Design, Synthesis, and Biological Evaluation of Novel Camphor-Based Hydrazide and Sulfonamide Derivatives as Laccase Inhibitors against Plant Pathogenic Fungi/Oomycetes

Design, synthesis, antifungal evaluation and mechanism study of myrtenal derivatives as potential laccase inhibitors

BACKGROUND

To develop novel natural product-derived fungicides, four series of myrtenal oxadiazole/thiadiazole-hydrazide/amide derivatives were designed, synthesized and evaluated for their inhibitions toward several crop pathogenic fungi/oomycetes.

RESULTS

In the in vitro antifungal experiments, some myrtenal derivatives showed good antifungal activity against Botryosphaeria dothidea, Sclerotinia sclerotiorum and Fusarium graminearum. Among them, compound 6d exhibited the best antifungal activity against B. dothidea, with EC50 value of 0.052 mg/L, which was significantly stronger than carbendazim (EC50 = 0.40 mg/L). In vivo antifungal activity assay on apple fruits and branches showed that 6d displayed significant protective and curative effects, both superior to carbendazim. In the preliminary antifungal mechanism study, 6d can damage the surface morphology of mycelia, destroy the integrity and permeability of cell membrane, increase intracellular ROS content, thereby inhibiting the growth of B. dothidea. Compound 6d also significantly inhibited laccase with IC50 value of 8.45 μM, much stronger than cysteine and PMDD-5Y. Molecular docking also confirmed the binding affinity and interaction mode of 6d with laccase.

CONCLUSION

This study presented a promising lead compound for the study on novel laccase inhibitors as fungicidal agrochemicals, which demonstrated significant anti-B. dothidea activity and laccase inhibitory activity. © 2025 Society of Chemical Industry.

A highly sensitive caffeic acid fluorescent probe for detecting laccase in grape juice and mushrooms

Laccase is a kind of copper-containing polyphenol oxidase obtained with similar properties from different sources, and its catalytic reaction requires only oxygen and releases only water. The most widely used method for laccase detection is spectrophotometric assay with limited sensitivity. Herein, we report a highly sensitive fluorescence method for detecting the activity of laccase. The reported probe shows high selectivity, reliable accuracy and a good linear relationship toward different concentrations of laccase. Upon reaction, a new absorption peak appears around 420 nm accompanied by a significant fluorescence increase at 470 nm, which was consistent with the conjugation extension after the polyphenol structure was oxidized to quinone. The effects of different inhibitors and metal ions on the activity of laccase were assessed and the probe was compared with a commercial laccase detection kit and it showed higher sensitivity. Furthermore, this method was successfully used for the determination of laccase in grape musts with different degrees of fermentation as well as in three kinds of mushroom extracts.

Structural Simplification of Luotonin F: Discovery of Quinoline Derivatives as Novel Antifungal Agents for Plant Protection

Inspired by the natural quinoline alkaloid Luotonin F, a series of quinoline-3-hydrazide derivatives were designed and synthesized by structure simplification and evaluated for their fungicidal activities against plant phytopathogenic fungi. The bioassay results showed that these compounds exhibited broad-spectrum and highly efficient antifungal activities in vitro. Among them, compound W9 exhibited broad-spectrum inhibitory activities against R. solani, S. sclerotiorum, B. cinerea, F. graminearum, F. oxysporum, and P. capsici with EC50 values of 0.471, 0.752, 0.570, 0.329, 0.960, and 0.504 μg/mL, respectively. In particular, compound W9 was more effective against B. cinerea than the reference drugs Pyrimethanil (EC50 = 3.54 μg/mL) and Boscalid (EC50 = 1.37 μg/mL). Therefore, the preliminary mechanism of action of compound W9 on B. cinerea was explored. The results showed that it could affect the mycelial morphology, disrupt the integrity of the cell membrane, and increase the reactive oxygen species content. Furthermore, compound W9 exhibited excellent in vivo protection efficacy against B. cinerea compared to the reference drug Pyrimethanil. Thus, compound W9 is expected to be a novel broad-spectrum fungicidal candidate, exhibiting excellent fungicidal activities, which is worthy of further investigation.

Synthesis and antifungal activity of arecoline derivatives containing amino acid fragments

A series of new arecoline derivatives containing amino acid fragments were synthesized, and their fungicidal activities were investigated. All synthesized compounds were characterized by 1H NMR, 13CNMR, and HRMS. Preliminary bioactivity assays demonstrated that Compounds 3k, 3n, 3p, 3q, 3r, and 3s exhibited significant antifungal activity against Botryosphaeria cactivora, Botryosphaeria dothidea, and Fusarium pseudograminearum at a concentration of 100 μg/mL. Among them, Compound 3s displayed the highest inhibitory activity against Botryosphaeria dothidea (96.63%), surpassing the commercial fungicide chlorothalonil (91.30%). To explore the underlying mechanisms of the compounds, preliminary investigations into the antifungal mechanism involved molecular docking study, scanning electron microscopy and fluorescence microscopy observations, assessments of membrane permeability, and measurements of malondialdehyde content were carried out, respectively. The findings demonstrated that Compound 3s effectively inhibits fungal hyphal growth by compromising the integrity of the hyphal cell membrane. These results indicate that arecoline derivatives containing amino acid benzyl esters have potential as promising fungicides.

