Fungicidal Action of the Triphenylphosphonium-Driven Succinate Dehydrogenase Inhibitors Is Mediated by Reactive Oxygen Species and Suggests an Effective Resistance Management Strategy

Rational design and discovery of novel hydrazide derivatives as potent succinate dehydrogenase inhibitors inspired by natural d/l-camphor

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) have rapidly become one of the fastest-growing categories of fungicides used against plant pathogenic fungi. Recent research advancements have emphasized that structural modifications of SDHIs using naturally sourced scaffolds represent an innovative strategy for developing new, highly effective, broad-spectrum fungicides. A novel series of d/l-camphorhydrazide derivatives potentially targeting fungal succinate dehydrogenase (SDH) were designed, synthesized and evaluated for their antifungal effects against Rhizoctonia solani, Fusarium graminearum, Valsa mali and Botrytis cinerea.

RESULTS

Amongst them, compounds A1-7 (d-camphor) and A2-7 (l-camphor) displayed excellent in vitro activity against R. solani with median effective concentration (EC50) values of 0.38 and 0.48 μg mL-1, which were obviously superior to that of boscalid (0.87 μg mL-1). A2-5 (l-camphor, EC50 = 3.27 μg mL-1) exhibited good activity against V. mali. A2-7 (2.13 μg mL-1), A2-21 (5.2 μg mL-1) and A1-5 (5.15 μg mL-1) showed good antifungal activity against F. graminearum with EC50 values below that of boscalid (5.85 μg mL-1). Preliminary mechanistic studies, using scanning and transmission electron microscopy, indicated that compound A1-7 induced disordered entanglement of hyphae, shrinkage of hyphal surfaces, and vacuole swelling and rupture, which disrupted normal hyphal growth. Additionally, compound A1-7 induced the production and accumulation of reactive oxygen species, disrupted mitochondrial membrane potential, and effectively inhibited the germination and formation of sclerotia in R. solani. Moreover, the molecular docking results and SDH enzyme assays yielded promising outcomes.

CONCLUSION

In this study, the designed and optimized compounds A1-7 and A2-7 emerged as promising candidates for SDH-targeting fungicides, demonstrating strong antifungal activity. These compounds hold potential as new antifungal agents for further research. © 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.

Phytopathogenic Fungicidal Activity and Mechanism Approach of Three Kinds of Triphenylphosphonium Salts

The triphenylphosphonium (TPP) cation has been widely used as a carrier for mitochondria-targeting molecules. We synthesized two commonly employed targeting systems, namely, ω-triphenylphosphonium fatty acids (group 2) and ω-triphenylphosphonium fatty alcohols (group 3), to assess the impact of the TPP module on the biological efficacy of mitochondria-targeting molecules. We evaluated their fungicidal activities against nine plant pathogenic fungi in comparison to alkyl-1-triphenylphosphonium compounds (group 1). All three compound groups exhibited fungicidal activity and displayed a distinct "cut-off effect", which depended on the length of the carbon chain. Specifically, group 1 compounds showed a cut-off point at C10 (compound 1-7), while group 2 and 3 compounds exhibited cut-off points at C15 (compound 2-12) and C14 (compound 3-11), respectively. Notably, group 1 compounds displayed significantly higher fungicidal activity compared to groups 2 and 3. However, group 2 and 3 compounds showed similar activity to each other, although susceptibility may depend on the pathogen tested. Initial investigations into the mechanism of action of the most active compounds suggested that their fungicidal performance may be primarily attributed to their ability to damage the membrane, as well as uncoupling activity and inhibition of fungal respiration. Our findings suggest that the TPP module used in delivery systems as aliphatic acyl or alkoxyl derivatives with carbon chains length < 10 will contribute negligible fungicidal activity to the TPP-conjugate compared to the effect of high level of accumulation in mitochondria due to its mitochondria-targeting ability. These results provide a foundation for utilizing TPP as a promising carrier in the design and development of more effective mitochondria-targeting drugs or pesticides.

