Plant Growth Regulators – Types, Characteristics, Functions, Examples

What is Plant growth regulator?

• Plant growth regulators (PGRs), also known as phytohormones, are organic compounds that influence various physiological functions in plants. These substances are either naturally synthesized by the plant or can be artificially produced. They play a crucial role in regulating plant growth, development, and responses to environmental stimuli, even when present in very small amounts.
• The term “phytohormone” was first suggested by Thimmann in 1948, highlighting that these hormones are produced in specific parts of the plant but act on different areas, governing essential processes. Plant growth regulators encompass a wide range of chemicals, including auxins, gibberellins, cytokinins, ethylene, and growth inhibitors.
• Auxins were the first phytohormones identified, leading to further discoveries of other groups like gibberellins and cytokinins. Each of these hormones has unique functions that influence plant growth and development. For instance, auxins promote cell elongation, gibberellins are involved in stem elongation and seed germination, while cytokinins stimulate cell division.
• Plant growth regulators can be classified into two primary categories: plant growth promoters and plant growth inhibitors. Growth promoters, such as auxins, gibberellins, and cytokinins, encourage various developmental processes like cell division, elongation, and differentiation. On the other hand, growth inhibitors, like abscisic acid, slow down growth and facilitate processes like leaf fall and dormancy.
• Ethylene is an interesting compound, as it can function both as a growth promoter and inhibitor, depending on the context. It plays a role in fruit ripening, leaf abscission, and stress responses in plants.

Types of Plant Growth

There are several distinct types of plant growth, each characterized by specific processes and outcomes. These growth types define how plants develop in size, form, and function throughout their life cycle. Below is a detailed breakdown of the main types of plant growth:

1. Primary Growth
   • Primary growth refers to the elongation of the plant body, which results in an increase in height or length. This growth occurs through mitotic divisions in the meristematic cells located at the root and shoot apices. Therefore, the plant extends upward and downward, allowing it to explore new areas for resources like light, water, and nutrients.
2. Secondary Growth
   • Secondary growth refers to an increase in the diameter or girth of the plant body. This process occurs through the division of cells in the secondary meristem, leading to the thickening of stems and roots. Secondary growth is prominent in woody plants and is responsible for the formation of bark and wood.
3. Unlimited Growth
   • In unlimited growth, the plant continues growing throughout its life, from the germination stage until death. This type of growth is often observed in plant structures such as roots and shoots, which can continuously produce new cells without a defined stopping point.
4. Limited Growth
   • Limited growth occurs when a plant or its parts stop growing after reaching a certain size. This type of growth is commonly seen in structures like leaves, flowers, and fruits, which have a defined size and shape and cease growth once they mature.
5. Vegetative Growth
   • Vegetative growth involves the production of stems, leaves, and branches. This type of growth focuses on the development of the plant’s structural and support systems, excluding reproductive structures like flowers. It is essential for expanding the plant’s photosynthetic capacity and overall biomass.
6. Reproductive Growth
   • Reproductive growth occurs when the plant begins to produce flowers, signaling the onset of its reproductive phase. This growth is vital for the propagation of the species, as it leads to the formation of seeds and fruits, ensuring the continuation of the plant’s life cycle.

Factors Affecting Plant Growth

Plant growth is influenced by a range of factors that can be broadly categorized into external and internal factors. Each category includes specific elements that affect various physiological and developmental processes within plants.

External Factors

1. Temperature
   • Range and Variability: Temperature is a critical environmental factor influencing plant growth. The optimal temperature range varies among species. For instance, winter cereals can grow at temperatures between 34°F and 40°F, whereas pumpkins and melons are unable to grow in this range. Tropical plants generally thrive in temperatures between 30°C and 35°C, whereas temperate species have optimal growth at 25°C to 30°C.
   • Growth Rate: As temperature rises from the minimum threshold, growth initially accelerates until reaching an optimum level. Beyond this point, growth slows and can eventually halt if temperatures become too high.
   • Soil Temperature: Soil temperature affects root and shoot growth. Generally, plants grow better with cooler night temperatures compared to daytime temperatures. The concept of thermo-periodicity refers to the impact of alternating temperatures between day and night on plant growth and reactions.
2. Light
   • Intensity: Light intensity significantly impacts plant growth. Full sunlight usually promotes vigorous growth and flowering in many plants. For example, crops like wheat and corn grow robustly under full sun, whereas in lower light intensities, these plants exhibit elongated growth and reduced flowering.
   • Quality: Different wavelengths of light affect plant development differently. Full-spectrum light promotes overall growth and increases dry weight. Blue and violet light often result in dwarf plants, while red light tends to produce taller, spindly plants. Green light is less effective for photosynthesis, leading to reduced growth.
   • Duration: The length of light exposure affects both photosynthesis and growth rate. Short days in winter typically slow growth, while longer days in spring accelerate it. Light duration also influences dormancy and flowering. Plants are categorized based on light requirements for flowering: long-day plants, short-day plants, and day-neutral plants.
3. Oxygen Supply
   • Soil Aeration: Oxygen availability is crucial for plant growth. In poorly aerated soils, particularly in waterlogged conditions, plants suffer from reduced mineral and water absorption, leading to retarded growth. Proper soil aeration is essential for root health and overall plant vigor.
4. Water
   • Availability: Adequate water supply is fundamental for plant growth. Plants grow best with sufficient moisture, though excessive water can be detrimental. Ideal soil-water content is just above the wilting percentage. Below this level, growth slows, and at the permanent wilting percentage, growth ceases. Some plants, like radishes and peppers, exhibit noticeable wilting and growth cessation under water stress, while others may show changes in leaf color or curling.
5. Soil Nutrients
   • Nutrient Levels: The presence and concentration of soil nutrients—both macro and micronutrients—affect plant growth. Nutrients must be available in ionic form or as components of molecules for optimal plant development. Nutrient deficiency or imbalance can significantly impact growth and development.

Internal Factors

1. Growth Regulators
   • Types: Plant growth regulators include both growth promoters (e.g., auxins, gibberellins, cytokinins) and growth inhibitors (e.g., abscisic acid, ethylene). These regulators can be either naturally occurring or synthetic. They play crucial roles in various developmental processes, such as cell division, elongation, and differentiation.
2. Carbon/Nitrogen Ratio
   • Growth Patterns: The ratio of carbohydrates to nitrogenous compounds influences plant growth. A higher carbohydrate ratio generally promotes robust vegetative growth, flowering, and fruiting. Conversely, a higher nitrogen ratio tends to enhance vegetative growth at the expense of flowering and fruiting.
3. Genotype and Genetic Factors
   • Genetic Control: Plant growth and development are governed by the genotype, including both nuclear and extra-nuclear genes. Gene expression, controlled by both genetic factors and environmental conditions, dictates the synthesis of structural and enzymatic proteins essential for various physiological processes.

