4.2 Plant Hormones - The Science of Plants (2023)

learning goals

learning goals

At the end of this lesson you will be able to:

  • Understand the role of the five main groups of hormones in plant growth and development.
  • Realize that cells, tissues and organs are uniquely competent to respond to specific hormones.
  • Link specific hormones to plant responses and how they are used in plant reproduction.

The five major groups of plant hormones—auxins, cytokinins, gibberellins, ethylene, and abscisic acid—are distinguished by their chemical structures and the response they elicit in the plant (see Table 4.1). For a cell to respond to a hormone, it must be able to sense the chemical. Some cells just don't have the ability to "see" the hormone and don't respond to its presence. The ability to sense a hormone depends on the cell's physiology when the hormone is present. When the hormone is sensed, its unique chemical structure causes a chain reaction, or signal transduction, that involves changes in gene expression and cell morphology. These cellular responses to hormones can result in the changes we see in the plant, such as: B. a movement towards light, a transition from vegetative growth to flowering or the closing of leaf crevices due to water stress.

Hormone perception takes place both in cells and in a tissue or organ, depending on the location of the hormone, the concentration of the hormone, the stage of development and the physiological state of the cell. Many cells within a tissue can respond in a coordinated manner, leading to plant-wide changes. Hormones are often produced in one cell and transported to other cells where they are sensed, and the response can occur far from the site of hormone synthesis. Responses to hormones are studied by applying the chemical exogenously to plant tissue - the hormone is applied to the outer (exo) part of the plant and observations are made of how the plant responds. For gaseous hormones like ethylene, this means that the hormone can be transferred from one plant to another. Plants essentially talk to each other using a variety of molecules.

(Video) Plant Hormones | Biology | Chapter 5 | CBSE | NCERT | NEET | Class 10

Experiments in which hormones are applied exogenously to a plant show how plants respond to hormones; Much of our knowledge of the role hormones play in plant growth comes from this type of experimentation. However, when a hormone is administered exogenously, it also interacts with any hormones present in the plant. These areendogenousHormone (Endomeans internal), and the cell reacts according to the sum of all hormones in its presence. This can complicate the interpretation of responses to exogenous hormone use.

Hormones used in plant reproduction can be natural and found in many plants, or they can be synthetic or synthesized to mimic the structure and response of a natural hormone. Synthetic hormones are often used over natural versions because they are cheaper to source, can produce stronger or longer lasting reactions, and may be less prone to breakdown in the plant and during storage. Because the exogenous application of hormones plays a role in manipulating or arresting plant growth, they are used extensively as herbicides (herbicides) and can be targeted to specific plant species based on how specific species respond to different structures.


Auxins are a group of related molecules involved in almost every aspect of the plant life cycle. Auxins stimulate growth through cell elongation, which is an integral part of plant responses to environmental change. Auxins are responsible for two types of growth responses:Phototropismus, the bending or growing of a bud towards the light, andGravitropismus, a growth change that occurs after a change in gravitational force. The chart below shows indole acetic acid (IAA, shown with pink dots), a naturally occurring auxin, moving from the sunny side to the shaded side of a bud tip. The differential accumulation of auxin on the shaded side of the bud causes these cells to increase growth and bend the bud tip towards the light. The stimulation of cell growth by auxin is also important for wound healing and callus formation after circumcision.

Auxin's ability to regulate growth can be used against weeds (plants out of place). The synthetic auxin 2,4-dichlorophenoxyacetic acid or 2,4-D is a widely used herbicide that disrupts normal growth regulation when applied to the plant, resulting in leaf fall and death. Because dicotyledonous (two-cotton) plants are more competent to respond to 2,4-D, 2,4-D can be used as a selective herbicide to kill dicot weeds in lawns and corn fields that are resistant to monocotyledonous grasses (single-cotyledonous plants). Grass is left unharmed due to its lower responsiveness, while dicots are killed.

