Disease development: Role of enzymes, toxins and growth regulators.

Presented By:
Prosenjit Bhowmick
M.Sc. 4th Semester(Botany), Roll No. 11
Dept. of Life Science & Bioinformatics, AUS
Role of Enzymes, Toxins & Growth Regulators

The branch of botany which deals with the study of the understanding
of the nature, development and control of plant disease is called Plant pathology
or Phytopathology (phyton=plant; pathos=suffering; logos=knowledge)
Stakman & Harrar(1957) defined disease as “a physiological disorder
or structural abnormality that is harmful to the plant or to any of its parts or
products that reduces the economic value”.
The process of disease development is very complicated. Environmental
factors & Stress often can accelerate this process.
INTRODUCTION
Disease infected corn leafHealthy corn leaf
Image source: Google images

HISTORY OF PLANT PATHOLOGY
Plant pathology, as a branch of science, existed in Ancient India much
before the time of the Greek Philosopher-Theophrastus(370-286BC).
Ancient Indian books like Rigveda, Atharveda(1500-
500BC),Artha Shashtra of Kautilya (321-186BC) have clearly
mentioned plant diseases &their controls.
Theophrastus(370BC-286BC) had the opinion that diseases
originated from plants or from environment and cite din his book
“Enquiry into Plants”.
During 1830-1850, devastating late blight of potato appeared
and starvation forced de Bary (1831-1888) a German Mycologist
to prove Phytophthora infestans as the causal organism of the
disease.
From then he is regarded as the ‘Father of Plant Pathology’
and the branch developed into a mature science with all its
ramifications.
Theophrastus(370-286BC)
de Bary(1831-1888)

To gain insight into the process of disease development, it is very
essential to understand the role of various pathogens such as:
 The adaptive modifications they have,
 The functional importance of the target tissue,
 interaction between pathogen and host at the target tissue level,
 Pathogenicity mechanism of pathogens etc.
The sequence of disease development will to a large extent depend
on the:
 Nature of pathogen (parasite, bacteria, virus, fungi)
 Environmental factors
 Size of the host
 Pathogen load or intensity per unit area or unit weight of the host.
DISEASE DEVELOPMENT

To study the overall process of disease development, the concept of Disease
triangle and different factors and stages of disease development are essential:
 Disease Triangle
 Factors for Disease Development
1. Properties of Pathogen
2. Properties of Host
3. Properties of Environment
 Stages in Disease Development
1. Inoculation
2. Penetration
3. Infection
4. Growth and Reproduction
5. Dissemination
OUTLINE OF DISEASE DEVELOPMENT PROCESS
Image source: Wikipedia

The disease triangle drawing was first published by Stevens in 1960.
The disease triangle concept explains that, the existence of a disease caused
by a biotic agent requires the interaction of
 The Host
1. Susceptibility & General health
2. Growth stage and form
3. Population density and structure
 The Pathogen and
1. Virulence and Aggressiveness
2. Adaptability & dispersal efficiency
3. Survivability & Reproductive fitness
 The Environment
1. Temp. & rainfall
2. Leaf wetness period
3. Wind & fire history (for native forests)
4. Soil temperature, water content, fertility, organic matter content
DISEASE TRIANGLE
Image source: Wikipedia

To occur a disease all the three factors are essential.
Conversely, plant disease is prevented upon elimination of any
one of these three causal components.

Environmental conditions frequently determine whether a particular
disease will occur or not.
Most common environmental factors that have considerable influence on
development of plant disease are temperature, moisture, wind, light,
soil pH, soil structure etc.
Effect of Temperature:
 Each pathogen has an optimum temperature for its growth.
 Pathogen differs in their preference for higher or lower temperature.
 Rapid disease development occurs when temp. is optimum for
pathogen development and is below or above the optimum for host
development.
 Effects of temp. may mask symptoms of certain viral and mycoplasmal
diseases and making them more difficult to detect.
EFFECT OF ENVIRONMENTAL FACTORS ON
DISEASE DEVELOPMENT

Effect of moisture
 Moisture influences the initiation and development of infectious plant diseases
in many interrelated ways.
 Moisture is indispensable for the germination of fungal spores and penetration
of the host by germ tube.
 It is also indispensable for the activation of bacterial, fungal and nematode
pathogens before they can infect the plant.
 Moisture also increases the succulence of host plants and thus their
susceptibility to certain pathogens, which affects the extent and severity of
disease.
Effect of rainfall
 The occurrence of many diseases in a particular region is closely correlated
with the amount and distribution of rainfall within year.
 Late blight of potato, apple scab, downy mildew of grapes and fire blight are
found or are severe only in areas with high rainfall or high relative humidity
during the growing season.
 In powdery mildews, spore germination and infection are actually lower in the
presence of free moisture on the plant surface than they are in its absence.

