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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 85
conditions and their metabolic activity is enhanced. These cells may
repair the damage sustained by the injured tissue. In response to injury,
the host tissue is capable of forming protective "barriers" composed of
tightly packed cells, but such barrier formation takes place only when
the injured tissue is still capable of cell division, or the cells surrounding
the wound can produce and deposit lignin and suberin in their walls
(Eckert, 1978). These compounds protect the host from pathogen
penetration or from the action of cell-wall degrading enzymes secreted by
the pathogen. As a result of wounding, the production of antimicrobial
polyphenolic compounds can also contribute to wound protection.
Phytoalexins are other toxic compounds that can be formed at the wound
area following inducement by initial infection.
Ray and Hammerschmidt (1998) found that inoculation of potato
tubers with Fusarium sambucinum, the fungal pathogen of potato dry
rot, resulted in an increase in phenolic acids suggesting that phenolic
acid biosynthesis was induced. Following such inducement, free phenolic
acids are removed as they are converted into lignin or are cross-linked to
cell walls. Lignin production is mediated by peroxidase, which is strongly
induced in the infected tuber in a number of isoforms. While a lignin
barrier may inhibit the advance of the fungus, microscopy has indicated
that lignin barriers are repeatedly breached or circumvented as dry rot
progresses. This may suggest that lignin is not in itself an effective
defense against F. sambucinum or, alternatively, that it cannot be
induced quickly enough to defend the tissue (Ray and Hammerschmidt,
1998). Furthermore, the phenolic acid and total peroxidase contents are
not higher in tubers of the resistant line than in more susceptible
genotypes, suggesting that neither is critical to tuber infection
resistance.
Suberization of cells at the wound area, a process that heals the
wound with a layer of phenolic and aliphatic compounds (Mohan and
Kolattukudy, 1990), is an important anti-infection defense mechanism of
the wounded tuber. When superficially wounded tubers are stored at
high relative humidity and moderate temperature, suberin is formed in
the walls and intercellular spaces of the living cells surrounding the
injury. Under optimal environmental conditions this takes place within
24 h after wounding (Fox et al., 1971). The suberized periderm which
forms after several days is composed of tightly packed cells, the walls of
which are impregnated with suberin and contain only a small amount of
pectin (Fox et al., 1971). On the other hand, storing tubers at too low
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