Chapter 10
Chapter Title: Mechanisms of Infection and Invasion of Plants
Author: Leila Geagea, PhD
Page 1: Overview
Introduction to mechanisms of plant infection and invasion.
Page 2: Conditions for Plant Diseases
Three conditions for disease:
A virulent organism.
A susceptible host-plant.
Favorable climatic conditions.
Page 3: Plant Cell Structure
Components of plant cells:
Cell wall, cell membranes, cytoplasm, nucleus, and organelles.
Pathogen Interaction:
Pathogens evolved to utilize substances produced by host plants, relying on them for survival.
Page 5: Pathogen Penetration
Critical factors for pathogen invasion:
Favorable climatic conditions before penetration.
Must penetrate the cuticle and cell walls.
Further invasion requires overcoming additional cell walls.
Page 6: Requirements for Infection
A pathogen must:
Enter and traverse the plant.
Obtain and assimilate nutrients, often requiring breakdown of complex substances.
Neutralize plant defense mechanisms.
Page 7: Penetration Mechanism
Methodologies for invasion:
Exert mechanical force on plant cell walls.
Secrete chemicals affecting plant metabolic processes.
Page 8: Fungal Infection Mechanics
Adherence Requirement:
Fungi must adhere to plant surfaces before penetrating.
Some fungi use mucilaginous substances to enhance adherence via surface contact forces.
Page 9: Fungal Adhesion Mechanisms
Fungi often develop:
Adhesion pads from hydrated spores.
Adhesive tip substances for effective attachment.
Page 10: Formation of Appressorium
Appressorium Development:
After adhesion, the hyphal tip enlarges and forms an appressorium.
Increases contact area with the plant and facilitates penetration through barriers.
Page 11: Enzymatic Assistance in Penetration
Fungal penetration is often aided by enzymes secreted to soften or dissolve plant barriers.
Page 12: Other Pathogens
Nematodes:
Utilize a stylet to penetrate cell walls.
Bacteria:
Enter through wounds or natural openings.
Viruses:
Introduced by vectors.
Page 13: Pathogen Effects on Plants
Effects stem from biochemical reactions between pathogen secretions and plant substances.
Major pathogen-secreted substances include:
Enzymes.
Toxins.
Growth regulators.
Polysaccharides.
Page 14: Enzymatic Action
Role of Enzymes:
Large protein molecules catalyze reactions; first contact commonly occurs at plant surfaces.
Enzymes break down substances found on plant surfaces.
Page 15: Specific Enzymatic Actions
Key Enzymes:
Cutinases: Break down cutin molecules, essential for fungi penetrating the cuticle.
Pectinases: Cause tissue softening and rotting (e.g., Botrytis cinerea).
Page 16: Cellulose Degradation
Cellulases:
Breakdown cellulose, leading to cell wall disintegration.
Page 17: Importance of Toxins
Toxins are extremely potent, effective in low concentrations, damaging or killing host cells.
Mycotoxins affect consumers rather than the host plants directly.
Page 18: Nonhost-Selective Toxins
Definition and Impact:
Affect multiple plant species, increasing disease severity beyond the immediate host.
Page 19: Examples of Nonhost-Selective Toxins
Tabtoxin: Produced by Pseudomonas syringae pv. tabaci, causing necrosis in multiple hosts.
Phaseolotoxin: Produced by Pseudomonas syringae pv. phaseolicola, affecting legumes.
Page 20: Additional Nonhost-Selective Toxins
Tentoxin: Produced by Alternaria alternata, causing tissue damage across various species.
Page 21: Host-Selective Toxins
Characteristics:
Toxic specifically to certain hosts; necessary for the pathogen's ability to cause disease.
Page 22: Example of Host-Selective Toxin
Victorin (Hv-toxin):
First identified host-specific toxin from Cochliobolus victoriae, toxic to specific oat variety.
Page 23: Growth Regulators Overview
Role in Plant Growth:
Regulate growth through naturally occurring compounds (hormones) acting in low concentrations.
Page 24: Production of Growth Regulators by Pathogens
Pathogens may produce:
More of the same growth regulators as plants.
Inhibitors of plant growth regulators.
Page 25: Auxins
Functions of Auxins:
Promote cell elongation and differentiation; crucial in plant growth.
Excess auxins can cause abnormal plant growth (galls).
Page 26: Pathogens and Auxins
Pathogens can:
Induce increased levels of auxins or produce their own.
Inhibit breakdown of existing auxins.
Page 27: Gibberellins
Effects of Gibberellins:
Promote elongation, flowering, and root growth.
Page 28: Gibberellins in Plant Pathology
Gibberellins can activate previously off genes in infected plants, leading to exaggerated growth.
Page 29: Gibberellins Application
Spraying diseased plants with gibberellins can temporarily alleviate symptoms, but the pathogen remains active.
Page 30: Cytokinins Importance
Roles of Cytokinins:
Essential for cell growth, differentiation, and delaying senescence (aging).
Page 31: Impact of Polysaccharides
Mechanical Blockage:
Large polysaccharides produced by pathogens can block vascular tissues, leading to wilting.
Page 32: Suppressors
Pathogenicity Factors:
Suppressors hinder expression of plant defense mechanisms, facilitating pathogen establishment.