Biological characteristics and resistance analysis of the novel fungicide SYP-1620 against Botrytis cinerea

The G143A/S substitution of mitochondrially encoded cytochrome b (Cytb) in Magnaporthe oryzae confers resistance to quinone outside inhibitors

BACKGROUND

Rice blast, caused by Magnaporthe oryzae, is a destructive disease threatening the production of staple foods worldwide. Quinone outside inhibitors (QoIs) are a group of chemicals exhibiting excellent activity against a majority of plant pathogens, with the disadvantage that pathogens can easily develop resistance to QoIs.

RESULTS

Here, we investigated the activity of picoxystrobin against M. oryzae, which showed a great inhibitory effect on 100 strains of M. oryzae with half-maximal effective concentrations (EC₅₀ ) ranging from 0.0251 to 0.1337 μg ml^-1 . The EC₅₀ values showed a continuous unimodal distribution that was identical to the normal distribution, suggesting the potency of our study to represent baseline sensitivity. In addition, nine resistant mutants were obtained by exposing M. oryzae to a high dosage of picoxystrobin in the laboratory; all of them showed cross-resistance to the other five QoI fungicides. Although some mutants showed a decreased resistance factor after ten successive cultures on fungicide-free medium, the resistance to picoxystrobin was still inheritable. Amino acid substitution of G143S was detected in eight of nine picoxystrobin-resistant mutants, and G143A was detected in only one of nine mutants. A fitness penalty was found in the mutants carrying G143S rather than G143A.

CONCLUSION

Our findings suggested that M. oryzae had a mid to high risk of resistance to picoxystrobin. Considering this, we should be vigilant to the resistance risk and apply picoxystrobin sensibly in the field. © 2022 Society of Chemical Industry.

Fusarium graminearum FgSdhC1 point mutation A78V confers resistance to the succinate dehydrogenase inhibitor pydiflumetofen

BACKGROUND

Fusarium head blight (FHB) caused by Fusarium graminearum complex (Fg) is a devastating disease of cereal crops worldwide. The succinate dehydrogenase inhibitor, pydiflumetofen, was registered for management of FHB in China in 2019. Previously, laboratory-induced pydiflumetofen-resistant (PyR) mutants of Fg have been characterized. However, resistance situation of Fg to pydiflumetofen in the field remains largely unknown.

RESULTS

After screening 6468 isolates of Fg from various regions of China, six PyR isolates were identified. All six resistant isolates exhibited no fitness penalties based on mycelial growth, conidiation and virulence. However, no cross-resistance between pydiflumetofen and azoxystrobin, tebuconazole or fludioxonil in Fg was detected. Genome-sequencing revealed that all six PyR isolates contained a point mutation A78V in FgSdhC1 (FgSdhC1^A78V ). Genetic replacement assay further confirmed that FgSdhC1^A78V conferred resistance of Fg to pydiflumetofen. Based on this, a mismatch allele-specific polymerase chain reaction was developed for rapidly detecting the PyR isolates containing the FgSdhC1^A78V mutation in Fg.

CONCLUSION

This is the first time that resistance of Fg to pydiflumetofen in the field was reported and point mutation FgSdhC1^A78V conferring resistance of Fg to pydiflumetofen was confirmed. This study provides critical information for monitoring and managing pydiflumetofen resistance in Fg.