Characteristics of Plants Growth

Plant growth is a multifaceted process characterized by several key attributes and stages. Understanding these characteristics is crucial for comprehending how plants develop and thrive. The following points outline the essential features of plant growth:

1. Assimilation and Protoplasm Formation
   • Assimilation: Plants transform various nutrients, including minerals, proteins, carbohydrates, fats, vitamins, and hormones, into protoplasm. This process, known as assimilation, results in the formation of new cellular material and supports overall growth.
   • Protoplasm: The newly formed protoplasm contributes to the development of new cell walls and tissues, thereby promoting plant growth.
2. Growth Stages
   • Initial Stage (Cell Division): The first stage of growth involves the production of new protoplasm, which is most evident in regions of active cell division, such as meristems.
   • Intermediate Stage (Tissue Expansion): The second stage is characterized by the absorption of water, leading to tissue expansion. This stage results in an increase in plant size without a significant increase in dry weight.
   • Final Stage (Maturation): The third stage involves the accumulation of building materials, mainly carbohydrates, into expanded tissues. This stage increases the plant’s dry weight without further visible growth.
3. Differentiation
   • Cell, Tissue, and Organ Differentiation: Differentiation occurs at various levels, from individual cells to entire organs. For example:
     • Flower Structure: Flowers exhibit differentiation with sepals for protection, petals for attraction, stamens for male gametes, and carpels for ovules.
     • Whole Plant: Angiosperms have roots for absorption, stems and branches for support and photosynthesis, and fruits for seed protection and dispersal.
   • Functional Specialization: Differentiation results in specialized structures and functions, enhancing the plant’s ability to perform essential biological processes.
4. Development
   • Definition: Development encompasses a series of qualitative changes from the zygote stage to plant death. It includes processes such as cell division, enlargement, differentiation, morphogenesis, and senescence.
   • Phases:
     • Embryonal Stage: Begins with the fertilized egg, developing into an embryo within the seed.
     • Seedling Stage: Following dormancy, the embryo resumes growth, developing into a seedling.
     • Vegetative Phase: The seedling matures into a vegetative plant, focusing on growth and development of leaves, stems, and roots.
     • Reproductive Phase: The plant produces flowers and fruits, leading to seed formation.
     • Senescence: The final phase involves aging and eventual death of the plant.
5. Growth in Unicellular and Multicellular Organisms
   • Unicellular Organisms: Growth involves an increase in cell size due to protoplasm synthesis. Cell division results in the multiplication of individuals.
   • Simple Multicellular Organisms: In organisms like Spirogyra, growth includes both cell division and enlargement.
   • Flowering Plants: Growth comprises three phases—cell division, enlargement, and differentiation—resulting in a complex structure with specialized functions.
6. Growth Regions
   • Localized Growth: In higher plants, growth is localized to specific regions known as meristems. These include:
     • Apical Meristems: Located at the tips of roots and shoots, contributing to elongation.
     • Intercalary Meristems: Found in regions between mature tissues, facilitating elongation.
     • Lateral Meristems: Located along the sides of stems and roots, responsible for secondary growth.
   • Diffuse Growth: In lower plants, growth can be diffuse, with each cell capable of dividing and enlarging independently.

Characteristics of Plant Growth Regulators

Below are the main characteristics of plant growth regulators:

1. Diverse Chemical Composition
   • Plant growth regulators can vary in chemical structure, including gases like ethylene, terpenes such as gibberellic acid, or carotenoid derivatives like abscisic acid. Each type interacts differently with plant cells, enabling them to regulate various physiological processes. Therefore, PGRs can have a broad spectrum of effects depending on their chemical nature.
2. Classification Based on Function
   • Plant Growth Promoters: These regulators enhance growth by promoting cell division, cell enlargement, flowering, fruit formation, and seed development. Examples of growth promoters include:
     • Auxins: Stimulate cell elongation and are crucial for root development.
     • Gibberellins: Promote stem elongation, seed germination, and flowering.
     • Cytokinins: Encourage cell division and delay the aging of leaves.
   • Plant Growth Inhibitors: These substances slow down or inhibit growth processes, often inducing dormancy or causing the shedding of leaves and fruits. The primary example is:
     • Abscisic Acid: This regulator promotes dormancy and helps plants cope with stress by closing stomata and inhibiting growth.
     • Ethylene: Though unique in that it can act as both a growth promoter and inhibitor, ethylene is primarily considered an inhibitor, influencing processes like fruit ripening and leaf abscission.
3. Specificity in Action
   • PGRs act in very minute quantities and are highly specific in their functions. They are often synthesized in one part of the plant and transported to another area where they exert their effect. This specificity allows plants to precisely control processes like growth direction, flowering time, and response to environmental conditions.

Phases of Plant Growth

Plant growth is a complex process that can be understood through the examination of three fundamental phases: cell division, cell enlargement, and cell differentiation. Each phase contributes to the overall development and maturation of plant tissues and organs.

1. Phase of Cell Division (Formative Phase)
   • Mitosis: This is the foundational process for plant growth, involving the division of pre-existing cells to produce new cells. Mitosis is a key mechanism in the formative phase and includes two main stages:
     • Karyokinesis: The division of the nucleus, ensuring that genetic material is evenly distributed.
     • Cytokinesis: The division of the cytoplasm, resulting in two daughter cells.
   • Process: The cell undergoes a series of stages—prophase, metaphase, anaphase, and telophase. During these stages, the genetic material is replicated and equally distributed to the daughter cells. The result is two genetically identical cells.
   • Meristematic Regions: In higher plants, cell division predominantly occurs in meristematic regions such as the apical meristem. These regions are responsible for the continuous increase in cell number. Some daughter cells retain their ability to divide, while others transition to the next growth phase.
2. Phase of Cell Enlargement
   • Mechanism: Following cell division, cells enter the phase of enlargement. This phase is crucial for increasing tissue and organ size. It involves:
     • Protoplasm Synthesis: The production of new cellular material.
     • Hydration: The absorption of water, leading to cell expansion.
     • Vacuole Development: Formation and expansion of vacuoles within the cell, contributing to cell size.
     • Cell Wall Formation: The addition of new material to the cell wall, which becomes thicker and more permanent as the cell stretches.
   • Types of Enlargement: Cell enlargement can be linear or occur in multiple directions, depending on the growth pattern required for the plant.
3. Phase of Cell Differentiation or Cell Maturation
   • Specialization: In the final phase, cells that have enlarged undergo differentiation. This process involves:
     • Biochemical Changes: Alterations in cellular biochemistry to support specific functions.
     • Physiological Changes: Adaptations in cellular processes that align with the cell’s role.
     • Morphological Changes: Structural modifications that define the cell’s shape and function.
   • Formation of Tissues: Differentiated cells develop into various types of simple and complex tissues, each specialized for particular functions within the plant. These tissues contribute to the formation of organs and overall plant structure.

Hormone groups/Examples of Plant Growth Regulators

Plant hormones, also known as phytohormones, are classified into distinct groups based on their chemical composition and specific functions in regulating plant growth and development. Each group of hormones influences various physiological processes in plants. Below is a detailed breakdown of the major hormone groups:

1. Auxins
   • Auxins are a class of hormones that primarily stimulate cell elongation, particularly in the stems and roots. They resemble indole-3-acetic acid (IAA), the naturally occurring auxin in plants. Auxins play a crucial role in the elongation of coleoptiles (the protective sheath covering the emerging shoot) and are vital for processes like phototropism (growth towards light) and gravitropism (growth in response to gravity).
2. Gibberellins
   • Gibberellins are diterpenoid compounds that primarily promote stem elongation, especially in certain dwarf or rosette plant species. They help elongate the stem in green seedlings and are also involved in breaking seed dormancy, stimulating germination, and inducing flowering. Besides, gibberellins are crucial for the growth of fruits and regulation of various developmental processes.
3. Cytokinins
   • Cytokinins are substituted adenines that resemble zeatin, the naturally occurring cytokinin found in Zea mays (corn). These hormones promote cell division, or cytokinesis, particularly in plant tissues. Cytokinins work in conjunction with auxins to regulate cell growth, differentiation, and development. They also help delay senescence (aging) in leaves and other plant organs.
4. Ethylene
   • Ethylene is a gaseous hormone that acts as a regulator for various growth and developmental processes. It stimulates diametric (lateral) growth, especially in the apices (growing tips) of dicot seedlings. Ethylene plays a critical role in fruit ripening, leaf abscission (shedding), and stress responses in plants, particularly under conditions of drought, flooding, or mechanical damage.
5. Inhibitors (Abscisic Acid)
   • Growth inhibitors, such as abscisic acid (ABA), primarily act to slow down or depress plant growth. ABA is involved in promoting dormancy in seeds and buds, helping plants survive under unfavorable conditions like drought by inducing stomatal closure to reduce water loss. It also regulates responses to stress, making it crucial for plant survival.

