PPHYS 531 Hormonal Regulation of Plant Growth and Development 3(2+1)

Definition and classification of plant growth regulators- Hormones

Plant growth regulators (PGRs), also known as plant hormones or phytohormones, are naturally occurring organic compounds that influence physiological processes in plants at very low concentrations. These substances play crucial roles in regulating various aspects of plant growth and development. Plant hormones can be classified into several major groups based on their functions and chemical structures:

1. Auxins:

   • Function: Auxins are involved in the regulation of cell elongation, apical dominance, and tropisms (response to environmental stimuli like light and gravity). They also play a role in root development and the formation of lateral roots.
   • Examples: Indole-3-acetic acid (IAA), NAA (1-Naphthaleneacetic acid).

2. Cytokinins:

   • Function: Cytokinins promote cell division and influence shoot development. They are essential for the control of apical dominance, leaf senescence, and the formation of chloroplasts.
   • Examples: Zeatin, kinetin.

3. Gibberellins:

   • Function: Gibberellins are involved in the regulation of stem elongation, seed germination, and flowering. They also influence fruit development and play a role in breaking seed dormancy.
   • Examples: Gibberellic acid (GA3), Gibberellin A1.

4. Abscisic Acid (ABA):

   • Function: ABA is primarily associated with stress responses and the regulation of seed dormancy and germination. It also plays a role in stomatal closure, helping plants conserve water during drought conditions.
   • Examples: Abscisic acid.

5. Ethylene:

   • Function: Ethylene is involved in various aspects of plant growth and development, including fruit ripening, senescence, and response to environmental stresses.
   • Example: Ethylene.

6. Brassinosteroids:

   • Function: Brassinosteroids are involved in promoting cell elongation, cell division, and differentiation. They also play a role in seed germination and responses to environmental stresses.
   • Examples: Brassinolide, castasterone.

7. Jasmonates:

   • Function: Jasmonates are involved in plant defense responses to biotic stress (such as herbivory and pathogen attack). They also play a role in root growth and development.
   • Examples: Jasmonic acid, methyl jasmonate.

8. Salicylic Acid (SA):

   • Function: Salicylic acid is involved in the regulation of plant defense responses against pathogens. It plays a key role in systemic acquired resistance (SAR).
   • Example: Salicylic acid.

9. Polyamines:

• Function: Polyamines are involved in cell division, elongation, and differentiation. They also play a role in stress responses and senescence.
• Examples: Putrescine, spermidine, spermine.

Endogenous growth substances and synthetic chemicals

Plant growth substances, also known as plant hormones or phytohormones, can be broadly categorized into two groups: endogenous growth substances and synthetic chemicals.

1. Endogenous Growth Substances: Endogenous growth substances are naturally occurring compounds within plants that regulate various physiological processes. The major types of endogenous growth substances are plant hormones, which were mentioned in the previous response. These include auxins, cytokinins, gibberellins, abscisic acid, ethylene, brassinosteroids, jasmonates, and salicylic acid. These substances are produced by plants in response to internal developmental signals or external environmental cues. They play essential roles in coordinating growth, development, and responses to environmental stimuli.

2. Synthetic Chemicals: Synthetic chemicals refer to artificially produced substances that can influence plant growth and development. These chemicals are often created to mimic the effects of natural plant hormones or to modify specific aspects of plant physiology. Some synthetic chemicals are designed for agricultural and horticultural applications to enhance crop productivity, control plant growth, or provide protection against pests and diseases. Here are a few examples:

• Plant Growth Regulators (PGRs): These are synthetic chemicals that mimic the functions of natural plant hormones. Common PGRs include synthetic auxins (e.g., 2,4-D), synthetic cytokinins (e.g., kinetin), and synthetic gibberellins.

• Herbicides: These are chemicals designed to control or eliminate unwanted vegetation. Some herbicides may interfere with specific plant hormones or metabolic pathways, disrupting normal growth processes.

• Fungicides: Fungicides are chemicals used to control fungal diseases in plants. They may target specific metabolic pathways or cellular processes in fungi, protecting plants from infection.

• Insecticides: Insecticides are substances that control or eliminate insect pests. They can target various physiological processes in insects, disrupting their development, feeding, or reproductive capabilities.

