Point Mutations in FgSdhC2 or in the 5' Untranslated Region of FgSdhC1 Confer Resistance to a Novel Succinate Dehydrogenase Inhibitor Flubeneteram in Fusarium graminearum

Functional Differentiation of the Succinate Dehydrogenase Subunit SdhC Governs the Sensitivity to SDHI Fungicides, ROS Homeostasis, and Pathogenicity in Fusarium asiaticum

Succinate dehydrogenase (SDH) is an integral component of the tricarboxylic acid cycle (TCA) and respiratory electron transport chain (ETC), targeted by succinate dehydrogenase inhibitors (SDHIs). Fusarium asiaticum is a prominent phytopathogen causing Fusarium head blight (FHB) on wheat. Here, we characterized the functions of the FaSdhA, FaSdhB, FaSdhC1, FaSdhC2, and FaSdhD subunits. Deletion of FaSdhA, FaSdhB, or FaSdhD resulted in significant growth defects in F. asiaticum. The FaSdhC1 or FaSdhC2 deletion mutants exhibited substantial reductions in fungal growth, conidiation, virulence, and reactive oxygen species (ROS). The FaSdhC1 expression was significantly induced by pydiflumetofen (PYD). The ΔFaSdhC1 mutant displayed hypersensitivity to SDHIs, whereas the ΔFaSdhC2 mutant exhibited resistance against most SDHIs. The transmembrane domains of FaSdhC1 are essential for regulating mycelial growth, virulence, and sensitivity to SDHIs. These findings provided valuable insights into how the two SdhC paralogues regulated the functional integrity of SDH, ROS homeostasis, and the sensitivity to SDHIs in phytopathogenic fungi.

Resistance mechanism of Phomopsis longicolla to fludioxonil is associated with modifications in PlOS1, PlOS4 and PlOS5

Phomopsis longicolla, a causal agent of soybean root rot, stem blight, seed decay, pod and stem canker, which seriously affects the yield and quality of soybean production worldwide. The phenylpyrrole fungicide fludioxonil exhibits a broad spectrum and high activity against phytopathogenic fungi. In this study, the baseline sensitivity of 100 P. longicolla isolates collected from the main soybean production areas of China to fludioxonil were determined. The result showed that the EC50 values of all the P. longicolla isolates ranged from 0.013 to 0.035 μg/ml. Furthermore, 12 fludioxonil-resistance (FluR) mutants of P. longicolla were generated from 6 fludioxonil-sensitive (FluS) isolates. and the resistance factors (RF) of 12 FluR mutants were >3500. Sequence alignment showed that multiple mutation types were found in PlOS1, PlOS4 or/and PlOS5 of FluR mutants. All the FluR mutants exhibited fitness penalty in mycelial growth, conidiation, virulence and osmo-adaptation. Under fludioxonil or NaCl treatment condition, the glycerol accumulation was significantly increased in FluS isolates, but was slightly increased in FluR mutants, and the phosphorylation level of most FluR mutants was significantly decreased when compared to the FluS isolates. Additionally, positive cross-resistance was observed between fludioxonil and procymidone but not fludioxonil and pydiflumetofen, pyraclostrobin or fluazinam. This is first reported that the baseline sensitivity of P. longicolla to fludioxonil, as well as the biological and molecular characterizations of P. longicolla FluR mutants to fludioxonil. These results can provide scientific directions for controlling soybean diseases caused by P. longicolla using fludioxonil.

