Mechanisms of some Toxins
Prepared by
Fahd Al-Dhabaan
Ph.D. student
Faculty of Science
King Saud University
Pathotoxin
Pathotoxin is a chemical of biological origin other than an enzyme that
plays an important role in a plant disease.
Most pathotoxins are produced by plant pathogenic fungi such as Alternaria
alternata or bacteria such as Pseudomonas
seruginosa, but some are produced by higher
plants, and one has been reported to be the product of an interaction between a
plant and a bacterial pathogen.
Properties of Pathotoxins
1.Some pathotoxins are selective in their action
but others are nonselective.
2.Some of pathotoxins act as plant growth regulators.
3.Pathotoxins are considered microtoxins due to small size of their
molecules.
4.Pathotoxins cause plant diseases as well as
fungal-fungal destruction.
Chemistry and mode of action of pathotoxins
Pathotoxins mean toxins which cause pathogenicity to organisms. Pathotoxins group included various members
of toxins each of which has specific manner such as gibberellins, fusicoccin, and tentoxin.
Gibberellins
The gibberellins were produced by the fungus Gibberella
fujikuroi and cause disease of rice. This
fungus which produces gibberellins infects the plant and form long internodes,
which typify the effects of this group of plant hormones.
Fusicoccin
Fusicoccin produced by the fungus Fusicoccum
amygdali, which causes a wilt disease of
peach and almond trees, is another nonselective pathotoxin with growth-regulator
properties.
This chemical also causes stomata
to open.
Tentoxin
Tentoxin is a product of Alternaria
alternata, it causes a striking variegated
chlorosis in cucumber, cotton, lettuce,
and many other sensitive plants, but has no effect on tobacco, radish, willow,
or other insensitive species. Tentoxin binds specifically to coupling factor 1 in chloroplasts of
sensitive species, and presumably acts by disrupting energy generation in the photophosphorylative electron transport system.
Figure
Leaf spots and chlorosis caused by the Alternaria alternata toxin
Leaf spots and chlorosis caused by the Alternaria alternata toxin
Victorin (HV-toxin)
Victorin is produced by the fungus Cochliobolus
(Helminthosporium)
victoriae. Cochliobolus
victoriae
infects the basal portions of
susceptible oat plants and produces a toxin that is carried to the leaves,
causes a leaf blight, and destroys the entire plant. Toxin production in the
fungus is controlled by a single gene (Vb).
Victorin cause loss of electrolytes from
cells and increased respiration. Subsequently leads to decreased both of growth
and protein synthesis.
The toxin not only produces all
the external symptoms of the disease induced by the pathogen, but it also
produces similar histochemical and biochemical changes in the host, such as changes in cell
wall structure
Victorin has been purified and its
chemical structure has been determined to be a complex chlorinated, partially
cyclic pentapeptide.
The primary target of the toxin
seems to be the cell plasma membrane where victorin seems to bind to several
proteins. The possible site of action of victorin seems to be the glycine decar-boxylate complex.
Fusaric acid
Fusaric acid is a picolinic acid derivative produced from
various Fusarium species such as Fusarium
oxysporum, and has been proposed for a
various therapeutic applications. However, it is primarily used as a research
tool.
Its mechanism of action is not
well understood. It likely inhibits Dopamine beta-hydroxylase (the enzyme that converts
dopamine to norepinephrine). It may also have other actions, such as the inhibition of
cell proliferation and DNA synthesis.
Factors affecting the toxicity
1.Toxin dose: Toxicity depends on the dose of
the poison introduced into the host, so that the proportional relationship
between the dose of poison and toxicity. The more toxic dose increased the
effect of toxicity.
2.Time of poison
abuse: Toxicity increases over time from the beginning of the poison
abuse where the toxin spreads and dips in the body of the host and be able to
cause damage or death.
3. Age of host: Age of host is a significant
factor which depend upon it the extent of toxicity. High toxicity be on two
phases of the life on opposite sides either in the early or later stages of age
due to weakened immunity and little or no resistance to the poison. Youth is
the stage of power that can be faced the toxicity correlated with other
factors.
4. Nutrition of
host: The host can resists the influence of the poison and slows
down its spreading if there is proper nutrition because vitamins and proteins
delay the activity of the toxin. And vice versa, toxicity increases if the host
body was weak due to malnutrition.
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