• Biostimulants: These are substances that enhance plant growth, yield, and stress tolerance. Biostimulants may include synthetic or naturally derived compounds that improve nutrient uptake, root development, and overall plant health.

Endogenous growth regulating substances other than hormones. tricontanol, Phenols – polyamines, jasmonates, concept of death hormone.

Beyond the well-known plant hormones, there are other endogenous growth-regulating substances that play important roles in plant growth, development, and responses to various stimuli. Here are some examples:

1. Tricontanol:

   • Function: Tricontanol is a fatty alcohol that has been found to influence various aspects of plant growth, including seed germination, root development, and stress responses. It is known to enhance photosynthesis, increase chlorophyll content, and promote overall plant growth.

2. Phenols:

   • Function: Phenols are a diverse group of compounds that include various plant secondary metabolites. They play roles in plant defense against pathogens and herbivores, as well as in the regulation of growth and development. Phenolic compounds can act as antioxidants, and some are involved in lignin formation and other structural components of plants.

3. Polyamines:

   • Function: Polyamines, such as putrescine, spermidine, and spermine, are organic compounds that participate in cell division, elongation, and differentiation. They are involved in various physiological processes, including seed development, flowering, and stress responses. Polyamines also play a role in stabilizing cellular structures and protecting plants from environmental stresses.

4. Jasmonates:

   • Function: Jasmonates, including jasmonic acid and its derivatives, are involved in plant defense responses against herbivores, pathogens, and various stresses. They regulate the expression of genes related to defense mechanisms, and their signaling pathways intersect with those of other hormones like salicylic acid and ethylene.

5. Concept of Death Hormone:

• Concept: The term "death hormone" is not a widely accepted or recognized scientific term. However, in some contexts, it may be used metaphorically to describe substances or processes associated with programmed cell death (apoptosis) or senescence in plants. Ethylene, a plant hormone, is often associated with senescence and the ripening of fruits, but it is not commonly referred to as a "death hormone" in scientific literature.

Site of synthesis, biosynthetic pathways and metabolism and the influence on plant growth and development of individual group of hormones- Auxins

Auxins are a group of plant hormones that play crucial roles in regulating various aspects of plant growth and development. The site of synthesis, biosynthetic pathways, metabolism, and the influence on plant growth and development for auxins can be outlined as follows:

Site of Synthesis:

1. Apical Meristems: The shoot apical meristem (SAM) is a primary site of auxin synthesis in young developing shoots. The auxins produced here help regulate cell division and elongation in the growing tips of shoots.
2. Young Leaves: Young leaves are also a site of auxin synthesis, contributing to the control of leaf expansion and development.
3. Root Tips: In the root apical meristem (RAM), auxins are synthesized to regulate root growth, including cell elongation and differentiation.

Biosynthetic Pathways:

1. Tryptophan-Dependent Pathway: The primary precursor for auxin biosynthesis is the amino acid tryptophan. The tryptophan-dependent pathway involves several enzymatic steps leading to the synthesis of the primary auxin, indole-3-acetic acid (IAA). Key enzymes include TAA (tryptophan aminotransferase) and YUC (yucca), which convert tryptophan to IAA.

Metabolism:

1. Conjugation: Auxins can undergo conjugation with other molecules, such as amino acids or sugars, leading to the formation of inactive storage forms. This conjugation is reversible, allowing for the release of active auxins when needed.
2. Oxidation and Inactivation: Auxins can be metabolized through oxidation, leading to the formation of inactive compounds. This helps regulate the concentration of active auxins and prevent excessive growth stimulation.

Influence on Plant Growth and Development:

1. Cell Elongation: Auxins promote cell elongation by increasing the plasticity of the cell wall, allowing it to stretch more easily. This is crucial for the growth of stems and roots.
2. Apical Dominance: Auxins inhibit the growth of lateral buds in favor of the apical bud, promoting apical dominance and the upward growth of the plant.
3. Tropisms: Auxins play a role in phototropism (response to light) and gravitropism (response to gravity) by redistributing auxin to the shaded side of the stem or root, leading to differential growth.
4. Root Development: Auxins are involved in root initiation, elongation, and branching. They also contribute to the development of lateral roots.