Functional Plasticity, Redundancy, and Specificity of Lanosterol 14α-Demethylase in Regulating the Sensitivity to DMIs in Calonectria ilicicola

Cytochrome P450 sterol 14α-demethylase (CYP51) is a key enzyme involved in the sterol biosynthesis pathway and serves as a target for sterol demethylation inhibitors (DMIs). In this study, the 3D structures of three CPY51 paralogues from Calonectria ilicicola (C. ilicicola) were first modeled by AlphaFold2, and molecular docking results showed that CiCYP51A, CiCYP51B, or CiCYP51C proteins individually possessed two active pockets that interacted with DMIs. Our results showed that the three paralogues play important roles in development, pathogenicity, and sensitivity to DMI fungicides. Specifically, CiCYP51A primarily contributed to cell wall integrity maintenance and tolerance to abiotic stresses, and CiCYP51B was implicated in sexual reproduction and virulence, while CiCYP51C exerted negative regulatory effects on sterol 14α-demethylase activity within the ergosterol biosynthetic pathway, revealing its genus-specific function in C. ilicicola. These findings provide valuable insights into developing rational strategies for controlling soybean red crown rot caused by C. ilicicola.

The natural polycyclic tetramate macrolactam HSAF inhibit Fusarium graminearum through altering cell membrane integrity by targeting FgORP1

Fusarium graminearum is a dominant phytopathogenic fungus causing Fusarium head blight (FHB) in cereal crops. Heat-stable antifungal factor (HSAF) is a polycyclic tetramate macrolactam (PoTeM) isolated from Lysobacter enzymogenes that exhibits strong antifungal activity against F. graminearum. HSAF significantly reduces the DON production and virulence of F. graminearum. Importantly, HSAF exhibited no cross-resistance to carbendazim, phenamacril, tebuconazole and pydiflumetofen. However, the target protein of HSAF in F. graminearum is unclear. In this study, the oxysterol-binding protein FgORP1 was identified as the potential target of HSAF using surface plasmon resonance (SPR) combined with RNA-sequence (RNA-seq). The RNA-seq results showed cell membrane and ergosterol biosynthesis were significantly impacted by HSAF in F. graminearum. Molecular docking showed that HSAF binds with arginine 1205 and glutamic acid 1212, which are located in the oxysterol-binding domain of FgORP1. The two amino acids in FgORP1 are responsible for HSAF resistance in F. graminearum though site-directed mutagenesis. Furthermore, deletion of FgORP1 led to significantly decreased sensitivity to HSAF. Additionally, FgORP1 regulates the mycelial growth, conidiation, DON production, ergosterol biosynthesis and virulence in F. graminearum. Overall, our findings revealed the mode of action of HSAF against F. graminearum, indicating that HSAF is a promising fungicide for controlling FHB.

Mitochondria-Associated Protein FgNdk1 Regulates the Development, Pathogenicity, and SDHI Fungicide Sensitivity of Fusarium graminearum by Interacting with Succinate Dehydrogenase

Nucleoside diphosphate kinase (NDK) plays an important role in many cellular processes in all organisms. In this study, we functionally characterized a nucleoside diphosphate kinase (FgNdk1) in Fusarium graminearum, a causal agent of Fusarium head blight (FHB). FgNdk1 was involved in the generation of energy in the electron-transfer chain by interacting with succinate dehydrogenase (FgSdhA, FgSdhC1, and FgSdhC2). Deletion of FgNdk1 not only resulted in abnormal mitochondrial morphology, decreased ATP content, defective fungal development, and impairment in the formation of the toxisome but also led to the suppressed expression level of DON biosynthesis enzymes, decreased DON biosynthesis, and declined pathogenicity as well. Furthermore, deletion of FgNdk1 caused increasing transcriptional levels of FgSdhC1 and FgdhC2, in the presence of pydiflumetofen, related to the decreased sensitivity to SDHI fungicides. Overall, this study identified a new regulatory mechanism of FgNdk1 in the pathogenicity and SDHI fungicide sensitivity of Fusarium graminearum.

