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:
First line of defense: epidermis and periderm
Breaches from wounds or stomata allow pathogens in
PAMP-Triggered Immunity (PTI):
Plants recognize pathogen-associated molecular patterns (PAMPs) (e.g., bacterial flagellin)
Trigger signal transduction leading to:
Phytoalexins: broad-spectrum antimicrobial chemicals
Strengthening of cell walls
Effector-Triggered Immunity (ETI):
Some pathogens deliver effectors to disable PTI
Plants counter with R proteins, encoded by R genes, that detect effectors
Triggers strong responses like:
Hypersensitive Response (HR): local cell death forming a "ring of death"
Systemic Acquired Resistance (SAR): long-lasting, plant-wide immunity
Hypersensitive Response (HR)
A localized response to effectors
Leads to:
Cell wall reinforcement
Lignin deposition
Production of antimicrobial compounds
Programmed cell death around the infection site to starve the pathogen
Result: lesions on infected tissues prevent pathogen spread while preserving the rest of the plant
Systemic Acquired Resistance (SAR)
Triggered by HR
Uses the signaling molecule methylsalicylic acid, transported throughout the plant
Converted to salicylic acid in distant tissues
Activates defense gene expression plant-wide
Provides protection for several days against a broad range of pathogens
Defense Against Herbivores
Herbivory causes physical damage and makes plants vulnerable to infection. Plants respond with layered defensesacross biological levels:
Molecular:
Produce toxins (e.g., alkaloids, terpenoids, tannins)
Some mimic insect hormones, causing developmental disruption
Cellular:
Specialized cells (e.g., idioblasts) store chemicals or contain sharp raphide crystals to irritate herbivores
Tissue:
Sclerenchyma fibers make tissues tough to chew
Organ:
Thorns, spines, and leaf mimicry deter feeders
Organismal:
Behavior change: e.g., wild tobacco switches from moth to hummingbird pollinators when attacked
Population:
Some plants warn neighbors via chemical signals
Masting: synchronous seed production overwhelms herbivores
Community:
Some plants recruit predators: damaged leaves release volatiles that attract parasitoid wasps to caterpillars
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.