Plant hormones (types and functions)

  • E. Starling coined the term “hormone” in connection with the discovery of Secretin.
  • Hormones are organic compounds secreted in small quantities that regulate the plant’s activities after being translocated to the organ or site of reaction where they show their specific effect.
  • Five major kinds of plant hormones are as follows:
  1. Auxins
  2. Cytokinins
  3. Gibberellins
  4. Ethylene
  5. Abscisic acid


  1. Auxins:
  • Existence of growth substance in plants was proposed by Charles Darwin in his book “The power of movements in plants” while working on Canary grass (Phalaris cannariensis).
  • W. Went discovered auxins by Avena curvature test.
  • In 1931, Kogl and Haagen Smit isolated an active substance from human urine and named as Auxin-A (Auxintriolic acid).
  • In 1934, they isolated similar active substance from corn grain oil and was named as Auxin-B (Auxinolonic acid). Neither of these two Auxins has ever been isolated again.
  • Re-examination of human urine by Kogl and examination of Rhizopus culture by Thimann (1935) led to the isolation of different substances, which was named as heteroauxins.
  • Heteroauxins are identical with indole-3-acetic acid.
  • Indole-3-acetic acid is considered as true natural auxin, which is found in most plants including Avena
  • Synthetic auxins are: Nephthalene acid (NAA), indole butyric acid (IBA), indole propionic acid (IPA), 2,4-Dichlorophenoxyacetic acid (2,4-D), 2,4,5-Trichlorobenzoic acid (2,4,5-T)etc.
  • Synthesis of auxin takes place from the amino acid tryptophan.
  • Auxin shows polar transport.

Funtions of Auxins:

  • Elongation of the cell.
  • Apical dominance: suppresses the growth of lateral buds.
  • Root initiation: high concentration of auxins inhibits the elongation of roots but increases the number of lateral roots.
  • IAA induces rooting.
  • NAA, IBA, IPA induce rooting of cuttings.
  • 2,4-D and 2,4,5-T both are weed killers.
  • Methyl ester of Nephthalene acetic acids is used for preventing sprouting of potato.
  • Foliar spray of NAA and 2,4-D causes flowering of litchi and pineapple.
  • Auxin controls abscission (the natural detachment of parts of plant, normally dead leaves and ripe fruits) of fruits.
  • Auxins also stimulate cell wall synthesis and parthenocarpy.
  1. Cytokinins (kinetins):
  • They are also called kinetins.
  • Cytokinin was extracted from old stock of degraded DNA by Miller and Skoog.
  • Chemically, cytokinin is 6-furfuryl amino-purine.
  • Overbeek gave the name cytokinin because it stimulates cytokinesis (cell division).
  • It has been isolated from plant extracts like milk of coconut and old DNA extract.
  • Zeatin belonging to kinin group of hormones was extracted from maize kernel.
  • They are produced in actively growing tissues such as embryos, developing fruits and roots.

Functions of Cytokinins:

  • Activates cell division.
  • Formation of callus occurs if ratio of Auxin and Cytokinin is equal.
  • More Auxin and less Cytokinin results in root initiation whereas more Cytokinin and less Auxin results in bud formation.
  • Induces initiation of interfascicular cambium.
  • Stimulates cell elongation resulting in the expansion of cotyledons and foliage leaves (shoot development).
  • Counteraction of apical dominance (induces growth of lateral buds).
  • Prevents ageing and senescence in plants.
  1. Gibberellins:
  • They were first isolated from a fungus Gibberella fugikuroi/ Fusarium moniliforme which caused a disease known as foolish seedling or bakanae disease in rice plant.
  • Up to date, more than 100 naturally occurring gibberellins have been identified.
  • Widely studied Gibberellin is Gibberellic acid (GA3).

Functions of Gibberellins:

  • Seed germination: induces seed germination in light sensitive seeds even in dark after its treatment with the hormone. E.g. lettuce and tobacco.
  • Gibberellins stimulate stem elongation and leaf expansion.
  • Genetically dwarf plant grows tall after gibberellins treatment. E.g. corn, pea.
  • Causes parthenocarpy in pome fruits.
  • Helps to break dormancy in potato tubers and tree buds in winter.
  • Induces flowering in long day plants in short days.
  • In monoecious plants, spraying of gibberelins increases the number of male flowers.
  • De novo (a new) synthesis of α-amylase in the alurone layer of epidermis of cereal grains during germination.
  1. Ethylene:
  • It is the only gaseous hormone.
  • It consists of an amino acid, methionine.
  • O2 is a promoter and CO2 is an inhibitor for its synthesis.
  • It is mostly released from roots, shoot meristems and ripening fruits.

Functions of Ethtylene:

  • It induces ripening of fruits. Mature fruits also release ethylene.
  • It accelerates abscission of leaves, stems, flowers and fruits.
  • It induces flowering in pineapple.
  • Treatment of plants with ethylene increases the number of female flowers and fruits in cucumbers.
  • Induces epinasty (the outward and often downward bending of plant parts) in leaves.
  1. Abscisic acid (ABA):
  • A substance strongly antagonistic to growth was isolated by Addicott from young cotton fruits and named it Abscisin II.
  • Wareing et. Al pointed the presence of an active substance in birch leaves which inhibited growth and induced dormancy of buds. It was named as Dormin.
  • Later on, both Abscisin II and Dormin were found to be same and named as Abscisic acid.
  • It is mainly produced in mature leaves, stem, fruits and seeds.

Functions of Abscisic acid:

  • It accelerates abscission.
  • Maintains dormancy of seeds.
  • It also accelerates senescence in leaves.
  • It causes the closing of stomata.

Plant hormones (types and functions)