Fungal Endophyte Production of Reactive Oxygen Species is Critical for Maintaining the Mutualistic Symbiotic Interaction Between Epichloe festucae and Perennial Ryegrass

Key requirements for microbes to initiate and establish mutualistic symbiotic interactions with plants are evasion of potential host defense responses and strict control of microbial growth. We have recently shown that reactive oxygen species (ROS) produced by a specific fungal NADPH oxidase isoform NoxA, have a critical role in regulating hyphal growth in the mutualistic interaction between Epichloe festucae and perennial ryegrass. Regulation of ROS production in the symbiosis requires two additional components, NoxR and RacA, homologues of the mammalian p67phox and Rac2. Perennial ryegrass host plants containing noxA or noxR mutants lose apical dominance, become severely stunted, and undergo precocious senescence. Our working model proposes that hyphal tip growth and branching is controlled by localized bursts of ROS catalysed by NoxA, following recruitment of NoxR and RacA from the cytosol to the membrane in response to signaling from the grass host.