Plant Growth Promoters

1. Auxins
2. Gibberellins
3. Cytokinins (kinetin)

1. Auxins

Auxins represent a critical class of plant hormones responsible for regulating various growth and developmental processes. Their discovery and subsequent study have significantly enhanced our understanding of plant biology.

1. Discovery and Historical Background

• Initial Observations: The discovery of auxins can be traced back to the pioneering observations of Charles Darwin and his son, Francis Darwin. Their studies on canary grass coleoptiles revealed that these structures exhibit phototropism, the tendency to grow toward light sources. They noted that the bending of the coleoptile occurred due to a substance located at its tip.
• Isolation of Auxins: F. W. Went furthered this research by isolating the first auxin from the coleoptile tips of oat seedlings. This landmark discovery established auxins as fundamental regulators of plant growth.

2. Types of Auxins

• Natural Auxins: The most well-known natural auxins include Indole-3-acetic acid (IAA) and Indole butyric acid (IBA). These compounds are produced in growing stems and roots and then transported to their sites of action within the plant.
• Synthetic Auxins: In addition to natural auxins, synthetic auxins such as Naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) have been developed. These compounds are often used in agricultural and horticultural practices due to their growth-regulating properties.

Physiological effects of auxin

Auxin, a pivotal plant hormone, plays a crucial role in regulating various physiological processes. Below are the detailed physiological effects of auxin:

1. Cell Division and Elongation
   • Auxin primarily influences cell division and elongation in plant shoots. This hormone is instrumental in secondary stem growth and the differentiation of vascular tissues such as xylem and phloem. Auxin promotes cell elongation by increasing cell wall plasticity and reducing wall elasticity. It achieves this by breaking cross-links in cell wall components, which enhances water and solute uptake and contributes to overall cell expansion.
2. Apical Dominance
   • Apical dominance refers to the suppression of lateral bud growth by the terminal bud. This phenomenon occurs because auxin, produced in the apical bud, is transported downward through the stem. This auxin inhibits the growth of lateral buds by preventing their activation. Skoog and Thimmann’s experiments demonstrated that removing the apical bud or replacing it with an agar block could stimulate lateral bud growth. Conversely, replacing it with an auxin-containing agar block maintained suppression of lateral buds.
3. Root Initiation
   • Auxin’s effect on root growth varies with concentration. While high auxin levels inhibit root elongation, they promote the development of lateral roots. For practical applications, applying auxin to the cut end of a young stem, often in a lanolin paste, facilitates rapid and extensive root formation. This property is widely utilized in the propagation of economically valuable plants through cuttings.
4. Prevention of Abscission
   • Auxins help prevent abscission, the process by which leaves, flowers, and fruits detach from the plant. Natural auxins inhibit the formation of the abscission layer, which otherwise leads to the shedding of these plant parts. High auxin concentrations suppress the formation of this layer, whereas a reduction or neutralization of auxin concentration triggers abscission.
5. Parthenocarpy
   • Parthenocarpy is the development of fruit without fertilization, which can be induced by auxins. In parthenocarpic fruits, auxin concentrations in the ovaries are higher compared to fertilized fruits, where auxin levels increase post-pollination. This hormonal effect is exploited to produce seedless fruits, such as bananas and tomatoes, without requiring pollination.
6. Respiration
   • Auxin stimulates respiration in plants. This is linked to the hormone’s role in growth, as auxin can increase the rate of respiration by enhancing ATP utilization in expanding cells. The subsequent increase in ADP availability drives ATP formation, thus supporting the energetic demands of cell growth.
7. Callus Formation
   • Auxins are critical in callus formation, especially in tissue cultures. For sustained callus growth, auxins must be present. This hormone stimulates both cell division and elongation, facilitating the development of callus tissue, which is essential for various plant regeneration processes.
8. Eradication of Weeds
   • Synthetic auxins, such as 2,4-D and 2,4,5-T, are used as herbicides to control and eradicate weeds. At elevated concentrations, these synthetic auxins cause uncontrolled growth in weeds, leading to their demise.
9. Flowering and Sex Expression
   • Auxins generally inhibit flowering but can induce flowering under specific conditions. For instance, in pineapple and lettuce, auxins like 2,4-D can promote uniform flowering. This effect is utilized in horticulture to manage and enhance flowering patterns.

Distribution of auxin in plants

Auxin, particularly Indole-3-acetic acid (IAA), is a crucial plant hormone involved in various developmental processes. Its distribution within plants is essential for understanding its functional roles. The distribution pattern of auxin can be outlined as follows:

1. Synthesis and Primary Locations
   • Auxin is primarily synthesized in the meristematic regions of plants, including the growing tips of shoots and roots. Therefore, the highest concentrations of IAA are found in these areas:
     • Growing Shoot Tips: The apical meristems of shoots are major sites for auxin production. Auxin levels are highest here, where they facilitate cell elongation and differentiation.
     • Young Leaves: In developing leaves, auxins are present in significant concentrations, supporting growth and developmental processes.
     • Developing Axillary Shoots: Auxins also accumulate in the young axillary shoots, influencing their growth and potential for branching.
2. Auxin Distribution in Monocots and Dicots
   • Monocot Seedlings: In monocots, such as grasses, the highest concentration of auxin is observed in the coleoptile tip. This concentration diminishes progressively towards the base of the coleoptile.
   • Dicot Seedlings: In dicots, auxin is concentrated in the growing regions of the shoot, including young leaves and developing axillary shoots. This distribution supports the general growth and development of the plant.
3. Forms of Auxin
   • Free Auxins: These are auxins that are readily extracted using organic solvents such as diethyl ether. Free auxins are active forms of the hormone that directly participate in growth regulation and other physiological functions.
   • Bound Auxins: Unlike free auxins, bound auxins are complexed with other molecules and require more intensive extraction methods like hydrolysis or enzymolysis. Bound auxins are associated with carbohydrates (e.g., glucose, arabinose), proteins, amino acids (e.g., aspartate, glutamate), or inositol. These complexes play a role in auxin storage and transport within the plant.

