In general, disease resistance in plants depends on the ability to tolerate a pathogenic agent or to restrict its development. Tolerance as one type of disease resistance was discussed in Chapter 1.C, and as a factor in insensitivity to selective pathotoxins in Chapter 2.B.III.3. In some cases, preformed, physical or chemical barriers prevent potential pathogens from becoming established (Chapter 2.A.II.3). It is also possible that papillae and cell-wall modifications formed in response to infection retard or prevent some pathogens from penetrating or becoming established within the plant (Chapter 3.A.II.1). In a few cases, preformed toxic materials or deficiency of a nutrient required by the pathogen appear to confer resistance. Very often, however, penetration and initial development of the pathogen take place as readily in resistant tissues as in those that are susceptible. In such cases, disease reactions apparently depend on the type of response induced by the pathogen.
KeywordsHypersensitive Response Shikimic Acid Fire Blight Disease Reaction Carrot Root
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