This document discusses how plant pathogens cause disease in plants. It explains that pathogens produce enzymes and other substances that allow them to degrade the structural barriers in plant cell walls like cutin, cellulose, pectin, and lignin. This allows the pathogens to enter plant tissues and obtain nutrients. It provides examples of specific enzymes produced by different pathogenic fungi, bacteria, and nematodes to degrade wax, cutin, cellulose, hemicellulose, pectin, protein, starch, and lipids. Degrading these plant cell wall components weakens plant tissues and interferes with their metabolic processes.
3. 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
PATHOGEN
v/s
PLANT
4. 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
6. How Pathogens affect Plants
There are many ways in which plant disease
pathogens can affect plants
By utilizing host cell contents
By 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.
7. •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
8. • Pathogen produce these substances
– In normal course of their activities :constitutively
– When the grow on certain substrates such as their
host plants : inducible
9. • 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
10. • 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
11. 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).
12. • 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.
13. 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
14. 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
16. • 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
17. Enzymes degrade cuticular wax
• In the cuticle of many
aerial plant parts
• Some fungi produce
enzymes that degrade
waxes
• Eg: Puccinia hordei
18. 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
19. • 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
20. • 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)
21. 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
22.
23. • 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
24. • 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
25. • 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
27. • 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
28. 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 called
Xylanase
Glucanase
Galactanase
Arabinase
Mannase
29. • 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
30. 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
31. • 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.
33. 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
35. 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
36. 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
37. • 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