Site of synthesis, biosynthetic pathways and metabolism and the influence on plant growth and development of individual group of hormones- Gibberlins

that play essential roles in regulating various aspects of plant growth and development. Here is an overview of the site of synthesis, biosynthetic pathways, metabolism, and the influence on plant growth and development of gibberellins:

Site of Synthesis:

1. Meristematic Regions: The actively dividing cells in the apical meristems of shoot tips and root tips are primary sites of gibberellin synthesis.
2. Young Leaves: Gibberellins are also synthesized in young leaves.
3. Embryo: Developing embryos within seeds are another site of gibberellin synthesis.

Biosynthetic Pathways:

1. Terpenoid Pathway: Gibberellins are derived from the mevalonic acid pathway, a terpenoid biosynthesis pathway. The precursors for gibberellin biosynthesis are geranylgeranyl diphosphate (GGDP) and ent-kaurene. Several enzymatic steps lead to the production of various bioactive gibberellins.

Metabolism:

1. Inactivation: Gibberellins can be inactivated through processes like oxidation and conjugation. Inactive forms are often found in storage, and active gibberellins can be released when needed.

Influence on Plant Growth and Development:

1. Stem Elongation: Gibberellins promote cell elongation in stems by inducing the synthesis of enzymes that break down cell wall components, allowing for cell expansion.
2. Seed Germination: Gibberellins play a crucial role in breaking seed dormancy and promoting germination. They stimulate the synthesis of enzymes that mobilize stored reserves in the seed.
3. Flowering: Gibberellins influence flowering, particularly in long-day plants where they promote the transition from the vegetative to the reproductive phase.
4. Fruit Development: Gibberellins can stimulate fruit development and influence fruit size by promoting cell division and expansion.
5. Dwarfism and Height Control: Some dwarf varieties of plants are characterized by a deficiency in gibberellins or a reduced response to them. Conversely, applications of gibberellins can be used to increase plant height in certain crops.

Agricultural Applications:

1. Seedless Fruit Production: Gibberellins are used in agriculture to induce seedless fruit development in certain crops, such as grapes.
2. Increased Fruit Size: Gibberellins are sometimes applied to increase the size of fruits, particularly in seedless varieties.
3. Promotion of Germination: Gibberellin treatments can be used to break seed dormancy and promote uniform germination.

Site of synthesis, biosynthetic pathways and metabolism and the influence on plant growth development of individual group of hormones- and cytokinins

Cytokinins are a group of plant hormones that play a key role in regulating cell division and differentiation. Here's an overview of the site of synthesis, biosynthetic pathways, metabolism, and the influence on plant growth and development of cytokinins:

Site of Synthesis:

1. Root Apical Meristems (RAM): The primary site of cytokinin synthesis is in the root apical meristems, where new cells are actively dividing.
2. Developing Fruits and Seeds: Cytokinins are also produced in developing fruits and seeds, where they play a role in seed development and maturation.

Biosynthetic Pathways:

1. Adenylate-Ipso-Type Pathway: The biosynthetic pathway for cytokinins involves the conversion of adenosine triphosphate (ATP) to isopentenyl adenine or trans-zeatin through a series of enzymatic reactions. The enzymes involved include isopentenyl transferases (IPT) and cytochrome P450 monooxygenases.

Metabolism:

1. Conjugation: Cytokinins can undergo conjugation with sugars, forming inactive storage forms. This conjugation allows for the regulation of cytokinin levels within the plant.
2. Degradation: Cytokinins can be degraded by enzymes such as cytokinin dehydrogenase, leading to the formation of inactive breakdown products.

Influence on Plant Growth and Development:

1. Stimulates Cell Division: Cytokinins work in conjunction with auxins to promote cell division and are essential for the growth of shoot meristems.
2. Delay in Senescence: Cytokinins delay the aging (senescence) of plant organs by promoting chloroplast development and inhibiting the breakdown of chlorophyll.
3. Promotes Lateral Bud Growth: Cytokinins help in breaking apical dominance by promoting the outgrowth of lateral buds, leading to bushier plants.
4. Role in Root Development: Cytokinins influence root development, especially in the differentiation of vascular tissues and lateral root formation.
5. Seed Germination: Cytokinins are involved in breaking seed dormancy and promoting germination.