Resistance risk, resistance mechanism and the effect on DON production of a new SDHI fungicide cyclobutrifluram in Fusarium graminearum

Fusarium head blight in wheat is caused by Fusarium graminearum, resulting in significant yield losses and grain contamination with deoxynivalenol (DON), which poses a potential threat to animal health. Cyclobutrifluram, a newly developed succinate dehydrogenase inhibitor, has shown excellent inhibition of Fusarium spp. However, the resistance risk of F. graminearum to cyclobutrifluram and the molecular mechanism of resistance have not been determined. In this study, we established the average EC50 of a range of F. graminearum isolates to cyclobutrifluram to be 0.0110 μg/mL. Six cyclobutrifluram-resistant mutants were obtained using fungicide adaptation. All mutants exhibited impaired fitness relative to their parental isolates. This was evident from measurements of mycelial growth, conidiation, conidial germination, virulence, and DON production. Interestingly, cyclobutrifluram did not seem to affect the DON production of either the sensitive isolates or the resistant mutants. Furthermore, a positive cross-resistance was observed between cyclobutrifluram and pydiflumetofen. These findings suggest that F. graminearum carries a moderate to high risk of developing resistance to cyclobutrifluram. Additionally, point mutations H248Y in FgSdhB and A73V in FgSdhC1 of F. graminearum were observed in the cyclobutrifluram-resistant mutants. Finally, an overexpression transformation assay and molecular docking indicated that FgSdhBH248Y or FgSdhC1A73V could confer resistance of F. graminearum to cyclobutrifluram.

FgSdhC Paralog Confers Natural Resistance toward SDHI Fungicides in Fusarium graminearum

Fusarium graminearum exhibited natural resistance to a majority of succinate dehydrogenase inhibitor fungicides (SDHIs) and the molecular mechanisms responsible for the natural resistance were still unknown. Succinate dehydrogenase subunit C (SdhC) is an essential gene for maintaining succinate-ubiquinone oxidoreductase (SQR) function in fungi. In F. graminearum, a paralog of FgSdhC named as FgSdhC1 was identified. Based on RNA-Seq and qRT-PCR assay, we found that the expression level of FgSdhC1 was very low but upregulated by SDHIs treatment. Based on reverse genetics, we demonstrated that FgSdhC1 was an inessential gene in normal growth but was sufficient for maintaining SQR function and conferred natural resistance or reduced sensitivity toward SDHIs. Additionally, we found that the standard F. graminearum isolate PH-1 had high sensitivity to a majority of SDHIs. A single nucleotide variation (C to T) in the FgSdhC1 of isolate PH-1, resulting in a premature termination codon (TAA) replacing the fourth amino acid glutamine (Q), led to the failure of FgSdhC1 to perform functions of conferring nature resistance. These results established that a dispensable paralogous gene determined SDHIs resistance in natural populations of F. graminearum.

Resistant risk and resistance-related point mutation in SdhC1 of pydiflumetofen in Fusarium pseudograminearum

BACKGROUND

Fusarium pseudograminearum is one of the dominant pathogens of Fusarium crown rot (FCR) worldwide. Unfortunately, no fungicides have yet been registered for the control of FCR in wheat in China. Pydiflumetofen, a new-generation succinate dehydrogenase inhibitor, exhibits excellent inhibitory activity to Fusarium spp. A resistance risk assessment of F. pseudograminearum to pydiflumetofen and the resistance mechanism involved have not yet been investigated.

RESULTS

The median effective concentration (EC50 ) value of 103 F. pseudograminearum isolates to pydiflumetofen was 0.0162 μg mL-1 , and the sensitivity exhibited a unimodal distribution. Four resistant mutants were generated by fungicide adaption, which possessed similar or impaired fitness compared to corresponding parental isolates based on the results of mycelial growth, conidiation, conidium germination rate, and virulence determination. Pydiflumetofen showed strong positive cross-resistance with cyclobutrifluram and fluopyram but no cross-resistance with carbendazim, phenamacril, tebuconazole, fludioxonil, or pyraclostrobin. Sequence alignment revealed that pydiflumetofen-resistant F. pseudograminearum mutants had two single-point mutations of A83V or R86K in FpSdhC1 . Molecular docking further confirmed that point mutation of A83V or R86K in FpSdhC1 could confer resistance of F. pseudograminearum to pydiflumetofen.

