494 Part V: Fruits, Vegetables, and Cereals
the binding of ethylene to its receptor and with initi-
ation of ethylene action (Davies et al. 1988). Thus,
there is a link between ethylene, PG synthesis, and
fruit softening.
Genetic engineering of tomato with the objective
of regulating PG activity has yielded complex re-
sults. In the rin mutant of tomato, which lacks PG
and does not soften, introduction of a PG gene re-
sulted in the synthesis of an active enzyme; how-
ever, this did not cause fruit softening (Giovannoni
et al. 1989). As a corollary to this, introduction of
the PG gene in the antisense orientation resulted in
near total inhibition of PG activity (Smith et al.
1988). In both these cases, there was very little
effect on fruit softening, suggesting that factors oth-
er than pectin depolymerization may play an inte-
gral role in fruit softening. Further studies using
tomato cultivars such as UC82B (Kramer et al.
1992) showed that antisense inhibition of ethylene
biosynthesis or PG did indeed result in lowered PG
activity, improved integrity of cell wall, and in-
creased fruit firmness during fruit ripening. As well,
increased activity of pectin methylesterase, which
removes the methyl groups from esterified galactur-
onic acid moieties, may contribute to the fruit soft-
ening process.
The activities of pectin-degrading enzymes have
been related to the incidence of physiological disor-
ders such as “mealiness” or “wooliness” in mature,
unripe peaches that are stored at a low temperature.
The fruits with such a disorder show a lack of juice
and a dry texture. Deesterification of pectin by the
activity of pectin methyl esterase is thought to be
responsible for the development of this disorder.
Pectin methyl esterase isozymes with relative mo-
lecular masses in the range of 32 kDa have been ob-
served in peaches, and their activity increases after 2
weeks of low temperature storage. Polygalacturon-
ase activity increases as the fruit ripens. The ripen-
ing fruits, which possess both polygalacturonase
and pectin methyl esterase do not develop mealy
symptoms when stored at low temperature, implicat-
ing the potential role of pectin degradation in the
development of mealiness in peaches.
There are two forms of polygalacturonase in
peaches, the exo- and endopolygalacturonases. The
endopolygalacturonases (endo-PG) are the predomi-
nant forms in the freestone type of peaches, whereas
the exopolygalacturonases (exo-PG) are observed in
the mesocarp of both freestone and clingstone vari-
eties of peach. As the name implies, exopolygalac-
turonases remove galacturonic acid, moieties of pec-
tin from the terminal reducing end of the chain,
whereas the endopolygalacturonases can cleave the
pectin chain at random within the chain. The activi-
ties of these enzymes increase during the ripening
and softening of the fruit. Two exo-PG isozymes
having a relative molecular mass of near 66 kDa
have been identified in peach. The exo enzymes are
activated by calcium. Peach endo-PG is observed to
be similar to the tomato endo-PG. The peach endo-
PG is inhibited by calcium. Freestone peaches pos-
sess enhanced activities of both exo-PG and endo-
PG, leading to a high degree of fruit softening.
However, the clingstone varieties with low levels of
endo-PG activity do not soften as do the freestone
varieties. In general, fruits such as peaches, toma-
toes, strawberries, and pears that soften extensively
possess high levels of endo-PG activity. Apple fruits
that remain firm lack endo-PG activity.Starch DegradationStarch is the major storage form of carbohydrates.
During ripening, starch is catabolized into glucose
and fructose, which enter the metabolic pool, where
they are used as respiratory substrates or further
converted to other metabolites (Fig. 21.2). In fruits
such as banana, the breakdown of starch into simple
sugars is associated with fruit softening. There
are several enzymes involved in the catabolism of
starch. -amylase hydrolyzes amylose molecules by
cleaving the -1,4 linkages between sugars to pro-
vide smaller chains of amylose, termed dextrins. -
amylase, another enzyme that acts upon the glucan
chain, releases maltose, a diglucoside. The dextrins
and maltose can be further catabolized to simple
glucose units by the action of glucosidases. Another
enzyme, starch phosphorylase, mediates the phos-
phorylytic cleavage of terminal glucose units at the
nonreducing end of the starch molecule using inor-
ganic phosphate, thus releasing glucose-1-phosphate.
The amylopectin molecule is degraded in a similar
manner to amylose, but its degradation also involves
the action of debranching enzymes that cleave the -
1,6 linkages in amylopectin and release linear units
of the glucan chain.
In general, starch is confined to the plastid com-
partments of fruit cells, where it exists as granules
made up of both amylose and amylopectin molecules.