Design, synthesis and antifungal/anti-oomycete activity of pyrazolyl oxime ethers as novel potential succinate dehydrogenase inhibitors

3D-QSAR model-oriented optimization of Pyrazole β-Ketonitrile derivatives with diphenyl ether moiety as novel potent succinate dehydrogenase inhibitors

BACKGROUND

Succinate dehydrogenase inhibitor (SDHI) fungicides play important roles in the control of plant fungal diseases. However, they are facing serious challenges from issues with resistance and cross-resistance, primarily attributed to their frequent application and structural similarities. There is an urgent need to design and develop SDHI fungicides with novel structures.

RESULTS

Aiming to discover novel potent SDHI fungicides, 31 innovative pyrazole β-ketonitrile derivatives with diphenyl ether moiety were rationally designed and synthesized, which were guided by a 3D-QSAR model from our previous study. The optimal target compound A23 exhibited not only outstanding in vitro inhibitory activities against Rhizoctonia solani with a half-maximal effective concentration (EC50) value of 0.0398 μg mL-1 comparable to that for fluxapyroxad (EC50 = 0.0375 μg mL-1), but also a moderate protective efficacy in vivo against rice sheath blight. Porcine succinate dehydrogenase (SDH) enzymatic inhibitory assay revealed that A23 is a potent inhibitor of SDH, with a half-maximal inhibitory concentration of 0.0425 μm. Docking study within R. solani SDH indicated that A23 effectively binds into the ubiquinone site mainly through hydrogen-bonds, and cation-π and π-π interactions.

CONCLUSION

The identified β-ketonitrile compound A23 containing diphenyl ether moiety is a potent SDH inhibitor, which might be a good lead for novel fungicide research and optimization. © 2024 Society of Chemical Industry.

Design, Synthesis, and Antifungal Activities of Novel Potent Fluoroalkenyl Succinate Dehydrogenase Inhibitors

The development of new fungicide molecules is a crucial task for agricultural chemists to enhance the effectiveness of fungicides in agricultural production. In this study, a series of novel fluoroalkenyl modified succinate dehydrogenase inhibitors were synthesized and evaluated for their antifungal activities against eight fungi. The results from the in vitro antifungal assay demonstrated that compound 34 exhibited superior activity against Rhizoctonia solani with an EC50 value of 0.04 μM, outperforming commercial fluxapyroxad (EC50 = 0.18 μM) and boscalid (EC50 = 3.07 μM). Furthermore, compound 34 showed similar effects to fluxapyroxad on other pathogenic fungi such as Sclerotinia sclerotiorum (EC50 = 1.13 μM), Monilinia fructicola (EC50 = 1.61 μM), Botrytis cinerea (EC50 = 1.21 μM), and also demonstrated protective and curative efficacies in vivo on rapeseed leaves and tomato fruits. Enzyme activity experiments and protein-ligand interaction analysis by surface plasmon resonance revealed that compound 34 had a stronger inhibitory effect on succinate dehydrogenase compared to fluxapyroxad. Additionally, molecular docking and DFT calculation confirmed that the fluoroalkenyl unit in compound 34 could enhance its binding capacity with the target protein through p-π conjugation and hydrogen bond interactions.

Discovery of novel piperidine-containing thymol derivatives as potent antifungal agents for crop protection

BACKGROUND

Plant fungal diseases pose a significant threat to crop production. The extensive use of chemical pesticides has led to growing environmental safety risks and pesticide resistance of various plant pathogens. Therefore, it is an urgent task to explore novel eco-friendly fungicidal agents with high efficacy to combat fungal infection.

RESULTS

In this study, we rationally designed a series of novel thymol derivatives by incorporation of the sulfonamide moiety and evaluated their biological activities against plant pathogenic fungi. The bioassay results underscored the remarkable in vitro antifungal activity of compounds 5m and 5t against Phytophthora capsici (P. capsici), with EC50 values of 8.420 and 8.414 μg/mL, respectively. Their efficacies were superior to that of widely used commercial fungicides azoxystrobin (AZO, 20.649 μg/mL) and cabendazim (CAB, 251.625 μg/mL). Furthermore, compound 5v exhibited excellent in vitro antifungal activity against Sclerotinia sclerotiorum (S. sclerotiorum), with an EC50 value of 12.829 μg/mL, significantly outperforming AZO (63.629 μg/mL). In vivo bioassays demonstrated the impactful activity of compound 5v against S. sclerotiorum, achieving over 98% curative and protective efficacies at the concentration of 200 μg/mL. Further mechanistic investigations unveiled that compound 5v induced mycelial shrinkage and collapse in S. sclerotiorum, resulting in organelle damage and the accumulation of antioxidant enzyme activity.

