A study on the molecular mechanism of resistance to amicarthiazol in Xanthomonas campestris pv. citri

Design, Synthesis, and Biological Profiles of Novel 1,3,4-Oxadiazole-2-carbohydrazides with Molecular Diversity

To unceasingly expand the molecular diversity of 1,3,4-oxadiazole-2-carbohydrazides, herein, small fragments (including -CH₂-, -OCH₂-, and -SCH₂-) were incorporated into the target compounds to screen out the potential succinate dehydrogenase inhibitors (SDHIs). The bioassay results showed that the antifungal effects (expressed by EC₅₀) against Sclerotinia sclerotiorum, Botryosphaeria dothidea, Fusarium oxysporum, and Colletotrichun higginsianum could reach 1.29 (10a), 0.63 (8h), 1.50 (10i), and 2.09 (10i) μg/mL, respectively, which were slightly lower than those of carbendazim (EC₅₀ were 0.69, 0.13, 0.55, and 0.80 μg/mL, respectively). Especially, compound 10h was extremely bioactive against Gibberella zeae (G. z.) with an EC₅₀ value of 0.45 μg/mL. This outcome was better than that of fluopyram (3.76 μg/mL) and was similar to prochloraz (0.47 μg/mL). In vivo trials against the corn scab (infected by G. z.) showed that compound 10h had control activity of 86.8% at 200 μg/mL, which was better than that of boscalid (79.6%). Further investigations found that compound 10h could inhibit the enzymatic activity of SDH in the G. z. strain with an IC₅₀ value of 3.67 μM, indicating that potential SDHIs might be developed. Additionally, the other biological activities of these molecules were screened simultaneously. The anti-oomycete activity toward Phytophthora infestans afforded a minimal EC₅₀ value of 3.22 μg/mL (10h); compound 4d could strongly suppress the growth of bacterial strains Xanthomonas axonopodis pv. citri and Xanthomonas oryzae pv. oryzae with EC₅₀ values of 3.79 and 11.4 μg/mL, respectively; and compound 10a displayed some insecticidal activity toward Plutella xylostella. Given their multipurpose features, these frameworks could be actively studied as potential pesticide leads.

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.

Design, synthesis, fungicidal activities and structure-activity relationship studies of (-)-borneol derivatives containing 2-aryl-thiazole scaffold

A series of (-)-borneol derivatives containing 2-aryl-thiazole scaffold were designed, synthesized, and characterized by ¹H NMR, ¹³C NMR, and HRMS. The fungicidal activities of these novel compounds against Fusarium oxysporum, Magnaporthe grisea, Botrytis cinerea, and Penicillium digitatum were evaluated. The results indicated that (1S,2R,4S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl(Z)-4-oxo-4-(((2-phenylthiazol-4-yl)methyl)amino)but-2-enoate (6a) displayed potential fungicidal activities with broad spectrum. Especially, 6a exhibited an IC₅₀ value of 48.5 mg/L against P. digitatum, which has higher fungicidal activity than commercial products hymexazol and amicarthiazol. Moreover, (1S,2R,4S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl-4-oxo-4-(((2-phenylthiazol-4-yl)methyl)amino)butanoate (5a) possesses an IC₅₀ value of 24.3 mg/L against B. cinerea, comparable to hymexazol and far superior to amicarthiazol. Furthermore, the superficial structure-activity relationship was discussed, which might be helpful for discovering novel fungicides.

The relationship between features enabling SDHI fungicide binding to the Sc-Sdh complex and its inhibitory activity against Sclerotinia sclerotiorum

BACKGROUND

A new generation of succinate dehydrogenase inhibitors (SDHIs) with high efficiency and broad-spectrum antifungal activity has been frequently used in crop production. Sclerotinia stem rot is a major disease of various plants and crops caused by Sclerotinia sclerotiorum. Although benzovindiflupyr and isopyrazam reportedly have high activity against S. sclerotiorum, little is known about the bioactivity of different SDHIs classes against S. sclerotiorum or the mechanism of their differential antifungal activity.