Preliminary Study of Resistance Mechanism of Botrytis cinerea to SYAUP-CN-26

SYAUP-CN-26 (1S, 2R-((3-bromophenethyl)amino)-N-(4-chloro-2-trifluoromethylphenyl) cyclohexane-1-sulfonamide) is a novel sulfonamide compound with excellent activity against Botrytis cinerea. The present study sought to explore the mutant of B.cinerea resistant to SYAUP-CN-26 using SYAUP-CN-26 plates. Moreover, the cell membrane functions of B.cinerea, histidine kinase activity, relative conductivity, triglyceride, and cell membrane structure were determined, and the target gene histidine kinase Bos1 (AF396827.2) of procymidone was amplified and sequenced. The results showed that compared to the sensitive strain, the cell membrane permeability, triglyceride, and histidine kinase activity of the resistant strain showed significant changes. The relative conductivity of the sensitive strain increased by 6.95% and 9.61%, while the relative conductivity of the resistant strain increased by 0.23% and 1.76% with 26.785 µg/mL (EC₉₅) and 79.754 µg/mL (MIC) of SYAUP-CN-26 treatment. The triglyceride inhibition rate of the resistant strain was 23.49% and 37.80%, which was 0.23% and 1.76% higher than the sensitive strain. Compared to the sensitive strain, the histidine kinase activity of the resistant strain was increased by 23.07% and 35.61%, respectively. SYAUP-CN-26 significantly damaged the cell membrane structure of the sensitive strain. The sequencing of the Bos1 gene of the sensitive and resistant strains indicated that SYAUP-CN-26 resistance was associated with a single point mutation (P348L) in the Bos1 gene. Therefore, it was inferred that the mutant of B.cinerea resistant to SYAUP-CN-26 might be regulated by the Bos1 gene. This study will provide a theoretical basis for further research and development of sulfonamide compounds for B. cinerea and new agents for the prevention and control of resistant B. cinerea.

Comparative Analysis of Botrytis cinerea in Response to the Microbial Secondary Metabolite Benzothiazole Using iTRAQ-Based Quantitative Proteomics

Benzothiazole is a microbial volatile compound with strong antifungal activity against the phytopathogenic fungus Botrytis cinerea, but its mode of action against fungi remains largely unknown. Understanding the molecular mechanisms underlying its activity could aid the design and synthesis of similar compounds against pathogenic fungi. Based on the results of morphological and antifungal activity assays, B. cinerea was exposed to 2.5 µl/liter of benzothiazole for 12, 24, and 48 h, and an isobaric tags for relative and absolute quantitation-based quantitative proteomic analysis showed that 378 out of 5,110 identified proteins were differentially expressed proteins (DEPs). The majority of these DEPs were associated with carbohydrate metabolism, oxidation reduction processes, and energy production. Further analysis showed that benzothiazole inhibited mitochondrial membrane organization and decreased the mitochondrial membrane potential of B. cinerea. In addition, the key enzymes of the glyoxylate cycle were downregulated after benzothiazole treatment, and a biochemical analysis indicated that inhibition of the glyoxylate cycle by benzothiazole blocked nutrient availability and interfered with adenosine triphosphate generation. This study provides markers for future research of the molecular responses of B. cinerea to benzothiazole stress.

Design, synthesis and antifungal evaluation of novel mandelic acid derivatives containing a 1,3,4-oxadiazothioether moiety

A series of novel mandelic acid derivatives containing a 1,3,4-oxadiazothioether moiety were designed and synthesized. Bioassay results showed that some target compounds exhibited certain antifungal activity against six kinds of pathogenic fungi in vitro. Among the compounds, the EC₅₀ values of T₄₁ against Gibberella saubinetii, Verticillium dahlia and Sclerotinia sclerotiorum were 31.0, 27.0 and 32.1 μg/ml, respectively, and the EC₅₀ value of T₁₄ against S. sclerotiorum was 14.7 μg/ml. The antifungal activity against the resistant fungus S. sclerotiorum indicated that this series of target compounds may have the similar action modes or sites as the commercialized succinate dehydrogenase inhibitor carboxin. A morphological study with fluorescence microscope demonstrated that T₄₁ can significantly destroy the membrane integrity of G. saubinetii.

