Plant Physiology Forum

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Plant Physiology Forum

• Co-ordinators Phillip Joy
• Kathy Pickle

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Insect-Induced Conifer Defense.

• Miller, B., Madilao, L.L, Ralph, S., and Bohlman,
  J. 2005. Plant Physiol. 137 369-382.

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Forestry Problems

• Bark invading insects are major economic and
  ecological concerns
• Beetles and their vectored fungal pathogens are
  the most destructive agents of conifer forests
  worldwide.
• Pest management practices in agriculture do not
  apply to forestry.
• Conifers have evolved defense strategies towards
  these invaders
• overlapping mechanical and chemical defenses and
  the capacity to up-regulate additional defenses.

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• Adult white pine weevil

• White pine showing death of leader and top
  lateral growth from weevil attack

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Chemical Defense

• Diverse group of terpenoids
• derived from isoprene C5H8 units
• different terpenoids have different multiples of
  units
• Mono-, di-, and, sesqui-
• Major components of oleresins
• Function as repellents or toxins to herbivores or
  pathogens

Myrcene, a monoterpene
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Resin

• Secretion formed in special resin canals
• Exuded in soft drops from wounds
• Resins seal the plant's wounds, kill insects and
  fungi, and allow the plant to eliminate excess
  metabolites.

Conifers such as this Douglas fir ignite like a
torch during fire storms due to the combustible
terpene oleoresins
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The Experiment

• Purpose
• To determine if stem-boring insects and methyl
  jasmonate (MeJA) treatments invoke similar
  defenses in conifers.
• Analyzed
• Oleoresin mixtures
• Emissions of terpenoid volatiles
• Expression of terpenoid synthase (TPS) genes

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Methyl-Jasmonate (MeJA)

• Among the most potent plant signaling molecules
• Damaged or infested plants produce higher levels
  of MeJA
• Defense mechanisms include
• Interference with herbivore digestive system
  deterring insect feeding.
• Volatile organic molecules
• that serve as attractants for predators of the
  herbivores.
• act as a messenger to neighboring undamaged
  plants, prompting them to produce defensive
  chemicals
• Treatment with MeJA induced
• formation of resin ducts
• increased accumulation of oleoresin terpenoids
• Unclear how well MeJA treatment mimics the
  effects of insects in inducing defense.

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Results

• Trepenoid accumulation in stems
• Both Weevil and MeJA induced traumatic resin duct
  formation
• Increase of Terpenoids after treatment with MeJA
  or Weevil attack

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Results

• TPS Transcript Accumulation in Stems
• Both MeJA and Weevil induce mono-TPS
• Both MeJA and Weevil induce di-TPS
• Weevil induces Sesqui-TPS
• Spatial Patterns of Weevil defense
• Weevils on top no increase of terpenoids in
  distal tissue
• Weevils on bottom an increase of TPS in upper
  tissue

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Results

• Emissions
• MeJA
• Max at 3d
• Diurnal
• Weevil
• Max at 6-12hr
• Not diurnal
• Active mechanisms, not passive release from ducts
  and blisters
• Larger variety of emissions where ID

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Results

• Whats going on in the needles?
• Monotrepeniods, specifically (-)- linaloon (an
  alcohol) is formed and released from the needles
• No Ditrepeniods are found

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Discussion

• Both Weevil and MeJA induce similar chemical
  defense
• Weevil defense is stronger and more complex
• They activated similar TPS genes, but Weevils
  induced more and with more strength
• So, can MeJA be used to jump-start Weevil
  defense??

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Discussion

• Both Weevil and MeJA induce Traumatic resin ducts
  to be formed
• Stronger for Weevils
• Due to the wounding factor?
• Due to continues feeding?
• Is MeJA releasing the passive/stored terpenoids?
• More likely activating their production
• What is the role of the stored terpenoids?

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Discussion

• Weevil feeding causes some signal translocation,
  will MeJA?
• What other Octadecanoid pathway intermediates are
  involved?
• JA synthesis pathway
• What about other xylem regulators (forms TDs)?
• Ethylene?

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Conclusion

• MeJA induces some of the same defenses as Weevil
  attack, therefore it may be able to activate
  Weevil defense prior to attack
• The role of TDs are still not truly understood
• Contain terpenoids, are formed as a defense
  mechanism, fills with resin
• Are the emissions inter-plant communication?

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Questions Comments