Agricultural and Horticultural Applications:

1. Promoting Shoot Formation in Tissue Culture: Cytokinins are commonly used in plant tissue culture to promote the development of shoots from explants.
2. Delaying Senescence: Foliar application of cytokinins can be used to delay senescence in certain crops, extending the shelf life of harvested produce.
3. Enhancing Fruit Development: Cytokinins can be applied to enhance fruit development and improve fruit quality.

Site of synthesis, biosynthetic pathways and metabolism and the influence on plant growth and development of individual group of hormones-. Abscisic acid

Abscisic acid (ABA) is a plant hormone that plays a crucial role in various physiological processes, particularly in response to environmental stresses. Here's an overview of the site of synthesis, biosynthetic pathways, metabolism, and the influence on plant growth and development of abscisic acid:

Site of Synthesis:

1. Leaves: The primary site of ABA synthesis is in mature leaves, especially in response to environmental stressors such as drought, high salinity, or low temperatures.
2. Roots: ABA is also synthesized in the roots, where it regulates water uptake and interacts with other hormones to modulate root growth.

Biosynthetic Pathways:

1. Carotenoid Pathway: ABA is derived from carotenoids, which are pigments involved in photosynthesis. The precursor for ABA biosynthesis is violaxanthin, which undergoes a series of enzymatic reactions, including the action of enzymes like 9-cis-epoxycarotenoid dioxygenase (NCED), to produce xanthoxin. Xanthoxin is then converted to ABA through several steps.

Metabolism:

1. Catabolism: ABA can be catabolized through processes such as oxidation, leading to the formation of inactive breakdown products. This catabolism allows for the regulation of ABA levels within the plant.

Influence on Plant Growth and Development:

1. Seed Dormancy: ABA is a key regulator of seed dormancy, preventing premature germination. It inhibits the action of gibberellins and promotes the accumulation of storage reserves in seeds.
2. Stomatal Closure: ABA induces stomatal closure in response to water deficit, reducing water loss through transpiration and helping plants cope with drought stress.
3. Root Growth Inhibition: ABA can inhibit root growth, particularly under conditions of water stress. This helps to balance water uptake with water availability.
4. Response to Environmental Stress: ABA is involved in the plant's response to various environmental stresses, including drought, salinity, and cold stress. It helps plants adapt to and survive unfavorable conditions.

Agricultural and Horticultural Applications:

1. Drought Stress Tolerance: Understanding the role of ABA in drought response has implications for developing crops with improved drought stress tolerance.
2. Seed Germination Control: ABA can be applied to control seed germination in agriculture, ensuring that germination occurs under favorable conditions.

Site of synthesis, biosynthetic pathways and metabolism and the influence on plant growth and development of individual group of hormones- Ethylene

Ethylene is a gaseous plant hormone that plays a significant role in various aspects of plant growth and development. Here's an overview of the site of synthesis, biosynthetic pathways, metabolism, and the influence on plant growth and development of ethylene:

Site of Synthesis:

1. Various Plant Tissues: Ethylene is synthesized in various plant tissues, including:
   • Meristematic Tissues: Apical meristems and root apical meristems.
   • Senescing Tissues: During senescence, particularly in aging leaves and flowers.
   • Developing Fruits: Especially during ripening.

Biosynthetic Pathways:

1. Methionine Pathway: The primary precursor for ethylene biosynthesis is the amino acid methionine. The biosynthetic pathway involves a series of enzymatic reactions, and the key enzyme in this process is 1-aminocyclopropane-1-carboxylic acid (ACC) synthase. The precursor, ACC, is converted to ethylene by ACC oxidase.

Metabolism:

1. Oxidation: Ethylene is oxidized in plant tissues, primarily by the enzyme ethylene oxidase, leading to the formation of ethylene oxide and other compounds. This process helps regulate ethylene levels and prevent excessive accumulation.

