Seminars

Reactive oxygen species (ROS) are produced in response to pathogens and microbe-associated molecular patterns (MAMPs), as exemplified by the rapid extracellular oxidative burst dependent on the NADPH oxidase isoform RBOHD in Arabidopsis thaliana [1-2]. We used the H2 O2 biosensor roGFP2-Orp1 and the glutathione redox state biosensor GRX1-roGFP2 targeted to various organelles to reveal unsuspected oxidative events during the pattern-triggered immune response to flagellin (flg22) and after inoculation with Pseudomonas syringae [3]. roGFP2-Orp1 was oxidised in a biphasic manner one hour and six hours after treatment, with a more intense and faster response in the cytosol compared to chloroplasts, mitochondria and peroxisomes. Peroxisomal and cytosolic GRX1-roGFP2 were also oxidised in a biphasic manner. Interestingly, our results suggest that bacterial effectors partially suppressed the second phase of roGFP2-Orp1 oxidation in the cytosol. Pharmacological and genetic analyses indicated that the MAMPs-induced cytosolic oxidation required the BAK1 and BIK1 signalling components involved in the immune response but was largely independent of NADPH oxidases RBOHD and RBOHF, and apoplastic peroxidases PRX33 and PRX34. The initial apoplastic oxidative burst measured with luminol was followed by a second oxidation burst, both of them preceded the two waves of cytosolic oxidation. In contrast to the cytosolic oxidation, these bursts were RBOHD-dependent. Our results reveal complex oxidative sources and dynamics during the pattern-triggered immune response. The cytosolic oxidation is largely independent of the preceding extracellular oxidation events.