AN ASSIGNMENT

ON BIO - WARFARE DURING HOST - PATHOGEN INTERACTIONS IN INDIGENOUS CROP PLANTS

Course No.: P.Path.- 501 Course Title: Plant Pathogenesis and Genetics of plant pathogens

SUBMITED TO SUBMITED BY

DEPARTMENT OF PLANT PATHOLOGY BANGLADESH AGRICULTURAL UNIVERSITY

MYMENSINGH

Dr. A. Q. M. Bazlur Rashid

Professor

Department of Plant Pathology

Bangladesh Agricultural

University

Mymensingh

Md. Kamaruzzaman ID No. 11 Ag.P.Path. JJ 07 M Reg. No. 33141 Department of Plant Pathology

Bangladesh Agricultural University

Mymensingh

Ph.- +8801722449614

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Abstract

Plants represent a rich source of nutrients for many organisms including bacteria, fungi,

protists, insects, and vertebrates. Although lacking an immune system comparable to

animals, plants have developed a stunning array of structural, chemical, and protein-based

defenses designed to detect invading organisms and stop them before they are able to

cause extensive damage. Humans depend almost exclusively on plants for food, and

plants provide many important non-food products including wood, dyes, textiles,

medicines, cosmetics, soaps, rubber, plastics, inks, and industrial chemicals.

Understanding how plants defend themselves from pathogens and herbivores is essential

in order to protect our food supply and develop highly disease-resistant plant species.

Plant diseases caused by fungi and oomycetes result in significant economic losses every

year. Although phylogenetically distant, the infection processes by these organisms share

many common features. These include dispersal of an infectious particle, host adhesion,

recognition, penetration, invasive growth, and lesion development. Bacteria pathogenic

for plants are responsible for devastating losses in agriculture. The use of antibiotics to

control such infections is restricted in many countries due to worries over the evolution

and transmission of antibiotic resistance. The advent of genome sequencing has enabled a

better understanding, at the molecular level, of the strategies and mechanisms of

pathogenesis, evolution of resistance to plant defence mechanisms, and the conversion of

non-pathogenic into pathogenic bacteria.

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Introduction:

A pathogen is a microorganism that is able to cause disease in a plant, animal or insect.

Pathogenicity is the ability to produce disease in a host organism. Microbes express their

pathogenicity by means of their virulence, a term which refers to the degree of

pathogenicity. Hence, the determinants of virulence of a pathogen are any of its genetic or

biochemical or structural features that enable it to produce disease in a host.

The relationship between a host and a pathogen is dynamic, since each modifies the

activities and functions of the other. The outcome of such a relationship depends on the

virulence of the pathogen and the relative degree of resistance or susceptibility of the

host, due mainly to the effectiveness of the host defense mechanisms.

Plant-interacting micro-organisms can establish either mutualistic or pathogenic

associations. Although the outcome is completely different, common molecular

mechanisms that mediate communication between the interacting partners seem to be

involved. Specifically, nitrogen-fixing bacterial symbiosis of legume plants, collectively

termed rhizobia, and phytopathogenic bacteria have adopted similar strategies and genetic

traits to colonize, invade and establish a chronic infection in the plant host. Quorum-

sensing signals and identical two-component regulatory systems are used by these

bacteria to coordinate, in a cell density-dependent manner or in response to changing

environmental conditions, the expression of important factors for host colonization and

infection. The success of invasion and survival within the host also requires that rhizobia

and pathogens suppress and/or overcome plant defense responses triggered after

microbial recognition, a process in which surface polysaccharides, antioxidant systems,

ethylene biosynthesis inhibitors and virulence genes are involved.

In view of the above facts, the present study was undertaken to achieve the following

objectives –

1. To know about host- pathogen interaction.

2. To get knowledge about bio-warfare and penetration mechanisms.

3. To know about various weapons of fungi, bacteria, viruses and nematodes.

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Plant pathogens:

Plant pathology (also phytopathology) is the scientific study of plant diseases caused by

pathogens (infectious diseases) and environmental conditions (physiological factors).

Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses,

viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants.