Biosynthesis of auxin (IAA) in plants

Indole-3-acetic acid (IAA) is a key plant hormone, commonly referred to as auxin. Its biosynthesis involves the conversion of the amino acid tryptophan into IAA through distinct biochemical pathways. The following outlines the primary steps involved in auxin biosynthesis:

1. Tryptophan as the Precursor
   • Tryptophan, an essential amino acid, serves as the primary precursor for IAA synthesis in plants. The conversion of tryptophan into IAA occurs through two main biochemical pathways.
2. Pathway 1: Indole-3-Pyruvic Acid Route
   • Deamination of Tryptophan: The first step involves the deamination of tryptophan, resulting in the formation of indole-3-pyruvic acid. This reaction is catalyzed by the enzyme tryptophan deaminase.
   • Decarboxylation of Indole-3-Pyruvic Acid: Indole-3-pyruvic acid is then decarboxylated to yield indole-3-acetaldehyde. This reaction is facilitated by the enzyme indole pyruvate decarboxylase.
   • Oxidation to IAA: Indole-3-acetaldehyde is subsequently oxidized to form IAA. This oxidation is carried out by the enzyme indole-3-acetaldehyde dehydrogenase.
3. Pathway 2: Tryptamine Route
   • Decarboxylation of Tryptophan: In this pathway, tryptophan undergoes decarboxylation to produce tryptamine. This step is mediated by the enzyme tryptophan decarboxylase.
   • Deamination of Tryptamine: Tryptamine is then deaminated to form indole-3-acetaldehyde. The enzyme involved in this reaction is tryptamine oxidase.
   • Oxidation to IAA: Similar to the first pathway, indole-3-acetaldehyde is oxidized to IAA by indole-3-acetaldehyde dehydrogenase.
4. Enzymatic Roles
   • Tryptophan Deaminase: Catalyzes the deamination of tryptophan.
   • Indole Pyruvate Decarboxylase: Facilitates the decarboxylation of indole-3-pyruvic acid.
   • Tryptophan Decarboxylase: Converts tryptophan into tryptamine.
   • Tryptamine Oxidase: Converts tryptamine into indole-3-acetaldehyde.
   • Indole-3-Acetaldehyde Dehydrogenase: Converts indole-3-acetaldehyde into IAA.

2. Gibberellins

Discovery

The discovery of gibberellins, a class of plant hormones, originated from observations of unusual growth patterns in rice seedlings. The following outlines the key events leading to the identification of gibberellins:

1. Initial Observation
   • Japanese scientist E. Kurosawa was investigating the effects of fungal infections on rice seedlings. He observed that seedlings infected with the fungus Gibberella fujikuroi exhibited abnormal growth. These plants grew excessively tall but appeared thin and pale, a condition known as “bakane” disease.
2. Isolation of the Active Substance
   • Kurosawa isolated an active substance from the infected seedlings responsible for these growth abnormalities. This substance was subsequently named gibberellin, marking the first identification of a compound linked to abnormal plant growth.
3. Identification of Gibberellic Acid
   • Further research revealed that the active substance causing the symptoms of “bakane” disease was gibberellic acid, a specific type of gibberellin. Kurosawa confirmed this by treating uninfected rice seedlings with sterile filtrates of the Gibberella fujikuroi fungus. The treated seedlings developed similar growth abnormalities, thereby validating the role of gibberellic acid.
4. Biosynthesis of Gibberellins
   • Gibberellins are synthesized from acetate, a fundamental building block. The biosynthetic pathway involves several key steps:
     • Acetate to Acetyl CoA: Acetate is converted into Acetyl CoA.
     • Mevalonic Acid Formation: Acetyl CoA is transformed into mevalonic acid.
     • Formation of Mevalonic Acid Pyrophosphate: Mevalonic acid is then converted to mevalonic acid pyrophosphate.
     • Isopentanyl Pyrophosphate Formation: Mevalonic acid pyrophosphate is converted into isopentanyl pyrophosphate.
     • Geranyl Pyrophosphate Formation: Isopentanyl pyrophosphate is used to form geranyl pyrophosphate.
     • Gibberellin Biosynthesis: Geranyl pyrophosphate is further converted into gibberellins through the intermediate kaurene.

Physiological effects of gibberellins

Gibberellins are crucial plant hormones that significantly impact various physiological processes. Below are the primary effects of gibberellins on plant growth and development:

1. Seed Germination
   • Gibberellins play a vital role in seed germination, particularly in seeds that are light-sensitive, such as those of lettuce and tobacco. These seeds often exhibit poor germination in darkness. Exposure to light typically stimulates germination, but gibberellic acid can override the need for light, enabling seeds to germinate even in the dark.
2. Dormancy of Buds
   • In temperate regions, buds that form in autumn enter a period of dormancy due to cold temperatures. Gibberellins can break this dormancy, allowing buds to resume growth in the spring. Additionally, in potatoes, which have a dormant phase after harvest, gibberellin application can stimulate vigorous sprouting.
3. Root Growth
   • The effects of gibberellins on root growth are relatively minimal. While gibberellins have little influence on root development, high concentrations can inhibit root growth. This inhibition is particularly evident in isolated cuttings, where gibberellins can markedly suppress root initiation.
4. Elongation of Internodes
   • Gibberellins are notably effective in promoting the elongation of internodes. This action is particularly beneficial for plants exhibiting genetic dwarfism, such as dwarf pea and dwarf maize. By applying gibberellins, these plants can overcome their genetic constraints and achieve normal growth patterns.
5. Bolting and Flowering
   • Gibberellins induce bolting—a process where the stem elongates rapidly and transitions into a flowering stage—in many herbaceous plants. This effect can be observed even under conditions that do not typically induce bolting, such as short days. For example, in Hyoscyamus niger, a long-day plant, gibberellins can promote bolting and flowering under short days. Conversely, in long-day plants, gibberellins often lead to early flowering. The effect of gibberellins on flowering in short-day plants can vary, potentially inhibiting or activating flowering depending on the specific plant species.
6. Parthenocarpy
   • Gibberellins can stimulate pollen grain germination and promote the growth of fruit. They are particularly effective in inducing parthenocarpy, the formation of seedless fruits without fertilization. This is especially useful in producing seedless and fleshy fruits like tomatoes and large-sized seedless grapes. In cases where auxins are ineffective, gibberellins have successfully induced parthenocarpy.
7. Synthesis of α-Amylase
   • During seed germination, gibberellins induce the synthesis of α-amylase in the aleurone layer of cereal grains. This enzyme is crucial for the hydrolysis of starch into simple sugars, which are then transported to the growing embryo to provide an energy source necessary for development.

Distribution of gibberellins in plant

Gibberellins, a class of plant hormones, are distributed throughout all parts of higher plants, with varying concentrations and forms. Their distribution and concentration play crucial roles in plant growth and development.

1. General Distribution
   • Gibberellins are present in all major plant organs, including:
     • Shoots: Involved in promoting growth and elongation.
     • Roots: Although gibberellins have minimal effects on root growth, they are still present.
     • Leaves: Participate in various growth processes.
     • Flowers: Critical for reproductive development.
     • Petals: Influence flowering and developmental processes.
     • Anthers: Essential for pollen development.
     • Seeds: Particularly high concentrations are found in seeds, where they contribute to germination.
2. Concentration Variations
   • Reproductive Parts: Gibberellins are generally found in higher concentrations in reproductive tissues compared to non-reproductive parts. This higher concentration in flowers and seeds is vital for their development and function.
   • Immature Seeds: These seeds are notably rich in gibberellins, with concentrations ranging from 10 to 100 mg per gram of fresh weight. This high gibberellin content supports seed development and germination.
3. Forms of Gibberellins
   • Free Gibberellins: These are the biologically active forms of gibberellins that exert their effects directly on plant growth and development.
   • Bound Gibberellins: Typically occur as gibberellin-glycosides. In this form, gibberellins are bound to sugars, rendering them inactive until they are hydrolyzed into free gibberellins.

