Contrasting roles of fungal siderophores in maintaining iron homeostasis in Epichloë festucae

The symbiosis between Epichloё festucae and its host perennial ryegrass (Lolium perenne) is a model system for mutualistic interactions where the fungal endophyte grows between plant shoot cells and acquires host nutrients to survive. E. festucae synthesises the siderophore ferriepichloënin A (FEA) via SidN, a non-ribosomal peptide synthetase (NRPS). FEA is involved in the acquisition of iron, an essential micronutrient, as part of the process of maintaining the symbiotic interaction. Here, we mutated a different NRPS gene sidC and showed that it is required for production of a second siderophore ferricrocin (FC). Furthermore mutations in sidA, encoding an L-ornithine N5-monooxygenase, abolished both FEA and FC production. Axenic growth phenotypes of the siderophore mutants were altered relative to wild-type (WT) providing insights into the roles of E. festucae siderophores in iron trafficking and consequently in growth and morphogenesis. During iron-limitation, FEA is the predominant siderophore and in addition to its role in iron acquisition it appears to play roles in intracellular iron sequestration and oxidative stress tolerance. FC in contrast is exclusively located intracellularly and is the dominant siderophore under conditions of iron sufficiency when it is likely to have roles in iron storage and iron transport. Intriguingly, FEA acts to promote but may also moderate E. festucae growth (depending on the amount of available iron). We therefore hypothesise that coordinated cellular iron sequestration through FC and FEA may be one of the mechanisms that E. festucae employs to manage and restrain its growth in response to iron fluxes and ultimately persist as a controlled symbiont.