Download How Plants Defend Themselves against Pathogens

8/17/2011
How Plants Defend
Themselves Against
Pathogens
Genetically-resistant apple showing the HR response (localized host cell
death and accumulation of phenolic compounds that autofluoresce under
UV light) to attempted invasion by Ventura inaequalis spores. The V.
inaequalis spore can be observed at the center of the autofluorescence.
(School of Biological Sciences, University of Auckland, NZ)
Two Categories of Defensive Weapons
• Structural Barriers
Inhibit Entrance or Spread of Pathogen
• Biochemical Reactions
Toxins
Pathogen Growth Inhibitors
Plants May Possess One or More of the
Weapons
Combination of Defensive Weapons
Depends on Various Factors
• Species
• Age of Plant
• Organ or Tissue Attacked
• Nutritional Condition of Plant
• Weather Conditions
First Line of Defense Is Surface
of the Plant
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Wax
– Water-Repellent Surface
– Prevents Film of Water Needed to Germinate
(Fungi) or Multiply (Bacteria)
Cuticle
– Thick Cuticle May Increase Resistance to
Infection but Not Always Correlated with
Resistance
Epidermal Cell Walls
– Thick, Tough Outer Walls Make Direct
Penetration Difficult or Impossible
– However, If Plant Invaded through Wound
Site, Inner Tissues Easily Invaded
• Fungal Spores Germinate at Night, then
Enter Plant Stomata when They Open Early
in the Morning
• Some Pathogens Can Force Their Way
through Closed Stomata, But Some, eg
Stem Rust of Wheat, Can Enter Only When
Stomata Are Open
• Varieties of Wheat (Triticum aestivum)
Resistant to Stem Rust Don‟t Open Stomata
Until Late in Day
•
Stomata
Regulation
Some Species of Fungi and Bacteria Enter via
Stomata
• Some Plant Species Resistant to Certain
Bacterial Pathogens Because of Stomata
Structure
– Very Narrow Entrance
– Broad, Elevated
Guard Cells
caliban.mpiz-koeln.mpg.de/.../ snapdragon/anatomy/bract.html
– Germinated Spores Waiting for Stomata to
Open…Dry Out in Heat of Day, Die
Other Anatomical Features
• Sclerenchyma Bundles
in Stems
– Sclerenchyma fibers usually
occur in bundles. The
individual cells are highly
elongated with overlapping
end walls. They are usually
lignified. Lignin makes cell
walls extremely strong and
inflexible. This makes
sclerenchyma a good
support tissue.
– Sclerenchyma is usually
associated with vascular
tissues and may completely
surround them.
• Cell Wall Thickness, Toughness May Sometimes
Inhibit Advance of the Pathogen
• Xylem, Bundle Sheath and Sclerenchyma Cells of
Leaf Veins Effectively Block Spread of Some
Fungal, Bacterial and Nematode Pathogens that
Cause Angular Leaf Spots
– Pathogens Able to Spread Only into Areas Between, but
Not Across, Veins
SEM photo of a Fiber Bundle
www.botany.hawaii.edu/.../CellTissOrgan/ CellTissOrgan-8.htm
Angular leaf spot on pumpkin (left) and strawberry (center and right).
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Plants Release
Inhibitors into
Environment
Fungitoxic Exudates
Inhibitors in Plant Cells
• Tomatoes vs. Botrytis
• Sugar Beets vs. Cercospora (Fungal
Leaf Spot Disease)
• Red Onions vs. Onion Smudge
(Colletotrichum circinans)
www-plb.ucdavis.edu/.../Tomato/
Stems/epidermis1.html
• Pathogens Secrete Enzymes to Break down
certain Plant Substances such as Pectin
– Pectin Is a Gel-Like Polysaccharide, Found in Fruit and
Vegetables
• Phenolics and Tannins Inhibit Enzymes that
Pathogens Use to Break down Pectin
– Phenolics Are Reactive Acidic Compounds
– Tannins Act As Astringents, Shrinking Tissues and
Contracting Structural Proteins
• Saponins
– Attack (Break down) Fungal Membranes
– Checkmate?
