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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Factors Affecting disease Development 53 6. The hypersensitive response, which is thought to function by isolating and kiUing microorganisms in a small region of the plant tissue by localized cell death. 7. The induction of host resistance by the formation of pathogenesis-related proteins, such as chitinase and P-1,3 glucanase, in pathological situations or in response to other stresses. 8. The production of active oxygen in response to pathogens and elicitors; this oxygen has an antimicrobial effect and plays a role in various defense mechanisms. Attack mechanisms of the pathogen and defense mechanisms of the host will be discussed separately in the following chapters. http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE CHAPTER 5 ATTACK MECHANISMS OF THE PATHOGEN A. ENZYMATIC ACTIVITY Many plant pathogens produce extracellular products that may influence their growth, be determinant factors in their pathogenic capability, or contribute to their pathogenicity or virulence. Such products may include enzymes capable of hydrolyzing the cuticle layer of the epidermic plant cells (Kolattukudy, 1985), cell wall degrading enzymes, toxins, hormones, siderophores, DNA and signaling molecules (Salmond, 1994), or factors capable of degrading host chemical defense agents (Staples and Mayer, 1995). We will concentrate on the secretion of extracellular enzymes and toxins, the two major attack means with which the pathogen may be equipped, and the ability of the pathogen to detoxify host defense compounds. Cutinase The enzyme cutinase, which hydrolyzes the primary alcohol ester linkages of the polymer, cutin (Kolattukudy, 1985), has been found in many plant pathogenic fungi, including Botrytis cinerea (Shishiyama et al., 1970). However, the importance of this enzyme for penetration of the cuticle seems to be a matter of debate (Stahl and Schafer, 1992). Many lines of evidence suggest that cuticular penetration of Colletotrichum gloeosporioides into the host is associated with the induction of extracellular cutinase production, and that insertion of the cutinase gene into this pathogen facilitates infection of an intact host (Dickman et al., 1989). Following the finding that cutinase secretion was also directed towards the region that penetrates the host, the infection peg of an appressorium produced by Colletotrichum spores (Podilia et al., 1995), it was suggested that the inhibition of this enzyme by active inhibitors could result in delay in the penetration of germinating appressoria into the host (Prusky, 1996). Secretion of extracellular cutinase has been reported for Aspergillus flavus (Guo et al., 1996) when grown on purified cutin as the sole carbon source. Pretreatment of corn kernels with A flavus culture filtrate or with bacterial cutinase promoted increased levels of aflatoxin production http://arab2000.forumpro.fr
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