Plant hormones, also known as phytohormones, are vital in regulating various aspects of plant growth, development, and responses to environmental stimuli. These chemical messengers are synthesized in specific plant tissues and then transported to other parts of the plant, where they exert their effects. The intricate network of interactions between different hormones helps coordinate plant physiological processes and ensures optimal growth and adaptation to changing conditions.
Functions of peptide hormones in plant development (Gancheva et al., 2019).
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Auxins
Function:
- Cell Elongation: Auxins are primarily known for promoting cell elongation in plant stems and coleoptiles (the protective sheath covering the emerging shoot). This enables upward growth towards light sources and supports plant height.
- Root Development: Auxins play a crucial role in stimulating root growth and branching, enhancing the plant's ability to absorb water and nutrients from the soil.
- Phototropism: Auxins regulate phototropism, bending plant stems towards light sources. This ensures that leaves receive maximum light exposure for efficient photosynthesis.
Examples: The most common auxin found in plants is indole-3-acetic acid (IAA).
Mechanism:
Receptor Binding: Auxins bind to specific receptors located in the plasma membrane of plant cells. The binding of auxins to these receptors initiates a signaling cascade.
Auxin-Inducible Proteins: The receptor-auxin complex promotes the degradation of Auxin/Indole-3-Acetic Acid (Aux/IAA) proteins through the ubiquitin-proteasome pathway. Aux/IAAs are transcriptional repressors that inhibit the expression of auxin-responsive genes.
Auxin Response Factors (ARFs): Degradation of Aux/IAAs releases Auxin Response Factors (ARFs). These are transcriptional activators that bind to auxin-responsive elements (AREs) in the promoters of target genes.
Gene Regulation: ARFs, once activated, stimulate the expression of target genes involved in cell elongation, root development, and phototropic responses. This leads to changes in cell growth and organ development.
Cytokinins
Function:
- Cell Division: Cytokinins promote cell division in various plant tissues, contributing to growth and development.
- Shoot Growth: They regulate lateral shoot growth, preventing apical dominance, where the main shoot inhibits the growth of lateral shoots. This leads to branching and increased biomass.
- Delaying Senescence: Cytokinins delay senescence, the natural aging process of leaves, extending the photosynthetic period and supporting overall plant vigor.
Examples: Isopentenyl adenine and zeatin are common cytokinins found in plants.
Mechanism:
Cytokinins bind to receptors located on the endoplasmic reticulum (ER) membrane of plant cells, triggering the activation of histidine kinases in a two-component signaling system. These activated histidine kinases then transfer phosphate groups to histidine phosphotransfer proteins (AHPs). Phosphorylated AHPs, in turn, transfer the phosphate group to type-B response regulators (RRs), which function as transcription factors. Once activated, the type-B RRs enter the nucleus and regulate the expression of specific target genes involved in cell division, shoot growth, and apical dominance. This regulatory process ultimately leads to increased cell division and lateral shoot growth.
Gibberellins
Function:
- Stem Elongation: Gibberellins are crucial for stem elongation by promoting cell division and expansion in plant stems.
- Seed Germination: They break seed dormancy and initiate germination by activating hydrolytic enzymes, which break down stored nutrients in seeds, providing energy for growth.
- Flowering Induction: Gibberellins play a pivotal role in the transition from vegetative to reproductive growth, triggering the flowering process in many plant species.
Examples: Gibberellic acid (GA) is a well-known gibberellin.
Mechanism:
Gibberellins bind to receptors within the cytoplasm of plant cells, forming a receptor-gibberellin complex. This complex then translocates to the nucleus, where it interacts with the gibberellin-delusion conjugate (GID1), a nuclear protein. Through this interaction, DELLA proteins are targeted for degradation via the proteasome pathway. As negative regulators of gibberellin signaling, the degradation of DELLA proteins relieves the repression of growth-promoting genes. Consequently, this process promotes stem elongation, seed germination, and flowering induction.
Abscisic Acid (ABA):
Function:
- Stress Response: ABA is a key hormone in plant responses to various stresses, such as drought, salinity, and extreme temperatures. It enhances stress tolerance and helps plants cope with adverse conditions.
- Stomatal Closure: ABA regulates stomatal closure, controlling the opening and closing of stomata, tiny pores on the leaf surface. This mechanism helps plants conserve water during water scarcity.
- Seed Dormancy and Germination Inhibition: ABA induces seed dormancy and inhibits premature seed germination, ensuring seeds germinate under optimal conditions.
Examples: Abscisic acid (ABA) is the primary hormone responsible for these functions.
Mechanism:
Receptor Binding: ABA interacts with specific receptors associated with G-proteins located on the plasma membrane of plant cells.
Secondary Messengers: Upon ABA binding, G-proteins are activated, initiating the production of secondary messengers like calcium ions and reactive oxygen species.