CONCLUSION

The significant antifungal efficacy of the prepared thymol derivatives shall encourage further exploration of compound 5v as a promising candidate to develop novel fungicides for crop protection. © 2024 Society of Chemical Industry.

Oxime chemistry in crop protection

An overview is given on the significance of the oxime moiety in crop protection chemistry. This review focuses on the two most important aspects of agrochemical oximes, which are the occurrence and role of oxime groups in compounds with herbicidal, fungicidal and insecticidal activity, as well as the application of oxime derivatives as intermediates in the synthesis of crop protection agents not bearing any oxime function. Especially noteworthy is the fact, that in the synthesis of agrochemicals, oximes can be cyclized to isooxazoline, isoxazole, oxadiazole, oxazine, pyrrole, isothiazole and imidazole rings. © 2024 Society of Chemical Industry.

Discovery of Novel Pyrazol-5-yl-benzamide Derivatives Containing a Thiocyanato Group as Broad-Spectrum Fungicidal Candidates

In an effort to promote the development of new fungicides, a series of 48 novel N-(1-methyl-4-thiocyanato-1H-pyrazol-5-yl)-benzamide derivatives A1-A36 and B1-B12 were designed and synthesized by incorporating a thiocyanato group into the pyrazole ring, and their fungicidal activities were evaluated against Sclerotinia sclerotiorum, Valsa mali, Botrytis cinerea, Rhizoctonia solani, and Phytophthora capsici. In the in vitro antifungal/antioomycete assay, many of the target compounds exhibited good broad-spectrum fungicidal activities. Among them, compound A36 displayed the best antifungal activity against V. mali with an EC50 value of 0.37 mg/L, which was significantly higher than that of the positive controls fluxapyroxad (13.3 mg/L) and dimethomorph (10.3 mg/L). Meanwhile, compound B6 exhibited the best antioomycete activity against P. capsici with an EC50 value of 0.41 mg/L, which was higher than that of azoxystrobin (29.2 mg/L) but lower than that of dimethomorph (0.13 mg/L). Notably, compound A27 displayed broad-spectrum inhibitory activities against V. mali, B. cinerea, R. solani, S. sclerotiorum, and P. capsici with respective EC50 values of 0.71, 1.44, 1.78, 0.87, and 1.61 mg/L. The in vivo experiments revealed that compounds A27 and B6 presented excellent protective and curative efficacies against P. capsici, similar to that of the positive control dimethomorph. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that compound B6 could change the mycelial morphology and severely damage the ultrastructure of P. capsici. The results of the in vitro SDH enzymatic inhibition experiments indicated that compounds A27 and B6 could effectively inhibit the activity of P. capsici SDH (PcSDH). Furthermore, molecular docking analysis demonstrated significant hydrogen bonds and Pi-S bonding between the target compounds and the key amino acid residues of PcSDH, which could explain the probable mechanism of action. Collectively, these studies provide a valuable approach to expanding the fungicidal spectrum of pyrazol-5-yl-benzamide derivatives.

Design, Synthesis, and Antifungal/Antioomycete Activity of Thiohydantoin Analogues Containing Spirocyclic Butenolide

