Phytohormone Metabolism and Homeostasis

Phytohormone Metabolism and Homeostasis • Auxin Biosynthesis: Indole-3-pyruvate o primary intermediate. • Gibberellin Synthesis: Derived from diterpene ent-kaurene. • Cytokinins: Adenine derivatives with isoprene side chains. • Ethylene Production: Synthesized from methionine via the intermediate ACC. • Abscisic Acid: Synthesized from a carotenoid precursor. • Brassinosteroids: Derived from the sterol campesterol. • Strigolactones: Synthesized from β-carotene. Signal Transmission and Cell-Cell Communication • Hormonal Signaling Pathways: o Cytokinin and ethylene signal transduction ▪ derived from bacterial two-component regulatory systems. o Receptor-like kinases regulate brassinosteroid and auxin signaling. o ABA signaling involves phosphatases and kinases. o Plant hormone signaling commonly employs negative regulation. o Several plant hormone receptors function in ubiquitination-mediated protein degradation. o Plants have evolved mechanisms to switch off or attenuate signaling responses. o Cellular response to a signal is often tissue-specific. o Cross-regulation integrates multiple signaling pathwaysPlant physiology. Temporal and Spatial Aspects of Signaling • Signal transduction in plants allows both rapid and long-term responses. • cell-autonomous (signal and response in the same cell) • non–cell autonomous (signal in one location, response in another). • Signaling events can be tightly localized or spread systemically to coordinate developmental processes. Signal Perception and Amplification • Receptors: o Found on the plasma membrane, cytosol, endomembrane system, and nucleus. o Detect diverse signals such as hormones, light, and stress. o Some receptors, like receptor kinases, are membrane-bound and mediate extracellular signaling. o • GPCR-like functions in plants remain unclearPlant physiology. Intracellular Signal Amplification: o Signaling strength can be lost without amplification due to dilution in the cytosol. o Signal strength is maintained through phosphorylation cascades and second messengers. • MAP Kinase Signal Amplification Cascade: o Highly conserved in eukaryotes, regulating hormone responses, abiotic stress, and defense pathways. o Involves a three-step phosphorylation relay: MAP3K → MAP2K → MAPK. o Activated MAPKs regulate transcription factors and proteins to modify gene expressionPlant physiology. • Second Messengers: o Ca²⁺: Universal secondary messenger involved in numerous responses. o H⁺ (pH changes): Alters cytosolic and cell wall conditions for stress and hormonal responses. o Reactive Oxygen Species (ROS): Act as secondary messengers in environmental and developmental responses. o Lipid Signaling Molecules: Mediate cellular communication and regulation Plant physiology. Hormones and Plant Development • Hormones as Chemical Messengers: o Act at very low concentrations to induce significant physiological responses. o Transported between cells to coordinate growth and adaptationPlant physiology. • Major Plant Hormones: o o o o Auxin: ▪ Discovered in studies on phototropism in coleoptiles. ▪ Regulates cell elongation, apical dominance, and differentiation. Gibberellins: ▪ Identified from "foolish seedling disease" in rice. ▪ Promote stem elongation, seed germination, and flowering. Cytokinins: ▪ Promote cell division and differentiation. ▪ Discovered through tissue culture studiesPlant physiology. Ethylene: ▪ A gaseous hormone that regulates fruit ripening, leaf abscission, and stress responses. o Abscisic Acid (ABA): ▪ o Controls seed dormancy, stomatal closure under drought stress. Brassinosteroids: ▪ Lipid-soluble hormones regulating photomorphogenesis, germination, and stress responses. o Strigolactones: ▪ Suppress lateral shoot branching. ▪ Involved in rhizosphere interactions with fungi and parasitic plants Plant physiology.