In general, auxins are produced in the young leaves of a plant and translocated down to older tissues. This downward translocation controls apical dominance where axillary bud growth is suppressed. Removing (pinching) the tip of the stem where auxin is produced, as shown in the three mint photos below, frees the axillary buds from apical dominance and they begin to grow. This is a common horticultural practice that increases branching and flower production. Pinching is often used on plant seedlings such as basil or zinnias to achieve spherical shapes in a pot rather than large single stem plants.

One of the most important uses of auxin in plant propagation is to stimulate the growth of adventitious roots—roots that arise anywhere on the plant except the roots—in scion cuttings. The photo below shows sections of two different onesAcer ginnala(Amur Maple) Plants that have different abilities to form adventitious roots. Both cuttings were treated with auxin, but only the competent plant forms adventitious roots (left). A cutting from a plant that is unable to respond to auxin will not root (right) and will eventually die.

(Video) Which plant hormone promotes dormancy in seeds and buds? MCQ Science- Biology Plant Hormone

This form of asexual (clonal) reproduction is used by both horticultural professionals and hobbyists. Rooting competency for cuttings can be species specific or seasonal. Collecting stalks of a plant to use as seedlings can be most successful in the growing season, as with the Amur maples shown above. However, for plants like grapes, the cuttings are taken and rooted during the winter when the vines are not actively growing. The following video shows how shoots are taken from Amur maples, treated with auxin and incubated in a high humidity environment for several weeks to form adventitious roots. Exogenous auxin application is not required for adventitious rooting of all plants. Some plants can form many adventitious roots without exogenous applications because the naturally occurring endogenous auxin in the shoot is sufficient for root formation.


Like auxins, cytokinins are a group of related molecules that regulate growth and development. However, the plant response to cytokinin is very different from the response to auxin. Cytokinin comes from the word cytokinesis, which means cell division. You will learn about cytokinesis, especially mitosisChapter 13.

Cytokinins promote cell division, with one cell dividing and two new daughter cells being formed. Cytokinins are important regulators of plant growth and development.

Cytokinins have an interesting interaction with auxins in plants. In the 1950s, Skoog and Miller studied the growth ofNO. tobaccoStrains in Tissue Culture. They found that they could use specific ratios of an auxin (IAA) and a cytokinin (kinetin) to control the growth of stem tissue in culture. A high cytokinin to auxin ratio led to the formation of sprouts, a higher auxin level led to the formation of roots and equal amounts of each produced callus growth, which is the undifferentiated growth of plant cells. The transformative discovery of Skoog and Miller formed the basis of "MS" plant medium, which remains popular for tissue culture plant propagation.

Watch this video to learn more about plant propagation on synthetic media with exogenous hormones in tissue culture.

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Gibberellins or gibberellic acid (GA) are a group of more than 100 molecules that are primary regulators of stem elongation and seed germination. They were discovered while researching the cause of "silly seedling disease" in rice. The disease, characterized by tall plants with few grains, is caused by infection withGibberella fujikora, a parasitic fungus that produces GA in the rice shoots, leading to increased stem elongation.

EmChapter 9.2, on seed physiology, you will learn that some seeds are dormant and will not germinate even if the right environment is in place. Seed dormancy, which has many causes and evolutionary advantages, always has the common trait of preventing seed germination until the time, season, or seed physiology is right. Planting a dormant seed or a dead seed produces the same result: no germination. For plant growers, the dormant period can be confusing, leading to the question, "Are my seeds dead or dormant?" Any condition prevents the germination and reproduction of the plant. Seed treatment with GA is a common method to break dormancy and facilitate germination. GA treatment ofYellow gentian(Bitterroot), for example, increases germination from 0% (no germination) to over 80% when treated with 100 parts per million (ppm) GA (see germination chart at left). If a multiplierG. luteaknew nothing about seed dormancy, they may have assumed their bitter roots were dead. For most plants, GA is the endogenous hormone that triggers seed germination.

abscisic acid

While GA facilitates seed germination, abscisic acid (ABA) inhibits it. Abscisic acid is a unique molecule that regulates germination and a plant's response to reduced water availability during water stress. ABA levels increase when there is less water available to the plant, causing several responses including stomata closure. The closing of the stomata slows transpiration (also called evapotranspiration), the movement of water in the plant from the root, through the stem, to the leaf, and out through the stomata to the atmosphere. Read more about stomata and water movement inChapter 11, Plants and Water.

