Plant Hormones

Phytohormones, Growth Regulators, and Environmental Cues

Factors Involved in Plant Growth

  • Light: Source of energy from the sun.

  • Carbon Dioxide: Obtained from the air, used in photosynthesis.

  • Water and Minerals: Absorbed from the soil, essential for various physiological processes.

  • Plant Hormones (Phytohormones):- Derived from the Greek word "horman" meaning "to set in motion."

    • Mediate interactions between plants and the environment, serving as chemical messengers.

    • Definition by Frits Went and Kenneth Thimann (1937): "…characterized by the property of serving as chemical messengers, by which the activity of certain organs is coordinated with that of others."

Phytohormones

  • Control cellular activities (division, elongation, differentiation) and processes like:- Pattern formation

    • Sex determination

    • Organogenesis

    • Responses to abiotic (non-living) and biotic (living) stressors.

Environmental Cues for Plant Growth

  • Influential factors include:- Light:

    • Intensity

    • Wavelength

    • Duration

    • Gravity

    • Temperature

    • Water

    • Nutrients

    • Touch

    • Other Organisms

  • Types of responses:- Morphogenic responses

    • Tropic responses (e.g., phototropism, gravitropism)

    • Nastic responses (non-directional movements)

    • Localized cellular responses

    • Systemic responses (affecting the whole plant).

Phytohormones and the Plant Life Cycle

  • Phytohormones regulate all stages of the plant life cycle including:- Fertilization and fruit formation

    • Seed dormancy

    • Embryogenesis

    • Fruit ripening

    • Germination

    • Flower development

    • Growth and branching.

Characteristics of Plant Hormones

  • Nature: - Small organic compounds that affect physiological processes at low concentrations (less than 10−710^{-7} Molar).

    • Can act where they are produced or be transported to other parts of the plant.

  • Processes Regulated:- Cell division

    • Seed dormancy

    • Cell enlargement

    • Germination

    • Cell differentiation

    • Senescence

    • Flowering

    • Leaf abscission

    • Fruit maturation

    • Stomatal conductance

    • Movement (tropisms).

Inorganic Compounds vs. Phytohormones

  • Distinction:- Plant hormones are NOT inorganic compounds.

    • Minerals (e.g., Ca2+Ca^{2+} and K+K^{+}) can cause physiological responses but are not synthesized by the plant; hence not hormones.

    • Example of Sugars:

    • Sucrose is not a hormone but must be present in high concentrations for growth (greater than 1 mM up to 50 mM).

Plant vs. Animal Hormones

  1. Plant hormones are small, simple molecules; animal hormones tend to be larger and more complex (proteins, peptides).

  2. Plant hormones are not produced in specialized glands; they can be distributed across the plant.

  3. Different tissues may respond differently to the same hormone.

  4. Plant hormones interact with each other to determine the overall response.

  5. Unlike animal hormones, plant hormones are not regulated by a central nervous system.

Mechanism of Hormone Action

  1. Synthesis:- Hormones are often stored as inert conjugates.

  2. Binding (Docking):- Hormones bind to specific protein receptors typically located on the cell membrane.

    • Cells lacking the receptor will not respond to the hormone.

  3. Signal Transduction:- Initiated by receptor binding, involving various proteins and enzymes within the cell.

    • Calcium (Ca2+^{2+}) often acts as a second messenger.

  4. Physiological Response:- Changes in gene expression, enzyme activities, and ion gradients may occur.

Classes of Phytohormones

  • Old Timers and Newcomers:- Auxin

    • Cytokinin

    • Gibberellin

    • Abscisic Acid

    • Ethylene

    • Brassinosteroid

    • Salicylic Acid

    • Jasmonic Acid

    • Strigolactone

Key Characteristics of Plant Hormones

  1. Name of the five major classes of hormones.

  2. Location of production.

  3. Mechanism of transport.

  4. Chemical precursors.

  5. Major physiological effects.

Auxin (Indoleacetic Acid)

  • Production: Synthesized in shoot and root apical meristems, young leaves, seeds in developing fruits.

  • Synthesis: Derived from the amino acid tryptophan.

  • Functions:- Cell elongation and expansion

    • Suppression of lateral bud growth

    • Initiation of adventitious roots

    • Vascular differentiation

    • Tropisms (photo-, gravi-, thigmo-).

Auxin and Phototropism

  • Early experiments exhibited that the shoot tip acts as a receptor for light.

  • Conclusion: A substance produced in the plant tip diffuses into the shoot and induces bending toward light.

Demonstration of Auxin Activity

  • Experiments by Frits Went (1920's):- Identified a growth-promoting chemical from coleoptile tips, named auxin (from "auxein" meaning "to increase").

    • Indoleacetic acid (IAA) was later identified as the major auxin in plants.

Mechanism of Auxin Action: The Acid Growth Hypothesis

  • Upon auxin binding to its receptor, it activates a H$^+$-ATPase pump, expelling protons from the cell.

  • The increase in acidity activates an enzyme that loosens cellulose fibers in the cell wall, allowing for cell expansion under turgor pressure.

Polar Transport of Auxin

  • Auxin is transported downwards from the shoot apex.

  • In light-exposed seedlings, IAA concentration increases on the shaded side of the shoot, leading to curvature towards the light.

