IMPORTANCES OF MICROBIAL TOXINS IN PLANT DISEASES.pptx

In living plant cells several interdependent biochemical reactions are taking place concurrently or in a well defined succession to sustain life. Disturbance of any of these metabolic reactions causes disruption of the physiological processes leading to the development of disease. The harmful substances produced by the pathogenic microorganisms which are responsible for such disturbances are called toxins. Toxins are effective in very low concentrations. Some are unstable or react quickly and are bound tightly to specific sites within the plant cell.
Some toxins act as general protoplasmic poisons and affect many species of plant of different families termed as non specific toxins, other are toxic to only a few plant species or varieties and are harmless to others. Many toxins exists in multiple forms that have different potency.

1. SUBRATA RAHA, Ph. D PROFESSOR & H.O.D IN BOTANY, S.K.B.U. PURULIA, W.B MICROBIAL TOXINS IN PLANT DISEASES

2. Three broad classes of toxins : PHYTOTOXIN ( Luke & Gracens, 1972) : Any compound produced by micro-organisms which is toxic to plant is termed as Phytotoxin Rudolph (1973) preferred to use Phytotoxic compounds]. They are non specific, incite few or none of the symptoms that are incited by the pathogen. In most cases shows no relation between toxin production and pathogenecity . Lycomarasmine & Alternaric Acid . VIVOTOXIN (Dimond & Waggoner, 1953) : Microbial toxin produced in the infected host by the pathogen and/or its host which functions in the production of disease but is not itself the initial inciting agent. Fusaric Aci d . PATHOTOXIN (Wheeler & Luke, 1963) :It is a host-specific toxin which includes all the typical disease symptoms. The production of toxin is correlated with pathogenecity. These toxins are restricted only to a particular host species of the microbial pathogen and It damage the tissues of susceptible host but have little or no effect on resistant cultivars. Piricularin.

3. Modes of Action Toxin Vs Enzyme Actions on plasma membrane and tonoplast (Pathotoxins) Inhibition of Enzyme activity either by chelate formation or chemical reaction. Toxin may form complex with some essential metabolite (due to structural homology), thus put it out of its normal biochemical activities. Plant suffer sdue to deficiency of metabolite. Toxins are different from enzymes in that they do not attack the structural integrity of the tissue but affect the metabolism in a subtle manner. They act directly on the protoplast of the cells. The toxin of Clostridium welchii (Clostridium perfringens) , lecithinase, even though an enzyme, is a toxin because it disrupts cell membrane. Such enzymes are toxins.

4. TOXINS HOST- SPECIFIC TOXIN NON- SPECIFIC TOXIN A Brief Idea About Different toxins In living plant cells several interdependent biochemical reactions are taking place concurrently or in a well defined succession to sustain life. Disturbance of any of these metabolic reactions causes disruption of the physiological processes leading to the development of disease. The harmful substances produced by the pathogenic microorganisms which are responsible for such disturbances are called toxins. Toxins are effective in very low concentrations. Some are unstable or react quickly and are bound tightly to specific sites within the plant cell. Some toxins act as general protoplasmic poisons and affect many species of plant of different families termed as non specific toxins, other are toxic to only a few plant species or varieties and are harmless to others. Many toxins exists in multiple forms that have different potency.

5. NON - SPECIFIC TOXINS Tabtoxin / Wild fire toxin ( Pseudomonas syringae pv. Tabaci ): Cause wild fire of Tobacco, also occurring on Bean, Soybean. Cause necrotic spots on leaves, surrounded by yellow halo. Toxin is a Dipeptide (Threonine & Tabtoxinine) , on hydrolysis produce tabtoxinine which inactivate the enzyme Glutamine synthetase, which lowers glutamine level and leads to the accumulation of toxic ammonia. The latter destroys the thylakoid membrane of chloroplast, causing chlorosis and eventually necrosis. May affect RNA metabolism and hamper protein synthesis in host (Lovrekovich et al ., 1964).

6. Phaseolotoxin ( Pseudomonas syringae pv.phaseolicola ): Cause Halo blight of Bean and other legumes. The principal symptom produced by Phaseolotoxin - producing strains is a chlorotic zone or halo around the necrotic infection site. This protoxin is a modified Ornithine – Alanine – Arginine tripeptide carrying a phosphosulfinyl group. After the excretion of tripeptide from bacterial cell into the plant cell, plant enzyme cleaves the peptide bonds and release alanine, arginine and phosphosulfinyl ornithine. The last one is the functional moiety of the toxin. Toxin binds with Ornithine Carbomoyl Transferase which catalyses conversion of ornithine to citrulline which is the precursor of arginine. Thus the toxin causes accumulation of ornithine and depleted level of citrulline & arginine. The toxin seems to also inhibit the pyrimidine nucleotide biosynthesis, reduce activity of ribosome, interfere with lipid synthesis and changes the permeability of the membrane.

7. Tentoxin ( Alternaria alternata , Previously A.tenuis ): Cause spots and chlorosis of many species. The toxin is a Cyclic tetrapeptide. Binds and inactivates a protein involved in energy transfer into chloroplasts and also inhibit light-dependent phosphorylation of ADP to ATP. It also inactivates the activity of Polyphenol oxidase, enzyme involved in host resistance. Pyricularia toxin ( Pyricularia oryzae ) Cause Blast disease. α -Picolinic acid & Piricularin cause characteristic symptoms, now a third substance has isolated - pyriculol. Picolinic acid affects catalase, chelates iron in respiratory enzymes. Piricularin inhibit respiration at high concentration. Inhibits the enzymes catalase, Peroxidase, Cytochrome oxidase and Ascorbic acid oxidase.

