Biological Characteristics and Molecular Mechanisms of Fludioxonil Resistance in Fusarium graminearum in China

Characterization of Bacillus amyloliquefaciens BA-4 and its biocontrol potential against Fusarium-related apple replant disease

Apple replant disease (ARD), caused by Fusarium pathogens, is a formidable threat to the renewal of apple varieties in China, necessitating the development of effective and sustainable control strategies. In this study, the bacterial strain BA-4 was isolated from the rhizosphere soil of healthy apple trees in a replanted orchard, demonstrating a broad-spectrum antifungal activity against five crucial apple fungal pathogens. Based on its morphology, physiological and biochemical traits, utilization of carbon sources, and Gram stain, strain BA-4 was tentatively identified as Bacillus amyloliquefaciens. Phylogenetic analysis using 16S rDNA and gyrB genes conclusively identified BA-4 as B. amyloliquefaciens. In-depth investigations into B. amyloliquefaciens BA-4 revealed that the strain possesses the capacity to could secrete cell wall degrading enzymes (protease and cellulase), produce molecules analogous to indole-3-acetic acid (IAA) and siderophores, and solubilize phosphorus and potassium. The diverse attributes observed in B. amyloliquefaciens BA-4 underscore its potential as a versatile microorganism with multifaceted benefits for both plant well-being and soil fertility. The extracellular metabolites produced by BA-4 displayed a robust inhibitory effect on Fusarium hyphal growth and spore germination, inducing irregular swelling, atrophy, and abnormal branching of fungal hyphae. In greenhouse experiments, BA-4 markedly reduced the disease index of Fusarium-related ARD, exhibiting protective and therapeutic efficiencies exceeding 80% and 50%, respectively. Moreover, BA-4 demonstrated plant-promoting abilities on both bean and Malus robusta Rehd. (MR) seedlings, leading to increased plant height and primary root length. Field experiments further validated the biocontrol effectiveness of BA-4, demonstrating its ability to mitigate ARD symptoms in MR seedlings with a notable 33.34% reduction in mortality rate and improved biomass. Additionally, BA-4 demonstrates robust and stable colonization capabilities in apple rhizosphere soil, particularly within the 10-20 cm soil layer, which indicates that it has long-term effectiveness potential in field conditions. Overall, B. amyloliquefaciens BA-4 emerges as a promising biocontrol agent with broad-spectrum antagonistic capabilities, positive effects on plant growth, and strong colonization abilities for the sustainable management of ARD in apple cultivation.

Survey of prothioconazole sensitivity in Fusarium pseudograminearum isolates from Henan Province, China, and characterization of resistant laboratory mutants

BACKGROUND

Fusarium crown rot (FCR) is one of the most significant diseases limiting crop production in the Huanghuai wheat-growing region of China. Prothioconazole, a triazole sterol 14α-demethylation inhibitor (DMI) fungicide developed by the Bayer Crop Protection Company, is mainly registered for the prevention and control of wheat powdery mildew and stripe rust (China Pesticide Information Network). It is known to exhibit high activity against F. pseudograminearum, but further research, particularly regarding the potential for fungicide resistance, is required before it can be registered for the control of FCR in China.

RESULTS

The current study found that the baseline sensitivity of 67 field isolates of F. pseudograminearum collected between 2019 and 2021 ranged between 0.016-2.974 μg/mL, with an average EC50 value of 1.191 ± 0.720 μg/mL (mean ± SD). Although none of the field isolates exhibited signs of resistance, three highly resistant mutants were produced by repeated exposure to prothioconazole under laboratory conditions. All of the mutants were found to exhibit significantly reduced growth rates on potato dextrose agar (PDA), as well as reduced levels of sporulation, which indicated that there was a fitness cost associated with the resistance. However, inoculation of wounded wheat coleoptiles revealed that the pathogenicity of the resistant mutants was little affected or actually increased. Molecular analysis of the genes corresponding to the prothioconazole target protein, FpCYP51 (FpCYP51A, FpCYP51B, and FpCYP51C), indicated that the resistant mutants contained three conserved substitutions (M63I, A205S, and I246V) that were present in the FpCYP51C sequence of all three mutants, as well as several non-conserved substations in their FpCYP51A and FpCYP51B sequences. Expression analysis revealed that the presence of prothioconazole (0.1 μg/mL) generally resulted in reduced expression of the three FpCYP51 genes, but that the three mutants exhibited more complex patterns of expression that differed in comparison to their parental isolates. The study found no evidence of cross-resistance between prothioconazole and any of the fungicides tested including three DMI fungicides tebuconazole, prochloraz, and flutriafol.

