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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
CHAPTER 6
HOST PROTECTION AND DEFENSE MECHANISMS
A. THE CUTICLE AS A BARRIER AGAINST INVASION
The cuticle provides a barrier to pathogen penetration. Wounding and
treatments that disrupt or dissolve the cuticle result in more rapid
infection by various pathogens (Elad and Evensen, 1995). The cuticle
may, however, function not only as a physical barrier but also as a
chemical barrier, since it may contain substances antagonistic to fungi
(Martin, 1964). The cuticle thickness has been correlated with the
resistance of tomato fruit to Botrytis cinerea (Rijkenberg et al., 1980), or
of peach cultivars to Monilinia fructicola (Adaskaveg et al., 1989, 1991).
Biggs and Northover (1989) showed that the cuticle and cell-wall
thicknesses were correlated with longer incubation periods of the brown
rot in cherry fruits, accompanied by a lower incidence of infection. Elad
and Evenson (1995) listed several ways by which a thicker cuticle might
enhance host resistance: (a) by being more resistant to cracking and thus
to penetration of wound pathogens (Coley-Smith et al., 1980); (b) by
providing mechanical resistance to penetration (Howard et al., 1991);
(c) by presenting a greater quantity of substrate to be degraded by fungal
enzymes, and preventing diffusion of cellular solutions, thus limiting the
access of water and nutrients to the microenvironment where they are
required for spore germination and the infection process (Martin, 1964).
Studies with various genotypes of peaches clearly showed that
genotypes resistant to brown rot caused by Monilinia fructicola had a
thicker cuticle and denser epidermis than susceptible genotypes. Their
resistance was correlated with both a delay in fungal penetration and a
longer incubation period for infection initiation, and resulted in the
extension of the quiescent period of the pathogen (Adaskaveg et al., 1989,
1991).
B. INHIBITORS OF CELL WALL-DEGRADING ENZYMES
Pathogen development within the host is correlated in many cases
with the activity of cell wall-degrading enzymes, which are responsible
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