WELCOME

MECHANISM OF ATTACK: ROLE OF ENZYMES

By,Bhavyasree R KDepartment of plant breeding and geneticsPUNJAB AGRICULTURAL UNIVERSITY, LUDHIANA

Infectious agent cause disease or illness to

the host

PATHOGENESIS: step by step

development of a disease

Structural changes

mechanisms to avoid the entry

and attack of pathogens

physical barriers- cell wall, cuticle,

wax

chemical barriers-

hypersensitivity, Antibiotics

Fungi, bacteria, virus etc

Functional changes

PATHOGENv/s

PLANT

Pathogen attack plants when they acquire the ability to overcome these barriers during their evolutionary development

Mechanical forces exerted by pathogens on host tissues

• By pre softening by enzymatic secretions• Adhesion of pathogen• Appressorium• Penetration peg• Nematode-through stylet • After entering –enzymes produced to make

penetration easier

Magnaporthe grisea pathogen appressorium

Nem

atod

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How Pathogens affect PlantsThere are many ways in which plant disease

pathogens can affect plantsBy utilizing host cell contentsBy killing host or by interfering with its metabolic

processes through their enzymes, toxins etc.By weakening the host due to continuous loss of

the nutrients.By interfering with the translocation of the food,

minerals land water.

•Enzymes•Toxins•Growth regulators•polysaccharides

Chemical weapons of pathogen

•All plant pathogens except viruses and viriods can probably produce enzymes growth regulators and polysaccharides•They don’t produce substances themselves but induce the host to produce certain substances

• Pathogen produce these substances

– In normal course of their activities :constitutively

– When the grow on certain substrates such as their host plants : inducible

• First contact of pathogen with host –on plant surface

• Aerial plant part surfaces consist primarily of cuticle and/or cellulose

• root cell wall surfaces consist only of cellulose.

Bacteria on leaf surface

• Cuticle consists primarily of cutin• Lower layer consisting predominantly of pectic

substances• Polysaccharides of various types are often

found in cell walls.• epidermal cell walls may also contain suberin

and lignin.• Complete plant tissue disintegration involves

the breakdown of lignin. • Pathogen attack the plant by degrading these

substances by several enzymes

Cell wall composition

• Functionally cell wall is divided into 3 regions– middle lamella (made

of pectins)– primary wall

(cellulose, pectic substances)

– secondary cell wall (entirely cellulose).

• Middle lamella -intercellular cement which binds the cells together in tissue system.

• Besides these two major components, ie. Pectin and cellulose, other components such as hemicelluloses, lignin and some amount of protein is also present.

CELL WALL STRUCTURAL PROTEINS– 5 classes• Extensins (0.5%normal, but 5- 15 % on infection of

fungi)• Proline rich proteins (PRPs)• Glycine rich proteins (GRPs)• Solanaceous lectins• Arabinogalactan proteins (AGPs)

– Role in not clear : thought to accumulate in response to elicitor released by fungi and role in plant defence response

ENZYMES IN PLANT DISEASES

• Enzymes are large protein molecules that catalyse all the interrelated reactions in a living cell

• Some enzymes are present in cells at all times (constitutive).

• Many are produced only when they are needed by the cell in response to internal or external gene activators (induced)

• Anton de Bary reported the role of enzymes and toxins in tissue disintegration caused by Sclerotinia sclerotiorum

Soft rot fungus : Sclerotinia sclerotiorum

• Cutinases, cellulases, pectinases and lignases are often secreted by the pathogenic organism.

• Fungi, nematodes and bacteria are all known to produce one or more of the above

• Enzymes in specific pathogen-host combinations.

• Viruses and viroids are generally not considered to secrete enzymes

Enzymes degrade cuticular wax

• In the cuticle of many aerial plant parts

• Some fungi produce enzymes that degrade waxes

• Eg: Puccinia hordei

Enzymes degrade cutin• Cutin- main component of

cuticle• Many fungi and bacteria

produce cutinase- degrade cutin

• Fungi constantly produce cutinase in small amount come in contact with cutin release small amount of monomers

• These enter the pathogen cell and trigger the expression of cutinase gene

• Presence of glucose suppresses the expression of cutinase gene

• Enzymes reaches the higher concentration at penetration point of germtube and infection peg of appressorium forming fungi

• Pathogens produce higher amount of cutinase seems to be more pathogenic than others

• Cutinase inhibitors or antibodies applied to plant surface protect plants from fungal pathogens

• Eg: Fusarium Penicillium spinulosum (rotting of leaves)

Pectic substances• Main component of middle lamella-intracellular cement• Pectin-polysaccharides with side chains• Enzyme degrade pectic substances are PECTINASES or

PECTOLYTIC ENZYMES• Examples

– Pectin methyl esterases- remove small branches of pectic chains and promote the attack of chain splitting pectinases

– Polygalacturonases – split the pectic chain by adding a molecule of water

– Pectin lyases – split the chain by removing a molecule of water

• Pectinase: 2 types– Endopectinase : break pectin chains at random sites– Exopectinase :only break terminal linkage

• Pathogen produce a small amount of pectin all the time

• When contact with the pectin in the host they produce small amount of monomers,dimers or oligomers

• These induce the pectinase production in large amount

• Pectin degradation produce many diseases particularly those characterised by soft rotting of tissues

• Pectic enzymes are produced by germinating spores and act together with other pathogen enzymes – helps in penetration of host

