plant responses

• Apical dominance


  The inhibition of the growth of lateral buds by auxins, leading to the dominance of the apical bud
• Role of plant hormones in leaves

  • Cytokinins delay leaf aging/senescence by maintaining nutrient supply
  • Auxins inhibit leaf abscission (falling off)
• Leaf senescence and abscission in deciduous plants

  1. Cytokinins and auxin levels decrease in winter

  2. Leaf senescence occurs

  3. Leaves fall off (abscission)
• Abscisic acid


  Hormone that causes stomata to close in leaves when water availability is low, reducing water loss through transpiration
• Gibberellins

  • Control stem elongation
  • Control seed germination
• Gibberellin-induced stem elongation

  1. Fungus secretes gibberellic acid

  2. Gibberellic acid causes dwarf plants to grow very tall

  3. GA1 gibberellin directly stimulates stem elongation

  4. GA20 is converted to GA1 by an enzyme encoded by the LE gene
• Auxins


  A type of plant hormone or plant growth factor responsible for controlling the plant's phototropic and geotropic responses by regulating cell elongation
• Gibberellin-induced seed germination

  1. Gibberellins released by seed embryo when seed absorbs water

  2. Gibberellins enable production of amylase enzyme

  3. Amylase breaks down starch to glucose

  4. Glucose used for respiration and protein synthesis to allow seed germination and growth
• Auxins

  • They don't focus on mitosis but rather on cell elongation
  • They can encourage or inhibit cell elongation
• Gibberellins are a family of plant hormones
• Indole acetic acid (IAA, a type of auxin)

  • Causes shoots to grow towards light (positive phototropism)
  • Causes roots to grow away from light (negative phototropism)
• Auxins inhibit the growth of lateral buds, leading to apical dominance
• Indole acetic acid (IAA, a type of auxin)

  • Causes shoots to grow away from gravity (negative geotropism)
  • Causes roots to grow towards gravity (positive geotropism)
• Stimulus


  A detectable change in the external or internal environment of an organism leading to a response from that organism
• Cytokinins promote bud growth and nutrient supply in leaves
• Mechanism of auxin-induced cell elongation (acid growth hypothesis)

  1. Auxin binds to cell

  2. Auxin activates proton pump to transport protons into cell wall spaces

  3. Protons activate expansin proteins

  4. Expansins loosen cellulose in cell wall

  5. Cell wall becomes more plastic and cell elongates
• Types of stimuli

  • External stimuli (anything outside of the organism's body)
  • Internal stimuli (from inside the body)
• Abscisic acid causes stomata to close in leaves when water availability is low
• Organisms need to respond to both external and internal stimuli to avoid harm and maintain optimal internal environment
• Mechanism of phototropism


  1. Light shines unilaterally on plant

  2. Auxin moves away from illuminated side to shaded side of shoot

  3. Higher auxin concentration on shaded side causes faster cell elongation

  4. Shoot bends towards light (positive phototropism)
• Plant responses to environmental changes

  • Producing chemicals
  • Changing and altering their growth
• Abiotic components


  Non-living components in the environment (e.g. sunlight, humidity, soil pH)
• Mechanism of root phototropism


  1. Light shines on root

  2. Auxin moves away from illuminated side to shaded side of root

  3. Higher auxin concentration on shaded side inhibits cell elongation

  4. Cells on illuminated side elongate more, causing root to bend away from light (negative phototropism)
• Biotic components


  Living components in the environment (e.g. other plants, animals, microorganisms)
• Mechanism of geotropism


  1. Gravity pulls auxin to lower side of root

  2. Higher auxin concentration on lower side inhibits cell elongation

  3. Cells on upper side elongate more, causing root to grow downwards (positive geotropism)
• Chemical defences in plants

  • Tannins
  • Alkaloids
• Pheromones


  Chemicals released by plants that affect the behaviour or physiology of other organisms
• Mechanism of shoot geotropism


  1. Gravity pulls auxin to lower side of shoot

  2. Higher auxin concentration on lower side encourages cell elongation

  3. Cells on lower side elongate more, causing shoot to grow upwards (negative geotropism)
• Tropism


  A directional growth response of a plant to an external stimulus
• Auxin is produced at the tips of shoots and roots
• Phototropism


  Growth response of a plant towards or away from light
• Auxin initially diffuses evenly through the plant tissues
• Unilateral light or gravity causes auxin to redistribute within the plant tissues
• Geotropism


  Growth response of a plant towards or away from gravity
• Positive tropism


  Growth towards a stimulus
• Negative tropism


  Growth away from a stimulus
• Chemotropism


  Growth response of a plant towards the location of chemicals
• Nastic response


  A non-directional plant response to a stimulus
• Thigmonastic response


  A non-directional response to touch
• Plant hormones/growth factors


  • Control and coordinate plant responses
  • Influence cell division, elongation, and differentiation