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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 91
species has not yet been clarified, but they are considered to play a role
in various defense mechanisms, including lignin production, lipid
peroxidation, phytoalexin production and hypersensitive responses.
However, active oxygen can be difficult to monitor in plant cells because
many of the "active oxygen species" are short lived and are subject to
cellular antioxidant mechanisms such as superoxide dismutases,
peroxidases, catalase and other factors (Baker and Orlandi, 1995).
A first report on the production of active oxygen in potato tubers
undergoing a hypersensitive response was given by Doke (1983), who
demonstrated that O2 production occurred in potato tissues upon
inoculation with an incompatible race of Phytophthora infestans (i.e., a
race causing a hypersensitive response), but not after inoculations with a
compatible race (i.e., a disease-causing race). Following this early report
the connection between active oxygen species and phytoalexin synthesis
has been found in several host-pathogen interactions (Devlin and
Gustine, 1992).
In a recent study Beno-Moualem and Prusky (2000) found that the
level of reactive oxygen species in freshly harvested unripe avocado fruit,
which is resistant to infection, was higher than that in the susceptible
ripe fruit. Moreover, inoculation of resistant fruits with Colletotrichum
gloeosporioides further increased their reactive oxygen production,
whereas inoculation of susceptible fruits had no such effect. It was also
indicated that reactive oxygen production could be induced by avocado
cell cultures treated with cell-wall elicitor of C. gloeosporioides.
When isolated avocado pericarp tissue was treated with H2O2 (1 mM)
the reactive oxygen production was enhanced and, in parallel, a rapid
increase in the levels of epicatechin was detected. Epicatechin appears to
play a key role in fruit susceptibility during ripening, by indirectly
controlling the levels of the antifungal compounds naturally present in
the peel of unripe fruits (Prusky et al., 1982); phenylalanine ammonia
lyase (PAL), one of the enzymes involved in epicatechin synthesis, also
increased, in parallel with the enhanced production of reactive oxygen
species. On the other hand, pre-incubation of avocado tissue in the
presence of protein kinase inhibitors inhibited the release of H2O2 from
avocado cell cultures and PAL activity was no longer induced.
Beno-Moualem and Prusky (2000) suggested that the inducement of
reactive oxygen species by fungal infection of unripe fruits may modulate
fruit resistance and lead to the inhibition of fungal development and the
continuation of the quiescent stage of the pathogen.
http://arab2000.forumpro.fr