Plants have evolved clever ways to defend themselves against hungry herbivores. From physical barriers like thorns to chemical weapons like toxins, they use a variety of strategies to stay alive and thrive.

These defenses can be always-on or activated when needed. Some even call for backup, releasing smells that attract predators to eat the herbivores. It's a complex dance of survival in the plant world.

Plant Defense Strategies

Types of Plant Defenses

• Constitutive defenses are always present in the plant, regardless of whether the plant is under attack by herbivores or pathogens
  • These defenses are typically physical barriers or chemical compounds that deter or prevent damage from herbivores (thorns, trichomes, thick cuticles)
• Inducible defenses are activated only in response to damage or stress, such as herbivory or pathogen attack
  • These defenses often involve the production of toxic or deterrent compounds, or the release of volatile organic compounds that attract natural enemies of the herbivore (protease inhibitors, polyphenol oxidases, volatile organic compounds)

Modes of Action for Plant Defenses

• Direct defenses directly affect the herbivore by deterring feeding, reducing digestibility, or causing toxicity
  • Examples include physical barriers like thorns and trichomes, as well as chemical defenses like alkaloids, terpenes, and phenolics that can be toxic or unpalatable to herbivores
• Indirect defenses promote the effectiveness of the natural enemies of herbivores, such as predators and parasitoids
  • Plants emit volatile organic compounds (VOCs) that attract predators and parasitoids of the herbivores, increasing the mortality of the herbivores and reducing plant damage (methyl salicylate, methyl jasmonate, green leaf volatiles)

Physical and Chemical Defenses

Structural Defenses

• Trichomes are hair-like structures on the surface of leaves, stems, and other plant organs that can deter herbivores by physically impeding their movement or by releasing toxic or irritating compounds
  • Glandular trichomes contain cells that produce and secrete secondary metabolites, while non-glandular trichomes act as physical barriers (tomato plants, stinging nettles)

Chemical Defenses

• Secondary metabolites are compounds produced by plants that are not essential for primary metabolic processes but play important roles in defense against herbivores and pathogens
  • These compounds can be constitutively expressed or induced upon damage and include alkaloids, terpenoids, phenolics, and glucosinolates
  • Secondary metabolites can deter feeding, reduce digestibility, or cause toxicity to herbivores (nicotine in tobacco, capsaicin in chili peppers)

Chemical Signaling

Volatile Organic Compounds (VOCs)

• VOCs are small molecular weight compounds that easily evaporate at ambient temperatures and play important roles in plant-plant, plant-herbivore, and plant-predator interactions
  • Plants release VOCs in response to herbivore damage, which can induce defense responses in neighboring plants, attract natural enemies of the herbivores, or directly deter herbivore feeding (methyl jasmonate, methyl salicylate, green leaf volatiles)

Herbivore-Induced Plant Volatiles (HIPVs)

• HIPVs are a specific class of VOCs that are emitted by plants in response to herbivore damage and serve to attract predators and parasitoids of the herbivores
  • The composition of HIPVs can vary depending on the plant species, the herbivore species, and the type of damage inflicted
  • HIPVs can provide important cues for natural enemies to locate their prey or hosts, leading to increased herbivore mortality and reduced plant damage (maize plants emit HIPVs that attract parasitic wasps when attacked by armyworms)