Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 171
with fruit softening (DellaPenna et al., 1986; Biggs and Handa, 1989; Brummell and Harpster,
2001). However, molecular genetic studies have shown that even though the PG is
responsible for polyuronide depolymerization and solubilization, it makes only a partial
contribution to fruit softening.
Structural alterations in the cell walls of ripening fruit take place within its semisoluble
pectin matrix and within the pectin-rich middle lamellae that cement walls of adjacent cells.
Pectins in fruits represent over half of the cell wall polysaccharides present (Brummell
and Harpster, 2001). Ripening-associated changes in pectin porosity and ion-facilitated
gel formation regulate cell wall hydration status and the mobility of resident enzymes.
Biologically active cell wall fragments are generated during plant processes, including
defense against pathogens (Vorwerk et al., 2004).
Homogalacturonans are initially synthesized and secreted into plant cell wall with a
high degree of methylesterification (Carpita and McCann, 2000), which declines during
development due to the action of apoplastic pectin methylesterase (PME) (Willats et al.,
2001a). A reduction in degree of pectin methylesterification is a common feature of most
aspects of plant development, which is most noticeable during fruit ripening. Large decrease
in the degree of pectin methylesterification has been reported during fruit development in
tomato (Koch and Nevins, 1989; Tieman et al., 1992), kiwifruit (Redgwell et al., 1990),
papaya (Paull et al., 1999), avocado (Wakabayashi et al., 2000), peach (Brummell et al.,
2004a), and grapes (Barnavon et al., 2001). In general, alkaline PMEs from plants are
thought to deesterify contiguous Gal residues in homogalacturonans by the so-called single
chain mechanism (Denes et al., 2000), in which a PME deesterifies contiguous Gal residues
in a pectin chain in a linear fashion to produce blocks of pectin with free carboxyl groups.
PMEs can deesterify more limited runs of Gals using a multiple attack mechanism also
(Grasdalen et al., 1996; Denes et al., 2000). PME action pattern affects block length of
deesterified HGA and its propensity for calcium-cross-linked gel formation.
PMEs may play important roles in determining the extent to which demethylated polygalacturonans
are accessible to degradation by PGs, releasing galacturonic acid (exo-PG) or
oligogalacturonate (endo-PG), and the availability of homogalacturonan carboxylic group
for calcium ion binding, resulting in supramolecular assemblies and gels. The formation of
these calcium-mediated pectin gels significantly affects the mechanical properties of cell
wall and adds rigidity to the wall (Thakur et al., 1997; Brummell and Harpster, 2001; Rose
et al., 2003). Although PMEs play a little role in fruit softening during ripening, their effect
on tissue integrity is substantial (Brummell and Harpster, 2001). Low PME expression
in Cnr tomato mutants is speculated to be responsible for maintaining a strong cell wall,
highlighting its role in maintaining fruit cell wall integrity (Eriksson et al., 2004).
Peach genotype that show limited flesh softening during ripening were characterized by
lower losses of neutral sugars, especially those of arabinose and galactose, higher ethylene
production during ripening, higher levels of uronic acids, and increased capacity for calcium
binding in the water-insoluble pectin fraction compared with fruits of the extensive fleshsoftening
genotypes during ripening. The limited softening character can be attributed to the
decreased activity of PME combined with higher levels of calcium in the water-insoluble
pectin fraction and reduced solubility of cell wall pectin (Manganaris et al., 2006). During
ripening of nectarine, there was a temporal coincidence among higher rates of ethylene
production, higher PME and PG activities, and lower firmness. PME and PG activities