School of Engineering and Science - Jacobs University
School of Engineering and Science - Jacobs University
School of Engineering and Science - Jacobs University
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Introduction<br />
the following necrosis <strong>of</strong> the adjacent tissue cuts the pathogen <strong>of</strong>f from its<br />
nutrients. It thus is unable to spread <strong>and</strong> multiply <strong>and</strong> will finally die.<br />
(Hutcheson, 2001; Klement, 1963). The localized response is followed by a<br />
systemic defense reaction, which is mediated by the plant messenger<br />
molecules salicylic acid (Baker et al., 1997). This reaction is referred to as<br />
systemic acquired resistance (SAR; (Ross, 1962; van Loon et al., 1998a).<br />
A pathogen that is recognized by the plant is called “incompatible” to the<br />
respective host plant. The plant is non-susceptible. In case <strong>of</strong> a so-called<br />
“compatible” pathogen host interaction, the pathogen suppresses or modifies<br />
plant defense in such a way, that it is not attacked (Staskawicz et al., 2001).<br />
It proliferates causing visible disease symptoms as it nourishes on plant<br />
material. New inoculum is spread from the infested tissue <strong>and</strong> the disease cycle<br />
continues (Fig.2).<br />
Fig. 2. Life cycle <strong>of</strong> plant pathogens.<br />
The simplified life cycle <strong>of</strong> plant pathogenic<br />
microorganisms divides into two parts.<br />
During the epiphytic phase (left half <strong>of</strong> the<br />
cycle) the pathogen is distributed to new<br />
locations were it colonizes plant surfaces.<br />
Under favourable environmental conditions<br />
<strong>and</strong> in the presence <strong>of</strong> suitable entry sites,<br />
the pathogen invades the plant tissue. In<br />
an incompatible system the pathogen is<br />
recognized by the host <strong>and</strong> plant defence<br />
is activated. During hypersensitive<br />
response (HR) the pathogen is killed <strong>and</strong><br />
the respective plant tissue becomes<br />
necrotic. The surviving plant enters a state<br />
<strong>of</strong> enhanced disease resistance (SAR).<br />
In a compatible system the pathogen<br />
remains undetected or is able to<br />
manipulate plant defence reactions. After<br />
achieving a threshold density it enters into<br />
the pathogenic phase (right half <strong>of</strong> the<br />
cycle). By expression <strong>of</strong> virulence factors it<br />
attacks surrounding tissue <strong>and</strong> further<br />
proliferates on the nutrients gained from<br />
the plant. The pathogen is distributed from<br />
the infected tissue <strong>and</strong> again enters into<br />
the epiphytic phase.<br />
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