This document discusses the role of toxins in plant pathogenesis. It defines toxins as low molecular weight compounds produced by microorganisms that are toxic to plants. Toxins can injure host cells by disrupting osmotic relations or enzymatic reactions. Most toxins directly or indirectly affect plant respiration. The document then classifies toxins into three categories: phytotoxins, which are toxic but do not cause disease symptoms; vivotoxins, which are produced in infected plants and cause disease symptoms; and pathotoxins, which directly cause disease symptoms and determine pathogenicity. It provides examples of host-specific and non-host-specific toxins produced by various plant pathogens.

1. ROLE OF TOXINS IN
PATHOGENESIS
N. H. SHANKAR REDDY
2nd, Ph.D. Plant Pathology
Annamalai University

2. Toxin – Any compound (low molecular weight) produced by the
microorganisms which is toxic to plants.
• Toxins are extremely poisonous and effective in very low
concentrations.
• Toxins injure host cell either by affecting osmotic relations or by
affecting enzymatic reactions going on in the plant cells.
• Most toxins have a direct or indirect effect on the respiration of the
plant.
• Toxins which directly act on living host protoplast and kill the cells
of the plant.

3. Wheeler and Luke (1963) classified toxins into 3 categories
Phytotoxins,
Vivotoxins and
Pathotoxins

4. Phytotoxins -
• Compound produced by the microorganism which is toxic
to plants is called phytotoxin Produce few or no
symptoms
• Phytotoxins has no role in pathogenicity
• Eg – Alternaric acid, Cochliobolin

5. Vivotoxins -
• Diamond and Waggoner (1953) coined the term “vivotoxin” and
defined as substance produced in the infected plants by the
pathogen and/ or its host which functions in the production of
disease but is not itself the initial inciting agent of the disease.
• It produces majority of the symptoms which determines the
virulence
• Vivotoxin is usually non specific
• Eg – Fusaric acid

6. Pathotoxins
• Toxins produced by the plant pathogens which plays
major role in disease production and development
• Pathotoxin mainly determines the pathogenicity and
produce characteristics symptoms of the disease
• Eg – Pyricularin, ten toxin, tab toxin and victorin

7. Host specific –
Host specific toxin (HST) is a metabolite produced by the
pathogen which is selectively toxic to susceptible host only,
little or non toxic to the non suceptable hosts.
Host non-specific
Toxins produced the pathogen not only diseases host plant,
but also affects/ targets non host plants and other species of
plants

8. Host specific Host non specific
Name of the toxin Produced by the pathogen
HV – toxin/ Victorin Helminthosporium victoriae
HC – toxin H. carbonum/ Cochliobolus
carbonum
HS – toxin H. sacchari (eye spot of
sugarcane)
HM-T toxin H. maydis race T
ACL – toxin Alternaria citri
AL toxin A. lycopersici
AK – toxin A. kikuchiana
AM – toxin A. mali
PC – toxin Periconia circinata
PM – toxin Phyllosticta maydis
Name of the toxin Produced by the pathogen
Alternaric acid, Zinniol,
Alternariol
Alternaria
Fumaric acid Rhizopus spp
Oxalic acid Aspergillus, Sclerotiumand
Sclerotiniai spp
Thaxtomins Streptomyces
Syringomycin,
Syringotoxin, Tagetoxin
Pseudomonas syringae pv.
syringae
Phaseolotoxin Pseudomonas syringae pv.
phaseolicola
Tab toxin (wild fire toxin) Pseudomonas syringae pv.
tabaci
Ten toxin Alternaria tenuis
Ophiobolin Cochiliobolus spp,
Helminthospporium oryzae