Phytohormones- Introduction, Auxins, Gibberellins, Cytokinins, Functions


Phytohormones, often known as plant hormones, are naturally occurring small chemical molecules or substances that play key functions in plant function. At very low concentrations, it effects physiological processes in plants such as growth, development, reproductive functions, longevity, responses to environmental stimuli, and even death. In other words, phytohormones are chemical messengers that play a role in the coordination of cellular processes in plants.

Plant hormone biosynthesis, catabolism, and translocation are influenced by environmental signals such as light, temperature, and moisture. Environmental circumstances are also likely to influence plant hormone sensitivity.


Plant hormones were discovered in the late 1800s, when Charles Darwin and his son Francis suggested the concept of plant sensitivity to external stimuli. Frits Went discovered auxin, the first known plant hormone, in the early 1900s, sparking further research that led to the discovery of additional hormones like as cytokinins, gibberellins, abscisic acid, and ethylene. Plant hormone research is a flourishing subject that is constantly discovering novel hormones and their complicated interactions with the environment. Plant hormone discovery has significantly enhanced our understanding of plant biology and has practical uses in agriculture and horticulture.


Plant hormones are classified into various classes based on their structural and chemical diversity, including auxins, cytokinins (CKs), abscisic acid (ABA), gibberellins (GAs), ethylene, jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BRs), and strigolactones (SLs).

The two major categories of phytohormones are:

Growth-promoting hormones

Auxins, gibberellins, and cytokinins are plant growth and development hormones. They participate in activities such as cell division, cell elongation, and differentiation, as well as the development of organs such as roots, stems, leaves, and flowers.

Stress-related hormones

This category includes abscisic acid (ABA) and ethylene, which are involved in plant responses to environmental stresses. ABA regulates the plant’s response to water stress, while ethylene is involved in responses to biotic and abiotic stresses such as mechanical damage, pathogen attack, and flooding. These hormones help plants to survive and adapt to adverse environmental conditions.

Structure of different types of plant hormones

Fig: Structure of different types of plant hormones


Auxin is a type of plant hormone that plays a critical role in plant growth and development. It was the first plant hormone to be discovered and is named after its ability to promote the elongation of plant cells. Auxin is produced in the tips of plant shoots and roots and is transported throughout the plant in a process known as polar auxin transport. The most abundant and well-known form of auxin is indole-3-acetic acid (IAA). The chemical structure of IAA consists of a central indole ring connected to a side chain containing a carboxyl group (-COOH) and a terminal methyl group (-CH3). Two indolic auxins other than IAA have been isolated from plants, indole-3-butyric acid (IBA) and 4-chloro-indole-3-acetic acid (4-C1-IAA).

Mechanism of actions

Auxin is a plant hormone that is essential for plant growth and development. The mechanism of action of auxin comprises an intricate relationship of multiple processes at the molecular, cellular, and organismal levels.

  • At the molecular level, auxin works by binding to specific receptors located on the surface of plant cells. The TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) auxin co-receptors, the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors, and the AUXIN RESPONSE FACTOR (ARF) transcription factors are the core components of the auxin signaling machinery. Auxin enhances the interaction of TIR1/AFB with Aux/IAA proteins, resulting in Aux/IAA degradation and the release of ARF repression.
  • Auxin affects a variety of cellular activities, including cell elongation, division, and differentiation. Auxin, for example, stimulates cell elongation by relaxing the cell wall and promoting water intake into the cell. Auxin also regulates cell division orientation, which serves as to the overall structure of the plant.
  • At the organismal level, auxin is engaged in a variety of developmental activities, including the development of roots, shoots, and leaves. Auxin gradients are established in the plant, with larger quantities of auxin present at the plant’s tips. This uneven auxin distribution is critical for several aspects of plant development, including the beginning of lateral roots, the bending of shoots towards light, and the differentiation of leaf cells.


  • Promotes cell elongation by relaxing the cell wall and promoting water intake.
  • Supports the growth of new roots, especially in stem cuttings.
  • Regulates the growth of the main shoot by suppressing the growth of lateral shoots, a process known as apical dominance.
  • It is involved in the bending of plant stems towards a light source, known as phototropism, as well as the regulation of plant development in response to gravity, known as gravitropism.
  • It is important in the development of fruit, especially in the early phases of fruit growth.

Cytokinins (CKs):

Cytokinins (CKs) are a class of plant hormones that are involved in various physiological processes in plants, including cell division, shoot and root growth, and delay of senescence. They are synthesized in the root tips and transported to the shoots, where they promote cell division and differentiation.

Cytokinin is synthesized in roots and transferred via xylem to shoots and leaves, where it promotes organogenesis.

Cytokinins are structurally similar to adenine, a nitrogenous base found in DNA and RNA, and they can act in concert with other plant hormones such as auxins to regulate plant growth and development.


  • Cytokinins are involved in cell division and differentiation.
  • Cytokinins play a number of crucial roles in the growth and morphogenesis of plants. They interact with auxins to control apical dominance, lateral branching, and the root-shoot ratio in intact plants as well as in tissue culture. They are also involved in the regulation of cell division.
  • It controls not just how plants grow and develop but also how well they can withstand the stress of drought.
  • Cytokinins can regulate nutrient uptake and use by plants by influencing the expression of genes involved in nutrient transport and metabolism.

Abscisic acid (ABA):

Abscisic acid (ABA) is a naturally occurring plant hormone, probably present in all higher plants and is also produced by a variety of phytopathogenic fungi, bacteria, and metazoans. The Latin term “abscisio,” which means “cutting off,” is the source of the name ABA, which was first used to describe a chemical that suppresses plant growth. In reaction to stress, roots synthesize ABA, which is then transferred to the leaves, but leaves can also produce ABA.


  • Many aspects of plant growth and development have been found to be regulated by ABA, including embryo maturation, seed dormancy, germination, cell division and elongation, floral induction, and responses to environmental challenges like as drought, salinity, cold, pathogen attack, and UV radiation.
  • ABA plays a role in regulating root development and growth. High levels of ABA inhibit root growth, while low levels of ABA promote root growth.
  • ABA is involved in the regulation of leaf senescence, the process through which leaves age and eventually die. ABA causes chlorophyll and other photosynthetic pigments to degrade, resulting in leaf yellowing.

Gibberellins (GAs):

Gibberellic acids are diterpene plant hormones that are biosynthesized from geranylgeranyl diphosphate, a typical C20 precursor for diterpenoids. Gibberellic acids, class of the terpenoid family, regulate several aspects of plant growth and development, including seed germination, stem elongation, flowering, and fruit synthesis. It is generated by fungus and bacteria as well as higher plants. Gibberellic acids are hypothesized to be secondary metabolites in fungi and bacteria that act as signaling molecules to form interactions with host plants.

Early in the twentieth century, gibberellins were identified as the cause of the symptoms of overgrowth in rice plants. The phytopathogenic fungus Gibberella fujikuroi was shown to be the cause of the infection.


  • It lengthens stems and enhances the shape and quality of fruits and vegetables.
  • It aids in the transport of stored nutrients from the endosperm to the developing embryo.
  • By regulating gene expression and other physiological processes, it aids plants in adapting to adverse situations such as excessive salt, drought, and extreme temperatures.
  • It has the ability to influence floral formation and development, including flowering timing, flower number, and blossom size.


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