Plant Hormones: Definition, Characteristics, Types, Discovery, Distribution, Biosynthesis, Transport, Mechanism of Action, and Functions

Plant Hormones: Definition, Characteristics, Types, Discovery, Distribution, Biosynthesis, Transport, Mechanism of Action, and Functions


The term “Phytohormones” also refers to Plant hormones. Phytohormones are organic compounds made by higher plants naturally that control growth or other physiological processes at a location far from the origin of synthesis and are only active in very small amounts. Because these hormones are produced in plants, Thimmann (1948) proposed the term “Phytohormone.” Among the hormones produced by plants are Auxins, Gibberellins, Cytokinins, Ethylene, Growth inhibitors, and Growth retardants. The first hormones found in plants were Auxins, followed by the discoveries of Gibberellins and Cytokinins.


1. The word hormone is Greek in origin and means to set in action. Cellular division, growth, and gene expression are all affected by plant hormones.

2. they are naturally produced within plants. Although fungi and bacteria also create very similar compounds that can impact plant growth,

3. Plant hormones are chemicals, not nutrients, that, in little doses, encourage and impact the growth, development, and differentiation of cells and tissues.

Types of Plant Hormones

It is generally agreed that there are five main categories of plant hormones, some of which can have a wide range of chemical compositions from one plant to the next.

The five main categories are:

a) Auxin  

b) Gibberellin

c) Cytokinin

d) Ethylene

e) Abscisic acid


Auxins are a group of Phytohormones that are produced in the shoot and root apices and migrate from the apex to the zone of elongation. Auxins encourage plant growth along the plant’s longitudinal axis, thus the name (auxeing : to grow). Kogl and Haagen-Smit coined the name “auxin” (1931). Avena coleoptile or curvature test was used by Went (1928) to isolate auxin from the tips of the plant’s coleoptiles, and he concluded that growth is impossible without it. Auxins are broadly distributed throughout the plant, but are particularly abundant in their growth tips, including the coleoptile tip, buds, root tips, and leaves. The only auxin that exists naturally in plants is called indole acetic acid (IAA). The synthetic auxins include,

NAA: Naphthalene Acetic Acid,

IBA: Indole Butyric Acid

MENA: Methyl ester of Naphthalene acetic acid

MCPA: 2 Methyl 4 chloro phenoxy acetic acid

TIBA : 2, 3, 5 Tri iodo benzoic acid

2, 4-D: 2, 4 dichloro phenoxy acetic acid

2, 4, 5-T: 2, 4, 5 – Trichloro phenoxy acetic acid

Natural Auxins can exist as bound auxins, which are released from plant tissues only after hydrolysis, autolysis, or enzymolysis, or as free auxins, which can freely migrate or diffuse out of the plant tissues with ease.

Auxin’s Physiological Effects

1. Cell elongation and division

Auxin primarily causes cell division and cell elongation in the shoots, which are physiological effects. It is crucial for stem secondary growth and xylem and phloem tissue differentiation.

2. Apical dominance

In many plants, the growth of lateral buds immediately below the terminal bud stays repressed as long as it is healthy and growing. The lateral buds grow quickly once the apical bud is removed. Apical dominance is the occurrence in which the apical bud predominates over the lateral buds and prevents the lateral buds from developing.

Skoog and Thimmann (1948) noted that Auxin, produced at the terminal bud and transmitted downhill through the stem to the lateral buds and inhibits growth, maybe in charge of the apical dominance. The apical bud was removed, and an agar block was put in its place. This caused the lateral buds to expand quickly. However, the lateral buds remained inhibited and did not expand when the apical bud was substituted with an auxin-containing agar block.

3. Root initiation

A higher auxin concentration, in contrast to the stem, slows root elongation but significantly increases the number of lateral roots, or auxin induces more lateral branch roots. Applying IAA in lanolin paste—a soft fat made from wool that is a good auxin solvent—to the cut end of a young stem causes an early and extensive rooting. This knowledge has been widely used to encourage the growth of roots in commercially valuable plants that are propagated via cuttings.

4. Abscission prevention

Natural auxins stop the development of the abscission layer, which would otherwise cause the falling of fruits, flowers, and leaves.

5. Parthenocarpy

The production of parthenocarpic fruits can be induced by auxin (fruit formation without pollination and fertilization). Compared to plants that produce fruits only after fertilization, parthenocarpic fruits have ovaries that contain a higher concentration of auxin. After pollination and fertilization, the auxin concentration in the ovaries rises in the latter situations.

