Molecular characterization, fitness and mycotoxin production of Fusarium graminearum laboratory strains resistant to benzimidazoles

Identification of glutathione metabolic genes from a dimorphic fungus Talaromyces marneffei and their gene expression patterns under different environmental conditions

Talaromyces marneffei is a human fungal pathogen that causes endemic opportunistic infections, especially in Southeast Asia. The key virulence factors of T. marneffei are the ability to survive host-derived heat and oxidative stress, and the ability to convert morphology from environmental mold to fission yeast forms during infection. Glutathione metabolism plays an essential role in stress response and cellular development in multiple organisms. However, the role of the glutathione system in T. marneffei is elusive. Here, we identified the genes encoding principal enzymes associated with glutathione metabolism in T. marneffei, including glutathione biosynthetic enzymes (Gcs1 and Gcs2), glutathione peroxidase (Gpx1), glutathione reductase (Glr1), and a family of glutathione S-transferase (Gst). Sequence homology search revealed an extended family of the TmGst proteins, consisting of 20 TmGsts that could be divided into several classes. Expression analysis revealed that cells in conidia, mold, and yeast phases exhibited distinct expression profiles of glutathione-related genes. Also, TmGst genes were highly upregulated in response to hydrogen peroxide and xenobiotic exposure. Altogether, our findings suggest that T. marneffei transcriptionally regulates the glutathione genes under stress conditions in a cell-type-specific manner. This study could aid in understanding the role of glutathione in thermal-induced dimorphism and stress response.

Prevalence of Carbendazin Resistance in Field Populations of the Rice False Smut Pathogen Ustilaginoidea virens from Jiangsu, China, Molecular Mechanisms, and Fitness Stability

Rice false smut (RFS), caused by Ustilaginoidea virens, is an important fungal disease of rice. In China, Methyl Benzimidazole Carbamates (MBCs), including carbendazim, are common fungicides used to control RFS and other rice diseases. In this study, resistance of U. virens to carbendazim was monitored for three consecutive years during 2018 to 2020. A total of 321 U. virens isolates collected from Jiangsu Province of China were tested for their sensitivity to carbendazim on PSA. The concentration at which mycelial growth is inhibited by 50% (EC50) of the carbendazim-sensitive isolates was 0.11 to 1.38 µg/mL, with a mean EC50 value of 0.66 μg/mL. High level of resistance to carbendazim was detected in 14 out of 321 isolates. The resistance was stable but associated with a fitness penalty. There was a statistically significant and moderate negative correlation (r= −0.74, p < 0.001) in sensitivity between carbendazim and diethofencarb. Analysis of the U. virens genome revealed two potential MBC targets, Uvβ1Tub and Uvβ2Tub, that putatively encode β-tubulin gene. The two β-tubulin genes in U. virens share 78% amino acid sequence identity, but their function in MBC sensitivity has been unclear. Both genes were identified and sequenced from U. virens sensitive and resistant isolates. It is known that mutations in the β2-tubulin gene have been shown to confer resistance to carbendazim in other fungi. However, no mutation was found in the Uvβ2Tub gene in either resistant or sensitive isolates. Variations including point mutations, non-sense mutations, codon mutations, and frameshift mutations were found in the Uvβ1Tub gene from the 14 carbendazim-resistant isolates, which have not been reported in other fungi before. Thus, these results indicated that variations of Uvβ1Tub result in the resistance to carbendazim in field isolates of Ustilaginoidea virens.

Identification of the Causal Agent of Brown Leaf Spot on Kiwifruit and Its Sensitivity to Different Active Ingredients of Biological Fungicides

Kiwifruit (Actinidia chinensis) is an important commercial crop in China, and the occurrence of diseases may cause significant economic loss in its production. In the present study, a new pathogen that causes brown leaf spot disease on kiwifruit was reported. The fungus was isolated from an infected sample and identified as Fusarium graminearum based on morphological and molecular evaluation. Koch's postulates were confirmed when the pathogen was re-isolated from plants with artificially induced symptoms and identified as F. graminearum. Based on the biological characteristics of the pathogen, it was determined that: its optimal growth temperature was 25 °C; optimal pH was 7; most suitable carbon source was soluble starch; most suitable nitrogen source was yeast powder; and best photoperiod was 12 h light/12 h dark. Further investigations were conducted by determining 50% effective concentrations (EC50) of several active ingredients of biological fungicides against F. graminearum. The results showed that among the studied fungicides, tetramycin and honokiol had the highest antifungal activity against this pathogen. Our findings provide a scientific basis for the prevention and treatment of brown leaf spot disease on kiwifruit.

