Reactive oxygen species dosage in Arabidopsis chloroplasts can improve resistance towards Colletotrichum higginsianum by the induction of WRKY33

Arabidopsis plants overexpressing glycolate oxidase in chloroplasts (GO5) and loss-of-function mutants of the major peroxisomal catalase isoform, cat2-2, produce increased hydrogen peroxide (H2 O2 ) amounts from the respective organelles when subjected to photorespiratory conditions like increased light intensity. Here, we have investigated if and how the signaling processes triggered by H2 O2 production in response to shifts in environmental conditions and the concomitant induction of indole phytoalexin biosynthesis in GO5 affect susceptibility towards the hemibiotrophic fungus Colletotrichum higginsianum. Combining histological, biochemical, and molecular assays, we found that the accumulation of the phytoalexin camalexin was comparable between GO genotypes and cat2-2 in the absence of pathogen. Compared with wild-type, GO5 showed improved resistance after light-shift-mediated production of H2 O2 , whereas cat2-2 became more susceptible and allowed significantly more pathogen entry. Unlike GO5, cat2-2 suffered from severe oxidative stress after light shifts, as indicated by glutathione pool size and oxidation state. We discuss a connection between elevated oxidative stress and dampened induction of salicylic acid mediated defense in cat2-2. Genetic analyses demonstrated that induced resistance of GO5 is dependent on WRKY33, but not on camalexin production. We propose that indole carbonyl nitriles might play a role in defense against C. higginsianum.