The bZIP transcription factor Fgap1 mediates oxidative stress response and trichothecene biosynthesis but not virulence in Fusarium graminearum

Abstract

Redox sensing is of primary importance for fungi to cope with oxidant compounds found in their environment. Plant pathogens are particularly subject to the oxidative burst during the primary steps of infection. In the budding yeast Saccharomyces cerevisiae, it is the transcription factor Yap1 that mediates the response to oxidative stress via activation of genes coding for detoxification enzymes. In the cereal pathogen Fusarium graminearum, Fgap1 a homologue of Yap1 was identified and its role was investigated. During infection, this pathogen produces mycotoxins belonging to the trichothecenes family that accumulate in the grains. The global regulation of toxin biosynthesis is not completely understood. However, it is now clearly established that an oxidative stress activates the production of toxins by F. graminearum. The involvement of Fgap1 in this activation was investigated. A deleted mutant and a strain expressing a truncated constitutive form of Fgap1 were constructed. None of the mutants was affected in pathogenicity. The deleted mutant showed higher level of trichothecenes production associated with overexpression of Tri genes. Moreover activation of toxin accumulation in response to oxidative stress was no longer observed. Regarding the mutant with the truncated constitutive form of Fgap1, toxin production was strongly reduced. Expression of oxidative stress response genes was not activated in the deleted mutant and expression of the gene encoding the mitochondrial superoxide dismutase MnSOD1 was up-regulated in the mutant with the truncated constitutive form of Fgap1. Our results demonstrate that Fgap1 plays a key role in the link between oxidative stress response and F. graminearum secondary metabolism.

Figure 1. Radial growth assay to evaluate stress tolerance in F. graminearum wild-type strain and mutants.

F. graminearum wild-type strain, ΔFgap1, ΔFgap1:ap1, and Fgap1 ^c _trunc were grown 7 days under oxidative stress with cadmium chloride 150 µM (C_dCl₂) or H₂O₂ (15 mM), osmotic stress with sorbitol (1 M) or NaCl (1 M).

Figure 2. Fgap1 plays a role in toxin accumulation and Tri genes expression.

(A) DON + 15ADON yields (in ng of toxins produced per mg of dry fungal biomass) in the wild-type and the mutant strains after 3, 5 and 14 days of growth, ND stands for not detected. Error bars represent the standard deviation of three biological replicates. (B) Expression ratio mutant/wild-type (in log2 ratio scale) for Tri genes after 5 days of culture. The star indicates a significant difference compared to the wild-type (p<0.05).

Figure 3. Wheat ears inoculated with F. graminearum wild-type strain, ΔFgap1, ΔFgap1:ap1, and Fgap1 ^c _trunc.

Wheat ears 21 days after point inoculation of two central spikelets. Bleaching indicates successful infection. Infection assays were performed with 10 replicates for each strain. For each strain, calculated disease index (% of infected spikelets) was: WT = 90.2 ± 25.6, ΔFgap1 = 94.2 ± 11.1, ΔFgap1:ap1 = 90.0 ± 22.4, Fgap1 ^c _trunc = 77.9 ± 25.2.

Figure 4. Toxin production and Tri genes expression in the wild-type strain and in the ΔFgap1 and Fgap1 ^c _trunc mutants after exposure to oxidative stress by H₂O₂.

F. graminearum wild-type or mutants were cultured up to 14 days in liquid medium supplemented or not supplemented with 0.5 mM H₂O₂. (A) DON + 15ADON yield (ng of toxins per mg of dry fungal biomass) after 3, 5 or 14 days of growth. Error bars represent the standard deviation of three biological replicates. The star indicates a significant difference for the treated condition compared to the control condition (p<0.05). (B) Tri genes expression (expression ratios treated/not treated in log2 scale) in 5 day-old cultures. Tri4, Tri5, Tri6, Tri10, Tri12, and Tri101 were considered. The star indicates a significant difference of expression between treated vs. not treated (p<0.05).

Figure 5. Expression of genes encoding antioxidant activities in the wild-type strain and in the ΔFgap1 and Fgap1 ^c _trunc mutants.

(A) Antioxidant genes expression (expression ratio treated/not treated in log2 scale) in the wild-type strain in 5-day old cultures. The star indicates a significant difference between treated vs. not treated (p<0.05). ND stands for not detected. (B) Antioxidant genes expression in the mutants strain compared to wild-type strain (expression ratios mutant/wild-type strain in log2 scale) in 5-day old cultures. The star indicates a significant difference for mutant compared to wild-type (p<0.05).