3D-QSAR-Directed Synthesis of Halogenated Coumarin-3-Hydrazide Derivatives: Unveiling Their Potential as SDHI Antifungal Agents

The pursuit of new succinate dehydrogenase (SDH) inhibitors is a leading edge in fungicide research and development. The use of 3D quantitative structure-activity relationship (3D-QSAR) models significantly enhances the development of compounds with potent antifungal properties. In this study, we leveraged the natural product coumarin as a molecular scaffold to synthesize 74 novel 3-coumarin hydrazide derivatives. Notably, compounds 4ap (0.28 μg/mL), 6ae (0.32 μg/mL), and 6ah (0.48 μg/mL) exhibited exceptional in vitro effectiveness against Rhizoctonia solani, outperforming the commonly used fungicide boscalid (0.52 μg/mL). Furthermore, compounds 4ak (0.88 μg/mL), 6ae (0.61 μg/mL), 6ah (0.65 μg/mL), and 6ak (1.11 μg/mL) showed significant activity against Colletotrichum orbiculare, surpassing both the SDHI fungicide boscalid (43.45 μg/mL) and the broad-spectrum fungicide carbendazim (2.15 μg/mL). Molecular docking studies and SDH enzyme assays indicate that compound 4ah may serve as a promising SDHI fungicide. Our ongoing research aims to refine this 3D-QSAR model further, enhance molecular design, and conduct additional bioactivity assays.

Novel 5-Sulfonyl-1,3,4-thiadiazole-Substituted Flavonoids as Potential Bactericides and Fungicides: Design, Synthesis, Three-Dimensional Quantitative Structure-Activity Relationship Studies

Flavonoids, ubiquitous natural products, provide sources for drug discovery owing to their structural diversity, broad-spectrum pharmacological activity, and excellent environmental compatibility. To develop antibacterial and antifungal agents with novel mechanisms of action and innovative structures, a series of novel 5-sulfonyl-1,3,4-thiadiazole-substituted flavonoids were designed and synthesized, and their biological activities against seven agriculturally common phytopathogenic microorganisms were evaluated. The results of the antimicrobial bioassay showed that most of the target compounds displayed excellent inhibitory effects against Xanthomonas oryzae, Rhizoctonia solani, and Colletotrichum orbiculare. Compounds 1, 3, 7, 9, 13, and 14 exhibited remarkable antibacterial activity against X. oryzae pv. oryzae with EC50 values below 10 μg/mL, which were superior to bismerthiazol (70.89 μg/mL). Compound 2 (EC50 = 0.41 μg/mL) displayed the most effective inhibitory potency against R. solani in vivo, comparable protective effects with the positive control carbendizam. Preliminary mechanistic studies indicated that compound 2 induced disordered entanglement of hyphae, shrinkage of hyphal surfaces, extravasation of cellular contents, and vacuole swelling and rupture, which disrupted normal hyphal growth. Subsequently, compounds 35-53 with good antifungal activity were designed and synthesized based on reliable three-dimensional quantitative structure-activity relationship (3D-QSAR) models. Compound 49 showed high efficacy and superior antifungal activity against R. solani, with an EC50 value of 0.28 μg/mL and a half-maximal effective concentration of 0.46 μg/mL.

A Novel Mitochondrial Targeted Compound Phosundoxin Showing Potent Antifungal Activity against Common Clinical Pathogenic Fungi

The current increase in resistance to antifungal drugs indicates that there is an urgent need to explore novel antifungal drugs with different mechanisms of action. Phosundoxin is a biphenyl aliphatic amide using a TPP-targeting strategy which targets mitochondria. To provide insights into the antifungal activities of phosundoxin, the antifungal susceptibility testing of phosundoxin was conducted on 158 pathogenic fungi and compared to that of traditional azole drugs. Phosundoxin displayed a broad-spectrum antifungal activity on all the tested yeast-like and filamentous fungi ranging from 2 to 16 mg/L. In particular, azole-resistant clinical isolates of Candida albicans were susceptible to phosundoxin with the same MICs as azole-susceptible C. albicans. Transcriptome analysis on azole-resistant C. albicans identified 554 DEGs after treatment with phosundoxin. By integrating GO and KEGG pathway enrichment analysis, the antifungal activity of phosundoxin was related to impairment of mitochondrial respiratory chain function. Acute oral and percutaneous toxicity of phosundoxin to rats showed that the compound phosundoxin were mild toxicity and LD50 was above 5000 mg/kg body weight in rats. This study demonstrated the potential of phosundoxin as an antifungal agent for the treatment of common fungal infection and contributed to providing insights into the mechanisms of action of phosundoxin against C. albicans.