• Some Pathogens Produce Saponinase Enzymes to
Break down the Saponin
– Host Range of Fungus Depends on whether the
Plant Produces Saponin or whether the Fungus
Produces Saponinase
• Lectins
– Proteins that Bind to certain Sugars and Inhibit
Fungal Growth
• Enzymes
– Break down Pathogen‟s Cell Walls
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Lack of Recognition
Between Host and
Pathogen
Recognition Sites or Factors on
Surface of Plant Cell Walls
• Specific Molecules or Structures on or Embedded
in Cell Walls
–
–
–
–
Oligosaccharides
Polysaccharides
Proteins
Glycoproteins
• If Plant Cell Surface Lacks certain Molecules that
the Pathogen „Recognizes,‟ the Pathogen may not
Recognize the Plant as One of Its Hosts
– Pathogen may not Become Attached to the Plant
– May not Produce Infection Substances (Enzymes) or
Structures (Appressoria, Penetration Pegs, Haustoria)
Necessary for Establishment of Infection
Lack of Essential
Substances for
Pathogen Survival
• Rhizoctonia Needs a Substance from Plants
to Grow Its Hyphal Cushion
• In Some Host-Pathogen Combinations,
Disease Develops but amount of Disease Is
Reduced because certain Host Substances
Are Present at lower Concentrations
– Bacterial Soft Rot of Potatoes Is less Severe on
Potatoes with low Reducing Sugar Content than
on Potatoes High in Reducing Sugars
To Trigger any Induced Change:
Plant Begins to Receive Signal Molecules to
Indicate Presence of Pathogen as soon as
Pathogen Establishes Physical Contact with
Plant
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Cytoplasmic Defense
Reaction
• Cytoplasm Surrounds Hyphae
• Cell Nucleus Stretches, Breaks
• Dense, Granular Cytoplasm Full of New
Particles, Structures
• May Stop Invasion of Hyphae of SlowGrowing, Weakly Pathogenic Fungi and the
Mycorrhizal Fungi
Cell Wall Defense Structures
(Limited Effectiveness)
• Plant cells are usually enclosed by a
more or less rigid cell wall containing
cellulose
• Structure of cell walls can be
compared to structure of reinforced
concrete
– The scaffolding substance is cellulose
– The iron is embedded in an amorphous
ground substance
• Lignin (a phenolic compound) in wood
• Suberin in cork
• The cell wall has a number of
functions
–
–
–
–
–
Cellulose
Lends the cell stability
Determines its shape
Influences its development
Protects the cell against pathogens
Counterbalances the osmotic pressure
Cell Wall Defense Structures
Cellulose = linear chains of
glucose residues
Very stable chemically and
extremely insoluble
The primary cell wall consists a
glucose polymer of roughly
6,000 glucose units
Cellulose-Microfibrils in Isolated
Walls of the Green Alga Oozystis
solitaria
http://www.biologie.unihamburg.de/b-online/e26/3.htm
The secondary wall is contains
13,000 to 16,000 glucose units
Cellulose chains form
crystalline structures called
microfibrils (diameter of 20-30
nm which contains about 2,000
molecules)
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Three Main Types:
Swelling of Wall Outer Layer
with Synthesis of New Amorphous
Material to Surround and Trap
Bacteria
Phenolics Embedded in NewlyDeposited Cellulose
Callose Sheaths around Hypha
• Produced by Cells within Minutes after
Wounding and 2-3 Hours after
Inoculation by Microbes
• Main Function of Callose Papillae Seems to
Be Repair of Cellular Damage,
Sometimes They Prevent Pathogen from
Penetrating the Cell further
• Sometimes Callose later Becomes Infused
with Phenolic Substances
Other Anatomical Defense
Structures
Cork
Layers
Abscission
Layers
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Tyloses
Gum Deposition
in
Anatomical and Physiological Responses of
Bark Tissues to Mechanical Injury
Alan R. Biggs, West Virginia University
Xylem
“…Between 8 and 12 days after wounding, the
primary walls and the middle lamella in the
boundary zone exhibited an increase in electron
density. This is due probably to the deposition
of phenolic polysaccharide material in the wall.
In peach, these substances, usually referred to
as gum, are produced nonspecifically in
response to wounds or infections…”
Vessels
Hypersensitive Response
• Biochemical Response with Visible
Cellular Responses
– aka Necrotic Defense Reaction
• In Fungal Invasions, Cell Suicide
Occurs
• In Bacterial Invasions, Cell
Membranes Destroyed, Followed By
Desiccation and Necrosis of Invaded
Leaf Tissues
Agrios text, Fig. 5.9.
Cowpea (Vigna
unguiculata)
Hypersensitive
Response to Tobacco
Ringspot Virus.
Agrios text, Fig. 5.8. Stages in development of necrotic defense reaction in a cell of
a very resistant potato variety infected by Phytophthora infestans (Late Blight of
Potato).
Hypersensitive Response
(Not always Visible)
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Active Oxygen Radicals
• Upon Attack, Get Rapid and Transient
Generation of Superoxide (O2-),
Hydrogen Peroxide (H2O2), Hydroxyl
Radical (•OH)
• Destroy Plant Cell Membranes
• May Destroy Pathogen Membranes
Reinforce Host Cell Walls
with Strengthening Molecules
Phenolics:
Suberin and Lignin
Callose
Glycoproteins
Mineral Elements:
Silicon and Calcium
• Part of Hypersensitive Response
• May Oxidize Phenolics = Make them Active or
more Toxic
Produce Proteins Toxic to
Invading Pathogens
• Inhibit Spore Release, Germination
• Break Down of some Fungi and
Bacteria Cell Walls
Detoxify Pathogen Toxins
Phytoalexins (Substances
Toxic to Microbes)
Phenolics
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