Bio2 Unit5 11/19/25

Plant Sensing and Response to Environment

  • Importance of plants sensing their environment
    • Plants must respond to various environmental stimuli to survive
    • Includes avoiding predation and seeking optimal growth conditions (e.g., light)

Chameleon Vine Example

  • Two photographs illustrating a vine (V) and a tree (T)
    • Both represent the same species of vine growing in different trees
    • The vine mimics the tree's leaf shape, known as the "chameleon vine"
  • Purpose of mimicry
    • Camouflage to avoid being eaten by herbivores using visual cues
    • Increases the chance of survival by blending into the background
    • Demonstrates a biochemical response to the environment
    • Vine can sense the type of tree it is growing on

Plant Hormones

  • Overview of plant hormones (phytome hormones)
    • Initially not recognized as hormones, but later studied and found comparable to animal hormones
    • Active at very low concentrations; small amounts lead to significant effects on plant shape and growth
    • Can travel throughout the plant via xylem and phloem, targeting specific cells with receptors
  • Major plant hormones discussed:
    • Auxins:
    • Promote cell elongation, especially in stems and roots
    • Cytokinins:
    • Drive cell division (cytokinesis) and promote overall growth
    • Gibberellins (GAs):
    • Stimulate stem elongation, leaf growth, and play roles in fruit development and seed germination
    • Abscisic Acid (ABA):
    • Acts as a stress hormone, inhibiting growth, promoting seed dormancy, and closing stomata during drought
    • Ethylene:
    • Involved in fruit ripening, leaf abscission, and responds to mechanical stress

Plant Growth Responses

  • Plants exhibit shape changes in response to environmental cues
    • Example: Phototropism - plants grow towards light sources
  • Mechanism of phototropism
    • Seedlings bend towards light; differential growth of stem cells is essential
    • Cells on the illuminated side are shorter; underlying chemical signaling leads to differential growth
  • Light sensing in plants
    • Plants can detect light direction and quality (e.g., blue vs. red light)
    • Plants possess biophysical mechanisms to respond without eyes or photoreceptors

Tropism Types

  • Phototropism: Growth in response to light
  • Gravitropism: Growth in response to gravity
  • Thigmotropism: Growth in response to touch or physical stimuli

Darwin's Contributions

  • Charles Darwin's experiments on plant response to light
    • Early studies on phototropism and plant behavior
    • Removed tips of seedlings to investigate light reception leading to curvature
    • Concluded that the stem tip contains the sensing mechanism for light
    • Use of gelatin to provide evidence of chemical action mediating growth principles

Auxin and Growth Mechanism

  • Auxin's role in curvature:
    • Accumulates on the shaded side of a stem, promoting elongation on that side
    • This results in bending toward the light source
  • Acid Growth Hypothesis: Mechanism of auxin action
    • Auxin causes cell wall acidification, activating enzymes that weaken cell wall structure
    • Leads to increased water uptake and cell expansion through osmosis

Auxin Functions

  • Involved in:
    • Differential growth leading to phototropism
    • Leaf spatial arrangement (phyllotaxy)
    • Apical dominance: auxin from the apical meristem inhibits nearby axillary buds

Cytokinins Overview

  • Promote cell division, affecting growth and development
  • Play notable roles in:
    • Counteracting aging and protein breakdown
    • Stimulation of root and shoot growth and development

Gibberellins Overview

  • Involve stem and leaf growth and critical during:
    • Rapid flowering (bolting)
    • Seed germination, by aiding starch breakdown via alpha-amylase enzyme activation

Abscisic Acid (ABA) Overview

  • Acts as a stress regulator during drought
    • Closes stomata to minimize water loss
    • Opposes growth; inhibiting germination under unfavorable conditions

Ethylene Overview

  • Signaling hormone involved in:
    • Leaf senescence and abscission processes
    • Promotes fruit ripening, can be induced commercially

Abscission Process

  • Ethylene's role in signal transduction during leaf loss in autumn
    • Involves apoptosis of leaf cells, nutrient recycling before leaf drop

Environmental Applications of Hormones

  • Use of ethylene in agricultural practices:
    • Helping farmers control fruit ripening timing and preventing losses due to over-ripening or under-ripeness

Summary Table of Hormones

  • Key patterns identified across hormones include:

    • Functions often relate to cell elongation and division
    • Exceptions exist with ABA and ethylene - associated with retarding growth functions

  • Emphasis that understanding plant hormones offers insight into agricultural practices and strategies for enhancing plant health and productivity.