Novel fungicidal agents were designed based on the combination of two privileged scaffolds, thiohydantoin and spirocyclic butenolide, which are widely found in natural products. The synthesized compounds were characterized by ¹H NMR, ¹³C NMR, and high-resolution electrospray ionisation mass spectrometry. The in vitro antioomycete activity evaluation showed that most of the compounds exhibited excellent inhibitory activities against different developmental stages in the life cycle of pathogenic oomycete Phytophthora capsici. Compound 5j could inhibit the mycelial growth, sporangium production, zoospore release, and cystospore germination significantly with EC₅₀ values of 0.38, 0.25, 0.11, and 0.026 μg/mL, respectively. The in vivo antifungal/antioomycete bioassay results revealed that the series of compounds generally showed outstanding control efficacies against the pathogenic oomycete Pseudoperonospora cubensis, and compounds 5j, 5l, 7j, 7k, and 7l possessed broad-spectrum antifungal activities against the test phytopathogens. The in vivo protective and curative efficacies against P. capsici of the representative compound 5j were excellent, which were better than those of azoxystrobin. More prominently, 5j significantly promoted the biomass accumulation of the root system and reinforced the cell wall by callose deposition. The pronounced upregulation of immune response-related genes indicated that the active oomycete inhibitor 5j also functioned as a plant elicitor. Transmission electron microscopy observation and the enzyme activity test demonstrated that the mechanism of action of 5j was to bind to the pivotal protein, complex III on the respiratory chain, which resulted in a shortage of energy supply. Molecular docking results exhibited that compound 5j appropriately matched with the Qo pocket and had no interaction with the most commonly mutated site Gly-142, which may be of significant benefit in Qo fungicide resistance management. Compound 5j showed great advantages and potential in oomycete control, resistance management, and induction of disease resistance. A further investigation of 5j with a unique structure might have direct implications for the creation of novel oomycete inhibitors against plant-pathogenic oomycetes.

Rational Exploration of Novel SDHI Fungicide through an Amide-β-ketonitrile Bioisosteric Replacement Strategy

The identification of succinate dehydrogenase inhibitor (SDHI) fungicides bearing a novel scaffold is of great importance to control pathogenic fungi. Difluoromethyl-pyrazole β-ketonitrile derivatives were rationally designed through an innovative amide-β-ketonitrile bioisosteric replacement strategy and evaluated for their antifungal activities. In preliminary fungicidal screening, our new β-ketonitrile compounds showed outstanding in vitro activity. Compounds A7 and A14 exhibited EC₅₀ values of 0.116 and 0.165 μg/mL against Sclerotinia sclerotiorum, respectively, and A14 also displayed an EC₅₀ of 0.0774 μg/mL against Rhizoctonia solani. Furthermore, A14 exhibited moderate in vivo protective activity against rice sheath blight on rice plants. Results from SDH enzymatic assays demonstrated that A14 possesses significant inhibitory effect toward porcine heart SDH, with an IC₅₀ value of 0.183 μM, which was 20-fold more potent than that of fluxapyroxad (IC₅₀ = 3.76 μM). A docking study indicated that H-bonds, cation-π interactions, and edge-to-face π-π interactions play key roles in the binding of A14 with R. solani SDH. The CoMSIA model guided the approach to further structural optimizations and indicated that hydrophobic and steric substituents on the benzene ring have decisive effects on the fungicidal activity against R. solani. The present work describes for the first time the successful bioisosteric replacement of the common SDHI amide moiety by a β-ketonitrile group and highlights the potential of β-ketonitriles as an innovative novel SDHI subclass.

Occurrence and mechanism of target-site resistance to bensulfuron-methyl in Monochoria korsakowii from China

Monochoria korsakowii is an increasingly significant threat to rice production across China, particularly in Liaoning province. Few studies have reported herbicide resistance in M. korsakowii, and resistance status and mechanisms are poorly understood. Here, thirty field populations of M. korsakowii were collected from 11 rice-growing regions of Liaoning, and 97% of populations had evolved resistance to bensulfuron-methyl (BM), with majority (24 of 28) showing high resistance levels (RI > 10). The first in-depth analysis of molecular features of AHAS1 and AHAS2 in BM-resistant populations showed that four Pro197 mutations (Pro197 to His, Ala, Leu or Ser) in AHAS1 and one mutation (Pro197Ser) in AHAS2 were identified. Notably, novel double Pro197Ser mutations co-occurred in both AHAS1 and AHAS2 in the most resistant line LN-20. Furthermore, resistant mutants were used to investigate the effect of Pro197 mutations on AHAS functionality, binding modes, gene expression and cross-resistance in M. korsakowii. All the detected Pro197 mutations considerably reduced in vitro AHAS sensitivity to BM by weakening hydrogen bonds and hydrophobic interactions in the predicted BM-AHAS complexes, especially the double Pro197Ser mutations. This novel resistance mutation combination slightly impacted the extractable AHAS activity, and increased the affinity and catalytic rate of pyruvate. Also, the AHAS expression level was significantly up-regulated. Moreover, all mutations provided resistance only to other sulfonylureas herbicides but not triazolopyrimidine or pyrimidinyl-benzoates herbicides. In conclusion, bensulfuron-methyl resistance in M. korsakowii was grim in Liaoning, China, and amino acid mutations on AHAS isozymes were the primary resistance mechanism. Double Pro197Ser mutations in both AHAS1 and AHAS2 confer higher herbicide resistance than single mutations in AHAS1. Thus, this work deepens our understanding of resistance status and mechanisms of M. korsakowii.

