Chapter 15 Cell differentiation as a response to oxidative stress

Abstract

Abstract Photosynthetic dioxygen (O 2 ) discharge and accumulation in the atmosphere compelled cells to adapt or to die in the presence of a reactive compound and to protect themselves from it and from the inevitable formation of more reactive oxygen species (ROS). At the same time, by using O 2 as final electron acceptor, early heterotrophs could obtain more energy from reduced carbon and attain faster growth. However, fast growing aerobes were made more dependent on the availability of reduced carbon. In the fight for food, microorganisms colonized different niches including other organisms, which they parasitized or ate. With time, life-damaging ROS became signals used by cells to regulate growth and proliferation, cell differentiation and cell death. We have proposed that cell differentiation is a response to oxidative stress. In different eukaryotic microorganisms, the presence of ROS and the induction of antioxidant mechanisms are associated with development. Moreover, the superoxide-producing NADPH-oxidases proved essential for some developmental processes. The small GTPase RAS-1 also regulates the production of ROS and different mutations in the corresponding gene affect cell differentiation. The process of asexual reproduction (conidiation) in Neurospora crassa involves three morphogenetic transitions, each preceded by a hyperoxidant state. An accentuated redox imbalance, the oxidation and degradation of oxidized protein, oxidative inactivation of different enzyme activities, and increased production of ROS characterize the hyperoxidant state. Null mutants in catalase genes show increased levels of development. Strains without superoxide dismutase express a conidiation rhythm, similar to the “band” strain, which contains a semi-dominant ras -1 mutation. Mutation of NADPH-oxidase genes reduced or completely abolished different developmental processes. Together, these results support our view on cell differentiation as a response to oxidative stress.