EPWS 310 - Plant Pathology
Lectures - Fall 2002
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
Characteristics-
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
EXAMPLES
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
4) ETHYLENE - CH2=CH2
Effects:
- 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