Use of chemosensitization to overcome fludioxonil resistance in Penicillium expansum

A Secondary Metabolite Secreted by Penicillium citrinum Is Able to Enhance Parastagonospora nodorum Sensitivity to Tebuconazole and Azoxystrobin

Parastagonospora nodorum causes glume and leaf blotch of wheat, a harmful disease resulting in serious losses in grain yield. In many countries including Russia, fungicidal formulations based on triazoles and on triazoles combined with strobilurins are used to control this fungus. However, their prolonged application may promote the selection of fungicide-resistant strains of P. nodorum leading to significant attenuation or even loss of fungicidal effect. Chemosensitization of plant pathogenic fungi with natural compounds represents a promising strategy for mitigating fungicide resistance and other negative impacts of fungicides. In this work, we applied a chemosensitization approach towards P. nodorum strains non-resistant or resistant to tebuconazole or azoxystrobin using 6-demethylmevinolin (6-DMM), a metabolite of Penicillium citrinum. The resistant strains were obtained by the mutagenesis and subsequent culturing on agar media incorporated with increasing doses of Folicur® EC 250 (i.e., tebuconazole) or Quadris® SC 250 (i.e., azoxystrobin). Test strains m8-4 and kd-18, most resistant to tebuconazole and azoxystrobin, respectively, were selected for sensitization experiments. These experiments demonstrated that combining 6-DMM with Folicur® enhanced fungicidal effectiveness in vitro and in vivo in addition to attenuating the resistance of P. nodorum to tebuconazole in vitro. 6-DMM was also found to augment Quadris® efficacy towards kd-18 when applied on detached wheat leaves inoculated with this strain. Experiments on P. nodorum sensitization under greenhouse conditions included preventive (applying test compounds simultaneously with inoculation) or post-inoculation spraying of wheat seedlings with 6-DMM together with Folicur® at dose rates (DR) amounting to 10% and 20% of DR recommended for field application (RDR). Combined treatments were run in parallel with using the same DR of the fungicide and sensitizer, alone. A synergistic effect was observed in both preventive and post-inoculation treatments, when the sensitizer was co-applied with the fungicide at 10% of the RDR. In this case, disease reduction significantly exceeded the protective effect of Folicur® at 10% or 20% of the RDR, alone, and also a calculated additive effect. Collectively, our findings suggest that 6-DMM is promising as a putative component for formulations with triazole and strobilurin fungicides. Such new formulations would improve fungicide efficacy and, potentially, lower rates of fungicides needed for plant pathogen control.

Antimicrobial Efficacy of Edible Mushroom Extracts: Assessment of Fungal Resistance

Antimicrobial efficacy of the water or methanolic extracts of three medicinal mushrooms Taiwanofungus camphoratus, Agaricus blazei Murrill, and Ganoderma lucidum (Curtis) P. Karst were investigated against yeast and filamentous fungal pathogens as well as against commensal and pathogenic bacteria. The methanolic extract of T. camphoratus (TcM) exhibited both potent antifungal and antibacterial activity, while the water extract of T. camphoratus (TcW) showed limited antibacterial activity against Listeria monocytogenes. Neither the methanolic nor water extracts of A. blazei and G. lucidum exhibited antimicrobial activity. In the risk assessment testing monitoring the development of fungal tolerance to mushroom extracts in food matrices, two P. expansum mitogen-activated protein kinase (MAPK) mutants exhibited a tolerance to TcM. In a proof-of-concept bioassay using the natural benzoic salicylaldehyde (SA), P. expansum and A. fumigatus MAPK antioxidant mutants showed similar tolerance to SA, suggesting that natural ingredients in TcM such as benzoic derivatives could negatively affect the efficacy of TcM when antioxidant mutants are targeted. Conclusion: TcM could be developed as a food ingredient having antimicrobial potential. The antimicrobial activity of TcM operates via the intact MAPK antioxidant signaling system in microbes, however, mutants lacking genes in the MAPK system escape the toxicity triggered by TcM. Therefore, caution should be exercised in the use of TcM so as to not adversely affect food safety and quality by triggering the resistance of antioxidant mutants in contaminated food.