RESULTS

The in vitro tests revealed that the pyrazole-4-carboxamides of SDHIs (benzovindiflupyr, isopyrazam, fluxapyroxad, pydiflumetofen) had the highest activity against S. sclerotiorum followed by pyridine carboxamides (boscalid), pyridinyl-ethyl benzamides (fluopyram) and thiazole carboxamides (thifluzamide), and of these thifluzamide showed poor antifungal activity with EC₅₀ values greater than 6.01 mg L^-1 . The pyrazole-4-carboxamides of SDHIs showed satisfactory protective and curative activity against Sclerotinia stem rot. After treatment with the pyrazole-4-carboxamides of SDHIs, mitochondrial function in S. sclerotiorum decreased significantly. The enzyme activity assays revealed a lower affinity between thifluzamide and the Sc-Sdh complex than was observed for the other six fungicides, with IC₅₀ values ranging from 0.0036 to 1.2088 μmol L^-1 . Additionally, the docking positions of fungicides were similar, yet binding energies were different in the docking study with the Sdh complex. The correspondingly weaker hydrogen bonds may be responsible for the poor activity of thifluzamide against S. sclerotiorum.

CONCLUSION

Understanding different binding features of various SDHIs classes with the Sc-Sdh complex might be beneficial for the design and development of highly effective broad-spectrum fungicides to ensure high yield and quality in crops by reducing fungicide use. © 2020 Society of Chemical Industry.

Fabrication of Furan-Functionalized Quinazoline Hybrids: Their Antibacterial Evaluation, Quantitative Proteomics, and Induced Phytopathogen Morphological Variation Studies

The limited number of agrochemicals targeting plant bacterial diseases has driven us to develop highly efficient, low-cost, and versatile antibacterial alternatives. Herein, a novel type of simple furan-functionalized quinazolin-4-amines was systematically fabricated and screened for their antibacterial activity. Bioassay results revealed that compounds C₁ and E₄ could substantially block the growth of two frequently mentioned pathogens Xanthomonas oryzae pv oryzae and X. axonopodis pv citri in vitro, displaying appreciable EC₅₀ values of 7.13 and 10.3 mg/L, respectively. This effect was prominently improved by comparing those of mainly used agrochemicals. An in vivo experiment against bacterial blight further illustrated their viable applications as antimicrobial ingredients. Quantitative proteomics demonstrated that C₁ possessed a remarkable ability to manipulate the upregulation and downregulation of expressed proteins, which probably involved d-glucose and biotin metabolic pathways. This finding was substantially verified by parallel reaction monitoring analysis. Scanning electron microscopy images and fluorescence spectra also indicated that the designed compounds had versatile capacities for destroying the integrity of bacteria. Given these remarkable characteristics, furan-functionalized quinazoline hybrids can serve as a viable platform for developing innovative antibiotic alternatives against bacterial infections.

The Effect of Citrus Essential Oils and Their Constituents on Growth of Xanthomonas citri subsp. citri

Citrus bacterial canker (CBC) caused by Xanthomonas citri subsp. citri (Xcc), is the most devastating of the citrus diseases worldwide. During our study, we found that Essential oils (EOs) of some citrus cultivars are effective on Xcc. Therefore, it prompted us to determine the plant metabolites responsible for the antibacterial properties. We obtained EOs from some locally cultivated citrus by using a Clevenger apparatus and their major constituents were identified by gas chromatography/mass spectrometry (GC-MS). The effect of Citrus aurantium, C. aurantifolia, Fortunella sp. EOs and their major constituents were evaluated against Xcc-KVXCC1 using a disk diffusion assay. Minimal inhibitory and bactericidal concentration of the EOs and their constituents were determined using the broth microdilution method. C. aurantium, C. aurantifolia Eos, and their major constituents including citral, linalool, citronellal, geraniol, α-terpineol, and linalyl acetate indicated antibacterial effects against Xcc. The C. aurantifolia EO and citral showed the highest antibacterial activity among the tested EOs and constituents with inhibition zones of 15 ± 0.33 mm and 16.67 ± 0.88 mm, respectively. Synergistic effects of the constituents were observed between α-terpineol-citral, citral-citronellal, citral-geraniol, and citronellal-geraniol by using a microdilution checkerboard assay. Transmission electron microscopy revealed that exposure of Xcc cells to citral caused cell wall damage and altered cytoplasmic density. We introduced C. aurantifolia and C. aurantium EOs, and their constituents citral, α-terpineol, citronellal, geraniol, and linalool as possible control agents for CBC.

