7) Plant Defense

Big idea framing

  • “Plants feed the world”:

    • all non-plants depend on plant food

    • every plant is under constant threat of attack (herbivory)

  • Herbivory:

    • widespread plant–animal interaction, often least visible; insects feeding day/night

A) Primary vs secondary compounds (super testable)

  • Primary compounds (basic metabolism):

    • products of photosynthesis/protein synthesis

    • carbohydrates, proteins, starches, cellulose, lignin

    • essential for normal cellular function

  • Secondary compounds:

    • no known direct function in basic metabolism

    • huge chemical complexity; strict control of synthesis

    • role = chemical weapons vs herbivores

  • Examples listed:

    • cyanide, strychnine, caffeine, mustard oil, morphine, cannabinol, quinine, atropine, THC, resveratrol, cocaine, limonene, jasmonate

B) Defense types (the classification the lecture uses)

Defense can be:

  • Systemic = always present

  • Induced = triggered by attack
    And it can be:

  • Physical (mostly systemic)

  • Chemical (often induced)
    So the full grid is:

  • systemic physical

  • systemic chemical

  • induced physical

  • induced chemical

C) Examples of systemic physical defense (shown)

  • Thorns and spines

  • Trichomes (hair density) vs herbivory study shown

  • Visual deception/mimicry examples shown as defense:

    • fake insect eggs; fake bird droppings; fake herbivory; fake damage spots

  • “Pebble plants” (Lithops) = passive hiding; rarely eaten

  • “Sensitive plant” = active hiding (folding leaves quickly), signal travels through phloem via electric currents

D) Examples of systemic chemical defense (shown)

  • Poison ivy example slide

  • Stinging nettle noted as systemic physical AND chemical

E) Induced defense examples (physical + chemical)

  • Induced physical:

    • grass defense vs grazing pressure (study shown)

    • holly “de-induced” example shown

  • Induced chemical:

    • plant makes enzyme after feeding that reduces caterpillar nutrient utilization and growth

    • enzyme concentration highest near wound (corn–armyworm example)

    • ant-bodyguard mutualism:

      • Cecropia-tree “calls” for help and pays with food reward

      • injured leaves → emits gas + more honey → attracts ants → ants limit herbivores

F) Arms race (plants vs herbivores)

  • Herbivores evolve to circumvent defenses → co-evolutionary arms race

  • Example shown:

    • milkweed toxins harm small mammals; monarch is immune and uses toxins for its own protection vs birds

  • Why plants don’t always defend at max:

    • “Defense is costly”

    • Jasmonic acid used as a plant-plant signal warning of herbivore attack (“gear up defense”)