Plant Hormones

What are Plant Hormones?

Plant hormones (also known as phytohormones) are signal molecules that exist at extremely low concentrations and are formed within plants. From embryogenesis, organ size regulation, pathogen protection, stress tolerance, and reproductive development, plant hormones regulate all aspects of plant growth and development. There are some intrinsic variables that control plant growth and development.

  • Some examples of plant hormones are Indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid), and gases (Ethylene).

  • In nearly all parts of the plant, these hormones are produced and are transferred to different parts of the plant.

  • Plant hormones may act individually or synergistically. It may be complementary or antagonistic to the functions of various hormones.

  • Hormones, along with extrinsic influences, play an important role in processes such as vernalization, seed germination, phototropism, dormancy, etc.

  • In regulated crop production, synthetic plant hormones are exogenously added.

Phototropism in the canary grass coleoptiles was first observed by Charles Darwin and auxin was first isolated from the coleoptiles of oat seedlings by F.W Went.

Main Functions of Plant Hormones

All the growth and development activities such as cell division, enlargement, flowering, seed formation, dormancy, and abscission are regulated by plant hormones and are the main functions of plant hormones.

Plant hormones are categorized into two groups based on their action:

  • Plant Growth Promoters.

  • Plant Growth Inhibitors.

Types of Plant Hormones and their Functions

  1. Auxin

Auxin means "to be able to grow." In agricultural and horticultural practice, they are widely used. They are found in roots and stems in rising apices and then move to other parts of the plant.

Example of plant hormone Auxin:

  • Natural: Indole butyric acid (IBA), and Indole-3-acetic acid (IAA)

  • Synthetic: NAA (Naphthalene acetic acid) and 2,4-D (2,4-Dichlorophenoxyacetic acid)

Functions of Plant Hormone Auxin:

  • Elongation of cells of stems and roots.

  • IAA in apical buds suppresses the development of lateral buds with apical dominance.

  • Induces parthenocarpy, i.e. fruit growth without fertilization, for example, in tomatoes.

  • Prevent leaves, flowers, and fruits from dropping prematurely.

  • Useful in stem cuts and grafting where rooting is initiated.

  • Promotes flowering in, for example, pineapple.

  • 2,4-D is commonly used to destroy unwanted dicot weeds without harming monocot plants.

  • Contributes to cell division and xylem differentiation.

  1. Gibberellin:

There exist more than a hundred (100) known Gibberellins (GA1, GA2, GA3..). In nature, they are found to be acidic. These kinds of hormones are majorly observed in higher plants and fungi.

Functions of Plant Hormone Gibberellin:

  • Promotes bolting, i.e. sudden internode elongation just before flowering, as seen in cabbage, beet, and others.

  • Brings delays in senescence.

  • Stimulates parthenocarpy.

  • Stem elongation and reversing dwarfism.

  • In plants such as cannabis, it induces maleness.

  • Promotes the synthesis of hydrolytic enzymes such as amylase, lipase, in the endosperm of germinating barley seeds and cereal grains.

  1. Cytokinins:

Cytokinins play a major role in the mechanism of cytokinesis. rIn plants where rapid cell division occurs, cytokinins are naturally synthesized, such as root apices, shoot buds, young fruits, etc. Cytokinins show basipetal and polar movement.

Example of plant hormone Cytokinins:

  • Natural: isopentenyl adenine and Zeatin (corn kernels, coconut milk).

  • Synthetic: Benzyladenine, Kinetin, thidiazuron, and diphenylurea.

Functions of Plant Hormone Cytokinins:

  • This promotes lateral and adventitious shoot growth and is used in culture to initiate shoot production.

  • Assists in resolving auxin-induced apical dominance.

  • Stimulate the production of chloroplast in the leaves.

  • Promoting the mobilization of nutrients and slowing leaf senescence.

  1. Abscisic Acid:

It is a hormone that has the role in inhibiting the growth of plants. ABAs serve as an opponent to GAs. It prevents the metabolism of plants and controls abscission and dormancy. It is often referred to as a 'stress hormone' as it improves plant tolerance.

Plant Hormones Diagram - Abscisic Acid

(image will be uploaded soon)

Functions of Plant Hormone Abscisic Acid:

  • Induces abscission of fruits and leaves.

  • Inhibits germination of seeds.

  • Induces leaf senescence.

  • Accelerates seed dormancy, which is useful for storage purposes.

  • Stimulates stomatal closure to stop transpiration under water stress.

  1. Ethylene:

Ethylene acts as both inhibitors as well as a growth promoter. It Occurs in gaseous form. The synthesis of ethylene occurs in the maturing fruit and tissues which are undergoing senescence. Many physiological procedures are regulated by this hormone and are one of the most commonly used hormones in agriculture.

Functions of Plant Hormone Ethylene:

  • It speeds up the fruit ripening.

  • Controls leaf epinastine.

  • Breaks the bud and seed dormancy.

  • Facilitates rapid elongation of  internodes and petioles.

  • Senescence and abscission of leaves and flowers are encouraged.

  • This raises the absorption surface by inducing root growth and root hair formation.

  • Stimulates femininity in plants that are single.

  • Formation of the apical hook in dicot seedlings.

Application of Plant Hormones:

  • Tissue Culture.

  • Somatic Cell Hybridization.

  • Introduction of recombinant DNA into protoplasts.


The answer to the question ‘what are plant hormones’ can be stated by the Plant hormone definition. Plant hormones are organic substances that control plant growth and development (also known as phytohormones). Different plant hormones may include Auxins, abscisic acid (ABA), gibberellins (GA), cytokinins (CK), ethylene (ET), salicylic acid (SA), jasmonates (JA), peptides, and brassinosteroids (BR). All these examples of plant hormones carry certain applications. The application of plant hormones may include Tissue Culture, the Introduction of recombinant DNA into protoplasts and Somatic Cell Hybridization, and many others. Some of the functions of plant hormones are to stimulate the production of chloroplast in the leaves, to stimulate parthenocarpy, to promote elongation of cells of stems and roots, to induce leaf senescence, etc.

FAQs (Frequently Asked Questions)

Q1. Which of the Hormones are Present in the Form of Gases?

Ans. Ethylene is the hormone that is present in the form of gases. Ethylene is known to be a multifunctional phytohormone that controls both development and senescence. Depending on its concentration, the timing of application, and the plant species, it promotes or inhibits growth and senescence processes.

Q2. Name the Most Important Plant Hormone?

Ans. Abscisic acid (also called ABA) is the most crucial plant, growth regulator. Abscisic acid (ABA) is a significant phytohormone that controls the development, growth, and stress responses of plants. It plays a vital role in various plant physiological processes, including stomatal closure, accumulation of cuticular wax, leaf senescence, dormancy of buds, germination of seeds, osmotic control, and inhibition of growth, among many others. Abscisic acid regulates downstream responses by both transcriptional and posttranscriptional mechanisms to abiotic and biotic environmental changes.