Design, Synthesis and Bioactivity of Novel Pyrimidine Sulfonate Esters Containing Thioether Moiety

Pesticides play an important role in crop disease and pest control. However, their irrational use leads to the emergence of drug resistance. Therefore, it is necessary to search for new pesticide-lead compounds with new structures. We designed and synthesized 33 novel pyrimidine derivatives containing sulfonate groups and evaluated their antibacterial and insecticidal activities. Results: Most of the synthesized compounds showed good antibacterial activity against Xanthomonas oryzae pv. Oryzae (Xoo), Xanthomonas axonopodis pv. Citri (Xac), Pseudomonas syringae pv. actinidiae (Psa) and Ralstonia solanacearum (Rs), and certain insecticidal activity. A₅, A₃₁ and A₃₃ showed strong antibacterial activity against Xoo, with EC₅₀ values of 4.24, 6.77 and 9.35 μg/mL, respectively. Compounds A₁, A₃, A₅ and A₃₃ showed remarkable activity against Xac (EC₅₀ was 79.02, 82.28, 70.80 and 44.11 μg/mL, respectively). In addition, A₅ could significantly improve the defense enzyme (superoxide dismutase, peroxidase, phenylalanine ammonia-lyase and catalase) activity of plants against pathogens and thus improve the disease resistance of plants. Moreover, a few compounds also showed good insecticidal activity against Plutella xylostella and Myzus persicae. The results of this study provide insight into the development of new broad-spectrum pesticides.

Novel (Z)/(E)-1,2,4-triazole derivatives containing oxime ether moiety as potential ergosterol biosynthesis inhibitors: design, preparation, antifungal evaluation, and molecular docking

Inspired by the highly effective and broad-spectrum antifungal activity of ergosterol biosynthesis inhibitions, a series of novel 1,2,4-triazole derivatives containing oxime ether moiety were constructed for screening the bioactivity against phytopathogenic fungi. The (Z)- and (E)-isomers of target compounds were successfully separated and identified by the spectroscopy and single crystal X-ray diffraction analyses. The bioassay results showed that the (Z)-isomers of target compounds possessed higher antifungal activity than the (E)-isomers. Strikingly, the compound (Z)-5o exhibited excellent antifungal activity against Rhizoctonia solani with the EC₅₀ value of 0.41 μg/mL in vitro and preventive effect of 94.58% in vivo at 200 μg/mL, which was comparable to the positive control tebuconazole. The scanning electron microscopy observation indicated that the compound (Z)-5o caused the mycelial morphology to become wizened and wrinkled. The molecular docking modes of (Z)-5o and (E)-5o with the potential target protein RsCYP51 were especially compared. And the main interactions between ligands and amino acid residues were carefully analyzed to preliminarily explain the mechanism leading to the difference of activity between two isomers. The study provided a new lead molecular skeleton for developing novel triazole fungicides targeting ergosterol biosynthesis.

Methods for Fungicide Efficacy Screenings: Multiwell Testing Procedures for the Oomycetes Phytophthora infestans and Pythium ultimum

Oomycetes-borne diseases represent a serious problem for agriculture sustainability due to the high use of chemical products employed for their control. In recent years, increasing concerns on side effects associated with fungicide utilization have led to the reduction of the permissible modes of action, with the remaining ones continuously threatened by the increase of resistant strains in the pathogen populations. In this context, it is mandatory to develop new generation fungicides characterized by high specificity towards the target species and low environmental impact to guarantee the sustainability, productivity, and quality of food production. Fungicide discovery is a lengthy and costly process, and despite these urgent needs, poor description and formalization of high-throughput methodologies for screening the efficacy of active compounds are commonly reported for these kinds of organisms. In this study, a comprehensive picture of two high-throughput practices for efficient fungicide screening against plant-pathogenic oomycetes has been provided. Different protocols using multiwell plates were validated on approved crop protection products using Phytophthora infestans and Pythium ultimum as the model species. In addition, detailed statistical inputs useful for the analysis of data related to the efficacy of screenings are included.

