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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Novel Approaches for Enhancing Host Resistance 259
persistence of defense enzymes in the tissue following elicitation by
chitosan may also contribute to the delay in the reactivation of quiescent
infections, which naturally takes place when tissue resistance declines.
Because of these features, chitosan has been considered a promising
means for enhancing disease resistance and has been offered as a possible
alternative to synthetic fungicides (El Ghaouth, 1994).
Salicylic Acid and Defense Reactions
Plant infection by fungi, bacteria and viruses may lead to a
hypersensitive response which prevents pathogen spread, by the
development of necrotic lesions at the site of infection. In addition, the
localized infection may induce enhanced resistance against further
infection by a variety of pathogens. This phenomenon is known as
systemic acquired resistance (Raskin, 1992; Buchel and Linthorst, 1999).
Along with the development of a hypersensitive reaction and systemic
acquired resistance to subsequent pathogen attack, the systemic
synthesis of low-molecular-weight pathogenesis-related (PR) proteins has
been recorded (Carr and Klessing, 1989). Among the various PR proteins
discovered, we find the enzymes, P-l,3-glucanases and chitinases, which
are associated with defense responses.
Although PR proteins are induced in pathological situations, they can
also be induced by the application of certain chemicals that partly mimic
the effects of pathogen infection, as well as by wounding or other stresses
(Buchel and Linthorst, 1999). Salicylic acid is a natural compound capable
of inducing such proteins even in the absence of pathogenic organisms.
Salicylic acid, which belongs to the diverse group of plant phenolics,
has ubiquitous distribution in plants. Many plant scientists have used
salicylic acid and aspirin (acetylsalicylic acid) interchangeably in their
experiments. Aspirin undergoes spontaneous hydrolysis to salicylic acid,
although it has not been identified as a natural plant product.
Increasing evidence suggests that endogenous salicylic acid plays an
important role in the activation in plant tissues, of defense responses
against pathogen attack (Kessman et al., 1994; Buchel and Linthorst,
1999). It is no wonder that recent development in the area of new
antifungal compounds has focused on this compound and its functional
analogs. Exogenic application of salicylic acid or aspirin has been found
to induce PR proteins in leaves of some crops (Renault et al., 1996); no
information, however, is available on the ability of such treatments to
induce PR proteins in fruits, where they might be induced in reaction to
wounding or other stresses. On the other hand, studies with pear cell
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