Reducing water content is one of the final steps in seed maturation and is important to seed longevity by keeping metabolism to a minimum, which is the dormant nature of mature seeds. Increasing endogenous ABA levels in seeds prepares them to survive lower water levels, is important for seed maturation, and prevents early germination (voviparity). Seeds with low ABA levels during seed development may germinate prematurely. Low ABA levels can result from a genetic mutation or environmental causes. Viviparity is not uncommon in some fruits and can occur during fruit storage in supermarkets.


Ethylene is known as the gaseous hormone of maturation. It also regulates seedling growth and root hair formation, and can lead to epinasty — the bending of branches downwards.

Many plants are sensitive to the effects of ethylene on fruit ripening. Iconic examples are the tomato and the banana. These fruits are climacteric - they continue to ripen after harvest. The perception of ethylene by the cells that make up the fruit triggers the ripening process and the production of more ethylene. As the ethylene concentration increases, the rate of ripening also increases. Harvesting unripe or immature fruit allows for long-distance transportation because the fruit is firmer and less prone to damage in transit. The unripe fruit can then be treated with ethylene from an ethylene generator (right) to accelerate ripening.

(Video) Auxin and effect on cell elongation

In other fruit crops, the introduction or production of ethylene should be avoided to prevent overripening and spoilage. To prevent the formation of ethylene during fruit storage, ethylene is removed from the air by an air filtration system. A reduction in ethylene concentration means slower ripening and less spoilage.

The senescence process is also triggered by the production of ethylene and is important in the cut flower industry. Keeping cut flowers away from gases with ethylene-like activity will help keep floral arrangements looking fresh. Reducing the effect of ethylene extends the shelf life of many cut flowers as well as fruit storage.

The five major groups of plant hormones control many aspects of plant growth and development and have important applications in plant reproduction. But many other molecules are also essential for the plant's response to its environment. These very different signaling molecules modulate plant physiology through complex interactions. A cell's response to many different hormones is a sum of its genetic makeup, physiology, and environment.

Table 4.1
HormonesStructuresynthesized versionsgeneral answersApplication
Auxin (Indolessigsäure; IAA)indole butyric acid (IBA); Naphthaleneacetic Acid (NAA), 2,4-Dichlorophenoxyacetic Acid (2,4-D)Accidental rooting, tropisms, apical dominanceAccidental rooting of cuttings
Citocinina (Zeatine)Benzyladenin (BA, BAP oder Benzylaminopurin), Thidiazuron (TDZ), Kinetincell divisionShoot formation in plant cultures
Giberelina (GA3 Mostrado)Over 100 types named by GAnumber (e.g. GA3)Promotes seed germination and stem elongationdormant seed germination
abscisic acidNone, although it can be synthesizedSeed dormancy, reaction to water stress, leaf fallGenetic engineering for drought resistance
ethyleneNatural gas, propane and their by-products of combustionFruit ripening, epinasty, root hair formationPromote or prevent fruit ripening

review questions

  • Describe the plant's general response to each of the five major plant hormones and the factors that affect a plant's response.
  • Explain the difference between endogenous and exogenous plant hormones.
  • Describe an application for each of the plant hormones in plant reproduction specifically or in horticulture in general.
(Video) F.W. Went experiment isolating Auxins from coleoptile tips


What are plant hormones question answer? ›

Plant hormones are chemical compounds present in very low concentration in plants. They are derivatives of indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid) and gases (Ethylene).