CONCLUSION

Fusarium pseudograminearum shows an overall moderate risk of developing resistance to pydiflumetofen, and point mutation FpSdhC1 A83V or FpSdhC1 R86K could confer pydiflumetofen resistance in F. pseudograminearum. This study provided vital data for monitoring the emergence of resistance and developing resistance management strategies for pydiflumetofen. © 2023 Society of Chemical Industry.

Study on Mechanisms of Resistance to SDHI Fungicide Pydiflumetofen in Fusarium fujikuroi

Rice bakaenii disease (RBD) is a widespread and devastating disease mainly caused by Fusarium fujikuroi. Pydiflumetofen (Pyd) is a novel succinate dehydrogenase inhibitor (SDHI) with strong inhibitory activity against F. fujikuroi, but the mechanism of resistance to Pyd has not been well studied for this pathogen. Through fungicide adaption, a total of 12 Pyd-resistant mutants were obtained and the resistance level could be divided into three categories of high resistance (HR), moderate resistance (MR), and low resistance (LR) with resistance factors (RF) of 184.04-672.90, 12.63-42.49, and <10, respectively. Seven genotypes of point mutations in FfSdh genes (FfSdhBH248L, FfSdhBH248D, FfSdhBH248Y, FfSdhC2A83V, FfSdhC2H144Y, FfSdhDS106F, and FfSdhDE166K) were found in these mutants, among which genotype FfSdhBH248L and FfSdhC2A83V mutants showed HR, genotype FfSdhBH248D, FfSdhBH248Y, FfSdhC2H144Y, and FfSdhDE166K mutants showed MR, and genotype FfSdhDS106F mutants showed LR. Moreover, all the substitutions of amino acid point mutations including FfSdhBH248L/D/Y, FfSdhC2A83V,H144Y, and FfSdhDS106F,E166K conferring resistance to Pyd in F. fujikuroi were verified by protoplast transformation. Additionally, a positive cross-resistance was detected between Pyd and another SDHI fungicide penflufen, while no cross-resistance was detected between Pyd and phenamacril, prochloraz, azoxystrobin, carbendazim, or fludioxonil. Although pathogenicity of the mutants was increased compared with that of the wild-type parental strains, the mycelial growth rate and spore production levels of the resistant mutants were significantly decreased (P < 0.05), indicating significant fitness cost of resistance to Pyd in F. fujikuroi. Taken together, the risk of resistance to Pyd in F. fujikuroi might be moderate, and appropriate precautions against resistance development in natural populations should be taken into account when Pyd is used for the control of RBD.

Multiple Mutations in SDHB and SDHC2 Subunits Confer Resistance to the Succinate Dehydrogenase Inhibitor Cyclobutrifluram in Fusarium fujikuroi

Fusarium fujikuroi is one of the dominant phytopathogenic fungi causing rice bakanae disease worldwide. Cyclobutrifluram is a novel succinate dehydrogenase inhibitor (SDHI), which shows strong inhibitory activity against F. fujikuroi. The baseline sensitivity of 112 F. fujikuroi to cyclobutrifluram was determinated with a mean EC₅₀ value of 0.025 μg/mL. A total of 17 resistant mutants were obtained by fungicide adaptation and displayed equal or slightly weaker fitness than parental isolates, which suggests that the resistance risk of F. fujikuroi to cyclobutrifluram is medium. A positive cross-resistance was detected between cyclobutrifluram and fluopyram. The amino acid substitutions H248L/Y of FfSdhB and G80R or A83V of FfSdhC₂ conferred cyclobutrifluram resistance in F. fujikuroi, which was validated by molecular docking and protoplast transformation. The results indicate that the affinity between cyclobutrifluram and FfSdhs obviously decreased after point mutations, causing the resistance of F. fujikuroi.

