Plant Hormones - BIO 306 2025 PDF

**Plant hormones** Plant hormones (also known as phytohormones) are [chemicals](https://en.wikipedia.org/wiki/Chemical) that regulate [plant](https://en.wikipedia.org/wiki/Plant) growth. Plant [hormones](https://en.wikipedia.org/wiki/Hormones) are [signal molecules](https://en.wikipedia.org/wiki/Si...

**Plant hormones** Plant hormones (also known as phytohormones) are [chemicals](https://en.wikipedia.org/wiki/Chemical) that regulate [plant](https://en.wikipedia.org/wiki/Plant) growth. Plant [hormones](https://en.wikipedia.org/wiki/Hormones) are [signal molecules](https://en.wikipedia.org/wiki/Signal_molecule) produced within the [plant](https://en.wikipedia.org/wiki/Plant), and occur in extremely low [concentrations](https://en.wikipedia.org/wiki/Concentration). Hormones regulate cellular processes in targeted [cells](https://en.wikipedia.org/wiki/Cell_%28biology%29) locally and, moved to other locations, in other functional parts of the plant. [Hormones](https://en.wikipedia.org/wiki/Hormones) also determine the formation of [flowers](https://en.wikipedia.org/wiki/Flowers), [stems](https://en.wikipedia.org/wiki/Plant_stem), [leaves](https://en.wikipedia.org/wiki/Leaves), the [shedding](https://en.wikipedia.org/wiki/Moulting) of [leaves](https://en.wikipedia.org/wiki/Leaves), and the development and ripening of [fruit](https://en.wikipedia.org/wiki/Fruit). Plants, unlike [animals](https://en.wikipedia.org/wiki/Animal), lack [glands](https://en.wikipedia.org/wiki/Gland) that produce and [secrete](https://en.wikipedia.org/wiki/Secrete) hormones. Instead, each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of [flowering](https://en.wikipedia.org/wiki/Flowering), the sex of flowers, [senescence](https://en.wikipedia.org/wiki/Senescence) of leaves, and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant [longevity](https://en.wikipedia.org/wiki/Longevity), and even plant [death](https://en.wikipedia.org/wiki/Death). Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones. Phytohormones are found not only in [higher plants](https://en.wikipedia.org/wiki/Higher_plant) but in [algae](https://en.wikipedia.org/wiki/Algae), showing similar functions, and in [microorganisms](https://en.wikipedia.org/wiki/Microorganism), such as unicellular [fungi](https://en.wikipedia.org/wiki/Fungus) and [bacteria](https://en.wikipedia.org/wiki/Bacteria), but in these cases they play no hormonal or other immediate physiological role in the producing organism and can, thus, be regarded as [secondary metabolites](https://en.wikipedia.org/wiki/Secondary_metabolites). Plant hormones are not [nutrients](https://en.wikipedia.org/wiki/Nutrient), but [chemicals](https://en.wikipedia.org/wiki/Chemical) that in small amounts promote and influence the growth, development, and differentiation of cells and [tissues](https://en.wikipedia.org/wiki/Tissue_%28biology%29). The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized. Plants lack glands to produce and store hormones, because, unlike animals-which have two circulatory systems ([lymphatic](https://en.wikipedia.org/wiki/Lymphatic) and [cardiovascular](https://en.wikipedia.org/wiki/Cardiovascular)) powered by a [heart](https://en.wikipedia.org/wiki/Heart) that moves fluids around the body---plants use more passive means to move chemicals around their bodies. Plants utilize simple chemicals as hormones, which move more easily through their tissues. They are often produced and used on a local basis within the plant body. Plant cells produce hormones that affect even different regions of the cell producing the hormone. Hormones are transported within the plant by utilizing four types of movements. For localized movement, [cytoplasmic](https://en.wikipedia.org/wiki/Cytoplasm) streaming within cells and slow diffusion of [ions](https://en.wikipedia.org/wiki/Ion) and [molecules](https://en.wikipedia.org/wiki/Molecule) between cells are utilized. Vascular tissues are used to move hormones from one part of the plant to another; these include [sieve tubes](https://en.wikipedia.org/wiki/Sieve_tube) or [phloem](https://en.wikipedia.org/wiki/Phloem) that move [sugars](https://en.wikipedia.org/wiki/Sugar) from the leaves to the [roots](https://en.wikipedia.org/wiki/Root) and flowers, and [xylem](https://en.wikipedia.org/wiki/Xylem) that moves water and mineral solutes from the roots to the [foliage](https://en.wikipedia.org/wiki/Foliage). **Classes** In general, it is accepted that there are five major classes of plant hormones, some of which are made up of many different chemicals that can vary in structure from one plant to the next. The chemicals are each grouped together into one of these classes based on their structural similarities and on their effects on plant physiology. Other plant hormones and growth regulators are not easily grouped into these classes; they exist naturally or are synthesized by humans or other organisms, including chemicals that inhibit plant growth or interrupt the physiological processes