Design, Synthesis, and Antifungal Evaluation of Novel Cuminic Acid Derivatives as Potential Laccase Inhibitors

In search of novel natural product-based fungicides, 49 cuminic acid derivatives were designed, synthesized, and screened for their in vitro antifungal effects toward seven phytopathogenic fungi and oomycetes. Consequently, several derivatives exhibited strong antifungal activities toward Fusarium graminearum, Botryosphaeria dothidea, and Valsa mali. Among them, compound 2b exhibited the most potent antifungal activity toward B. dothidea (EC50 = 0.96 mg/L), more powerful than chlorothalonil. The in vivo assay against B. dothidea found that the protective and curative effects of 2b were comparable to chlorothalonil. Meanwhile, SEM and TEM observations indicated that 2b could ruin the integrity of mycelial morphology and organelles of B. dothidea. Preliminary mechanism research showed that 2b increased the cell membrane permeability and intracellular ROS level, as well as conspicuously decreased the mycelial dry weight and cell wall chitin contents of B. dothidea. The phytotoxicity test revealed that 2b showed good safety on seeds of mung bean and radish. The in vitro laccase inhibitory activity assay and molecular docking study demonstrated that 2b could be a promising laccase inhibitor. This type of cuminic acid hydrazide derivative would provide valuable inspiration for developing novel fungicides against B. dothidea.

Thin layer chromatography assay to detect laccase inhibitors

Thin-layer chromatography (TLC) hyphenated to bioassays is a modern tool used for discovery of biologically active compounds from complex mixtures. The first bioautographic assay for detecting laccase inhibitors on a TLC plate was developed in this study. The on-plate reaction of laccase with colourless ABTS that renders the blue ABTS∙+ radical was optimised. Combination of the enzymatic TLC-assay with a control TLC-assay, wherein ABTS∙+ radical is chemically generated and then applied on the TLC, allowed to differentiate between the pure laccase inhibitor sodium azide and radical scavengers such as gallic and kojic acids. The limit of detection and quantification for the method were 54.9 and 166 ng of sodium azide respectively. The methodology was applied successfully to a recently discovered laccase inhibitor chemotype: hydrazones. A model hydrazone was compared with several hydrazones synthesized for this study. For the first time, laccase inhibitors separated on a TLC plate can be detected individually.

l-Isoleucine-Derived Amide-hydrazide Compounds Evaluated as a Novel Potential Agricultural Fungicide

Building upon previous structure-activity relationships about the fungicidal amide and hydrazide lead structures, 24 novel amide-hydrazide compounds were designed and synthesized with L-isoleucine as the initial skeleton to explore the impact of substituents in the hydrazide bridge on the fungicidal activity. Among these compounds, A5 exhibited excellent and broad spectrum inhibitory activity, along with satisfactory in vivo protective efficiency against R. solani at concentrations of 200 and 50 μg·mL-1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that compound A5 induced significant morphological changes in the R. solani mycelium coupled with vacuole rupture and cytoplasmic inhomogeneity in cellular structures. Transcriptomic and metabolomic analyses indicated that, following A5 treatment, the differentially expressed genes and metabolites were significantly enriched in carbohydrate metabolism-related pathways as well as in lipid metabolism-associated pathways, including glycerophospholipid metabolism, steroid biosynthesis, arachidonic acid metabolism, and sphingolipid metabolism. Additionally, compound A5 demonstrated low toxicity to zebrafish, with survival rates of 100% and 60% at concentrations of 1 and 10 μg·mL-1, respectively, over a period of 7 days. The above results provide theoretical guidance for the development of novel green hydrazide fungicidal candidates.