Novel Camphor Sulfonohydrazide and Sulfonamide Derivatives as Potential Succinate Dehydrogenase Inhibitors against Phytopathogenic Fungi/Oomycetes

To discover novel fungicidal agrochemicals for treating wheat scab, 39 novel camphor sulfonohydrazide/sulfonamide derivatives 4a-4t and 6a-6s were designed and synthesized. In the in vitro antifungal/antioomycete assay, compounds 4g, 4n, and 4o displayed significant inhibitory activities against Fusarium graminearum, Botryosphaeria dothidea, and Phytophthora capsici. Among them, 4n exhibited the best antifungal activity against F. graminearum with an EC₅₀ value of 0.41 mg/L, which was at the same level as that of pydiflumetofen. The in vivo experiment revealed that 4n presented excellent protective and curative efficacy toward F. graminearum. In the antifungal mechanism study, 4n could increase the cell membrane permeability and reduce the exopolysaccharide and ergosterol content of F. graminearum. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed that 4n could significantly damage the surface morphology and the cell ultrastructure of mycelia to interfere with the growth of F. graminearum. Furthermore, 4n exhibited potent succinate dehydrogenase (SDH) inhibitory activity in vitro with an IC₅₀ value of 3.94 μM, which was equipotent to pydiflumetofen (IC₅₀ = 4.07 μM). The molecular dynamics simulation and docking study suggested that compound 4n could well occupy the active site and form strong interactions with the key residues of SDH. The above-mentioned results demonstrated that the title camphor sulfonohydrazide/sulfonamide derivatives could be promising lead compounds for further succinate dehydrogenase inhibitor (SDHI) fungicide development.

Anti-Fungal Activity of Moutan cortex Extracts against Rice Sheath Blight (Rhizoctonia solani) and Its Action on the Pathogen's Cell Membrane

Rice sheath blight (RSB) caused by Rhizoctonia solani is one of the most destructive diseases of rice (Oryza sativa). Although chemical fungicides are the most important control methods, their long-term unreasonable application has brought about problems such as environmental pollution, food risks, and non-target poisoning. Therefore, considering the extraction of fungistatic substances from plants may be an alternative in the future. In this study, we found that the Moutan cortex ethanol extract has excellent antifungal activity against R. solani, with a 100% inhibition rate at 1000 μg/mL, which aroused our great exploration interest. In-depth exploration found that the antifungal active ingredients of M. cortex were mainly concentrated in the petroleum ether extract of the M. cortex ethanol extract, which still maintained a 100% inhibition rate with 250 μg/mL, and its effective medium concentration (EC₅₀) was 145.33 μg/mL against R. solani. Through the measurement of extracellular relative conductivity and OD₂₆₀, the petroleum ether extract induced leakage of intracellular electrolytes and nucleic acids, indicating that the cell membrane was ruined. Therefore, we preliminarily determined that the cell membrane may be the target of the petroleum ether extract. Moreover, we found that petroleum ether extract reduced the content of ergosterol, a component of the cell membrane, which may be one of the reasons for the cell membrane destruction. Furthermore, the increase of MDA content would lead to membrane lipid peroxidation, further aggravating membrane damage, resulting in increased membrane permeability. Also, the destruction of the cell membrane was observed by the phenomenon of the mycelium being transparent and broken. In conclusion, this is the first report of the M. cortex petroleum ether extract exhibiting excellent antifungal activity against R. solani. The effect of the M. cortex petroleum ether extract on R. solani may be on the cell membrane, inducing the disorder of intracellular substances and metabolism, which may be one of the antifungal mechanisms against R. solani.

Deciphering the Biomolecules from Bacillus atrophaeus NMB01 Untangles the Anti-Oomycetes Action of Trioxsalen and Corynan-17-ol, Against Phytophthora infestans Inciting Late Blight of Potato