3. Cytokinins (kinetin)

Discovery of Cytokinins (Kinetin)

The discovery of cytokinins, particularly kinetin, marks a significant advancement in plant physiology. This hormone plays a crucial role in cell division and growth. The following points outline the key aspects of its discovery:

1. Initial Observations
   • F. Skoog and Colleagues: The discovery began with observations made by F. Skoog and his research team. They studied a mass of undifferentiated cells known as ‘callus’ derived from tobacco plants.
   • Callus Proliferation: The callus cells exhibited proliferation, but this growth was contingent upon the presence of specific conditions in the nutrient medium.
2. Role of Auxins and Yeast Extract
   • Nutrient Medium Composition: Skoog and his team noted that the callus cells proliferated only when the nutrient medium included both auxins and yeast extract or extracts from vascular tissues.
   • Auxins: These hormones, known for their role in promoting cell elongation and differentiation, were essential but not sufficient alone for the observed proliferation.
3. Identification of Kinetin
   • Active Substance: Through their research, Skoog and Miller isolated and identified the active substance responsible for the callus proliferation.
   • Naming: This substance was named ‘kinetin.’ It was later understood to be a type of cytokinin, a class of plant hormones that stimulate cell division.
4. Significance of the Discovery
   • Function of Kinetin: Kinetin, as a cytokinin, plays a crucial role in promoting cell division and growth. Its discovery helped elucidate the complex interactions between different plant hormones.
   • Impact on Plant Science: The identification of kinetin contributed significantly to the understanding of plant growth regulation and led to further research into other cytokinins and their functions.

Types of Cytokinins (Kinetin)

Cytokinins, initially identified as kinetin, are a class of plant hormones known for their role in regulating cell division and growth. While kinetin itself is not naturally occurring, various natural and synthetic cytokinins have been discovered and characterized. The following outlines the key types of cytokinins and their sources:

1. Kinetin
   • Discovery: Kinetin was the first cytokinin discovered and is known for its ability to promote cell division. However, it does not occur naturally in plants.
   • Synthetic Origin: It was synthesized from the adenine base and is used primarily in research to study cytokinin activity.
2. Zeatin
   • Natural Occurrence: Zeatin is one of the most well-known naturally occurring cytokinins. It is primarily found in the root apices and developing shoot buds where rapid cell division occurs.
   • Sources: Zeatin can be extracted from a variety of plant tissues including coconut milk, which is a rich source due to its high cytokinin content, and from fruits and vegetables such as tomatoes and pears.
3. Coconut Milk Factor
   • Description: This is a term used to describe the cytokinin activity present in coconut milk. It was one of the early sources of cytokinins identified and has been used extensively in tissue culture to promote cell division and growth.
   • Role in Plant Growth: The coconut milk factor is significant in promoting cell division and growth in various plant tissues.
4. Natural Sources of Cytokinins
   • Plant Tissues: Cytokinins can be extracted from diverse plant parts including:
     • Fruits: For example, Pyrus malus (apple), Pyrus communis (pear), and Prunus cerasiferae (plum).
     • Vegetables: Lycopersicum esculentum (tomato) and Bhendi.
     • Cambial Tissues: Found in species such as Pinus radiata (radiata pine), Eucalyptus regnans, and Nicotiana tabacum (tobacco).
     • Immature Fruits: Of Zea mays (corn), Juglans species (walnut), and Musa species (banana).
     • Female Gametophytes: Such as those from Ginkgo biloba.
     • Seedlings and Roots: Pisum sativum (pea) and Helianthus annuus (sunflower).
     • Tumor Tissues: Of tobacco, which are known to have high cytokinin content.
5. Cytokinin in t-RNA
   • Identification: Cytokinins have been identified as components of t-RNA (transfer RNA), reflecting their integral role in protein synthesis and cell function.
6. Skoog and Armstrong’s Classification
   • Reported Types: According to Skoog and Armstrong (1970), at least seven well-established types of cytokinins have been identified, reflecting the diversity and complexity of these plant hormones.

Biosynthesis of Cytokinins (Kinetin)

Cytokinins, including kinetin, are crucial plant hormones involved in various growth and developmental processes. Their biosynthesis involves several key sites and mechanisms within the plant. The following outlines the biosynthetic pathway and movement of cytokinins:

1. Sites of Synthesis
   • Root Tips: The primary site of cytokinin synthesis is the root tip. This region is responsible for the production of cytokinins, which then participate in regulating plant growth and development.
   • Developing Seeds: Cytokinins are also synthesized in developing seeds. The hormone plays a role in seed development and maturation.
   • Cambial Tissues: Another significant site for cytokinin biosynthesis is the cambial tissues. These tissues are involved in the secondary growth of plants and contribute to the production of cytokinins.
2. Biosynthetic Pathway
   • Purine Metabolism: Cytokinins are synthesized from purines, similar to other nucleic acids in plants. This suggests that the biosynthetic pathway for cytokinins involves modifications of purine structures.
   • Complex Pathways: The exact biochemical pathway for cytokinin synthesis includes several steps, often starting from basic purine precursors and involving various enzymatic transformations.
3. Movement and Translocation
   • Upward Movement: Cytokinins are believed to move upwards through the plant, likely transported via the xylem stream. This upward translocation helps distribute cytokinins to various growing tissues.
   • Basipetal Movement: Besides the upward movement, cytokinins also exhibit basipetal (downward) movement in petioles and isolated stems. This movement ensures that cytokinins reach different parts of the plant where they are needed.
   • Auxin Interaction: Auxins can enhance the movement (translocation) of cytokinins. Research by Seth et al. (1966) demonstrated that the presence of auxins facilitates the transport of kinetin in bean stems, indicating a synergistic relationship between these hormones.

Physiological Effects of Cytokinins

Cytokinins, including kinetin, exert a wide range of physiological effects on plant growth and development. These effects are integral to various plant processes, and their application can influence multiple aspects of plant physiology.

1. Cell Division
   • Induction of Cell Division: The primary and most notable effect of cytokinins is their ability to promote cell division. This effect is particularly observed in tissues such as tobacco pith callus, carrot root tissue, soybean cotyledons, and pea callus. Cytokinins stimulate mitotic activity in these tissues, facilitating their growth and proliferation.
2. Cell Enlargement
   • Promotion of Cell Enlargement: Similar to auxins and gibberellins, cytokinins can induce cell enlargement. This effect is evident in various plant parts, including the leaves of Phaseolus vulgaris, pumpkin cotyledons, tobacco pith cultures, and cortical cells of tobacco roots. The enlargement of cells contributes to the overall growth and size of the plant organs.
3. Apical Dominance
   • Counteracting Apical Dominance: Cytokinins can influence apical dominance, a phenomenon where the growth of lateral buds is suppressed by the presence of a dominant apical bud. By applying cytokinins externally, the growth of lateral buds is promoted, reducing the effect of apical dominance and encouraging more branching.
4. Seed Dormancy
   • Breaking Seed Dormancy: Cytokinins can break the dormancy of certain light-sensitive seeds, such as those of lettuce and tobacco. This effect parallels that of gibberellins, facilitating seed germination under conditions that would otherwise inhibit it.
5. Delay of Senescence (Richmond-Lang Effect)
   • Postponement of Leaf Senescence: Cytokinins delay leaf senescence, which typically involves the loss of chlorophyll and the rapid breakdown of proteins. The Richmond-Lang effect, observed by Richmond and Lang (1957), demonstrates that kinetin treatment can extend the vitality of leaves by enhancing RNA synthesis, which subsequently promotes protein synthesis and delays aging.
6. Flower Induction
   • Induction of Flowering: Cytokinins are effective in inducing flowering in short-day plants. This capability is utilized to promote blooming under non-inductive conditions, facilitating controlled flowering and fruit production.
7. Morphogenesis
   • Regulation of Morphogenesis: Cytokinins play a critical role in morphogenesis. High levels of auxin combined with low levels of kinetin typically lead to root formation, whereas high levels of kinetin combined with low levels of auxin promote the formation of shoot buds. This balance is crucial for proper plant development and organ differentiation.
8. Accumulation and Translocation of Solutes
   • Enhanced Solute Management: Cytokinins aid in the active accumulation and translocation of solutes within the plant. They facilitate solute movement in the phloem, which is essential for nutrient distribution and overall plant health.
9. Protein Synthesis
   • Increased Protein Synthesis: Cytokinins have been shown to enhance the rate of protein synthesis. Osborne (1962) demonstrated that kinetin treatment leads to increased protein synthesis, which supports various physiological processes and improves plant growth.
10. Other Effects
    • Resistance and Stress Tolerance: Cytokinins can enhance plant resistance to high temperatures, cold, and diseases. They also play a role in flowering by substituting for photoperiodic requirements and can stimulate the synthesis of enzymes involved in photosynthesis.
11. Commercial Applications
    • Practical Uses: Cytokinins are applied commercially to extend the shelf life of fruits, accelerate root induction, enhance root system efficiency, and increase yield and oil content in crops such as groundnut. Their diverse applications underline their importance in agricultural practices.