Design, Synthesis, and Antifungal Evaluation of Novel Pyrazole-5-sulfonamide Derivatives for Plant Protection

To develop further novel environmentally friendly antifungal agents with high efficacy, a series of pyrazole-5-sulfonamide derivatives were designed and synthesized by using the active molecules synthesized in previous works as lead compounds. Their antifungal activities were evaluated in vitro against ten highly destructive plant pathogenic fungi. The bioassay results indicated that more than half of the target compounds displayed potent antifungal activities (inhibition rate ≥85%) against Valsa mali and Sclerotinia sclerotiorum at 20 mg/L. Among them, compound C22 exhibited significant broad-spectrum antifungal activities against V. mali, S. sclerotiorum, Rhizoctonia solani, Botrytis cinerea, and Trichoderma viride, with EC50 values of 0.45, 0.49, 3.06, 0.57, and 1.43 mg/L, respectively. Moreover, compounds C21 and C22 exhibited remarkable protective effects on apple Valsa canker similar to tebuconazole (89.5%) at 50 mg/L. Preliminary antifungal mechanism investigations demonstrated that compound C22 may have inhibited V. mali mycelial growth by inducing oxidative damage to the mycelium and compromising the integrity of the cell membrane. Meanwhile, compounds C21 and C22 exhibited no obvious toxicity to worker bees (Apis mellifera ligustica). Taken together, these pyrazole-5-sulfonamide derivatives, particularly compound C22, possess huge potential to be developed as novel environmentally friendly fungicides with high efficacy.

Design and Synthesis of 3-Carene-Derived Amide-Thiourea/Nanochitosan Complexes with Excellent Laccase Inhibitory Activity and Sustained Releasing Performance for Crop Protection

The discovery of natural product-derived novel nanopesticide systems can effectively address the adverse effects caused by the improper use of traditional fungicides. In this research, 33 novel 3-carene-derived amide-thiourea derivatives 5a-5zg were designed using laccase as the biological target, synthesized from natural renewable forest biomass resource 3-carene as the starting material, and structurally confirmed by Fourier-transform infrared spectroscopy, nuclear magnetic resonance, high-resolution mass spectrometry, and single crystal X-ray diffraction. The antifungal activity of the target compounds against eight plant pathogenic fungi was evaluated, and the results presented that target compound 5g exhibited excellent and broad-spectrum antifungal activity against the eight tested phytopathogenic fungi. Furthermore, the important contribution of the gem-dimethylcyclopropane structure in the antifungal activity of compound 5g was revealed through two negative controls without the gem-dimethylcyclopropane structure. Besides, compound 5g also demonstrated a prominent laccase inhibitory activity. The fluorescence quenching of the laccase with compound 5g, the chelating characteristics of compound 5g, and the interaction mode between the laccase and compound 5g presented that the target compound 5g probably exhibited excellent antifungal activity by acting on the laccase target. Cytotoxicity assay revealed that compound 5g had a low cytotoxicity for LO2 and HEK293T cell lines. On the other hand, to further improve the application potential of compound 5g, the 3-carene molecular skeleton containing gem-dimethylcyclopropane ring was grafted onto chitosan, and two nanopesticide carriers CACS and CATCS with sustained releasing performance were synthesized for loading compound 5g. 3-Carene-derived nanochitosan carrier CATCS showed a relatively regular, loose, and porous reticular structure, which displayed high dispersibility and good thermostability. In addition, this carrier had a higher drug-loading capacity and sustained releasing performance than that of the unmodified chitosan. This research identified that the target compound 5g could be used as a promising lead compound for fungicide against the laccase target, meanwhile, the complex 5g/CATCS deserved further study as a nanopesticide candidate.

Pinonic Acid Derivatives Containing Thiourea Motif: Promising Antifungal Lead Compound Targeting Cellular Barrier of Colletotrichum fructicola

In order to explore novel antifungal lead compounds from plant essential oil, thirty-two pinonic acid derivatives containing thiourea groups were designed and synthesized using α-pinene as a raw material. One of these pinonic acid derivatives compound 3a exhibited noteworthy in vitro antifungal activity against Colletotrichum fructicola (EC50 = 9.22 mg/L), which was comparable to that of the positive control kresoxim-methyl (EC50 = 9.69 mg/L). Structure-activity relationship (SAR) studies demonstrated that the introduction of thiourea groups, F atoms, and Cl atoms into the structure of pinonic acid derivatives significantly improved their antifungal activity. The in vivo antifungal test revealed that compound 3a could effectively control pear anthracnose. It also proved that compound 3a showed low acute oral toxicity to honeybees (LD50 > 100 μg/bee) and low or no cytotoxicity to LO2 and HEK293 cell lines. The preliminary mechanism of action studies revealed that compound 3a caused mycelium deformity, increased cell membrane permeability, blocked the normal process of phospholipase C on the cell membrane, and reduced mycelium protein content. The results of molecular docking studies demonstrated the stable binding of compound 3a to phospholipase C and chitin synthetase. This study suggested that compound 3a could be used as a promising lead compound for the development of novel antifungal agents targeting the cellular barrier of C. fructicola.

Design, synthesis, antifungal evaluation and mechanism study of novel norbornene derivatives as potential laccase inhibitors

BACKGROUND

To discover novel fungicide candidates, five series of novel norbornene hydrazide, bishydrazide, oxadiazole, carboxamide and acylthiourea derivatives (2a-2t, 3a-3f, 4a-4f, 5a-5f and 7a-7f) were designed, synthesized and assayed for their antifungal activity toward seven representative plant fungal pathogens.