Amino Acid Polymorphism in Succinate Dehydrogenase Subunit C Involved in Biological Fitness of Botrytis cinerea

Succinate dehydrogenase (SDH) is an important respiratory enzyme which participates in the tricarboxylic acid cycle and oxidative phosphorylation. A previous study of the baseline sensitivity of Botrytis cinerea against SDH inhibitors (SDHIs) showed that intrinsic sensitivity of the small population against the SDHIs exhibited significant differences. In the sequencing assay, we found five kinds of amino acid polymorphism in SDH subunit C (SdhC) of B. cinerea isolates which were never exposed to the SDHIs. To validate that amino acid polymorphism in the SdhC of B. cinerea confers intrinsic sensitivity against the SDHIs, the replacement mutants containing each kind of amino acid polymorphism of SdhC exhibited phenotype differences in intrinsic sensitivity to SDHIs, mycelial growth, sporulation, virulence, oxidative stress response, and carbon source utilization. These results indicated that SdhC of B. cinerea experienced positive selection during evolution and resulted in amino acid polymorphism which is involved in intrinsic sensitivity to SDHIs and biological fitness.

Baseline Sensitivity and Action Mechanism of Propamidine Against Alternaria brassicicola, the Causal Agent of Dark Leaf Spot on Cabbage

In the current study, a total of 53 isolates of Alternaria brassicicola collected from Shaanxi Province of China were characterized for their sensitivity to propamidine. The EC₅₀ (50% effective concentration) values for propamidine inhibiting mycelial growth and spore germination ranged from 0.515 to 3.247 µg/ml and 0.393 to 2.982 µg/ml, with average EC₅₀ values of 1.327 ± 0.198 µg/ml and 1.106 ± 0.113 µg/ml, respectively. In greenhouse experiments, propamidine at 100 µg/ml provided >90% efficacy against dark leaf spot on cabbage, which was higher than the efficacy obtained by azoxystrobin at the same concentration. After treatment with propamidine, fungal growth distortions were observed in the form of excess mycelial branching, thickened cell walls, decreased cell membrane permeability, and increased chitin content. Interestingly, colony color faded after treatment with propamidine compared with that of the untreated parental isolates. Importantly, the expressions of melanin biosynthesis-associated genes Amr1, Scd1, Brn1, and Brn2 were downregulated at different levels. The obtained baseline sensitivity and control efficacy data suggested that propamidine inhibited not only growth of A. brassicicola but also melanin biosynthesis, which could reduce the biocompatibility of A. brassicicola in the field. These biological characteristics encourage further investigation of the mechanism of action of propamidine against A. brassicicola.

Effects of SHAM on the Sensitivity of Sclerotinia sclerotiorum and Botrytis cinerea to QoI Fungicides

It is a common practice to add salicylhydroxamic acid (SHAM) into artificial medium in the in vitro sensitivity assay of fungal phytopathogens to the quinone outside inhibitor (QoI) fungicides. The rationale for adding SHAM is to inhibit fungal alternative oxidase, which is presumed to be inhibited by secondary metabolites of plants. Therefore, the ideal characteristics of SHAM should be almost nontoxic to phytopathogens and have no significant effect on control efficacy of fungicides. However, this study showed that the average effective concentration for 50% inhibition (EC₅₀) of mycelial growth values of SHAM were 97.5 and 401.4 μg/ml for Sclerotinia sclerotiorum and Botrytis cinerea, respectively. EC₅₀ values of the three QoI fungicides azoxystrobin, kresoxim-methyl, and trifloxystrobin in the presence of SHAM at 20 and 80 μg/ml for S. sclerotiorum and B. cinerea, respectively, declined by 52.7 to 78.1% compared with those without SHAM. For the dicarboximide fungicide dimethachlone, the average EC₅₀ values in the presence of SHAM declined by 18.2% (P = 0.008) for S. sclerotiorum and 35.9% (P = 0.012) for B. cinerea. Pot experiments showed that SHAM increased control efficacy of the three QoI fungicides against the two pathogens by 43 to 83%. For dimethachlone, SHAM increased control efficacy by 134% for S. sclerotiorum and 86% for B. cinerea. Biochemical studies showed that SHAM significantly inhibited peroxidase activity (P = 0.024) of B. cinerea and esterase activity (P = 0.015) of S. sclerotiorum. The strong inhibitions of SHAM per se on mycelial growth of B. cinerea and S. sclerotiorum and significant influences on the sensitivity of the two pathogens to both the QoI fungicides and dimethachlone as well as inhibitions on peroxidase and esterase indicate that SHAM should not be added in the in vitro assay of sensitivity to the QoI fungicides.