within plants. Each class has positive as well as inhibitory functions, and most often work in tandem with each other, with varying ratios of one or more interplaying to affect growth regulation. The five major classes are: **Abscisic acid** [Abscisic acid](https://en.wikipedia.org/wiki/Abscisic_acid) (also called ABA) is one of the most important plant growth regulators. The name \"abscisic acid\" was given because it was found in high concentrations in newly abscissed or freshly fallen leaves. It acts as an inhibitory chemical compound that affects [bud](https://en.wikipedia.org/wiki/Bud) growth, and seed and bud dormancy. It mediates changes within the apical meristem, causing bud dormancy and the alteration of the last set of leaves into protective bud covers.. Abscisic acid\'s effects are degraded within plant tissues during cold temperatures or by its removal by water washing in and out of the tissues, releasing the seeds and buds from dormancy. In plants under water stress, ABA plays a role in closing the [stomata](https://en.wikipedia.org/wiki/Stomata). **Auxins** https://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/Indol-3-ylacetic\_acid.svg/155px-Indol-3-ylacetic\_acid.svg.png The auxin indole-3-acetic acid [Auxins](https://en.wikipedia.org/wiki/Auxin) are compounds that positively influence cell enlargement, bud formation and root initiation. They also promote the production of other hormones and in conjunction with [cytokinins](https://en.wikipedia.org/wiki/Cytokinin), they control the growth of stems, roots, and fruits, and convert stems into flowers. Auxins were the first class of growth regulators discovered. They affect cell elongation by altering cell wall plasticity. The most common auxin found in plants is [indole-3-acetic acid](https://en.wikipedia.org/wiki/Indole-3-acetic_acid) or IAA. **Cytokinins**  The cytokinin [zeatin](https://en.wikipedia.org/wiki/Zeatin), the name is derived from [*Zea*](https://en.wikipedia.org/wiki/Teosinte) [maize](https://en.wikipedia.org/wiki/Maize), in which it was first discovered in immature kernels. [Cytokinins](https://en.wikipedia.org/wiki/Cytokinin) or CKs are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay [senescence](https://en.wikipedia.org/wiki/Senescence) of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. 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; they in conjunction with ethylene promote abscission of leaves, flower parts, and fruits. **Ethylene** https://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/Ethene-2D-flat.png/100px-Ethene-2D-flat.png [Ethylene](https://en.wikipedia.org/wiki/Ethylene) is a gas that forms through the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant. Its effectiveness as a plant hormone is dependent on its rate of production versus its rate of escaping into the atmosphere. Ethylene is produced at a faster rate in rapidly growing and dividing cells, especially in darkness. As the new shoot is exposed to light, reactions by [phytochrome](https://en.wikipedia.org/wiki/Phytochrome) in the plant\'s cells produce a signal for ethylene production to decrease, allowing leaf expansion. **Gibberellins**  **Main function:** Initiate mobilization of storage materials in seeds during germination, cause elongation of stems, stimulate bolting in biennials, stimulate pollen tube growth. [Gibberellins](https://en.wikipedia.org/wiki/Gibberellin), or GAs, include a large range of chemicals that are produced naturally within plants and by fungi. They promote flowering, cellular division, and in seeds growth after germination. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA. **Development of Roots** 1\. Embryonic Development: The root system develops from the radicle, a structure that forms during embryogenesis. 2\. Primary Root Growth: The primary root grows downward into the soil, driven by cell division and elongation in the root apical meristem. 3\. Secondary Root Growth: Secondary roots develop from the primary root, increasing the root system\'s surface area and absorption capacity. 4\. Root Hairs: Root hairs are specialized cells that increase the root\'s surface area, allowing for greater water and nutrient absorption. 5\. Root Cap: The root cap protects the root apical meristem and helps guide the root\'s growth downward. **Development of Stems** 1\. Embryonic Development: The shoot system develops from the hypocotyl, a structure that forms during embryogenesis. 2\. Primary Stem Growth: The primary stem grows upward, driven by cell division and elongation in the shoot apical meristem. 3\. Secondary Stem Growth: Secondary stems develop from the primary stem, increasing the plant\'s height and branching. 4\. Node and Internode Formation: Nodes are the points on the stem where leaves attach, while internodes are the stem segments between nodes. 5\. Leaf Formation: Leaves develop from the shoot apical meristem, with the leaf primordia forming at the node. **Development of Leaves** 1\. Leaf Primordia Formation: Leaf primordia form at the node, driven by cell division and differentiation. 