14: Plant Defenses + Their Use by Humans

• Plant defense categories:

  • Mechanical
  • Chemical
• Bark: impenetrable barrier (mechanical)
• Waxy Cuticle (Mechanical):

  • Multi-layer structure of waxes cutin (a polyester) + a cutan (a hydrocarbon polymer), polysaccharides
  • Hydrophobic barrier
  • Reduces water loss
  • Blocks pathogens from entering
• Trichomes (mechanical + chemical):

  • Tiny hair outgrowths of epidermis
  • Obstruct insect movement + feeding, making it harder for insects to feed on leaves
  • May be longer than insect proboscis, protecting plant
• Trichomes may be high density --> fuzzy plants
• Non-glandular trichomes: short, stiff, provide physical protection
• Glandular Trichomes (chemical):

  • Diverse in shape, cell number, + type of secreted metabolites
  • Many metabolites have role in plant defense + adaptation to biotic + abiotic stresses
• Glandular trichomes of Gorgons dewstick:

  • Traps insects in sticky resin
  • Can't digest the insects --> relies on a symbiotic relationship with an assassin bug that digests the insects + deposits feces on the leaf surfaces providing nutrients to the plant
• Peppermint (chemical):

  • Oils released by trichomes are high in menthol and menthone + repels aphids and other insects
• Lemons + other citrus fruits (chemical):

  • Oils released by trichomes are high in menthol and menthone + repels aphids and other insects
  • Citral (acyclic monoterpene aldehyde)
  • Insecticidal activity: slows respiration rates, locomotor activity, + avoidance response
• Pine tree pitch (chemical):

  • Contains pinene (bicyclic monoterpenes) that are highly repellent to insects
• Tannins (chemical):

  • Produced by many plant species
  • Produced by chloroplast-derived organelle (tannosome)
  • Antinutritional properties: inhibit herbivore digestion by binding to proteins, making them more difficult to digest + making the animals sick
  • Use by humans:
  • Tanning hides for leather
  • Helping preserve iron artefacts
• Pyrethroid (chemical):

  • Produced by chrysanthemums
  • Toxic to many insects by preventing closure of voltage-gated Na+ channels (doesn't bind efficiently to human Na+ channels)
  • Use by humans:
  • Pesticide
  • Medicine (parasitic crustaceans)
  • Mosquito control
• Caffeine (chemical):

  • Disrupt the nervous systems of insects + vertebrates
  • Causes insects to stop producing a protein required for nervous system --> paralysis + death
• Nicotine (chemical):

  • Binds nicotinic acetylcholine receptors (nAChRs) in the insect's nerve cells, blocking action potentials
• Opium (chemical):

  • Disrupts the nervous systems of insects + vertebrates
  • Human use: pain reduction
• Stinging Nettle (chemical):

  • Hollow trichomes, which act like hypodermic needles, injecting formic acid, histamine, and other chemicals that produce a stinging sensation upon contact
  • Deters herbivores
  • Flavour similar to spinach when cooked --> soaking in water or cooking removes the stinging chemicals
• African Nightshade (chemical):

  • Trichomes trap tomato red spider mites + when the mites disturb the trichomes, foul smelling chemicals are released that prevent the mites from laying eggs
• Evolutionary Arms Race: as plants evolve defense mechanisms (mechanical, chemical), animals evolve methods to evade + feast on plants

  • Never ending