this about plant defense mechanism against plant pathogens...,

1. K. Ramalingam M.Sc., (Agri)
Asst. Professor (Plant Pathology)
RVS AGRICULTURAL COLLEGE
Usilampatti, Thanjavur, Tamil Nadu, India

2. RESISTANCE
The ability of an organism to exclude or overcome
completely or in some degree, the effect of a pathogen, or
other damaging factor.
Gene For Resistance
They posses gene for resistance (R gene) directed
against the gene for virulence of the pathogen.
TYPES
1. Horizontal resistance
2. Vertical resistance
INTRODUCTION

3. 1. Horizontal resistance
Such resistance is also called non-specific, general, quantitative,
adult plants, field or durable resistance but it is most commonly
known as horizontal resistance.
Horizontal resistance is controlled by many type of genes so it is
also called Polygenic resistance.
2. Vertical resistance
Such resistance is also called strong, major, specific, qualitative
or differential resistance but it is most commonly known as
vertical resistance.
Vertical resistance is always controlled by one gene so it is also
called monogenic resistance.

4. BiochemicalStructural
Defense
Pre-existing Induced Pre-existing Induced

5. STRUCTURAL DEFENSES
Pre-existing structural defense
1. Wax
2. Thick cuticle
3. Thickness and toughness of
the outer wall of epidermal
cells
4. Sclerenchyma cells
5. Natural openings
 Stomata
 Lenticels
 Hydathodes
 Nectaris
Induced structural defense
1.Cytoplasmic defense reaction
• Cell wall defense structure
2.Histological defense structure
• Formation of cork layer
• Formation of abscission layer
• Formation of tyloses
• Deposition of gums

6. BIOCHEMICAL DEFENSES
Pre-existing bio-chemical
defense
1.Inhibitors
 Released by plant in it’s
environment
 Present in plant cells before
infection
2.Phenolics
 Tannins
 Glucanases
 Dienes
 Chitinase
Induced bio-chemical defense
1.Hypersensitivity response
(HR)
2. Production of Antimicrobial
substances
• Pathogenesis- Related
Protein (PR Protein)
• Phytoalexins
• Systemic acquired
Resistance

7. PREEXISTING STRUCTURAL DEFENSE
Some structural defense are present in the plant even before the
pathogen comes in contact with the plant.
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1. WAXES
 Waxes is a general term used to the mixture of long-chain
apolar lipids forming a protective coating (cutin in the
cuticle) on plant leaves and fruits
Synthesized by epidermal cells
Extremely hydrophobic
Waxes form outer coating of the cuticle
Often crystallizes in pattern of rods, tubes or plates

8. 2. THICK CUTICLE
Plant cuticle are protective, hydrophobic, waxy covering
produced by the epidermal cells of leaves, young shoots and all
other aerial plant organ.
Thickness of cuticle e.g. in resistant flax Melampsora lini,
Barberry-Puccinia graminis tritici

9. 3. EPIDERMAL CELLS
The thickness and toughness of outer wall of
epidermal cells.
For example Barberry- Puccinia graminis tritici resistance.
4. STOMATA
• Size of stomata - Have small stomata Xanthomonas
campestris)
• Shape- narrow entrance and broad gaurd cells- Bacteria
• opening and closing (resistant wheat vars. – Rust).

10. 5. SCLERENCHYMA CELLS
Composed of walls thickened with lignin
mechanical support to plants
These cells effectively blocks pathogen - angular leaf spot
e.g.,wheat –resistant to stem rust-stem contain high
Sclerenchyma cells.
6. LENTICEL
Lenticels are airy cells in the bark of stem and roots
The shape and internal structure of lenticels can increase and
decrease the incidence of fruit disease
Lenticels of small size restrict entry of pathogen.

11. INDUCED STRUCTURAL DEFENSES
1. Cytoplasmic Defense Reaction
Some of the defense structures formed involve the cytoplasm of the cells
under attack, and the process is called cytoplasmic defense reaction.
Cell Wall Defense Structure
• Three main types of such structures have been observed in plant diseases.
• The outer layer of the cell wall of parenchyma cells coming in contact
with incompatible bacteria swells and produced an amorphous, fibrillar
material that surrounds and traps the bacteria and prevents them from
multiplying.
• Cell walls thicken in response to several pathogens by producing
what appears to be a cellulosic material.
• Callose papillae are deposited on the inner side of cell walls in response
to invasion by fungal pathogen.

12. 2 . Histological Defense
Structures
2.1. Formation Of Cork Layer
The formation of corky layer can limit the size of lesions, and
consequently the extent of damage that can be caused by a single
infection.
e.g. Cork layer Rhizoctonia solani canker in potato
Abscission layer Xanthomonas pruni shot hole
Provide protection by -
Inhibiting the further
spread of pathogen
Block the spread of
toxic substances of the
pathogen
stop the flow of
nutrients to infection
point

13. 2.2 Tyloses
Tyloses are overgrowth of living cells that protrude via pits into xylem
vessels blocking the vascular system.
eg. Vascular wilt diseases
Their formation is triggered by a “stress condition”.

