Agriculture Reference
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increased during ripening, with a highly negative linear correlation between activities of
the two enzymes and firmness (Artes and Salmeron, 1996).
In rambutan the firmness of skin increased toward harvest with decrease in water-
soluble pectic substances, hemicellulose, and cellulose. Although PE activity in the pulp
did not change during fruit development, endo-PG increased toward harvest, suggesting
that endo-PG may relate to cell wall polyuronide degradation in the pulp (Kondo et al.,
2002).
An overall increase of cell wall material was observed during green bean pod de-
velopment, and major changes were detected in the pectic polymers. Initially, cell walls
contained large amounts of neutral, sugar-rich pectic polymers (rhamnogalacturonan), and
during elongation, more galactose-rich pectic polymers were deposited into the cell wall,
and the level of linear homogalacturonan steadily increased. During maturation of the pods,
galactose-rich pectic polymers were degraded and there was an increase in the amount of
ionically complexed pectins at senescence. The most abundant enzymes were PME, per-
oxidase,
-arabinosidase with PG present only in very small amounts
throughout pod development (Stolle-Smits et al., 1999). Ebbelaar et al. (1996) also ob-
served low endo-PG or exo-PG activities in early developmental stages, while PE activities
were measurable during all stages of pod and seed development. These results do not
favor a possible synergistic action of PE and PG. PE gene expression levels varied sig-
nificantly in pods from different cultivars suggesting its involvement in determining pod
morphology.
Pectate lyases are the other pectin-degrading enzymes that randomly cleave
β
-galactosidase, and
α
(1-4)
linkages between galacturonosyl residues, generating 4,5-unsaturated oligogalacturonates
by
β
-elimination (Willats et al., 2001b). In strawberries an antisense PL gene caused an
increase in fruit firmness and reduced the postharvest softening, without affecting weight,
color, and soluble solid content of the fruit, indicating that PLs play an important role
in fruit softening (Jimenez-Bermudez et al., 2002). In ripening strawberry, accumulation
of a PL gene product is absent in green fruit but subsequently appears at the white stage
and further during ripening (Medina-Escobar et al., 1997). Constitutive downregulation
of corresponding gene expression in transgenic strawberry reduced softening as measured
by the presence of higher numbers of red fruit within fruit populations showing enhanced
internal and external firmness (Jimenez-Bermudez et al., 2002). Firmness of transgenic fruit
was preferentially enhanced during the developmental transition from white to red fruit,
with red transgenic fruit populations being approximately twofold firmer than populations
of ripe nontransformed controls. Populations of ripe transgenic fruit also maintained greater
levels of overall firmness during several days of postharvest storage (Jimenez-Bermudez
et al., 2002).
The action of the endogenous enzyme from banana pulp tissue revealed a significant
increase in calcium-dependent pectate lyase activity during ripening. The enhanced levels
of enzyme activity corresponded with an increase in soluble polyuronides from banana pulp
(Marin-Rodriguez et al., 2003).
In some fruit, pectin solubilization can also be nonenzymatic. In ripening kiwifruit, it
is proposed that pectin solubilization may be due to nonenzymically regulated cell wall
swelling (Newman and Redgwell, 2002). It has also been proposed that ascorbate, copper
ion, and H 2 O 2 , naturally produced in cell walls, can solubilize fruit pectin via the production
of hydroxyl radicals (Dumville and Fry, 2003).
β
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