The article by J.G. Scandalios discusses the role of molecular oxygen (O₂) and reactive oxygen species (ROS) in cellular functions and their potential to cause oxidative stress. O₂, while essential for aerobic life, can form ROS such as superoxide (O₂•⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (OH•) under various conditions. These ROS can damage cellular components and lead to cell death if not properly managed. Organisms have evolved both non-enzymatic and enzymatic antioxidant defenses to mitigate the damaging effects of ROS, including catalases, peroxidases, superoxide dismutases (SOD), and glutathione S-transferases (GST). The article highlights the complex regulatory mechanisms involved in the perception and transduction of ROS signals to trigger antioxidant gene defenses. In prokaryotes and yeast, transcription factors like OxyR and SoxRS play crucial roles in regulating antioxidant gene expression. In higher eukaryotes, the response to oxidative stress is more complex and involves multiple regulators, including nuclear factor κB (NF-κB) and activator protein-1 (AP-1). The antioxidant-responsive element (ARE) is a common motif found in the promoters of antioxidant genes in mammals and plants. The article also discusses the pleiotropic roles of ROS, which can act as signaling molecules and participate in various cellular processes. For example, H₂O₂ can activate transcription factors and modulate gene expression, while O₂•⁻ can serve as a broad-spectrum antibiotic against invading microbes. The induction of antioxidant genes in response to ROS is regulated by a variety of biotic and abiotic stressors, and the use of microarray technology has helped identify a large number of genes responsive to oxidative stress. Finally, the article emphasizes the importance of understanding the integrated view of oxidative stress responses, including the identification of regulatory pathways and the role of ROS in maintaining cellular homeostasis. The paradox of oxygen, where it is both essential for life and highly toxic in its reduced forms, underscores the need for a comprehensive understanding of how organisms perceive and respond to oxidative stress.The article by J.G. Scandalios discusses the role of molecular oxygen (O₂) and reactive oxygen species (ROS) in cellular functions and their potential to cause oxidative stress. O₂, while essential for aerobic life, can form ROS such as superoxide (O₂•⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (OH•) under various conditions. These ROS can damage cellular components and lead to cell death if not properly managed. Organisms have evolved both non-enzymatic and enzymatic antioxidant defenses to mitigate the damaging effects of ROS, including catalases, peroxidases, superoxide dismutases (SOD), and glutathione S-transferases (GST). The article highlights the complex regulatory mechanisms involved in the perception and transduction of ROS signals to trigger antioxidant gene defenses. In prokaryotes and yeast, transcription factors like OxyR and SoxRS play crucial roles in regulating antioxidant gene expression. In higher eukaryotes, the response to oxidative stress is more complex and involves multiple regulators, including nuclear factor κB (NF-κB) and activator protein-1 (AP-1). The antioxidant-responsive element (ARE) is a common motif found in the promoters of antioxidant genes in mammals and plants. The article also discusses the pleiotropic roles of ROS, which can act as signaling molecules and participate in various cellular processes. For example, H₂O₂ can activate transcription factors and modulate gene expression, while O₂•⁻ can serve as a broad-spectrum antibiotic against invading microbes. The induction of antioxidant genes in response to ROS is regulated by a variety of biotic and abiotic stressors, and the use of microarray technology has helped identify a large number of genes responsive to oxidative stress. Finally, the article emphasizes the importance of understanding the integrated view of oxidative stress responses, including the identification of regulatory pathways and the role of ROS in maintaining cellular homeostasis. The paradox of oxygen, where it is both essential for life and highly toxic in its reduced forms, underscores the need for a comprehensive understanding of how organisms perceive and respond to oxidative stress.