Influence on Plant Growth and Development:

1. Fruit Ripening: Ethylene is a key regulator of fruit ripening. It induces the expression of genes associated with the breakdown of cell wall components, softening of fruit, and the production of aroma compounds.
2. Senescence: Ethylene promotes senescence in aging tissues. It is involved in the breakdown of chlorophyll and other cellular components during leaf senescence.
3. Apical Hook Formation: In seedlings, ethylene induces the formation of the apical hook, which protects the shoot apical meristem during seedling emergence through the soil.
4. Root Hair Development: Ethylene inhibits root elongation but promotes the development of root hairs.
5. Abscission: Ethylene is involved in the abscission (shedding) of leaves, flowers, and fruits. It promotes the production of enzymes that weaken cell walls, leading to organ detachment.

Agricultural and Horticultural Applications:

1. Fruit and Flower Ripening: Ethylene is often used in the agricultural and horticultural industries to induce uniform ripening of fruits and promote flower opening.
2. Postharvest Storage: Ethylene is used to regulate the postharvest storage of fruits and vegetables by controlling ripening and senescence processes.

Site of synthesis, biosynthetic pathways and metabolism and the influence on plant growth and development of individual group of hormones- brassinosteroids

Brassinosteroids (BRs) are a group of steroid hormones that play crucial roles in regulating various aspects of plant growth and development. Here's an overview of the site of synthesis, biosynthetic pathways, metabolism, and the influence on plant growth and development of brassinosteroids:

Site of Synthesis:

1. Young Tissues: Brassinosteroids are synthesized in young, actively growing tissues, such as shoot and root tips, as well as developing seeds.
2. Apical Meristems: The apical meristems of plants, including the shoot apical meristem (SAM) and root apical meristem (RAM), are primary sites of brassinosteroid synthesis.

Biosynthetic Pathways:

1. Campesterol Pathway: The biosynthesis of brassinosteroids involves multiple steps. It starts with the conversion of campesterol, a sterol compound, into castasterone, which is then further modified to produce various bioactive brassinosteroids. Key enzymes involved in this pathway include BR6OX1 (brassinosteroid-6-oxidase 1) and CPD (constitutive photomorphogenesis and dwarfism).

Metabolism:

1. Inactivation: Brassinosteroids can undergo inactivation through processes such as oxidation, leading to the formation of inactive breakdown products. The inactivation of brassinosteroids is crucial for maintaining hormonal balance within the plant.

Influence on Plant Growth and Development:

1. Cell Elongation: Brassinosteroids promote cell elongation and expansion, contributing to overall plant growth. They influence the cell wall structure and composition, allowing cells to elongate more efficiently.
2. Cell Division: Brassinosteroids stimulate cell division in the cambium and other meristematic tissues, contributing to increased organ size and biomass.
3. Vascular Tissue Differentiation: Brassinosteroids play a role in the differentiation of vascular tissues, contributing to the development of xylem and phloem.
4. Root Growth: Brassinosteroids influence root growth and development, including root elongation and lateral root formation.
5. Seed Germination: Brassinosteroids promote seed germination by enhancing the mobilization of stored reserves and facilitating seedling growth.

Agricultural and Horticultural Applications:

1. Crop Yield and Quality: Exogenous application of brassinosteroids has been explored to enhance crop yield and improve the quality of harvested produce.
2. Stress Tolerance: Brassinosteroids play a role in plant responses to various environmental stresses, including drought and salinity. Their application can enhance stress tolerance in plants.

Signal perception. Transduction and effect at functional gene level of different hormones Auxins- cell elongation, Gibberellins-, germination of dormant seeds

The action of hormones like auxins and gibberellins involves signal perception, transduction, and effects at the functional gene level. Let's explore how these processes occur for each hormone:

Auxins (e.g., Indole-3-Acetic Acid, IAA):

1. Signal Perception:

   • Auxin receptors are localized at the cell membrane. The primary auxin receptor is the F-box protein TRANSPORT INHIBITOR RESPONSE 1 (TIR1).
   • Auxin perception involves the binding of auxin to the TIR1 receptor, leading to the formation of the SCF^TIR1 ubiquitin ligase complex.

2. Transduction:

   • Auxin binding to TIR1 leads to the ubiquitination of AUX/IAA proteins (Auxin/Indole-3-Acetic Acid proteins).
   • Ubiquitinated AUX/IAA proteins are targeted for degradation by the 26S proteasome.
   • Degradation of AUX/IAA proteins releases the transcriptional repressor, allowing auxin-responsive genes to be activated.

