Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 39.5 Plants Respond to Attacks by Pathogens and Herbivores
Plants are constantly under threat—from pathogens like bacteria and fungi to herbivores like insects and mammals. Though they lack a circulatory immune system, plants possess a robust multi-level defense system that helps them survive and thrive.
Defense Against Pathogens
- Physical Barriers:
- Epidermis and periderm act as the first line of defense.
- Pathogens enter through wounds or stomata.
- PAMP-Triggered Immunity (PTI):
- Recognition of PAMPs (e.g., flagellin) activates defenses.
- Responses include:
- Phytoalexins: antimicrobial compounds
- Cell wall strengthening
- Effector-Triggered Immunity (ETI):
- Some pathogens inject effectors to suppress PTI.
- Plants use R proteins from R genes to recognize effectors.
- Triggers:
- Hypersensitive Response (HR): localized cell death
- Systemic Acquired Resistance (SAR): whole-plant defense
Hypersensitive Response (HR)
- Localized response at infection site
- Includes:
- Cell wall reinforcement
- Lignin deposition
- Antimicrobial compound production
- Programmed cell death to contain spread
- Results in lesions that isolate infection
Systemic Acquired Resistance (SAR)
- Triggered by HR
- Uses methylsalicylic acid as a mobile signal
- Converted to salicylic acid in distant tissues
- Activates defense genes across the plant
- Provides long-term protection against diverse pathogens
Defense Against Herbivores
Plants deploy multi-level defenses when grazed or damaged:
- Molecular:
- Production of toxins (alkaloids, tannins)
- Hormone mimics to disrupt insect development
- Cellular:
- Idioblasts contain chemicals or sharp raphides
- Tissue:
- Sclerenchyma fibers make tissues hard to chew
- Organ:
- Thorns, spines, leaf mimicry deter herbivory
- Organismal:
- Wild tobacco shifts pollinators after herbivore attack
- Population:
- Plants warn neighbors via airborne signals
- Masting: synchronized seed production to overwhelm herbivores
- Community:
- Recruit predators by emitting volatiles (e.g., parasitoid wasps)
In a Nutshell
Plants are not passive victims. They have evolved sophisticated immune-like systems to fight pathogens and a layered arsenal of defenses against herbivores. Through chemical signaling, structural barriers, and even community-level cooperation, plants defend themselves in diverse and powerful ways—without ever lifting a limb.