Synergistic Interactions of Schizostatin Identified from Schizophyllum commune with Demethylation Inhibitor Fungicides

Botrytis cinerea, which causes gray mold disease in more than 200 plant species, is an economically important pathogen that is mainly controlled by synthetic fungicides. Synergistic fungicide mixtures can help reduce fungicide residues in the environment and mitigate the development of fungicide-resistant strains. In this study, we screened microbial culture extracts on Botrytis cinerea to identify an antifungal synergist for tebuconazole. Among the 4,006 microbial extracts screened in this study, the culture extract from Schizophyllum commune displayed the most enhanced activity with a sub-lethal dosage of tebuconazole, and the active ingredient was identified as schizostatin. In combination with 5 μg/ml tebuconazole, schizostatin (1 μg/ml) showed disease control efficacy against gray mold on tomato leaf similar to that achieved with 20 μg/ml tebuconazole treatment alone. Interestingly, schizostatin showed demethylation inhibitor (DMI)-specific synergistic interactions in the crossed-paper strip assay using commercial fungicides. In a checkerboard assay with schizostatin and DMIs, the fractional inhibitory concentration values were 0.0938-0.375. To assess the molecular mechanisms underlying this synergism, the transcription levels of the ergosterol biosynthetic genes were observed in response to DMIs, schizostatin, and their mixtures. Treatment with DMIs increased the erg11 (the target gene of DMI fungicides) expression level 15.4-56.6-fold. However, treatment with a mixture of schizostatin and DMIs evidently reverted erg11 transcription levels to the pre-DMI treatment levels. These results show the potential of schizostatin as a natural antifungal synergist that can reduce the dose of DMIs applied in the field without compromising the disease control efficacy of the fungicides.

Naturally Occurring Phenols Modulate Vegetative Growth and Deoxynivalenol Biosynthesis in Fusarium graminearum

To assess the in vitro activity of five naturally occurring phenolic compounds (ferulic acid, apocynin, magnolol, honokiol, and thymol) on mycelial growth and type B trichothecene mycotoxin accumulation by Fusarium graminearum, three complementary approaches were adopted. First, a high-throughput photometric continuous reading array allowed a parallel quantification of F. graminearum hyphal growth and reporter TRI5 gene expression directly on solid medium. Second, RT-qPCR confirmed the regulation of TRI5 expression by the tested compounds. Third, liquid chromatography-tandem mass spectrometry analysis allowed quantification of deoxynivalenol (DON) and its acetylated forms released upon treatment with the phenolic compounds. Altogether, the results confirmed the activity of thymol and an equimolar mixture of thymol-magnolol at 0.5 mM, respectively, in inhibiting DON production without affecting vegetative growth. The medium pH buffering capacity after 72-96 h of incubation is proposed as a further element to highlight compounds displaying trichothecene inhibitory capacity with no significant fungicidal effect.

Uncertainty surrounding the mechanism and safety of the post-harvest fungicide fludioxonil

Fludioxonil is a phenylpyrrole pesticide that is applied to fruit and vegetable crops post-harvest to minimize losses to mold, both during transport and at point of sale. Its effectiveness is reflected in the dramatic increase in its production/usage since its introduction in 1994, an increase that has peaked in recent years as it became licenced for use abroad. Recently, doubts as to the nature of its mechanism of action have been raised. Given that the pesticide has long been known to induce stress intermediates in target and non-target organisms alike, the lack of a firmly established mechanism might be cause for concern. Troubling reports further delineate a capacity to disrupt hepatic, endocrine and neurological systems, indicating that fludioxonil may represent a health threat to consumers. In the absence of a clear, safe mechanism of action, fludioxonil should be re-evaluated for its potential to impact human health.

The Response of Rhodotorula mucilaginosa to Patulin Based on Lysine Crotonylation

