2. 2
Contents
• Plant Immune System
• Recognizing self from non self
• Plant Resistance Mechanism
• Post - Infectional Structural Defense Mechanisms
• Plant disease
• Disease process
• History of Plant diseases
• Useful plant pathogens
• Plant pathogen as medicinal
• Type of Plant Disease Resistance
Horizontal resistance
Vertical resistance
Defense Signaling in Non host resistance
Role of metabolic defense
3. 3
• Basal Resistance (MAMP- Triggered Immunity)
• Pathogen Effectors
• Bacterial MAMP or PAMP
• Fungal MAMP or PAMP
• Suppression of PTI by Pathogen Effectors
• Gene for Gene Hypothesis
• Disease Resistance or susceptibility
• Pathogenicity genes in phytopathogen
• Virulent and avirulent gene
• R gene and protein
• Direct Interaction Model
• Indirect Interaction Model
Guard Hypothesis
Decoy Hypothesis
4. 4
• The Hypersensitive response
• Thermo Regulation
• Cell to cell propagation
• Oxidative Burst in HR
• Systemic Acquired Resistance
• Historical Perspective
• Characteristic of SAR
• SAR and SA (Salicylic acid)
• SAR vs ISR
• Induced Systemic Resistance
• Localized Induced resistance
• Systemic induced resistance
• Type of Localized induced resistance
• Properties of PGPR
• The Negative effects of ISR
• Plant-microbe Interaction
• Bacteria
• Viruses
• Fungi
• MAPK Cascades in Plant Disease Resistance Signaling
6. 6
Exam Pattern
• Objectives (30%)
Mcq’s, mind maps, Identify the mistakes, label the
diagram etc.
• Subjective (50%)
Short question, medium question and concept tester.
1. Quiz will be taken after every two lectures.
2. Class activity will be performed after three lecture.
7. 7
Recommended reading, including textbooks,
reference books
• Molecular Biology of the Cell by Bruce Albert and Dennis Bray. Garland Publishing Inc, New
York and London.
• Handbook of Cell Signaling Vol 1-3, by Ralph Bradshaw, and Edward Dennis.
• Cell Signalling, by John T. Hancock, 2nd
ed. Oxford University Press.
• Apoptosis, Cell Signaling, and Human Diseases: Molecular Mechanisms, Volume 2 by Rakesh
Srivastava.
• Cell Signaling and Growth Factors in Development: From Molecules to Organogenesis by Klaus
Unsicker (Editor), Kerstin Krieglstein.
• Signal Transduction by Bastien D. Gomperts.
• The Biochemistry of Cell Signalling by Ernst J. M. Helmreich.
9. Do Plants Have an Immune System?
• It Depends on How You Define “Immune System”
• Plants get sick. That is, they can be infected by pathogens.
• But after hundreds of millions of years of pathogen attacks, plants are still here. So,
they must have ways to get well after being sick.
• Plants can defend themselves against disease-causing organisms (pathogens) such
as viruses, bacteria and fungi.
• They do so by producing physical barriers (e.g., plant cell walls), some antibiotic
compounds (e.g., phytoalexins), and even enzymes that perturb pathogens.
• In a broad sense, these are all part of a plant’s immune response, that is,
biological processes that an organism uses to defend itself against disease.
10. Do plants have an immune system
similar to that in animals? One that can
“remember” exposure to specific
pathogens?
11. Recognizing (and Remembering) Self from Non-Self
• Humans, along with most other vertebrates, have a multifaceted immune system
called an adaptive immune system, which is the culmination of complex
interactions at the biochemical, genetic and cellular levels.
• Key parts of this adaptive system are the organism’s ability to
1. biochemically distinguish between it’s own cells (self) and foreign (non-self)
entities
2. “remember” specific features of the foreigner.
• All pathogens – from viruses to fungi – have so-called macromolecules on their
surfaces that distinguish them.
• Adaptive immune systems (AIS) use these macromolecules as antigens. That is,
the immune system uses these characteristic surface features as a way to
specifically identify foreign (non-self) entities.
12. • The AIS uses the antigens to generate specific antibodies,
which are used to tag the “foreigner” for destruction by
specialized blood cells called lymphocytes.
• These specific antibodies then allow for the rapid detection
of subsequent infections with a particular pathogen, which
allows for relatively quick defensive responses.
• Although plants don’t possess such a sophisticated AIS,
there are instances of self/non-self recognition in plants.
13. Plants Have an Innate (Passive) Immune
System
• A more generic, non-specific response to infection characterizes a plant’s
immune system.
• This type of response is called an innate immune system, in contrast to
AIS.
• Plants don’t have antibodies or special cells that search for and destroy
pathogens.
• Plants do, however, have cell-surface receptors to identify certain
patterns characteristic of pathogens.
• Such receptors, when activated, trigger the production of chemical
signals, such as methyl jasmonate that may elicit both local and
systemic defense responses.
14. • Local defensive responses included the so-called hypersensitive
response characterized by the self-destruction of the plant cells in a
localized area around the site of infection.
• Plants also possess inducible systemic defense responses when
locally infected by pathogens. That is, a single, localized infection
may elicit defensive responses throughout the plant.
