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Write an account of how plants defend themselves against attack by pathogens and parasites.

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Clare Johnson

Queens College

17th January 2004

Write an account of how plants defend themselves against attack by pathogens and parasites

Parasitic organisms such as bacteria, viruses, fungi, nematodes and insects have evolved ways to exploit the food, shelter and in the case of viruses, the replication machinery of plants as well as animals.  Animals have a circulating adaptive immune system to protect them from pathogens but plants, which have no such system have evolved other forms of anti-microbial defence.  In general, plants defend themselves against pathogens by a combination of two forms: structural characteristics that act as physical barriers and prevent the pathogen from entering the plant, and biochemical reactions which face the pathogen if it manages to breach the initial basal defenses and succeeds in penetrating the plant.  The structural characteristics are passive and non-specific where as the biochemical reactions are more host specific with the cells and tissues of the plant producing substances which are either toxic to the pathogen or create conditions that inhibit the growth of the pathogen.  Plant resistance can be described as the inability of a pathogen to grow or multiply and therefore spread in the plant it infects.

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  • Cork layer formation which prevents the pathogen or its toxins from spreading beyond the initial lesion and also prevent the pathogen from receiving any of the plant’s nutrients.  E.g. Cork cells in sweet potato reduce Rhizopus soft rot.
  • Rapid wound healing
  • Abscission layer formation which is when the plant excises a portion of itself in order to benefit the rest
  • Formation of tyloses which are overgrowths of the protoplasts of adjacent living parenchyma cells that extend into the xylem vessels.  They are formed in response to xylem invading pathogens.
  • Gum deposition in areas surrounding the most concentrated area of infection so that the pathogen becomes fully enclosed, isolated and eventually dies.
  • Lignification e.g. in root cells of infected sunflowers
  • Suberisation

Cellular defense structures are characterised by morphological changes occurring in the cell wall.

  • Callose thickening of the cell wall
  • Necrotic defense reaction achieved by the hypersensitive response.  This is when infection of a pathogen stimulates the nucleus of the cell to migrate towards the pathogen and disintegrates.  Meanwhile, brown granules form in the cytoplasm that spread and kill the invading organism.

Biochemical induced defense mechanisms produce toxic w

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Pythium and Phytophthora.  Tolerance to disease is the plant’s ability to produce a good crop despite being infected with a pathogen.  The plant achieves this by either lacking receptor sites for the pathogen or by being able to overcome the excretions of the pathogen.

In conclusion plants have many varied mechanisms of basal and induced resistance to parasites and pathogens and an important part of governing this resistance is by plant gene recognition of parasite genes.


Agrious GN (1997)

Plant Pathology.

Academic Press. 4th Edition

Strange RN (2003)

Introduction to Plant Pathology

Wiley Press

Jackson and Taylor (1996)

Plant-Microbe Interactions: Life and Death at the Interface

The Plant Cell (1996) vol. 8 no. 10 pp1651 – 1668

De Wit P (2002)

Plant Biology: On Guard

Nature 416, 801 – 803

Odjakova and Hadjiivanova (2001)

The Complexity of Pathogen Defense in Plants

Plant Physiology 27 (1-2) 101-109

Dangl and Jones

Plant pathogens and integrated defense responses to infection

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