Not included are ectoparasites like insects, mites, vertebrate or other pests that affect

plant health by consumption of plant tissues. Plant pathology also involves the study of

pathogen identification, disease etiology, disease cycles, economic impact, plant disease

epidemiology, plant disease resistance, how plant diseases affect humans and animals,

pathosystem genetics, and management of plant diseases. On the other hand, plant

pathogen is an organism that causes a disease on a plant. Although relatives of some plant

pathogens are human or animal pathogens, most plant pathogens only harm plants. Some

plant pathogens make immuno-depressed people also sick. Organisms that cause plant

diseases reduce our ability to produce food and support the economy. All plants from

citrus to grains to ornamental plants are susceptible to plant diseases. Plant diseases cause

billions of dollars’ worth of direct and indirect losses every year (Citrus greening

example). Emerging plant pathogens require preparation and planned, scientifically-based

response to lessen the impact on our farmers and the economy. Management of plant

diseases includes management of overall plant health. Healthy plants are less likely to get

diseases, just like healthy humans. You can help reduce the impact of both emerging and

endemic plant pathogens by remembering not to transport unhealthy plant parts or

products. An endemic pathogen is one that has become established in a new environment

and can no longer be eradicated. At that point, response switches from keeping it out or

eradicating it to managing it through plant health, antimicrobial chemistries and

monitoring production. The major pathogen of plant are as follows-

Fungi:

The majority of phytopathogenic fungi belong to the Ascomycetes and the

Basidiomycetes. The fungi reproduce both sexually and asexually via the production of

spores and other structures. Spores may be spread long distances by air or water, or they

may be soil borne. Many soil inhabiting fungi are capable of living saprotrophically,

carrying out the part of their lifecycle in the soil. These are known as facultative

saprotrophs. Fungal diseases may be controlled through the use of fungicides and other

agriculture practices, however new races of fungi often evolve that are resistant to various

fungicides. A successful infection requires the establishment of a parasitic relationship

between the pathogen and the host, once the host has gained entry to the plant. There are

two broad categories of pathogens –

Biotrophs:

Those that establish an infection in living tissue. Biotrophs are less dangerous than

necrotrophs.

Necrotrophs:

Those that kill cells before colonising them, by secreting toxins that diffuse ahead of the

advancing pathogen.

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These two kinds of pathogens are also sometimes known as 'sneaks' and 'thugs', because

of the tactics they use to acquire nutrients from their hosts.

Hemibiotrophic:

Hemibiotrophic begin frist biotrophic phase, then necrotrophic, intermediate host

range e.g., Phytophthora (potato blight disease)

Bacteria:

Bacteria are microscopic, single-celled prokaryotic organisms, without a defined

nucleus, that reproduce asexually by binary fission (one cell splitting into two). They

occur singly or in colonies of cells. Bacteria are classified into two main groups based on

cell wall structure, which can be determined by a simple staining procedure called the

Gram stain. Gram negative bacteria stain red or pink and Gram positive bacteria stain

purple. The difference in color is directly related to the chemical composition and

structure of their cell walls. The cells can be rod-shaped, spherical, spiral-shaped, or

filamentous. Only a few of the latter are known to cause diseases in plants. Most bacteria

are motile and have whip-like flagella that propel them through films of water.. Most

plant pathogenic bacteria are rod shaped (bacilli).

Fig. Crown gall disease caused by Agrobacterium Fig. Infected stem

Fig. Powdery mildew (Biotrophic) & Rice blast (necrotrophic)

fungus

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Viruses:

Plant viruses are pathogens which are composed mainly of a nucleic acid (genome)

normally surrounded by a protein shell (coat); they replicate only in compatible cells,

usually with the induction of symptoms in the affected plant. Viroids are among the

smallest infections agents known. Their circular, single-stranded ribonucleic acid (RNA)

molecule is less than one-tenth the size of the smallest viruses. There are many types of

plant virus, and some are even asymptomatic. Normally plant viruses only cause a loss of

crop yield. Therefore it is not economically viable to try to control them, the exception

being when they infect perennial species, such as fruit trees.Most plant viruses have

small, single stranded RNA genomesPlant viruses must be transmitted from plant to plant

by a vector. This is often by an insect (for example, aphids), but some fungi, nematodes

and protozoa have been shown to be viral vectors.

