HOW PLANT DEFEND THEMSELVES AGAINST

PATHOGENS

Zohaib ul Hassan (Plant Pathology)

zuhaib1144@gmail.com

+92 343 6331144

University College of Agriculture, University of Sargodha

Sargodha, Pakistan

HOW PLANTS DEFEND THEMSELVES

AGAINST PATHOGENSPLANT:

Plants, also called green plants , are multi cellular, auto-

trophic eukaryotes of the kingdom Plantae.

MAJOR PLANT PATHOGENS:

A. Bacteria

B. Virus

C. Fungi

D. Nematodes

PLANT CAN DEFEND THEMSELVES

1. Structural characteristics

Physical barriers & inhibit the pathogen from gaining

entrance.

2. Bio-Chemical Reaction:

Different substances produce by plant cells or tissues.

(inhibit the growth of pathogen in it).

Plant resistance:

Generally controlled by the genes.

RESISTANCE

Ability of host/plant to overcome or exclude the attack of

pathogen.

Resistance is of different types:

1. Monogenic

2. Polygenic

3. Systemic acquired resistance

When pathogen attack on plant.

The genes of pathogen are activated (produce all their weapons of attack e.g. enzymes, toxins etc.)

To infect the plant.

Plant show resistance in different ways

Polygenic

Monogenic

Induced

NON-HOST RESISTANCE

Unsuccessful plant/pathogen interactions

Pathogen---- >Non-Host

e.g. Apple trees are not infected by tomato pathogens

POLYGENIC RESISTANCE

• Many genes include in such type of resistance.

Some plant genes code for the chemical substances.

Toxic to pathogens or neutralize the toxin of pathogen.

• Some structural genes also present which slow down the

entry of pathogen.

Plant defense (polygenic)

• Different chemical substances

• Pre-existing defense structures

• Chemicals and different defense structures made after

attack of pathogen or A-biotic stress.

Polygenic resistance

• Plant pathogen attack.

• Plant shows resistance partially or incompletely.

• Different genes are involved.

• Polygenic, quantitative, multi-genic, horizontal, durable,

field or minor gene resistance.

Monogenic, r-gene resistance

• Host Plant:

Have one resistance genes per pathogen capable of

attacking it.

• “each pathogen carries matching genes for

a-virulence (A) for each of the R genes of the host plant.”

Pathogen Host

A-virulence gene in pathogen

Trigger the R-gene in the host.

• Host will show the resistance

The Hypersensitive Response

• Localized induced cell defense.

• Limiting the growth of the pathogen.

• initiated by the recognition by the plant of specific

pathogen-produced signal molecules, known as elicitors.

Preexisting structures

• Waxes

• Cuticle

• Stomata

• Cell wall (toughness,hardness)

Preexisting chemical defenses

• Resistance of a plant against pathogen attacks also

depends on the substances produced in its cells before or

after infection.

• These chemicals inhibits the growth of pathogen/

suppress the pathogen infection.

• In some cases these chemicals check the disease ,when

physical barriers are absent.

Inhibitors released by plants in its

environment• Fungitoxic exudates:

• inhibit the germination of spores of fungi Botrytis

and Cercospora, respectively, in tomato.(leaves)

• onion smudge, caused by the fungus Colletotrichum

circinans

Reduced by phenolic compounds protocatechuic

acid and catechol.

Inhibitors Present in Plant Cells before

Infection• In young fruits, leaves and stems

• Several phenolic compounds,tannins, and some fatty

acid-like compounds such as dienes.

• Resistance to Botrytis spp.

• Also determine the host range of fungus e.g; the presence

or absence of saponin in a host and of saponinase in a

fungus determines the host range of the fungus.

DEFENSE THROUGH LACK OF

ESSENTIAL FACTORS• Lack of Recognition between Host and Pathogen

• recognition factors (specific molecules or structures)

that can be recognized by pathogen.

It is thought that Various types of oligosacchrides,

polysacchrides and proteins are involved in it. (but

actual factor is still unknown).

Lack of Host Receptors and Sensitive

Sites for Toxins• Pathogen (usually fungus produces) host-specific toxins.

• Specific toxins specific receptor (symptoms)

• Lack of host receptors and sensitive sites for toxins

No symptoms will produce

INDUCED STRUCTURAL AND

BIOCHEMICAL DEFENSES• Recognition of the Pathogen by the Host Plant

• Pathogen Elicitors

A. Non specific elicitors:

• ( toxins, glycoproteins, carbohydrates, fatty acids,

peptides and enzymes like proteases, pectnases)

B. Specific elicitor:

• (avr gene products, hrp gene products, and

suppressor molecules).

• Host receptors

Recognize the elicitor , which trigger its defense

mechanism.

• it induces the expression of all defense related

genes tested and also induced resistance.

INDUCED STRUCTURAL DEFENSES

• cytoplasmic defense reaction:

• Defense structures formed, involve the cytoplasm of the

plant.

• cell wall defense structures:

• Involve the walls of invaded cells.

• Histological defense structures:

• Deeper into the plant.

• necrotic or hypersensitive defense reaction:

Histological Defense Structures

• Formation of Cork Layers:

• Formation of Abscission Layers:

• Formation of Tyloses:

• Present in xylem. Tyloses have cellulosic walls and may,

by their size and numbers, clog the vessel

completely.

• Defense through the Hypersensitive Response:

• As soon as the pathogen establishes contact with the cell,

the nucleus moves toward the invading pathogen and

soon disintegrates.

IMMUNIZATION OF PLANTS AGAINST

PATHOGENS• Defense through Plantibodies:

• Such antibodies, encoded by animal genes but produced

in and by the plant, are called plantibodies.

• Mostly present in transgenic plants.

SYSTEMIC ACQUIRED RESISTANCE

• Generalized resistance in response to infection.

• By a pathogen or

• Treatment with certain natural or synthetic chemical

compounds.

• Systemic acquired resistance acts nonspecifically.

• Reduces the severity of disease.

Defense Mechanisms in plants