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

  1. Functions of Plant Hormones

  2. Auxin Hormone

  3. Gibberellins Hormone

  4. Cytokinins Hormone

  5. Abscisic Acid Function

  6. Ethylene Plant Hormone

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)

Auxin Indole 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.

Gibberellins

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

Cytokinins

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.

Abscisic Acid (ABA)")

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.

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