Emergence of succinate dehydrogenase inhibitor resistance of Pyrenophora teres in Europe

BACKGROUND

Net blotch caused by Pyrenophora teres is an important disease of barley worldwide. In addition to strobilurins (quinone ouside inhibitors) and azoles (demethylation inhibitors), succinate dehydrogenase inhibitors (SDHIs) are very effective fungicides for net blotch control. Recently, SDHI-resistant isolates have been found in the field. Intensive sensitivity monitoring programmes across Europe were carried out to investigate the situation concerning SDHI resistance in P. teres.

RESULTS

The first isolates with a lower sensitivity to SDHIs registered in barley were found in Germany in 2012 and carried the B-H277Y substitution in the succinate dehydrogenase enzyme. In 2013 and 2014, a significant increase in isolates with lower SDHI sensitivity was detected mainly in France and Germany, and the range of target-site mutations increased. Most of the resistant isolates carried the C-G79R substitution, which exhibits a strong impact on all SDHIs in microtitre tests. All SDHIs tested were shown to be cross-resistant. Other substitutions are gaining in importance, e.g. C-N75S in France and D-D145G in Germany. So far, no double mutants in SDH genes have been detected. Glasshouse tests showed that SDHI-resistant isolates were still controlled by the SDHI fluxapyroxad when applied preventively. To date, most isolates with C-G79R substitution have not simultaneously carried the F129L change in cytochrome b, which causes resistance towards QoI fungicides at low to moderate levels.

CONCLUSION

Several target-site mutations in the genes of subunits SDH-B, SDH-C and SDH-D with different impact on SDHI fungicides were detected. The pattern of mutations varied from year to year and between different regions. Strict resistance management strategies are recommended to maintain SDHIs as effective tools for net blotch control, especially in areas with low frequencies of resistant isolates. © 2016 Society of Chemical Industry.

Photochemical degradation of bismerthiazol: structural characterisation of the photoproducts and their inhibitory activities against Xanthomonas oryzae pv. oryzae

BACKGROUND

Bismerthiazol is a commonly used bactericide against rice bacterial leaf blight in China. Although previous research determined that bismerthiazol is susceptible to photolytic degradation, the photodegradation pathway and degradation products, except for 2-amino-5-mercapto-1,3,4-thiadiazole, have remained unknown.

RESULTS

The photodegradation of bismerthiazol was investigated after 4 and 8 h of irradiation in a solar simulator. Inhibition of Xanthomonas oryzae pv. oryzae (Xoo) was greater with a photolysed solution than with a non-photolysed solution of bismerthiazol. Six photoproducts of bismerthiazol were characterised by liquid chromatography coupled with mass spectrometry, and, based on these products, a photodegradation pathway was inferred. Inhibition of Xoo was significantly greater with bismerthiazol and 2-amino-5-mercapto-1,3,4-thiadiazole than with 5-amino-1,3,4-thiadiazole. In addition, Xoo strain 2-1-1 was bismerthiazol- and 2-amino-5-mercapto-1,3,4-thiadiazole resistant in vivo.

CONCLUSION

Photodegradation increased the inhibitory activity of bismerthiazol against Xoo. The photodegradation pathway was inferred on the basis of the photoproducts of bismerthiazol. In vitro assays indicated that the sulfhydryl group was crucial for the inhibition of Xoo by bismerthiazol and its photoproducts. Bismerthiazol and 2-amino-5-mercapto-1,3,4-thiadiazole might have a similar mode action in vivo and in vitro.