Fungicide Resistance in Fusarium graminearum Species Complex

Fusariosis affects cereal grain crops worldwide and is responsible for devastating crops, reducing grain quality and yield, and producing strong mycotoxins. Benzimidazoles and triazoles were recommended to combat fusariosis; however, there were reports of resistance, making it necessary to reflect on the reasons for this occurrence. The purpose of this review was to evaluate the fusariosis resistance to the main agricultural fungicides, to observe whether this resistance can cause changes in the production of mycotoxins, and to verify the influence of resistance on the cereal grain production chain. Scientific articles were selected from the ScienceDirect, Scopus, and Pubmed databases, published at maximum 10 years ago and covering the main fungicide classes that combat phytopathogenesis and mycotoxin production. A high occurrence of resistance to carbendazim was found, while few reports of resistance to triazoles are available. The effectiveness of strobilurins is doubtful, due to an increase of mycotoxins linked to it. It is possible to conclude that the large-scale use of fungicides can select resistant strains that will contribute to an increase in the production of mycotoxins and harm sectors of the world economy, not only the agriculture, but also sanitation and foreign trade.

Design, synthesis and biological evaluation of novel evodiamine and rutaecarpine derivatives against phytopathogenic fungi

Evodiamine and rutaecarpine are two alkaloids isolated from traditional Chinese herbal medicine Evodia rutaecarpa, which have been reported to have various biological activities in past decades. To explore the potential applications for evodiamine and rutaecarpine alkaloids and their derivatives, various kinds of evodiamine and rutaecarpine derivatives were designed and synthesized. Their antifungal profile against six phytopathogenic fungi Rhizoctonia solani, Botrytis cinerea, Fusarium graminearum, Fusarium oxysporum, Sclerotinia sclerotiorum, and Magnaporthe oryzae were evaluated for the first time. Furthermore, a series of modified imidazole derivatives of rutaecarpine were synthesized to investigate the structure-activity relationship. The results of antifungal activities in vitro showed that imidazole derivative of rutaecarpine A1 exhibited broad-spectrum inhibitory activities against R. solani, B. cinerea, F. oxysporum, S. sclerotiorum, M. oryzae and F. graminearum with EC₅₀ values of 1.97, 5.97, 12.72, 2.87 and 16.58 μg/mL, respectively. Preliminary mechanistic studies showed that compound A1 might cause mycelial abnormalities of S. sclerotiorum, mitochondrial distortion and swelling, and inhibition of sclerotia formation and germination. Moreover, the curative effects of compound A1 were 94.7%, 81.5%, 80.8%, 65.0% at 400, 200, 100, 50 μg/mL in vivo experiments, which was far more effective than the positive control azoxystrobin. Significantly, no phytotoxicity of compound A1 on oilseed rape leaves was observed obviously even at a high concentration of 400 μg/mL. Therefore, compound A1 is expected to be a novel leading structure for the development of new antifungal agents.

Removal of prothioconazole using screened microorganisms and identification of biodegradation products via UPLC-QqTOF-MS

Degradation of the prothioconazole by three strains of microorganisms isolated from activated sludge obtained from a pesticide factory was assessed, and an ultrahigh-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QqTOF-MS) method for the determination of prothioconazole and its metabolites was established. The optimal conditions for the degradation of prothioconazole were determined by single factor optimization experiments. A degradation rate of 93.32% is achieved when the prothioconazole is co-cultured with the strain W313 at a cultivation time of 60 h, a cultivation temperature of 30 °C, a pH of 6.33, a prothioconazole concentration of 50 mg L^-1, a microorganism volume of 10%, and a dextrose volume of 4%. The three effective microorganism strains were identified by morphological and molecular biology to be Candida tropicalis, Enterobacter cloacae, and Pseudomonas aeruginosa. UPLC-QqTOF-MS analysis allowed the identification of 62 different prothioconazole degradation products produced by the strain cultures, with prothioconazole-desthio, prothioconazole-dechloropropyl, and oxidizing prothioconazole being the main products. In addition, degradation products from different strains and conditions were compared. The results of scatter plot (S-Plot) analysis indicated that C₉H₇NO, C₁₀H₁₇N₇, and C₁₂H₁₃ClN₂O were only detected in the products incubated with Enterobacter cloacae. Thus, this study demonstrates that Enterobacter cloacae and Pseudomonas aeruginosa possesses high potential for bioremediation of prothioconazole-contaminated environments.