2\. Leaf Expansion: Leaves expand through cell division and elongation, increasing their surface area. 3\. Leaf Differentiation: Leaves differentiate into distinct tissues, including the epidermis, mesophyll, and vascular tissue. 4\. Leaf Venation: Leaf veins form through the differentiation of vascular tissue, allowing for the transport of water, nutrients, and sugars. 5\. Leaf Morphogenesis: Leaves undergo morphogenesis, taking on their final shape and form through a combination of cell division, expansion, and differentiation. **Apical Growth** 1\. Definition: Apical growth refers to the growth of plants from the tips of roots and shoots, driven by cell division and elongation in the apical meristems. 2\. Apical Meristems: Apical meristems are regions of undifferentiated cells located at the tips of roots and shoots, responsible for producing new cells that contribute to plant growth. 3\. Types of Apical Growth: There are two types of apical growth: 1\. Primary Growth: Primary growth occurs in the primary meristems, resulting in the elongation of roots and shoots. 2\. Secondary Growth: Secondary growth occurs in the secondary meristems, resulting in the widening of roots and shoots. **Apical Dominance** 1\. Definition: Apical dominance refers to the phenomenon where the apical meristem inhibits the growth of lateral buds, resulting in the dominance of the main shoot or root. 2\. Hormonal Regulation: Apical dominance is regulated by plant hormones, particularly auxins, which are produced in the apical meristem and transported to the lateral buds, inhibiting their growth. 3\. Types of Apical Dominance: There are two types of apical dominance: 1\. Shoot Apical Dominance: Shoot apical dominance refers to the inhibition of lateral bud growth by the apical meristem of the shoot. 2\. Root Apical Dominance: Root apical dominance refers to the inhibition of lateral root growth by the apical meristem of the root. **Factors Affecting Apical Dominance** 1\. Light: Light intensity and quality can affect apical dominance, with high light intensities promoting apical dominance. 2\. Nutrient Availability: Nutrient availability can affect apical dominance, with nutrient deficiencies reducing apical dominance. 3\. Hormone Balance: Hormone balance, particularly auxin and cytokinin balance, can affect apical dominance. 4\. Pruning and Training: Pruning and training can affect apical dominance, with pruning reducing apical dominance and promoting lateral bud growth. **Importance of Apical Dominance** 1\. Plant Architecture: Apical dominance plays a crucial role in determining plant architecture, with apical dominance influencing the shape and size of plants. 2\. Yield and Productivity: Apical dominance can affect yield and productivity, with apical dominance influencing the number and size of fruits and flowers. 3\. Plant Defense: Apical dominance can affect plant defense, with apical dominance influencing the production of defense compounds. **Growth Rhythms and Correlations** 1\. Growth Rhythms: Plants exhibit rhythmic growth patterns, influenced by internal biological clocks and external environmental factors. 2\. Circadian Rhythms: Daily growth rhythms, regulated by light-dark cycles, influence stem elongation, leaf movement, and stomatal opening. 3\. Ultradian Rhythms: Short-term growth rhythms, occurring within a 24-hour period, influence cell division and expansion. 4\. Growth Correlations: Correlations exist between different plant parts, such as between root and shoot growth, influenced by hormonal signals and nutrient availability. **Methods of Growth Analysis** 1\. Growth Rate Analysis: Measures the rate of growth over time, using parameters such as increase in length, weight, or area. 2\. Growth Stage Analysis: Identifies specific stages of growth, such as germination, seedling establishment, and reproductive development. 3\. Allometric Analysis: Studies the relationships between different plant parts, such as between leaf area and stem length. 4\. Morphometric Analysis: Examines the shape and size of plant structures, such as leaf morphology and branching patterns. **Factors Controlling Plant Growth: Temperature** 1\. Temperature Effects: Temperature influences plant growth, with optimal temperatures varying among species. 2\. Temperature Thresholds: Plants have minimum, optimum, and maximum temperature thresholds for growth, beyond which growth is impaired. 3\. Thermal Time: The accumulation of heat units over time influences plant growth and development. 4\. Chilling and Freezing Injury: Low temperatures can damage or kill plants, depending on the severity and duration of the cold stress. **Factors Controlling Plant Growth: Tropisms** 1\. Phototropism: Growth response to light, influencing stem elongation and leaf orientation. 2\. Gravitropism: Growth response to gravity, influencing root growth and stem orientation. 3\. Thigmotropism: Growth response to touch, influencing stem twining and leaf movement. 