Baseline sensitivity and resistance risk assessmemt of Rhizoctonia cerealis to thifluzamide, a succinate dehydrogenase inhibitor

During 2010-2012, a total of 120 isolates of Rhizoctonia cerealis were collected from wheat with symptoms of sharp eyespot in four provinces (Henan, Shandong, Anhui and Jiangsu) in China. All the isolates were determined for baseline sensitivity to thifluzamide, a succinate dehydrogenase inhibitor (SDHI) with strong antifungal activity. The sampled pathogenic populations, never exposed to SDHIs, had similar sensitivity to trifluzamide (0.025-0.359 µg/ml) in the four regions and over the two years. The baseline sensitivity was distributed as a skewed unimodal curve with a mean EC50 value (effective concentrations for 50% inhibiting mycelial growth) of 0.064 ± 0.013 µg/ml. The resistance risk of R. cerealis to thifluzamide was further evaluated in vitro. Two thifluzamide-resistant mutants of R. cerealis were obtained by culturing on thifluzamide-amended plates. The resistance factors (RF = EC50 value of a mutant/EC50 value of the wild type progenitor of the mutant) were 120 and 40 for two R. cerealis mutants, respectively. All the mutants exhibited similar fitness after 10 successive transfers when compared to their wild-type parents in mycelial growth, sclerotia production, and virulence. However, the two thifluzamide-resistant mutants differed significantly in sensitivity to boscalid and flutolanil. Therefore, a low-to-moderate risk of resistance development was recommended for thifluzamide.

A Botrytis cinerea Population from a Single Strawberry Field in Germany has a Complex Fungicide Resistance Pattern

Gray mold, caused by the fungus Botrytis cinerea, is one of the most important diseases of strawberry in Germany. The application of site-specific fungicides remains the main strategy to reduce disease incidence and severity in the field. Isolates (n = 199) were collected from fungicide-treated strawberry fruit at a German research site with a long history of fungicide efficacy trials against gray mold. Sensitivities to the six site-specific botryticides registered in Germany were determined using microtiter assays. Values for the concentration of a fungicide at which fungal development is inhibited by 50% (EC₅₀) ranged from 0.03 to ≥30 ppm for the succinate dehydrogenase inhibitor boscalid, 0.015 to ≥10 ppm for the hydroxyanilide fenhexamid, 0.009 to 0.739 ppm for the phenylpyrrole fludioxonil, 0.55 to 43.45 ppm for the dicarboximide iprodione, 0.021 to ≥3 ppm for the quinone outside inhibitor pyraclostrobin, and 0.106 to ≥30 ppm for the anilinopyrimidine pyrimethanil. Pyrosequencing revealed that amino acid substitutions in the target proteins Bos1 (I365S/N, V368F + Q369H), CytB (G143A), Erg27 (F412S), and SdhB (P225F, N230I, and H272R/Y) were associated with reduced sensitivity levels to the corresponding fungicide classes. In most cases, isolates with a decreased sensitivity to fludioxonil showed a reduced sensitivity to tolnaftate. This reduction is considered to be an indication of multidrug efflux pump activity. The amino acid change I365S, I365N, or V368F + Q369H in Bos1 and H272R in SdhB by itself showed EC₅₀ values of 3.99 to 14.73 ppm, 3.87 to 5.37 ppm, 4.81 to 15.63 ppm, and 2.071 to ≥30 ppm, respectively. When isolates that contained one of these mutations were also multidrug resistant, the ranges of EC₅₀ values shifted to 6.47 to 43.45 ppm for I365S, 7.28 to 29.84 ppm for I365N, 6.89 to 26.67 ppm for V368F + Q369H, and ≥30 ppm for H272R. The reported data suggest that the combination of multidrug resistance and an amino acid change in the target site may result in a lower sensitivity to the fungicides than one resistance mechanism by itself. Although 20% of the population analyzed was sensitive to all six different chemical classes, the majority showed reduced sensitivity to one (6%), two (13%), three (23%), four (17%), five (11%), and six (11%) different fungicides.

Discovery of 1,2,4-triazole-1,3-disulfonamides as dual inhibitors of mitochondrial complex II and complex III

1,2,4-Triazole-1,3-disulfonamide derivatives as dual function inhibitors of mitochondrial complex II (SQR) and complex III (cyt bc₁) were discovered.