RESULTS

In the in vitro antifungal assay, some title norbornene derivatives presented good antifungal activity against Botryosphaeria dothidea, Sclerotinia sclerotiorum and Fusarium graminearum. Especially, compound 2b exhibited the best inhibitory activity toward B. dothidea with the median effective concentration (EC50) of 0.17 mg L-1, substantially stronger than those of the reference fungicides boscalid and carbendazim. The in vivo antifungal assay on apples revealed that 2b had significant curative and protective effects, both of which were superior to boscalid. In the preliminary antifungal mechanism study, 2b was able to injure the surface morphology of hyphae, destroy the cell membrane integrity and increase the intracellular reactive oxygen species (ROS) level of B. dothidea. In addition, 2b could considerably inhibit the laccase activity with the median inhibitory concentration (IC50) of 1.02 μM, much stronger than that of positive control cysteine (IC50 = 35.50 μM). The binding affinity and interaction mode of 2b with laccase were also confirmed by molecular docking.

CONCLUSION

This study presented a promising lead compound for the study of novel laccase inhibitors as fungicidal agrochemicals, which demonstrate significant anti-B. dothidea activity and laccase inhibitory activity. © 2024 Society of Chemical Industry.

Design and synthesis of bis(indolyl)-hydrazide-hydrazone derivatives and their antifungal activities against plant pathogen fungi

A series of bis(indolyl)-hydrazide-hydrazone derivatives were synthesised, and their structures were characterised using 1H-NMR and HRMS. The antifungal activity of the prepared compounds was evaluated against Pyricularia oryzae Cav., Colletotrichum -gloeosporioides Penz., Botrytis cinerea Pers.: Fr. and Rhizoctonia solani Kühn using the mycelial growth rate method. The preliminary bioassays revealed that most of the synthesised compounds exhibited antifungal activity against the four tested fungi and displayed a remarkable inhibitory effect on the mycelium growth of R. solani. In particular, compounds 3b, 3c, and 3k demonstrated significant antifungal activity against R. solani, with EC50 values of 26.42, 20.74, and 22.41 μM, respectively, outperforming the positive control shenqinmycin (47.18 μM) and carvacrol (49.13 μM).

Antifungal Activity and Mechanism of Camphor Derivatives against Rhizoctonia solani: A Promising Alternative Antifungal Agent for Rice Sheath Blight

Rice sheath blight, caused by the fungus Rhizoctonia solani, poses a significant threat to rice cultivation globally. This study aimed to investigate the potential mechanisms of action of camphor derivatives against R. solani. Compound 4o exhibited superior fungicidal activities in vitro (EC50 = 6.16 mg/L), and in vivo curative effects (77.5%) at 500 mg/L were significantly (P < 0.01) higher than the positive control validamycin·bacillus (66.1%). Additionally, compound 4o exhibited low cytotoxicity and acute oral toxicity for adult worker honeybees of Apis mellifera L. Mechanistically, compound 4o disrupted mycelial morphology and microstructure, increased cell membrane permeability, and inhibited both PDH and SDH enzyme activities. Molecular docking and molecular dynamics analyses indicated a tight interaction of compound 4o with PDH and SDH active sites. In summary, compound 4o exhibited substantial antifungal efficacy against R. solani, serving as a promising lead compound for further optimization of antifungal agents.

Isolating Antipathogenic Fungal Coumarins from Coriaria nepalensis and Determining Their Primary Mechanism In Vitro

Through bioassay-guided isolation, eight undescribed coumarins (1-8), along with six reported coumarins (9-14), were obtained from Coriaria nepalensis. The new structures were determined by using IR, UV, NMR, HRESIMS, and ECD calculations. The results of the biological activity assays showed that compound 9 exhibited broad spectrum antifungal activities against all tested fungi in vitro and a significant inhibitory effect on Phytophthora nicotianae with an EC50 value of 3.00 μg/mL. Notably, compound 9 demonstrated greater curative and protective effects against tobacco balack shank than those of osthol in vivo. Thus, 9 was structurally modified to obtain new promising antifungal agents, and the novel derivatives (17b, 17j, and 17k) exhibited better effects on Sclerotinia sclerotiorum than did lead compound 9. Preliminary mechanistic exploration illustrated that 9 could enhance cell membrane permeability, destroy the morphology and ultrastructure of cells, and reduce the exopolysaccharide content of P. nicotianae mycelia. Furthermore, the cytotoxicity results revealed that compound 9 exhibited relatively low cytotoxicity against HEK293 cell lines with an inhibition rate of 33.54% at 30 μg/mL. This research is promising for the discovery of new fungicides from natural coumarins with satisfactory ecological compatibility.