Nematodes:

Nematodes are small, multicellular wormlike creatures. Many live freely in the soil, but

there are some species which parasitize plant roots. They are a problem in tropical and

subtropical regions of the world, where they may infect crops. Potato cyst nematodes

(Globodera pallida and G. rostochiensis) are widely distributed in Europe and North and

South America and cause $300 million worth of damage in Europe every year. Root knot

nematodes have quite a large host range, whereas cyst nematodes tend to only be able to

infect a few species. Nematodes are able to cause radical changes in root cells in order to

facilitate their lifestyle.

Fig. Tobacco mosaic virus

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Protozoa:

There are a few examples of plant diseases caused by protozoa. They are transmitted as

zoospores which are very durable, and may be able to survive in a resting state in the soil

for many years. They have also been shown to transmit plant viruses. When the motile

zoospores come into contact with a root hair they produce a plasmodium and invade the

roots.

Fig. Leishmania donovani, (a species of protozoa) in a bone marrow cell

Parasitic plants:

Parasitic plants such as mistletoe and dodder are included in the study of phytopathology.

Dodder, for example, is used as a conduit for the transmission of viruses or virus-like

agents from a host plant to either a plant that is not typically a host or for an agent that is

not graft-transmissible.

Fig. Cuscuta europaea on Sambucus ebulus

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Economic significance:

Plant diseases caused by fungi and oomycetes result in significant economic losses every

year. Although phylogenetically distant, the infection processes by these organisms share

many common features. These include dispersal of an infectious particle, host adhesion,

recognition, penetration, invasive growth, and lesion development. Diseases are important

to humans because they cause damage to plants and plant products, commonly with an

associated economic effect, either positive or negative. Negative economic effects include

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Germination failure: Basic and primary loss of crop production due to seed

borne pathogen. In diseased seed, pathogen kills the sprouting plumule in the

seed. When farmer sow the seed under favorable condition, it germinates within

24 hours. On the same time pathogen germinate rather than more quickly. This

pathogenic inoculum inhibit the germination ability of the seedlings.

Seedling diseases: In case of infected seed, when it germinate some seedlings are

found healthy. But after some days this seedlings are infected by diseases.

Infected seeds encourage different pathogen to make diseases. This is the major

reason for destruction of seedlings but sometimes this infection symptom does not

appeared immediately in case of seed borne diseases.

Adult plant infection: Adult plant are infected by the seed borne pathogen and

almost all times yields are drastically reduced. In rice some pathogens like

Pyricularia, Drechslera, Xanthomonas etc. which causes serious yield loss in other

rice grown areas are common in Bangladesh and assumed to cause severe damage

to the crop here also. Ahmed (1968) reported that stem rot alone damages 5 lakh

bales of jute fibers. The value of five lakh bales of jute is TK. 800 million

approximately at present world market.

crop failure: Diseases are responsible for the destruction of crop losses, in this

why this is very important to know about all of the responsible causes responsible

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for diseases. Ultimate goal of all kinds of diseases are lowering of diseases and we

should try to control the pathogenicity of the pathogens.

Incremental loss from lower quality or failure to meet market standards:

Diseases infestation results in contamination and loss of quantity in the crops; the

quality losses may be due to relation in nutritional value. Or in marketability

(lowering of grade). Loss is easily overlooked; this is the type of damage done to

stored grain. A more common loss of quantity is the effect of diseases on the

appearance of the crop, for example skeletonized or discolored vegetables or

other crops have a lower market value than intact ones.

Plant diseases are also responsible for the creation of new industries to develop control

methods. Newly developed pesticide, fungicide, bio control agent helps to provide

employment opportunity.

Description of various weapons of fungi, bacteria, viruses and nematodes used in the

bio-warfare:

Biological warfare (BW) also known as germ warfare are bacteria, viruses, fungi, or

biological toxins, used to kill or incapacitate humans, animals or plants as an act of war.

Biological weapons (often termed "bio-weapons" or "bio-agents") are living organisms or

replicating entities (viruses) that reproduce or replicate within their host victims.

Entomological (insect) warfare is also considered a type of BW. Here , the objectives of

pathogen to develop diseases at any cost and plant trying to kill / stop pathogenic

organisms.

