1

PRESENTED BYMISS SHIKHA SHARMA

ASSISTANT PROFESSOR (PL. PATH.)COLLEGE OF AGRICULTURE, BALAGHAT

MURJHAD FARM, WARASEONI

ROLE OF TOXINS IN

PLANT PATHOGENESIS

OUTLINE INTRODUCTIONHISTORYTOXINSCLASSIFICATION OF TOXINS TARGET SITES OF THE TOXINSEXPLANATION OF FEW IMPORTANT TOXINSCONCLUSION

INTRODUCTION Three classic cases often cited are:

1. Oat production in North America (1946 to 1948)

2. Bengal famine in India (1942-43)

3. Maize production in South America(1970 -1971)

The most dangerous effect of the fungi is the production of mycotoxins that causes a significant health hazard to humans and animals.

St Antony’s fire in humans

Rye grass staggers in sheep

Helminthosporium sp.

Ergot Alkaloid

HISTORY

“Micro-organisms are pathogenic only if they are toxicogenic: in other words, the agents responsible for diseases can damage their hosts only if they form toxins-microbial poisons that penetrate into host tissue”. (Gaumann, 1954)“A toxin as a product of a microorganism or of a microorganism host complex which acts on living host protoplast to influence disease development or symptoms”. (Ludwig, 1960)

“Toxins as injurious substances produced by organisms”. (Wheeler,1975)

What is TOXIN ?

1. A toxin should produce all symptoms characteristic of the disease.

2. Sensitivity to toxin will be correlated with susceptibility to

pathogen.

3. 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.

Toxin hypothesis

(Luke and Wheeler, 1955)

Target sites of Toxin in Plant cell

1. SOURCE

OF ORIGIN

Sheffer & Briggs, 1981

3.Chemistry

Eg. Trichothecenes and Fusicoccin

Ptr toxin and ergot alkaloids

Oxalic acid fatty acids e.g. aflatoxin, cercosporin and T-toxin

HC-Toxin and Vicorin

Scheffer and Briggs, 1981

VICTORIN

FIG.3 : CHLORINATED, PARTIALLY CYCLIC PENTAPEPTIDE

Fig. 4 The current model of victorin-induced cellular responses.

Tada et al. (2005)

Race T of Helminthosporium (Cohliobolus) heterostropus, the cause of southern blight of Maize in USA, 1968.It attacks corns that have Texas male sterile (Tms) cytoplasm.

Fig 6 : T-toxin is a mixture of long (35-45carbon) Polyketoles.

T-Toxin

Fig. 7: The Effect of T-toxin From Cochliobolus heterostrophus on T-cms maize

HC- Toxin

Race 1 of Bipolaris zeicola causes northern leaf spot and ear rot disease in maize.

Tox-2 Governs the production of Toxin.

Hm1 gene confers insensitivity to HC-toxin and hence resistance to B. zeicola

Fig. 8 : Cyclic Polypeptide

Tabtoxin (Non-Selective)

Pseudomonas syringae pv. tabaci (Wildfire diseases tobacco) (non-toxic)

Hydrolized In the Host

Cell

Fig. 9: Structure of Tabtoxin

Pseudomonas syringae pv. phaseolicola ( halo blight of beans)

Fig 10: Ornithine-alanine-arginine-tripeptide ( Phosphosulfinyl group)

Phaseolotoxin

Ornithine

Citrullin n

Arginine

Starch

TENTOXIN

Fig 11: Cyclic tetrapeptide

Alternaria alternata ( spots and chlorosis)

Energy

Cercosporin

Fig. 12: Structure of Cercosporin

Grey leaf spot of Corn

Fig. 13: Mode of action of Cercosporin

Ergot alkaloid

The most prominent member of this group is Claviceps purpurea.

This fungus grows on rye and related plants and produces alkaloids. Cause ergotisms in humans and other mammals who consume grains contaminated with its fruiting structure called ergot sclerotium.

Fig.:16 Structure of Ergotamine

Aflatoxin

Aspergillus flavus, Aspergillus nomius and Aspergillus parasiticus. There are four kinds of aflatoxins such as aflatoxin B1, B2, G1 and

G2, in which aflatoxin B1 (AFB1) is highly toxic and carcinogenic (Leontopoulos et al., 2003).

Aflatoxins are known to be potent carcinogenic agents that pose serious hazards to human and animal health (Sidhu et al., 2009).

In addition, aflatoxin also has an impact on agricultural economy through the loss of crop production (Wu, 2004).

Food and Agriculture Organization of United Nations (CAST, 2003) shows that 25% feedstuffs is polluted by mycotoxin in the world, and it results in over 1 billion dollars loss for poultry industry annually .

Trichothecenes

Fusarium species that are pathogenic to economically important crops such as wheat, barley, and maize.

