NPS6, Encoding a Nonribosomal Peptide Synthetase Involved in Siderophore-Mediated Iron Metabolism, Is a Conserved Virulence Determinant of Plant Pathogenic Ascomycetes

NPS6, encoding a nonribosomal peptide synthetase, is a virulence determinant in the maize (Zea mays) pathogen Cochliobolus heterostrophus and is involved in tolerance to H2O2. Deletion of NPS6 orthologs in the rice (Oryza sativa) pathogen, Cochliobolus miyabeanus, the wheat (Triticum aestivum) pathogen, Fusarium graminearum, and the Arabidopsis thaliana pathogen, Alternaria brassicicola, resulted in reduced virulence and hypersensitivity to H2O2. Introduction of the NPS6 ortholog from the saprobe Neurospora crassa to the delta nps6 strain of C. heterostrophus restored wild-type virulence to maize and tolerance to H2O2, demonstrating functional conservation in filamentous ascomycete phytopathogens and saprobes. Increased sensitivity to iron depletion was identified as a conserved phenotype of delta nps6 strains. Exogenous application of iron enhanced the virulence of nps6 strains of C. heterostrophus, C. miyabeanus, F. graminearum, and A. brassicicola to each host. Delta NPS6 is responsible for the biosynthesis of extracellular siderophores by C. heterostrophus, F. graminearum, and A. brassicicola. Application of the extracellular siderophore of A. brassicicola restored wild-type virulence of the delta nps6 strain to Arabidopsis. It is proposed that the role of extracellular siderophores in fungal virulence to plants is to supply an essential nutrient, iron, to their producers in planta and not to act as phytotoxins, depriving their hosts of iron.