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NADPH oxidases/RBOHs catalyze apoplastic ROS production and act as key signaling nodes, integrating multiple signal transduction pathways regulating plant development and stress responses. Although RBOHs have been suggested to be activated by Ca²⁺ binding and phosphorylation by various protein kinases, a mechanism linking Ca²⁺ binding and phosphorylation in the activity regulation remained elusive. Chitin‐triggered ROS production required cytosolic Ca²⁺ elevation and Ca²⁺ binding to MpRBOHB in a liverwort Marchantia polymorpha. Heterologous expression analysis of truncated variants revealed that a segment of the N‐terminal cytosolic region highly conserved among land plant RBOHs encompassing the two EF‐hand motifs is essential for the activation of MpRBOHB. Within the conserved regulatory domain, we have identified two Ser residues whose phosphorylation is critical for the …