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Since 1937, gibberellin (GA), ethylene, cytokinin, and abscisic acid (ABA) have been identified alongside auxin as phytohormones, collectively known as the "classical five."
What are the main functions of plant hormones?
Plant hormones, or phytohormones, regulate all plant growth and development activities, including cell division, enlargement, flowering, seed formation, dormancy, and abscission.
What are plant hormones called?
Plant hormones, also known as phytohormones, are organic substances that control plant growth and development.
Which is the plant stress hormone?
The plant stress hormone is abscisic acid (ABA). ABA acts as a growth inhibitor in extreme environmental conditions, such as extreme heat. It induces seed dormancy and closes stomata in plant leaves to reduce transpiration.
Is insulin a plant hormone?
No, insulin is an animal hormone. Cytokinin is an example of a plant hormone.
What is IAA?
Indole-3-acetic acid (IAA) is a plant growth regulator or hormone. It is a type of auxin.
Plant Hormones, Types, Functions, Examples and Mechanisms
Plant Hormones are signal molecules produced in tiny amounts and regulate all aspects of plant growth and development. NEET Notes on Plant Hormones are provided below for detailed studies of various types.
Krati Saraswat30 May, 2025
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Plant Hormones: Plant hormones, also known as phytohormones, are molecules produced in very small amounts by plants. They regulate many aspects of plant growth and development, including embryogenesis, pathogen defense, organ size regulation, stress tolerance, and reproduction. tolerance, and reproduction. Unlike animals, which produce hormones in specialized glands, every plant cell can produce hormones.
Phytohormones, including algae, are found throughout the plant kingdom, where they perform similar functions as vascular plants. As secondary metabolites, some phytohormones are also present in unicellular fungi and bacteria, although they do not have the same hormonal properties. This article contains detailed information about plant hormones, including their types, functions, and more.
Plants rely on external factors such as sunlight, water, oxygen, and minerals for growth and development. Internal factors known as plant hormones, or "phytohormones," also play an important role in these processes. Plant hormones are chemical compounds found in minute amounts within plants. They come from many different sources, such as gases (ethylene), carotenoids (abscisic acid), terpenes (gibberellins), and indole (auxins). These hormones are produced in nearly every part of the plant and transported to various locations as needed. These hormones can function together or independently, and their roles can be complementary or antagonistic. They play an important role in many processes, including the initiation of spring growth (vernalisation), light-oriented growth (phototropism), seed germination, dormancy, and others. They operate in tandem with external factors. Plant hormones are used to control crop production through external applications. Charles Darwin discovered phototropism in canary grass coleoptiles, and F.W. Went isolated auxin from oat seedling coleoptiles.
Plant hormones play a essential role in regulating various growth and development processes in plants. These processes include cell division, enlargement, flowering, seed formation, dormancy, and abscission. Plant hormones are categorised into two groups based on their functions:
Plant Growth Promoters: This group includes hormones such as auxins, gibberellins, and cytokinins.
Plant Growth Inhibitors: This group includes Abscisic acid and ethylene hormones.
Plant Growth Promoters and Inhibitors : Ethylene can act as a plant growth inhibitor and promoter.
Auxin Hormone
Auxins are plant hormones that promote growth. They are produced in the growing tips of roots and stems and then move to other parts of the plant.
Receptor Binding: Auxins bind to specific receptors in plant cell membranes.
Protein Degradation: This binding triggers the degradation of Auxin/Indole-3-Acetic Acid (Aux/IAA) proteins.
Gene Activation: The degradation releases Auxin Response Factors (ARFs) which activate target genes involved in growth and development.
Gibberellins Hormone
Gibberellins are plant growth regulators that control various developmental processes in plants, including stem elongation, germination, flowering, and enzyme induction.
Function of Gibberellins
Stem Elongation: Gibberellins stimulate stem elongation by promoting cell division and expansion in plant stems.
Seed Germination: They break seed dormancy and initiate germination by activating hydrolytic enzymes that break down stored nutrients in seeds.
Flowering Induction: Gibberellins trigger the transition from vegetative to reproductive growth, initiating the flowering process in many plant species.
Example: Gibberellic acid (GA) is a well-known gibberellin.
Mechanism of Giberellins
Gibberellins bind to receptors in the cytoplasm of plant cells, forming a complex.
This complex translocates to the nucleus and interacts with gibberellin-insensitive dwarf (GID) proteins.
The interaction targets DELLA proteins, negative regulators of gibberellin signaling, for degradation via the proteasome pathway.
Degradation of DELLA proteins relieves repression of growth-promoting genes, promoting stem elongation, seed germination, and flowering induction.