Effect of soil moisture
 Soil moisture influences the initiation and development of infectious
plant diseases.
 High or low soil moisture may be a limiting factor in the development of
certain root rot diseases.
 High soil moisture levels favours development of destructive water
mould fungi, such as species of Aphanomyces, Pythium and
Phytophthora.
 Overwintering by decreasing oxygen and raising carbon-dioxide levels in
the soil makes roots more susceptible to root rotting organisms.
Effect of wind
 Most plant diseases that occurs in epidemic portions and spread in large
areas are caused by fungi, bacteria and viruses that are spread either
directly by wind or indirectly by insects which can travel long distances
with the wind.
 Uredospores and many conidia are transported to many kilometers by
wind.
 Wind becomes more important when it is accompanied by rain.
 Wind blown rain splashes can help in spread of bacteria from the infected
tissues.

Effect of light
 Light intensity and duration may either increase or decrease the
susceptibility of plants to infection and also the severity of disease.
 Light mainly cause production of etiolated plants due to reduced light
intensity which in turn increases the susceptibility of plants to non-
obligate parasites but decreases the susceptibility of plants to obligate
parasites.
 It also enhances the plants’susceptibility to viral infections.
Effect of soil pH
 Soil pH is a measure of acidity or alkalinity and it markedly influences
occurrence of soil borne pathogens.
 Growth of potato scab (Streptomyces scabies) pathogen is suppressed at
a pH of 5.2 or slightly below but is more severe at a pH 5.2 to 8.0 or
above.
Effect of soil type
 Certain pathogens are favored by loam soils and others by clay soils.
 Nematodes are also most damaging in lighter soils that warm up
quickly.

In an infectious disease there is a series of more or less distinct events
which occur in sequence and lead to development and perpetuation of
the disease and the pathogen. This chain of events is called the disease
cycle.
The main events of a disease cycle include:
i. Inoculation
ii. Pre-penetration
iii. Penetration
iv. Infection(including invasion)
v. Growth & reproduction of the pathogen
vi. Dissemination of the pathogen and
vii. Seasonal carry-over of the pathogen.
STAGES IN DISEASE DEVELOPMENT

Before a pathogen can infect a plant, it must be introduced to the plant. This
process is called inoculation.
The part of the pathogen, coming in contact with the host to cause infection
is called inoculum. Mostly carried by air, wind & water.
INOCULATION
Types of Inoculum:
1. Primary Inoculum: It is the one that survives
during unfavourable conditions and causes
original infection or primary infection.
2. Secondary inoculum: Produced from primary
inoculum and causes secondary infection.
The inoculum exists in the form of virus particles, bacterial cells,
active mycelium (Rhizoctonia), dorment mycelium (Ustilago
tritici) & various kinds of spores.

The inoculam may cause successful infection only when the
following conditions are fulfilled:
1. The holt should be susceptible and have disease proneness.
2. The pathogen should be aggressive, capable of fast multiplication
and have proper inoculum potential.
3. The environmental conditions should be favourable for the
pathogen but not for the host.
Inoculum Potential:
The term was coined by Horsfall(1932) and defined as “the
number of infecting particles present in the environment of the
infected host.
Inculum potential is the most important deciding factor to
ensure successful infection.

This stage includes the interaction of host and pathogen before
penetration of the pathogen into the host.
 In case of virus and bacteria where the whole pathogen as a whole acts
as an inoculum, there is no pre-propagation stage.
 The fungal pathogen produces various kinds of inoculum such as
dormant mycelium, active mycelium, sclerotia, conidia, zoospores,
oidia, sporengiospores, chlamydospores, ascospores,
basidiospores, uredospores, aeciospores, etc.
 Thus the penetration stage is important for fungal pathogens.
 In fungi the pre-penetration stage involves two stages:-
(1) Spore germination
(2) Growth of germ tube
PRE-PENETRATION