Protein Phosphorylation: The secondary messengers initiate a cascade of protein phosphorylation events, leading to modifications in gene expression.
Gene Regulation: The altered gene expression activates stress-responsive genes and triggers the closure of stomata, enabling plants to conserve water during drought and respond effectively to various stress conditions.
Ethylene:
Function:
- Fruit Ripening: Ethylene is a key regulator of fruit ripening. It triggers the breakdown of cell walls, leading to softening and color changes in fruits.
- Senescence and Leaf Abscission: Ethylene induces leaf and flower senescence, promoting aging and facilitating the shedding of leaves and flowers.
- Root Growth and Gravitropism: Ethylene influences root growth and gravitropism, the response of roots to gravity, affecting root orientation.
Examples: Ethylene is a gaseous hormone synthesized from the amino acid methionine.
Mechanism:
Receptor Binding: Ethylene is a gaseous hormone that diffuses through the cell membrane and binds to receptors in the endoplasmic reticulum (ER) of plant cells.
Constitutive Triple Response: The binding of ethylene to its receptors triggers the activation of the transcription factor EIN3 (ethylene-insensitive 3). EIN3 induces the expression of genes associated with the "constitutive triple response," causing the plant to display a unique growth pattern characterized by a thickened and shortened stem, radial swelling, and horizontal growth.
Downstream Effects: The constitutive triple response helps the plant adapt to mechanical stress and ensure survival in challenging environments.
Brassinosteroids
Function:
Brassinosteroids play a vital role in promoting cell elongation and division, contributing significantly to plant growth and development. Additionally, these hormones enhance plant tolerance to diverse stresses, including heat, cold, and salinity, empowering plants to withstand challenging environmental conditions.
Examples: Brassinolide is a well-known brassinosteroid.
Mechanism:
Receptor Binding: Brassinosteroids initiate their signaling process by binding to receptors present on the cell surface, triggering the activation of a receptor complex.
Brassinosteroid Signaling Kinase (BSK): The activated receptor complex sets off a series of phosphorylation events involving Brassinosteroid Signaling Kinase (BSK).
BRI1-Associated Kinases (BAK1): BSK, in turn, phosphorylates and activates BRI1-Associated Kinases (BAK1).
Transcription Factors: As a result, BAK1 phosphorylates and activates the transcription factor BES1 (Bri1-EMS-Suppressor 1).
Gene Regulation: Once activated, BES1 regulates the expression of specific target genes involved in cell elongation, cell division, and stress responses, leading to enhanced growth and improved stress tolerance in plants.
Jasmonates
Function:
Defense Response: Jasmonates play a crucial role in plant defense responses against herbivores and pathogens. They induce the production of defensive compounds, such as secondary metabolites and proteinase inhibitors, to protect plants from potential threats.
Examples: Jasmonic acid and its derivatives are common jasmonates.
Mechanism:
Receptor Activation: Jasmonates are perceived by cell membrane receptors, which initiate a signaling cascade.
Jasmonate ZIM-Domain Proteins (JAZ): Upon receptor activation, Jasmonate ZIM-Domain Proteins (JAZ) undergo degradation through the ubiquitin-proteasome pathway.
Transcription Factors: The degradation of JAZ proteins releases transcription factors, including MYC2, which regulate the expression of genes involved in defense responses.
Defense Response: As a result, the activated transcription factors trigger the production of defense-related compounds, such as secondary metabolites and proteinase inhibitors, to fortify the plant against herbivores and pathogens.
Salicylic Acid
Function:
Systemic Acquired Resistance (SAR): Salicylic acid triggers systemic acquired resistance (SAR), an important defense response in plants. It enables plants to defend against pathogens by priming other parts of the plant for a faster and stronger defense response.
Examples: Salicylic acid (SA) is the primary hormone responsible for SAR.
Mechanism:
Pathogen Perception: Salicylic acid is often produced in response to pathogen attack or infection.
Gene Expression: Salicylic acid induces the expression of various pathogenesis-related (PR) genes, encoding proteins that play a role in defense against pathogens.
Systemic Acquired Resistance (SAR): Salicylic acid triggers SAR, where the plant induces a broad-spectrum defense response throughout the entire organism, providing resistance to future pathogen attacks.
Understanding the intricate mechanisms of plant hormone action is critical for manipulating plant growth and responses in agriculture and horticulture. Researchers continue to explore these pathways to uncover new insights into the complex world of plant signaling and physiology. As our knowledge deepens, we gain valuable tools to improve crop productivity, enhance stress tolerance, and contribute to sustainable agriculture and environmental conservation.
Reference
- Gancheva, M. S., et al. "Plant peptide hormones." Russian Journal of Plant Physiology 66 (2019): 171-189.
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