Biological weapons may be employed in various ways to gain a strategic or tactical

advantage over an adversary, either by threat or by actual deployment. Like some of the

chemical weapons, biological weapons may also be useful as area denial weapons. These

agents may be lethal or non-lethal, and may be targeted against a single individual, a

group of people, or even an entire population. They may be developed, acquired,

stockpiled or deployed by nation states or by non-national groups

Fig. Logo of bio warfare

Description of various weapons:

Modified hypha: Hyphae may be modified in many different ways to serve specific functions. Some

parasitic fungi form haustoria that function in absorption within the host cells. The

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arbuscules of mutualistic mycorrhizal fungi serve a similar function in nutrient exchange,

so are important in assisting nutrient and water absorption by plants. Hyphae are found

enveloping the gonidia in lichens, making up a large part of their structure. In nematode-

trapping fungi, hyphae may be modified into trapping structures such as constricting rings

and adhesive nets. Mycelial cords can be formed to transfer nutrients over larger

distances.

Fig. Modified hypha penetrating host cell

Haustoria:

A specialized absorbing structure of a parasitic plant, such as the rootlike outgrowth of

the dodder, that obtains food from a host plant. In parasitic fungi, haustoria are

specialized hyphae that penetrate the cells of other organisms and absorb nutrients

directly from them. Fungi in all major divisions form haustoria. Haustoria take several

forms. Generally, on penetration, the fungus increases the surface area in contact with

host plasma membrane releasing enzymes that break down the cell wall, enabling greater

potential movement of organic carbon from host to fungus. Thus, an insect host a

parasitic fungus such as Cordyceps may look as though it is being "eaten from the inside

out" as the haustoria expand inside of it.

Fig. Haustoria with conidium

Appressorium:

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An appressorium is a flattened, hyphal "pressing" organ, from which a minute infection

peg grows and enters the host, using turgor pressure capable of punching through even

Mylar. Fungi that exhibit appressorial formation include the necrotroph Pyrenophora

teres.

Appressorium are the tube which enters the host, puts out branches between the cells of

the host, and forms a mycelial network within the invaded tissue. The germ tubes of some

fungi produce special pressing organs called appressoria, from which a microscopic,

needlelike peg presses against and punctures the epidermis of the host; after penetration, a

mycelium develops in the usual manner. Many parasitic fungi absorb nutrient through

appressoria from host body.

Capsules: The cell capsule is a very large structure of some prokaryotic cells, such as bacterial cells.

It is a layer that lies outside the cell wall of bacteria. It is a well-organized layer, not

easily washed off, and it can be the cause of various diseases. .some bacteria form an

organized glycocalyx called a capsule around their cell walls which increases the

virulence of the species. The capsule helps resist host defenses by interfering with

phagocytosis. If the human body produces antibodies against the capsule, this can allow

destruction of the bacteria by

phagocytosis.

Fig. Appressoria

Fig. Bacteria with capsule

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Stylet:

The stylet or stomatostyle, is the primitive mouth-parts of some nematodes and some

nemerteans. It actually presents as a hardened protrusible opening to the stomach.

The stylet is adapted for the piercing of cell walls, providing the operative organism with

access to the nutrients contained within the prey cell. All plant-parasitic nematodes have a

stylet or mouth-spear that is similar in structure and function to a hypodermic needle. The

nematode uses the stylet to puncture plant cells, and then inject digestive juices and ingest

plant fluids through it. All of the plant-parasitic nematodes that are important turfgrass

pests feed on roots.

Fig. stylet of nematode

Toxins:

Pathogens often benefit by producing toxins, which kill the tissue in advance of

enzymatic degradation. In many pathogens, particularly non-obligate pathogens, toxins

cause the majority of damage to the host

Enzymes Some of the pathogen Produce enzymes that break down key structural components of

plant cells and their walls by soft-rotting bacteria that degrade the pectin ayer that holds

plant cells together.

Mechanism of penetration & host tissue disintegration: Successful infection of a host plant by a pathogen involves the movement of the pathogen

toward the host, attachment of the pathogen to the plant surface, penetration of the host

by the pathogen, and the proliferation of the pathogen inside the host immediately

following entrance.

Microorganisms have various strategies to establish an infection in a host. Some micro-

organisms recognize molecules on the surface of the host cell, and use these as receptors.

The binding of bacteria or viruses to receptors brings the microorganism in close contact

with the host surface.

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Fig. The mode of penetration by pathogenic organisms

Fungi: Penetration of a host by an invading fungus gives rise to the potential establishment of

physiological contact between the two organisms. A fungus will usually use a

combination of methods to gain access to host tissue.

