Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2006
Publication Date: 12/1/2006
Citation: Oide, S., Moeder, W., Krasnoff, S., Gibson, D.M., Haas, H., Yoshioka, K., Turgeon, G. 2006. Nps6, encoding a non-ribosomal peptide synthetase involved in siderophore-mediated iron metabolism, is a conserved virulence determinant of plant pathogenic ascomycetes. The Plant Cell. 18:2836-2853. Interpretive Summary: Genome-wide searches of fungal systems have revealed a number of genes involved in secondary metabolites for which no products are known. One type of gene family involved in forming secondary metabolites are nonribosomally derived peptide synthetases (NRPSs), and these genes are known to encode small peptides that may play major roles in fungal virulence. In this study, we sought to define the products of a particular gene, NRPS6, and understand its role in fungal virulence, fungal morphology and development, and sensitivity to oxidative stress. Since structural analyses suggested that NPS6 is conserved among diverse species of fungi, this study employed 3 different fungal strains and their plant hosts. We found that NPRS6 is responsible for the synthesis of a related family of siderophores, coprogens/fusarinines, known to be involved in extracellular iron acquisition. The results of this study establish a clear role for siderophores in plant-pathogen interactions by providing the essential nutrient iron to the fungus during infection.
Technical Abstract: NPS6, encoding a non-ribosomal peptide synthetase, is a virulence determinant in the corn pathogen Cochliobolus heterostrophus and is also involved in resistance to oxidative stress, generated by hydrogen peroxide. Deletion of NPS6 orthologs in the rice pathogen, Cochliobolus miyabeanus, the cereal pathogen, Fusarium graminearum, and the Arabidopsis thaliana pathogen, Alternaria brassicicola, resulted also in reduced virulence to each host and hypersensitivity to H2O2. Introduction of the NPS6 ortholog from a saprobe Neurospora crassa, to the 'nps6 strain of C. heterostrophus, restored wild-type virulence to corn and tolerance to H2O2, demonstrating functional conservation in pathogens and saprobes. Further characterization identified increased sensitivity to iron depletion as a conserved phenotype of the Dnps6 strains. HPLC and MS analyses of C. heterostrophus, F. graminearum and A. brassicicola revealed that the NPS6 protein is responsible for biosynthesis of extracellular siderophores in all three species. Exogenous application of iron enhanced virulence of 'nps6 strains of C. heterostrophus, C. miyabeanus, F. graminearum, and A. brassicicola, to each host. Application of the extracellular siderophore of A. brassicicola restored WT virulence of the DAbnps6 strain to A. thaliana. Overall, the results demonstrate that extracellular siderophores are virulence determinants, conserved among filamentous ascomycete phytopathogens. We propose 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.