Abstract
Soilborne fungal pathogens cause devastating yield losses and are highly persistent and difficult to control. During the infection process, these organisms must cope with limited availability of iron. Here we show that the bZIP protein HapX functions as a key regulator of iron homeostasis and virulence in the vascular wilt fungus Fusarium oxysporum. Deletion of hapX does not affect iron uptake but causes derepression of genes involved in iron-consuming pathways, leading to impaired growth under iron-depleted conditions. F. oxysporum strains lacking HapX are reduced in their capacity to invade and kill tomato (Solanum lycopersicum) plants and immunodepressed mice. The virulence defect of ΔhapX on tomato plants is exacerbated by coinoculation of roots with a biocontrol strain of Pseudomonas putida, but not with a siderophore-deficient mutant, indicating that HapX contributes to iron competition of F. oxysporum in the tomato rhizosphere. These results establish a conserved role for HapX-mediated iron homeostasis in fungal infection of plants and mammals.
Highlights
Soilborne fungal pathogens are ubiquitous, highly persistent, and extremely difficult to control
Alignment of the amino acid sequence revealed the presence of all the characteristic domains of this class of transcription factors, including an N-terminal region essential for the interaction with the CCAAT binding complex (Hortschansky et al, 2007), a bZIP domain, and several Cys-rich motifs putatively involved in iron sensing (Hortschansky et al, 2007; Schrettl et al, 2010)
The DhapX strains showed no growth defects on rich media, but mycelial growth was markedly reduced under iron-limiting conditions and was almost undetectable in the presence of the iron chelator bathophenanthrolinedisulfonic acid disodium salt (BPS) (Figures 1A and 1B)
Summary
Soilborne fungal pathogens are ubiquitous, highly persistent, and extremely difficult to control. They cause root rots, wilts, stunting, and seedling damping-off in a wide range of plant species, leading to devastating losses in field and greenhouse crops both in industrialized and developing countries. Agricultural practices, such as crop rotation, resistance breeding, and application of fungicides, are insufficient to prevent root diseases of important crop plants (Haas and Défago, 2005).
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