Functional analysis of endophyte genes required for maintaining the mutualistic symbiosis between Epichloë festucae and perennial ryegrass

Abstract

  Endophytic fungi of the Epichloë group systemically colonize the intercellular spaces of grass plants to establish a symbiotic association. Key requirements for microbes to initiate and establish mutualistic symbiotic interactions with plants are evasion of potential host defense responses and strict control of the growth of the fungal endophyte throughout the host plant. We have recently shown that reactive oxygen species (ROS) produced by a specif c fungal NADPH oxidase isoform NoxA, have a critical role in regulating hyphal growth in the mutualistic interaction between E. festucae and perennial ryegrass. Regulation of ROS production in the symbiosis requires two additional components, NoxR and RacA, homologues of the mammalian p 67^phox and Rac 2. Perennial ryegrass host plants containing noxA, noxR or racA 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 production of ROS catalyzed by NoxA, following recruitment of NoxR and RacA from the cytosol to the membrane in response to signaling from the grass host.