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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Attack Mechanisms of the Pathogen 55
typical of this fungus in wounded kernels. Furthermore, cutinase activity
was strongly inhibited by an inhibitor specific to fungal cutinase,
supporting the suggestion that cutinase may have a role in the
pathogenicity of A flavus.
In a recent study, the involvement of phenolic acids, which are found
in the peach fruit surface, in disease resistance to Monilinia fructicola,
the fungal cause of brown rot, has been related to their effect on cutinase
production by the fungus (Bostock et al., 1999). Chlorogenic and caffeic
acids, the major phenolic acids in the epidermis and subtending cell
layers of peach fruits, were found in higher concentrations in a peach
genotype with a high level of resistance; they declined as the fruit
matured, with a corresponding increase in disease susceptibility. These
phenolic compounds, however, did not show any suppressive effects on
fungal development. On the other hand, in cultures amended with either
of these phenolic acids, the cutinase activity of the fungus was markedly
reduced. It was, therefore, suggested that the relationship between the
high concentration of the phenolic acid in the immature peach fruit and
the resistance of this fruit to infection may be because they can interfere
with the capability of the pathogen to degrade the host polymers. (See
also the chapter on Host Protection and Defense Mechanisms -
Preformed Inhibitory Compounds.)
Pectolytic and Cellulolytic Enzymes
The pectic substances, which constitute an important component of the
primary cell wall of plants, and form the major component in the middle
lamella (Bateman and Basham, 1976), are responsible for the tight link
between the cells and the integrity of the plant tissue. These substances
are composed mainly of a high-molecular weight polymer comprising
linear chains of D-galacturonic acid (1,4-a-galacturonic acid) which
underwent various degrees of methylation, combined with side chains of
neutral sugars (Bateman and Basham, 1976). Non-methylated chains are
termed pectic acid; chains with about 75% or more of the units
methylated are termed pectin, while chains with smaller percentages of
the galacturonic acid units methylated are referred to as pectinic acid.
The development of disease within the harvested fruit or vegetable is
dependent, to a great extent, on the ability of the pathogen to secrete
pectolytic enzymes, which are capable of decomposing the non-soluble
pectic compounds and so causing cell separation and tissue
disintegration. This process leads to enhanced permeability of the
plasma membranes of attacked cells and to cell death, and facilitates the
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