Resistance Risk and Resistance-Related Point Mutation in SdhB and SdhC1 of Cyclobutrifluram in Fusarium pseudograminearum

Cyclobutrifluram is a novel succinate dehydrogenase inhibitor (SDHI) developed by Syngenta and helps to inhibit Fusarium pseudograminearum. Here, the potential for cyclobutrifluram resistance in F. pseudograminearum and the resistance mechanism involved were evaluated. Baseline sensitivity of F. pseudograminearum to cyclobutrifluram was determined with a mean EC₅₀ value of 0.0248 μg/mL. Fungicide adaption generated five resistant mutants, which possess a comparable or a slightly impaired fitness compared to corresponding parental isolates. This indicates that the resistance risk of F. pseudograminearum to cyclobutrifluram might be moderate. Cyclobutrifluram-resistant isolates also demonstrated resistance to pydiflumetofen but sensitivity to carbendazim, phenamacril, tebuconazole, fludioxonil, or pyraclostrobin. Additionally, point mutations H248Y in FpSdhB and A83V or R86K in FpSdhC₁ were found in cyclobutrifluram-resistant F. pseudograminearum mutants. Molecular docking and overexpression transformation assay revealed that FpSdhB^H248Y and FpSdhC₁^A83V or FpSdhC₁^R86K confer the resistance of F. pseudograminearum to cyclobutrifluram.

Design, Synthesis, Inhibitory Activity, and Molecular Modeling of Novel Pyrazole-Furan/Thiophene Carboxamide Hybrids as Potential Fungicides Targeting Succinate Dehydrogenase

To discover new fungicides targeting succinate dehydrogenase (SDH), 36 new furan/thiophene carboxamides containing 4,5-dihydropyrazole rings were designed, synthesized, and characterized. The crystal structure of compound 5l was determined with the X-ray diffraction (XRD) of single crystals. The antifungal activity of these compounds was studied against Botrytis cinerea, Pyricularia oryzae, Erysiphe graminis, Physalospora piricola, and Penicillium digitatum. Bioassay results were that most compounds had obvious inhibitory activity at 20 μg/mL. Compounds 5j, 5k, and 5l possessed outstanding inhibitory activity against B. cinerea. Their EC₅₀ values were 0.540, 0.676, and 0.392 μg/mL, respectively. They owned better effects than fluxapyroxad (EC₅₀ = 0.791 μg/mL). In the meantime, the inhibitory activity of 16 compounds was evaluated against SDH. It turned out that these compounds displayed excellent activity. The IC₅₀ values of compounds 5j, 5k, and 5l reached 0.738, 0.873, and 0.506 μg/mL, respectively, whereas the IC₅₀ value of fluxapyroxad was 1.031 μg/mL. The results of molecular dynamics (MD) simulation showed that compound 5l possessed a stronger affinity to SDH than fluxapyroxad.

Novel Pyrazole Carboxamide Containing a Diarylamine Scaffold Potentially Targeting Fungal Succinate Dehydrogenase: Antifungal Activity and Mechanism of Action

