Classification, Function and Mechanism of Action of Plant Hormones 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 developmentFunctions of peptide hormones in plant development (Gancheva et al., 2019). 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.