High-affinity iron uptake is required for optimal Epichloe festucae colonization of Lolium perenne and seed transmission

Epichlo & euml; festucae uses a siderophore-mediated system to acquire iron, which is im-portant to maintain endophyte- grass symbioses. Here we investigate the roles of the alternative iron acquisition system, reductive iron assimilation (RIA), via disruption of the fetC gene, which encodes a multicopper ferroxidase, either alone (i.e., AfetC) or in combination with disruption of the gene sidA, which encodes a siderophore bio-synthesis enzyme (i.e., AfetC/AsidA). The phenotypic characteristics of these mutants were compared to AsidA and wild- type (WT) strains during growth under axenic cul-ture conditions (in culture) and in symbiosis with the host grass, perennial ryegrass (in planta). Under iron deficiency, the colony growth rate of AfetC was slightly slower than that of WT, while the growth of AsidA and AfetC/AsidA mutants was severely suppressed. Siderophore analyses indicated that AfetC mutants hyperaccumulate fer-riepichlo & euml;nin A (FEA) at low iron concentrations and ferricrocin and FEA at higher iron concentrations. When compared to WT, all mutant strains displayed hyperbranch-ing hyphal structures and a reduced ratio of Epichlo & euml; DNA to total DNA in planta. Furthermore, host colonization and vertical transmission through infection of the host seed were significantly reduced in the AfetC/AsidA mutants, confirming that high-affinity iron uptake is a critical process for Epichlo & euml; transmission. Thus, RIA and siderophore iron uptake are complementary systems required for the maintenance of iron metabolism, fungal growth, and symbiosis between E. festucae and perennial ryegrass.

Automated summary

Epichloë festucae utilizes a siderophore-mediated system and reductive iron assimilation (RIA) to acquire iron, which is crucial for maintaining endophyte-grass symbioses. Disruption of the fetC gene, encoding a ferroxidase, and the sidA gene, encoding a siderophore biosynthesis enzyme, revealed that RIA and siderophore iron uptake are essential for iron metabolism, fungal growth, and transmission in Epichloë festucae. Mutants lacking AfetC/AsidA displayed inhibited growth, hyperaccumulation of siderophores, abnormal hyphal structures, reduced DNA ratio in planta, and decreased host colonization and vertical transmission.