Synthesis and Antiphytopathogenic Activity of Novel Oxazolidine-2,4-diones Bearing Phenoxypyridine Moiety

In the present study, we conducted optimization of pyramoxadone and synthesized a series of novel oxazolidinediones. Antifungal assays showed that these compounds exhibited moderate to excellent antifungal activity against various pathogens. Further SAR analysis revealed that the introduction of substituents to the benzene ring of the phenoxy group or the inclusion of bulky groups, such as tert-butyl, on the aniline moiety, had a detrimental effect on the activity. However, the inclusion of fluorine atoms in the aniline moiety significantly enhanced the antifungal efficacy. Notably, compound 2-4 displayed significantly higher activity compared to both pyramoxadone and famoxadone against R. solani, B. cinerea, S. sclerotiorum, and P. oryzae, where it demonstrated EC50 values of 1.78, 2.47, 2.33, and 2.23 μg/mL, respectively. Furthermore, compound 2-4 exhibited potent protective and curative effects against the tomato gray mold in vivo. A mechanistic investigation revealed that compound 2-4 significantly impacted the mycelial morphology, inhibited spore germination, and impeded mycelial respiration, ultimately leading to the inhibition of pathogenic fungus growth. These findings indicate that compound 2-4 has the potential to serve as a cyt bc1 inhibitor and should be further investigated for development.

Triphenylphosphonium (TPP)-Conjugated Quinolone Analogs Displayed Significantly Enhanced Fungicidal Activity Superior to Its Parent Molecule

Although 1-hydroxy-4-quinolone derivatives, such as 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), aurachin C, and floxacrine, have been reported as effective cytochrome bc₁ complex inhibitors, the bioactivity of these products is not ideal, presumably due to their low bioavailability in tissues, particularly their poor solubility and low mitochondrial accumulation. In order to overcome the drawbacks of these compounds and develop their use as agricultural fungicides acting by cytochrome bc₁ inhibition, in this study, three novel mitochondria-targeting quinolone analogs (mitoQNOs) were designed and synthesized by conjugating triphenylphosphonium (TPP) with quinolone. They exhibited greatly enhanced fungicidal activity compared to the parent molecule, especially mitoQNO₁₁, which showed high antifungal activity against Phytophthora capsici and Sclerotinia sclerotiorum with EC₅₀ values of 7.42 and 4.43 μmol/L, respectively. In addition, mitoQNO₁₁ could inhibit the activity of the cytochrome bc₁ complex of P. capsici in a dose-dependent manner and effectively depress its respiration and ATP production. The greatly decreased mitochondrial membrane potential and massively generated reactive oxygen species (ROS) strongly suggested that the inhibition of complex III led to the leakage of free electrons, which resulted in the damage of the pathogen cell structure. The results of this study indicated that TPP-conjugated QNOs might be used as agricultural fungicides by conjugating them with TPP.

The Effect of Terbinafine and Its Ionic Salts on Certain Fungal Plant Pathogens

Terbinafine, an inhibitor of squalene epoxidase in ergosterol biosynthesis, is chiefly utilized as an antifungal medication with potential uses in pesticide applications. This study explores the fungicidal efficacy of terbinafine against prevalent plant pathogens and confirms its effectiveness. To augment its water solubility, five ionic salts of terbinafine were synthesized by pairing them with organic acids. Among these salts, TIS 5 delivered the most impressive results, amplifying the water solubility of terbinafine by three orders of magnitude and lessening its surface tension to facilitate better dispersion during spraying. The in vivo experiments on cherry tomatoes showed that TIS 5 had a superior therapeutic activity compared to its parent compound and two commonly used broad-spectrum fungicides, pyraclostrobin and carbendazim. The results highlight the potential of terbinafine and its ionic salts, particularly TIS 5, for use as fungicides in agriculture due to their synergistic effects with furan-2-carboxylate.

