Lecture 17 - Plant Defense Mechanisms | Plant Metabolism - Botany PDF Download

 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.