CR

9, Plant Defense and Sensory Systems

Plant Defense

Chapter 38 Overview

  • Plants face numerous threats from abiotic factors and biotic threats such as pathogens and invasive species.

Physical Defenses

  • Abiotic Factors: Fire and harsh weather can threaten plants. Some plants may utilize new ground after a fire to regenerate.
  • Plant Pests: Pathogenic agents like viruses, bacteria, fungi, and animals can exploit plant nutrients, causing cell death and necrosis.
  • Invasive Species: Non-native species like the emerald ash borer can devastate ecosystems in the absence of natural predators.

Dermal Tissue System

  • First-line Defense: The outer layer of plants, including the epidermis, secretes protective waxes to minimize water loss and fend off attacks.
    • Cutin: Covers above-ground parts and provides additional protection.
    • Suberin: Found in subterranean plant parts (roots).
    • Other structures: Silica inclusions, trichomes, bark, and thorns enhance protection.

Invaders and Dermal Defenses

  • Damage to dermal tissue creates entry points for pathogens:
    • Parasitic Nematodes: Produce tumors on roots and weaken plant defenses.
    • Fungi: Seek entrance through stomata, using wind-blown spores.

Phases of Fungal Invasion

  1. Spore lands on leaves.
  2. Spore germinates, forming an adhesion pad.
  3. Hyphae penetrate the cell walls.
  4. Haustoria develop, enabling nutrient extraction from the host plant.

Chemical Defenses

  • Plant Toxins: Plants produce chemical compounds to deter herbivores, often unique to species.
    • Defensins: Small peptides with antimicrobial properties that may inhibit protein synthesis or damage cellular structure.

Secondary Metabolites

  • Alkaloids: Toxic compounds like caffeine and nicotine inhibit herbivore feeding.
  • Tannins: Bind proteins, inactivating them and deterring feeding.
  • Plant Oils: Repel insects through strong odors, particularly those in the mint family.
  • Varied Diet: Animals mitigate their risk of toxicity by consuming a diverse diet of plant materials.

Allelopathic Plants

  • Plants secrete chemicals to inhibit growth or seed germination of nearby competitors, reducing resource competition (e.g., black walnut trees).

Ricin Poison

  • Source: Produced by the castor bean plant; a potent toxin that can be lethal.
  • Mechanism: Ingested ricin activates in the digestive tract, cleaving ribosomal RNA and halting protein synthesis.

Animals that Protect Plants

  • Ants and Acacia Trees: Ants protect Acacia trees from herbivores in exchange for food (extrafloral nectaries) and shelter (hollow thorns).
  • Parasitoid Wasps: Wounding response from caterpillars attracts wasps that lay eggs in them, resulting in predation of the caterpillar by wasp larvae.

Chapter 39: Plant Sensory Systems

Responses to Light

  • Pigments: Molecules that absorb light, facilitating photosynthesis and other light responses.
  • Photomorphogenesis: Developmental changes triggered by light.
  • Phototropism: Directional growth towards light sources.

Phytochrome

  • Present in all plants; exists in two forms:
    • Pr: Absorbs red light (660 nm).
    • Pfr: Absorbs far-red light (730 nm).
  • Signaling Pathways: Phytochromes activate gene expression for growth regulation.

Phototropic Responses

  • Light stimulates growth and bending toward light through auxin distribution.

Gravitropism

  • Plant response to gravity:
    • Shoots (negative gravitropism): Grow upwards.
    • Roots (positive gravitropism): Grow downwards.

Mechanism of Gravity Perception

  1. Gravity detected via falling amyloplasts.
  2. Mechanical signals transduced into physiological responses.
  3. Differential growth occurs in response.

Auxins

  • Discovered by Charles and Francis Darwin; play a crucial role in plant growth and directional responses to stimuli.
  • Indoleacetic acid (IAA): The primary natural auxin, aiding in cell elongation and shaping growth towards light.