WELCOME"Defense responses of plants to pathogen"

 

INTRODUCTION

• Every plant bears its own defense mechanism(s) against the

development of diseases caused by various fungi, bacteria, viruses,

nematode and other microorganisms.

• 'The attribution of the hosts that reduce the chances of infection or the

further development of the pathogen are considered to be defense

mechanism'.

• The defense may be used against pathogen at any stage of infection i.e.

during pre penetration, penetration or post penetration.

• In nature disease resistance seems to be the rule rather than the

exception.

DEFENSE MECHANISM IN PLANTS AGAINST DISEASES

STRUCTURAL BIOCHEMICAL

PRE EXISTING DEFENSE

STRUCTURE

INDUCED DEFENSE

STRUCTURE

PRE EXISTING DEFENSE

INDUCED DEFENSE

PRE EXISTING DEFENSE

STRUCTURE

INDUCED DEFENSE

STRUCTURE

1) Cultivar wax2) Thickness of cuticle3) Structure of epidermal cell wall4) Structure of natural openings

1) Cytoplasmic defense reaction2) Cellular defense structure3) Histological defense structure a) Cork layer formation b) Tylose formation c) Abscission layer formation d) Gum deposition4) Necrotic defense reaction through HR

STRUCTURAL

I) STRUCTURAL DEFENSE

A)PRE-EXISTING DEFENSE STRUCTURE1) Presence of cuticular wax:Make the leaf surface hydrophobic Preventing deposition

and further germination(fungi) and multiplication (bacteria).

2) Thickness of cuticle:Act as barrier for directly penetrating pathogens.e.g.In linseed cuticle acts as a barrier against Melampsora

lini.3) Structure of epidermal cell wall: Thick and tough outer wall of epidermal cells due to

lignification or the presence of salicle acid.e.g. Rice blast - C.O. - Pyricularia oryzae.

4) Structure of natural opening:

Small and narrow openings surrounded by raised

broad-lipped structures.

e.g. Szinkum variety of mandarin resistant to citrus

canker bacterium X. campestris pv. citri.

5) Internal structural barrier :

Vascular bundles or extended areas of Sclerenchyma

cell leaf veins.

e.g. Angular leaf spot of cotton.

INDUCED DEFENSE

Step I: Recognition of pathogen by host:Plants begin to receive pathogen elicitors.

e.g. glycoproteins, carbohydrates, fatty acids and peptides and oligomers or monomers released after breakdown of host cell wall or pathogen cell wall polysaccharides.

Recognition of pathogen:Induction of biochemical reactions and structural

changes.

Step II: Signal transduction:

Signals sendout to host protein and nuclear genes for their

activation.

Production of substances inhibitory to pathogens.

Sometimes systemic signal transduction.

Signal transducers - Protein kinases, calcium ions,

phosphorylases and phospholipases. AT pases, H2O2, ethylene.

Systemic signal transducer - Salicylic acid, jasmonic acid,

systemin, fatty acids, Oligogalacturonides, ethylene.

B) INDUCED DEFENSE STRUCTURES1) Cytoplasmic defense reaction:

Cytoplasm surrounds invading hyphae, nucleus is stretched and cytoplasm

become granular and dense mycelium disintegrates.

2) Cellular defense structure:

i) Swelling of the cell wall:

Outer wall of epidermal cells as well as sub-epidermal cells swells and inhibit

pathogen penetration. e.g. Pea leaves affected by Botrytis cinerea.

ii)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:

i)Cork layer formation:

Multilayered cork cells are formed beyond the point of infection.Inhibit further

pathogen invasion and blocks pathogen toxins.Also blocks nutrient and water to

infected area Necrotic lesions formed.

e.g.Potato tubers infected by Rhizoctonia.

ii)Abscission layer formation:

Gap formation between infected cells and adjacent healthy cells.

Develop in young leaves of stone fruits at the point of infection gap formation

Shot hole effect.

e.g.Xanthomonas pruni on peach leaves.

Healthy area Diseased area

Abscission layer

iii) Tylose formation:Overgrowths of the protoplast of adjacent living parenchymatous

cells into xylem vessel.Formed in stress or in response to invasion by

vascular pathogens. Clog the vessels and check further

e.g.In most plants against vascular wilt pathogens.

iv) Gum deposition:

Along the borders of diseased lesions Produced in

advance of fungus in relatively short-time.

e.g. In rice varieties resistant to Helminthosporium

leaf spots.

4) Necrotic defense reaction through HR -

Nucleus moves towards pathogen and disintegrates

brown, resin-like granules in cytoplasm death of cell and

invading hypha digenerate.