Plant Growth Inhibitors

1. Ethylene
2. Abscisic Acid

1. Ethylene

Discovery of Ethylene

Ethylene, a gaseous plant growth regulator, was discovered through an empirical observation of fruit ripening. The discovery process and subsequent understanding of ethylene’s role in plant physiology can be detailed as follows:

1. Initial Observation
   • Observation of Ripening: The initial discovery of ethylene began with a notable observation by a group of cousins. They noted that a gaseous substance released from ripe oranges accelerated the ripening of unripe oranges. This observation was pivotal in linking a gaseous substance to the ripening process.
2. Identification of the Substance
   • Characterization as Ethylene: Through further investigation, the substance responsible for hastening the ripening process was identified as ethylene. Ethylene is a simple gaseous molecule (C₂H₄) that acts as a plant growth regulator. Its role in influencing fruit ripening and other physiological processes was established based on its effects observed in fruit tissues.
3. Role in Ripening and Senescence
   • Ethylene Production: Ethylene is produced in significant quantities by ripening fruits and tissues undergoing senescence. This natural production is a key factor in the regulation of various stages of plant development, including the ripening of fruits and the aging of plant tissues.
4. Impact on Plant Physiology
   • Influence on Ripening: Ethylene’s primary role is to facilitate the ripening process of fruits. It accelerates biochemical processes involved in fruit maturation, such as the breakdown of cell walls, changes in texture, and development of characteristic flavors and aromas.
   • Senescence: Besides ripening, ethylene also plays a crucial role in the senescence of plant tissues. Its production increases as tissues age, contributing to the overall process of aging and degradation.
5. Conclusion
   • Significance of Discovery: The discovery of ethylene as a plant growth regulator marked a significant advancement in understanding plant physiology. Ethylene’s role in ripening and senescence highlights its importance in agricultural practices and plant management. Its identification as a gaseous hormone opened new avenues for research and practical applications in crop production and post-harvest handling.

Important Physiological Effects of Ethylene

Ethylene is a key gaseous plant hormone with a range of significant physiological effects on plant growth and development. Its actions span several critical processes, as outlined below:

1. Ripening of Fleshy Fruits
   • Effect: Ethylene plays a central role in accelerating the ripening of fleshy fruits.
   • Examples: This effect is evident in fruits such as bananas, apples, pears, tomatoes, and citrus fruits. Ethylene facilitates biochemical changes that lead to the softening of fruit texture, color changes, and development of characteristic flavors and aromas.
2. Senescence and Abscission of Leaves
   • Effect: Ethylene promotes the aging (senescence) and detachment (abscission) of leaves.
   • Mechanism: It influences the degradation of cellular components and the weakening of cell adhesion, leading to leaf drop and tissue aging.
3. Induction of Flowering
   • Effect: Ethylene is effective in inducing flowering, particularly in certain crops.
   • Examples: It is used to induce flowering in pineapples, where it helps synchronize flowering and fruit set.
4. Inhibition of Root Growth
   • Effect: Ethylene can inhibit root growth.
   • Implications: This effect may affect overall plant development and nutrient uptake, especially in certain conditions where ethylene levels are elevated.
5. Formation of Adventitious Roots
   • Effect: Ethylene stimulates the formation of adventitious roots.
   • Applications: This promotes root development from non-traditional locations on the plant, which can be beneficial for plant propagation and stability.
6. Fading of Flowers
   • Effect: Ethylene induces the fading of flowers.
   • Mechanism: This includes the alteration of flower color and the initiation of the senescence process, leading to petal drop.
7. Epinasty of Leaves
   • Effect: Ethylene causes epinasty, which is the downward curling of leaves.
   • Implications: This effect can impact leaf orientation and overall plant morphology.
8. Breaking Seed and Bud Dormancy
   • Effect: Ethylene can break dormancy in seeds and buds.
   • Applications: For example, it can initiate germination in peanut seeds and promote sprouting of potato tubers.
9. Rapid Petiole Elongation
   • Effect: Ethylene promotes rapid elongation of petioles in deep water rice plants.
   • Implications: This adaptation allows plants to reach above the water surface for optimal light capture.
10. Enhancing Respiration Rate
    • Effect: Ethylene increases the respiration rate in ripening fruits, a phenomenon known as the “respiratory climactic.”
    • Mechanism: This accelerated respiration contributes to the ripening process and overall fruit development.
11. Impact on Seedling Growth
    • Effect: Ethylene affects the horizontal growth of seedlings and causes swelling of the axis in dicot seedlings.
    • Implications: This influence can alter seedling architecture and growth patterns.
12. Increased Root Hair Formation
    • Effect: Ethylene enhances root growth and root hair formation.
    • Benefits: Increased root hair development improves the plant’s absorption surface area, aiding in nutrient uptake.

2. Abscisic Acid

Discovery of Abscisic Acid

The discovery of abscisic acid (ABA) is a significant milestone in plant physiology, marking the identification of a crucial growth inhibitor in plants. The historical development of our understanding of ABA can be summarized through the following key points:

1. Initial Identification of Inhibitors
   • Early Research: In the early 1960s, three independent research groups reported the isolation and characterization of substances that exhibited growth-inhibitory properties. These substances were initially identified as Inhibitor B, Abscission II, and Dormin.
   • Chemical Identity: Subsequent research revealed that these three substances were chemically identical, leading to the conclusion that they represented a single compound.
2. Naming and Characterization
   • Naming: The compound was subsequently named abscisic acid. This designation was derived from its role in abscission (the process of leaf and fruit drop) and its chemical nature as an acid.
   • Additional Functions: Besides its role in abscission, abscisic acid was also noted for its ability to induce dormancy in buds, further contributing to its nomenclature. This dual function led to the initial name “Dormin” being used interchangeably with abscisic acid.
3. Key Research Contributions
   • Addicott’s Contribution: In 1963, Addicott and his team isolated a growth-inhibitory substance from young cotton fruits. This substance was found to be strongly antagonistic to growth, aligning with the later findings of abscisic acid. Addicott named this substance Abscissin II, which was later revised to abscisic acid.
   • Antagonism to Gibberellic Acid: Abscisic acid was recognized for its role as an antagonist to gibberellic acid, a growth promoter. This antagonistic relationship is critical in understanding plant growth regulation and responses to environmental conditions.
4. Functional Role in Plants
   • Growth Inhibition: Abscisic acid is known primarily as a growth inhibitor. It plays a crucial role in regulating various physiological processes, including seed dormancy and stress responses.
   • Bud Dormancy: By inducing dormancy in buds, abscisic acid helps plants manage their growth and development in response to environmental conditions.

