Plant basal defenses

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Plant basal defenses
1. Pre-existing
2. Induced
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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.

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Plant pathogenic bacteria are extracellular
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How do pathogens enter the apoplast?
Fungi
Bacteria
penetration peg
Illustrated glossary of plant pathology
www.apsnet.org/
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Pathogen-induced responsesIts 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.
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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.
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Cell polarization and papilla formation upon
fungal infection
Schmelzer (2002). Trends in Plant Science 7
411-415
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Vesicles carrying antimicrobial compounds can be
observed under the microscope
Snyder and Nicholson, Science (1990)
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Aggregation of vesicles in response to fungal
infection
Plant proteins can be visualized by tagging
them with fluorescent markers (GFP, YFP, etc)
Polarization of microfilaments in response to
fungal infection
Koh et al., The Plant Journal (2005)
Shimada et al., MPMI (2006)
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Induced basal defenses (Innate immunity)
Bacterium
Signal transduction cascade
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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)

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Q4 How can microbes be successful pathogens?
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• Successful pathogens are able to
• Suppress or evade host basal defenses
• Interfere with host cell metabolism, altering it
  to their own advantage

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Strategies used by bacterial pathogens
Abramovitch et al. Nature Reviews Molecular Cell
Biology 7, 601611 (August 2006)
doi10.1038/nrm1984
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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 AlfanoCellular Microbiology 6 (11), 10
27-1040.
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Bacterial virulence effectors suppress host
innate immunity
Bacterium
Signal transduction cascade
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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)