Investigating the Efficiency of Hydroxycinnamic Acids to Inhibit the Production of Enniatins by Fusarium avenaceum and Modulate the Expression of Enniatins Biosynthetic Genes

Enniatins (ENNs) that belong to the group of emerging mycotoxins are widespread contaminants of agricultural commodities. There is currently insufficient evidence to rule out health concerns associated with long-term exposure to ENNs and efforts must be strengthened to define a control strategy. While the potential of plant compounds to counteract the contamination with legislated mycotoxins has been reported, little remains known regarding ENNs. The present study evidenced for the first time the efficiency of hydroxycinnamic acids to inhibit the fungal growth and ENNs yield by Fusarium avenaceum. Notably, 0.5 mM of exogenous ferulic, caffeic, and p-coumaric acids led to a drastic reduction of ENNs synthesis in pH4 broths, with ferulic acid being the most potent. The ENNs production inhibitory activity of ferulic acid was shown to be associated with a significant down-regulation of the expression of ENNs biosynthetic genes. To further investigate the bioactivity of ferulic acid, its metabolic fate was characterized in fungal broths and the capacity of F. avenaceum to metabolize it through a C2-cleavage type degradation was demonstrated. Overall, our data support the promising use of ferulic acid in ENNs control strategies, either as part of an environmentally friendly plant-care product or as a biomarker of plant resistance

The bZIP Transcription Factor Fgap1 Mediates Oxidative Stress Response and Trichothecene Biosynthesis But Not Virulence in <i>Fusarium graminearum</i>

<div><p>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 <i>Saccharomyces cerevisiae</i>, 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 <i>Fusarium graminearum</i>, 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 <i>F. graminearum</i>. The involvement of <i>Fgap1</i> in this activation was investigated. A deleted mutant and a strain expressing a truncated constitutive form of <i>Fgap1</i> were constructed. None of the mutants was affected in pathogenicity. The deleted mutant showed higher level of trichothecenes production associated with overexpression of <i>Tri</i> genes. Moreover activation of toxin accumulation in response to oxidative stress was no longer observed. Regarding the mutant with the truncated constitutive form of <i>Fgap1</i>, 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 <i>Fgap1</i>. Our results demonstrate that <i>Fgap1</i> plays a key role in the link between oxidative stress response and <i>F. graminearum</i> secondary metabolism.</p> </div

Wheat ears inoculated with <i>F. graminearum</i> wild-type strain, <i>ΔFgap1, ΔFgap1:ap1</i>, and <i>Fgap1</i>^c_trunc.

<p>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, <i>ΔFgap1</i> = 94.2 ± 11.1, <i>ΔFgap1:ap1</i> = 90.0 ± 22.4, <i>Fgap1</i>^<i>c</i>_trunc = 77.9 ± 25.2. </p

Toxin production and <i>Tri</i> genes expression in the wild-type strain and in the <i>ΔFgap1</i> and <i>Fgap1</i>^c_trunc mutants after exposure to oxidative stress by H₂O₂.

<p><i>F. graminearum</i> 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) <i>Tri</i> genes expression (expression ratios treated/not treated in log2 scale) in 5 day-old cultures. <i>Tri4, Tri5, Tri6, Tri10, Tri12</i>, and <i>Tri101</i> were considered. The star indicates a significant difference of expression between treated <i>vs</i>. not treated (p<0.05).</p

Expression of genes encoding antioxidant activities in the wild-type strain and in the <i>ΔFgap1</i> and <i>Fgap1</i>^c_trunc mutants.

<p>(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 <i>vs</i>. 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).</p

<i>Fgap1</i> plays a role in toxin accumulation and <i>Tri</i> genes expression.

<p>(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 <i>Tri</i> genes after 5 days of culture. The star indicates a significant difference compared to the wild-type (p<0.05).</p

Radial growth assay to evaluate stress tolerance in <i>F. graminearum</i> wild-type strain and mutants.

<p><i>F. graminearum</i> wild-type strain, <i>ΔFgap1</i>, <i>ΔFgap1:ap1</i>, and <i>Fgap1</i>^<i>c</i>_<i>trunc</i> 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). </p

Interaction between the Accumulation of Cadmium and Deoxynivalenol Mycotoxin Produced by Fusarium graminearum in Durum Wheat Grains

International audienceDurum wheat is one of the cereal crops that accumulates the highest concentrations of cadmium (Cd) and deoxynivalenol (DON) mycotoxin in its grains, thereby affecting the safety of products made of durum wheat grains (pasta and semolina). This study investigates in planta the interaction between Cd and Fusarium graminearum, the main causal agent of DON accumulation in grains. A pot experiment was designed to characterize the response of durum wheat to F. graminearum infection at three levels of Cd exposure: 0.1, 2, and 10 mg Cd kg(-1) soil, which showed that the accumulation of Cd and DON resulted from interacting processes. On the one hand, plant exposure to Cd reduced the concentration of DON in grains. The mitigating effect of Cd on DON accumulation was attributed to the restricted growth of F. graminearum, which could result from enhanced plant resistance to the fungal pathogen induced by Cd exposure. On the other hand, F. graminearum infection of durum wheat increased the Cd concentration in the grains. The promoting effect of Fusarium infection on Cd accumulation was attributed to decoupling of the allocation of Cd and photoassimilates to the grains and to the reduced strength of the grain sink for photoassimilates caused by the fungus. Provided that this result is confirmed in field conditions, it suggests that in Cd-contaminated soils, particular attention should be paid to agronomic practices that affect Fusarium head blight disease to avoid further increase in the risk of exceeding the regulatory limit set by the European Union for Cd in durum wheat