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Plant Response. Signal Reception/Transduction Pathways… the plant response to external stimuli determines the i nternal patterns of development Etiolation … adaptations for growing in darkness Elongation of roots to obtain substrates

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Plant Response

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plant response
Plant Response
  • Signal Reception/Transduction Pathways… the plant response to external stimuli determines the internal patterns of development
    • Etiolation … adaptations for growing in darkness
      • Elongation of roots to obtain substrates
        • Root shoots dominate over extensive root systems as transpiration is low
      • Underdeveloped leaves
        • Small size & lacking in chlorophyll
    • De-etiolation… process of greening when light is available
      • Leaves expand and production of chlorophyll increases
      • Elongation of stems (previously root shoots)
signal reception transduction response
Signal Reception-Transduction-Response
  • Studies in de-etiolation demonstrate the model
    • Reception… the availability of light activates phytochrome in the cytoplasm of the leaf
    • Transduction… even the slightest amount of light causes signal amplification from the phytochrome
      • cGMP opens Ca+ channels in the cell membrane
        • Activates various kinases that signal production of de-etiolation response proteins

Response …

    • Transcription Regulation
      • By activation of specific positive transcription factors by cGMP & Ca+
    • Post-translational Modification
      • Modification of existing proteins by phosphorylation (kinase activity)
      • Protein phosphatases turn off process through dephosphorylating if light is no longer available
plant hormones
Plant Hormones
  • Hormones affect plant growth by affecting growth, elongation, and differentiation.
    • Due to positive or negative tropism
      • Phototropism causes the dark side of the grass to grow faster due to a release in a growth hormone (auxin) resulting in bending toward light
      • Each hormone has multiple effects causing a profound effect on plant development
        • Most through signal amplification or redirection of metabolism
hormone types
Hormone types
  • Auxins… stimulate elongation (growth)
    • Travel from tip to root … polar travel from apical to basal end of the cell
      • Used in broadleaf herbicides as they are taken in by leaves causing a hormone overdose
  • Cytokinins…activate growth through cell division (cytokinesis)
    • Produced in roots (travel up)
    • Works in concert with auxins
      • The ratio of each determines whether roots or shoots develop
  • Apical dominance is exhibited in plants as auxins dominate.
    • Apical inhibition is exhibited through decapitation as cytokinins to dominate causing later bud formation
hormone types1
Hormone types
  • Gibberellins (GA)… active in growth in leaves and stems, fruit growth, and seed germination
    • More than 100 different forms identified
    • Facilitates the penetration of expansion proteins through the cell wall
    • Work in concert with auxins to promote fruit formation
      • Sprayed on fruit to increase size for market
    • Activated in the seed after imbibition to signal embryogenisis
hormone types2
Hormone types
  • Brassinosteroids… promote cell elongation and differentiation on stems and seedlings.
    • Similar to auxins
    • Retard abscission (loss of leaves)
  • Abscisic Acid (ABA)… antagonizes the other growth hormones to retard growth
    • Ratio determines effect
    • Present in concentrated amounts in the seed to prevent maturation in the absence of water or light
      • “washed out” during imbibition
      • The ratio of ABA to GA determines germination
    • Helps in drought by causing stomata to stay closed decreasing transpiration
hormone types3
Hormone types
  • Ethylene… produced by the plant in times of stress (drought, flooding, mechanical pressure, infection, …) Production causes both cell death and fruit to ripen
    • Triple response… produced in high concentrations when a seedling reaches an obstruction causing it to grow sideways
    • Apoptosis… a burst of ethylene production is associated with deciduous leave removal in autum and the death of an annual plant
      • Process of abscission causing hydrolyzation of polysaccharides holding the leave stem
      • Recycling of nutrients and auxins signal new leaves in spring
    • Fruit ripening… a burst of production signals ripening by softening the cell walls and sweetening of the fruit through acid and sugar production.
      • Storing unripened fruit in a paper bag concentrates ethylene gas to speed ripening
      • Commercial fruits are stored in CO2 ventilated containers (retards ethylene production) and exposed to ethylene gas just prior to market
      • A rotten apple does spoil the bunch by over production of ethylene causing nearby fruit to spoil