Synthesis, Fungicidal Activity and SAR of 2-Thiazolamide/Pyrazolamide-Cyclohexylsulfonamides against Botrytis cinerea

In order to explore more efficient sulfonamides against Botrytis cinereal, 36 novel cyclohexylsulfonamides were synthesized by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDCI) and 1-hydroxybenzotriazole (HOBt) condensation reaction using chesulfamide as a lead compound, introducing thiazole and pyrazole active groups. Their structures were characterized by ¹H-NMR, ¹³C-NMR, mass spectrum (MS), and elemental analysis. Compound III -31 was further confirmed by X-ray single crystal diffraction. The in vitro and in vivo fungicidal activities against B. cinerea were evaluated by three bioassay methods. The results of mycelial growth demonstrated that median effective concentration (EC₅₀) values of nine compounds were close to boscalid (EC₅₀ = 1.72 µg/mL) and procymidone (EC₅₀ = 1.79 µg/mL) against B. cinerea (KZ-9). In the spore germination experiment, it was found that compounds III-19 and III-31 inhibited germination 93.89 and 98.00%, respectively; at 10 µg/mL, they approached boscalid (95.97%). In the tomato pot experiment, the control effects of two compounds (III-21 and III-27) were 89.80 and 87.90%, respectively, at 200 µg/mL which were significantly higher than boscalid (81.99%). The structure–activity relationship (SAR) was also discussed, which provided a valuable idea for developing new fungicides.

Evaluating the Potential Value of Natural Product Cuminic Acid against Plant Pathogenic Fungi in Cucumber

Fusarium wilt and anthracnose are two major diseases which limit the yield and quality of cucumber worldwide. Cuminic acid was extracted from the seed of Cuminum cyminum L. The mean EC50 values of cuminic acid for inhibiting mycelial growth and zoospore germination of five Fusarium oxysporum f. sp. cucumerinum strains were 25.66 ± 3.02 μg/mL and 15.99 ± 2.19 μg/mL, and of four Colletotrichum lagenarium (Pass.) Ellis and Halsted strains were 29.53 ± 3.18 μg/mL and 18.41 ± 2.78 μg/mL, respectively. In greenhouse experiments, cuminic acid at 2000 μg/mL exhibited 70.77% protective and 62.63% curative efficacies against F. oxysporum, and 65.43% protective and 55.46% curative efficacies against C. lagenarium. Moreover, the translocation behavior of cuminic acid, determined by high performance liquid chromatography (HPLC), showed that it could be readily absorbed and transported upward and downward in cucumber. Importantly, superoxide dismutase (SOD) and pyphenol oxidase (PPO) activities of cucumber leaves treated with cuminic acid increased significantly. All results indicated that cuminic acid showed antifungal activity, and could be used as a botanical fungicide in disease management. This study encourages further investigation on the mechanism of action of cuminic acid and the development of alternative antifungal drugs.

Resistance Mechanisms and Molecular Docking Studies of Four Novel QoI Fungicides in Peronophythora litchii

Peronophythora litchii is the causal agent of litchi downy blight. Enestroburin, SYP-1620, SYP-2815 and ZJ0712 are four novel QoI fungicides developed by China. Eight mutants of P. litchii resistant to these QoI fungicides and azoxystrobin (as a known QoI fungicide) were obtained in our preliminary work. In this study, the full length of the cytochrome b gene in P. litchii, which has a full length of 382 amino acids, was cloned from both sensitive isolates and resistant mutants, and single-site mutations G142A, G142S, Y131C, or F128S were found in resistant mutants. Molecular docking was used to predict how the mutations alter the binding of the five QoI fungicides to the Qo-binding pockets. The results have increased our understanding of QoI fungicide-resistance mechanisms and may help in the development of more potent inhibitors against plant diseases in the fields.