6. Respiration

There is a link between auxin-induced growth and respiration, which is stimulated by auxin. By quickly consuming ATP in the growing cells, auxin may indirectly enhance the rate of respiration through an increase in the availability of ADP.

 7. Callus development

Auxin might be involved in cell division in addition to cell elongation. The continuing growth of such a callus only occurs with the addition of auxin in many tissue cultures where the callus growth is pretty normal.

8. Eliminating weeds

When used in higher quantities, several synthetic auxins, particularly 2, 4- D and 2, 4, 5-T, are effective at eliminating weeds.

9. Expression of sex and flowering

Auxins often prevent flowering, however, they encourage uniform flowering in pineapple and lettuce.

Auxin distribution in plants

Auxin (IAA) is produced in meristematic or growing regions of plants, from which it is transmitted to other plant tissues. Therefore, growing shoot tips, immature leaves, and developing auxiliary shoots have the maximum concentration of IAA. The coleoptile tip of monocot seedlings contains the largest concentration of auxin, which gradually diminishes toward the base.

The largest concentration is detected in developing auxiliary shoots, young leaves, and growing shoot regions in dicot seedlings. Auxin can exist in two different forms within plants. specifically, bound auxins and free auxins. Free auxins are those that can be conveniently extracted using different organic solvents, including diethyl ether. For the extraction of auxin from bound auxins, however, more extreme techniques like hydrolysis, autolysis, enzymolysis, etc. are required. Bound auxins are found in plants as complexes with proteins or amino acids like aspartate, glutamate, or inositol as well as carbohydrates like glucose, arabinose, or sugar alcohols.

Auxin (IAA) biosynthesis in plants

Tryptophan, an amino acid, is transformed into Indole 3 acetic acid, according to Thimann (1935). The main precursor of IAA in plants is tryptophan. Tryptophan can be converted to IAA in two separate methods.

1. Tryptophan is deaminated to form indole-3-pyruvic acid, which is then decarboxylated to form indole-3-acetaldehyde. Tryptophan deaminase and indole pyruvate decarboxylase are the relevant enzymes.

2. Using the enzymes tryptophan decarboxylase and trypamine oxidase, tryptophan is first converted to tryptamine, which is then converted to indole-3-acetaldehyde. When indole 3-acetaldehyde dehydrogenase is present, indole 3-acetaldehyde can easily oxidize to indole 3-acetic acid (IAA).

Auxin transport in plants

Auxin is primarily transported polarly. Auxin is transported polarly in stems in a basipetal fashion, moving from the apex to the base. Naphthyl thalamic acid and 2, 3, 5 Triiodobenzoic Acid (TIBA) inhibit the polar transport of auxin (NPA). Antiauxins is the name given to the compounds.

Plant auxin destruction or inactivation

The enzyme IAA oxidase oxidizes auxin in the presence of oxygen to destroy it.

IAA + H2O2 + O2——IAA Oxidase———————- 3-methylene oxindole + H2O + CO2

Gamma and x-ray radiation can also quickly inactivate a substance. In plants, auxin levels are also decreased by UV light. IAA photooxidation refers to the inactivation or breakdown of IAA by light.

Action Mechanism

IAA improves the cell walls’ plasticity, allowing for simple cell stretching in response to turgor pressure. IAA may affect the synthesis of mRNA by acting on DNA, according to some research. Specific enzymes needed for cell wall expansion are encoded by the mRNA. Recent data suggest that IAA enhances oxygen absorption by increasing oxidative phosphorylation during respiration. It has been shown that auxin causes the formation of ethylene in plants, which may also contribute to growth stimulation.



Scientist Kurosawa from Japan discovered that infected rice seedlings of the fungus Gibberella fujikuroi grow higher and become incredibly thin and pale. From the infected seedlings, an active compound was identified and given the name Gibberellin. Gibberellin biosynthesis in plants

Acetate serves as the main starting material for the production of gibberellins.

Acetate + COA → Acetyl COA → Mevalonic acid → MA pyrophosphate → Isopentanyl pyrophosphate → Geranyl pyrophosphate → GGPP → Kaurene → Gibberellins

Physiological effects of Gibberellins

1. Germination of seeds

Certain seeds that are sensitive to light, like lettuce and tobacco, have poor germination in low light. If these seeds are exposed to light or red light, germination begins quickly. If the seeds are treated with gibberellic acid in the dark, this requirement for light is overcome.

2. Buds’ dormancy

In locations with extreme cold, the autumn-formed buds are dormant until the following spring. Treatments with gibberellin can remove the buds’ dormancy. After harvest, potatoes similarly go dormant, but the administration of gibberellin causes the roots to sprout vigorously.

