Page 1
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
0
Lesson Prepared Under MHRD project “National Mission on
Education Through ICT”
Discipline: Botany
Paper: Plant Metabolism
National Coordinator: Prof. S.C. Bhatla
Lesson: Plant Defense Mechanisms
Lesson Developer: Dr. Dhara Arora Research Scholar
Department/College: Department of Botany
University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla, Department of Botany
University of Delhi
Language Editor: Ami Choubey
Department/College: Department of Genetics, University
of Delhi, South Campus
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Page 2
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
0
Lesson Prepared Under MHRD project “National Mission on
Education Through ICT”
Discipline: Botany
Paper: Plant Metabolism
National Coordinator: Prof. S.C. Bhatla
Lesson: Plant Defense Mechanisms
Lesson Developer: Dr. Dhara Arora Research Scholar
Department/College: Department of Botany
University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla, Department of Botany
University of Delhi
Language Editor: Ami Choubey
Department/College: Department of Genetics, University
of Delhi, South Campus
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
1
Table of contents
Lesson: Plant defense mechanisms
? Introduction
? Various plant defense mechanisms
? Structural defenses- First line of defense
? Surface structures/ Special appendages- Thorns, spines,
prickles and trichomes
? Mineral crystals in specialised cells like idioblasts, sclereids,
crystalliferous cells and silica cells
? Cell wall
? Biochemical defences
? Constitutive
? Secondary metabolites
? Terpenoids
? Phenolics
? Alkaloids
? Induced
? Hypersensitive responses
? Systemic acquired resistance (SAR) and induced
systemic resistance (ISR)
? Jasmonic acid and ethylene have role in
plant defences against pathogen
infection
? Role of systemin against insect attack
? Role of various proteins and enzymes to counteract
pathogen attack
? Defensins
? Digestive enzyme inhibitors
? Proteinase inhibitors
? Hydrolytic enzymes/PR proteins
? Genetic basis of plant-pathogen interaction
? Summary
? Exercise/ Practice
? Glossary
Page 3
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
0
Lesson Prepared Under MHRD project “National Mission on
Education Through ICT”
Discipline: Botany
Paper: Plant Metabolism
National Coordinator: Prof. S.C. Bhatla
Lesson: Plant Defense Mechanisms
Lesson Developer: Dr. Dhara Arora Research Scholar
Department/College: Department of Botany
University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla, Department of Botany
University of Delhi
Language Editor: Ami Choubey
Department/College: Department of Genetics, University
of Delhi, South Campus
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
1
Table of contents
Lesson: Plant defense mechanisms
? Introduction
? Various plant defense mechanisms
? Structural defenses- First line of defense
? Surface structures/ Special appendages- Thorns, spines,
prickles and trichomes
? Mineral crystals in specialised cells like idioblasts, sclereids,
crystalliferous cells and silica cells
? Cell wall
? Biochemical defences
? Constitutive
? Secondary metabolites
? Terpenoids
? Phenolics
? Alkaloids
? Induced
? Hypersensitive responses
? Systemic acquired resistance (SAR) and induced
systemic resistance (ISR)
? Jasmonic acid and ethylene have role in
plant defences against pathogen
infection
? Role of systemin against insect attack
? Role of various proteins and enzymes to counteract
pathogen attack
? Defensins
? Digestive enzyme inhibitors
? Proteinase inhibitors
? Hydrolytic enzymes/PR proteins
? Genetic basis of plant-pathogen interaction
? Summary
? Exercise/ Practice
? Glossary
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
2
? References/ Bibliography/ Further Reading
Learning outcomes:
The students will be able to
? Understand how efficiently plants have evolved defence mechanisms
? Explain the various approaches by which plants protect themselves against
pathogens
? Understand the importance of certain plant hormones in the induction of plant
defense responses
? Know about PR proteins, the genes which encode them and the importance of PR
proteins in plant defense
? Explain how pathogens elicit systemic acquired plant defense responses
? Differentiate between systemic acquired resistance (SAR) and induced systemic
resistance (ISR)
? Understand the genetic basis of plant defense
? Explain the role of genes behind the plant-pathogen interactions
? Understand the concept of avirulence and virulence genes
Page 4
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
0
