The role of reactive oxygen species in the virulence of wheat leaf rust fungus Puccinia triticina

ROS, or Reactive Oxygen Species are a suite of highly damaging oxygen molecul species that play an important role during the infection of a host plant by a pathogen, and are often an indication of induced host defense responses. In this study, we demonstrate for the first time that the leaf rust fungus, Puccinia triticina (Pt), generates ROS during its infection of its wheat host plant. We experiment with chemical compounds that inhibit various known mechanisms of ROS production and could show that these various Reactive Oxygen Species are critical for the spores of this fungus to germinate when they land on the wheat leaf surface, and for the subsequent infection. These processes are inhibited when using these chemical compounds. By performing a large-scale analysis comparing different stages of the infection, we identified 291 fungal genes that are potentially responsible for the various known mechanisms of ROS production. Thirty-seven of these genes produce known proteins and we could show that the fungus uses five of them (named PtNoxA, PtNoxB, PtNoxR, PtCat and PtSod) when its spores germinate just before the infection. A bit later, when infecting the wheat leaves, the expression of four genes (PtNoxA, PtNoxB, PtNoxR and PtCat) is switched off, meaning they are less likely to play a role during that phase of the infection. However, the expression of PtSod is increased up to 120 hours after infection, showing its further importance. We conclude that these Reactive Oxygen Species play a critical role for the full virulence of this fungus, but that a reduction one ROS compound, nitric oxide, is necessary for successful infection in wheat. These studies reveal potential targets for pathogen control.