3. introduction
Anton de Bary in 1886, first recognized that the pathogen damages
their host by secreting toxins.
He is considered as discoverer of “Toxin”.
Hutchinson in 1913, tried to prove the cause behind pathogenicity of
Psudomonas solanacearum, but he failed.
Gottleib in 1943, reported the role of toxins produced by Fusarium
spp.
Clawson in 1944, characterized “Lycomersmin” in Fusarium wilt of
tomato.
Gaumann in 1954, wrote that “ microorganisms are pathogenic only
if they are toxigenic.”
4. Toxincan be defined as-
A microbial metabolite excreted (exotoxin) or released by
lysed cells (endotoxin) are produced as a result of
interaction between host and pathogen. They are:
Proteinaceous in nature.
Low molecular weight, mobile in plant.
Effective at low concentration.
Act directly on host protoplast and influence the cause
of disease development and symptom expression.
6. Toxin hypothesis
According to Luke and Wheeler, 1955—
A toxin should produce all symptoms characteristic of
the disease.
Sensitivity to toxin will be correlated with susceptibility
to pathogen.
Toxin production by the pathogen will be directly related
to its ability to cause disease.
Except, Victorin, the toxic metabolite of Cochliobolus
victoriae, the vast majority of toxins associated with plant
diseases fail to exhibit all the above characters.
8. Pathotoxins—
Wheeler and Luke(1963) gave the concept of
Pathotoxins.
These are host specific toxins.
Most of these toxins are produced by pathogens during
pathogenesis.
Ex: Victorin: Cochliobolus victoriae(Helminthosporium
victoriae), the causal agent of Victoria blight of oats.
9. Vivotoxins—
The concept of vivotoxin was first introduced by Dimond
and Waggoner(1953).
It is usually non-host specific.
EX. Fusaric acid – Fusarium sp.
Lycomarasmin – Fusarium oxysporum f.sp. lycopersici
Piricularin – Pyricularia oryzae
10. Phytotoxins—
They are host and non-host specific.
There is no relationship between toxin production and
pathogenicity of disease causing agent.
Ex: Alternaric acid – Alternaria solani caused early
blight of Potato.
11. Classification based on specificity
toxin
Host specific /
Host selective
toxins
Non-host specific /
Non-selective
toxins
12. Host specific toxin —
These are metabolic products of pathogens.
They are selectively toxic only to the susceptible
host of the pathogen.
HSTs affect only plants that are host of toxin
producing microbial pathogens.
It is essential for disease development.
EX. Victorin or HV- Toxin
T- Toxin
AK- Toxin
AM- Toxin
HMT- toxin
Amylovorin
13. Victorin or HV-Toxin—
It is the first host specific toxin.
Reported by Meehan and Murphy in 1947.
It is produced by Helminthosporium victoriae causing
leaf blight or Victoria blight disease of Oats.
This toxin change the cell wall structure.
14. AK- Toxin—
This toxin is produced by Alterneria kikuchiana.
It is also host specific toxin.
Caused black leaf spot of Pirus serotine.
It is also called as “Altenin”.
15. AM-Toxin—
This toxin is produced by fungus Alternaria mali.
It is host specific in nature.
Causes disease Alternaria blotch of apple.
Due to this toxin rapid loss of chlorophyll occurs
which induces damage to plasmalemma and
chloroplast lamella.
16. Non-host specific toxins—
These are the metabolic products of the pathogen.
Don’t have host specificity.
It affect the protoplasm of many unrelated plant species that
are normally not infected by the pathogen.
EX. Tab-toxin
Ten-toxin
Cercosporin
Phaseolotoxin
Alterneric acid
Pyricularin
17. Tabtoxin—
It is first reported by Johnoson and Marwin in 1925.
It is confirmed by Clayton in 1934.
It is produced by bacterial pathotype Psudomonas
syringae pv. tabaci caused wildfire blight of tobacco.
It is dipeptide composed of two amino acid Threonin
and Tabtoxinin.
18. Tentoxin—
It is produced by fungus Alternaria alternata.
Causing spot & Chlorosis in seedlings of many crops.
The chlorosis including principle was isolated and
characterized by Templeton et al 1967 and named
“Tentoxin”.
It is tetrapeptide bind with bind with protein and inactivates
the protein.
It also inhibit light dependent phosphorylation ADP to ATP.
19. Fumeric acid—
Fumaric acid is produced by Rhizopus spp.
It cause Hull rot disease of Almond is associated with
the rotting of the fruit mesocarp of almond.
Fumaric acid is translocated from diseased fruits and
aids in blight of adjacent leaves and twigs.
20. Diffrence between host andnon-host specifictoxin
Host specific Non-host specific
Selectively toxic only to
susceptible host of the
pathogen.
No host specificity and can
also affect the physiology of
those plants that are
normally not infected by the
pathogen
Primary determinants of
disease.
Secondary determinants of
disease
Produce all the essential
symptoms of the disease.
Produce few or none of the
symptoms of the disease.
Ex: Victorin, T- toxin Ex: Tentoxin, Tabtoxin
21. Effect of toxins on host tissues
Changes in cell permeability--
Toxins kill plant cells by altering the permeability of
plasma membrane.
It causes loss of water and electrolytes and also unrestricted
entry of substances including toxins.
Cellular transport system, especially, H+ / K+ exchange at
the cell membrane is affected.
22. Disruption of normal metabolic processes—
Toxin disturb salt balance of the host cell, so that it may
cause—
Increase in respiration rate.
Malfunctioning of enzyme system.
Death of host cells.
Hydrolysis of protoplasmic protein.
23. Interfere with the growth regulatory system—
EX. Bakane disease or Foolish disease of Rice.
24. conclusion
Toxins are microbial metabolites involved in development
of plant diseases.
Toxins are the weapon of microbes, that helps in injuring
and killing the host cell.
They cause severe damage to the plant and produce
different symptoms.
It inhibits enzymatic reactions of the host.
It is effective at low concentration and directly act upon the
host protoplast.