Cytokinins are crucial for the cytokinesis process, which involves cell division. They are naturally synthesized in plants where rapid cell division occurs, such as in root apices, shoot buds, and young fruits. The movement of cytokinins within plants is basipetal and polar.
Natural Sources: Zeatin (found in corn kernels and coconut milk), isopentenyladenine
Cell Division: Cytokinins promote cell division in various plant tissues, aiding in growth and development.
Shoot Growth: They regulate lateral shoot growth, preventing apical dominance, which occurs when the main shoot prevents lateral shoots from growing. This promotes branching and increased biomass.
Delaying Senescence: Cytokinins delay senescence, the natural aging process of leaves, by extending the photosynthetic period and supporting overall plant vigour.
Examples: Isopentenyl adenine and zeatin are common cytokinins found in plants.
Cytokinins bind to receptors on the endoplasmic reticulum (ER) membrane of plant cells, activating histidine kinases in a two-component signalling system.
Activated histidine kinases transfer phosphate groups to histidine phosphotransfer proteins (AHPs).
Phosphorylated AHPs then transfer the phosphate group to type-B response regulators (RRs), which act as transcription factors.
Activated type-B RRs enter the nucleus and regulate the expression of specific target genes involved in cell division, shoot growth, and apical dominance.
This regulatory process ultimately leads to increased cell division and lateral shoot growth.
Abscisic Acid Hormone
Abscisic acid (ABA) is an essential plant hormone that regulates germination and the plant's response to reduced water availability, particularly during drought stress. As water becomes scarce, ABA levels rise, triggering various responses, including the closure of stomates. This closure reduces transpiration, the movement of water from the plant's root to stem to leaf and out through the stomates into the atmosphere.
Stress Response: ABA plays a vital role in plant responses to stresses like drought, salinity, and extreme temperatures, enhancing stress tolerance and aiding plants in coping with adverse conditions.
Stomatal Closure: ABA regulates the opening and closing of stomata, the tiny pores on the leaf surface, helping plants conserve water during periods of water scarcity.
Seed Dormancy and Germination Inhibition: ABA induces seed dormancy and prevents premature seed germination, ensuring that seeds germinate under optimal conditions.
Examples: Abscisic acid (ABA) is primarily responsible for these functions in plants.
Mechanism of Abscisic Acid
Receptor Binding: ABA interacts with specific receptors linked to G-proteins on the plasma membrane of plant cells.
Secondary Messengers: ABA binding activates G-proteins, initiating the production of secondary messengers such as calcium ions and reactive oxygen species.
Protein Phosphorylation: These secondary messengers trigger a series of protein phosphorylation events, leading to changes in gene expression.
Gene Regulation: The altered gene expression activates stress-responsive genes and induces stomatal closure, enabling plants to conserve water during drought and respond effectively to various stresses.
Ethylene is a gaseous hormone known for its role in fruit ripening. It also regulates seedling growth, root hair formation, and can cause epinasty, the downward bending of branches.
Function of Ethylene
Fruit Ripening: Ethylene triggers the breakdown of cell walls in fruits, leading to softening and colour changes.
Senescence and Leaf Abscission: It induces leaf and flower senescence, aiding in aging and the shedding of leaves and flowers.
Root Growth and Gravitropism : Ethylene influences root growth and gravitropism, affecting root orientation in response to gravity.
Examples: Ethylene is synthesised from the amino acid methionine.
Mechanism of Ethylene
Receptor Binding: Ethylene diffuses through the cell membrane and binds to receptors on the endoplasmic reticulum of plant cells.
Constitutive Triple Response: Ethylene binding activates the transcription factor EIN3, which induces genes associated with the "constitutive triple response," causing specific growth patterns.
Downstream Effects: This response helps plants adapt to stress and survive in challenging environments.
Plant Hormones Functions
Plant hormones, or phytohormones, are chemical messengers produced in small amounts within plants. They regulate growth, development, and responses to environmental stimuli. Unlike animal hormones, which come from specific glands, plant hormones can be produced in various plant tissues. They travel throughout the plant and affect target cells, often in low concentrations.
Key Functions:
Cell Division and Enlargement: Auxin, cytokinin, and gibberellins stimulate cell division and enlargement.
Stem Elongation: Auxin and gibberellins primarily elongate stems.
Root Development: Auxin promotes root initiation and growth.
Tropisms : Auxin regulates tropisms, such as responses to light or gravity.
Dormancy and Seed Germination: ABA induces seed and bud dormancy, while gibberellins promote germination.
6 . Flowering and Fruit Development: Auxin, gibberellins, and cytokinins influence flower and fruit formation.
Leaf Senescence and Abscission: Ethylene and ABA contribute to leaf aging and shedding.
Stress Tolerance: Brassinosteroids help plants tolerate stresses like drought and salinity.
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