PRE-EXISTING BIOCHEMICAL

DEFENSE

INDUCED BIOCHEMICAL

DEFENSE

1)Inhibitors released by plants in its environment 2) Inhibitors in plant cell

3)Lack of essential factors a) Lack of recognition b)Host receptor and sensitivities for toxins c)Nutrients

1) HR2) Disruption of host cell membrane3) Strengthening of host cell resistance4) Release of antimicrobials a) Phenolics b) Phytoalexins c) Phenolics from non- toxic glycosides d) Polyphenol oxidase e) PR - proteins5) Detoxification of pathogen toxins6) Aquired resistance with microbs and chemicals 7) Plantibodies8) Genetically engineered disease resistant plant a) Plant derived gene b) Pathogen derived gene

BIOCHEMICAL

II) BIOCHEMICAL DEFENSE

A) PRE-EXISTING BIOCHEMICAL DEFENSE :1) Inhibitors released by the plants in its environment:Plants exude some inhibitory compounds in phyllosphere and rhizosphere.e.g. Resistance in red scaled onion variety to smudge, caused by

Colletotrichum circinans is attributed to the presence of phenolics - protocatechuic acid and catechol.

2) Inhibitors present in plant cell before infection : Phenolics, tannins and some fatty acid like compounds in high

concentration in cells of young fruits, leaves or seeds resistance against Botrytis.

Other –Saponins-Tomatin in tomato, avinacin in oats --Antifungal activity.

Hydrolytic enzymes-Glucanases chitinases--Breakdown of cell wall components of

pathogens.

3) Defense through lack of essential factors:a) Lack of recognition between host and pathogen:Plant surface lacking specific recognition factors escapes pathogen infection.Recognition factors - various types of oligo and polysaccharides and proteins or glycoproteins.b) Lack of host receptors and sensitive sites for toxins:Plants lacking sensitive receptors for toxins escape infection.c) Lack of nutrients essential for the pathogens:Plants which do not produce one of the nutrients. e.g. Rhizoctonia infect only the plants with substances required for formation of hyphal cushion.

B) INDUCED BIOCHEMICAL DEFENSE 1) Hypersensitive response (HR):First used by Stakman (1915)What - Localised induced cell death in the host at the site of infection.How - Leaf area become water soaked, then necrotic and collapsed within

8-12 hrs.When - Virulent strains into non host plant or resistant varieties and

avirulent strains into susceptible cultivars. It is culmination defense response and after recognition of pathogen

activates cascade of biochemical reactions. The most common reactions -i) Rapid burst of oxidative reactionsii) Increased ion movement (K+ and H+) through cell mumbraneiii) Diruption of cell membrane and loss of compartmentalizationiv) Cross linking of phenolics with cell wall components and

strengthening of cell wallv) Production of phytoalexinsvi) Production of PR - Proteins.

2) Disruption of host cell membrane:Some structural and permeability changes in host cell membrane play a role in defense. Most common are -Release of molecules in signal transduction around cell and systemically.Release and accumulation of reactive oxygen 'radicals' and lipoxygenase enzyme.Activation of phenol oxidases and oxidation of phenolic due to loss of comport meutalisation. 3) Oxidative burst and release of Nitric oxide (NO):Rapid generation of activated oxygen radicals, superoxide (O2-), Hydrogen peroxide (H2O2), Hydroxylradical (OH) released by multisubunit NADPH enzymes complex of cell membrane. Hydroperoxidation of membrane phospholipids to lipid hydroperoxides. Disrupt cell membrane and induce cell colapse and death.

4) Strengthening of host cell wall resistance:Accumulation of defense related substances in cell wall. e.g. Callose, glycoproteins rich in hydroxy proline, phenolics (lignin, suberin) and mineral like silicon and calcium.Form complex polymer and cross link with one-another forming insoluble cell wall structures. 5) Production of anti-microbials in attacked cells:a)Common phenolics-Produced and accumulated after infection in a resistant variety.Chlorogenic acid, caffeic acid and ferulic acidCombined effect, rather than individual.

b) Phytoalexins-Muller and Borger (1940) first used the term

phytoalexins.Low molecular weight antimicrobial compounds. Produced in healthy cells adjacent to damaged or necrotic cells. One or more phytoalexin accumulation cause infection.