3. Effect at the Functional Gene Level (Cell Elongation):

   • The activation of auxin-responsive genes leads to the expression of various genes involved in cell elongation.
   • Auxins promote cell elongation by activating genes related to cell wall loosening, cell expansion, and water uptake.
   • The increased expression of expansins and other cell wall-modifying proteins contributes to cell elongation.

Gibberellins (e.g., Gibberellic Acid, GA3):

1. Signal Perception:

   • Gibberellin receptors are localized in the cytoplasm and nucleus. The GIBBERELLIN INSENSITIVE DWARF1 (GID1) receptor is a key player in gibberellin perception.
   • Gibberellin binding to GID1 induces a conformational change in the receptor.

2. Transduction:

   • The gibberellin-GID1 complex interacts with DELLA proteins, which are negative regulators of gibberellin responses.
   • Gibberellin binding promotes the degradation of DELLA proteins via the ubiquitin-proteasome pathway.

3. Effect at the Functional Gene Level (Germination of Dormant Seeds):

• Gibberellins promote the germination of dormant seeds by activating the expression of genes involved in breaking seed dormancy.
• Gibberellins induce the expression of hydrolytic enzymes such as α-amylase, which breaks down stored starch into sugars.
• Sugars serve as an energy source for germinating seeds, facilitating the growth of the embryonic axis and the emergence of the radicle.

Signal perception. Transduction and effect at functional gene level of different hormones cytokinins- cell division. Retardation of senescence of plant parts, Abscisic acid- stomatal closure and induction of drought resistance, Ethylene- fruit ripening.

Certainly, let's explore the signal perception, transduction, and effects at the functional gene level for cytokinins, abscisic acid (ABA), and ethylene:

Cytokinins:

1. Signal Perception:

   • Cytokinin receptors are histidine kinases located on the endoplasmic reticulum membrane.
   • The binding of cytokinins to receptors leads to autophosphorylation of the receptors.

2. Transduction:

   • Phosphorylated receptors transfer phosphate groups to histidine phosphotransfer proteins (AHPs).
   • AHPs transfer phosphate groups to response regulators (ARRs).
   • Phosphorylated ARRs act as transcription factors, regulating the expression of cytokinin-responsive genes.

3. Effect at the Functional Gene Level (Cell Division):

   • Cytokinins promote cell division and differentiation.
   • Genes related to cell cycle progression and cell division, such as cyclins and cyclin-dependent kinases (CDKs), are activated by cytokinins.
   • Increased expression of genes involved in the synthesis of proteins and nucleic acids contributes to enhanced cell division.

Abscisic Acid (ABA):

1. Signal Perception:

   • ABA receptors include PYR/PYL/RCAR proteins localized in the cytoplasm.
   • ABA binding to receptors leads to the inhibition of protein phosphatases.

2. Transduction:

   • Inhibition of protein phosphatases prevents the dephosphorylation and activation of SNF1-related protein kinases (SnRK2s).
   • Activated SnRK2s phosphorylate and activate downstream targets, including the transcription factors known as ABFs (ABRE-binding factors).

3. Effect at the Functional Gene Level (Stomatal Closure and Drought Resistance):

   • ABA induces stomatal closure by promoting the efflux of ions and water from guard cells.
   • Genes encoding ion channels and transporters involved in ion efflux are upregulated.
   • ABA also induces the expression of genes related to stress tolerance, such as LEA proteins, contributing to drought resistance.

Ethylene:

1. Signal Perception:

   • Ethylene receptors, such as ETR1 and EIN4, are located on the endoplasmic reticulum membrane.
   • In the absence of ethylene, receptors are active and inhibit downstream signaling.

2. Transduction:

   • Ethylene binding inactivates the receptors, allowing the downstream signaling pathway to proceed.
   • This leads to the activation of the transcription factor EIN3 (Ethylene Insensitive 3).

3. Effect at the Functional Gene Level (Fruit Ripening):

• Ethylene induces the expression of genes involved in fruit ripening, such as those encoding enzymes like pectinases and cellulases.
• These enzymes contribute to the softening of fruit and the breakdown of cell wall components.
• Ethylene also regulates the expression of genes involved in the synthesis of volatile compounds responsible for aroma and flavor.

