Molecular Mechanisms in Stress Responses PDF

**Molecular Mechanisms in Stress Responses** **1. Role of Hormones in Stress Responses** Plant hormones, or phytohormones, play critical roles in modulating stress responses, often acting as signals that trigger specific defense mechanisms. The primary hormones involved include: - **Abscisic Ac...

**Molecular Mechanisms in Stress Responses** **1. Role of Hormones in Stress Responses** Plant hormones, or phytohormones, play critical roles in modulating stress responses, often acting as signals that trigger specific defense mechanisms. The primary hormones involved include: - **Abscisic Acid (ABA):** Known as the \"stress hormone,\" ABA is central to responding to drought and salinity stress by regulating stomatal closure to reduce water loss. ABA also modifies gene expression to enhance stress tolerance. - **Ethylene:** Under abiotic stresses like flooding, ethylene accumulates, inducing adaptations such as programmed cell death in certain root cells to form air spaces, allowing better oxygen supply. - **Jasmonic Acid (JA):** JA is essential for defense against herbivores and pathogens. It can also activate antioxidant defenses during drought or temperature stress. - **Salicylic Acid (SA):** SA mediates plant responses to pathogens and also plays a role in abiotic stress, such as temperature fluctuations, by activating defense-related genes. - **Auxins, Gibberellins, and Cytokinins:** While primarily associated with growth, these hormones modulate responses to environmental stress. For instance, cytokinins delay leaf senescence under stress conditions, contributing to longevity and tolerance. **2. Signal Transduction Pathways** Signal transduction pathways translate external stress signals into cellular responses, involving multiple steps: - **Perception of Stress Signals:** Stress signals are detected by receptors, often located on the plasma membrane. These receptors recognize environmental changes such as drought, salinity, or pathogen invasion. - **Secondary Messengers:** Once stress is perceived, secondary messengers like calcium ions (Ca²⁺), reactive oxygen species (ROS), and cyclic AMP (cAMP) are activated. These messengers amplify the stress signal, initiating a more robust response. - **Activation of Protein Kinases and Phosphatases:** Protein kinases, particularly mitogen-activated protein kinases (MAPKs), play a central role by phosphorylating target proteins to relay the signal downstream. Phosphatases act to balance this activation and ensure response specificity. - **Transcriptional Regulation:** The signal transduction cascade ultimately leads to the activation of transcription factors, which then regulate the expression of stress-responsive genes. These genes produce stress proteins, antioxidants, osmoprotectants, and enzymes that combat stress effects. - **Crosstalk Between Pathways:** These pathways often interact, creating a network that allows plants to prioritize responses based on the specific stress condition. This crosstalk ensures a tailored and efficient response to combined stress factors.