8. Cercosporin ( Cercospora personata ) Leaf spot and blight of many crop (Zinnia, Corn). It is a photosensitizing perylenequinone absorb light energy and converted to activated state and then reacts with molecular oxygen and forms activated oxygen . The latter react wit protein, lipid, nucleic acid of plant cell & damage or destroy host cell membrane.

9. Lycomarasmine - Fusarium oxysporum f.sp. lycopersici . Tomato wilt etc. Lycomarasmine is an amino acid derivative (A Dipeptide). Chelate iron, disturb permeability of cells. Inactivate growth factor Strepogenin. Fusaric acid is a picolinic acid derivative. Produced by Fusarium moniliformae, F. oxysporum etc. Adversely affect membrane, respiration, phosphorylation. Inhibit activity of poly phenol oxidase, porphyrin oxidase etc.

10. NAME OF THE TOXIN PATHOGEN & DISEASE FUMARIC ACID Rhizopus sp., Almond hull rot disease. OXALIC ACID Sclerotium sp. & Sclerotinia sp. cause several diseases. Cryphonectria parasitica cause Chestnut blight. ALTERNARIC ACID, ALTERNARIOL, ZINNIOL Alternaria sp. leaf spot disease of many plants. CERATOULMIN Ophiostoma ulmi . Dutch elm disease. FUSICOCCIN Fusicoccum amygdali. ( Phomopsis amygdali ) Twig blight of Almond & peach. OPHIOBOLINS Cochliobolus sp. diseases of grain crops. CORNATINE P. Syringae pv. atropurpurea infect grasses and Soybean. SYRINGOMYCIN P. Syringae pv. syringae. Leaf spot of many plants. SYRINGOTOXIN P. Syringae pv. syringae in Citrus plants. TAGETITOXIN P. Syringae pv. tagetis in Marigold leaf spot. THAXTOMINS Streptomyces sp. cause root and tuber rots. OTHER NONHOST-SPECIFIC TOXINS (ISOLATED FROM CULTURE)

11. Almond hull rot disease caused by Rhizopus sp. Peanut stem blight Sclerotium formation Sclerotinia stem blight of Soybean southern blight (Sclerotium rolfsii ) on tobacco

12. Early blight by Alternaria solani Brown leaf spot by Alternaria alternata Early blight, Tomato Conidia of Alternaria DUTCH ELM DISEASE Hylurgopinus rufipes Ophiostoma ulmi

13. Fusicoccum amygdali. ( Phomopsis amygdali ) Twig blight of Almond & peach. HOST - SPECIFIC TOXINS Host specific toxins are produced by certain fungi like Cochliobolus, Alternaria, Periconia, Phyllosticta, Corynespora and Hypoxylon. Although certain bacterial polysaccharides from Pseudomonas and Xanthomonas have been reported to be host specific.

14. HV Toxin ( Victorin) - Helminthosporium victoriae on Oat – Chlorinated partially Cyclic Pentapeptide - Damage plama membrane binds to several proteins, disrupt chlororoplast and other , loss of electrolyte, increases respiration, decrease growth and protein synthesis. The possible site of action seems to be the Glycine decarboxylate complex which is a key component of the Photorespiratory cycle. Evidences prove that Victorin functions as an elicitor that induces resistance response that include features of hypersensitive response and lead to programmed cell death

15. HC Toxin – Helminthosporium carbonum (Bipolaris zeicola) – Toxin is a cyclic tetrapeptide, (L-Aoe - D- prolyl - L-alanyl - D-alanyl) where Aoe stands for 2-amino-8-oxo-9,10-epoxidecanoic acid. C auses northern leaf spot and ear rot disease in Corn. Toxic only on specific maize lines. Resistant maize lines have a gene (Hm1) coding for an enzyme called HC toxin reductase that reduces and thereby detoxifies the toxin. Susceptible maize lack this gene. – Cyclic Polypeptide – less strong than HV toxin – affect nitrate uptake and reduction. HS Toxin - Helminthosporium sacchari – Blast of Sugercane – Glycoside (helminthosporiside) – Disrupt plasma membrane, chloroplast lamellae.

16. PC Toxin (Periconin) – Periconia cercinata causes Milo disease of Sorghum vulgare var subglabrescens - Polypeptide – Interfere membrane function, rapid loss of electrolyte, decrease ability to incorporate amino acids, increase respiration. AK Toxin – Alternaria kikuchiana cause black spot of Pear – Not chemically identified – Damage cell membrane. AM Toxin – Alternaria mali cause leaf blotch of Apple – Cyclic depsipeptide (A depsipeptide is a peptide in which one or more of its amide, -C(O)NHR-, groups are replaced by the corresponding ester, -C(O)OR,)– Action like AK toxin, also induces rapid loss of chlorophyll.

17. T Toxin – Cochliobolus (Helminthosporium) heterostrophus race T toxin [ Bipolaris maydis ] Cause Southern Corn leaf blight. Long linear Polyketides with 35-45 C (Large class of diverse compounds that are characterized by more than two carbonyl groups connected by single intervening carbon atoms, derived from repeated condensation of acetyl coenzyme A). Attack only corn that have Texas male sterile (Tms) cytoplasm which is inherited maternally. Corn with normal cytoplasm is resistant to fungus & toxin. T toxin acts specifically on mitochondria, thus inhibit ATP synthesis. Toxin binds with a receptor protein (URF 13) located on inner mitochondrial membrane of sensitive plant.

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