15: Plant Defenses + Their Use by Humans

• Constituent Defenses: when herbivores are always present (e.g. tropical environments), defense chemicals may always be produced
• Inducible Defenses: plants can turn on defenses only in the presence of feeding herbivores (e.g. stop production of defense chemicals in winter when insects are not present)
• Defenses require energy that could be used for plant growth and reproduction
• Evolutionary Arms Race:

  • Most plant pathogens are specific, infecting only one or a few host species
  • Coevolution of the pathogen and the host
  • Pathogen evolves a mechanism to invade the host + the host may evolve new methods to defend itself from attack
  • In response, pathogens may evolve alternate strategies to evade detection by the host
  • Never-ending
• Plant chemical effect on herbivores may be grouped into one of three categories:

  1. Antixenosis - disrupts normal insect behavior
  2. Tolerance - insect tolerates the chemical
  3. Antibiosis - adversely affects insect physiology (survival, development, reproduction)
• Milkweed + Monarch butterfly:

  • Milkweed produces bitter-tasting steroids (cardenolides) that disrupts Na+ - K+ flux in insects
  • Monarch larvae exclusively feed on milkweed, sequestering cardenolides as an anti-predator defense from birds and other insects
  • The monarch adapted to be more tolerant of cardenolides than other insects and the colour pattern warns predators
• Some plants contain cyanogenic glycoside, which transforms into cyanide when cells are destroyed by a predating insect

  • Cyanide disrupts cytochrome-c oxidase, shutting down the e- transport chain + cellular respiration
  • Aphids evolved specialized sucking styli which sucks phloem say, avoiding disruption of tissues containing cyanogenic glucosides and their spatially separated hydrolysis enzymes
• Six-spot (insect) evolved feeding behavior adaptation: feeds quickly and leaves large portions of the plant tissue intact to reduce exposure to cyanogenic glucoside hydrolysis
• Plant must get past the surface of a plant to cause disease --> most enter through stomata or wounds

  • Sensory proteins detect invading pathogens
  • Signaling pathways carry out their defense
  • Successful pathogens have evolved elaborate mechanisms to evade these defenses
• Hypersensitive response (a signal cascade when attacked by a pathogen):

  • Stomata close
  • Toxins targeting the pathogen are produced
  • STPs (sugar transport proteins) and SUTs (sugar transporters) retrieve apoplasmic sugars back into the cytosol, thereby starving pathogens
  • Cells walls are reinforced to prevent pathogen movement
  • Cells in infected area rapidly die by apoptosis
• Systemic Acquired Resistance (SAR): a slower, widespread set of events that is triggered by a hormone produced at the infection site

  • Primes roots and shoots to resist assault
  • Signal results in the expression of a suite of pathogenesis-related (PR) genes
• Pathogenesis-related (PR) genes:

  • Antimicrobial proteins that attack cell wall of bacteria or fungi
  • Function as signals that spread "news" of the infection to nearby cells
  • Stimulate strengthening (cross-linking) of cell wall and deposition of lignin
• Hypersensitive Response (Tobacco + aphids):

  • Tobacco plant recognizes cathepsin B protease in saliva of feeding aphids
  • Protease binds EDR1-like protein (enhanced disease resistance)
  • Triggers release of reactive oxygen species (ROS) accumulation in phloem, reducing aphid feeding
  • Aphids successful at feeding on tobacco have reduced cathepsin B production in their saliva
• Proteinase inhibitors: block proteinase enzymes in the mouths and stomachs of herbivores

  • Herbivores ingest proteinase inhibitor + get sick
  • They learn to avoid plants with high amount of proteinase inhibitors
• Systemin: a signaling hormone
• Process: How systemin functions:

  1. When a plant is under attack, the damaged cells produce the hormone, systemin
  2. Systemin binds to receptors on the membranes of undamaged cells
  3. A series of reactions produces jasmonic acid
  4. Jasmonic acid activates transcription of proteinase inhibitors that will deter the herbivore from further attacks
• Prtoeinase inhibitors can inhibit mammalian enzymes. Some may help treat human illnesses.
• "Talking Trees":

  • When insects are feeding on a leaf, volatile compounds are emitted that travel through the air
  • Neighbouring plants sense these volatiles and increase their own defenses
  • Exposing plants to volatiles in the absence of any insects produe proteinase inhibitors
• Pheromones: chemical messengers synthesized by an individual + released into the environment to elicit a response from a different individual

  • Plants being eaten release pheromones that attract parasitic wasps
  • Parasitoids are free living as adults, but parasitic as larvae (kill their host organism to complete development)