Physical mechanisms Natural Openings: Plants have several types of natural openings utilized by fungi. The

most common are stomates. Some fungi sense the location of openings by chemical or

thigmatropic (touch) stimuli. Other natural openings include lenticels (open pores on

woody stems).

2. Wounds: Damage to a plant surface may result from animal and insect activities,

environmental causes (e.g. hail), and mechanical injury (tree falling against stem,

pruning). These sites provide ideal penetration sites for some types of fungi.

3. Direct Penetration: After contact between a germ tube and the plant surface, the direct

penetration of plant cells requires a combination of mechanical force and enzymatic

Fig. open stomata

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softening of the cuticle. Mechanical force is often achieved by a bulbous appressorium

and penetration peg.

Fig. Direct penetration through penetration peg

Chemical Mechanisms

Enzymatic penetration of cell walls:

During germination and penetration, fungi generally secrete a mixture of hydrolytic

enzymes including cutinases, cellulases, pectinases, and proteases. Although these

enzymes are also required by saprophytes, their structures and biosynthetic regulation

may be adaptated to the specific needs of pathogens. For instance, different cutinase

isozymes are expressed during saprophytic and parasitic stages of Alternaria brassicicola.

Many fungal genes encoding various hydrolytic enzymes have been cloned. Usually,

however, the infection phenotype of gene disruption/replacement mutants does not differ

from wild-type. In particular, enzymatic degradation of cutin, the structural polymer of

the plant cuticle, has been postulated to be crucial for fungal pathogenicity and cutinase to

be a key player in the penetration process.

Bacteria

Bacteria are one of the many harmful germs throughout the body and in the environment.

Germs and bacteria are almost everywhere in the world. Bacteria gets into the body when

there is an open wound or an open patch of skin. When a knee is scraped and a person has

an open wound, it gives bacteria a chance to come directly into the body.

Methods by which bacteria cause disease-

Adhesion: Many bacteria must first bind to host cell surfaces. Many bacterial and host

molecules that are involved in the adhesion of bacteria to host cells have been identified.

Often, the host cell receptors for bacteria are essential proteins for other functions.

Colonization: Some virulent bacteria produce special proteins that allow them to

colonize parts of the host body. Helicobacter pylori is able to survive in the acidic

environment of the human stomach by producing the enzyme urease. Colonization of the

stomach lining by this bacterium can lead to Gastric ulcer and cancer. The virulence of

various strains of Helicobacter pylori tends to correlate with the level of production of

urease.

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Invasion: Some virulent bacteria produce proteins that either disrupt host cell

membranes or stimulate endocytosis into host cells. These virulence factors allow the

bacteria to enter host cells and facilitate entry into the body across epithelial tissue layers

at the body surface.

Immune response inhibitors: Many bacteria produce virulence factors that inhibit the

host's immune system defenses. For example, a common bacterial strategy is to produce

proteins that bind host antibodies. The polysaccharide capsule of Streptococcus

pneumoniae inhibits phagocytosis of the bacterium by host immune cells.

Toxins: Many virulence factors are proteins made by bacteria that poison host cells

and cause tissue damage. For example, there are many food poisoning toxins produced by

bacteria that can contaminate human foods. Some of these can remain in "spoiled" food

even after cooking and cause illness when the contaminated food is consumed. Some

bacterial toxins are chemically altered and inactivated by the heat of cooking.

Most bacterial pathogen damage host cell by in four main ways:

1. By using host's nutrients (mainly iron)

2. By causing direct damage in the immediate area of the invasion

3. By producing toxins that may be transported by blood and lymph to

damage sites far from the original invasion

4. By causing the host to react with a hypersensitivity reaction

Viruses Viruses require living cells for their replication. Some viruses, such as tobacco mosaic

virus (TMV) and cucumber mosaic virus, are found in many plant species.

The replication of a plant virus appears to proceed according to the following general

scheme: introduction of the virus to a plant through a wound, release of the nucleic acid

from the protein coat, association of viral RNA (or messenger RNA of DNA viruses) with

cellular ribosomes for its translation to the proteins required for virus synthesis,

replication of the nucleic acid and production of coat protein, and assembly of the nucleic

acid and coat protein to form complete virus particles.

