Plants have both pre-existing and induced defense mechanisms against pathogens. Pre-existing defenses include structural barriers like the cuticle, cell walls, and natural openings. Biochemical pre-existing defenses include a lack of recognition factors for pathogens and an absence of receptors for pathogen toxins. When a plant is challenged by a pathogen, it induces additional structural defenses like cork layers and gum deposition. Induced biochemical defenses include the hypersensitive response, which causes cell death around the infection site, production of antimicrobial compounds, and detoxification of pathogen toxins. Both pre-existing and induced defenses help plants resist pathogen infection and disease development.

1. Chapter: 5 Defense mechanism in plants
 Analysis of most of the host parasite relationships reveals
that on the pattern of pathogenesis, the plants on their
part, do exhibit defense mechanisms (structural and
chemical) as soon as challenged by the pathogen.
 The plants due to hereditary characters have several
naturally occurring physical and chemical barriers
(preexisting) resisting penetration, and
 If at all the penetration occurs, the host reacts by
different means resulting in formation of physical and
chemical barriers.

2. DEFENSE MECHANISM IN PLANTS AGAINST DISEASES
STRUCTURAL
BIOCHEMICAL
PRE EXISTING
DEFENSE
INDUCED
DEFENSE
PRE EXISTING
DEFENSE
INDUCED
DEFENSE

3. Constitutive defense -
always present
Constitutive vs. Induced Defenses
Induced defense - synthesized in response to challenge

5. PRE EXISTING
DEFENSE
STRUCTURE
INDUCED
DEFENSE
STRUCTURE
1) Cuticular wax
2) Thickness of cuticle
3) Structure of epidermal
cell wall
4) Structure of natural openings
1) Cytoplasmic defense
reaction
2) Cellular defense structure
3) Histological defense
structure
a) Cork layer formation
b) Tylose formation
c) Abscission layer
formation
d) Gum deposition
4) Necrotic defense reaction
through HR
STRUCTURAL

6. 6.1 structural defenses
6.1.1 A Pre-existing Structural Defenses
 The first line of defense in plants is present in its surface.
 Several characters of the plants surface function as barriers to penetration
which pathogen must breach to enter the host.
 The pathogens enter the plant host by penetrating the epidermis along
with cuticle and cuticular wax and number of natural openings existing
before the onset of the pathogenesis can obstruct penetration.
 If the pathogen succeeds in penetration; it encounters pre-existing internal
structural barriers.
 The external and internal structural barriers existing before pathogen
attack are also called Pre-existing defense structures or passive/static or
anit-infection structures
Thickness of cuticle and wax
 The cuticle covers the epidermal cells of plants and consists of pectin
layer, a cutinized layer and a wax layer
 Cutin is composed of fatty acids. Waxes are mixture of long chain
aliphatic compounds which prevent the retention of water on plant surface
essential for spore germination.
Act as barrier for directly penetrating pathogens. Example: In linseed cuticle
acts as a barrier against Melampsora lini

7. Epidermal layer
 Epidermis is the first layer of living host cells that comes in contact with
attacking microbes.
 The toughness of epidermis is due to the polymers of cellulose,
hemicelluloses, lignin mineral substances, polymerized organic
compounds, suberin etc.
 Potato tubers resistant to Pythium debaryanum contain higher fiber.
 Suberization of epidermis confers protection against
Hydathodes
 Are natural openings on the edges of leaves and serve to excrete excess
water from the interior. They are easy entry pints of bacterial pathogens
such as X.capestris pv. capestis (black rot of cabbage), Similar to
Hydathodes are the-
nectarthodes in inflorescence of many plants.
 They secrete sugary nectar and this serves as barrier to those organisms
that cannot tolerate this condition and thus, can enter through nectarines.
 Leaf hairs on leaves and on nectarines also resist entry of pathogens.
Lenticles
 Are opening in outer walls involved in gaseous exchange. They are weak
points in defense unless the cork cells within them are suberized. After
Suberization and periderm formation, lenticels are more resistant to
invasion by pathogens.

10. 6. 1 .2. Induced Defense Structure
1) Cytoplasmic defense reaction
The cytoplasm becomes granular and dense, various particles or structures appear in
it
Then, the mycelium of the fungal pathogen disintegrates and the invasion stops
2) Cell wall defense structures
Involve morphological changes in the cell wall
The following main types of such structures have been observed in plant diseases.
The outer layer of the cell wall of parenchyma cellular combining in contact with
incompatible bacterial cell and produce an amorphous,
Swelling of the cell wall: outer wall of epidermal cells as well as sub-epidermal
cells swells and inhibit pathogen penetration.
 Sheathing of hyphae: penetrating hyphae enveloped in a sheath formed by
extension of cell wall, delays penetration. E.g. Hyphal sheathing of F.
oxysporum f.sp. lini
3)Histological defense structures
 Involve tissues ahead of the pathogen (deeper into the plant)

11. i. Cork layer formation
 Also blocks nutrient and water to infected area and necrotic lesions formed.
 Infection by pathogens induces plants to form multilayered cork cells beyond the
point of infection as a result of stimulation of the host cells by substances
secreted by the pathogen. It inhibits further invasion by the pathogen beyond the
initial lesion.

