EPWS 310 - Plant Pathology
Lectures - Fall 2002
Readings – Chapters 4, 6
HOW PLANTS DEFEND THEMSELVES AGAINST PATHOGENS
Two basic ways that plants defend themselves against pathogens.
I. Structural defense-
A. Pre-existing defense structures
B. Defense structures formed in response to pathogen invasion
1. Histological defense structures-
*Tyloses-overgrowths of the protoplasts of adjacent living parenchymatous cells which protrude into xylem vessels through pits.
C. Cellular defense structures-morphological changes in the cell wall or changes derived from the cell wall.
Example- Callose papillae-
D. Cytoplasmic defense-
II. Metabolic (Biochemical defense)
A. Pre-existing biochemical defenses-
*Example-Onion smudge disease caused by Colletotrichum circinans.
B. Defense through lack of essential factors
1. Lack of recognition between host and pathogen.
2. Lack of host receptors and sensitive sites for toxins.
3. Lack of essential nutrients for the pathogen.
III. Metabolic defense induced by the attacking pathogen-
A. Defense through the hypersensitive reaction.
*It occurs only in incompatible combinations.
*Results in death and collapse of the few infected cells and a few surrounding cells.
B. Defense through increased levels of phenolic compounds.
1. Phytoalexins- toxic substances produced in response to infection and mechanical injury.
Phytoalexins are stimulated by pathogen substances called elicitors. Elicitors are usually part of the fungal cell wall (glucans, chitosan, glycoproteins, and polysaccharides). In a susceptible response the pathogen is thought to have suppressors.
*What about the virulent pathogen? How do they respond?
2. Fungitoxic phenolics released from nontoxic phenolic complexes- nontoxic glycosidase that cleaves phenol and then renders it toxic.
3. The role of phenol-oxidizing enzymes in disease resistance- Polyphenoloxidase (PPO)-; usually higher in resistant plants than in susceptible plants. PPO oxidizes phenolics to quinones (more toxic to microbes). Peroxidases
4. The role of induced synthesis of enzymes.
Chitinase, b-glucanases, PG-inhibitors.
5. Defense through inactivation of pathogen enzymes- caused by phenols and proteins.
6. Defense through release of fungitoxic cyanides from non-toxic complexes. Cyanides-released by decompartmentalized hydrolytic enzymes.
7. Defense through detoxification of the pathogen toxins.