Reductive iron assimilation and intracellular siderophores assist extracellular siderophore-driven iron homeostasis and virulence.

Iron is an essential nutrient and prudent iron acquisition and management are key traits of a successful pathogen. Fungi use nonribosomally synthesized secreted iron chelators (siderophores) or reductive iron assimilation (RIA) mechanisms to acquire iron in a high affinity manner. Previous studies with the maize pathogen Cochliobolus heterostrophus identified two genes, NPS2 and NPS6, encoding different nonribosomal peptide synthetases responsible for biosynthesis of intra- and extracellular siderophores, respectively. Deletion of NPS6 results in loss of extracellular siderophore biosynthesis, attenuated virulence, hypersensitivity to oxidative and iron-depletion stress, and reduced asexual sporulation, while nps2 mutants are phenotypically wild type in all of these traits but defective in sexual spore development when NPS2 is missing from both mating partners. Here, it is reported that nps2nps6 mutants have more severe phenotypes than both nps2 and nps6 single mutants. In contrast, mutants lacking the FTR1 or FET3 genes encoding the permease and ferroxidase components, respectively, of the alternate RIA system, are like wild type in all of the above phenotypes. However, without supplemental iron, combinatorial nps6ftr1 and nps2nps6ftr1 mutants are less virulent, are reduced in growth, and are less able to combat oxidative stress and to sporulate asexually, compared with nps6 mutants alone. These findings demonstrate that, while the role of RIA in metabolism and virulence is overshadowed by that of extracellular siderophores as a high-affinity iron acquisition mechanism in C. heterostrophus, it functions as a critical backup for the fungus.