12. ii. Abscission layer formation
 Formed on young, active leaves of fruit trees after infection by pathogens.
 Gap formation between infected cells and adjacent healthy cells.
 On infection, the middle lamella between these two layers of cells is dissolved
throughout the thickness of the leaf, completely cutting of the central area from
the rest of the leaf. e.g. Xanthomonas pruni on peach leaves.
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13. iii. Gum deposition
 Various types of gums are produced by many plants around
lesions after infection by pathogens or injury.
 Their defense role is by quickly depositing in the intercellular
spaces and within the cells surrounding the locus of
infection-forming an impenetrable barrier that completely
encloses the pathogen.
 The pathogen then becomes isolated, starves, and dies.
 Along the borders of diseased lesions penetration.

14. 6.2 Biochemical defense mechanism
 The induced biochemical changes in host plants are the last
line of host defense.
 This may condition a plant or plant tissue from susceptible to
resistant to immune status as per their genetic potential.
 The role of bio chemical factor in host defense is based on the
following four attributes:
1. The substance is associated with protection against disease at
the site where protection occurs.
2. The substance can be isolated from the host showing
protection against the disease.
3. Introduction of isolated substance to the appropriate
susceptible host confers protection.
4. The nature of protection so induced resembles that of the
natural agents of a resistant plant.

15. 6.2.1 Pre-existing biochemical defense
 Plants liberate different chemicals, which interfere with
activities of the pathogen and pathogenesis, thereby
preventing or reduce infection.
 These chemicals and the biochemical conditions that develop
may act either directly through toxic or lytic effect on the
invader or indirectly through stimulating antagonistic plant
surface micro flora.
 A particular pathogen may not infect plants without structural
barriers
Lack of recognition between host and pathogen
 A plant species is a host for a particular pathogen (wheat -stem
rust fungi) or not a host for that pathogen (tomato-wheat stem
rust fungus)

16.  How does a pathogen recognize that the plant
with which it comes in contact is a host or non
host?
 A species or variety of plants may not become infected
by a pathogen if their surface cells lack specific
recognition factors (specific molecules or structures)
that can be recognized by the pathogen.
 Such as-oligosaccharides, polysaccharides, proteins.
 If the pathogen does not recognize the plant as one of
its host plants, it may not become attached to the
plant or may not produce infection substances, such as
enzymes, structures, such as appressoria, penetration
pegs and haustoria necessary for the establishment of
infection.

17. b) Lack of host receptors and sensitive sites for toxins
 In host-pathogen combinations in which the pathogen produce a
host specific toxin which is responsible for the symptoms is
thought to attach and react with a specific receptors or sensitive
sites in the cell
 Plants with sensitive receptors or sites become diseased
 Plants lacking sensitive receptors for toxins escape infection
 Plants species or varieties that lacks sensitive receptors remain
resistance to the toxin and develop no symptoms
c) Lack of nutrients essential for the pathogens
 Species or varieties of plants that for some reason don’t produce
one of the substance essential for the survival of an obligate
parasite or for development of infection by any parasite would be
resistant to the pathogen that requires it.
 E.g., for Rhizoctonia to infect a plant, it needs to obtain from the
plant a substance necessary for the formation of a hyphal cushion
from which the fungus sends into the plant its penetration hyphae
or peg.

18. 6.2.2 Induced Biochemical Defense
1. Necrotic defense reaction through HR
 The death of the invaded cell may protect the plant from
further invasion.
 It is considered biochemical rather than structural defense
mechanisms.
 The necrotic tissues–isolates the pathogen from the plant
living substances.
 Signifies the concentration of numerous biochemical cell
responses and anti-microbial substances that neutralize the
pathogen
HR occur only in specific host pathogen combination in which
the host and the pathogen are incompatible that the pathogen
fails to infect the host

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20. 3) Detoxification of pathogen toxins
 Cases where pathogen produces toxin, detoxification is the disease resistance or in
plant disease- caused by toxins produced by a pathogen- resistance to disease is the
same as resistance to toxins
 The amount of such substances formed is proportional to the disease resistance.
 E.g. detoxification of HC-toxin and pyriculerin-produced by the fungus.
4. Local and systemic acquired resistance
Induction of plant defense with artificially inoculation with microbes or by treatment
with chemicals. Induced resistance- is first localized around the point of necrosis caused
by infection or by the chemical is called local acquired resistance (LAR).
Subsequently, resistance spread systemically and develops in distal untreated part of the
plant is called- systemic acquired resistance (SAR). When a plant survives the infection
of a pathogen at one site it can develop increased resistance to subsequent attacks.
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23. 5. Release of anti-microbial compounds
 Plants while growing and developing release gases as well as organic
substances, from leaves and roots (leaf and root exudates), containing
sugars, amino acid, organic acids, enzymes, glycoside etc.
 These materials have profound effect on the nature of surrounding
environment,
 particularly the phyllosphere, rhiizosphere microflora and fauna.
 Although these substances are ideal nutrients for microbes and help in
germination and growth of several saprophytes and parasites number
of inhibitory substances is also present in these exudates.
 Theses inhibitory substances directly affect the microorganism, or
encourage certain groups to dominate the environment and function as
antagonists of the pathogen.

24. Thank You Very Much!