this presentation provides an idea on how plants act when they are infected by a pathogen.specially post infection structural mechanisms of plants have been discussed.

1. STRUCTURAL DEFENSE
MECHANISMS
POST-INFECTIONAL STRUCTURAL DEFENSE MECHANISMS
S.G.S.Harshana
AG/2011/2012/160
Faculty of Agriculture
Rajarata University of Sri
Lanka

2. PLANT RESISTANCE MECHANISM
• STRUCTURAL DEFENSE
• BIOCHEMICAL DEFENSE
Pre-existing structural defense
mechanism
Post infectional structural defense
mechanism

3. 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

4. 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: Peronospora parasitica, Alternaria japonica
Potato: Phytophtora infestans
Wheat: Septoria nodorum
Cucumber: Cladosporium cucumerium, Colletorichum lagenarium
Carrot: Botrytis cineria

5. 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

6. 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. I.E. 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.

7. 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 facto
(TIF) up the stem.
• Examples :
Sweet potato: Fusarium oxysporum f. Sp. batatas.

8. 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.

9. 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.

10. INDUCED CELLULAR DEFENSE STRUCTURES
• The cellular defense structures, I.E. 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.

11. 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 .

12. HYPERSENSITIVE/NECROTIC DEFENSE REACTION
• 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.
• The necrotic tissue isolates the obligate parasite from the living substances and

14. REFERENCES
• Pandey, B.P. And P, B.P. (2001) plant pathology: pathogen and plant disease. India: S
chand & co.(Pandey and P, 2001)
• Deverall, B.J. (1977). Defense mechanisms of plants. Cambridge university press.
• Brown, j.F. (1980). Mechanisms of resistance in plants to infection by pathogens, n J. F.
Brown (ed.) Plant protection pp. 254-266. Australian vice-chancellors' committee,
canberra.
• Agrios, G.N. (2005) plant pathology. 5th edn. Amsterdam: elsevier academic press.
(Agrios, 2005)
• Society, T.A.P. (2016) welcome to APS. Available at:
http://www.Apsnet.Org/pages/default.Aspx (accessed: 24 october 2016).