Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 175
loss of avocado mesocarp firmness during fruit ripening may also be linked to the onset of
cellulase activity (O’Donoghue et al., 1994), but no relationship between cellulase activity
and softening of banana fruit was observed (Xue et al., 1995).
Pectin solubilization and release of side chain-derived galactose have been shown to be
temporally coincident during early ripening in avocados and melons (Sakurai and Nevins,
1997; Rose et al., 1998). During ripening of multiple fruits, polyuronides are released from
loose associations, whereas most Gal lost is derived from tightly held cell wall components
enriched in rhamnogalacturonans. Therefore, in many cases, cell wall sources of solubilized
pectins during ripening may not represent polymers targeted by β-galactosidase (β-Gal)
(Redgwell et al., 1997b).
With advancement in ripening of papaya fruit, increased structural solubilization
and concomitant depolymerization of pectin are observed (Manrique and Lajolo, 2004).
A decline in the levels of galactose, galacturonic acid, and nonglucose monosaccharides
indicated the association between polysaccharides from matrix and microfibrillar
phases.
The β-D-galactoside galactohydrolase enzymes were capable of differentially hydrolyzing
the cell wall of papaya as evidenced by increased pectin solubility, pectin depolymerization,
and degradation of the alkali-soluble hemicelluloses. Hemicellulose seemed
to be hydrolyzed more extensively than the pectins. The ability of the β-galactanases to
markedly hydrolyze pectin and hemicellulose suggests that galactans provide a structural
cross-linkage between the cell wall components (Lazan et al., 2004).
α-Galactosidase is one of the exoglycosidases capable of hydrolyzing α-1,6-linked
α-galactoside residues. α-Galactosidases remove galactosyl moieties from stored galactomannan
polysaccharides in germinating seeds, and can be used to change their rheological
properties (Gao and Schaffer, 1999). Although α-galactosidase activity was observed to
increase during ripening in tomatoes (Jagadeesh et al., 2004b), ethylene-responsive Cecona
apricots and ethylene-resistant San Castrese apricot (Botondi et al., 2003), the exact role of
α-galactosidase in these fruits is yet to be established.
α-Galactosidase activity increased concomitantly with firmness loss in papaya, and this
increase was largely ascribed to α-gal 2 (Soh et al., 2006). The protein level of α-gal 2 was
low in developing fruits and generally increased with ripening. α-Galactosidase 2 markedly
increased pectin solubility and depolymerization, while the polymers were still structurally
attached to the cell walls. The close correlation between texture changes, α-gal 2 activity,
and protein levels as well as capability to modify intact cell walls suggest that the enzyme
might contribute to papaya fruit softening during ripening.
In cellulose and the hemicellulose xyloglucan interactions, which typically comprise
about two thirds of the dry wall mass, xyloglucan binds noncovalently to cellulose, coating
and cross-linking adjacent cellulose microfibrils (McCann et al., 1992). The resulting extensive
xyloglucan-cellulose network is thought to act as the major tension-bearing structure in
the primary wall. Xyloglucan-metabolizing enzymes therefore represent a potentially important
mechanism for controlling wall strength and extensibility. Cleavage of load-bearing
xyloglucan cross-links by hydrolytic enzymes might be a means of achieving rapid wall
loosening. Enzymes, capable of splitting and reconnecting xyloglucan molecules in rapidly
growing plant tissues, were named as xyloglucan endotransglycosylase (XET) (Smith and
Fry, 1991), while Nishitani and Tominaga (1992) described them as endoxyloglucan transferase
(EXT, later redesignated as EXGT).