Physiological Effects of Abscisic Acid

Abscisic acid (ABA) is a crucial plant hormone known for its role in regulating various physiological processes. Its effects are diverse, impacting several aspects of plant growth and development. The primary physiological effects of ABA can be categorized into the following points:

1. Geotropism in Roots
   • Mechanism: ABA influences root geotropism, the directional growth of roots in response to gravity. This effect is mediated through the translocation of ABA towards the root tip in a basipetal direction.
   • Impact: The movement of ABA affects the curvature of roots, ensuring proper growth orientation in relation to gravitational forces.
2. Stomatal Closing
   • Synthesis and Storage: ABA is synthesized and stored in the mesophyll chloroplasts of plant leaves.
   • Response to Water Stress: Under conditions of water stress, the chloroplast membrane loses its permeability, leading to the release of ABA into the cytoplasm of mesophyll cells.
   • Action on Guard Cells: ABA then diffuses into the guard cells surrounding the stomata, promoting stomatal closure. This response helps reduce water loss through transpiration.
3. Other Physiological Effects
   • Bud and Seed Dormancy: ABA plays a critical role in inducing and maintaining dormancy in buds and seeds, aiding in survival during unfavorable conditions.
   • Tuberization: The hormone is involved in the formation of tubers, contributing to the plant’s ability to store energy.
   • Senescence and Abscission: ABA induces senescence (aging) of leaves and fruits, and promotes the abscission (detachment) of leaves, flowers, and fruits.
   • Frost Resistance: ABA increases the tolerance of temperate zone plants to frost injury, enhancing their survival in cold conditions.

Additional Functions

• Regulation of Abscission and Dormancy: ABA regulates the abscission of plant parts and maintains dormancy, ensuring plants can adapt to seasonal changes.
• Inhibition of Growth: The hormone inhibits plant growth, metabolism, and seed germination under stress conditions.
• Stress Response: ABA is often referred to as the ‘stress hormone’ due to its role in increasing plant tolerance to various stress factors.
• Seed Development and Maturation: It is crucial for seed development and maturation, helping seeds withstand desiccation and other unfavorable growth conditions.

6. Other Plant Growth Regulators

Plant growth regulators (PGRs) are crucial for modulating various physiological processes in plants. Among these, brassinosteroids, jasmonates, triacontanol, triazoles, and polyamines play significant roles in plant development and stress responses. This discussion elucidates their functions and effects on plant growth.

Brassinosteroids

• Nature and Occurrence: Brassinosteroids are steroid-based hormones found across various plant taxa, including monocotyledons, dicotyledons, gymnosperms, and algae. These hormones are pivotal in regulating plant growth and development.
• Types and Concentrations: Over 60 types of brassinosteroids have been identified, though only 19 have been thoroughly characterized. Despite their low concentrations in plant tissues—often in nanogram amounts—these hormones are crucial for plant function. For instance, concentrations of 0.5 ng to 10 ng in rice and beans have been shown to be effective.
• Functions: Brassinosteroids influence several growth processes, including seed germination, root development (rhizogenesis), flowering, and senescence. They are involved in basic cellular functions such as nucleic acid and protein synthesis, cell elongation, and stress tolerance. Additionally, brassinosteroids regulate fatty acid composition, amino acid balance, and enhance product translocation, thus aiding in stress resilience and improved fruit quality.

Jasmonates

• Presence and Synthesis: Jasmonates are present throughout the plant kingdom and are synthesized in response to environmental stress, such as wounding or water scarcity. They are found in seeds, flowers, and fruits of higher plants.
• Impact on Plant Processes: Jasmonates affect plant growth and development in various ways. They contribute to seed reserve mobilization, enhance defense mechanisms against pathogens and insects, and influence stress responses. Jasmonates regulate the synthesis of anti-fungal proteins, promote senescence, and modulate callus growth. Their effects are mediated through systemic signaling and interaction with plasma membrane receptors.
• Application and Effects: Exogenous application of jasmonates can improve plant defense mechanisms and stress tolerance. They play a role in inhibiting seedling growth and inducing stomatal closure to reduce water loss.

Triacontanol (TRIA)

• Discovery and Use: Triacontanol was first identified in alfalfa and has since been recognized for its beneficial effects on plant growth. It enhances nitrogen fixation, enzyme activities, and overall plant yield.
• Functions and Benefits: TRIA regulates photosynthesis, improves crop yield, and boosts the production of secondary metabolites in aromatic and medicinal plants. It helps plants maintain growth under abiotic stress conditions such as drought and salinity. By improving enzyme activities and photosynthetic efficiency, TRIA aids in maintaining plant health and productivity.
• Research and Application: The application of TRIA can enhance crop production and quality. Future research should focus on understanding its regulatory mechanisms and optimizing its use in various growth conditions.

Triazoles

• Role in Plant Protection: Triazoles are primarily used as fungicides and are part of plant protection technology. They are effective against fungal diseases while being relatively low in toxicity to mammals.
• Application and Benefits: Triazoles are employed to control plant diseases caused by fungi. Their use should be carefully managed to balance efficacy with environmental impact, ensuring they remain eco-friendly.

Polyamines

• Function and Impact: Polyamines are small, positively charged molecules involved in numerous plant processes. They help stabilize cell walls and membranes, possess antioxidant properties, and play roles in stress response.
• Stress Tolerance: Polyamines contribute to plant defense against various stresses, including drought, salinity, and oxidative stress. They regulate fundamental processes such as cell proliferation, DNA replication, and transcription, and support developmental processes like seed germination, flowering, and fruit ripening.
• Application: Exogenous application of polyamines can improve stress tolerance and enhance crop productivity by modulating stress responses and supporting plant growth under adverse conditions.