3. Root Growth

Gibberellins have negligible to no impact on root development. There may be some root development suppression at higher concentrations. In isolated cuttings, gibberellins significantly reduce the beginning of roots.

4. Internode elongation

The elongation of the internodes is the effect of gibberellins on the most noticeable plant growth. Therefore, gibberellins override the genetic dwarfism in numerous plants, including dwarf peas, dwarf maize, etc.

5. Flowering and bolting

Early growth in many herbaceous plants exhibits rosette habit with short stems and little leaves. While the rosette habit persists during short days, bolting occurs during long days, meaning the stem elongates quickly and transforms into polar axis-bearing flower primordia. Gibberellins can also be used to trigger bolting in such plants, even in non-inductive short days. Gibberellin administration produces bolting and flowering in Hyoscyamus niger (a long-day plant) under non-inductive short days. While the use of gibberellin typically leads to early blooming in long-day plants. Its impact on short-day plants can be highly diverse. It could either have no effect, stop or start blossoming.

6. Parthenocarpy

Gibberellins stimulate pollen grains to germinate, and they can also promote fruit growth and the development of parthenocarpic fruits when applied to plants. Gibberellins have shown success in numerous instances, such as pome and stone fruits, when auxins have been unable to promote parthenocarpy. Gibberellin treatments are used on a big scale to produce seedless, fleshy tomatoes and enormous seedless grapes.

7. Synthesis of the enzyme α – amylase

Gibberellins play a crucial role in the formation of the enzyme α – amylase in the aleurone layer of cereal grains’ endosperm during germination. This enzyme causes the starch to be hydrolyzed into simple sugars, which are then transferred to growing embryos as an energy source.

Gibberellins’ distribution in plants

Gibberellins can be found in the shoots, roots, leaves, petals, anthers, and seeds of higher plants. In comparison to the negative parts, reproductive parts often have substantially larger amounts of gibberellins. Gibberellin content is particularly high in immature seeds (10-100 mg per g fresh weight). Gibberellins are found in plants in two different forms: free gibberellins and bound gibberellins. Usually, bound gibberellins appear as gibberellin-glycosides.

Cytokinins (Kinetin)

Skoog and Miller (1950) isolated the chemical compound 6-furfuryl aminopurine from the tobacco pith callus and named it kinetin. It was known as cytokinins or kinetin because of its particular impact on cytokinesis (cell division). Letham coined the phrase “cytokinin” (1963). Because cytokinins are derived from plants, Fairley and Kingour (1966) referred to them as phytokinins. Chemically speaking, cytokinins are purine derivatives of kinins.

Cytokinins are thought of as natural plant growth hormones because, in addition to their primary impact on cell division, they also regulate growth. Zeatin and Coconut milk factor are two of the highly significant and well-known naturally occurring cytokinins. Additionally, cytokinin’s role as a component of t-RNA was discovered.

Naturally occurring cytokinins

The liquid endosperm of the coconut, tomato juice, the flowers and fruits of Pyrus malus, the fruits of Pyrus communis (the pear), the fruits of Prunus cerasiferae (the plum), and the fruits of Lycopersicum esculentum (the bhendi), the cambial tissues of Pinus radiata, Eucalyptus regnans, and Nicotiana tabacum, the immature fruits of Zea mays, Ju At least seven well-known kinds of cytokinins have been reported from the plants, according to Skoog and Armstrong (1970).


It is hypothesized that cytokinins are produced similarly to how plants produce purines (nucleic acid synthesis). An essential location for its synthesis is the root tip. However, cytokinin production also takes place in growing seeds and cambial tissues. According to Kende (1965), cytokinins migrate upward, maybe in the xylem stream. However, isolated stems and petiole basipetal movement are also seen. According to Seth et al. (1966), auxin promotes kinetin translocation in bean stems.

Physiological effects of cytokinins

1. Cell division

The most significant biological impact of kinetin on plants is the induction of cell division, particularly in the tobacco pith callus, carrot root tissue, soybean cotyledon, and pea callus, among other tissues.

2. Cell enlargement

The kinetin may also cause cell growth, much like auxins and gibberellins do. The cotyledons of pumpkins, tobacco pith culture, cortical cells of tobacco roots, and leaves of Phaseolus vulgaris have all shown significant cell expansion.

3. Apical dominance concentration

4. Dormancy of seeds

Similar to gibberellins, kinetin therapy can break the dormancy of several light-sensitive seeds, including lettuce and tobacco.

