Plant Hormones
Phytohormones are chemical compounds present in very low concentrations in plants. They regulate plant development, growth, longevity and reproductive processes. In this article, we will explore the structure and functions of different phytohormones such as auxins, gibberellins, cytokinins, ethylene and abscisic acid.
Table of Contents
Plant hormones are chemical substances produced naturally by plants, which act as signals in the regulation of growth and development. They are similar to hormones produced by animals, but they are produced in much smaller amounts and act on different processes.
Plants need sunlight, water, oxygen, and minerals for their growth and development, which are external factors. In addition to these, there are also “Phytohormones” - intrinsic factors that regulate the growth and development of plants.
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Plant hormones are chemical compounds present in very low concentrations in plants. These hormones include derivatives of indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid) and gases (Ethylene).
These hormones are produced in nearly every part of the plant and are distributed to different parts of the plant.
The roles of different hormones can either act synergistically or individually, and can be either complementary or antagonistic.
Hormones play a pivotal role in processes such as vernalisation, phototropism, seed germination, and dormancy, in addition to extrinsic factors.
Synthetic plant hormones are applied externally to regulate crop production.
Charles Darwin first observed phototropism in the coleoptiles of canary grass, while F.W. Went was the first to isolate auxin from the coleoptiles of oat seedlings.
The main functions of plant hormones are:
- Regulation of growth and development
- Stimulation of flowering
- Control of leaf and fruit senescence
- Regulation of root growth
- Stimulation of fruit and seed development
- Control of cell division and cell enlargement
Plant hormones regulate all growth and development processes such as cell division, enlargement, flowering, seed formation, dormancy, and abscission.
Based on their action, plant hormones are categorized into two categories:
Plant Growth Promoters
Plant Growth Inhibitors
Types of Plant Hormones
Check Out All Hormones and Their Functions Here
Auxin Hormone
Auxin is a term derived from the Latin word meaning “to grow”. It is widely used in agricultural and horticultural practices, and is found in the growing apices of roots and stems, where it then migrates to other parts to act.
Natural: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
Synthetic: 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid)
Functions:
Cell Elongation of Stems and Roots
Apical Dominance: The presence of IAA (auxin) in the apical bud suppresses the growth of lateral buds.
Induces parthenocarpy, which is the development of fruit without fertilization, such as in tomatoes.
Prevents premature fall of leaves, flowers, and fruits
Useful for initiating rooting in stem cuttings and grafting.
Encourages flowering, such as in pineapple
2,4-D is widely used as a herbicide to eliminate undesirable weeds of dicot plants without affecting monocot plants.
It assists in cell division and xylem differentiation
MCQs on Plant Hormones: A Must Read
Gibberellins Hormone
There are more than 100 gibberellins (GA1, GA2, GA3, etc.) known to exist. These gibberellins are acidic in nature and are found in higher plants and fungi.
Functions:
Promotes bolting, which is the sudden elongation of internodes just before flowering in rosette plants such as cabbage and beet.
Delaying Senescence
Induces Parthenocarpy
Elongation of the stem and reversal of dwarfism
Induces male characteristics in certain plants such as cannabis
Induces the formation of hydrolytic enzymes, such as lipase and amylase, in the endosperm of germinating cereal grains and barley seeds
Breaks Seed Dormancy
Cytokinins Hormone
Cytokinins play a pivotal role in the process of cytokinesis. These compounds are synthesized naturally in plants where rapid cell division takes place, such as root apices, shoot buds, and young fruits. Moreover, the movement of cytokinins is basipetal and polar.
+ Natural: Zeatin (corn kernels, coconut milk), Isopentenyladenine
Synthetic: Kinetin, Benzyladenine, Diphenylurea, Thidiazuron
Functions:
It encourages the growth of lateral and adventitious shoots and is used to initiate shoot growth in culture.
Provides a way to reduce the effects of auxins on apical dominance.
Encourage the development of chloroplasts in leaves
Encourages nutrient circulation and postpones leaf aging
Abscisic Acid Function
ABAs act as an antagonist to GAs, inhibiting plant metabolism and regulating abscission and dormancy. It is also known as a “growth-inhibiting hormone” and a “stress hormone” as it increases the tolerance of plants.
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Functions:
Promotes the shedding of leaves and fruits
Inhibits Seed Germination
Induces Senescence in Leaves
Accelerates dormancy in seeds which is beneficial for storage purposes
Stimulates closure of stomata to reduce transpiration under water stress
Ethylene Plant Hormone
It acts both as a growth promoter and an inhibitor, occurring in gaseous form. Synthesized in ripening fruits and senescing tissues, it regulates many physiological processes and is one of the most widely used hormones in agriculture.
![Ethylene]()
Functions:
It speeds up the ripening of fruits
Controls Epinasty of Leaves
Breaks Seed and Bud Dormancy
Stimulates rapid elongation of petioles and internodes
Encourages aging and shedding of leaves and flowers
Promotes root growth and root hair formation, thereby increasing the absorption surface
Stimulates Femininity in Monoecious Plants
Apical Hook Formation in Dicot Seedlings
Other than the main 5 hormones, there are other hormones such as brassinosteroids, salicylates, jasmonates, and strigolactones that affect the plant’s physiological processes.
