Calcium-dependent protein kinases participate in RBOH-mediated sustained ROS burst during plant immune cell death

The sensing of pathogen effector by an intracellular receptor called nucleotide-binding leucine-rich repeat receptor (NLR), induces a robust immune response, effector-triggered immunity (ETI). Sustained reactive oxygen species (ROS) production is accomplished by Nicotiana benthamiana RBOHB, an NADPH oxidase. However, molecular mechanisms connecting effector recognition and ROS production are unclear. Here, we show that calcium-dependent protein kinases (CDPKs) contribute to sustained ROS production downstream of NLR activation. We found that NbCDPK4 and NbCDPK5 directly phosphorylate NbRBOHB Ser-123 and provokes ROS production. In addition, constitutively active NbCDPKs upregulated NbRBOHB transcription. The phosphorylation of Ser-123 was significantly increased in a Ca²⁺-dependent manner during the ETI-like responses, which execute hypersensitive cell death. Moreover, transient expression of an autoactive helper NLR, NRC4, induced phosphorylation of Ser-123 dependent on its N-terminal conserved motif required for Ca²⁺ channel activity. These findings uncover a critical role for the NbCDPK-NbRBOHB module in regulating sustained ROS production during ETI.