Succinate dehydrogenase (SDH) is known as an ideal target for the development of novel fungicides. Over the years, a series of novel pyrazole carboxamides containing a diarylamine scaffold have been reported as potent SDH inhibitors (SDHIs) in our laboratory. Among them, compound SCU3038 (EC₅₀ = 0.016 mg/L) against in vitro Rhizoctonia solani was better than fluxapyroxad (EC₅₀ = 0.033 mg/L). However, its mechanism of action is still unclear. In this paper, in pot tests, bioactivity evaluation indicated that in vivo antifungal activity of compound SCU3038 (EC₅₀ = 0.95 mg/L) against R. solani was better than that of fluxapyroxad (EC₅₀ = 2.29 mg/L) and thifluzamide (EC₅₀ = 1.88 mg/L). In field trials, control efficacy of compound SCU3038 (74.10%) at 200 g ai/ha against rice sheath blight was better than that of thifluzamide (71.40%). Furthermore, target evaluation showed that compound SCU3038 could inhibit the fungal SDH from R. solani and fix in the binding site of SDH by molecular docking, thereby it could dissolve and reduce mitochondria of R. solani as observed by electron microscopy. In addition, transcriptome results showed that compound SCU3038 affected the TCA cycle pathway in mitochondria, and this was manifested in the downregulation of eight genes and upregulation of one gene. The most important phenomenon was the repressed expression of SDH2 confirmed by qRT-PCR. It was observed that compound SCU3038 was a potent SDHI, and these results afforded further research on pyrazole carboxamides.

Sterol 14α-Demethylase CaCYP51A and CaCYP51B are Functionally Redundant, but Differentially Regulated in Colletotrichum acutatum: Responsibility for DMI-Fungicide Resistance

Colletotrichum acutatum, the main pathogen causing anthracnose on chili worldwide, is controlled by tebuconazole [a sterol C14-demethylation inhibitor (DMI) fungicide, abbreviated as Teb] with excellent efficacy. Our previous study exhibited that all C. acutatum isolates were sensitive to Teb while the Colletotrichum gloeosporioides population had developed resistance to Teb on the same fungicide-pressure selection. Therefore, the assessment of Teb-resistance in C. acutatum is impending. Twenty Teb-resistant (Teb^R) mutants obtained by fungicide domestication and ultraviolet (UV)-mutagenesis displayed similar fitness compared to parental isolates. Data in the current study exhibited that mutations at CaCYP51A and/or overexpression of CaCYP51s were responsible for Teb-resistance. Furthermore, the deletion mutants ΔCaCYP51A and ΔCaCYP51B played different roles in sensitivities to DMIs. Taken together, this study first reported that mutations at CaCYP51A and/or overexpression of CaCYP51s conferred resistance to Teb in C. acutatum, CaCYP51A and CaCYP51B are functionally redundant, but differentially regulated in DMI resistance.

Reversible Regulation of Succinate Dehydrogenase by Tools of Photopharmacology

Succinate dehydrogenase (SDH) is extremely important in metabolic function and biological processes. Modulation of SDH has been reported to be a promising therapeutic target to SDH mutations. Current measures for the regulation of SDH are scarce, and precise and reversible modulation of SDH still remains challenging. Herein, a powerful tool for reversible optical control of SDH was proposed and evaluated utilizing the technology of photopharmacology. We reported photochromic ligands (PCLs), azobenzene-pyrazole amides (APAs), that exert light-dependent inhibition effects on SDH. Physicochemical property tests and biological assays were conducted to demonstrate the feasibility of modulating SDH. In this paper, common agricultural pathogens were used to develop a procedure by which our PCLs could reversibly and precisely control SDH utilizing green light. This research would help us to understand the target-ligand interactions and provide new insights into modulation of SDH.

Comprehensive Overview of Carboxamide Derivatives as Succinate Dehydrogenase Inhibitors

Up to now, a total of 24 succinate dehydrogenase inhibitors (SDHIs) fungicides have been commercialized, and SDHIs fungicides were also one of the most active fungicides developed in recent years. Carboxamide derivatives represented an important class of SDHIs with broad spectrum of antifungal activities. In this review, the development of carboxamide derivatives as SDHIs with great significances were summarized. In addition, the structure-activity relationships (SARs) of antifungal activities of carboxamide derivatives as SDHIs was also summarized based on the analysis of the structures of the commercial SDHIs and lead compounds. Moreover, the cause of resistance of SDHIs and some solutions were also introduced. Finally, the development trend of SDHIs fungicides was prospected. We hope this review will give a guide for the development of novel SDHIs fungicides in the future.