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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
266 Postharvest Diseases of Fruits and Vegetables
reduction in ethylene production and a marked inhibition of the ripening
of the tomato fruit, were recorded.
Inhibition of fruit ripening by these approaches has not only had a
direct effect of prolonging the physiological life of the harvested fruit, but
also an indirect effect on decay development, which is strongly related to
the stage of ripening. The relationship between the stage of maturity and
the resistance of harvested fruit to postharvest diseases is probably
associated with the declining ability of the mature tissue to produce
defensive compounds and structural barriers. Furthermore, the pathogen
is capable of producing and activating pectolytic enzymes only during
fruit ripening or in the mature fruit, where it decomposes the pectic
compounds of the fruit cell walls, thus causing tissue degradation.
Studies at the genetic level were also dedicated to direct enhancement
of fruit disease resistance (Martin et al., 1993). This was achieved in a
plant-pathogen system in which a single resistance gene in the plant
responded specifically to a single avirulence gene in the pathogen, i.e., in
a 'gene-for-gene' system. Such interactions have been described for
various plant-pathogen pairs (Keen and Buzzel, 1991). Susceptibility to
disease results in plant-pathogen systems in which either of two genes -
the plant resistance gene or the pathogen avirulence gene - is absent
from the interacting organisms.
Martin et al. (1993) used the relationship between tomato and the
bacterium Pseudomonas syringae pv. tomato, the causative agent of
bacterial speck, to isolate a plant resistance gene by map cloning. Since
the avirulence gene from the bacterium was found to induce resistance
specificity in tomato cultivars containing the plant resistance gene, it
was concluded that the interaction between the tomato and the
bacterium involves a 'gene-for-gene' system, and when tomatoes
susceptible to Pseudomonas are transformed with the plant resistance
gene they become resistant to the pathogen.
The tomato plant offers many advantages for the cloning of a
resistance gene on the basis of their position on the genetic linkage map;
since the tomato has been the subject of more than 50 years of plant
breeding, many loci have been identified that confer resistance to various
fungi, bacteria and viruses (Martin et al., 1993).
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