Plant Hormones

• Plant Hormones


  Chemical compounds present in very low concentrations in plants that regulate plant development, growth, longevity and reproductive processes
• Plant Hormones


  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic Acid
  • Ethylene
• Auxins


  • Cell elongation of stems and roots
  • Apical dominance
  • Induces parthenocarpy
  • Prevents premature fall of leaves, flowers, fruits
  • Useful in stem cuttings and grafting
  • Promotes flowering
  • Helps in cell division and xylem differentiation
• Auxin


  Means "to grow". They are widely used in agricultural and horticultural practices. They are found in growing apices of roots and stems and then migrate to other parts to act.
• Natural Auxins


  • Indole-3-acetic acid (IAA)
  • Indole butyric acid (IBA)
• Synthetic Auxins


  • 2,4-D (2,4-Dichlorophenoxyacetic acid)
  • NAA (Naphthalene acetic acid)
• Functions of Auxins


  • Cell elongation of stems and roots
  • Apical dominance, IAA in apical bud suppresses the growth of lateral buds
  • Induces parthenocarpy i.e. development of fruit without fertilisation e.g. in tomatoes
  • Prevents premature fall of leaves, flowers, fruits
  • Useful in stem cuttings and grafting where it initiates rooting
  • Promotes flowering e.g. in pineapple
  • 2,4-D is widely used as a herbicide to kill undesirable weeds of dicot plants without affecting monocot plants
  • Helps in cell division and xylem differentiation
• Gibberellins


  There are more than 100 gibberellins (GA 1 , GA 2 , GA 3 …..) that are known. They are acidic in nature. These are found in higher plants and fungi.
• Functions of Gibberellins


  • Promotes bolting, i.e. sudden elongation of internodes just before flowering in rosette plants like cabbage, beet
  • Delays senescence
  • Induces parthenocarpy
  • Elongation of the stem and reverses dwarfism
  • Induces maleness in certain plants like cannabis
  • Induces the formation of hydrolytic enzymes such as lipase, amylase in the endosperm of germinating cereal grains and barley seeds
  • Breaks seed dormancy
• Cytokinins


  Cytokinins play an important role in cytokinesis process. Cytokinins are naturally synthesised in the plants where rapid cell division occurs e.g. root apices, shoot buds, young fruits, etc. Movement of cytokinins is basipetal and polar.
• Natural Cytokinins


  • Zeatin (corn kernels, coconut milk)
  • isopentenyladenine
• Synthetic Cytokinins


  • Kinetin
  • benzyladenine
  • diphenylurea
  • thidiazuron
• Functions of Cytokinins


  • It promotes lateral and adventitious shoot growth and used to initiate shoot growth in culture
  • Helps in overcoming apical dominance induced by auxins
  • Stimulate the formation of chloroplast in leaves
  • Promotes nutrient mobilisation and delay leaf senescence
• Abscisic Acid


  It is a growth-inhibiting hormone. ABAs act as an antagonist to GAs. It inhibits plant metabolism and regulates abscission and dormancy. It is also called "stress hormone" as it increases the tolerance of plants.
• Functions of Abscisic Acid

  • Induces abscission of leaves and fruits
  • Inhibits seed germination
  • Induces senescence in leaves
  • Accelerates dormancy in seeds that is useful for storage purpose
  • Stimulates closure of stomata to prevent transpiration under water stress
• Ethylene


  It acts as a growth promoter as well as an inhibitor. Occurs in gaseous form. It is synthesised in the ripening fruits and tissues undergoing senescence. It regulates many physiological processes and one of the most widely used hormones in agriculture.
• Functions of Ethylene


  • It hastens the ripening of fruits
  • Controls epinasty of leaves
  • Breaks seed and bud dormancy
  • Stimulates rapid elongation of petioles and internodes
  • Promotes senescence and abscission of leaves and flowers
  • Induces root growth and root hair formation thereby increasing the absorption surface
  • Stimulates femaleness in monoecious plants
  • Apical hook formation in dicot seedlings
• Plant hormones


  Also known as plant growth factors, chemical substances that accelerate, inhibit or otherwise affect growth
• Action of indole-3 acetic acid (IAA), the most common form of auxin

  1. Produced at the apex of the stem under unilateral light

  2. Redistributed to the shaded side creating a gradient

  3. Travels between cells by diffusion

  4. Activates proton pumps lowering pH

  5. Activates enzymes expansins to break cell wall bonds

  6. Allows water entry and increases turgid pressure

  7. Causes elongation of cells on shaded side

  8. Bends shoot tips towards light
• Inserting glass plate below tip

  Auxin moved across to establish gradient, 70% found on shaded side
• Auxins are produced by cells at the shoot tip and transported downwards through the stem.
• In response to light, auxins accumulate on one side of the stem, causing it to bend towards the light source.
• The plant hormone auxin is involved in the growth and development of plants, including phototropism (bending towards light) and gravitropism (growing away from gravity).
• The concentration of auxin is higher on the lower (shaded) side than on the upper (sunlit) side due to differential absorption of light.
• The concentration of auxins is higher on the shaded side of the plant, leading to differential growth and curvature.
• Gibberellins promote seed germination and flowering, while cytokinins regulate cell division and differentiation.
• Phototropism is an example of tropisms where plants respond to environmental stimuli such as gravity or light.