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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 93
Peroxidases are another group of PR proteins whose activity has been
correlated with plant resistance against pathogens. Plant peroxidases,
which are glycoproteins that catalyze the oxidation by peroxide of many
organic and inorganic substrates, have been implicated in a wide range of
physiological processes, such as ethylene biosynthesis, auxin metabolism,
respiration, lignin formation, suberization, growth and senescence.
Findings of correlations between deposition of cell wall strengthening
materials, such as lignin, suberin and extensin, and peroxidase activities
are consistent with a role for this enzyme in defense through
wall-strengthening processes (Chitoor et al., 1999). The importance of
peroxidase lies in the fact that the host cell wall constitutes one of the first
lines of defense against pathogens, and peroxidase is a key enzyme in the
wall-building processes. Such processes include the accumulation of lignin
and phenolic compounds, which has been correlated with enhanced
resistance in various host-pathogen interactions, and suberization, which
leads to enhanced resistance by healing wounds with a layer of phenolic
and aliphatic compounds (Mohan and Kolattukudy, 1990). However, the
resistance against pathogens may also be related to the highly reactive
oxygen species such as H2O2 or oxygenase, which are likely to be toxic to
pathogens and which are formed by peroxidase activity during the
deposition of cell wall compounds (Goodman and Novacky, 1994).
In a recent study Ray and Hammerschmidt (1998) found that the
activity of peroxidase in potato tubers increased greatly following
infection with Fusarium sambucinum, the cause of potato dry rot. Such
an infection induces several isoforms of peroxidase whose role has not
been identified. Lignin levels also increased but the lignified zones could
be breached, allowing the infection to develop further into the tuber. The
increased peroxidase levels in infected tubers were expected to increase
their potential to synthesize lignin and so enhance their resistance to
infection. However, infection of tubers of transgenic potatoes following
introduction of a foreign peroxidase gene from cucumbers, which was
associated with an induced resistance response, had no effect on disease
caused by several potato pathogens (Ray et al., 1998). These results
indicated that the incorporation of a putative defense-associated
peroxidase and the formation of transgenic potato plants does not
necessarily result in the enhancement of resistance. It has been
suggested, however, that understanding the function of peroxidase and
determining the availability of the substrates needed for the enzyme
activity are important for using transgenic studies to evaluate the role of
peroxidase in defense responses (Ray et al., 1998, Chitoor et al., 1999).
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