14. 2.3 DEPOSITION OF GUM
 Various types of gums are produced by many plants
around lesions after infection by pathogen or injury.
 Gums secretion is most common in stone fruit trees
but occurs in most plants.
 E.g Phytophthora gummosis. On citrus plants
 Diplodia gummosis

15. 2.4 sheath formation
Hyphal tip of fungi penetrating the cell wall and growing
into the cell lumen are enveloped by cellulose material and
forming a sheath or lignintuber around the hyphae

16. 2.5 abscission layer formation
It consist of gap formed between two circular layers of
leaf cells sounding the locus infection.
Middle lamella dissolves

17. PREEXISTING CHEMICAL DEFENCE:
It is clear that the resistance of a plant against pathogen attack
depends not so much on its structural barriers as on the substances
produced in its cell before or after infection.
INHIBITORS RELEASED
ENVIRONMENT
Plants exude a variety of substances through the surface of their
above ground parts as well as through the surface of their roots.
 Fungitoxic exudates- on the leaves of some plants, e.g. tomato
and sugarbeet inhibit the germination of Botrytis and Cercospora
Oil capric acid on elm seed toxic to Ceratocystis ulmi spores
Phenolics: red scale onion against smudge –Colletotrichum
circinans
BY THE PLANT IN IT’S

18. Onion smudge
Susceptible
Resistant

19. INHIBITORS PRESENT IN PLANT CELLS BEFORE
INFECTION.
• Stored in vacuoles, lysogenous glands, heart wood periderm
of plants
Phenolics – onion (catechol and protocatechuic acid )
• Saponins (tomatine in tomato, avenacin in oats ) most of these
compounds inhibits hydrolytic enzymes produced by
pathogen. e.g. pectolytic enzymes (PME, PE, PGU)- Erwinia .
• Tannins, and some fatty acid-like compound such as dienes,
which are present in high concentrations in cells of young
fruits, leaves or seeds.
• Some plants also contains hydrolytic enzymes e.g.
glucanases, chitinases etc.

20. INDUCED CHEMICAL DEFENSE
1. HYPERSENSITIVE RESPONSE (HR)
Localized death of host cells at the site of infection.
It is the result of a specific recognition of a pathogen
attack by the host.
The HR is considered to be a type of programmed cell death.

21.  The hypersensitive response is the culumination of the plant
defense response initiated by:
 The recognition by the plant of specific pathogen-produced
signal molecules, known as elicitors.
 Recognition of the elicitors by the host plant activates a cascade
of biochemical reactions in the attacked and surrounding plant
cells, leads to new or altered cell functions and to new or greatly
activated defense- defense related compounds.
 The most common new cell functions and compound include:
 A rapid burst of reactive oxygen species, leading to a dramatic
increase of oxidative reaction.
 Increased ion movement, especially of K+ and H+ through the
cell membrane.

22. PRODUCTION OF ANTIMICROBIAL SUBSTANCES
PATHOGENESIS- RELATED PROTEINS (PRPROTEINS)
 Pathogenesis related proteins, called PR- proteins- A group of
plant coded proteins
 Are structurally diverse group toxic to invading pathogens.
 Produced under stress
 They are widely distributed in plants in trace amounts but are
produced in high concentration following pathogen attack or
stress.
 The PR proteins exist in plant cells intracellularly (acidic in
Apoplast & basic form in vacuoles) and also in the intercellular
spaces.
 Varying types of PR- proteins have been isolated from several
crop plants.

23. Groups of PR- proteins
 The better known PR protein are:
 PR- 1  ß-1,3-glucanases, chitinases, lysozymes
 PR 4 thaumatinelike proteins,
 PR 5- osmotin like proteins,
 PR 6 -cysteine-rich proteins, glycine-rich proteins,
 PR 7 - proteinase inhibitors,
 PR 8 - proteinases and
 PR 9 - peroxidases.
There are often numerous isoforms of each PR- protein
in various host plants.

24. PHYTOALEXINS
given by Borger & Muller in Potato- late blightConcept
interactions
Defined as antibiotics produced in plant- pathogen interaction or
as a response to injury or physiological stimuli
Paxton (1981) defined phytoalexins as low molecular weight
accumulates in plant cells after microbial infection.
Involves the role of elicitors in their production
Not produced during bio trophic infection.
Graminae,E.g. Leguminosae, Solanaceae, Malvaceae,
Compositae, Umbelliferae and Chenopodiaceae.
antimicrobial compounds which are synthesized by and

25.  SYSTEMIC ACQUIRED RESISTANCE (SAR)
 SAR confers broad-based resistance to different pathogens.
For example, primary inoculation with a fungal leaf spot
pathogen reduces susceptibility of the host plant to other fungi
as well as to bacterial and viral pathogens.
 Salicylic acid (chemical related to aspirin) is part of signaling
pathway involved in transmission of the defense response
throughout the plant to produce SAR. This has lead to the
development of synthetic chemicals that mimic the role of
salicylic acid.

26. Induction of Systemic Acquired Resistance
Production of H2O2 (plus antioxidants)
Hydrogen peroxide has been associated
secondary induction of SAR and direct toxic
activity to invading pathogens
Thickening of plant cell wall Production of
phenolic (lignin, tannic acid) that strengthen
walls and inhibit pathogen enzymes
Accumulation of pathogenesis related
proteins “PR-proteins” chitinases, ß-1,3
Glucanases.
These enzymes accumulate in vacuole of
plant cell. Upon attack, they directly degrade
fungal cell walls. Indirectly, their action
the release of fungal wall
that elicit additional defense
results in
components
reactions
Sprayed
inducer
(activator) that
mimics
salicyclic acid

27. CONCLUSION
 Under favourable condition the infection(susceptibility) or non-
infection (resistance) in each host-pathogen combination is
predetermined by the genetic material of the host and of the
pathogen.
 In some diseases however, particularly those caused by fungi,
such as potato late blight, apple scab, powdery mildews, tomato
leaf mould, and the cereal smuts and rust, and also in several
viral and bacterial disease of plants, considerable information
regarding the genetics of host-pathogen interaction is available.