Discovery of γ-Lactam Alkaloid Derivatives as Potential Fungicidal Agents Targeting Steroid Biosynthesis

Biological control of plant pathogens is considered as one of the green and effective technologies using beneficial microorganisms or microbial secondary metabolites against plant diseases, and so microbial natural products have played important roles in the research and development of new and green agrochemicals. To explore the potential applications for natural γ-lactam alkaloids and their derivatives, 26 γ-lactams that have flexible substituent patterns were synthesized and characterized, and their in vitro antifungal activities against eight kinds of plant pathogens belonging to oomycetes, basidiomycetes, and deuteromycetes were fully evaluated. In addition, the high potential compounds were further tested using an in vivo assay against Phytophthora blight of pepper to verify a practical application for controlling oomycete diseases. The potential modes of action for compound D1 against Phytophthora capsici were also investigated using microscopic technology (optical microscopy, scanning electron microscopy, and transmission electron microscopy) and label-free quantitative proteomics analysis. The results demonstrated that compound D1 may be a potential novel fungicidal agent against oomycete diseases (EC₅₀ = 4.9748 μg·mL^-1 for P. capsici and EC₅₀ = 5.1602 μg·mL^-1 for Pythium aphanidermatum) that can act on steroid biosynthesis, which can provide a certain theoretical basis for the development of natural lactam derivatives as potential antifungal agents.

The ASK1 gene regulates the sensitivity of Fusarium graminearum to carbendazim, conidiation and sexual production by combining with β2-tubulin

β-tubulin, a component of microtubules, is involved in a wide variety of roles in cell shape, motility, intracellular trafficking and regulating intracellular metabolism. It has been an important fungicide target to control plant pathogen, for example, Fusarium. However, the regulation of fungicide sensitivity by β-tubulin-interacting proteins is still unclear. Here, ASK1 was identified as a β-tubulin interacting protein. The ASK1 regulated the sensitivity of Fusarium to carbendazim (a benzimidazole carbamate fungicide), and multiple cellular processes, such as chromatin separation, conidiation and sexual production. Further, we found the point mutations at 50th and 198th of β₂-tubulin which caused carbendazim resistance decreased the binding between β₂-tubulin and ASK1, resulting in the deactivation of ASK1. ASK1, on the other hand, competed with carbendazim to bind to β₂-tubulin. The point mutation F167Y in β₂-tubulin broke the intermolecular H-bonds and salt bridges between β₂-tubulin and ASK1, which reduced the competitive effect of ASK1 to carbendazim and resulted in the similar carbendazim sensitivities in F167Y-ΔASK1 and F167Y. These findings have powerful implications for efforts to understand the interaction among β₂-tubulin, its interacting proteins and fungicide, as well as to discover and develop new fungicide against Fusarium.

Sulfur-containing natural hinduchelins derivatives as potential antifungal agents against Rhizoctonia solani

Aryl-oxazole alkaloids are an important class of heterocyclic natural products, and which has been demonstrated to exhibit broad biological functions. During the course of our research for highly active compounds from natural products, the natural hinduchelins A-D with typical aryl-oxazole unit have been synthesized and investigated. So, in order to develop highly potential functional molecules, a series of novel sulfur-containing aryl-oxazole compounds derived from natural hinduchelins was designed and synthesized, and their in vitro fungicidal activities against four common plant pathogenic fungi (oomycetes Phytophthora capsici, ascomycetes Sclerotinia sclerotiorum, deuteromycetes Botrytis cinerea and basidiomycetes Rhizoctonia solani) were evaluated, the results demonstrated that compounds 7b and 7c displayed good selectivity and specificity in vitro against basidiomycetes R. solani. In addition, the in vivo antifungal activities also indicated compounds 7b and 7c can protect the horsebean against infection by R. solani, and the possible mechanism of antifungal action for these compounds has also been investigated.

