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This article discusses the role of reactive oxygen species (ROS) and phosphatidic acid (PA) as important second messengers in plant immunity. The authors focus on the interaction between DGK5, a lipid kinase that regulates PA production, and RBOHD, an NADPH oxidase responsible for ROS production. They found that DGK5-derived PA stabilizes RBOHD, leading to enhanced ROS production and promoting plant immune responses. The authors also identified specific kinases that phosphorylate DGK5, regulating its enzymatic activity and maintaining PA homeostasis. These findings contribute to our understanding of the complex signaling network involved in plant immune responses. [Extracted from the article]

Biphasic production of reactive oxygen species (ROS) has been observed in plants treated with avirulent bacterial strains. The first transient peak corresponds to pattern-triggered immunity (PTI)-ROS, whereas the second long-lasting peak corresponds to effector-triggered immunity (ETI)-ROS. PTI-ROS are produced in the apoplast by plasma membrane-localized NADPH oxidases, and the recognition of an avirulent effector increases the PTI-ROS regulatory module, leading to ETI-ROS accumulation in the apoplast. However, how apoplastic ETI-ROS signaling is relayed to the cytosol is still unknown. Here, we found that in the absence of cytosolic ascorbate peroxidase 1 (APX1), the second phase of ETI-ROS accumulation was undetectable in Arabidopsis (Arabidopsis thaliana) using luminol-based assays. In addition to being a scavenger of cytosolic H2O2, we discovered that APX1 served as a catalyst in this chemiluminescence ROS assay by employing luminol as an electron donor. A horseradish peroxidase (HRP)-mimicking APX1 mutation (APX1W41F) further enhanced its catalytic activity toward luminol, whereas an HRP-dead APX1 mutation (APX1R38H) reduced its luminol oxidation activity. The cytosolic localization of APX1 implies that ETI-ROS might accumulate in the cytosol. When ROS were detected using a fluorescent dye, green fluorescence was observed in the cytosol 6 h after infiltration with an avirulent bacterial strain. Collectively, these results indicate that ETI-ROS eventually accumulate in the cytosol, and cytosolic APX1 catalyzes luminol oxidation and allows monitoring of the kinetics of ETI-ROS in the cytosol. Our study provides important insights into the spatial dynamics of ROS accumulation in plant immunity., Competing Interests: Conflict of interest statement. The authors declare that they have no conflict of interest., (© American Society of Plant Biologists 2022. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)