(1)Germination of Propagules:
 The fungal propgules are different types of spores, dormant & active
mycelium, sclerotia etc.
 Germination of propagules on the host surface involves a change from resting
or low metabolic stage to high metabolic active stage.
 Some propagules are thin walled & store less food material. They germinate
without resting period.
 Others are thick walled & store more food material. They can survive under
unfavourable conditions &germinate after resting period.
 Environmental factors such as temp., moisture, light etc effects germination.
 The microorganisms present around & chemicals secreted in the rhizosphere
& phyllosphere also influence the germination by antagonism, competition
& microparasitism.
Examples : The root exudate of turnip stimulates the germination of oospores
of Pythium manitatum

(2)Growth of germ tube:
 Most of the fungal propagules germinates to produce germ tubres.
The germ tube elongates over the surface
of host and either enters directly into the host
or prolifarets to produce mycelial hyphae.
In case of Rhizoctonia solanu, the hyphae
grow on the host surface to form infection
cushion, probably to achieve proper
numerical & chemical stregth to cause
infection.
In case the germ tube enters into the host,
the tip of germ tube usually forms an
appresorium from which the infection
threads arise and enter into the host.
Growth of germ tube and appresorium are influenced by the environmental
factors, type of plant surface, chemical substances secreted by the plant organs
and microorganisms of rhizosphere & phyllosphere.
Image source: Plant pathology by SN Shrivastava

Actual entry of the pathogen into the host is called penetration. Under
favourable environment the germ tube or any propagule enters the
susceptible host either by:
(1) Direct penetration or
(2) Indirect penetration
(a)Indirect penetration through natural openings &
(b) Indirect penetration through wounds.
(1)Direct penetration:
Direct penetrration of pathogen occurs through the
cuticularized surface of epidermis or uncutinized surfaces of root
tips, root hairs, buds,seedlings, stigma, anther, nectaries etc.
In this process the pathogen has to cross morphological,
biological & chemical barriers.
PENETRATION

 The barriers are so strong that the viruses can not cross them due to lack of physical
force and enzyme system of their own.
 The bacteria also lack physical forces to break mechanical barrier. They penetrate
directly only by their chemical action.
 The nematodes and fungi possess both the physical force as well as the capacity to
cross the physical barrier.
 Fungal pathogens penetrates directly through cuticularized epidermal wall by applying
physical force and chemical actions.
 The germ tube of fungal pathogen develops penetration hypha or appressorium. Which
exerts a strong mechanical pushing on the intact wall.
 The pressure results stretching of the epidermis which becomes thin.
 The infection peg pierces the delicate thin area. After passing through the cuticle, the
infection hypha comes in contact with the pectin substances & cellulosic wall.
 These are penetrated by the enzymatic action of the infection hypha so that the cell wall
becomes softened &the penetration is done.

Examples: Erysiphe graminis causing powdery mildaw of wheat,
Phytophthora infestans causing late blight of potato, Puccinia graminis of
barberry.
Figure: A-E, Direct penetration by germinating conidium of Erysiphe sp.

(2) Indirect penetration:
It occurs through natural openings such as stomata, lenticels, hydathodes,
etc. or through artificial openings such as wounds, insect punctures, etc.
(a) Indirect penetration through natural openings:
Through stomata: Usually the fungi and bacteria enter through stomata.
 The penetration through stomata requires the presence of a film of water which
extends from the exterior through stomatal apurture to the substomatal chamber.
 While entering through stoma, protoplasm of the germ tube migrates to the
appresorium which develops above the stomatal pore.
 Then the appresorium gives out an infection thread which passes through the
aperture into the stomatal cavity.
 The protoplasm of the appresorium migrates into the tip of the infection
threadwhich swells to form a sub-stomatal vesicle.
 The empty germ tube & appresorium are cut off by the formation of septa.
 Then the vesicle sends out thread like hyphae which make physical contact with the
living cells of host.

Examples: Uredospores of Puccinia graminis tritici causing black rust of wheat.
Through lenticles: Streptomyces scabies causing scab of potato.
Through hydathodes: Erwinia amylovora & Xanthomonas campestris
entersthrough hydathodes.
Through nectarthodes: Erwinia amylovora enters through nectaries of
apple and pear flowers.
Figure: A-C, stomatal penetration by germinating uredospores of Puccinia sp.

