Plant Hormones

Updated on

Signal molecules which exist at extremely low concentrations and are formed within plants are known as plant hormones (phytohormones). Plant hormones control all aspects of plant growth and development ranging from embryogenesis, organ size regulation, pathogen protection, stress tolerance, and reproductive development. There are certain inherent variables which control plant growth and development.

  • Indole (auxins), terpenes (Gibberellins), carotenoids (Abscisic acid), and gasses (Ethylene) are examples of plant hormones.
  • These hormones are produced in almost all parts of the plant, transferred to various parts of the plant.
  • Plant hormones may either act individually or synergistically ; complementary or antagonistic to the functions of various hormones.
  • Hormones, along with extrinsic influences, play crucial roles in vernalization, seed germination etc.
  • Synthetic plant hormones are exogenously added in regular crop production.

Charles Darwin was the first person to observe/identify phototropism in the canary grass coleoptiles. Similarly, in a first, F.W. Went was successful in isolating auxin from the coleoptiles of oat seedlings.

What are plant hormones?

Plants require natural aid like water, oxygen, sunlight, and various minerals and nutrients for their survival and growth. All these external factors help in the development of plants. There are certain intrinsic factors which help in their growth as well. They are basically called plant hormones/phytohormones. They primarily help in regulating the growth of plants. 

  • Chemical compounds which are found in a plant’s body in very low concentrations are called plant hormones. They are basically derived from indole (auxins), terpenes (Gibberellins), adenine (cytokinins), gasses (Ethylene) etc.
  • Plant hormones are produced in every part of the plant, circulated throughout a plant body.
  • Each hormone has a specific role to function. Different hormones carry out distinct functions, either individually or in combination with other hormones. 
  • With the help of external factors including sunlight, water and oxygen, hormones facilitate processes like vernalisation, phototropism, seed germination, dormancy etc.   
  • Synthetic hormones help in controlling crop production.

Major functions of plant hormones

Growth and development activities including cell division, enlargement, flowering, seed formation, dormancy etc are regulated by plant hormones.

Plant hormones are categorized into:

  • Plant growth promoters
  • Plant growth inhibitors

Types of plant hormones/functions

The meaning of auxin is ‘to be able to grow’. They are widely used in agricultural and horticultural practices. They are present in roots and stems in rising apices. They move to other parts of the plant from here.


  • Natural: Indole-3-acetic acid (IAA)
  • Synthetic: NAA (Naphthalene acetic acid)

Functions of Auxin

  • Facilitates elongation of cells of stems and roots.
  • Development of lateral buds are suppressed by IAA in apical buds, through apical dominance.
  • Incites parthenocarpy (fruit growth without fertilization like in tomatoes).
  • Helps in preventing leaves, flowers, and fruits from dropping prematurely. 
  • Handy in stem cuts and grafting where rooting is initiated.
  • Promotes flowering.
  • 2, 4-D is mostly used for destroying unwanted dicot weeds without harming monocot plants.
  • Helps in cell division and xylem differentiation.


There are over a 100 known Gibberellins (GA1, GA2, GA3 ..) They are acidic in nature. Such hormones are mostly present in higher plants and fungi.

Functions of Gibberellin

  • Boosts bolting – the sudden internode elongation just before flowering – like in cabbage, beet and others. 
  • Brings about delay in senescence. 
  • Boosts parthenocarpy.
  • Helps in stem elongation and in reversing dwarfism.
  • In cannabis, it induces maleness. 
  • Encourages synthesis of hydrolytic enzymes (amylase,lipase) in the endosperm of germinating barley seeds and cereal grains. 


Cytokinins play a crucial role in the mechanism of cytokinesis. They are naturally synthesized in plants where rapid cell division occurs (root apices, shoot buds etc). Cytokinins display basipetal and polar movement.


  • Natural: isopentenyl adenine and Zeatin.
  • Synthetic: Benzyladenine, Kinetin, and diphenylurea.

Functions of Cytokinins

  • Promotes lateral and adventitious shoot growth. Used in culture for initiating shoot production.
  • Helps in resolving auxin-induced apical dominance.
  • Encourages the production of chloroplasts in leaves.
  • Promotes mobilization of nutrients, slows leaf senescence. 


Ethylene functions as both an inhibitor as well as a growth promoter. It will be in gaseous form. The synthesis of ethylene occurs in the maturing fruit, tissues that undergo senescence. This hormone helps in regulating several physiological procedures. Ethylene is one of the most widely used hormones in agriculture.

Functions of Ethylene

  • Accelerates fruit ripening.
  • Regulates leaf epinastine. 
  • Disrupts the bud and seed dormancy.
  • Helps in rapid elongation of internodes and petioles.
  • Encourages senescence and abscission of leaves and flowers.
  • Hence raises the absorption surface by boosting root growth and root hair formation.
  • Helps in stimulating femininity in plants which are single.
  • Helps in forming apical hooks in dicot seedlings.    

Application of Plant Hormones

They are used in tissue culture, somatic cell hybridization and for introduction of recombinant DNA into protoplasts.


Plant hormones (phytohormones) are organic substances which regulate plant growth and development. Auxins, abscisic acid (ABA), gibberellins (GA), cytokinins (CK), ethylene (ET), salicylic acid (SA), and jasmonates (JA) are a few of the plant hormones. They are predominantly used in tissue culture, somatic cell hybridization etc. Functions of plant hormones include production of chloroplasts in leaves, inducing leaf senescence etc. 

Check Your Knowledge

 Indole (auxins), terpenes (Gibberellins), carotenoids (Abscisic acid), and gasses (Ethylene) are examples of plant hormones.

They are used in tissue culture, somatic cell hybridization and for introduction of recombinant DNA into protoplasts.

Leave a Reply

Your email address will not be published. Required fields are marked *