This document summarizes plant resistance to pathogens and the mechanisms involved. It discusses two main types of resistance - horizontal (polygenic) and vertical (monogenic). It also describes various pre-existing and induced structural defenses plants employ, such as waxes, thickened cell walls, and formation of cork layers. Biochemical defenses include inhibitors, phenolics, phytoalexins, pathogenesis-related proteins, and systemic acquired resistance mediated by salicylic acid. Overall, the document provides an overview of genetic and physiological factors that determine a plant's ability to resist pathogens.
The concept of gene for gene hypothesis was first developed by Flor in 1956 based on his studies of host pathogen interaction in flax, for rust caused by Melampsora lini. The gene for gene hypothesis states that for each gene controlling resistance in the host, there is corresponding gene controlling pathogenicity in the pathogen. The resistance of host is governed by dominant genes and virulence of pathogen by recessive genes. The genotype of host and pathogen determine the disease reaction. When genes in host and pathogen match for all loci, then only the host will show susceptible reaction. If some gene loci remain unmatched, the host will show resistant reaction. Now gene – for –gene relationship has been reported in several other crops like potato, sorghum, wheat, etc. The gene for gene hypothesis is also known as “Flor Hypothesis.”
Molecular basis of plant resistance and defense responses to pathogensSenthil Natesan
In response to pathogen attack, plants have evolved sophisticated defense mechanisms to delay or arrest pathogen growth.Unlike animals, plants lack a circulating immune system recognizing microbial pathogens. Plant cells are more autonomous in their defense mechanisms and rely on the innate immune capacity of each cell and systemic signals that disseminate from infection sites (Jones and Dangl, 2006). Plant innate immunity consists of preformed physical and chemical barriers (such as leaf hairs, rigid cell walls, pre-existing antimicrobial compounds) and induced defenses. Should an invading microbe successfully breach the pre-formed barriers, it may be recognized by the plant, resulting in the activation of cellular defense responses that stop or restrict further development of the invader.
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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.
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.
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
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 blightConcept
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.