Ultra performance liquid chromatography-tandem mass spectrometry for the determination of amicarthiazol residues in soil and water samples

A reliable and rapid method has been optimized to determine the residue of amicarthiazol in soil and environmental water samples. After extraction and evaporation, the extraction was carried out with solid phase extraction (SPE) cleanup using HLB cartridge (only soil samples) and for the quantitative determination by ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The resulting residues of amicarthiazol were analyzed by a gradient separation performed on a UPLC system with a C18 column, methanol and water containing 0.1% (v v(-1)) formic acid as the mobile phase in the mode of electrospray positive ionization (ESI(+)) and multiple reaction monitoring (MRM). Results showed that the recoveries for spiked samples were 74.4-97.1% and 72.1-109.9% for soil and water, respectively, with the relative standard deviation (RSD) less than 10.2% when fortified at 10, 100 and 1000μgL(-1). The limits of detection (LODs) and the limits of quantification (LOQs) for matrix matched standards ranged from 0.073-0.425μgL(-1) and 0.243-1.42μgL(-1). The intra-day precision (n=5) and the inter-day precision over 10 days (n=10) for the amicarthiazol in soils and water samples spiked at 100μgL(-1) was 7.9% and 15.9%, respectively. Results indicated that the developed method could be a helpful tool for the controlling and monitoring of the risks posed by amicarthiazol to human health and environment safety.

Exploring mechanisms of resistance to respiratory inhibitors in field strains of Botrytis cinerea, the causal agent of gray mold

Respiratory inhibitors are among the fungicides most widely used for disease control on crops. Most are strobilurins and carboxamides, inhibiting the cytochrome b of mitochondrial complex III and the succinate dehydrogenase of mitochondrial complex II, respectively. A few years after the approval of these inhibitors for use on grapevines, field isolates of Botrytis cinerea, the causal agent of gray mold, resistant to one or both of these classes of fungicide were recovered in France and Germany. However, little was known about the mechanisms underlying this resistance in field populations of this fungus. Such knowledge could facilitate resistance risk assessment. The aim of this study was to investigate the mechanisms of resistance occurring in B. cinerea populations. Highly specific resistance to strobilurins was correlated with a single mutation of the cytb target gene. Changes in its intronic structure may also have occurred due to an evolutionary process controlling selection for resistance. Specific resistance to carboxamides was identified for six phenotypes, with various patterns of resistance levels and cross-resistance. Several mutations specific to B. cinerea were identified within the sdhB and sdhD genes encoding the iron-sulfur protein and an anchor protein of the succinate dehydrogenase complex. Another as-yet-uncharacterized mechanism of resistance was also recorded. In addition to target site resistance mechanisms, multidrug resistance, linked to the overexpression of membrane transporters, was identified in strains with low to moderate resistance to several respiratory inhibitors. This diversity of resistance mechanisms makes resistance management difficult and must be taken into account when developing strategies for Botrytis control.

A single amino acid substitution in the SdhB protein of succinate dehydrogenase determines resistance to amicarthiazol in Xanthomonas oryzae pv. oryzae

BACKGROUND

Xanthomonas oryzae pv. Oryzae Ishiyama, a causal agent of rice bacterial leaf blight, was found to be sensitive in vitro to the systemic fungicide amicarthiazol (2-amino-4-methylthiazole -5-carboxanilide), which is a potent inhibitor of succinate dehydrogenase (SDH, EC 1.3.99.1). This paper aimed to determine the molecular resistance mechanism of X. oryzae pv. oryzae to amicarthiazol.

RESULTS

UV-induced resistant mutants of X. oryzae pv. oryzae to amicarthiazol were isolated. The activity of SDH in wild-type X. oryzae pv. oryzae was strongly inhibited by amicarthiazol, while that in resistant mutants was insensitive, although their SDH activity was decreased compared with the wild-type sensitive strain without amicarthiazol. A mutation of Histidine(229) (CAC) to Tyrosine(229) (TAC) was identified in sdhB, which encoded the iron-sulfur protein subunit of SDH. The sdhB from the mutant was ligated into a cosmid, pUFR034, to generate pUFR034RAX, which conferred resistance to amicarthiazol when transformed into the wild-type sensitive strain.

CONCLUSION

A mutation of His(229) (CAC) to Tyr(229) (TAC) in SdhB was responsible for determining amicarthiazol resistance. .