The production of reactive oxygen species (ROS) is one of the earliest cellular responses to pathogen recognition, often involving an oxidative burst. This response can be biphasic, with a transient initial phase followed by a sustained phase, and is correlated with disease resistance. ROS can directly strengthen host cell walls and cause lipid peroxidation, but they also serve as signaling molecules that mediate defense gene activation. The oxidative burst is primarily produced by NADPH oxidases and cell wall peroxidases, with the former being the predominant mechanism. ROS production can occur in various cellular compartments, and their levels are regulated by scavenging systems such as ascorbate peroxidases, glutathione, superoxide dismutases, and catalases. ROS play multiple roles in plant defense, including the formation of defensive barriers, the establishment of systemic defenses, and the regulation of calcium fluxes. They interact with other signaling molecules like salicylic acid (SA) and nitric oxide (NO), and their functions can vary depending on the context and the pathogen involved. Understanding the complex interplay between ROS and other signaling pathways is crucial for comprehending the diverse responses to pathogen infection.The production of reactive oxygen species (ROS) is one of the earliest cellular responses to pathogen recognition, often involving an oxidative burst. This response can be biphasic, with a transient initial phase followed by a sustained phase, and is correlated with disease resistance. ROS can directly strengthen host cell walls and cause lipid peroxidation, but they also serve as signaling molecules that mediate defense gene activation. The oxidative burst is primarily produced by NADPH oxidases and cell wall peroxidases, with the former being the predominant mechanism. ROS production can occur in various cellular compartments, and their levels are regulated by scavenging systems such as ascorbate peroxidases, glutathione, superoxide dismutases, and catalases. ROS play multiple roles in plant defense, including the formation of defensive barriers, the establishment of systemic defenses, and the regulation of calcium fluxes. They interact with other signaling molecules like salicylic acid (SA) and nitric oxide (NO), and their functions can vary depending on the context and the pathogen involved. Understanding the complex interplay between ROS and other signaling pathways is crucial for comprehending the diverse responses to pathogen infection.