Plant Hormones

Plant hormones, also known as phytohormones, are chemical messengers that regulate various physiological processes in plants. These hormones play a crucial role in plant growth, development, and responses to environmental stimuli. There are different types of plant  hormones and functions of plant hormones are as follows : 

1.0Auxin 

The discovery of auxin is credited to the pioneering work of Dutch scientist Frits Went in the early 20th century. The journey began with observations by Charles Darwin on phototropism, the bending of plant organs toward light. Peter Boysen-Jensen's 1913 experiment, where a translucent cap on a coleoptile tip did not hinder phototropic bending, suggested the involvement of a substance. Frits Went, in 1928, isolated and characterized this substance from oat seedlings, naming it "auxin" from the Greek word "auxein," meaning "to grow." Further research, including the identification of indole-3-acetic acid (IAA) as the primary naturally occurring auxin, revealed auxin's pivotal role in regulating diverse plant growth processes, marking a significant milestone in our understanding of plant physiology.

Types of Auxin

There are two types of auxin found in nature : 

1. Natural auxin : 

• Indole-3-Acetic Acid (IAA)
• Indole-3-Butyric Acid (IBA)
• 4-chloro-indole acetic acid
• Phenyl acetic acid

2. Synthetic auxin : 

• Naphthalene acetic acid (NAA)
• 2, 4-dichlorophenoxyacetic acid (2, 4-D)
• 2, 4, 5-trichlorophenoxy acetic acid (2, 4, 5-T) 

Both natural and synthetic auxins have been used extensively in agricultural and horticultural practices.

Function/ Role of Auxin

The functional role of auxin can be summarized as follow

1. Apical dominance : In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds. It is widely applied in tea plantations and in hedge-making.

2. Root initiation : Auxins help to initiate rooting in stem cuttings. This application is widely used for plant propagation.

3.  Flowering : Auxins promote flowering in pineapple.

4. Abscission : Auxins help to prevent fruit and leaf drop at early stages but promote the  abscission of older mature leaves and fruits.

5. Parthenocarpy : Auxin induces parthenocarpy in tomatoes.

6.Herbicide/weedicide : Auxins used as herbicides. 2,4 D widely used to kill dicotyledonous weeds. 2,4 D does not affect mature monocotyledonous plants. It is used to prepare weed free lawns by gardeners.

7. Xylem differentiation : Auxin controls xylem differentiation.

8. Cell division : Auxins helps in cell division.

9. Potato dormancy : MH (maleic hydrazide) and a NAA keep lateral buds of potato tubers dormant. Thus, potato tubers can be stored for longer durations.

2.0Gibberellins

Gibberellins (GAs) were first discovered in the fungus Gibberella fujikuroi, responsible for causing the 'bakanae' or foolish seedling disease in rice. Many GAs exist, but not all are biologically active; some are named GA1, GA2, and GA3, with gibberellic acid (GA) being the most common. Chemically, GAs are tetracyclic diterpenoids composed of four isoprene units (C5H8) and can be categorized into C20-gibberellins (20 carbon atoms) and C19-gibberellins (19 carbon atoms). These plant hormones are naturally synthesized in young leaves, buds, developing seeds, fruits, and roots. GAs play a crucial role in plant growth regulation. 

Function/ Role of  Gibberellins

The functional role of Gibberellins can be summarized as follows. 

1. Bolting : Gibberellins promote bolting (internodal elongation just prior to flowering) in beet, cabbages and many plants with rosette habits.

2. Overcome genetic dwarfism : Gibberellins promote internodal elongation in genetically dwarf plants like maize, Pisum, Vicia faba etc. Here only phenotype changes while genotype remains same.

3. Improve yield in sugarcane : Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem (internodal elongation), thus increasing the yield by as much as 20 tonnes per acre.

4. Improve shape in apples : Gibberellins cause fruits like apples to elongate and improve their shape. (by internodal elongation).

5. Improve yield in grapes : Ability of gibberellins to cause an increase in length of axis (internodal elongation) is used to increase the length of grape stalks.

6. Senescence : Gibberellins delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period.

7. Early maturity in conifers : Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.

8.  Brewing industry : GA3 is used to speed up the malting process in the brewing industry.

9. Dormancy : Gibberellins induce the synthesis of hydrolysing enzymes a-amylase, lipases and proteases. These enzymes hydrolyse the store food present in seeds and buds. This process results in breaking of their dormancy.