5. Delay of senescence ( Richmand – Lang effect)

Chlorophyll loss and quick protein breakdown are common side effects of a leaf’s senescence. By enhancing RNA synthesis first, followed by protein synthesis, kinetin therapy can postpone senescence for a few days. Kinetin was shown to be able to delay senescence for several days by Richmand and Lang (1957) when they were working on detached leaves of Xanthium.

6. Flower induction

It is possible to successfully use cytokinins to encourage flowering in short-day plants.

7. Morphogenesis

It has been demonstrated that high auxin and low kinetin only create roots while high kinetin and low auxin can encourage the development of shoot buds.

8. Accumulation and translocation of solutes

Plants use cytokinin to actively accumulate and translocate solutes inside the phloem.

9. Synthesis of proteins

Osborne (1962) showed that the translocation caused by kinetin therapy enhanced the rate of protein synthesis.

10. Other effects

Some plants have cytokinins that give them tolerance to cold, heat, and pathogens. They substitute for the photoperiodic requirements to aid in flowering as well. In some situations, they promote the production of several photosynthesis-related enzymes.

11. Commercial applications

Fruits’ shelf lives have been extended with cytokinins, which have also been used to speed up root induction, create effective root systems, and increase the yield and oil content of oil seeds like ground nuts.


The only naturally occurring plant growth hormone in gaseous form is ethylene.

Important Physiological Effects

1. Ethylene’s primary function is to enhance the ripening of fleshy fruits including bananas, apples, pears, tomatoes, and citrus.

2. It promotes leaf withering and abscission.

3. It successfully induces pine apple blossoming.

4. It inhibits the growth of roots.

5. It encourages the development of adven­ti­ous roots.

6. It encourages floral fading, and

7. It encourages leaf epinasty

Abscisic acid

A chemical from immature cotton fruits that Addicott (1963) isolated and termed Abscissin II was shown to be very hostile to growth. Abscisic acid was later given this name. This substance, also known as Dormin, causes buds to go into dormancy. One of nature’s growth inhibitors is called Abscisic acid.

Physiological effects

The two main physiological effects are

1. Geotropism in roots

2. Stomatal closing

3. Besides other effects

1. Geotropism in roots

The primary cause of the geotropic curvature of the root is ABA translocation in a basipetal direction towards the root tip.

2. Stomatal closing

The mesophyll chloroplast is where ABA is produced and stored for stomatal closure. When under water stress, the chloroplast membrane loses its permeability, allowing ABA to diffuse from the chloroplast into the cytoplasm of the mesophyll cells. It diffuses from mesophyll cells into guard cells, where it induces stomata to close.

3. Other effects

i. Including seed and bud dormancy;

ii. Including tuberization;

iii. Causing fruit ripening, abscission of leaves, flowers, and fruits;

iv. Increasing the resilience of temperate zone plants to frost harm.

Growth retardants

Several synthetic chemicals stop gibberellins from causing the usual reactions in plants, such as cell expansion or stem lengthening. Therefore, they are also known as growth retardants or anti-gibberellins. Those are

1. Cycocel (2- chloroethyl trimethyl ammonium chloride (CCC)

2.  AMO – 1618

3. Phosphon D – (2, 4 – dichlorobenzyl – tributyl phosphonium chloride)

4. Maleic hydrazide

5. Morphactins

Frequently Asked Questions

Question: What are the 5 hormones in plants?

The five main categories are

Ans: a) Auxin  b) Gibberellin c) Cytokinin d) Ethylene e) Abscisic acid

Question: What are Plant hormones called?

Ans: The term “Phytohormones” also refers to Plant hormones.

Question: How many hormones are in plants?

Ans: Mostly Five

Question: Who first discovered Plant hormones?

Ans: Frits W. Went (1903–1990),

Question: Who discovered Auxin?

Ans: In 1928, Dutch botanist Fritz W. Went

Question: What is the chemical name of Cytokinin?

Ans: Kinetin (C10H9N5O)

Question: Which one is a Stress hormone?

Ans: Cortisol

Question: What is the chemical name of Auxin?

Ans: indole-3-acetic acid

Question: What is the chemical name of Gibberellin?

Ans: Gibberellic acid (also called gibberellin A3, GA, and GA3)

Question: What is natural Auxin?

Ans: indole-3-acetic acid (IAA), indole butyric acid (IBA),

Question: What is the full form of IAA?

Ans: IAA stands for Indole Acetic Acid.

Question: Which hormone is present in coconut milk?

Ans: Cytokinins

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