The antagonistic Bacillus spp. is known well for the production of versatile antimicrobial biomolecules with broad spectrum of action against different types of plant pathogens. Considering the significance of metabolically active biomolecules, attempts were made to decipher the anti-oomycete nature of biomolecules produced by Bacillus atrophaeus NMB01 during di-trophic interaction with Phytophthora infestans. Ten biomolecules produced by B. atrophaeus NMB01 during di-trophic interaction with P. infestans were docked against the twelve target proteins of P. infestans. Molecular docking of biomolecules reported trioxsalen and corynan-17-ol,18,19-didehydro-10-methoxy-acetate(ester) as best hits with highest binding energy in the range of - 7.5 to - 5 kcal/mol against target proteins of P. infestans. Comparatively less binding energy was observed for commercially available fungicides mandipropamid and metalaxyl on docking against the target proteins of P. infestans. We also confirmed the direct impact of trioxsalen andcorynan-17-ol, on P. infestans under in vitro with 66% and 50% inhibition of mycelial growth of P. infestans, respectively. This is the first study attempted to untangle the role of bioactive anti-oomycete compounds produced by B. atrophaeus strain NMB01 during di-trophic interaction with P. infestans against late blight pathogen P. infestans infecting potato. From the present study, we conclude that the biomolecules, trioxsalen and corynan-17-ol, can be explored for the management of P. infestans, the incitant of late blight of potato.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01044-7.

Synthesis of (E)/(Z)-Verbenone Oxime Ethers and Photoresponsive Behavior to Herbicidal Activity

The accumulation of residual active herbicides in the environment can cause a series of problems. It is thus meaningful to explore a photoresponsive herbicide, whose activity can be weakened under the action of light to reduce the negative effect. To this purpose, a series of (E)/(Z)-verbenone oxime ethers were designed, synthesized, and characterized. Oxime ether groups were adopted as the trigger switches. The preliminary screening for herbicidal activity showed that some of them exhibited better or comparable effects than that of the commercial herbicide flumioxazin against Brassica campestris and Echinochloa crusgalli. Meanwhile, five pairs of the target compounds with significantly different herbicidal effects between E- and Z-forms were further investigated for their reversible isomerization reaction and the accompanying variation of herbicidal activity. As a result, the maximum conversion rates were around 50%, and the herbicidal effect of the resulting mixture of E- and Z-isomers decreased outstandingly. The phototransformation mechanism of a pair of isomers (E)-4a and (Z)-4a was preliminarily explored. Besides, a reasonable and effective 3D-quantitative structure-activity relationship model (r² = 0.984 and q² = 0.571) was established and the binding mode was also investigated by molecular docking.

Design, Synthesis, and Biological Evaluation of Novel Pyrazol-5-yl-benzamide Derivatives Containing Oxazole Group as Potential Succinate Dehydrogenase Inhibitors

A series of pyrazol-5-yl-benzamide derivatives containing the oxazole group were designed and synthesized as potential SDH inhibitors. According to the results of the bioassays, most target compounds displayed moderate-to-excellent in vitro antifungal activities against Valsa mali, Sclerotinia scleotiorum, Alternaria alternata, and Botrytis cinerea. Among them, compounds C13, C14, and C16 exhibited more excellently inhibitory activities against S. sclerotiorum than boscalid (EC₅₀ = 0.96 mg/L), with EC₅₀ values of 0.69, 0.26, and 0.95 mg/L, respectively. In vivo experiments on rape leaves and cucumber leaves showed that compounds C13 and C14 exhibited considerable protective effects against S. sclerotiorum than boscalid. SEM analysis indicated that compounds C13 and C14 significantly destroyed the typical structure and morphology of S. scleotiorum hyphae. In the respiratory inhibition effect assays, compounds C13 (28.0%) and C14 (33.9%) exhibited a strong inhibitory effect on the respiration rate of S. sclerotiorum mycelia, which was close to boscalid (30.6%). The results of molecular docking indicated that compounds C13 and C14 could form strong interactions with the key residues TRP O:173, ARG P:43, TYR Q:58, and MET P:43 of the SDH. Furthermore, the antifungal mechanism of these derivatives was demonstrated by the SDH enzymatic inhibition assay. These results demonstrate that compounds C13 and C14 can be developed into novel SDH inhibitors for crop protection.

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.

Design, Synthesis, and Fungicidal Evaluation of Novel 1,3-Benzodioxole-Pyrimidine Derivatives as Potential Succinate Dehydrogenase Inhibitors