Interaction of hormones in regulation of plant growth and development processes. Rooting of cuttings- Flowering. Apical dominance.

The regulation of plant growth and development is a complex process involving the interaction of multiple hormones. Here's an overview of how hormones interact in the context of rooting of cuttings, flowering, and apical dominance:

Rooting of Cuttings:

1. Auxins (e.g., Indole-3-Acetic Acid, IAA):

   • Role: Auxins play a central role in rooting of cuttings by promoting the initiation and development of roots.
   • Mechanism: The application of auxins, particularly indole-3-butyric acid (IBA) or naphthaleneacetic acid (NAA), stimulates the formation of adventitious roots from the stem cutting.
   • Interaction: The auxin gradient established by the cutting promotes the differentiation of root primordia, leading to the development of a new root system.

2. Cytokinins:

   • Role: Cytokinins can influence root development by interacting with auxins.
   • Mechanism: A balanced ratio of auxins to cytokinins is often important for optimal root formation. Too much cytokinin may inhibit root initiation.

Flowering:

1. Gibberellins (e.g., Gibberellic Acid, GA3):

   • Role: Gibberellins are involved in the promotion of flowering in certain plants, particularly those requiring long days (long-day plants).
   • Mechanism: Gibberellins stimulate the production of floral meristems, leading to the initiation of flowers.
   • Interaction: The balance between gibberellins and other hormones, such as photoperiod-sensitive hormones like phytochrome, determines the timing of flowering.

2. Florigens (FT and SOC1):

   • Role: Florigens are proteins that promote flowering.
   • Mechanism: These proteins are influenced by environmental signals, including photoperiod, and they activate flowering genes in the shoot apical meristem.

Apical Dominance:

1. Auxins:

   • Role: Auxins play a major role in apical dominance by inhibiting the outgrowth of lateral buds.
   • Mechanism: Auxins, produced in the apical meristem, move downward in the stem, suppressing the growth of lateral buds along the stem.
   • Interaction: The removal of the apical bud (apical dominance release) results in a decrease in auxin concentration, allowing lateral buds to grow.

2. Cytokinins:

   • Role: Cytokinins counteract the inhibitory effect of auxins on lateral bud growth.
   • Mechanism: Cytokinins promote lateral bud outgrowth and reduce apical dominance.
   • Interaction: The balance between auxins and cytokinins influences the growth of lateral buds and the establishment of new branches.

3. Strigolactones:

• Role: Strigolactones are involved in inhibiting lateral bud outgrowth and maintaining apical dominance.
• Mechanism: The production of strigolactones is influenced by the auxin flow from the apical bud. Strigolactones suppress lateral bud growth in response to high auxin levels.

Molecular aspects of control of reproductive growth and development.

The molecular control of reproductive growth and development in plants involves a complex interplay of genetic, hormonal, and environmental factors. Key molecular processes include the regulation of floral initiation, flower development, and the transition from vegetative to reproductive phases. Here's an overview of the molecular aspects involved:

1. Floral Initiation:

• Photoperiodic Pathway:

  • Molecular Components: Photoreceptors (e.g., phytochromes) sense day length.
  • Pathway: In long-day plants, exposure to a certain duration of light promotes flowering. CONSTANS (CO) is a key gene that integrates photoperiodic signals.
  • Downstream Effects: CO activates the expression of FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), promoting floral initiation.

• Vernalization Pathway:

  • Molecular Components: Vernalization involves exposure to prolonged cold.
  • Pathway: Cold induces the expression of genes like FLOWERING LOCUS C (FLC). Following vernalization, FLC repression occurs, allowing floral initiation.

2. Flower Development:

• ABC Model:

  • Molecular Components: Floral organ identity is determined by the ABC model genes (A, B, C).
  • Pathway: Homeotic genes (e.g., APETALA1, APETALA3, PISTILLATA, AGAMOUS) specify the identity of floral organs.
  • Combinations: Different combinations of A, B, and C gene activity determine the identity of sepals, petals, stamens, and carpels.

• Control of Floral Meristem Identity:

  • Molecular Components: Genes like LEAFY (LFY) and UNUSUAL FLORAL ORGANS (UFO) play roles in specifying floral meristem identity.
  • Pathway: Activation of LFY and UFO contributes to the transition of the shoot apical meristem into a floral meristem.

