Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 67
for cell death and the liberation of nutrients, which then become
available to the pathogen. Liberation of nutrients results in a stimulation
of pathogen growth and accelerated disease development. This may
explain the great importance attributed to compounds that are capable of
suppressing or preventing enzymatic activity.
Trials with various pathogens have emphasized the point that sugars
in the culture medium, which serve as available nutrients for the
pathogen and stimulate its growth, may inhibit pectolytic and cellulolytic
enzyme production and activity (Spalding et al., 1973; Biehn and
Dimond, 1973; Bahkali, 1995). It was found that the presence of glucose
in the culture medium, either as a sole source of carbon or in addition to
malic or citric acid, inhibited pectin lyase activity of Penicillium
expansum, while resulting in enhanced fungal growth (Spalding and
Abdul-Baki, 1973)
Polyphenols and tannins have been described by Byrde et al. (1973) as
inhibitors of polygalacturonase (PG) activity of Sclerotinia fructicola
(Monilinia fructicola) in apples and other pathogen/host combinations,
although they have no effect on the pathogen itself. Decay development
in many fruits and vegetables may be the result of the delicate balance
between active production of enzymes and their inhibition by
polyphenols, mainly in their oxidized condition, as a result of
polyphenol-oxidase activity (Dennis, 1987).
Another class of inhibitors of cell wall-degrading enzymes comprises
the PG-inhibitory proteins present in both infected and uninfected plant
tissue (Albersheim and Anderson, 1971; Abu-Goukh et al., 1983). A
PG-inhibiting protein from a given plant source may act on pectic
enzymes produced by different fungal species, both pathogenic and
non-pathogenic, or on various PG isozymes from the same fungus.
Research carried out with pepper fruits has shown that cell wall proteins
of the host inhibited pectolytic enzyme production by Glomerella
cinigulata, whereas the pectolytic activity of Botrytis cinerea was much
less affected by these proteins (Brown, A.E. and Adikaram, 1983). The
fact that B, cinerea can rot an immature pepper fruit whereas G.
cingulata can attack only the ripened fruit, suggested that protein
inhibitors might play a role in the quiescent infections of pepper fruits by
Glomerella,
Purified pear inhibitory proteins were found to inhibit various fungal
PGs, including that of B, cinerea, but did not affect endogenous PG
activity in pear fruits (Abu-Goukh and Labavitch, 1983). In a later study,
Stotz et al. (1993) reported on the molecular characterization of the
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