Plant responses

• Plant responses to abiotic stress and herbivory

  • Production of chemical defences (e.g. tannins, alkaloids, pheromones)
  • Physical movement of plant parts (e.g. folding in response to touch)
• Chemical defences


  • Tannins in oak leaves
  • Nicotine in tobacco plants
  • Volatile organic compounds in maize plants
• Folding response in Mimosa pudica

  • Mechanical stimulation: Touching or vibrating the leaflets triggers mechanical stimulation.
  • Action potential generation: This stimulation creates an electrical impulse in the leaflet's cells.
  • Potassium ion release: During this impulse, potassium ions (K+) are released from the cells, causing an imbalance in ion concentrations.
  • Decrease in cell turgor pressure: The release of potassium ions lowers the turgor pressure within the cells, leading to cell walls collapsing as the water pressure decreases.
  • Leaf movement: Causing the leaf to fold.
• Phototropism


  Growth movement of plants in response to light
• Geotropism


  Growth movement of plants in response to gravity
• Investigating phototropism


  1. Place potted plant in dark room with single light source

  2. Observe bending towards light

  3. Measure angle of bending
• Investigating geotropism


  1. Germinate seeds on horizontal paper towel

  2. Observe root growth downwards, shoot growth upwards

  3. Measure root and shoot angles
• Role of plant hormones

  • Leaf loss in deciduous plants
  • Seed germination
  • Stomatal closure
• Abscisic acid (ABA)


  Promotes leaf abscission, triggers stomatal closure
• Ethylene


  Enhances leaf abscission
• Gibberellins (GA)


  Stimulate hydrolytic enzyme production, promote embryonic axis elongation, weaken seed coat
• Auxins (IAA)


  Regulate radicle (embryonic root) growth and cell elongation
• Jasmonic acid (JA)


  Contributes to stomatal closure in response to biotic stress
• Removing apical bud


  Activates lateral bud growth
• Applying auxins to lateral buds

  Inhibits their growth
• Apical bud removal experiment

  1. Remove apical bud

  2. Observe lateral bud growth
• Auxin application experiment


  1. Apply exogenous auxin to lateral buds

  2. Observe inhibition of lateral bud growth
• Experimental evidence for gibberellin's role

  • Stem elongation
  • Seed germination
• Mutant plants with defects in gibberellin biosynthesis or signalling exhibit dwarfism
• Exogenous gibberellin application rescues the dwarfism phenotype
• Gibberellin stimulates cell elongation in stems
• Exogenous auxin application mimics the presence of auxins produced by the apical bud. The increased concentration of auxins in the lateral buds suppresses their growth, similar to the effect of endogenous auxins produced by the intact apical bud. This experiment confirms the inhibitory role of auxins in apical dominance.
• Gibberellin


  A plant hormone that promotes stem elongation and seed germination
• Experimental evidence for the role of gibberellin in stem elongation

  • Consistent demonstration through experimental studies, including genetic analyses and hormone application experiments
  • Mutant plants with defects in gibberellin biosynthesis or signalling pathways exhibit dwarfism, while exogenous gibberellin application rescues the phenotype
  • Genetic studies have identified key components of the gibberellin signalling pathway and their roles in regulating gene expression related to cell elongation
• Experimental evidence for the role of gibberellin in seed germination


  • Consistent results from both classical and molecular studies
  • Application of gibberellin to dormant seeds accelerates germination, while mutants deficient in gibberellin biosynthesis or signalling pathways exhibit delayed germination
  • Molecular studies have explained the gibberellin signalling pathway in seeds, revealing how gibberellin promotes the expression of genes involved in germination processes
• Experimental approaches used to explain the role of gibberellin

  1. Mutant analysis

  2. Hormone application experiments

  3. Molecular studies
• Mutant analysis for stem elongation

  1. Identify and characterise mutant plants with defects in gibberellin biosynthesis or signalling pathways

  2. Study the dwarfism phenotype of these mutants to infer the role of gibberellin in promoting stem elongation
• Mutant analysis for seed germination

  1. Identify mutants with impaired gibberellin pathways and compare their germination behaviour to wild-type seeds

  2. Assess the involvement of gibberellin in seed germination
• Hormone application experiments for stem elongation

  1. Apply exogenous gibberellin to dwarf mutants to restore normal stem elongation

  2. Apply inhibitors of gibberellin biosynthesis to wild-type plants to inhibit stem elongation
• Hormone application experiments for seed germination

  1. Apply gibberellin to dormant seeds to promote germination

  2. Apply inhibitors of gibberellin biosynthesis to delay germination
• Molecular studies


  1. Identify genes regulated by gibberellin and study their functions

  2. Explains the molecular mechanisms underlying gibberellin-mediated processes such as stem elongation and seed germination
• Plant hormones are used commercially to control ripening, promote rooting in cuttings, and as selective herbicides (weed killers)
• Controlling ripening


  Ethylene can be applied externally to induce or accelerate fruit ripening, extending shelf life and ensuring uniform ripening
• Rooting powders


  Contain synthetic auxins to stimulate root development in plant cuttings, facilitating propagation of new plants
• Hormonal weed killers


  Synthetic plant hormones (e.g. auxins) are used as selective herbicides to control weed growth while minimising harm to desirable crops or plants
• Benefits of using plant hormones in agriculture and horticulture

  • Improved crop yield and quality
  • Enhanced plant propagation
  • Effective weed management
  • Cost and resource efficiency
  • Environmental sustainability
• What is Abscission


  the process that trees and other plants undergo during the autumn to shed some or all of their leaves
• apical dominance

  
• Inhibits their growth


  When auxins are applied to lateral buds, they inhibit their growth. This is because auxins promote apical dominance, which is the tendency of the main stem to grow more vigorously than the lateral branches. By inhibiting the growth of lateral buds, auxins help to maintain the dominant growth of the main stem.
• Exogenous auxin application


  Application of auxin from an external source, which has a similar effect on lateral buds as auxins produced by the apical bud.