What are the 5 types of plant hormones? ›

There are five groups of plant-growth-regulating compounds: auxin, gibberellin (GA), cytokinin, ethylene, and abscisic acid (ABA).

What is plant hormones PDF? ›

Plant hormones are a group of naturally occurring, organic substances which influence physiological processes at low concentrations. The processes influenced consist mainly of growth, differentiation and development, though other processes, such as stomatal movement, may also be affected.

Which hormone is responsible for growth in plants? ›

Auxin and cytokinin are critical growth hormones in plant development and are naturally present within the plant at variable concentrations throughout the season.

How many hormones do plants produce? ›

There are five major types of plant hormones: auxins, cytokinins, gibberellins, ethylene and abscisic acid.

What hormones affect plants? ›

The classical plant hormones, such as auxin, abscisic acid (ABA), brassinosteroids (BRs), cytokinin (CK), salicylic acid (SA), jasmonate (JA), and ethylene (ET), integrate environmental stimuli and endogenous signals to regulate plant defensive response to various abiotic stresses, including heat.

What are plant hormones and their function? ›

These hormones help in regulation of the plant body by responding to the various signals from the plant and environment. The hormones are regulated in different tissues during the different development stages. There are five major hormones which are auxin, cytokinin, gibberellin, abscisic acid, and ethylene.

What are the plant hormones names? ›

The plant hormones that have been discovered so far are the auxins, the gibberellins (GAs), the cytokinins, ethylene, abscisic acid (ABA), jasmonic acid, the brassinosteroids, salicylic acid (SA), and the bioactive oligopeptides (such as CLE peptides).

What are the 2 main plant hormones? ›

Cytokinins and auxins often work together, and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime. Cytokinins counter the apical dominance induced by auxins; in conjunction with ethylene, they promote abscission of leaves, flower parts, and fruits.

What are plant 11 hormones? ›

Plant hormones are chemical compounds found in plants at very low concentrations. They are derivatives of indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid), and ethylene.

What are the 9 plant hormones? ›

According to structural and chemical diversity, plant hormones are grouped into several classes, including auxins, cytokinins (CKs), abscisic acid (ABA), gibberellins (GAs), ethylene, jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BRs), and strigolactones (SLs).

What hormone causes flowering? ›

Florigen, a proteinaceous hormone, functions as a universal long-range promoter of flowering and concurrently as a generic growth-attenuating hormone across leaf and stem meristems.

Which hormone is responsible for flowering? ›

Florigen, or flowering hormone, is thought to be a protein insulin particle that controls or enhances the flower initiation process in plants. These florigens are made in the leaf and are found in the bud's stem and root apical meristems and growing tips.

Which hormone promotes flowering? ›

Gibberellin is the hormone that is majorly found in plants and they help in the blooming of long-day plants.

How do hormones affect plants? ›

Plant hormones are chemicals in plants that regulate almost all aspects of plant growth and development. Hormones play a critical role in how plants respond to biotic and abiotic factors, including sunlight, soil conditions, soil water and nutrients.

Do all plants have the same hormones? ›

Plant growth is directed by molecules called auxins, which actively change their distribution within a plant in response to environmental cues such as light and gravity.

Which hormone inhibits growth in plants? ›

Abscisic acid is a plant growth hormone involved in seed dormancy by suppressing seed germination and growth as well as in responses of plants to stress.

How do plants use hormones to control growth? ›

Chemical Structures of the Plant Hormones

Ethylene gas promotes fruit ripening, senescence, and responses to pathogens and abiotic stresses. IAA (an auxin) regulates cell division and expansion, vascular differentiation, lateral root development, and apical dominance.

What are the effects of plant hormones on human health? ›

Plant hormones such as indole-3-acetic acid (IAA) works as an antitumor, anti-cancer agent, gibberellins help in apoptosis, abscisic acid (ABA) as antidepressant compounds and regulation of glucose homeostasis whereas cytokinin works as an anti-ageing compound.

