely correlated with fruit softening.
During ripening a progressive downshifts in the molecular weight of matrix glycans
have been observed in tomato (Tong and Gross, 1988; Maclachlan and Brady, 1994; Brummell
et al., 1999), pepper (Gross et al., 1986), melon (Rose et al., 1998), strawberry (Huber,
1984), avocado (Sakurai and Nevins, 1997), kiwifruit (Redgwell et al., 1991), and in many
other species. The depolymerization occurs in both the xyloglucan component and total
matrix glycans. Depolymerization of matrix glycan is thought to be a major contributor to
the reduced rigidity of cell wall and fruit softening. Due to its important structural role of
cross-linking cellulose in wall, depolymerization of xyloglucan may be one of the important
factors along with other polymer degradation. Loosening of the xyloglucan–cellulose
network may be also a part of cell wall swelling, which is correlated with pectin solubilization.
In general, depolymerization of matrix glycans begins during early ripening and continues
throughout, and these changes correlate with softening. Depolymerization may be
very slight (apple, strawberry, banana, and bell pepper); progressive, which begins slowly
during ripening but exhibits substantial depolymerization late in ripening (kiwifruit, tomato
avocado, and papaya); or abrupt, absent in early ripening and occurs rapidly during late
ripening (melon and melting flesh peach). The progressive loss of large arabinan and
galactan side chains of RG-I during ripening is likely to alter cell wall rigidity/flexibility
and intercellular attachment, increase cell wall porosity, and may affect pectin solubilization.
8.7 Enzymatic regulation of polyuronide depolymerization
During ripening, multiple enzymes promote disassembly of individual cell wall polysaccharides.
Most of the disassembly of the cell wall during ripening is due to the actions of a range
of polysaccharide-modifying enzymes secreted into the cell wall space from the symplast,
although nonenzymatic mechanisms mediated by free radicals may also be involved
(Fry et al., 2001, 2002; Dumville and Fry, 2003). The expression of many genes increases
during ripening, and the product of these genes may influence depolymerization or