How Pathogens Attack Plants
• Why Do Pathogens Attack Plants?
– Evolution
– Acquired Ability to Live Off Substances
Made by Plants
Pathogen’s Weapons
• Mechanical for ‘Big’ Pathogens
• Chemical/Enzymatic
• Hurdles for Successful Attack
– Penetration
– Neutralize Defense Reactions
– Convert Cell Components Into Food
http://www.itqb.unl.pt/Research/Biological_Chemistry/Protein_Modelling/Activities/?link=3
Fungi & Parasitic Plants
2. Must Penetrate
Surface
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1. Must Adhere to Surface
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Mucilage Around Hyphae, Radicles
Molecular Forces Between Organisms
Spore Forms an Adhesion Pad
Secretes Enzymes to Help It Stick
Tip of Hypha or Radicle Enlarges, Forms
Appressorium
– Increases Surface Area Between Organisms
– Adhesive
Adhesive pad covering tip of adhesive
conidia of the endoparasitic fungus
Drechmeria coniospora
Penetration Peg
• Threadlike Hyphal Strand
• Penetration Easy if Host
Wall Is Soft
• More Difficult if Wall Is
Hard
www.apsnet.org
– Pathogen Usually
Secretes Enzymes to
Soften Barrier
– Once Peg Penetrates
Cuticle, Hyphal Tube
Swells
http://www.ua.es/dpto/dcarn/fitopatologia/Appress.htm
invam.caf.wvu.edu/.../ Glomaceae/glomaceae.htm
Nematodes Attack
Directly
Mechanical Weapons
• Fungi Want Fruiting
Structures Above Surface
• Apply Mouth Suction
• Needlelike Stylet Thrusts
Back & Forth
• Rear Part of Body Sways
or Rotates
• Once It Pierces Wall,
Stylet or Entire Nematode
Enters Plant
spore-bearing fungal structures (SEM of
acervulus of Colletotrichum trifolii).
www.apsnet. org
– Hyphae, Fruiting Structures
Push Outward
– Cells Walls & Cuticle
Expand
– Rise as Blister & Finally
Break
Peter M. Sforza & Jon D. Eisenback
Department of Plant Pathology, Physiology, & Weed Science
Virginia Polytechnic Institute & State University
Blacksburg, Virginia, USA
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The Most Common
Method of Attack
• Secretions
– Enzymes
• Soft Rots
– Toxins
• Victoria Oats Blight
– Growth Regulators
• Crown Gall
– Polysaccharides
Fusarium oxysporum Causes Wilt Diseases in Tomato, Cabbage & Other Plants
• Some Pathogens Produce
Compounds to Suppress
Plant’s Defense
Responses
– Wheat Stem Rust Fungus
• Suppressor Chemical Found
in Infected Wheat Leaves
Viruses Don’t Produce Chemicals
• Induce Host Cells to Produce the Chemicals
• May/May Not Be a Chemical Already Made by
the Cell
Why Excrete the Chemicals?
• Enzymes
– Disintegrate Cell Structures
– Interfere With Cell Functions
• Toxins
– Interfere With Membrane Permeability, Functions
• Growth Regulators
Enzymes
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Large Protein Molecules
Catalyze Reactions in Living Cells
Already Present or Induced
Glucose Suppresses Production of
Cutinase
– Increase or Decrease Ability to Divide & Enlarge
• Polysaccharides
– Vascular Pathogens Block Translocation of Water
– May Be Used as Toxin
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Pectinases
Cutinase
• Cuticle Made of Cutin
– Cutin = Waxes on Top, Pectin &
Cellulose on Bottom
• Cutinase
• Hydrolyzes Cutin Molecules Into smaller
Pieces
• Fungi Constantly Produce Low Levels
• When Cutinase Encounters Cutin, Molecules
Get Broken
• Triggers Feedback Effect
• Liquefy Pectin
• Middle Lamella Is Made of
Pectic
Substances
• Pathogens Release Pectinases Constantly
• Also Has Feedback Effect
• Glucose Also Suppresses Pectinase
• Pectinase Triggers Defense Responses in
Resistant Plants
• Different Isozymes of Pectinase Can
Affect Host Specificity
Growth Regulators
Toxins
• Extremely Poisonous
• Effective in Low
Concentrations
• Injure Host Cells
Sclerotinia sclerotiorum on Soybean
– Affect Membrane
Permeability
– Deactivate or Inhibit Enzymes
– Induce Deficiency of
Essential Growth Factors
– Toxins May/May Not Be Host Specific
Pseudomonas syringae pv. tabaci
• Pathogen GRs May Cause Imbalance in
Plant Hormones
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Stunting
Overgrowth
Rosetting
Excessive Root Branching
Stem Malformation
Leaf Epinasty
Defoliation
Bud Growth Suppression
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Hormones
Growth Stimulation or Growth Inhibition
Auxins, Gibberellins, Cytokinins, Ethylene
Powerful at Low Concentrations
– Slight Deviation From Normal May Cause
Strikingly Different Plant Growth Patterns
• May Promote Synthesis of mRNA Molecules
– Trigger Formation of Enzymes to Control Plant
Biochemistry or Physiology
Polysaccharides
• Used by Vascular Pathogens to Block
Translocation of Water
• May Be Used as Toxin
http://www.ictvdb.rothamsted.ac.uk/ICTVdB/00.029.0.02.001.htm
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