Carbendazim Resistance of Fusarium graminearum From Henan Wheat

Fusarium head blight, also called scab, is caused by Fusarium graminearum and is one of the most important destructive diseases of wheat. The frequency of carbendazim resistance in 1,132 isolates of F. graminearum recovered from fields in different regions of Henan Province in 2016, 2017, and 2018 was determined. A total of 31 F. graminearum isolates resistant to carbendazim were detected, including 30 moderately resistant isolates and one highly resistant isolate. The frequency of resistance of F. graminearum isolates to carbendazim was 2.7%. The range of effective concentration (EC₅₀) values of 1,101 sensitive isolates and 30 moderately resistant isolates was 0.08 to 0.98 μg ml^-1 and 2.73 to 13.28 μg ml^-1, respectively. The mean ± SD EC₅₀ value was 0.55 ± 0.13 μg ml^-1 and 5.61 ± 2.58 μg ml^-1, respectively. The EC₅₀ value of the highly resistant isolate was 21.12 μg ml^-1. Point mutation types of the carbendazim-resistant isolates were characterized by cloning the β₂-tubulin gene of 31 resistant isolates. Three point mutation types at amino acids F167Y, E198Q, and E198L in the β₂-tubulin gene of resistant isolates were identified. Among 31 resistant isolates, the frequency of point mutation types in F167Y, E198Q, and E198L of the β₂-tubulin gene was 71.0, 25.8, and 3.2%, respectively. The data indicate that F. graminearum has developed resistance to carbendazim in Henan Province, and single point mutations at amino acid F167Y were the predominant type of mutation detected.

Preliminary Study on the Use of Chitosan as an Eco-Friendly Alternative to Control Fusarium Growth and Mycotoxin Production on Maize and Wheat

The objectives of the present study were to determine the combined effects of chitosan and water activity (aW) on growth and mycotoxin production in situ on the two most important Fusarium species (F. proliferatum and F. verticillioides) present on maize, and on F. graminearum, the main pathogen causing Fusarium head blight on wheat. Results showed that low-molecular-weight chitosan with more than 70% deacetylation at the lowest dose used (0.5 mg/g) was able to reduce deoxynivalenol (DON) and fumonisin (FBs) production on irradiated maize and wheat grains. Growth rates of F. graminearum also decreased at the lowest chitosan dose used (0.5 mg/g), while F. verticillioides and F. proliferatum growth rates were reduced at 0.98 aW at the highest chitosan dose used (2 mg/g). Since mycotoxins are unavoidable contaminants in food and feed chains, their presence needs to be reduced in order to minimize their effects on human and animal health and to diminish the annual market loss through rejected maize and wheat; in this scenario, pre- and post-harvest use of chitosan could be an important alternative.

Molecular characterization of carbendazim resistance of Fusarium species complex that causes sugarcane pokkah boeng disease

BACKGROUND

Pokkah boeng is one of the most serious and devastating diseases of sugarcane and causes significant loss in cane yield and sugar content. Although carbendazim is widely used to prevent fungal diseases, the molecular basis of Fusarium species complex (FSC) resistance to carbendazim remains unknown.

RESULTS

The EC50 (fungicide concentration that inhibits 50% of mycelial growth) values of carbendazim for 35 FSC isolates collected in cane growing regions of China were ranged from 0.5097 to 0.6941 μg mL- 1 of active ingredient (a.i.), in an average of 0.5957 μg a.i. mL- 1. Among carbendazim-induced mutant strains, SJ51M (F. verticillioides) had a CTG rather than CAG codon (Q134L) at position 134 of the FVER_09254 gene, whereas in the mutant strain HC30M (F. proliferatum) codon ACA at position 351 of the FPRO_07779 gene was replaced by ATA (T351I). Gene expression profiling analysis was performed for SJ51M and its corresponding wild type strain SJ51, with and without carbendazim treatment. The gene expression patterns in SJ51 and SJ51M changed greatly as evidenced by the detection of 850 differentially expressed genes (DEGs). Functional categorization indicated that genes associated with oxidation-reduction process, ATP binding, integral component of membrane, transmembrane transport and response to stress showed the largest expression changes between SJ51M and SJ51. The expression levels of many genes involved in fungicide resistance, such as detoxification enzymes, drug efflux transporters and response to stress, were up-regulated in SJ51M compared to SJ51 with and without carbendazim treatment.