A series of novel 1,3-benzodioxole-pyrimidine derivatives were designed and synthesized. The in vitro bioassay indicated that compounds 4e, 4g, 4n, 5c, and 5e displayed excellent fungicidal activities against test fungal strains. Especially, in the in vitro experiments, 5c exhibited a broad spectrum of fungicidal activity against Botrytis cinerea, Rhizoctonia solani, Fusarium oxysporum, Alternaria solani, and Gibberella zeae with EC₅₀ values of 0.44, 6.96, 6.99, 0.07, and 0.57 mg/L, respectively, which were significantly more potent than those of positive control boscalid (EC₅₀: 5.02, >50, >50, 0.16, and 1.28 mg/L). In vivo testing on tomato fruits and leaves showed that 5c displayed considerable protective and curative efficacy against A. solani. Scanning electron microscopy analysis indicated that 5c possessed a strong ability to destroy the surface morphology of mycelia and seriously interfere with the growth of the fungal pathogen. In the in vitro enzyme inhibition assay, 5c exhibited pronounced succinate dehydrogenase (SDH) inhibitory activity with an IC₅₀ value of 3.41 μM, equivalent to that of boscalid (IC₅₀: 3.40 μM). In addition, fluorescence quenching experiment further confirmed the strong interaction of 5c with SDH. Through chiral resolution, 5c was separated into two enantiomers. Among them, (S)-5c exhibited stronger fungicidal activity (EC₅₀: 0.06 mg/L) and SDH inhibitory (2.92 μM) activity than the R-enantiomer (EC₅₀: 0.17 mg/L and SDH IC₅₀: 3.68 μM), which was in accordance with the molecular docking study (CDOCKER Interaction Energy for (R)-5c and (S)-5c: -28.23 and -29.98 kcal/mol, respectively). These results presented a promising lead for the discovery of novel SDHIs as antifungal pesticides.

Rational Design, Synthesis, and Biological Evaluation of Fluorine- and Chlorine-Substituted Pyrazol-5-yl-benzamide Derivatives as Potential Succinate Dehydrogenase Inhibitors

To develop novel succinate dehydrogenase inhibitors (SDHIs), two series of novel N-4-fluoro-pyrazol-5-yl-benzamide and N-4-chloro-pyrazol-5-yl-benzamide derivatives were designed and synthesized, and their antifungal activities were evaluated against Valsa mali, Sclerotinia sclerotiorum, FusaHum graminearum Sehw, Physalospora piricola, and Botrytis cinerea. The bioassay results showed that some of the target compounds exhibited good antifungal activities in vitro against V. mali and S. sclerotiorum. Remarkably, compound 9Ip displayed good in vitro activity against V. mali with an EC₅₀ value of 0.58 mg/L. This outcome was 21-fold greater than that of fluxapyroxad (12.45 mg/L) and close to that of the commercial fungicide tebuconazole (EC₅₀ = 0.36 mg/L). In addition, in vivo experiments proved that compound 9Ip has good protective fungicidal activity with an inhibitory rate of 93.2% against V. mali at 50 mg/L, which was equivalent to that of the positive control tebuconazole (95.5%). The results of molecular docking indicated that there were obvious hydrogen bonds and p-π interactions between compound 9Ip and succinate dehydrogenase (SDH), which could explain the probable action mechanism. In addition, the SDH enzymatic inhibition assay was carried out to further prove its mode of action. Our studies suggest that compound 9Ip could be a fungicidal lead to discover more potent SDHIs for crop protection.

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.

Design, Synthesis and Antifungal/Nematicidal Activity of Novel 1,2,4-Oxadiazole Derivatives Containing Amide Fragments

Plant diseases that are caused by fungi and nematodes have become increasingly serious in recent years. However, there are few pesticide chemicals that can be used for the joint control of fungi and nematodes on the market. To solve this problem, a series of novel 1,2,4-oxadiazole derivatives containing amide fragments were designed and synthesized. Additionally, the bioassays revealed that the compound F15 demonstrated excellent antifungal activity against Sclerotinia sclerotiorum (S. sclerotiorum) in vitro, and the EC₅₀ value of that was 2.9 μg/mL, which is comparable with commonly used fungicides thifluzamide and fluopyram. Meanwhile, F15 demonstrated excellent curative and protective activity against S. sclerotiorum-infected cole in vivo. The scanning electron microscopy results showed that the hyphae of S. sclerotiorum treated with F15 became abnormally collapsed and shriveled, thereby inhibiting the growth of the hyphae. Furthermore, F15 exhibited favorable inhibition against the succinate dehydrogenase (SDH) of the S. sclerotiorum (IC₅₀ = 12.5 μg/mL), and the combination mode and binding ability between compound F15 and SDH were confirmed by molecular docking. In addition, compound F11 showed excellent nematicidal activity against Meloidogyne incognita at 200 μg/mL, the corrected mortality rate was 93.2%, which is higher than that of tioxazafen.