  • Auxin does not diffuse but exits the cell at its basal end through specific carrier proteins, undergoing polar transport.

Cytokinin

  • Production: Synthesized in root meristems, young leaves, fruits, and seeds.

  • Synthesis: Derived from purine base adenine.

  • Functions:- Cell division (cyclokinin)

    • Delays senescence (aging)

    • Promotes shoot development

    • Stimulates lateral bud growth

    • Regulates auxin action.

Effects of Cytokinins

  • Delay leaf senescence significantly in kinetin-treated leaves or leaves with higher cytokinin levels.

  • Interaction with Auxins: - Inhibits branching in the shoot, promotes branching in the root.

    • Acts synergistically and antagonistically with auxins to control apical dominance and branching patterns.

Control of Apical Dominance by Auxin and Cytokinin

  • Apical dominance refers to the suppression of lateral bud growth due to the activity of the shoot apical meristem.- Auxin (from the shoot tip) inhibits lateral bud growth.

    • Cytokinin (from the roots) stimulates growth of lateral buds and branches.

  • Practical note: Pinching shoot tips can enhance bushiness by promoting lateral branch growth.

Gibberellins

  • Associated with the "foolish seedling" disease in rice, which is caused by the Gibberella fungus.

  • Approximately 80 naturally occurring gibberellins.

  • Production: Present in all higher plants, mosses, algae, fungi, and bacteria.

  • Synthesis: Derived from the terpenoid pathway.

  • Functions: - Promotes stem elongation and growth

    • Stimulates seed and bud germination

    • Affects flowering and enhances fruit growth.

Gibberellin-Related Phenotypes

  • Gibberellin-deficient plants exhibit a dwarf phenotype.

  • Examples include genetically dwarf peas.

Effects of Gibberellin Application

  • Gibberellins can help produce seedless fruits, facilitate bolting, and impact many aspects of plant growth physiology.

  • Seed Germination Process:1. Water uptake triggers GA synthesis by the embryo.

    1. GA stimulates amylase production in the aleurone layer.

    2. Amylase catalyzes the breakdown of starch in the endosperm.

    3. Sugars generated fuel embryo growth.

Importance of Gibberellins in Agriculture

  • The manipulation of gibberellin levels is a vital part of modern agriculture.

  • Norman Borlaug (1914-2009), a distinguished breeder, developed semi-dwarf grain varieties that are deficient in gibberellin synthesis or response; a significant 20th-century achievement.

Abscisic Acid (ABA)

  • Function: Known as the stress hormone, primarily influencing leaf and stomatal closure under water stress.

  • Synthesis: Derived from carotenoids via the terpenoid pathway.

  • Characteristics:- General growth inhibitor.

    • Maintains dormancy in seeds and buds.

    • Promotes desiccation tolerance.

    • Exhibits antagonistic effects against gibberellin and ethylene.

Ethylene

  • Gaseous hormone that diffuses rapidly, affecting neighboring plants.

  • Production: Synthesized in ripening fruits, wilting flowers, and senescing leaves from the amino acid methionine.

  • Autocatalytic: Ethylene stimulates its own production.

  • Functions: - Involved in leaf abscission.

    • Ethephon can be used commercially to promote ripening.

Ethylene in Fruit Ripening

  • Ethylene promotes ripening in many plants, and its effects can be blocked by mutations affecting ethylene receptor genes in tomatoes.

  • Molecular Biology & Genetics: Used to study ripening modifications in tomatoes.

Summary of Phytohormone Effects on Plant Growth

  • Hormones like auxin, cytokinins, and gibberellins facilitate growth and developmental processes.

  • Auxin and cytokinin dictate branching patterns of plants.

  • Gibberellins and ethylene play roles in flowering processes.

  • Growth and ripening of fruits are modulated by auxins, gibberellins, and ethylene.

  • Seed maturation and germination are influenced by abscisic acid and gibberellins.

  • Senescence of leaves and fruits is regulated by cytokinins and ethylene; ABA is critical for stress responses.

Overview of Major Plant Hormones

Auxin

  • Production: Meristems of apical buds, embryo of seed, young leaves.

  • Major Functions: Stimulates cell elongation and is involved in multiple growth responses.

Cytokinin

  • Production: Synthesized in roots and transported to other tissues.

  • Major Functions: Stimulates cell division and delays leaf senescence.

Ethylene

  • Production: Found in ripening fruits and senescent leaves.

  • Major Functions: Drives fruit ripening, leaf and flower senescence, and abscission.

Abscisic Acid

  • Production: Synthesized in leaves, stems, fruits.

  • Major Functions: Inhibits growth, closes stomata, maintains dormancy.

Gibberellin

  • Production: Produced in meristems and influences shoot and fruit growth.

  • Major Functions: Stimulates shoot elongation and bolting in biennials, regulates enzyme production in grains.

Photoperiodism

  • Importance: Involves flowering, seed germination, and dormancy control based on day/night length.

  • Phytochrome: A protein pigment in plant cells that exists in active and inactive forms, enabling light detection necessary for regulating plant responses to photoperiod (short-day versus long-day plants).

Flowering and Photoperiodism

  • Flowering in many plants is influenced by the photoperiod, evidenced in species such as:- Short Day Plants: Aster, Chrysanthemum, Goldenrod

    • Long Day Plants: Lettuce, Spinach, Mustard.