Apoplastic ROS signaling in plant immunity

•NADPH oxidases are major enzymes synthesizing ROS in the apoplast.•NADPH oxidases are information processing center for diverse signal inputs.•Stomatal movement provides an excellent experimental system for ROS signaling studies.•Receptor-like kinases have emerged as key components in ROS signaling.

Reactive oxygen species (ROS) are widely produced in different cellular compartments under both biotic and abiotic stress conditions. ROS play a central role in plant signaling and regulate diverse cellular processes. Recent advances are shedding new light on sophisticated mechanisms controlling ROS biogenesis and signaling in plant immunity. In this review, we summarize our current understanding of the regulation of apoplastic ROS production in response to microbial molecular patterns and draw comparison with abscisic acid (ABA)-induced apoplastic ROS. We also discuss how ROS act as signal molecules to regulate cellular activities using stomatal movement as an example.