Low-Intensity, Short-Duration Proton Irradiation Enhances Oxidative Stress Sensitivity of Aspergillus nidulans, with Transcriptomic Data Indicating Downregulation of Antioxidative Enzyme Genes

Fungi regularly occur on spacecrafts, posing a serious risk to humans and equipment. In this study, we characterized how the model organism <i>Aspergillus nidulans</i> responds to low-intensity, short-duration proton irradiation designed to simulate a solar particle event, a common stress factor in space. The oxidative stress-sensitive <i>∆atfA</i> mutant exhibited a lower survival rate than the wild-type strain. Pretreatment of the wild-type strain with menadione sodium bisulfite (MSB), which activates oxidative stress defense mechanisms, increased tolerance to proton beam radiation. These data are consistent with the idea that oxidative defense contributes to cellular responses to ionizing radiation. Unexpectedly, the applied radiation decreased the tolerance to MSB. To understand this unusual behavior, we compared the transcriptomes of the irradiated and non-irradiated mycelia. As expected, proton beam irradiation upregulated many genes involved in DNA repair but downregulated a large number of antioxidant enzyme genes. The downregulation of three key antioxidant genes-<i>prxA</i> (thioredoxin peroxidase), <i>trxB</i> (thioredoxin reductase), and <i>gsh1</i> (γ-glutamylcysteine synthase)-was further confirmed by RT-qPCR analysis. One possible explanation is that, due to the rapid elimination of reactive oxygen species generated by water radiolysis, the effects of radiolysis-derived electrons could transiently dominate redox signaling. This shift may interfere with redox sensing in the fungus, resulting in reduced antioxidant gene expression and increased sensitivity to oxidative stress. Oxidative stress sensitivity caused by proton radiation may be the Achilles heel of cells that can survive this stress.