Plant Hormones

Plant hormones (also known as phytohormones) are signal molecules that exist at extremely low concentrations and are formed within plants. From embryogenesis, organ size regulation, pathogen protection, stress tolerance, and reproductive development, plant hormones regulate all aspects of plant growth and development. There are some intrinsic variables that control plant growth and development.

• Some examples of plant hormones are Indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid), and gases (Ethylene).

• In nearly all parts of the plant, these hormones are produced and are transferred to different parts of the plant.

• Plant hormones may act individually or synergistically. It may be complementary or antagonistic to the functions of various hormones.

• Hormones, along with extrinsic influences, play an important role in processes such as vernalization, seed germination, phototropism, dormancy, etc.

• In regulated crop production, synthetic plant hormones are exogenously added.

Phototropism in the canary grass coleoptiles was first observed by Charles Darwin and auxin was first isolated from the coleoptiles of oat seedlings by F.W Went.

What are Plant Hormones?

For their survival and growth plants require a lot of natural aid like water, oxygen, sunlight and various minerals and nutrients. All these are external factors that help in the development of plants . In addition to these, there are some intrinsic factors that help in their growth. These intrinsic factors that we just talked about are called plant hormones or Phytohormones. Plant Hormones help in regulating the growth of plants. 

Let us now understand what plant hormones are according to the NEET syllabus:

• Plant hormones are chemical compounds found in a plant's body in a very low concentration. They mainly derive from indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid) and gases (Ethylene).

• These are produced in every part of the plant and circulate throughout a plant’s body. 

• Each hormone has its own role in a plant’s body. Different hormones carry out different functions individually or in combination with other hormones.

• Hormones play a very important role in a plant’s body and facilitate processes like vernalisation, phototropism, seed germination, dormancy etc. with the help of  external factors like sunlight, water and oxygen.

• Crop production is controlled by the application of synthetic hormones.

Main Functions of Plant Hormones

All the growth and development activities such as cell division, enlargement, flowering, seed formation, dormancy, and abscission are regulated by plant hormones and are the main functions of plant hormones.

Plant hormones are categorised into two groups based on their action:

• Plant Growth Promoters.

• Plant Growth Inhibitors.

Types of Plant Hormones and their Functions

1. Auxin

Auxin means "to be able to grow." In agricultural and horticultural practises, they are widely used. They are found in roots and stems in rising apices and then move to other parts of the plant.

Example of plant hormone Auxin:

• Natural: Indole butyric acid (IBA), and Indole-3-acetic acid (IAA)

• Synthetic: NAA (Naphthalene acetic acid) and 2,4-D (2,4-Dichlorophenoxyacetic acid)

Functions of Plant Hormone Auxin:

• Elongation of cells of stems and roots.

• IAA in apical buds suppresses the development of lateral buds with apical dominance.

• Induces parthenocarpy, i.e. fruit growth without fertilisation, for example, in tomatoes.

• Prevent leaves, flowers, and fruits from dropping prematurely.

• Useful in stem cuts and grafting where rooting is initiated.

• Promotes flowering in, for example, pineapple.

• 2,4-D is commonly used to destroy unwanted dicot weeds without harming monocot plants.

• Contributes to cell division and xylem differentiation.

2. Gibberellin:

There exist more than a hundred (100) known Gibberellins (GA1, GA2, GA3..). In nature, they are found to be acidic. These kinds of hormones are majorly observed in higher plants and fungi.

Functions of Plant Hormone Gibberellin:

• Promotes bolting, i.e. sudden internode elongation just before flowering, as seen in cabbage, beet, and others.

• Brings delays in senescence.

• Stimulates parthenocarpy.

• Stem elongation and reversing dwarfism.

• In plants such as cannabis, it induces maleness.

• Promotes the synthesis of hydrolytic enzymes such as amylase, lipase, in the endosperm of germinating barley seeds and cereal grains.

3. Cytokinins:

Cytokinins play a major role in the mechanism of cytokinesis. rIn plants where rapid cell division occurs, cytokinins are naturally synthesised, such as root apices, shoot buds, young fruits, etc. Cytokinins show basipetal and polar movement.

Example of plant hormone Cytokinins:

• Natural: isopentenyl adenine and Zeatin (corn kernels, coconut milk).

• Synthetic: Benzyladenine, Kinetin, thidiazuron, and diphenylurea.

Functions of Plant Hormone Cytokinins:

• This promotes lateral and adventitious shoot growth and is used in culture to initiate shoot production.

• Assists in resolving auxin-induced apical dominance.

• Stimulate the production of chloroplast in the leaves.

• Promoting the mobilisation of nutrients and slowing leaf senescence.

4. Abscisic Acid:

It is a hormone that has a role in inhibiting the growth of plants. ABAs serve as an opponent to GAs. It prevents the metabolism of plants and controls abscission and dormancy. It is often referred to as a 'stress hormone' as it improves plant tolerance.

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Functions of Plant Hormone Abscisic Acid:

• Induces abscission of fruits and leaves.

• Inhibits germination of seeds.

• Induces leaf senescence.

• Accelerates seed dormancy, which is useful for storage purposes.

• Stimulates stomatal closure to stop transpiration under water stress.

5. Ethylene:

Ethylene acts as both inhibitor as well as a growth promoter. It Occurs in gaseous form. The synthesis of ethylene occurs in the maturing fruit and tissues which are undergoing senescence. Many physiological procedures are regulated by this hormone and are one of the most commonly used hormones in agriculture.

Functions of Plant Hormone Ethylene:

• It speeds up the fruit ripening.

• Controls leaf epinastine.

• Breaks the bud and seed dormancy.

• Facilitates rapid elongation of internodes and petioles.

• Senescence and abscission of leaves and flowers are encouraged.

• This raises the absorption surface by inducing root growth and root hair formation.

• Stimulates femininity in plants that are single.

• Formation of the apical hook in dicot seedlings.

Application of Plant Hormones:

• Tissue Culture.

• Somatic Cell Hybridization.

• Introduction of recombinant DNA into protoplasts.

Conclusion

The answer to the question ‘what are plant hormones’ can be stated by the Plant hormone definition. Plant hormones are organic substances that control plant growth and development (also known as phytohormones). Different plant hormones may include Auxins, abscisic acid (ABA), gibberellins (GA), cytokinins (CK), ethylene (ET), salicylic acid (SA), jasmonates (JA), peptides, and brassinosteroids (BR). All these examples of plant hormones carry certain applications. The application of plant hormones may include Tissue Culture, the Introduction of recombinant DNA into protoplasts and Somatic Cell Hybridization, and many others. Some of the functions of plant hormones are to stimulate the production of chloroplast in the leaves, to stimulate parthenocarpy, to promote elongation of cells of stems and roots, to induce leaf senescence, etc.