In-depth Notes on Plant Defense Mechanisms and Responses

PLANT DEFENSE

• Constitutive Defenses

  • Always immediately available; not induced
  • Mechanical Defense:
  • Physical structures: thorns, spines, hairs, grits
  • Chemical Defense:
  • Secondary metabolites that deter herbivores/pathogens

• Induced Defenses

  • Activated after herbivore/pathogen attack
  • Triggered by elicitors or signal molecules (e.g., herbivore saliva)

MECHANICAL DEFENSE

• Structures:
  • Spines: Modified leaves (e.g., Cactus)
  • Thorns: Modified branches
  • Prickles: Epidermal outgrowth (e.g., Roses)
  • Trichomes: Plant hairs for protection
  • Phytoliths: Silica crystals (e.g., Horsetails)
  • Idioblasts: Calcium oxalate crystals in specific cells (e.g., Agaves)

CHEMICAL DEFENSE

• Secondary Metabolites:
  • Organic compounds, not directly involved in growth/development
  • Functions: Defense, attractants for pollinators, allelopathy, plant-microbe interactions
  • Use in pharmaceuticals: >75% derived from plants
  • Includes terpenes, phenolics, and nitrogen-containing compounds

TERPENES

• Characteristics: Also referred to as terpenoids or isoprenoids
• Examples:
  • β-carotene: A precursor for Vitamin A
  • Phytoalexins: Stress-related metabolites providing antimicrobial properties
  • Scent and Flavor: Capsaicin (chili peppers), allicin (garlic)
  • Synthesized from isopentenyl diphosphate in plastids and cytosol

TERPENE SYNTHESIS

• Occurs in two main locations:
  • Cytosol:
  • Acetyl-CoA (C2) condensation, CO2 released
  • Plastid:
  • Pyruvate (C3) + glyceraldehyde 3-phosphate condensation, CO2 released
  • 5C units for each terpene type (monoterpenes, sesquiterpenes, etc.)

PHENOLIC COMPOUNDS

• Types:
  • Derived from phenylalanine and categorized into three groups:
  • Phenylpropanoids
  • Flavonoids
  • Tannins
• Function: Defense against herbivores, disease prevention, UV protection, and more.

ALLELOPATHY

• Definition: Release of chemicals by plants to inhibit neighboring plants’ growth
• Examples: Caffeic acid and ferulic acid, root exudates, and volatile compounds

NITROGEN-CONTAINING COMPOUNDS

• Types:
  • Alkaloids: Contain heterocyclic ring; examples include caffeine and nicotine
  • Cyanogenic Glycosides: Release hydrogen cyanide, inhibiting metals in respiration
  • Glucosinolates: Found in mustard family plants, have strong taste and aroma

INDUCED DEFENSES

• Triggering: Initiated post-damage by herbivores/pathogens; uses signal molecules (elicitors, e.g., saliva)
• Response Pathway: Elicitors bind to receptors (systemin) to activate defense response pathways
• Systemin: Short peptide signaling molecule; critical for defense against herbivores

PATHOGEN INTERACTIONS

• Types of Pathogens:
  • Necrotrophs: Kill plant cells, feeding on dead tissue (e.g., Botrytis cinerea)
  • Biotrophs: Live in host tissue without killing (e.g., Blumeria graminis)
  • Hemibiotrophs: Start as biotrophs and become necrotrophic (e.g., Phytophthora infestans)

SIGMA PATHWAYS IN DEFENSE

• Defense Response Activation: Elicitation causes influx of Ca2+, ROS production, and signaling pathways like MAP kinase cascade
• Systemic Acquired Resistance (SAR): Acknowledges local infection and triggers widespread resistance throughout the plant

DISEASE SYMPTOMS

• Blights: Rapid browning/death in infected tissue
• Galls: Overgrowth of cells due to altered hormonal balance
• Cysts: Formation of structures due to nematode infection, aiding reproduction

PARASITIC PLANTS

• Definition: Plants deriving nutrients from hosts; some are fully parasitic and lack photosynthesis (e.g., dodder)
• Types:
  • Hemiparasites: Partially photosynthetic (e.g., mistletoe)
  • Holoparasites: Completely rely on host (e.g., Indian pipe)