Plant Hormones and Responses

• Plant Hormones


  • Also called as Plant Growth Substance (PGS) and phytohormone
  • Compounds that act specifically to regulate growth and development of the plant
  • Each plant hormone regulates a variety of processes

  
• Differences between animal and plant hormones


  • Naturally occurring organic molecule exerting profound effect on physiological process
  • Active at low concentrations
  • Synthesized in a discrete organ or tissue remote from point of action
  • Transported in a circulatory system
  • Has one, or a few, functions
  • Require specific receptors in the cell to function
• Auxins


  • Derived from the Greek word auxein, which means "to grow"
  • Examples: indole-3-acetic acid (IAA), phenoxyacetic acid, indole 3-butyric acid
  • Effects: Elongation growth of stem and roots, Cell division and differentiation of undifferentiated cells, Apical dominant growth of the apical bud, Production of parthenocarpic (seedless) fruits, Inhibit senescence and abscission of mature leaves, fruits and flowers, Enhance abscission of young fruits

  
• Auxin shows polar transport (unidirectional) and moves basipetally (from the apex to the base)
• Small amounts of auxins produced at the root apex may also move acropetally (from the root tip up the root)

  
• Cytokinins


  • Regulate cell division in plants
  • Derivatives of adenine or amino purine
  • Synthesized in various tissues and organs specially in root apical meristem
  • Effects: Promotes growth and development and delay of senescence, Involve, together with auxin, in development of roots and shoots

  
• Gibberellins


  • Discovered in the 1930s by Japanese scientists
  • Nearly 100 gibberellins have been identified in plants though many do not have biological activity
  • Transported predominantly in the phloem and is non-polar
  • GA3 - most studied and probably the most significant gibberellin
  • Effects: Initiate the growth response such as stem elongation and flowering, Seed germination by breaking dormancy, Involved in regulating the transition from juvenile to mature growth form, Promotes fruit formation

  
• Abscisic acid


  • Found in all higher plants and mosses
  • Effects: Keeps some seeds dormant until the environment becomes suitable for growth, Regulates several plant stress responses, Promotes root growth and inhibits shoot growth
• Ethylene


  • A volatile gas (C2H4)
  • Effects: Promotes fruit ripening, Stimulates the conversion of starch and acids to sugars, Triggers leaf and fruit abscission, flower fading and dropping, Promotes germination in some cereals and sprouting of bulbs and potatoes
• Plant Responses


  • Phytochrome responses
  • Photoperiodism
  • Photomorphogenesis
  • Tropisms
• Phytochrome


  • Photoreceptor protein
  • Absorbs red light (666 nm) and is converted to Pfr
  • Pfr absorbs far-red light (720 nm), and initiates cell signaling events leading to photomorphogenesis
  • Phytochrome responses include: etiolation/de-etiolation, circadian rhythms such as leaf and petal movement, seed germination

  
• Photoperiodism


  • The response of a plant to length of day (dormancy and flowering)
  • Plants may be classified into: short-day plants (SDPs), long-day plants (LDPs), day-neutral species
  • Results from two processes: perception of light by the photopigment phytochrome in leaves, endogenous circadian rhythm

  
• Photomorphogenesis


  • The direct influence of light on growth and development of plant
  • Involves responses to certain wavelengths of light which are perceived by photoreceptor pigments

  
• Tropisms


  • Phototropism
  • Gravitropism
  • Hydrotropism
  • Thigmotropism
  • Chemotropism
• Phototropism


  • Orientation towards/away from a unilateral illumination
  • When a coleoptile is exposed to unilateral light for a short period (1–2 h) it bends towards the light
  • The action spectrum of phototropism shows sensitivity to blue light
  • Cholodny-Went hypothesis states that unilateral light causes auxin redistribution near the apex, with more auxin on the shaded side

  
• Gravitropism


  • Ensures that roots grow into the soil and that shoots grow toward sunlight
  • Negative gravitropism - Growth of the shoot apical tip upward
  • Positive gravitropism - growth of the roots downward
  • Amyloplasts or statoliths - specialized plastids that contain starch granules and settle downward in response to gravity