Lesson Prepared Under MHRD project “National Mission on
Education Through ICT”
Discipline: Botany
Paper: Plant Metabolism
National Coordinator: Prof. S.C. Bhatla
Lesson: Plant Defense Mechanisms
Lesson Developer: Dr. Dhara Arora Research Scholar
Department/College: Department of Botany
University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla, Department of Botany
University of Delhi
Language Editor: Ami Choubey
Department/College: Department of Genetics, University
of Delhi, South Campus
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
1
Table of contents
Lesson: Plant defense mechanisms
? Introduction
? Various plant defense mechanisms
? Structural defenses- First line of defense
? Surface structures/ Special appendages- Thorns, spines,
prickles and trichomes
? Mineral crystals in specialised cells like idioblasts, sclereids,
crystalliferous cells and silica cells
? Cell wall
? Biochemical defences
? Constitutive
? Secondary metabolites
? Terpenoids
? Phenolics
? Alkaloids
? Induced
? Hypersensitive responses
? Systemic acquired resistance (SAR) and induced
systemic resistance (ISR)
? Jasmonic acid and ethylene have role in
plant defences against pathogen
infection
? Role of systemin against insect attack
? Role of various proteins and enzymes to counteract
pathogen attack
? Defensins
? Digestive enzyme inhibitors
? Proteinase inhibitors
? Hydrolytic enzymes/PR proteins
? Genetic basis of plant-pathogen interaction
? Summary
? Exercise/ Practice
? Glossary
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
2
? References/ Bibliography/ Further Reading
Learning outcomes:
The students will be able to
? Understand how efficiently plants have evolved defence mechanisms
? Explain the various approaches by which plants protect themselves against
pathogens
? Understand the importance of certain plant hormones in the induction of plant
defense responses
? Know about PR proteins, the genes which encode them and the importance of PR
proteins in plant defense
? Explain how pathogens elicit systemic acquired plant defense responses
? Differentiate between systemic acquired resistance (SAR) and induced systemic
resistance (ISR)
? Understand the genetic basis of plant defense
? Explain the role of genes behind the plant-pathogen interactions
? Understand the concept of avirulence and virulence genes
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
3
Introduction
Plants have developed complex defense mechanisms to counteract a wide variety of
pathogens. Plants do not possess a immune system comparable to animals. However, plants
have developed a complex array of structural, chemical and genetic defense mechanisms
which enable them to detect invading organisms and stop them before they spread infection
to neighboring plant tissues. Plants have evolved several defense mechanisms including
constitutive defense that is associated with cell wall, waxy cuticle, and bark. Inducible
defenses involve plant recognition of invading pathogens and production of toxic chemicals,
pathogen degrading enzymes, etc. Plant defense system recognizes various effector
molecules displayed by plant pathogens and are termed as microbe-associated molecular
patterns (MAMPs), like specific proteins, lipopolysaccharides and cell wall components which
are present on several microbes. Mechanism of plant-pathogen interaction is a complex
phenomenon which requires better understanding, in order to develop highly disease
resistant plant species. Plants are indispensable for our survival on earth; we are dependent
on them for food as well as for non-food products like medicines, dyes, wood, textiles, ink,
soap, industrial chemicals etc. Better understanding of plant defense system is essential, in
order to protect them from variety of pathogens and to develop disease resistant plant
species.
Various plant defense mechanisms
Evolution of plant pathogens has lead to the development of different strategies through
which they can infect plant cells. However, in response to infection, plants also elaborate
various defense mechanisms to counteract the pathogen invasion.
Hypersensitive response (HR) is a common defense response used by plants against the
invading pathogens that primarily aims to prevent further spread of infection. In this
response, cells surrounding the infection site die out rapidly. Pathogens are thus unable to
derive nutrition from plant tissues, thereby preventing the spread of infection. The HR
response is followed by a rapid accumulation of reactive oxygen species (ROS) and nitric
oxide (NO). The HR response via reactive oxygen species contributes to host cell death or it
may directly kill the pathogen.