• Bottom Line: Although plants do have the ability to defend
themselves against disease-causing organisms (sort of a rudimentary
immune system), plants don’t have an immune system as complex as
humans.
17. POST INFECTIONAL DEFENSE MECHANISM
(INDUCED / ACTIVE)
• Occurs once after the infection of a plant by a pathogen.
• The activation or induction of defense mechanism may be both
specific and non-specific type.
• Several structural changes are known to be induced by a range of
biotic or abiotic elicitors.
• These dynamic defense mechanisms prevent further colonization or
spread of pathogen.
• Four types of induced/active structural defense mechanisms,
• Histological defense structures
• Cellular defense structures
• Cytoplasmic defense structures
• Hypersensitive/ Necrotic defense reaction
18. Histological Defense Structures
Even after the establishment of infection in plant cells, the host
defense system tries to create barriers for further colonization of
tissues. This may be at various levels.
1.Lignification
• Lignified cell wall provide effective barrier to hyphal penetration.
• They also act as impermeable barrier for free movement of nutrient
causing starvation of pathogen.
• Examples:
Radish: Alternaria japonica
Potato: Phytophtora infestans
Wheat: Septoria nodorum
Cucumber: Cladosporium cucumerium
Carrot: Botrytis cineria
19. 2. Suberization
• In several plants the infected cells are surrounded by suberized
cells.
• Thus, isolating them from healthy tissue. Corky layer formation
is a part of natural healing system of plants.
• Examples :
Common scab of potato
20. 3. Abscission layers
• Gap between host cell layers and devices for dropping –off older leaves and
mature fruits.
• Plant may use this for defense mechanism also. To drop-off infected or
invaded plant tissue or parts, along with pathogen.
• Shot holes in leaves of fruit trees is a common feature.
• Occurred due to the infection of fungi, bacteria, and viruses.
21. 4. Tyloses
• Formed by protrusion of xylem parachymatous cell walls,
through pits, into xylem vessels.
• The size and number of tyloses physically block the vessel.
• The tyloses are inductively formed much ahead of infection,
thus blocking the spread of pathogen.
• It suggests biochemical elicitors and movement of tyloses
inducing factors (TIF) up the stem.
• Examples :
Sweet potato: Fusarium oxysporum f. Sp. batatas.
22. 5. Gum deposition
The gums and vascular gels quickly accumulate and fill the
intercellular spaces or within the cell surroundings the infection
thread and haustoria, which may starve or die.
23. 6. Hyphal sheathing
• The fungal hyphae, which penetrate the cell wall are often unsheathed by
the extension of the cell wall.
• This delays contact between hypha and protoplasm.
• Later on, the hyphae penetrate the sheath and invade the lumen of the
cell.
25. 25
Induced Cellular Defense Structures
• The cellular defense structures, changes in cell walls, have only a
limited role in defense.
• Following types are commonly observed.
• Carhohydrate apposition (synthesis of secondary wall and
papillae formation)
• Callose deposition (hyphal sheathing just outside plasma lemma
around the haustorium which delays contact of pathogen
(Phytophythora infestans) with host cells.
• Structural proteins
• Induced cytoplasmic defense that present last line of host defense
and may effective against slow growing pathogens, weak parasites
or some symbiotic relationship.
26. 26
Cytoplasmic Defense Structures
• In a few cases of slowly growing , weakly pathogenic fungi that
induce chronic diseases or nearly symbiotic conditions , the
cytoplasm surrounds the clump of hyphae , and the nucleus is
stretched to the point where it breaks in two .
• In some cells , the cytoplasmic reaction is overcome and the
protoplast disappears while fungal growth increases.
• In some of the invaded cells , however , the cytoplasm and
nucleus enlarge .
• The cytoplasm becomes granular and dense , and various particles or
structures appear in it.
• Finally , the mycelium of the pathogen disintegrates and the
invasion stops .
27. 27
Hypersensitive / Necrotic Defense Reactions
• When the pathogen penetrates the cell wall, and establishes contact with the
protoplast of the cell, the nucleus of the cell moves toward the intruding
pathogen soon disintegrates forming resin-like brown granules in the
cytoplasm.
• Firstly, all the granules are formed around the pathogen and then throughout
the cytoplasm.
• Simultaneously the cell walls swell.
• As the browning of the cytoplasm continues and the necrosis is caused, the
invading hypha begins to disintegrate, and the further invasion of the pathogen is
stopped.
• Common in diseases caused by obligate fungal parasites, viruses and
nematodes.
#18:Lignification is a polymerization process in which lignin macromolecules grow from monomers via free radical coupling mechanisms.
https://www.embopress.org/doi/full/10.15252/embj.2019101948
Casparian strip organizer proteins CASPL1D1 and CASPL4D1 are required for pathogen-induced lignification.
#19:suberization (usually uncountable, plural suberizations) (botany) conversion of the cell walls into cork tissue by development of suberin; commonly taking place in exposed tissues, as when a callus forms over a wound.
#20:Abscission (from Latin ab- 'away', and scindere 'to cut') is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed.