4\. Chemotropism: Growth response to chemicals, influencing root growth and nutrient uptake. Other Factors Controlling Plant Growth 1\. Light: Intensity, quality, and duration of light influence plant growth and development. 2\. Water: Availability and quality of water influence plant growth, with drought and flooding affecting growth and survival. 3\. Nutrients: Availability and balance of essential nutrients influence plant growth, with deficiencies or excesses affecting growth and development. 4\. Hormones: Plant hormones, such as auxins, gibberellins, and cytokinins, regulate plant growth and development, influencing cell division, expansion, and differentiation. **Inhibitors/Inhibitions** 1\. Definition: Inhibitors are substances that slow down or stop plant growth and development. 2\. Types of Inhibitors: Abscisic acid (ABA), ethylene, and phenolic compounds are examples of inhibitors that affect plant growth. 3\. Effects of Inhibitors: Inhibitors can affect seed germination, root growth, stem elongation, and leaf expansion. **Osmotic Pressure** 1\. Definition: Osmotic pressure is the pressure exerted by a solution to prevent water from entering the solution through a semipermeable membrane. 2\. Importance in Plants: Osmotic pressure helps maintain cell turgor pressure, which is essential for plant growth and support. 3\. Factors Affecting Osmotic Pressure: Temperature, solute concentration, and membrane permeability affect osmotic pressure. **Water Potentials and Tension in the Cell Wall** 1\. Definition: Water potential is the potential energy of water in a system, while tension in the cell wall refers to the force exerted by the cell wall on the cell contents. 2\. Importance in Plants: Water potential and cell wall tension help maintain cell turgor pressure and support plant growth. 3\. Factors Affecting Water Potentials and Cell Wall Tension: Solute concentration, temperature, and cell wall properties affect water potentials and cell wall tension. **Permeability** 1\. Definition: Permeability refers to the ability of a membrane to allow substances to pass through. 2\. Importance in Plants: Permeability is essential for water and mineral absorption and movement in plants. 3\. Factors Affecting Permeability: Temperature, pH, and membrane properties affect permeability. **Water and Mineral Absorption and Movement in Plant Cell Walls** 1\. Pathways for Water and Mineral Absorption: Water and minerals enter plant cells through the apoplast (cell wall) or symplast (cytoplasm). 2\. Role of Cell Membranes: Cell membranes regulate the movement of water and minerals into and out of plant cells. 3\. Factors Affecting Water and Mineral Absorption: Temperature, pH, and membrane properties affect water and mineral absorption. **Transportation and Translocation in Plants** 1\. Transportation: The movement of water, minerals, and sugars throughout the plant body. 2\. Translocation: The movement of organic compounds, such as sugars and amino acids, from one part of the plant to another. **Types of Transportation:** \- Xylem Transport: Water and minerals are transported from roots to leaves through the xylem. \- Phloem Transport: Sugars and other organic compounds are transported from leaves to roots and other parts of the plant through the phloem. 4\. Mechanisms of Transportation: \- Osmosis: Water moves into plant cells through osmosis. \- Active Transport: Ions and other substances are transported against their concentration gradient using energy. **Flowering, Seed and Fruit Formation, and Fruit Ripening** 1\. Flowering: The process by which plants produce flowers, which are the reproductive structures of plants. 2\. Seed Formation: Seeds are formed after fertilization, and they contain the embryo of a new plant. 3\. Fruit Formation: Fruits are formed from the ovary of a flower, and they contain seeds. 4\. Fruit Ripening: Fruits ripen due to a series of biochemical reactions that involve the breakdown of cell walls and the production of ethylene gas. **Photoperiodism** 1\. Definition: Photoperiodism is the response of plants to the length of daylight. 2\. Effects of Light: \- Long-Day Plants: These plants require long days to flower. \- Short-Day Plants: These plants require short days to flower. \- Day-Neutral Plants: These plants flower regardless of the length of daylight. 3\. Mechanisms of Photoperiodism: \- Phytochrome: A pigment that responds to light and triggers a series of biochemical reactions. \- Circadian Rhythms: Internal biological clocks that regulate plant growth and development. **Abscission** 1\. Definition: Abscission is the process by which plants shed their leaves, fruits, or flowers. 2\. Mechanisms of Abscission: \- Ethylene Production: Ethylene gas is produced in response to stress or aging, triggering abscission. \- Cell Wall Breakdown: The cell walls of the petiole or pedicel break down, allowing the leaf or fruit to fall off. 3\. Importance of Abscission: Abscission allows plants to conserve energy and resources by shedding unnecessary or damaged tissues.