CONCLUSION

FSC was sensitive to carbendazim and had the potential for rapid development of carbendazim resistance. The transcriptome data provided insight into the molecular pathways involved in FSC carbendazim resistance.

Fusarium graminearum1H NMR metabolomics

Raw ¹H NMR spectra of Fusarium graminearum hyphae can be found at the website of the pesticide metabolomics group (PMG) of the Agricultural University of Athens at the address: http://www.aua.gr/pesticide-metabolomicsgroup/Resources/Fusarium_graminearum_NMR_spectra.html, accession number PMG-01–17. The data set support the research article “Implication of Fusarium graminearum Primary Metabolism in its Resistance to Benzimidazole Fungicides as revealed by ¹H NMR Metabolomics” [1].

Implication of Fusarium graminearum primary metabolism in its resistance to benzimidazole fungicides as revealed by 1H NMR metabolomics

Fungal metabolomics is a field of high potential but yet largely unexploited. Focusing on plant-pathogenic fungi, no metabolomics studies exist on their resistance to fungicides, which represents a major issue that the agrochemical and agricultural sectors are facing. Fungal infections cause quantitative, but also qualitative yield losses, especially in the case of mycotoxin-producing species. The aim of the study was to correlate metabolic changes in Fusarium graminearum strains' metabolomes with their carbendazim-resistant level and discover corresponding metabolites-biomarkers, with primary focus on its primary metabolism. For this purpose, comparative ¹H NMR metabolomics was applied to a wild-type and four carbendazim-resistant Fusarium graminearum strains following or not exposure to the fungicide. Results showed an excellent discrimination between the strains based on their carbendazim-resistance following exposure to low concentration of the fungicide (2 mg L^-1). Both genotype and fungicide treatments had a major impact on fungal metabolism. Among the signatory metabolites, a positive correlation was discovered between the content of F. graminearum strains in amino acids of the aromatic and pyruvate families, l-glutamate, l-proline, l-serine, pyroglutamate, and succinate and their carbendazim-resistance level. In contrary, their content in l-glutamine and l-threonine, had a negative correlation. Many of these metabolites play important roles in fungal physiology and responses to stresses. This work represents a proof-of-concept of the applicability of ¹H NMR metabolomics for high-throughput screening of fungal mutations leading to fungicide resistance, and the study of its biochemical basis, focusing on the involvement of primary metabolism. Results could be further exploited in programs of resistance monitoring, genetic engineering, and crop protection for combating fungal resistance to fungicides.

Glutathione transferase-mediated benzimidazole-resistance in Fusarium graminearum

Fusarium graminearum laboratory mutants moderately (MR) and highly (HR) benzimidazole-resistant, carrying or not target-site mutations at the β₂-tubulin gene were utilized in an attempt to elucidate the biochemical mechanism(s) underlying the unique BZM-resistance paradigm of this fungal plant pathogen. Relative expression analysis in the presence or absence of carbendazim (methyl-2-benzimidazole carbamate) using a quantitative Real Time qPCR (RT-qPCR) revealed differences between resistant and the wild-type parental strain although no differences in expression levels of either β₁- or β₂-tubulin homologue genes were able to fully account for two of the highly resistant phenotypes. Glutathione transferase (GST)-mediated detoxification was shown to be -at least partly- responsible for the elevated resistance levels of a HR isolate bearing the β₂-tubulin Phe200Tyr resistance mutation compared with another MR isolate carrying the same mutation. This benzimidazole-resistance mechanism is reported for the first time in F. graminearum. No indications of detoxification involved in benzimidazole resistance were found for the rest of the isolates as revealed by GST and glutathione peroxidase (GPx) activities and bioassays using monoxygenase and hydrolase detoxification enzyme inhibiting synergists. Interestingly, besides the Phe200Tyr mutation-carrying HR isolate, the remaining highly-carbendazim resistant phenotypes could not be associated with any of the target site modification/overproduction, detoxification or reduced uptake-increased efflux mechanisms.