(b) Indirect penetration through natural wounds:
Many types of wounds occur on the surface of plant parts. They are
caused by worker’s hands, tools, by animals, strong winds, storms, hails,
friction between plant parts, etc.
Examples: Stomatal penetration by the bacteria Erwinia amylovora ,
the causal organism of fire blight of apple.
Figure: Stomatal penetration by the bacteria Erwinia amylovora

After successful penetration, the pathogen proceeds further to establish proper
infection. This stage is called invasion.
Different types of pathogens invade the host in different ways and to different extents.
An infection can be local infection(localized)or it may infect the entire plant body
(systemic). These infections can be categorized as follows:
1. Ectoparasites (Ectotrophic)
2. Endoperasite with extreme mycelium.
3. Subcuticular parasites
4. Parasites in parenchyma
5. Parasites in vascular tissues
6. Endobiotic
7. Systemic infection
After successful infection, the pathogen derives its nourishment from the host. Viruses
do not require nutrients from hosts.
INVASION

 After entering the host cell, the pathogen faces multiple defence mechanisms of
host and once it gets established the growth phase of the disease cycle initiates.
 After successful establishment the pathogen derives nourishment from the host,
starts replication and secretes some chemical substances substances to express the
symptoms of the disease.
 Viruses enter the host cell protoplasm directly, disturb the normal functioning of
host cell nucleus and multiply them selves within the host.
 Bacteria and fungi require nutrition from host for their growth and reproduction.
 The fungal pathogens may colonize in the intercellular spaces of host tissues and
obtain their nourishment through the membrane of adjacent cells.
 Facultative saprophyte or facultative parasites produces haustoria, which absorbs
nutrients from the host and supply the same to the main body of the pathogen.
GROWTH & REPRODUCTION

Pathogens are disseminated by several ways:
 By air: While airborne spores touch wet surfaces get trapped &
air movement stops. From the air brought down by rain drops
 By wind: Winds also helps spreading by blowing away rain
splash droplets containing pathogens
 By water: Pathogens in the soil disseminate by rain, irrigation
water that moves on the surface through soil caries spores.
 By Insects: Nematodes & other Vectors; transmitted by insects
during feeding, movement from plant to plant. Nematodes can
also transmit virus internally
DISSEMINATION OF PATHOGEN

The establishment of pathogen in the host involves a close biochemical relationship
between two.It is accomplished in two ways
(1) By the destruction of host cells and tissues and/or
(2) By nutritional relationship.
 Almost all the stages of pathogenesis involve chemical interaction of pathogen
with the host.
 The pathogen brings several morphological, anatomical & physiological
disturbances in the host tissues mainly by secreting several injurious metabolic
products.
 These products may be of 3 main types
(1) Enzymes
(2) Toxins and
(3) Growth regulating substances
ROLE OF ENZYMES, TOXINS & GROWTH
REGUKATORS IN PATHOGENESIS

(I) ROLE OF ENZYMES
The most important chemical weapon which a pathogen(except
virus) possess are the hydrolytic enzymes which are secreted extracellularly
to disintegrate the protective layers of the host.
The various enzymes secreted by pathogens which degrade the cell
wall components of host are:
Cutin: Chemically it is 1-4 β N-acetyl glucosamine units. Two kinds of
cutin dissolving enzymes are cutin esterase & carboxycutin-peroxidase.
Example: Edwards & Allen(1970), the cutin degrading enzymes are
secreted from the tip of the infection peg by Erysiphe graminis f. sp.
hordei causing powdery mildew of barly.
Lignin: Lignin decomposing enzymes such as transmethylase are
produced by wood rotting fungi . This enzyme removes methoxy groups
and modifies lignin for further degradation.

Suberin: Enzymatic degradation of suberin is caused by fungal
pathogen attacking bark of trees.
Examples: In Armillarea mellea growing on bark of Barchystegia
spiciformis.
Pectin substances: There are 2 main groups of enzymes responsible
for degradation of pectin substances-
1. Pectin esterases: Hydrolyse & remove methyl group from pectin to
form pectic acid.
2. Chain splitting enzymes (Glycosidases & Transeliminases)
Cellulose: There are 2 groups of cellulolytic enzymes-
1. C1 cellulose enzymes: acts upon native cellulose & degrade its
crystalline structure to form reactive cellulose.
2. Cx endo gluconases: Acts upon the reactive cellulose &
hydrolytically claved to cellobiose.

