Biotic Stress and Plant Defense Responses Whos Minding the Fort

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Biotic Stress and Plant Defense Responses (Whos
Minding the Fort?)

• Land plants are attacked by
• Herbivores (insects, mammals, etc.)
• Pathogens

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Pathogen Strategies

• Necrotrophic plant tissue killed and then
  colonized broad host range
• e.g., rotting bacteria (Erwinia)
• Biotrophic plant cells remain alive, narrow
  host range (1 plant species)
• e.g., viruses, nematodes, fungal mildews
• Hemibiotrophic biotrophic phase, then
  necrotrophic, intermediate host range
• e.g., Phytophthora (potato blight disease)

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Major Pathogens

• 1) Viruses - most are RNA viruses
• Have a small compact genome which encodes
• coat protein
• RNA-dependent RNA polymerase (for replication)
• movement protein(s)
• Some plant RNA viruses also encode
• protease for processing polyprotein (Potyvirus)
• capping enzyme
• RNA helicase
• Some also exist as multiple particles, and a
  segmented RNA genome (e.g., Bromoviruses, 3
  particles needed for infection).

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An encapsulated SS RNA virus Tobacco mosaic
virus
DS DNA virus, Cauliflower Mosaic Virus
Fig. 21.10, Buchanan et al.
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Genome of Cauliflower Mosaic Virus (CaMV)
Fig. 21.11 , Buchanan et al.
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1a) Viroids- naked RNA infectious agents

• Single strands of RNA
• Lack the protein coat of viruses
• Do not code for any specific protein
• Replicate in the nuclei of infected cells
• e.g., Potato spindle tuber viroid was the
  first to be identified.

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Viroid structure
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• 2) Bacteria
• 3) Fungi - plant pathogens found among 4 major
  groups
• 4) Nematodes - major root parasites, also
  increase infection by microorganisms
• Insects and other herbivores can also transmit
  plant diseases.

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Distribution of Oak Wilt in the US
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Hill Country Oak
Leaves from Infected tree
Fungus - Ceratocystis fagacearum
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Modes of Transmission of Oak Wilt fungus
Sap beetle
Natural root grafts
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Plant Defenses

• 1) Physical barriers cuticle, thorns, cell walls
• 2) Constitutively produced chemicals (e.g.,
  phytoalexins) and proteins (e.g., Ricin)
• 1 and 2 together provide non-host resistance
  against most potential pathogens. However, some
  pathogens get around these protections (but only
  for certain plants), and generate induced
  responses, which work to limit the disease.
• 3) Induced responses

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Plant Defense Response
Compatible interaction ? disease Incompatible
interaction ? resistance

• 3 aspects of response
• Hypersensitive
• Local
• Systemic

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Hypersensitive response

• Rapid - within 24 h
• Not always needed for resistance
• Includes
• oxidative reactions (production of hydrogen
  peroxide)
• deposition of callose (related to cellulose)
• opening of ion channels
• apoptosis (programmed cell death)

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Disease symptoms (necrotic lesions surrounded by
chlorosis) caused by the phytobacterium
Pseudomonas syringae pv. tomato (Pst) strain
DC3000 on inoculated leaves of a susceptible
Arabidopsis thaliana Col-0 plant. Green leaves
were not inoculated. Photo by R. Thilmony
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Local responses

• Cessation of cell cycle
• Induction of genes that promote resistance
• Phenylpropanoid pathway induced products include
  salicylic acid (secondary inducer induces other
  pathogenesis-related proteins), lignins (cell
  wall), and flavonoids
• Pathogenesis-related (PR) proteins
• Phytoalexins increased
• Fortification of cell walls with lignin,
  hydroxyproline-rich glycoproteins (HRGPs), etc.

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Systemic Response or Systemic Acquired Resistance
(SAR)

• SAR takes 24-48 h to start, can last for months
• Involves gene activation and a transmitted
  signal.
• Genes induced
• chitinases
• ß 1,3- glucanases
• other PR proteins

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Signaling Cascade for Defense Responses

• Molecular nature of elicitors
• Cell wall proteins (e.g., Harpin)
• Intracellular proteins (defined genetically in a
  bacterium by cloning avirulent loci)
• Peptide derived from a larger protein (from a
  fungus)
• Heptaglucan (small oligosaccharide)

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Secondary Signals (3)

• Ca2 , required for subsequent steps
• May mediate phosphorylation-dephos. events
  involved in transcriptional or post-
  transcriptional gene regulation (there are a
  number of genes whose transcription increases,
  and some decrease)
• Some defense genes also induced by blue-UV light
  or other stresses

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2. H2O2 (hydrogen peroxide)

• Plays multiple roles
• induces defense-related genes
• induces apoptosis
• causes cross-linking of cell wall proteins (more
  resistant to wall-degrading enzymes)
• may directly kill pathogens

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3. Salicylic acid

• required for SAR
• levels increase locally and at distance from
  infection
• Systemic Signal? Probably not. Still unknown.

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Model derived mostly from studies in cell culture
using specific elicitors. However, there is
evidence for induction in inact plants by R
genes. Some aspects are also constitutive and
help block most microbes (non-host resistance).