EPWS 310 - Plant Pathology

Lectures - Fall 2002


Lecture 7

Microbial toxins in Plant Disease.

A. Definition-

Toxin-a non-enzymic metabolite of one organism which is injurious to another. Mycotoxin- a toxin produced by a fungus.


B. Mode of action?? -


C. Non-host specific toxins- Toxins that affect a wide range of host plants. These toxins affect many different plants and increase the extent of disease but are not essential for the pathogen to cause disease.

Example- Phaseolotoxin

*Phaseolotoxin-Caused by Pseudomonas syringae pv. phaseolicola, causes halo blight of bean.

It is a ornithine-alanine-arginine tripeptide carrying a phosphosulfamyl group. Plant enzymes cleave the peptide bonds and release alanine, arginine and phosphosulfamylornithine. Phosphosulfamylornithine is the active moiety of phaseolotoxin. PSO binds to the active site and inactivates the enzyme ornithine carbamoyl transferase which normally converts ornithine to citrulline, a precursor of arginine (an essential amino acids- what do a string of AA make????) Ornithine accumulates and depletes arginine.

D. Host-specific toxins- a substance produced by a pathogenic microorganism that, at physiological concentrations, is toxic only to the hosts of that plant pathogen and shows little or no toxicity against nonsusceptible plants. Most host-specific toxins must be present for disease to occur.

Examples- 1) Victorin or HV toxin- caused by Helminthosporium victoriae.

2) T-toxin- Cochliobolus heterostrophus (Helminthosporium maydis) race T

3) AK-toxin- Alternaria alternata

4) AM-toxin- Alternaria alternata (A. mali)

Example- T-toxin-Southern corn leaf blight

Resistance and susceptibility to C. heterostrophus T and its toxin are inherited maternally (in cytoplasmic genes). Virulence and T-toxin are controlled by the same gene in C. heterostrophus.

T-toxin acts on the mitochondria of susceptible cells. It causes early loss of matrix density, renders them nonfunctional, and inhibits ATP synthesis. T-toxin also causes selective uptake of certain ions, inhibition of root growth in seedlings and closure of stomata.

 Growth Regulators in Plant Disease

Growth regulators= hormones

1. Auxins

2. gibberellins

3. cytokinins

4. ethylene



1. Work in small concentrations

2. Usually synthesized a distance from the site of action.

3. Promotes syntheses of messenger-RNA molecules.

 1. AUXINS - Indole -3-acetic acid (IAA) is the auxin that occurs naturally in plants.

The effects of IAA?


A) Cause of auxin increase:

- plant

- pathogen stressing plant

- pathogen produced

- degradation of IAA oxidase - breaks down excess amount of IAA in normal plant metabolism


1) Pseudomonas solanacearum causes bacterial wilt of solanaceous plants

- Increased plasticity of the cell wall

- Inhibits lignification

2) Crown gall Agrobacterium tumefaciens

- Tumor cells contain higher than normal amounts of IAA and cytokinin.

3) Pseudomonas savastoni - knot disease of olive, oleander, and privet, produces IAA and induces gall formation.

 2. GIBBERELLINS - Effects:

- speed elongation of dwarf varieties

- promotes flowering

- stem and root elongation

- growth of fruit

- induces IAA formation

 3) CYTOKININ - necessary for cell growth and differentiation

- inhibits senescence

Cytokinin activity increases in clubroot galls, in crown galls, in smut and rust galls, and root infected bean and broad bean leaves

-Witches'-broom caused by fungi and bacteria



- chlorosis

- leaf abscission

- epinasty

- stimulation of adventitious roots

- fruit ripening

- increased permeability of cell membranes

- stimulates phytoalexins and enzymes that may play a role in increasing plant resistance to infection

- leaf epinasty, premature defoliation

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