Plant Growth Hormones: Introduction, Types and Functions

Plant Hormones


  • Hormones are chemical messenger and usually organic in nature that are produced by source cell and regulate the cellular physiology of another (target cell) by interacting with its specific receptor and initiate signal transduction pathways at very low concentration.
  • Hormones are responsible for regulation and coordination of metabolism, growth, development, morphogenesis, senescence and ultimately death.

Plant development is totally regulated by 7 endogenous phytohormones:

  1. Auxin
  2. Gibberellic acid
  3. Cytokinin
  4. Abscisic acid
  5. Ethylene
  6. Brassinosteroid
  7. Strigolactone


  • Auxin meaning “to increase” or “to grow”.

Auxin biosynthesis

  • Auxin mainly synthesized at shoot apex, young leaf, young fruit, RAM.
  • Auxin mainly synthesized from amino acid Tryptophan.
  • Major intermediate of auxin biosynthesis are Indole-3-Pyruvate.
  • In some plant like maize, auxin is synthesized from Tryptophan independent pathway.
  • Auxin biosynthesis show redundancy.
  • Natural auxin- Indole acetic acid(IAA), Phenol acetic acid, indole butyric acid(IBA), 4-chloro-indole acetic acid.
  • Artificial auxin- Naphthalene acetic acid (NAA), 2-methoxy-3,6- benzoic acetic acid, 2,4- Dichlorophenoxy acetic acid.
  • NAA and 2,4D mainly used as selective herbicide.
  • IBA used in horticulture.
  • Yucca gene encodes amino transferase and play important role in auxin biosynthesis.

Auxin catabolism

  • Auxin catabolism is completed by conjugation with Hexose sugar and amino acid.
  • Conjugated auxin is sequestered in SER by ABP-1 (Auxin binding protein).

Auxin transport

  • Auxin transported SAM to RAM, that means from top to bottom, and called Basipetal manner of transport.
  • Auxin transport show polar transport and unidirectional transport.

Auxin transport is completed into two steps:

Auxin efflux

    • Transport of auxin from cytosol to apoplastic space.
    • ABC pump is uniformly distributed through out the plasma membrane and regulate the activity of PIN protein.
    • PIN protein exclusively located at basal part of the plasma membrane and regulate auxin efflux only at the basal site.

Auxin influx

    • Transport of auxin from apoplastic space to cytosol.
    • Depending upon the pH of apoplastic space, auxin influx is of two types:
    • If pH of apoplastic space is less than 4.75 then, auxin is protonated and simply diffuse the plasma membrane.
    • If pH more than 4.75 , then auxin is deprotonated and enter in cytosol by using IAA/2H+  symporter.


  • Auxin promote apical dominance by suppression of lateral bud.
  • Auxin delay Senescence.
  • Auxin promote phototropism, geotropism, and thigmotropism.
  • Auxin promote parthenocarpic fruit development.
  • Auxin promote root development.
  • Auxin induce vascular bundle differentiation.
  • Auxin is responsible for sex-determination.

Gibberellic acid

  • Synthesized primarily in apical tissue and young leaves.
  • Highest level of Gibberellic acid are found in immature seeds and developing fruits.
  • It was discovered by plant pathologist Kurasawa.

Gibberellic acid biosynthesis

    • Gibberellic acid show Happlosufficient nature that means single copy of dominant gene that makes normal tall phenotype.
    • Gibberellic acid are synthesized by the condensation of four isoprenoids subunits, and IPP (isopentyl pyrophosphate) is the basic biological isoprene unit.
    • GA1, GA3, GA4 and GA7 are only biologically active gibberellin.
    • GA3 is mainly used in horticulture.


  • Gibberellic acid induce parthenocarpic fruit development.
  • It induce cell elongation without acidification.
  • Gibberellin induce internode elongation.
  • Gibberellic acid induce fruit production especially in grapes.
  • Gibberellin induce bolting.
  • Gibberellic acid induce seed germination by inhibiting della protein.
  • It induce flowering.
  • Gibberellic acid acts as positive regulator of SOC gene which is master switch of floral meristem gene.
  • Gibberellic acid responsible for sex-determination in plant.
  • It activate cyclin-cdk complex.