• Pectin degradation results in liquefaction of pectic substances that hold plant cells together- lead to weakening of cell wall – leads to tissue maceration

• Weakening of plant tissue and maceration lead to inter and intra cellular invasion of tissue by pathogen

• Eg – Brown rot (Monilinia fructicola) in plum– Bacterium Erwinia sp.– Psudomonas

CELLULOSE

• Cellulose :polysaccharide consists of chains of glucose molecules

• Cellulases degrade cellulose

• Soluble sugars serve as the food for pathogens• Cellulases produced by several phytopathogenic

fungi, bacteria, nematodes and parasitic higher plants

• Softening and disintegration of cell wall• Liberating large molecules to transpiration stream

which interfere with normal movement of water• eg:– Corn stalk rot fungus Fusarium– Rhizoctonia solani– Fusarium moniliforme – Sclerotium rolfsii

Hemicelluloses

• Complex mixture of polysaccharide polymers• Major constituent of secondary cell wall• Hemicellulose polymers include: xyloglucan,

glucomannan, galactomannans etc• Hemicellulases degrade hemicellulose• Depending on the monomer released after

degradation they are calledXylanaseGlucanaseGalactanase

ArabinaseMannase

• Fungal pathogens produce enzymes and oxidative agents (such as activated oxygen and other radicals) to degrade hemicellulose

• Eg.– Sclerotium rolfsii– Sclerotinia sclerotiorum– Diploidia viticola

Southern blight of tomato Dead arm of grapes

Lignin

• Found in middle lamella, secondary cell wall of xylem vessels and the fibers that strengthen the plants

• Only a small group of microorganism is capable of degrading lignin- more resistant to enzymatic degradation than other plant substances

• Ligninase degrade lignin

• Brown rot fungi degrade lignin but not utilise it• White rot fungi degrade and utilise lignin• Wood rotting basidiomycetes, several ascomycetes,

imperfect fungi and some bacteria produce small amount of lignin degrading enzymes and cause soft rot cavities in wood they colonise

• Eg.– Ceratocystis paradoxa (stem bleeding in coconut)– Polyporus versicolor– Some spp of Fusarium– Phellinus spp.

Enzymatic degradation of substances contained in the plant cell

• Proteins• Starch• Lipids

Protein

• Proteins have diverse role in the cellular reactions ( as enzymes) or as structural materials (in membranes and cell wall)

• Enzymes degrading proteins are called Proteases or proteinases or peptidases

• Proteolytic enzymes produced by most of the pathogens can affect the organisation and functions of host cells

Eg:• Pyricularia oryzae, • Phytophtora infestans • Helminthosporium oryzae

Rice blast

Starch

• Main reserve of polysaccharides found in plant cells• Starch is the glucose polymer containing 2 forms

amylose and amylopectin• Most pathogen utilise starch and other

polysaccharides in their metabolic activities• Degradation by enzyme Amylases• End product is glucose and it is used by pathogens

directly• Eg: Aspergillus spp

Lipids

• Many types of lipids• Oils and fats in many cells especially in seeds

as energy storage compounds• Wax lipids- in most aerial epidemal cells• Phospholipids and glycolipids along with the

proteins –in cell membranes• Lipolytic enzymes in several fungi, bacteria

and nematodes – Lipases, phospholipases

• Lipolytic enzymes hydrolyse fatty acids and liberate fatty acids utilised by pathogen

• Some times fatty acids are hyperoxidated by lipoxygenases or active oxygen molecules trigger the development of defence mechanism in plants

• Eg:– Sclerotium rolfsii, – Botrytis cinerea,– Thielaviopsis basicola

Grey mouldBlack root rot

SR.NO BIOMOLECULE DEGRADING ENZYMES

EXAMPLE FOR PATHOGEN PRODUCING ENZYME

1 Cuticular wax

Wax degrading enzymes Puccinia hordei

2 Cutin Cutinases Fusarium, Penicillium spinulosum

3 Pectic substances

Pectinases or pectyolytic enzymes

Monilinia fructicolaErwinia

Pseudomonas

SR.NO BIOMOLECULE DEGRADING ENZYMES

EXAMPLE FOR PATHOGEN PRODUCING ENZYME

4 Cellulose cellulasesRhizoctonia solani,

Fusarium moniliforme ,Sclerotium rolfsii

5 Hemi-cellulose

Hemi-cellulases

Sclerotium rolfsii, Sclerotinia

sclerotiorum, Diploidia viticola

6 Lignin Ligninases Trametes spp.,

Polyporus versicolor, Some spp of

Fusarium

SR.NO BIOMOLECULE DEGRADING ENZYMES

EXAMPLE FOR PATHOGEN PRODUCING ENZYME

7 Proteins Proteases or proteinases

Pyricularia oryzae, Phytophtora infestans

,Helminthosporium oryzae

8 Starch Amylases Aspergillus spp

9 Lipids LipasesSclerotium rolfsii, Botrytis cinerea,

Thielaviopsis basicola

REFERENCE • “PLANT PATHOLOGY” – George N Agrios : 5th

edition• “PLANT PATHOLOGY” – R S Mehrotra• “ENZYMES IN FUNGAL PATHOGENESIS” – Ales

Lebeda, Dacmar Jancova & Lenka Luhova: Plant physiology :vol 39

• Wikipedia encyclopedia

THANK YOU