3. Transition to Reproductive Phase:

• MiRNAs and Transcription Factors:

  • Molecular Components: MicroRNAs (miRNAs) and transcription factors regulate the transition to reproductive growth.
  • Pathway: miRNAs negatively regulate the expression of target genes involved in vegetative growth, ensuring the transition to reproductive growth.

• Gibberellins and Flowering:

  • Molecular Components: Gibberellins (GAs) influence flowering time.
  • Pathway: GAs interact with key regulatory genes, such as GIGANTEA (GI), to regulate the expression of flowering genes.

4. Seed Development:

• Embryo Development:

  • Molecular Components: Genes controlling seed development include LEC1 (LEAFY COTYLEDON 1), LEC2, and FUSCA3.
  • Pathway: These genes regulate the transition from embryonic to post-embryonic growth.

• Maturation and Dormancy:

• Molecular Components: Abscisic acid (ABA) and DELAY OF GERMINATION 1 (DOG1) are involved in seed maturation and dormancy.
• Pathway: ABA accumulates during seed maturation, promoting dormancy, while DOG1 contributes to seed dormancy release during germination.

Synthetic growth regulators- Classification, their effect on plant growth and development.

Synthetic growth regulators, also known as plant growth regulators (PGRs) or plant hormones, are chemical compounds designed to mimic the functions of natural hormones or regulate specific plant processes. These synthetic growth regulators can be classified based on their chemical nature and the effects they have on plant growth and development. Here is a general classification and an overview of their effects:

1. Auxin-like Compounds:

• Chemical Class: Synthetic auxins, such as 2,4-D (2,4-dichlorophenoxyacetic acid) and NAA (1-naphthaleneacetic acid).
• Effect on Plant Growth:
  • Stimulate Cell Elongation: Similar to natural auxins, synthetic auxins promote elongation of cells, leading to increased growth in stems and roots.
  • Weed Control: Synthetic auxins are commonly used as herbicides to control broadleaf weeds.

2. Gibberellin-like Compounds:

• Chemical Class: Gibberellin analogs, such as GA3 (gibberellic acid).
• Effect on Plant Growth:
  • Stimulate Stem Elongation: Gibberellin-like compounds promote stem elongation and can be used to induce internode elongation in certain crops.
  • Fruit Development: They may enhance fruit size and development in some plants.

3. Cytokinin-like Compounds:

• Chemical Class: Synthetic cytokinins, like kinetin and benzyladenine.
• Effect on Plant Growth:
  • Promote Cell Division: Synthetic cytokinins stimulate cell division and are used in tissue culture for shoot proliferation.
  • Delay Senescence: They can delay the senescence of leaves and promote longevity.

4. Abscisic Acid (ABA)-like Compounds:

• Chemical Class: Synthetic compounds with ABA-like effects.
• Effect on Plant Growth:
  • Stomatal Closure: ABA-like compounds induce stomatal closure, reducing water loss in plants.
  • Drought Resistance: They can enhance a plant's ability to tolerate drought stress.

5. Ethylene Releasers:

• Chemical Class: Compounds that release ethylene gas, such as ethephon.
• Effect on Plant Growth:
  • Fruit Ripening: Ethylene releasers are used to induce and accelerate fruit ripening.
  • Flowering: They can promote flowering in certain plants.

6. Plant Growth Retardants:

• Chemical Class: Compounds that inhibit gibberellin biosynthesis, such as paclobutrazol.
• Effect on Plant Growth:
  • Dwarfing Effect: Plant growth retardants inhibit stem elongation, resulting in compact and dwarfed plants.
  • Flowering Control: They can delay flowering and promote branching.

7. Synthetic Auxin Transport Inhibitors:

• Chemical Class: Compounds that inhibit auxin transport, such as TIBA (2,3,5-triiodobenzoic acid).
• Effect on Plant Growth:
  • Altered Growth Patterns: Inhibition of auxin transport disrupts normal growth patterns in stems and roots.

8. Brassinosteroid Analogues:

• Chemical Class: Compounds that mimic the action of brassinosteroids.
• Effect on Plant Growth:
  • Stimulate Growth: Brassinosteroid analogues can promote cell elongation and overall plant growth.