(II) ROLE OF TOXINS
Toxins are metabolites that are produced by invading
microorganisms and act directly on living host protoplast, seriously
damaging or killing the cells of the plant.
Some toxins act as a general protoplasmic poisons and affect many
species of plant representing different families.
Toxins are of two types based on their site of synthesis and release
A. Exotoxins: Secreted externally by pathogens.
B. Endotoxins: Secreted inside pathogen, liberated after death
Others are toxic to only a few plant species or varieties and are completely
harmless to others.
Many toxins exist in multiple forms that have different potency.
A. Non-host specific toxin
B. Host specific toxin

A. NON-HOST SPECIFIN TOXINS
Tab-toxin: Its produced by the bacterium Pseudomonas syringae pv
tabaci which causes the wildfire disease of tobacco, by strain of pv tabaci
occurring on other hosts such as bean and soybean and by other pathovars of
P. syringae such as those occurring on oats maize and coffee.
Phaseolo-toxin: Its produced by the bacterium Pseudomonas syringae,
the cause of halo blight of bean and some other legumes.
Ten-toxin: Its produced by the fungus Alternaria alternata which causes
spots and chlorosis in plants by many species.
Cercosporinits: produced by the fungus Cercospora and by several
other fungi. It causes damaging leaf spot and blight diseases of many crop
plants such as Cercospora leaf spot of Zinnia and gray leaf spot of corn.

B. HOST SPECIFIC OR HOST_SENSITIVE
TOXINS
Victorin or HV toxin: Its produced by the fungus Cochliobolus
Victoriae. This fungus 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.
T-toxin: T-toxin is produced by race T of Cochliobolus
heterostrophus, the cause of southern corn leaf blight. Race T is
indistinguishable from other all other C. heterostrophus races except for
its ability to produce the T toxin.
HC-toxin: It is produced by Race 1 of C. carbonum causing
northern leaf spot and ear rot disease in maize.

(III) ROLE OF GROWTH REGULATORS
Auxins: It occurs naturally in plants as indole-3-acetic acid (IAA).
Increased IAA levels occur in many plants infected by different pathogens,
although some pathogens seem to lower the auxin level of the host e.g
Exobasidium azalea causing azalea and flower gall, Ustilago maydis
causative organism of corn smut.
Cytokinins: Cytokinin activity increases in clubroot galls, in smut and
rust infected bean leaves. it is partly responsible for several bacterial galls of
leafy gall disease of sweet pea caused by bacterium Rhodococus fasciens.
Gibberellins: High level of gibberellins due to foolish seedling diseases of
rice, in which rice seedlings infected with the fungus Gibberella fujikuroi grow
rapidly and become taller than healthy plants.
Ethylene : In the fruit of banana infected with Ralstonia solanacearum,
the ethylene content increases proportionately with the (premature) yellowing
of the fruits, whereas no ethylene can be detected in the healthy fruits.

CONCLUSION
To develop any disease the concept of disease triangle must be fulfilled i.e. a
virulent pathogen, a susceptible host and a favourable environment.
The success of a pathogen to cause a disease is primarily dependent upon the
environmental factors such as temperature, humidity, rainfall, etc.
A pathogen must have to follow the disease cycle to success(Only viruses skips
the pre-penetration steps):
a) Inoculation
b) Pre-penetration
c) Penetration
d) Infection(including invasion)
e) Growth & reproduction of the pathogen
f) Dissemination of the pathogen and
g) Seasonal carry-over of the pathogen
 Every host-pathogen interaction have to possess the required enzymes, toxins
and growth regulators to develop a successful disease.

REFERENCES
Agrios, G. N. (2005), “Plant Pathology” (5th edition), Elsevier-Academic Press San Diego,
CA.
Burkholder WH (1948). "Bacteria as plant pathogens“, Annual Review of Microbiology. 2 (1
vol.): 389–412. doi:10.1146/annurev.mi.02.100148.002133. PMID 18104350.
B.P. Pandey (2010), “College Botany”, Vol.-1(Revised Edition); S. Chand & Company Ltd.,
New Delhi, ISBN-8121905931; Page: 268-273.
Francl, L.J. (2001), “The Disease Triangle: A plant pathological paradigm revisited”, The
Plant Health Instructor. DOI: 10.1094/PHI-T-2001-0517-01.
 H N Shrivastava (2002), Plant Pathology, ISBN-1234567144087; Pages: 54-68.
Hait, Bhattacharya & Ghosh (2007), “A Textbook of Botany”, Vol-1(Revised Edition), New
Central Book Agency (P) Ltd., London; ISBN- 9788173815478, Page:419-449.
J.N. Mitra et al. (2013), “Studies in Botany”, Vol-1(Revised Edition); Moulik Library,
Culcutta; ISBN-9789381676011; Page: 833-853.
L. N. Nair (2000), “Vistas in Mycology & Plant Pathology”; ISBN-9788171696253; Page:
310-353.

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Disease development: Role of enzymes, toxins and growth regulators.