Phytoalexins with their host and the pathogens against which they are producedPhytoalexin Host Pathogen

Pisatin Pisum sativum Sclerotinia fructicola

Phaseollin, Kievitone Phaseolus vulgaris Sclerotinia fructicola

Orchinol Orchis militaris Rhizoctonia repens

Ipomeamarone Ipomoea batatas Ceratocystis fimbriata

Isocoumarin Daucas carota Ceratocystis fimbriata

Gossypol Gossypium hirsutum Verticillium alboatrum

Rishitin Potato tuber Phytophthora infestans

Trifolirhizin Trifolium pratense Helmienthosporium turcicum

Medicarpin Medicago sativa H. turcicum

Cicerin Cicer arietinum Ascochyta rabiei

Glyceollin Soybean, Alfalfa, Clover Diaporthe phaselorum, Glomerella cingulata

Capsidiol Pepper Peronospora trifoliorum

Sharma,(2001)

c) Release of phenolics from non toxic glycocides-Enzyme glycosidase hydrolyses glycosides to phenolics.Glycosidase released from plant tissues on stimulation by pathogens. d) Polyphenol oxidase-Phenol oxidizing enzymes.More in infected tissues of resistant variety.Oxidise phenols to more toxic quinines.Another class is peroxidase which also liberate H2O2

e) Pathogenesis related proteins (PR - Proteins)-Present in trace amounts in plants cells intracellularly and also in intercellular spaces.Produced in greater amounts after pathogen entry.Highly acidic and extremely basic so highly soluble and reactive.Classified into several groups on the basis of function, serological relationship, amino acid sequence, molecular weight, etc. Some inhibit spore release and germination, others strengthen the host cell wall and it's outgrowths and papillae.

Family Type Member Properties

PR-1 Tobacco PR-1a Antioomycete and antifungal

PR-2 Tobacco PR-2 β-1, 3-glucanase

PR-3 Tobacco P, Q Chitinase types I, II, III, IV, V, VI, VII

PR-4 Tobacco R Chitinase types I, II

PR-5 Tobacco S Thaumatin-like

PR-6 Tomato inhibitor I Proteinase inhibitor

PR-7 Tomato P69 Endoproteinase

PR-8 Cucumber chitinase Chitinase type III

PR-9 Tobacco 'lignin-forming peroxidase' Peroxidase

PR-10 Parsley 'PR 1' Ribonuclease like

PR-11 Tobacco class V chitinase Chitinase, type I

PR-12 Radish Rs-AFP3 Defensin

PR-13 Arabidopsis ThI2.1 Thionin

PR-14 Barley LTP4 Lipid transfer protein

Fourteen families of PR-Proteins, their type members and properties

(Singh, 2005)

6) Detoxification of pathogen toxins:Cases where pathogen produces toxin, detoxification is the disease resistance.In resistant varieties toxins are metabolized rapidly or combined with other substances to form less or non-toxic compounds.The amount of such substances formed is proportional to the disease resistance.

e.g. Detoxificaiton of HC-toxin and pyricularin of Cochliobolus carbonum and Magnaporthe grisea in their host maize and rice, respectively.

7) Acquired resistance with microbes and chemicals:Plants develop resistance rather than producing antibodies Local aquired resistances - In the beginning (LAR)Systemic aquired resistance - Later (SAR)Chemicals like salicylic acid, arachidonic acid, 2, 6-dichloroisonicotinic acid induce LAR and SAR.Applied through root, leaf (spray) or stem (injection)SAR is produced after expression of HR in plants.Does not affect pre-penetration stage, but reduces penetration.

8) Plantibodies (Immunization of plants):Plants lack immune system and produce no antibodies. In 1990's transgenic plants, with incorporated mouse genes, produced antibodies, against certain pathogen.Such antibodies encoded by animal genes but produced in and by the plant are called plantibodies. e.g.Transgenic plants producing plantibodies against coat protein of viruses such as,artichoke mottle crinkle virus have been produced.9) Genetical engineered disease resistant plants:a)Plant derived genes:The plants incorporated with plant derived R gene showed the resistance.First 'R' gene isolated was Hm1 gene in corn in 1992 codes for enzyme detoxifying HC - toxin produced by Cochlibolus carbonum. contd…

In addition to R gene, other genes encoding proteins or enzyme are also incorporated.

e.g. Tobacco plants transformed with chitinase gene from bean become resistant to R. solani.b)Pathogen derived genes:Plants transformed by incorporating animal derived gene. e.g. In 1986 Tobacco plants transformed to express coat protein gene of TMV.

CONCLUSIONS1) Plants defened to pathogens at pre-germination and

germination stage with structural defense (pre-existing and induced).

2) When plants fails to defense pathogen with structural defense the biochemical defense mechanism is activated.

3) When pathogen elicitors reach to the host receptors, signal transduction takes place and a cascade of several structural and bio-chemical responses starts.

4) Plant defense is not the result of a single mechanism but the cumulative effect of more than one mechanisms.

5) All the defense exhibited by the plants are governed by it's genetic constitution.

Thank You