How are plant hormones produced? ›

With the deepening of research, some novel plant hormones such as brassinosteroid and salicylates have been found and identified. The plant hormone products are mainly obtained through plant extraction, chemical synthesis as well as microbial fermentation.

Which is the first hormone to be discovered in plants? ›

Auxin is the first growth hormone to be discovered in plants.

What is a Type 4 hormone? ›

4. Insulin. Insulin is a hormone produced by the pancreas. It has many functions, but its main responsibility is converting glucose (sugar) in the things we eat into a form the body can use for energy.

What hormone causes horniness? ›

Results: Testosterone has a primary role in controlling and synchronizing male sexual desire and arousal, acting at multiple levels.

What is the most important type of hormone? ›

1. Cortisol: The stress hormone. "One of the most, or the most important one—especially for anyone who has a uterus—is cortisol," said Jardim.

What are hormones made of? ›

Chemical Nature of Hormones

Chemically, hormones may be classified as either proteins or steroids. All of the hormones in the human body, except the sex hormones and those from the adrenal cortex, are proteins or protein derivatives.

What are the plant hormones explain? ›

1 Definition. Plant hormones (phytohormones) are chemicals produced by plants that regulate their growth, development, reproductive processes, longevity, and even death. These small molecules are derived from secondary metabolism and are responsible for the adaptation of plants to environmental stimuli.

What are plant hormones for competitive exams? ›

Some of the important plant hormones are – Auxin, Gibberellin, Ethylene, Abscisic acid, and Cytokinin. These hormones are involved in growth and development responses such as cell division, seed formation, flowering, seed formation and abscission.

What is a plant hormone quizlet? ›

hormones. compounds produced in one part of an organism that are transported to other locations where they produce specific responses. auxins. stimulates stem elongation; root growth, differentiation and branching, development of fruit; apical dominance; phototropism and gravitropism.

Where are hormones in plants? ›

The production of hormones occurs very often at sites of active growth within the meristems, before cells have fully differentiated. After production, they are sometimes moved to other parts of the plant, where they cause an immediate effect; or they can be stored in cells to be released later.

How many types of plant hormones and their functions? ›

There are five major hormones which are auxin, cytokinin, gibberellin, abscisic acid, and ethylene. Each hormone differs in its effects. The auxins, gibberellins, and cytokinins act as growth stimulators, whereas, abscisic acid and ethylene act as growth inhibitors.

Why are plant hormones important? ›

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to a wide range of biotic and abiotic stresses.

How do hormones affect plant growth? ›

Hormones respond to increased sunlight competition by stimulating increased stem elongation. Increased competition can cause plants to put more energy into stem elongation growth versus expanded leaf area.

Which plant hormone responsible for saving the crop from falling? ›

Auxin hormone saves the crops from falling i.e regulates the fall of leaves and fruits. In the presence of auxins, cytokinin hormones help in cell division and development. The most common naturally occurring plant hormone of the auxin class is acetic acid. The transport of auxin in plants is polar.

Are plant hormones steroids? ›

Plants produce numerous steroids and sterols, some of which are recognized as hormones in animals (Geuns, 1978; Jones and Roddick, 1988). Brassinolide (BL) is the most bioactive form of the growth-promoting plant steroids termed brassinosteroids (BRs). Grove et al.

Which hormone helps in blooming of flowers? ›

Florigen is known as the flowering hormone which is responsible for controlling the flowering in plants. The production of florigen is done in the leaves of the plants. It acts in the shoot of the plant and the growing tips of the apical meristem. It is considered graft transmissible and works between the species.

What hormone is stress? ›

Cortisol, the primary stress hormone, increases sugars (glucose) in the bloodstream, enhances your brain's use of glucose and increases the availability of substances that repair tissues. Cortisol also curbs functions that would be nonessential or harmful in a fight-or-flight situation.


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