Design, Synthesis, and Study of the Dual Action Mode of Novel N-Thienyl-1,5-disubstituted-4-pyrazole Carboxamides against Nigrospora oryzae

Due to the single target but extensive application of commercialized succinate dehydrogenase inhibitors (SDHIs), resistance problems have gradually become apparent in recent years. To solve this problem, a series of novel N-thienyl-1,5-disubstituted-1H-4-pyrazole carboxamide derivatives were designed and synthesized in this work based on the active skeleton 5-trifluoromethyl-4-pyrazole carboxamide. The bioassay results indicated that some target compounds exhibited excellent in vitro antifungal activities against the eight phytopathogenic fungi tested. Among them, the EC₅₀ values of T₄, T₆, and T₉ against Nigrospora oryzae were 5.8, 1.9, and 5.5 mg/L, respectively. The in vivo protective and curative activities of 40 mg/L T₆ against rice infected with N. oryzae were 81.5% and 43.0%, respectively. Further studies revealed that T₆ not only significantly inhibited the growth of N. oryzae mycelia but also effectively hindered spore germination and germ tube elongation. Morphological studies using scanning electron microscopy (SEM), fluorescence microscopy (FM), and transmission electron microscopy (TEM) found that T₆ could affect the mycelium membrane integrity by increasing cell membrane permeability and causing peroxidation of cellular lipids, and these results were further verified by measuring the malondialdehyde (MDA) content. The IC₅₀ value of T₆ against succinate dehydrogenase (SDH) was 7.2 mg/L, lower than that of the commercialized SDHI penthiopyrad (3.4 mg/L). Further, ATP content detection and the results after docking T₆ and penthiopyrad suggested that T₆ was a potential SDHI. These studies demonstrated that active compound T₆ could both inhibit the activity of SDH and affect the integrity of the cell membrane at the same time via a dual action mode, which is different from the mode of action of penthiopyrad. Thus, this study provides a new idea for a strategy to delay resistance and diversify the structures of SDHIs.

Discovery of triphenylphosphonium (TPP)-conjugated N-(1,1'-biphenyl)-2-yl aliphatic amides as excellent fungicidal candidates

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) are the fastest growing agricultural fungicides at present, but their rapidly growing resistance is a serious problem for their application. Previously, we screened out a fungicidal lead compound CBUA-TPP (1) through triphenylphosphonium (TPP)-driven mitochondrial-targeting strategy. The targeting led to the rapid accumulation of 1 in mitochondria and the saturation inhibition of complex II in a short time, resulting in electron leakage and the explosion of reactive oxygen species (ROS). However, the contribution of biphenyl-2-amines to the activity of these compounds and their structure-activity relationship are still unknown.

RESULTS

Two series of CBUA-TPP (1) analogues (series 2 and 3) were designed and synthesized. The bioassay results indicated that series 2 compounds generally showed much higher fungicidal activities than series 3, suggesting the crucial contribution of the biarylamine module in these targeted molecules and the pyridinyl substitution of phenyl is unfavorable to their activities. Interestingly, these two series of compounds displayed almost opposite substituent effects. Several compounds showed excellent fungicidal activities in vitro, among which compound 2-1 exhibited excellent field control efficacy on potato late blight.

CONCLUSION

By optimizing the lead compound, the contribution of biarylamine in CBUA-TPP (1) analogs to the fungicidal activity is clarified. Several compounds, represented by 2-1, have great potential as fungicide candidates. They exhibit high and broad-spectrum fungicidal activities and are highly effective against common pathogenic fungi infecting vegetables and fruits both in vitro and field control. It not only provided a new choice for controlling these diseases, but its low resistance tendency also provided a better scheme for resistance management. © 2023 Society of Chemical Industry.