Initially, most plant viruses multiply at the site of infection, giving rise to localized

symptoms such as necrotic spots on the leaves. Subsequently, the virus may be distributed

to all parts of the plant either by direct cell-to-cell spread or by the vascular system,

resulting in a systemic infection involving the whole plant. However, the problem these

viruses face in reinfection and recruitment of new cells is the same as they face initially -

how to cross the barrier of the plant cell wall. Plant cell walls necessarily contain

channels called plasmodesmata which allow plant cells to communicate with each other

and to pass metabolites between them.

Fig. Virus replication

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The replication of viroids is not clearly understood at present. Cell-to-cell spread of

viruses usually occurs, and eventually the virus spreads throughout the plant. In some

plants, the cells surrounding the initially infected cells die, and the virus usually does not

spread further.

Nematodes

Plant-parasitic nematodes have evolved diverse parasitic relationships with their host

plants to obtain nutrients that are necessary to support their development and

reproduction. There are different types of plant-parasitic nematodes with characteristic

patterns of plant infestation:

Ectoparasites feed on the outside of plant roots causing severe moisture stress and

a dramatic reduction in yield, e.g. sting, awl and stubby-root nematodes.

Endoparasites enter the plant roots and root hairs resulting in malformation and

yield reduction, e.g. reniform, cyst and root-knot nematodes

Fig. Endoparasitic nematode feed inside the host

Endoparasitic nematode species must penetrate host tissues directly, using

mechanical and/or biochemical methods. Secreted proteases from parasitic

nematodes appear to aid in the penetration and migration through animal tissues. For

plant-parasitic nematodes, a cell wall composed primarily of cellulose poses a

Fig. Ectoparasitic nematode feed outside the host

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formidable barrier to penetration. Thrusts of the nematode stylet combined with

esophageal gland secretions mediate penetration and migration through plant

tissues. Plant-parasitic nematodes possess an arsenal of hydrolytic enzymes for

digesting cell wall polymers. Genes encoding secreted cell-wall-modifying enzymes

have been localized to nematode esophageal gland cells including enzymes that

degrade the pectic polysaccharides (pectate lyases and polygalacturonases)

comprising the middle lamella between plant cells and enzymes that degrade the

cellulose (endoglucanases) and hemicellulose (xylanase) structural components of

the cell wall Interestingly, the cell-wall-modifying enzymes appear to be

active only in the subventral gland cells and, in the case of the cyst nematode

endoglucanases, they are only active during nematode migration within roots,

whereas plant endoglucanases upregulated in feeding sites probably modify

the walls comprising these specialized cells which named gall.

Disease development:

Pathogenicity is the ability of an organism to enter a host and cause disease. The degree

of pathogenicity, that is, the comparative ability to cause disease, is known as virulence.

The terms pathogenic and nonpathogenic refer to the relative virulence of the organism or

its ability to cause disease under certain conditions. This ability depends not only upon

the properties of the organism but also upon the ability of the host to defend itself (its

immunity) and prevent injury. The concept of pathogenicity and virulence has no

meaning without reference to a specific host.

While necrotrophs have little effect on plant physiology, since they kill host cells before

colonising them, biotrophic pathogens become incorporated into and subtly modify

various aspects of host physiology, such as respiration, photosynthesis, translocation,

transpiration and growth and development. The respiration rate of plants invariably

increases following infection by fungi, bacteria or viruses. The higher rate of glucose

catabolism causes a measurable increase in the temperature of infected leaves. An early

step in the plant's response to infection is an oxidative burst, which is manifested as a

rapid increase in oxygen consumption, and the release of reactive oxygen species, such as

hydrogen peroxide (H2O2) and the superoxide anion (O2-). The oxidative burst is involved

in a range of disease resistance and wound repair mechanisms Link to Rapid Active

Defense. In resistant plants, the increase in respiration and glucose catabolism is used to

produce defence-related metabolites via the pentose phosphate pathway. In susceptible

plants, the extra energy produced is used by the growing pathogen.