Plant Growth Regulator	Function
Auxins	– Stimulate cell elongation and differentiation – Promote apical dominance and lateral root initiation – Regulate phototropism and gravitropism – Involved in early plant development – Stimulate root formation in cuttings – Act as growth stimulants or inhibitors depending on concentration
Gibberellins	– Control stem internode elongation – Stimulate leaf elongation and cell division – Regulate flower development and seed fertility – Break seed dormancy – Enhance seed germination and growth under stress – Improve plant biomass and nitrogen metabolism under stress
Cytokinins	– Promote cell division and lateral growth – Enhance chloroplast maturation and callus formation – Stimulate bud release and root formation – Regulate response to water, light, and nutrient availability – Influence plant stress responses and nutrient mobilization
Abscisic Acid (ABA)	– Regulate stress responses (drought, salinity, etc.) – Control stomatal closure to reduce water loss – Influence seed dormancy and germination – Modulate gene expression and defense mechanisms during stress – Affect leaf abscission and freezing tolerance
Ethylene	– Control cell division and size – Regulate flowering, fruit ripening, and senescence – Stimulate root initiation and secondary metabolite production – Respond to environmental stress conditions (drought, salinity, etc.) – Influence seed germination and dormancy
Brassinosteroids	– Regulate cell elongation, division, and differentiation – Promote seed germination and root development – Enhance flowering, senescence, and fruit quality – Improve stress tolerance and nutrient uptake – Influence fatty acid and amino acid composition
Jasmonates	– Regulate defense responses to biotic stress (insects, pathogens) – Influence seed reserve mobilization and stress response – Promote senescence and stomatal closure – Affect cell mRNA populations, transcription, and translation – Modulate plant growth under stress

Quick Look of plant growth regulators (PGRs)

• Auxins
  • Structure: Indole-3-acetic acid (IAA), 4-Chloroindole-3-acetic acid (4-Cl-IAA)
  • Discovery/History: Discovered in the 1920s; Charles Darwin and his son Francis studied phototropism.
  • Types: Indole-3-acetic acid (IAA), Naphthaleneacetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D)
  • Biosynthesis: Synthesized in the apical meristems, young leaves, and developing fruits.
  • Physiological Effects: Regulates cell elongation, root formation, differentiation. Influences phototropism and gravitropism.
  • Applications and Effects: Used in rooting powders, seedless fruit production, and herbicides.
• Gibberellins
  • Structure: GA1, GA3, GA4, GA7
  • Discovery/History: Discovered in the 1930s from Gibberella fujikuroi.
  • Types: Gibberellic Acid (GA3), GA1, GA4, GA7
  • Biosynthesis: Produced in young tissues, seeds, and developing fruits.
  • Physiological Effects: Promotes stem elongation, seed germination, flowering. Affects fruit development and size.
  • Applications and Effects: Applied to increase crop yields, enhance seed germination, and induce flowering.
• Cytokinins
  • Structure: Zeatin, Kinetin
  • Discovery/History: Identified in the 1950s from coconut milk and maize.
  • Types: Zeatin, Kinetin, Benzyladenine (BA)
  • Biosynthesis: Synthesized in root tips, seeds, and developing fruits.
  • Physiological Effects: Stimulates cell division, promotes shoot development, delays leaf senescence.
  • Applications and Effects: Used to promote shoot growth, delay aging in plants, and enhance fruit set.
• Abscisic Acid (ABA)
  • Structure: C15H20O4
  • Discovery/History: Discovered in the 1960s from cotton plants.
  • Types: ABA, ABA-aldehyde
  • Biosynthesis: Synthesized in mature leaves, roots, and seeds.
  • Physiological Effects: Regulates stomatal closure, seed dormancy, and responses to stress.
  • Applications and Effects: Applied to manage water stress, improve seed storage, and enhance drought tolerance.
• Ethylene
  • Structure: C2H4
  • Discovery/History: Identified in the 1900s; studied extensively in the 1970s.
  • Types: Ethylene Gas
  • Biosynthesis: Produced in all plant tissues, especially during fruit ripening and stress conditions.
  • Physiological Effects: Influences fruit ripening, leaf abscission, and senescence.
  • Applications and Effects: Used to ripen fruits, control flowering, and manage plant growth.
• Triacontanol (TRIA)
  • Structure: C30H62O
  • Discovery/History: Discovered in alfalfa in 1977.
  • Types: Triacontanol
  • Biosynthesis: Synthesized in plants, particularly in leaves and stems.
  • Physiological Effects: Enhances nitrogen fixation, enzyme activities, and overall growth. Regulates photosynthesis and stress responses.
  • Applications and Effects: Applied to improve yield, growth, and quality of crops; beneficial for medicinal and aromatic plants under stress conditions.
• Triazoles
  • Structure: Various (e.g., Tebuconazole, Propiconazole)
  • Discovery/History: Developed as fungicides in the 1980s.
  • Types: Tebuconazole, Propiconazole
  • Biosynthesis: Chemically synthesized.
  • Physiological Effects: Controls fungal diseases; less toxic to mammals.
  • Applications and Effects: Used as fungicides in agriculture to protect crops from fungal infections.
• Polyamines
  • Structure: Putrescine, Spermidine, Spermine
  • Discovery/History: Discovered in the 1950s; roles in plant growth identified in the 1970s.
  • Types: Putrescine, Spermidine, Spermine
  • Biosynthesis: Synthesized from amino acids like arginine and ornithine.
  • Physiological Effects: Regulates growth processes, stress responses, and aging. Involved in cell division, seed germination, stress tolerance.
  • Applications and Effects: Applied to improve stress tolerance and crop productivity; supports plant development and stress management.

Functions of Plant Growth Regulators

Here’s he functions of various plant growth regulators (PGRs):

• Auxins
  • Cell Elongation: Promotes cell elongation in stems and roots.
  • Root Formation: Stimulates the formation of adventitious roots in cuttings.
  • Phototropism and Gravitropism: Regulates plant growth responses to light and gravity.
  • Fruit Development: Helps in the formation of seedless fruits.
  • Leaf Abscission: Influences the process of leaf and fruit drop.
• Gibberellins
  • Stem Elongation: Promotes internode elongation, leading to taller plants.
  • Seed Germination: Stimulates seed germination and breaking of dormancy.
  • Flowering: Induces flowering in some plants and can influence flower development.
  • Fruit Development: Enhances fruit size and growth, often used in seedless grape production.
• Cytokinins
  • Cell Division: Stimulates cell division and differentiation, leading to increased shoot growth.
  • Shoot Development: Promotes the growth of lateral buds and shoots.
  • Leaf Senescence: Delays leaf aging and promotes chlorophyll retention.
  • Nutrient Mobilization: Aids in the mobilization of nutrients within the plant.
• Abscisic Acid (ABA)
  • Stomatal Closure: Regulates the closure of stomata to reduce water loss during drought stress.
  • Seed Dormancy: Induces and maintains seed dormancy until conditions are favorable for germination.
  • Stress Responses: Mediates plant responses to environmental stresses such as drought, salinity, and cold.
• Ethylene
  • Fruit Ripening: Accelerates the ripening process in fruits.
  • Leaf Abscission: Promotes the shedding of leaves, flowers, and fruits.
  • Senescence: Regulates the aging process in plants, leading to increased senescence in various plant parts.
  • Stress Responses: Plays a role in plant responses to mechanical stress, wounding, and pathogen attack.
• Triacontanol (TRIA)
  • Growth Promotion: Enhances overall plant growth and yield.
  • Nitrogen Fixation: Improves nitrogen fixation in legumes.
  • Enzyme Activities: Stimulates enzyme activities related to growth and metabolism.
  • Stress Tolerance: Helps plants cope with abiotic stresses such as drought, salinity, and heavy metal toxicity.
  • Photosynthesis: Enhances photosynthetic efficiency and chlorophyll content.
• Triazoles
  • Fungal Disease Control: Effective in controlling fungal diseases by inhibiting fungal sterol synthesis.
  • Crop Protection: Used to protect crops from various fungal infections.
• Polyamines
  • Cell Growth and Division: Regulates cell proliferation, DNA replication, and membrane stability.
  • Stress Tolerance: Improves tolerance to abiotic stresses such as drought, salinity, and heavy metals.
  • Developmental Processes: Involved in seed germination, flowering, fruit ripening, and plant morphogenesis.
  • Anti-Senescence: Possesses antioxidant properties that help delay senescence and promote plant longevity.