Two broad categories of plant defense mechanisms are: constitutive and induced
Constitutive defenses are preformed structural barriers like, presence of cell wall, waxy
epidermal cuticle and bark. These substances play an important role in providing strength
and rigidity to plant cells. They also provide protection from pathogen invasion. In addition
to preformed barriers, all plant cells have the ability to detect invading pathogens and
respond with inducible defense strategies, which are also referred to as plant’s innate
Page 5
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
0
Lesson Prepared Under MHRD project “National Mission on
Education Through ICT”
Discipline: Botany
Paper: Plant Metabolism
National Coordinator: Prof. S.C. Bhatla
Lesson: Plant Defense Mechanisms
Lesson Developer: Dr. Dhara Arora Research Scholar
Department/College: Department of Botany
University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla, Department of Botany
University of Delhi
Language Editor: Ami Choubey
Department/College: Department of Genetics, University
of Delhi, South Campus
Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
1
Table of contents
Lesson: Plant defense mechanisms
? Introduction
? Various plant defense mechanisms
? Structural defenses- First line of defense
? Surface structures/ Special appendages- Thorns, spines,
prickles and trichomes
? Mineral crystals in specialised cells like idioblasts, sclereids,
crystalliferous cells and silica cells
? Cell wall
? Biochemical defences
? Constitutive
? Secondary metabolites
? Terpenoids
? Phenolics
? Alkaloids
? Induced
? Hypersensitive responses
? Systemic acquired resistance (SAR) and induced
systemic resistance (ISR)
? Jasmonic acid and ethylene have role in
plant defences against pathogen
infection
? Role of systemin against insect attack
? Role of various proteins and enzymes to counteract
pathogen attack
? Defensins
? Digestive enzyme inhibitors
? Proteinase inhibitors
? Hydrolytic enzymes/PR proteins
? Genetic basis of plant-pathogen interaction
? Summary
? Exercise/ Practice
? Glossary
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
2
? References/ Bibliography/ Further Reading
Learning outcomes:
The students will be able to
? Understand how efficiently plants have evolved defence mechanisms
? Explain the various approaches by which plants protect themselves against
pathogens
? Understand the importance of certain plant hormones in the induction of plant
defense responses
? Know about PR proteins, the genes which encode them and the importance of PR
proteins in plant defense
? Explain how pathogens elicit systemic acquired plant defense responses
? Differentiate between systemic acquired resistance (SAR) and induced systemic
resistance (ISR)
? Understand the genetic basis of plant defense
? Explain the role of genes behind the plant-pathogen interactions
? Understand the concept of avirulence and virulence genes
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
3
Introduction
Plants have developed complex defense mechanisms to counteract a wide variety of
pathogens. Plants do not possess a immune system comparable to animals. However, plants
have developed a complex array of structural, chemical and genetic defense mechanisms
which enable them to detect invading organisms and stop them before they spread infection
to neighboring plant tissues. Plants have evolved several defense mechanisms including
constitutive defense that is associated with cell wall, waxy cuticle, and bark. Inducible
defenses involve plant recognition of invading pathogens and production of toxic chemicals,
pathogen degrading enzymes, etc. Plant defense system recognizes various effector
molecules displayed by plant pathogens and are termed as microbe-associated molecular
patterns (MAMPs), like specific proteins, lipopolysaccharides and cell wall components which
are present on several microbes. Mechanism of plant-pathogen interaction is a complex
phenomenon which requires better understanding, in order to develop highly disease
resistant plant species. Plants are indispensable for our survival on earth; we are dependent
on them for food as well as for non-food products like medicines, dyes, wood, textiles, ink,
soap, industrial chemicals etc. Better understanding of plant defense system is essential, in
order to protect them from variety of pathogens and to develop disease resistant plant
species.
Various plant defense mechanisms
Evolution of plant pathogens has lead to the development of different strategies through
which they can infect plant cells. However, in response to infection, plants also elaborate
various defense mechanisms to counteract the pathogen invasion.
Hypersensitive response (HR) is a common defense response used by plants against the
invading pathogens that primarily aims to prevent further spread of infection. In this
response, cells surrounding the infection site die out rapidly. Pathogens are thus unable to
derive nutrition from plant tissues, thereby preventing the spread of infection. The HR
response is followed by a rapid accumulation of reactive oxygen species (ROS) and nitric
oxide (NO). The HR response via reactive oxygen species contributes to host cell death or it
may directly kill the pathogen.
Two broad categories of plant defense mechanisms are: constitutive and induced
Constitutive defenses are preformed structural barriers like, presence of cell wall, waxy
epidermal cuticle and bark. These substances play an important role in providing strength
and rigidity to plant cells. They also provide protection from pathogen invasion. In addition
to preformed barriers, all plant cells have the ability to detect invading pathogens and
respond with inducible defense strategies, which are also referred to as plant’s innate
Plant Defence Mechanisms
Institute of Lifelong Learning, University of Delhi
4
immune responses, and includes the production of toxic chemicals, pathogen-degrading
enzymes and deliberate cell suicide. Plant’s initial innate immune response is to recognize
specific conserved molecules present on many pathogens. These molecules are called as
elicitors and are also referred as microbial associated molecular patterns (MAMPs), or
pathogen associated molecular patterns (PAMPs). These molecules are specific
proteins, lipopolysaccharides, rhamnolipids, chitin oligomers and cell wall components which
are commonly found in microbes. In response to wound or damage, plants secrete certain
molecules, which are called as damage associated molecular patterns (DAMPs).
Systemin or oligogalacturonides are released as a result of cell wall damage. Plant immune
system recognizes these molecules and induces many basal responses, like callose
deposition on cell wall or advance defense responses against pathogen attack.
Check out the following links:
https://www.youtube.com/watch?v=zVbponLWnK8
https://www.youtube.com/watch?v=6KAsWPN-H2U
https://www.youtube.com/watch?v=S2jxH8WyTr0
Several means by which plants protect themselves from pathogen
attack
1. Structural defense
2. Chemical defense
3. Protein and enzymes
4. Hormonal defense
5. Genetic defense
Structural defenses
The organization of plant cell provides various for protection against pathogen attack,
invasion and infection.
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