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Tree Defence

IMPORTANT LECTURE​

Plants are not passive partners when getting munched on/hurt by disturbances

  • They are actually very dynamic

The way that plants and animals interact may have impacted nearly all animal evolution

Tree Defenses:

  • Lecture outline:
    • General introduction, the "green tree parodox"
    • Tree Defences
      • Types: Physical, Chemical, Symbioses, Natural Enemy Manipulation - Forms: Constitutive or Induced
    • The importance of redundant lines of defense: Conifer vs stem insects

The "green tree paradox": Why aren't all trees killed?​

An evolutionary arms race:

  • Trees evolve "slowly", but herbivorous insects can evolve "very quickly"
    • In 100 years:
      • ~Trees less than 1 generation;
      • ~Insects greater than 100 generations
        • Populations of target species started to evolve resistance to the insecticide DDT over only ~7 years

Why aren't all trees killed?​

  • Constraints on insect fitness:
    • Environment (e.g. climatic suitability)
    • Host availability
      • Physical availability
        • Alpha diversity (species)
        • Beta diversity (landscapes)
        • Gamma diversity (region/biomes)
      • Realized availability
        • Genetic diversity (quantity/quality of preferred tissues) Tree Defence

Tree defenses against herbivory: different types:​

  • Physical (e.g. trichomes, leaf toughness, etc.)
  • Chemical
  • Symbiotic relationships
  • Natural Enemy Manipulation

Two different forms:​

Constitutive:​

  • Passive preformed defenses
    • Physical
      • Bark
      • Stone cells
      • Thorns
      • Trichomes
      • Oleoresin/Latex
      • Leaf toughness
        • Cellulose
        • Mineral Inclusion
    • Chemical
  • Developed simply by growing
    • E.g., spines/thorns
    • E.g., resinous blisters on Abies

Induced:​

  • Active defensive response
    • Released when tree detects that it is being harmed
    • Mostly Chemicals
      • Energetically costly to produce
        • Suppressed trees or those without excess energy stores are at a disadvantage

Chemical Defense​

Chemical Defense (constitutive and induced)​

  • Secondary metabolites: organic compounds that do not have function in growth and development
    • Plant-plant competition (allelochemicals)
    • Plant microbe symbiosis
    • Attractants
      • Defense against pathogens

Constitutive chemical defense:​

  • Chemicals released from preformed structures due to injury
    • Deter/hinder feeding
    • Reduce digestibility of tissues (e.g. tannin)
  • Mostly carbon-based compounds
  • Terpenoids
    • Isoprene units
    • Monoterpenes = isoprene units C10H16
    • Other terpenes = multiples of C5 units (e.g. di-, tri-, sequi-)
  • Induced
    • Chemicals produced by living tissues in response to injury
      • Toxins intended to kill

Induced chemical defense:​

  • Active response to injury
  • Mostly nitrogenous compounds

Three groups of secondary metabolites

  • Nitrogen containing secondary products
    • E.g., nicotine, morphine, cocaine, caffeine, quinine
  • Phenolic compounds
    • Flavonoids
    • Anthocyanins
    • Salicylic acid
    • Tannin
  • Terpenoids
    • Limonoids
    • Saponins
    • Pinenes

Tree Defence: symbionts:​

Ants:​
  • e.g., swollen-thorn acacia
  • Pseudomyrmex ants protect the tree as it provides them living space (hallow thorns) and protein-rich food (Beltian bodies)
Epiphytes:​
  • Microorganisms on leaf surfaces
  • Bacteria and fungi
    • Prevents herbivory/infection
Endophytes​
  • primarily fungi
  • ubiquous in vascular plants
  • Systemic infections prevent herbivory
    • Herbivory
    • Drying
Natural enemy manipulation:​
  • Plants attract insect herbivore natural enemies
    • Herbivory stimulates release of volatile chemicals
    • Parasitoid wasps attracted to injured plant

Redundant lines of defense:​

-Conifer defense mechanisms redundancies:

  • First line (constitutive) = bark, stone cells
    • Physical
  • Second line (constitutive) = primary resinosis
    • Physical and chemical
  • Third line (induced) = secondary resinosis
    • Chemical

Effects of resinosis​

  • Physical barrier
    • Viscous
    • Rapid crystalization
    • Large volumes
  • Chemical barrier
    • Toxic monoterpenes
    • Fungistatic activity
    • Digestion inhibition

Primary resin​

  • Constitutive defense
  • Preformed compounds
    • Oleoresin
    • Mostly terpenoids
  • Species-specific deployment
    • Resin Canals (e.g. Pinus spp.)
    • Resin Blisters (e.g. Abies spp.)
    • Qualitative factors
      • Species and populative-specific terpenoid porilfes
      • Basis for resistance selection

Secondary resin​

  • Induced defense
    • Parenchyma cells lyse (rupture)
    • Resin canals increase in size and number
    • Qualitatively different from primary resin
      • Change in relative abundance of terpenoid consituents
      • More toxic compounds favoured

Induced defense: an effective barrier to invasion

  • Dramatic host response to insect/pathogen invasion
  • Formation of reaction zone
    • Rapid involvement of tissues surrounding points of penetration
    • Parenchyma cell death (autolysis)
    • Depletion of sugars
    • Formation of wound periderm
    • Secondary resin soaks area of response
  • Seals invading organisms in lesion of dead tissue
  • Occurs fast in healthy trees, slow in unhealthy trees