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
-wax, cuticle
B. Defense structures formed in response
to pathogen invasion
1. Histological defense structures-
*Cork layers-
*Abscission layers-
*Tyloses-overgrowths of the protoplasts of
adjacent living parenchymatous cells which protrude into xylem vessels through
pits.
*Gums-
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.
Diagram- Phenol-Quinone
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.