Triphenylphosphonium-Driven Targeting of Pyrimorph Fragment Derivatives Greatly Improved Its Action on Phytopathogen Mitochondria

Pyrimorph is a carboxylic acid amide (CAA) fungicide, which shows excellent activity against oomycetes such as pepper phytophthora blight, tomato late blight, and downy mildew of cucumber. It works mainly by inhibiting the biosynthesis of cell wall of oomycetes. However, pyrimorph also shows weak activity of inhibiting mitochondrial complex III, which is the first CAA fungicide found to act on mitochondria. To improve this effect on mitochondria and develop fungicides that may have a novel mechanism of action, in this paper, by disassembling pyrimorph and conjugating the fragments with the mitochondrial-targeted delivery system (triphenylphosphonium), three series of mitochondrial-targeting analogues of pyrimorph were designed and synthesized. The results show that the pyridine-containing 1,1-diaryl is the core module of inhibition mitochondrial function of pyrimorph. Among these conjugates, compound 3b with a short linker showed the highest and broad-spectrum fungicidal activity, strong respiratory inhibition activity, and adenosine 5'-triphosphate synthesis inhibition activity, suggesting its potential as a fungicide candidate. 3b exhibited greatly improved action on mitochondria, such as by destroying the mitochondrial function of pathogens, causing mitochondrial swelling, weakening its influence on cell wall morphology, and so on. More importantly, this study provides a method to strengthen the drugs or pesticides with weak mitochondrial action, which is of special significance for developing mitochondrial bioactive molecules with the novel action mechanism.

Discovery of N-Methoxy-(biphenyl-ethyl)-pyrazole-carboxamides as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) inhibitor is one of the research hotspots for the development of fungicides. Herein, we describe the design and synthesis of N-methoxy-(biphenyl-ethyl)-pyrazole-carboxamide derivatives with enhanced fungicidal activity by employing fragment combination strategy. The SDH enzymatic activity was evaluated for 24 title compounds, and compound 7s was identified as the highest activity against porcine SDH with an IC₅₀ value of 0.014 μM, 205-fold greater than that of fluxapyroxad. Furthermore, the greenhouse experiments showed that compound 7u exhibited potent fungicidal activity against wheat powdery mildew with an EC₅₀ value of 0.633 mg/L, higher activity than fluxapyroxad and benzovindiflupyr. The computational results showed that the fluorine atom substituted on the pyrazole ring formed an extra dipolar-dipolar interaction with C_S42 and then increased the van der Waals interaction between the compound and SDH. The structural and mechanistic insights obtained from the present work will provide a valuable clue to developing novel SDH inhibitors.

Mitochondrion-Targeted Triphenylphosphonium-Based Kresoxim-Methyl Analogues: Synthesis, Fungicidal Activity, and Action Mechanism Approach

β-Methoxyacrylate fungicides as complex III Q_o site inhibitors play a crucial role in the control of crop diseases. In this study, the triphenylphosphonium (TPP)-driven mitochondrion-targeting strategy was used to modify the kresoxim-methyl scaffold at the toxicophore or side chain to develop novel mitochondrion-targeted Q_oI fungicides. These kresoxim-methyl analogues exhibited different fungicidal activities, depending on the position of TPP conjugation and the linker length. Among them, 2A-5 and 2C-4 showed excellent characteristics superior to kresoxim-methyl as candidate fungicides, in which the activity enhancement against Phytophthora capsici was the most remarkable, with an EC₅₀ value of about 5 μM. Notably, both hyphal and zoospore structures of the pathogens were severely damaged after treatment with them. The action mechanism approach revealed that they might cause a significant decrease in ATP synthesis and ROS outbreak in different ways. The results also provided a new insight into the contribution of targeting group TPP to the fungicidal activity in TPP-driven fungicides.

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.

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.