STRUCTURAL DEFENSE MECHANISMS

POST-INFECTIONAL STRUCTURAL DEFENSE MECHANISMS

S.G.S.HarshanaAG/2011/2012/160Faculty of AgricultureRajarata University of Sri Lanka

PLANT RESISTANCE MECHANISM

• STRUCTURAL DEFENSE

• BIOCHEMICAL DEFENSE

Pre-existing structural defense mechanism

Post infectional structural defense mechanism

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

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

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

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.

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.

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.

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.

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.

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 .

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

the death is caused to the pathogen because of starvation.

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).