They are the causal agents of moldy-grain toxicoses in animals such as feed refusal, dermatitis, anemia, immunosuppresion and hemorrhagic

septicemia.

Typical examples of type A and type B trichothecenes are T-2 toxin produced by Fusarium

sporotrichioides and deoxynivalenol (DON) produced by Fusarium graminearum,

respectively.

FUMONISINS

Fusarium verticillioides and Fusarium proliferatum.

There are at least 28 different forms of fumonisins,

most designated as A-series, B series, C-

series and P-series.

Fumonisin B1 is the most common and

economically important form, followed by

B2and B3.

Maize is the most commonly contaminated crop.

Fumonisins are carcinogenic to laboratory

animals, and in humans, consumption of

fumonisin-contaminated maize is associated

with higher rates of esophageal cancer and

neural tube birth defects.

Fusarium ear rot

Ochratoxins

• Aspergillus and Penicillium. • Ochratoxin A is the most economically important

form of ochratoxin; ochratoxins B and C are less toxic and less common.

• Ochratoxin contamination is economically important in cereal grains, grapes, coffee, tree nuts, and figs.

Nadine and Hertweck, 2009

Figure 19: Overview on the cellular targets and the mode of action of several fungal phytotoxins

Table 1 : Example of Non-host selective Toxins

Non host selective toxin Fungus Diseases

Fumeric acid Rhizopous spp Almond hull rot

Oxalic acid Cryphonectria parasitica Chest nut blight

Alternaric acid, alternariol and zinniol

Alternaria species Leaf spot diseases

Ceratoulmin Ophistoma ulmi Dutch elm diseases

Fusicoccin Fusicoccum amygdali Twig blight diseases of Almond and peach trees

Ophiobolins Cochliobolous sp Grain crops

Pyricularin Pyricularia grisea Blast of rice

Lycomarasmin Fusarium oxysporum Wilt in tomato

Table 2 : Example of Non host selective bacterial toxin

Other non host toxin Bacteria Diseases

Coronatine Pseudomonas syringae pv atropurpurea

Grasses and soybean

Syringomysine Pseudomonas syringaePv syringae

Leaf spot in many plants

Syringotoxin Pseudomonas syringaePv syringae

Citrus plant

Tagetitoxin Pseudomonas syringaePv tagetis

Marigold leaf spot

Thaxomins Streptomyces sp Root an tuber diseases

Table 3 : Toxin produced by fungal plant pathogens

Toxin Pathogen Diseases Target of Function

PC Toxine (Peritoxin A and B)

Periconia ciriciana Milo diseases of sorghum

Plasma membrane

HS toxin A, B and C Bipolaris sacchari Eye spot of sugarcane Plasma membrane

PM Toxin A, B, C and D

Phyllosticta mayadis Yellow leaf blight of maize

mitochondria

Ptr- Toxin (Ptr chlorosis toxin)

Pyrenophora triricirepentis

Tan spot of wheat Chloroplast (Photosynthesis 14 KD Protein)

CC toxin Corynespora cassicola Tomato --

AK toxin Alternaria alternata black spot of japanese pear

Plasma membrane.

AAL toxin Alternaria alternata f. sp. Lycopersici

Leaf spot of many crops Sphingolipid and ethanolamine metaboloism.

AM Toxin Alternaria alternata (A. mali)

Leaf spot of many crops Chloroplast of plasma membrane

Contd.Toxin Pathogen Target of functionRhizobitoxine Pseudomonas andropogonis Β-Cystathionase, inactivating

homocysteine synthesis

corpeptin Pseudomonas corrugate Membrane active

Fuscopeptine Pseudomonas fuscovaginae Membrane active

viscosin Pseudomonas marginalis Membrane active

coronatine Pseudomonas syringae Molecular mimic of jasmonicacid, a plant signal molecule

phaseolotoxin Pseudomonas syringaePv phaeseoli

Inhibitor of ornithine carbamoyl transferase, inhibiting amino acid synthesis

Syringomysine Pseudomonas syringaePv syringae

Increase host membrane permeablity

Tabtotoxin Pseudomonas syringaePv tabaci and other

Glutamine synthetase

Tagetitoxin Pseudomonas syringaePv tagetis

Inhibitor of chloroplast RNA polymerase

Albicidin Xanthomonas albilineans Inhibitor of Plastid DNA replication

ConclusionPhytotoxins employ an array of strategies

to distress, weaken or kill the host plant in order to gain access to nutrients.

The captivating structural and mechanical diversity of the toxins teaches us a lesson on the complexity of pathogenic relationship.

understanding toxin biosynthesis pathways and their regulation, the mode of action and how this relates to fungal virulence will not only help to gain new insights into cellular processes in general but is also a stepping to develop ways to protect plants from fungal infections.

THANK YOU