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Plant basal defenses
1. Pre-existing
2. Induced
Pre-existing defenses: the first barrier
• Physical barriers involve properties of the plant
surface, that is, the cuticle, stomata and cell walls.
• Chemical barriers include compounds, such as
phytoanticipins that have antimicrobial activity, and
defensins, which interfere with pathogen nutrition
and retard their development.
Plant pathogenic bacteria are extracellular
How do pathogens enter the apoplast?
Fungi
Bacteria
penetration peg
Illustrated glossary of plant pathology
www.apsnet.org/
Pathogen-induced responses:
It’s a race!!!!
“As soon as a plant has recognized an attacking pathogen, the race is
on. The plant attempts to prevent infection and to minimize potential
damage, the pathogen attempts to gain access to nutrients for growth
and reproduction.”
Schmelzer, 2002
Question 3:
On your carbonless paper, make a model that may enable us to
follow the events that will occur in the plant cell during basal
defense against pathogens.
Signal
transduction
events
PAMPS
(Pathogen-Associated Molecular Patterns)
oligosaccharides, lipids, polypeptides
(flagellin), glycoproteins, etc…
Espinosa, Avelina & Alfano
Disabling surveillance: bacterial type III secretion system effectors that
suppress innate immunity.
Cellular Microbiology 6 (11), 1027-1040.
Cell polarization and papilla formation upon
fungal infection
Schmelzer (2002). Trends in Plant Science 7: 411-415
Vesicles carrying antimicrobial compounds can
be observed under the microscope
Snyder and Nicholson, Science (1990)
Plant proteins can be visualized by “tagging” them
with fluorescent markers (GFP, YFP, etc…)
Aggregation of vesicles in response to fungal infection
Polarization of microfilaments in
response to fungal infection
Koh et al., The Plant Journal (2005)
Shimada et al., MPMI (2006)
Induced basal defenses (Innate immunity)
Antimicrobial
compounds,
Defense proteins
Bacterium
Papilla
Signal
transduction
cascade
Secretory
pathway
Golgi
Hormones
(Salicylic acid,
jasmonic acid,
ethylene)
Nucleus
DNA
RNA
nucleus
Antimicrobial
compounds
New proteins
ER,
translation
Output of Induced basal defenses
Recognition events (elicitors, receptors)
• Signal transduction cascades
– MAP kinases, phosphorylation cascades
• Chemical changes:
– Synthesis of NO, ROSs, signaling molecules (SA, JA, Ethylene), etc…
• Gene expression changes (transcriptional regulation)
• Synthesis of antimicrobial compounds and proteins (phytoalexins, PR
proteins)
• Cytoskeletal rearrangements, vesicle trafficking, secretion
• Morphological changes (organelle redistribution, papilla deposition, cell wall
modifications)
Q4: How can microbes be successful pathogens?
Successful pathogens are able to:
1. Suppress or evade host basal defenses;
2. Interfere with host cell metabolism, altering
it to their own advantage
Strategies used by bacterial pathogens
Abramovitch et al. Nature Reviews Molecular Cell Biology 7, 601–611 (August 2006) | doi:10.1038/nrm1984
Plant pathogenic bacteria secrete proteins called
“virulence effectors” directly into the host cell
Bacteria use a sophisticated “injection” apparatus, called a Type III Secretion
System, to deliver virulence effector proteins directly in the cytoplasm of the
host cell.
Bacterial type III effectors disable host surveillance by suppressing innate
immunity.
Espinosa & Alfano
Cellular Microbiology 6 (11), 1027-1040.
Bacterial virulence effectors suppress host innate immunity
Bacterium
Antimicrobial
compounds,
Defense proteins
Papilla
Signal
transduction
cascade
Secretory
pathway
Golgi
Hormones
(Salicylic acid,
jasmonic acid,
ethylene)
Nucleus
DNA
RNA
nucleus
Antimicrobial
compounds
New proteins
ER,
translation
Some of the available tools for
dissecting plant-pathogen interactions:
- Pathogenesis Assays
(assessing symptom development and pathogen multiplication in the host)
- Microarrays
(analysis of global gene expression in the host plant)
- Genetic transformation
(expression of any given plant or pathogen gene in the host plant)
- Gene knock-out
(both in plant and pathogen)
- Fluorescent protein tagging and microscopy
(allows visualization of protein localization and cellular dynamics)