TOP 5 FUNCTIONS OF EVERY PHYTOHORMONE

1. AUXINS

Auxins were accidentally discovered by Charles Darwin and his son Francis Darwin when they observed the coleoptiles of Canary grass. In plants, Auxins were first isolated by E.W. Went from tips of coleoptiles of Oat seedlings, and in animals, these were first isolated from Human urine by Kogl and Smith.

Auxins refer to indole-3-acetic acid {IAA} and are generally produced by growing apices of the stems and roots wherefrom they migrate to the site of action.

IAA and IBA {indole butyric acid} are natural ones while as NAA {naphthalene acetic acid} and 2,4-D {2,4-dichlorophenoxyacetic} are synthetic auxins.

Functions of auxins:

Helps to initiate rooting in stem cuttings.
Promotes flowering like in Pineapples.
Prevents early leaf and fruit fall but promotes abscission of older mature leaves and fruits.
Promotes Apical dominance.
Induce parthenocarpy, e.g., in Tomatoes.

2. GIBBERELLINS

A disease of rice seedlings called ‘Bakanae’ [foolish seedling] was caused by fungal pathogen Gibberella fujikuroi.In 1926, E. Kurosawa reported that the disease symptoms came when the seedling was treated with sterile filtrates of the fungus. Later, the active substance was found to be gibberellic acid.

More than 100 Gibberellic acids have been reported and are denoted as GA₁, GA_2,GA_3,etc. GA₃ is the first discovered and most intensely studied form. All GA’s are acidic.

Functions of Gibberellins:

Their ability to increase the length of the stalk is used in increasing the size of Grape stalks.
Cause fruits like Apples to elongate and improve shape.
Delay senescence, thus fruits can stay longer on plants, thereby increasing the market period.
GA₃ is used to speed up the malting process in the brewing industry.
These also promote bolting [internode elongation just before flowering].

3.CYTOKININS

Skoog and his co-workers found that from the internodal segments of tobacco stems, the callus proliferated only if, in addition to auxins, the nutrient medium was supplemented with one of the following: extracts of vascular tissues, yeast extracts, coconut milk, or DNA. Miller et al. [1955] later identified and crystallized the cytokinesis promoting complex termed Kinetin.

Kinetin was first discovered from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. A first natural substance with cytokinin-like activities was Zeatin, isolated from corn kernels and coconut milk.

Functions of Cytokinin:

Helps to produce new leaves, chloroplast in leaves, lateral shoot growth, adventitious shoot formation.
Helps to overcome apical dominance.
Promote nutrient mobilization, thereby delaying leaf senescence [ Richmond land effect].
Prevents leaf fall.
Helps leaves to retain chlorophyll.

4. ETHYLENE

H.H. Cousins[1910] confirmed the release of a volatile substance from ripened oranges that hastened the ripening of unripe bananas. Later, this substance was found to be Ethylene, a gaseous phytohormone.

It is synthesized by tissues undergoing senescence and ripening fruits.

Functions of Ethylene:

Horizontal growth of seedlings, apical hook formation in dicotyledonous plants, and swelling of the axis.
Highly effective in fruit ripening.
Breaks seed and bud dormancy initiate germination in peanut seeds.
Rapid internode elongation in deepwater rice plants.
Helps upper shot parts to remain above water level.

5. ABSCISIC ACID

Three independent researchers reported the purification and chemical composition of different inhibitors: inhibitor-B, dormant, abscission-2. Later these were found to be chemically similar and were named Abscisic acid.

Functions of Abscisic acid:

General plant growth inhibitor and an inhibitor of plant metabolism.
Inhibits seed germination.
Increases tolerance of plants against stressful conditions [ stress hormone].
Acts as an antagonistic to Gibberellins.
Controls leaf fall and dormancy.

NOTE: 1. All phytohormone discoveries were accidental.

2.Ethylene can be regarded both as a promoter as well as an inhibitor.