Pathogens also affect photosynthesis, both directly and indirectly. Pathogens that cause

defoliation rob the plant of photosynthetic tissue, while necrotrophs decrease the

photosynthetic rate by damaging chloroplasts and killing cells. Biotrophs affect

photosynthesis in varying degrees, depending on the severity of the infection. A

biotrophic infection site becomes a strong metabolic sink, changing the pattern of nutrient

translocation within the plant, and causing net influx of nutrients into infected leaves to

satisfy the demands of the pathogen. The depletion, diversion and retention of

photosynthetic products by the pathogen stunts plant growth, and further reduced the

plant's photosynthetic efficiency. In addition, pathogens affect water relations in the

plants they infect. Biotrophs have little effect on transpiration rate until sporulation

ruptures the cuticle, at which point the plant wilts rapidly. Pathogens that infect the roots

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directly affect the plant's ability to absorb water by killing the root system, thus producing

secondary symptoms such as wilting and defoliation. Pathogens of the vascular system

similarly affect water movement by blocking xylem vessels. Growth and development in

general are affected by pathogen infection, as a result of the changes in source-sink

patterns in the plant. Many pathogens disturb the hormone balance in plants by either

releasing plant hormones themselves, or by triggering an increase or a decrease in

synthesis or degradation of hormones in the plant. This can cause a variety of symptoms,

such as the formation of adventitious roots, gall development, and epinasty (the down-

turning of petioles).

Factors that affect disease development

Pathogen Host Environment

Presence of pathogen

Pathogenicity

Adaptability

Dispersal efficiency

Survival efficiency

Reproductive fitness

Susceptibility

Growth stage & form

Population density &

structure

General health

Temperature

Rainfall / Dew

Leaf wetness period

Soil properties

Wind

Fire history

Air pollution

Herbicide damage

Overall diseases development process includes major steps. They are as follows-

Enzymatic degradation:

In their most basic form, pathogens secrete enzymes, which catalyze the breakdown of

host tissues, similar to the digestion of food in mammals.

Toxins:

Pathogens often benefit by producing toxins, which kill the tissue in advance of

enzymatic degradation. In many pathogens, particularly non-obligate pathogens, toxins

cause the majority of damage to the host.

Growth regulators:

Pathogens often find it advantageous to produce growth regulators (or cause the host to

produce them). The most common are those that cause translocation of nutrients to host

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cells and/or cause host cells to enlarge or divide in the vicinity of the pathogen, thus

providing an increase in food for the pathogen. Obligate pathogens are very good at this

technique because it allows the host to go on living, but still provides extra food for the

pathogen.

Genetic manipulations:

All viruses plus a few bacteria are able to force the plant to produce pathogen gene

products from pathogen genetic material. This starves plant cells and disrupts their

function.

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Conclusion:

Most of the pathogens can only cause disease on a relatively small group of host plants

because of the slightly different set of specialized genes and molecular mechanisms

required for each host-pathogen interaction. On the other hand, against pathogenicity,

plant show resistance. The cell wall is a major line of defense against fungal and bacterial

pathogens. It provides an excellent structural barrier that also incorporates a wide variety

of chemical defenses that can be rapidly activated when the cell detects the presence of

potential pathogens. All plant cells have a primary cell wall, which provides structural

support and is essential for turgor pressure, and many also form a secondary cell wall that

develops inside of the primary cell wall after the cell stops growing.

Many cell walls also contain lignin, a heterogeneous polymer composed of phenolic

compounds that gives the cell rigidity. Lignin is the primary component of wood, and cell

walls that become “lignified” are highly impermeable to pathogens and difficult for small

insects to chew. Cutin, suberin, and waxes are fatty substances that may be deposited in

either primary or secondary cell walls (or both) and outer protective tissues of the plant

body, including bark. So that biological warfare conduct in indigenous crop plant with

respective wapons. Growth and development in general are affected by pathogen

infection, as a result of the changes in source-sink patterns in the plant. Many pathogens

disturb the hormone balance in plants by either releasing plant hormones themselves, or

by triggering an increase or a decrease in synthesis or degradation of hormones in the

plant. This can cause a variety of symptoms, such as the formation of adventitious roots

and gall development. Minimizing plant disease requires understanding the mechanisms of survival and spread.

A competitive exclusion mechanism by beneficial organism can be effective in protection

against disease. (Biological Control of Plant Pathogens).

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References:

American Phytopathological Society. 2003. Microbial genomic sequencing. Perspectives

of the American Phytopathological Society (revised 2003). 21 pp.

Anderson RV, Byers JR (1975) Ultrastructure of the esophageal procorpus in the plant

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