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:

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 et.al 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.

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 et.al 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.

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.

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.

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:

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