3. Defense Mechanism in Plants
Constitutive or Induced
Structural ( Morphological) defense
Biochemical defense
Both the defenses are affected by:
Age of the plant
Type of organ infected
Nutritional status of the host
Environmental conditions
5. Non-host Resistance
When a plant resist the attack of pathogenic
organism which is otherwise not the host of that
pathogen is termed as non host resistance.
Eg: potato late blight pathogen do not infect
apple or wheat plant.
6. Host defenses
When a plant resist the attack of pathogenic
organism which is the host of that pathogen is
termed as host resistance or host defense.
Among the structural host defenses, it may be
Pre- existing structural defenses
Post existing structural defenses
7. Pre existing/ pre- infectional
structural barriers
Surface barrier
Presence of
Wax layers
Hairs
Lignin
Thickness of cuticle
Spines
Sub surface barriers
Epidermal cells
Stomata
Lenticels
Hydathodes
16. Cuticle defend the underlying tissues by,
Water repellent capacity
Negative electric charge
Toxicity to some pathogen
Mechanical obstructions.
18. Epidermis
May directly impede entrance of the pathogens.
The degree of impediment depends on,
i)thickness of the outer epidermis and
ii)toughness – due to variation in relative
lignifications and salicic acid deposition.
19. Eg:
Amount of salicic acid in leaves against blast in rice .
20. P. graminis tritici shows varied degree of
susceptibility i.e, young plants are more susceptible.
21. Formation of Appressorium and penetrating
Hyphae of Puccinia grisea on different cells of
Rice
EPIDERMAL
CELLS
PERCENT FORMATION
APPRESSORIA INFECTION
HYPHAE
Motor cells 53.0 63.7
Long cells 16.2 7.9
Short cells 12.8 6.1
Hairs 1.7 -
Stomata 6.0 -
22. Stomata
Mainly restricts bacteria
which enters through
passive mode.
Structure, number and
location decides extent of
resistance.
Eg:
Mandarins resistant to
Xanthomonas axanopodis
has stomata surronded by
raised broad -lip like
structure.
23. Location and shape of stomata, opening & closing
mechanisms -resistant wheat varieties for Rusts.
24. Size of stomata - Resistant citrus vars have small
stomata against X. campestris
25. Lenticels
Plant parasitic bacteria and fungus enters through
lenticels
The shape and internal structure plays a role .
Suberin layer beneath it restrict the entry.
26. Suberin
often within roots.
can protect against pathogens and other damage.
older parts of roots more suberized
endodermis has suberin side walls, water
must pass through plasma membrane to get to stele
27. Suberin can form transport barriers
between the soil and the roots
28. Hydathodes and Nectarthodes
Entry point for bacterial pathogens.
Sugary nectar serves as barrier to pathogens due
to high osmotic pressure.
29. Stem spines Colletia paradoxa Leaf spines- Opuntia invicta
Shoot spines- Dovyalis caffra
Otherwise known as kei apple
Drought tolerant
30. Post infectional defense
structures
Histological defense
Tylose formation e.g. wilts
Corky layers e.g. in potato- R. solani
Cellular defense
Cytoplasmic defense
Hypersensitive response.
31. Tylose formation
Tyloses are the
overgrowths
of living cells that
protrude via pits into
xylem vessels
blocking the vascular
system.
32. If they form
abundantly and
quickly, they can
stop the spread of
vascular wilt
pathogens.
Their formation is
triggered by a “stress
condition”.
36. The formation of abscission layers can,
Limit the size of lesions.
Delimits the extent of damage that can be caused by
a single infection.
Prevents the further spread of pathogen.
37. Schematic formation of
abscission layer around the
diseased spots.
Eg:
Xanthomonas pruni-shot
hole of Prunus leaf
38. 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.
39. Hypersensitive response
o Highest degree of resistance.
o Result in sudden death of the host cells in the vicinity
of the pathogen.
o Both structural & biochemical in nature.
o Common in obligate pathogens like fungi, viruses
and nematodes, also found in other fungal &
bacterial infection.
o Due to HR
– The necrotic tissues isolate the obligate
pathogen from living cells.
– Devoid the pathogen of nutrition, thus starved
and die.