Patulin (PAT) is a mycotoxin produced by some Penicillium, Aspergillus, and Byssochlamys species. Rhodotorula mucilaginosa is able to degrade PAT in vivo as well as in vitro, up till date, the process and molecular mechanism(s) involved patulin degradation still remains unknown. Protein lysine crotonylation (Kcr) plays an important role in regulating chromatin dynamics, gene expression, and metabolic pathways in mammals and eukaryotes. Investigation of the Kcr changes accompanying degradation of patulin in R. mucilaginosa were observed to investigate the mechanisms of patulin inhibition. Tandem mass tag (TMT) labeling and Kcro affinity enrichment, followed by high-resolution LC-MS/MS analysis, were used to perform quantitative lysine crotonylome analysis on R. mucilaginosa. Consequently, 1691 lysine crotonylation sites in 629 protein groups were identified, among which we quantified 1457 sites in 562 proteins. Among the quantified proteins, 79 and 46 crotonylated proteins were up-regulated and down-regulated, respectively. The differentially up expressed modified proteins were mainly involved in tricarboxylic acid cycle and gluconeogenic pathway. The differentially down expressed Kcr proteins were mainly classified to ribosome and carbohydrate transport and metabolism. Bioinformatic analyses were performed to annotate the quantifiable lysine crotonylated targets. Moreover, interaction networks and high confidence domain architectures of crotonylated proteins were investigated with the aid of bioinformatic tools, and these results showed that there was an increase in the number of yeasts with crotonylated proteins. The results also provided information on the various roles of crotonylation, which are involved in PAT degradation.

Chemosensitization of plant pathogenic fungi to agricultural fungicides

A common consequence of using agricultural fungicides is the development of resistance by fungal pathogens, which undermines reliability of fungicidal effectiveness. A potentially new strategy to aid in overcoming or minimizing this problem is enhancement of pathogen sensitivity to fungicides, or “chemosensitization.” Chemosensitization can be accomplished by combining a commercial fungicide with a certain non- or marginally fungicidal substance at levels where, alone, neither compound would be effective. Chemosensitization decreases the probability of the pathogen developing resistance, reduces the toxic impact on the environment by lowering effective dosage levels of toxic fungicides, and improves efficacy of antifungal agents. The present study shows that the antifungal activity of azole and strobilurin fungicides can be significantly enhanced through their co-application with certain natural or synthetic products against several economically important plant pathogenic fungi. Quadris (azoxystrobin) combined with thymol at a non-fungitoxic concentration produced much higher growth inhibition of Bipolaris sorokiniana, Phoma glomerata, Alternaria sp. and Stagonospora nodorum than the fungicide alone. The effect of Dividend (difenoconazole) applied with thymol significantly enhanced antifungal activity against B. sorokiniana and S. nodorum. Folicur (tebuconazole) combined with 4-hydroxybenzaldehyde (4-HBA), 2,3-dihydroxybenzaldehyde or thymol significantly inhibited growth of Alternaria alternata, at a much greater level than the fungicide alone. In addition, co-application of Folicur and 4-HBA resulted in a similar enhancement of antifungal activity against Fusarium culmorum. Lastly, we discovered that metabolites in the culture liquid of Fusarium sambucinum biocontrol isolate FS-94 also had chemosensitizing activity, increasing S. nodorum sensitivity to Folicur and Dividend.

Chemosensitization as a means to augment commercial antifungal agents

Antimycotic chemosensitization and its mode of action are of growing interest. Currently, use of antifungal agents in agriculture and medicine has a number of obstacles. Foremost of these is development of resistance or cross-resistance to one or more antifungal agents. The generally high expense and negative impact, or side effects, associated with antifungal agents are two further issues of concern. Collectively, these problems are exacerbated by efforts to control resistant strains, which can evolve into a treadmill of higher dosages for longer periods. This cycle in turn, inflates cost of treatment, dramatically. A further problem is stagnation in development of new and effective antifungal agents, especially for treatment of human mycoses. Efforts to overcome some of these issues have involved using combinations of available antimycotics (e.g., combination therapy for invasive mycoses). However, this approach has had inconsistent success and is often associated with a marked increase in negative side effects. Chemosensitization by natural compounds to increase effectiveness of commercial antimycotics is a somewhat new approach to dealing with the aforementioned problems. The potential for safe natural products to improve antifungal activity has been observed for over three decades. Chemosensitizing agents possess antifungal activity, but at insufficient levels to serve as antimycotics, alone. Their main function is to disrupt fungal stress response, destabilize the structural integrity of cellular and vacuolar membranes or stimulate production of reactive oxygen species, augmenting oxidative stress and apoptosis. Use of safe chemosensitizing agents has potential benefit to both agriculture and medicine. When co-applied with a commercial antifungal agent, an additive or synergistic interaction may occur, augmenting antifungal efficacy. This augmentation, in turn, lowers effective dosages, costs, negative side effects and, in some cases, countermands resistance.