3.0Cytokinins

The discovery of cytokinins originated from Haberlandt's 1913 experiment demonstrating that phloem sap could induce non-dividing potato tuber tissue to resume cell division. In the 1940s-1950s, F. Skoog's group at the University of Wisconsin studied tobacco stem tissue cultures and found that extracts like coconut milk stimulated cell division in the presence of auxin. C. O. Miller, working in Skoog's lab, identified the active substance as N6-furfurylaminopurine, naming it kinetin in 1956. Though an artifact of DNA isolation, kinetin spurred the search for natural cytokinins. In the early 1960s, Miller and D. S. Letham independently reported zeatin, a naturally occurring cytokinin, leading to the subsequent discovery of various cytokinins.

Function/ Role of Cytokinins

Cytokinin functions are as follows 

1. Cell division : Cytokinins have specific effects on cytokinesis (cytoplasmic division). It is considered as most

important biological effect of cytokinins.

2. Overcome apical dominance : Cytokinins promote the growth of lateral buds. This role of cytokinin is antagonist to auxin.

3. Senescence : Cytokinins promote nutrient mobilization towards young parts which helps in the delay of leaf senescence.

4. Help to produce new leaves.

5. Help to produce chloroplast in leaves.

6. Promote lateral shoot growth and adventitious shoot formation.

7. Induce opening of stomata.

8. Tissue culture :

• Cytokinin and auxin together cause dedifferentiation in differentiated parenchymatous cells in tissue culture.
• Morphogenesis :

High concentration of cytokinins to low concentration of auxin = shoot differentiation High concentration of auxin to low concentration of cytokinin = root differentiation

4.0Ethylene

In 1910, H. H. Cousins noted that volatile substances from ripe oranges hasten banana ripening, and in 1934, R. Gane definitively identified ethylene as the responsible volatile substance. This discovery marked a pivotal moment in understanding ethylene's role in plant physiology. Methionine is a precursor of ethylene plant hormone. Ethylene is synthesized in large amounts by tissues undergoing senescence and ripening fruits. Ethylene is one of the most widely used PGR in agriculture. The most widely used compound as source of ethylene is ethephon/CEPA (chloro ethyl phosphonic acid). Ethephon is an aqueous solution which is readily absorbed and transported within the plant and releases ethylene slowly. 

Function/ Role of Ethylene

1. Fruit ripening : Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in the rate of respiration is called respiratory climactic. e.g. tomato, apple, banana, oranges (Citrus).

2. Senescence and abscission : Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethephon is used to accelerate abscission in flowers and fruits (thinning of cotton, cherry, walnut).

3. Triple response : Triple response of ethylene on plants include :

• Horizontal growth of seedlings
• Swelling of the axis
• Apical hook formation in dicot seedlings.

4. Dormancy : Ethylene breaks seed and bud dormancy. Ethylene initiates germination in peanut seeds and sprouting of potato tubers.

5. Increase absorption surface : Ethylene promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.

6. Ethylene is used to initiate flowering and for synchronizing fruit-set in pineapples.

7. Ethylene induces flowering in mango.

8. Ethylene promotes female flowers in cucumbers (feminizing effect). This results in an increase of yield.

9. Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It help leaves/upper parts of the shoot to remain above water. 

5.0Abscisic Acid 

In 1953, Bennet-Clark and Kefford identified an inhibitory substance, inhibitor β, alongside IAA in plant extracts. Kefford suggested its role in apical dominance and potato dormancy. Concurrently, other researchers found inhibitors in buds correlating with woody plant dormancy. In 1964, P. F. Waring coined the term 'dormin' for these dormancy-inducing substances. Simultaneously, substances accelerating abscission, named 'abscission II,' were isolated. In the mid-1960s, three labs independently characterized and purified abscisin II, inhibitor β, and dormin, revealing their chemical identity. Disagreements over names ensued, leading to the adoption of "abscisic acid (ABA)" at the 1967 International Conference on Plant Growth Substances, becoming the universally accepted term. Later all the three were proved to be chemically identical and named abscisic acid (ABA).Carotenoid (violaxanthin) is the precursor of ABA.

Function/ Role of Abscisic Acid (ABA) 

Abscisic acid functions are as follows 

1. ABA acts as a general plant growth inhibitor. (Inhibitor of plant metabolism).

2. ABA inhibits seed germination.

3. ABA stimulates the closing of stomata in the epidermis and increases tolerance of plants to various kinds of stresses. Therefore, ABA is also called a stress hormone.

4. ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavorable for growth. 5. In most situations, ABA acts as an antagonist to GA.