The term toxin is generally used for substances, usually but not invariably of pathogen origin which are injurious to plants and directly or indirectly play a role in disease development. Thus harmful enzymes are also toxins. There are different terminologies :
Vivotoxin is a substance produced in the infected plant by the pathogen and or its host which functions in production of disease.
Pathotoxin is a general term used for phototoxic substances produced by live organism. Endotoxin is an extracellular toxin which diffuses from live bacterial cells.
Toxins may act as poisons on host cell protoplast: they may alter permeability of cell membranes; they may inhibit the activity of enzymes and may also act as antimetabolites (do not allow synthesis of a metabolite in plant cell). Some toxins act as general protoplasmic poison and affect many species of plants of different families. These are general toxins. Some other toxins are toxic to only a few plant species or varieties and completely harmless to others. These are known as host-specific toxins. Fungi and bacteria may produce toxins in infected plants as well as in culture medium. Toxins are extremely poisonous substances and are effective in very low concentrations. Some are unstable or react quickly and are tightly bound to specific site within the plant cell.
(i) General toxins or Non-host specific toxins
These produce all or part of the disease syndrome not only on the host plant but also on other species of plant that are not normally attacked by the pathogen in nature. Some of the toxins of this category are:
1. Wildfire toxin or tabtoxin: It is produced by the bacterium, Pseudomonas syringae pv tabaci, the cause of wildfire disease of tobacco. The toxin in sterile culture filtrate of the bacterium produces identical symptoms on many other plants also.
Besides P.syringae Pv.tabaci on tobacco, this toxin is now known to be produced by other strains of Pv tabaci occurring on other hosts such as bean and soybean. This is also produced by other pathovars (sub-species) of Ps. Syringae, such as those occurring on oats, maize and coffee. The toxin is responsible for including a characteristic necrotic spot an leaves, the spot surrounding by a yellow halo.
Tabtoxin is a dipetide of the amino acids threonine and so far unknown, tabtoxinine. Tabtoxin as such is not toxic. In the cell it becomes hydrolysed to release tabtoxinine (the active toxin), which inactivates the enzyme glutamine synthetase. It leads to depleted glutamine levels and reduced ability of plant to defend actively. Due to inactivation of glutamine synthetase NH3 accumulates during photorespiration, thus inhibiting photosynthesis and photorespiration and causing chlorosis and eventually necrosis.
2. Phaseolotoxin: This is produced by the bacterium pseudomonas syringae Pv phaseolicola, the cause of halo blight of bean and some other legumes. Plants treated with toxin produced symptoms identical to those on natural plants. Phaseolotoxin is an ornithine-arginine tripeotide carrying a phosphosulfamyl group. Soon after its production, the plant enzymes cleave the people bonds to release alanine, arginine and phosphosulfamylornithine (PSMO), the actual functional moiety of the toxin. The (PSMO) inactivates the enzyme ornithine carbamoyltransferase (which convents ornithine to citrulline, a precursor of arginine). The toxin thus causes accumulation of ornithine and depleted level of arginine. The mechanism of cholorosis induction is not understood.
3. Tentoxin: This is produced by the fungus Alternaria alternata (previously named A. tenuis), which causes chlorosis in seedlings of many plants. Tentoxin is a cyclic tetrapeptide that binds to and inactivates a protein (chloroplast-coupling factor) involved in energy transfer into chloroplasts and also inhibits photophosphorylation. It interferes with normal chloroplast development and chlorophyll synthesis (thus inducing chlorosis). It also inhabits the activity of polyphenyloxidases, involved in defense mechanisms of plants.
4. Fusicoccin: It is produced by the fungus Fusicoccum amygdale, cause of twig blight of almond and peach trees. It is a complex carbotricyclic terpene glycoside and affects the cellular transport systems, particularly the H+/K+ pumps. The toxin also stimulates stomatal opening, respiration and plant cell enlargement.
5. Other non-host specific toxins: A range of fungi and bacteria are known to produce several. These are listed below.
(ii) Host-specific toxins
A host-specific toxin is a substance produced by a pathogen that, at physiological concentrations, is toxic only to the hosts of that pathogen and shows little or no toxicity against non-host plants. Most host-specific toxins must be present for the producing pathogen to be cause disease. So far, such toxins are shown to be produced only by some fungi belonging to the genera, Helminthosporium , Alternaria, Periconia, Phyllosticta, Corynespora and Hypoxylon although some bacterial polysaccharides of pseudomonas and xanthomonas have also been reported to be host-specific,
1. Victorin or HV toxin. This is produced by the fungus Helminthosporium Victoriae, which appeared in 1945 on the introduced and widely basal part of plant and then produces the toxin, which is carried to the leaves, causing a leaf blight and destroying entire plant. The toxin produces histochemical and biochemical changes in plant, which include changes in cell wall structure, loss of electrolytes from cells, increased respiration, decreased growth and protein synthesis.
2. T-toxin (Helminthosporium maydis race T toxin): This is produced by race T of) Helminthosporium maydis, cause of southern corn leaf blight. The toxin is mixture of linear, long (35 to 45 carbon) polyketols. It acts specifically an mitochondria causing early loss of matrix density, rendering them non-functional.
3. AK-toxin: It is produced by a distinct pathotype of Alternaria alternate, previously to S kikuchiana, the cause of back leaf spot of Japanese pears (pyrus serotina). The toxin causes the cells to instantaneously lose K+ and phosphate.
4. AM- toxin: This produced by the apple pathotype of Altrnaria alternate previously referred to A mali, cause do Alternaria blotch of apple. The toxin is a cyclic depsipeptide. The site and mechanism of action are similar to AK-toxin but this also causes rapid loss of chlorophyll.
5. Other host-specific toxins: Some other host specific toxins produced by fungi are listed below.
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