Fungal responses to reactive oxygen species.

Reactive oxygen species (ROS) such as hydrogen peroxide, produced externally or during normal metabolism, can damage different cell components and usually trigger a counteracting antioxidant response. The fact that animals and humans utilize ROS and related nitrogen reactive species to prevent fungal infection has generated great interest in defining the components of the antioxidant response and studying their role as virulence determinants in fungi. Here we review the role of specific enzyme and non-enzyme mediated antioxidant mechanisms in virulence, as well as the signal transduction mechanisms that fungal cells use to perceive high ROS levels and induce gene expression. We focus on Schizosaccharomyces pombe antioxidant responses, which involve a prokaryotic-type multistep phosphorelay coupled to a stress-response MAP kinase pathway and an AP-1 type transcription factor, in relation to homologous mechanisms in Aspergillus nidulans and the human pathogen A. fumigatus. Compared to S. pombe and other unicellular fungi, filamentous fungi have additional mechanisms to handle ROS, such as the presence of a larger number of phosphorelay sensor kinases, antioxidant enzymes and secondary metabolites with antioxidant functions. In addition, filamentous fungi have enzymes like the NADPH oxidases, which regulate multicellular development through ROS production and therefore, offer a unique opportunity to study the interplay between ROS production, perception and detoxification, and the role of these processes in cell differentiation and pathogenesis.