  • It is responsible for continuous greenery on the earth, that’s why called greenery hormone.
  • Cytokinin gives chlorophyll preservation test.
  • Cytokinin first time discovered by Skog in yeast cell.
  • It is Adenine derivative.
  • It is mainly synthesized in root apex, and from root apex to whole plant body it is transported by xylem pathway small amount of Cytokinin is synthesized in new leaves.

Cytokinin biosynthesis

  • The common active Cytokinin are Zeatin and isopentenyl adenine.
  • Cytokinin biosynthesis starts with the isopentyl AMP formation.
  • Isopentyl AMP is then converted into isopentyl adenosine.
  • Finally it oxidized into Zeatin.
  • IPT is master regulator of Cytokinin biosynthesis.


  • Cytokinin is antagonistic to auxin, because it trigger more and more growth of auxiliary bud and inhibits apical growth.
  • Cytokinin promotes shoot development.
  • Cytokinin promote chloroplast and leaf development.
  • Cytokinin prevent abscission.


  • It is only gaseous hormone.
  • Derived from amino acid methionine.

Ethylene biosynthesis

    • It is synthesized by almost all parts of the plant.
    • Firstly it is converted in S-adenosylmethionine, by reaction of methionine and ATP catalysed by Ado-met synthase.
    • Then it is converted into 1-amino cyclo propane carboxylic acid, catalysed by enzyme ACC synthase.
    • Finally it is converted into ethylene by ACC oxidase.


  • It is main senescence hormone.
  • Ethylene promotes adventious root development in drought condition.
  • It promotes flowering in pineapple.
  • It promotes fruit ripening.
  • Ethylene promotes epinastic leaf development.
  • Ethylene promotes formation of aerenchyma and air chamber in hydrophytes.

Abscisic acid

  • Abscisic acid is also known as stress hormone.
  • Originally it was named as abscisin II, because it was thought that it involved in the abscission of fruits.
  • It i synthesized in almost all parts of the plant which having chloroplast.

Abscisic acid biosynthesis

  • ABA is 15 carbon compound called Sesquiterpene.
  • Sesquiterpene is synthesized by using tetraterpene zeaxanthin.
  • Biosynthesis starts with IPP and forms zeaxanthin which converted into violaxanthin.
  • Violaxanthin is then converted to form neoxanthin which is also converted into xanthinin which cleaved to form ABA aldehyde, and finally Abscisic acid.


  • ABA is called seed maturation hormone.
  • It induce abscission.
  • It induce tuburization of potato.
  • ABA delayed flowering.
  • ABA induce stomatal closure under drought condition, that’s why it is termed as anti-transpirant hormone.


  • It is the polyhydroxylated steroid hormone in plants.
  • It is synthesized from plant steroid called Compesterol.
  • Member of cyt-450 monoxygenase play important role in Brassinosteroid synthesis.
  • Its concentration is regulated by negative feedback mechanism.


  • It promotes pollen tube formation.
  • It induce chloroplast development,.
  • It induce shoot development.
  • Brassinosteroid stimulate stem elongation.
  • It Induce root development, similar to auxin.
  • It gives leaf inclination test.


  • It is one of the class of plant hormones which is first isolated plant exudates which involved in the stimulation of seed germination of striga, a parasitic plant.
  • Synthesized from carotenoid and major intermediate is carlactone.
  • Strigolactone synthesis is totally regulated by Max 1 and cyt-450 enzyme in SER.


  • It promotes rhizospheric interactions with arvascular myccorhizal fungi.
  • It also favours secondary growth and cambium activity.

Reference and Sources:

    25974#:~:text=The biosynthetic pathway begins with the C 5,xanthophyll, zeaxanthin, which is then
    converted to violaxanthin.

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