Plant Hormones

What are Plant Hormones?

For their growth and development, plants require sunlight, moisture, oxygen, and minerals. External factors are required. The growth and development of plants are regulated by a variety of intrinsic factors as well. We call these phytohormones. Plants produce and transmit these hormones in almost every part of the plant. Plant growth hormones may act synergistically or individually. Each hormone may play a complementary or antagonistic role. As well as extrinsic factors, hormones are involved in processes such as vernalization, phototropism, seeds germination, dormancy, and so forth. Controlled crop production is achieved through the exogenous application of plant hormones. Charles Darwin first observed phototropism within the coleoptiles of canary grass, and Frederick Going isolated auxin for the first time from the coleoptiles of oat seedlings.

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Which of the Following is a Plant Hormone?

A phytohormone is a chemical compound found in very low concentrations in plants. There are derivatives of indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid), and gases (Ethylene) and also examples of plant hormones.

What are the Two Types of Plant Hormones?

The growth and development activities of plants are controlled by hormones such as cell division, enlargement, flowering, seed formation, dormancy, and abscission.

There are two main types of plant hormones based on how they act:

  • Growth Promoters for Plants

  • Growth inhibitors for plants

Plant Hormones and Their Functions

Auxin Hormone 

To grow is what auxin means. Agricultural practices and horticultural practices use auxins extensively. The majority of them reside at the growing apices of roots and stems before migrating to other parts of the plant. 

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

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


  • Roots and stems lengthen as a result of cell division.

  • The growth of lateral buds is inhibited by apical dominance and IAA in apical buds.

  • It is inhibited by apical dominance and IAA in the apical buds that lateral buds grow.

  • Ensures that leaves, flowers, and fruits do not fall prematurely.

Gibberellins Hormone

Gibberellins are acidic compounds found in higher plants and fungi (GA1, GA2, GA3,....). There are more than 100 kinds of gibberellins (GA1, GA2, GA3,...) known.


  • Bolting in rosette plants like cabbage, beet induces sudden elongation of the internodes just before flowering.

  • Delays senescence.

  • Induces parthenocarpy. 

  • Elongates the stem and reverses dwarfism.

Cytokinins Hormone

Plants produce cytokinins when rapid cell division occurs, for example, at roots apices, shoot buds, young fruits, etc. The movement of cytokinins is basipetal and polar.

  • Naturally: Zeatin (corn kernels, coconut milk), isopentenyl adenine

  • Synthetically: Kinetin, benzyladenine, diphenylurea, thidiazuron


  • In cultures, it promotes the growth of lateral and adventitious shoots and is used to initiate the growth of shoots

  • Assists in overcoming apical dominance caused by auxin

  • Leaf chloroplasts should be stimulated

  • Activates nutrients and delays leaf senescence

Abscisic Acid Function 

Inhibiting plant growth regulates abscission and dormancy, affects plant metabolism, and increases plant tolerance to stress. It's also called the "stress hormone" because it increases plant tolerance to stress.


  • Leaf abscission and fruit abscission caused by this plant

  • Inhibition of seed germination

  • Senescent leaves are induced

  • Do not affect seeds' dormancy, so it is useful for storing seeds

Ethylene Plant Hormone

It regulates many physiological processes and is one of the most widely used agricultural hormones, acting as both a growth promoter and an inhibitor. It is produced in gaseous form by ripening fruits and tissues during senescence.


  • Fruits ripen more quickly when it is present

  • Leaves become less epinasty

  • Breaks dormancy of seeds and buds

  • Enhances the rapid extension of petioles and internodes

Besides the main 5 hormones, other hormones also affect the physiology of plants, such as brassinosteroids, salicylates, jasmonates, strigolactones, etc., are the other examples of plant hormones in the study of the application of plant hormones. In this way, plant hormones and their functions are described.

Did You Know

  • The oilseeds, pulses, and cereal crops each requires specific plant growth hormones.

  • By producing auxins and cytokinins as examples of plant hormones, plants can grow stronger, root and shoot growth can be promoted, and stress can be reduced.

FAQs (Frequently Asked Questions)

1. What are Chemical regulators of plant hormones and the application of plant hormones in basic research and agriculture?

Phytohormones are small molecules that help plants regulate their growth, development, and responses to the environment. In addition to classical methodologies, such as genetics, analytic chemistry, biochemistry, and molecular biology, chemical regulators of plant growth hormones functions have also been studied in such studies. The study and manipulation of biological systems using synthetic chemicals, including plant growth regulators, is collectively referred to as chemical biology. Here we summarize the chemical regulators available and their contribution to plant hormone studies. The study of plant hormones poses some questions that are still unanswered and could be resolved using chemical regulators.

2. What is the Role of plant hormones and their interplay in the development and ripening of fleshy fruits?

Various aspects of plant development are controlled by plant hormones, which have been extensively studied. In recent years, however, considerable progress has been made in the study of their action during development and ripening in dry fruits as well as fleshy fruits. A complex network of more than one plant growth hormone is involved in controlling various aspects of fruit development, and there is growing evidence that the relative role of plant hormones is not limited to a particular stage. Despite the extensive coverage in some areas, gaps remain in our understanding of hormonal networks and the interplay between different hormones during fruit growth, maturation, and other aspects of ripening. To understand their mechanism of action in fleshy fruits, we examine the relative role of plant hormones in tomato fruit development. To gain a deeper understanding of hormonal crosstalk during fruit development in other species, pertinent pieces of evidence are also discussed.