Location: Toxicology & Mycotoxin ResearchTitle: Characterization of two catalase-peroxidase-encoding genes in Fusarium verticillioides reveals differential responses to in vitro versus in planta oxidative challenges
Submitted to: Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2017
Publication Date: 10/24/2017
Citation: Gao, S., Gold, S.E., Glenn, A.E. 2017. Characterization of two catalase-peroxidase-encoding genes in Fusarium verticillioides reveals differential responses to in vitro versus in planta oxidative challenges. Molecular Plant Pathology. doi:10.1111/mpp.12591.
Interpretive Summary: The cells of many organisms, including plants, produce hydrogen peroxide (H2O2) and other so-called “reactive oxygen species” (ROS) as part of their normal cellular development, but ROS molecules are also produced as a defense mechanism against bacterial and fungal pathogens trying to infect the cells. In response, successful plant pathogens must be able to tolerate and detoxify ROS compounds. The catalase/peroxidases are a group of ROS-degrading enzymes in fungi that are interesting because one form of the enzyme is localized within fungal cells and is produced by a broad range of fungi, while a second form is localized outside of the cells and is produced only by fungi that are pathogens of plants. To better understand the function of each of these enzymes in Fusarium verticillioides, a prominent fungal pathogen of corn, the two genes encoding these enzymes, FvCP01 and FvCP02, respectively, were mutated individually. A strain having both genes mutated was also created. These strains and others were grown on media containing H2O2, and we found that the mutants were more sensitive to H2O2 than the normal strain, but the sensitivity differed depending on the form of the fungus used to inoculate the medium. The actively growing fungus (mycelium) was more tolerant of H2O2 than the dormant spores of the fungus. For the spores inoculated on agar medium with H2O2, the gene FvCP01 conferred more tolerance than FvCP02. In contrast, when the fungus was grown on germinating corn seed, data suggested FvCP02 was more active. Given the differential responses of these two F. verticillioides genes to in vitro versus in planta challenges, we proposed a model to illustrate the differing roles of FvCP01 and FvCP02 in protective responses against H2O2-derived oxidative stress.
Technical Abstract: Catalase/peroxidases (KatGs) are a superfamily of reactive oxygen species (ROS)-degrading enzymes believed to be horizontally acquired by ancient Ascomycota from bacteria. Subsequent gene duplication resulted in two KatG paralogs in ascomycetes: the widely distributed intracellular KatG1 group, and the phytopathogen-dominated extracellular KatG2 group. To functionally characterize FvCP01 (KatG1) and FvCP02 (KatG2) in the maize pathogen Fusarium verticillioides, single and double gene deletion mutants were examined in response to hydrogen peroxide (H2O2). Both 'FvCP01 and 'FvCP02 were more sensitive to H2O2 than the wild type in vitro, although their sensitivity differed depending on the type of inoculum. Inoculations using mycelial agar plugs demonstrated an additive effect of the mutants, with the 'FvCP01/'FvCP02 double deletion being most sensitive to H2O2. Conidia in general were much more sensitive to H2O2 than agar plugs, and conidial inoculations indicated FvCP01 conferred more ROS tolerance than FvCP02. Transcriptional analysis showed induction of FvCP01 but decreased expression of FvCP02 in both mycelia and spores in wild type after H2O2 exposure, but this trend was reversed when the fungus was grown on germinating maize seed. This interaction with the plant increased expression of FvCP02 but not FvCP01, indicating FvCP02 may be responsive to plant-derived H2O2. Yet, FvCP01 was induced >3-fold in the 'FvCP02 mutant grown on germlings, suggesting FvCP01 can compensate for the loss of FvCP02. Given the differential responses of these two F. verticillioides genes to in vitro versus in planta challenges, a model is proposed to illustrate the differing roles of FvCP01 and FvCP02 in protective responses against H2O2-derived oxidative stress.