CONCLUSIONS

Taken together these results not only provide new insight into the resistant mechanism and biological characteristics associated with prothioconazole resistance in F. pseudograminearum, but also strong evidence that prothioconazole could provide effective and sustained control of FCR, especially when applied in combination with other fungicides.

A new point mutation (D1158N) in histidine kinase Bos1 confers high-level resistance to fludioxonil in field gray mold disease

Gray mold, caused by the fungus Botrytis cinerea, is one of the most important plant diseases worldwide that is prone to developing resistance to fungicides. Currently, the phenylpyrrole fungicide fludioxonil exhibits excellent efficacy in the control of gray mold in China. In this study, we detected the fludioxonil resistance of gray mold disease in Shouguang City of Shandong Province, where we first found fludioxonil-resistant isolates of B. cinerea in 2014. A total of 87 single spore isolates of B. cinerea were obtained from cucumbers in greenhouse, and 3 of which could grow on PDA plates amended with 50 μg/mL fludioxonil that was defined as high-level resistance, with a resistance frequency of 3.4%. Furthermore, the 3 fludioxonil-resistant isolates also showed high-level resistance to the dicarboximide fungicides iprodione and procymidone. Sequencing comparison revealed that all the 3 fludioxonil-resistant isolates had a point mutation at codon 1158, GAC (Asp) → AAC (Asn) in the histidine kinase Bos1, which was proved to be the reason for fludioxonil resistance. In addition, the fludioxonil-resistant isolates possessed an impaired biological fitness compared to the sensitive isolates based on the results of mycelial growth, conidiation, virulence, and osmotic stress tolerance determination. Taken together, our results indicate that the high-level resistance to fludioxonil caused by the Bos1 point mutation (D1158N) has emerged in the field gray mold disease, and the resistance risk is relatively high, and fludioxonil should be used sparingly.

Efficacy of cyclobutrifluram in controlling Fusarium crown rot of wheat and resistance risk of three Fusarium species to cyclobutrifluram

Cyclobutrifluram (TYMIRIUM® technology), a new succinate dehydrogenase inhibitor (SDHI) fungicide, is currently being registered by SYNGENTA for controlling Fusarium crown rot (FCR) of wheat in China. The application of 15 or 30 g of active ingredient/100 kg seed of cyclobutrifluram significantly reduced pre-emergence damping-off, discoloration on the stem base and formation of whiteheads caused by FCR. The EC50 values of cyclobutrifluram for 60 isolates of F. pseudograminearum, 30 isolates of F. asiaticum and 30 isolates of F. graminearum ranged from 0.016 to 0.142 mg L-1, 0.010 to 0.041 mg L-1 and 0.012 to 0.059 mg L-1, respectively. One hundred and seven cyclobutrifluram-resistant (CR) mutants were obtained from three Fusarium species isolates, with ten types of mutations identified in Sdh genes. Three Fusarium species isolates exhibited similar resistance mechanisms, with the most prevalent mutations, SdhC1A83V and SdhC1R86K, accounting for 61.68% of mutants. The CR mutants possessed comparable or slightly impaired fitness compared to the corresponding parental isolates. The CR mutants carrying FpSdhBH248Y/Q/D exhibited increased sensitivity to fluopyram. An overall moderate risk of resistance development in three Fusarium species was recommended for cyclobutrifluram.

Risk assessment for resistance to fludioxonil in Corynespora cassiicola in Liaoning China

Cucumber corynespora leaf spot, caused by Corynespora cassiicola, is the primary disease of cucumber leaves in greenhouses in China. Fludioxonil is a phenylpyrrole fungicide that inhibits C. cassiicola growth. We studied the sensitivity of 170 isolates of C. cassiicola to fludioxonil and evaluated resistance risk. All of the isolates were sensitive to fludioxonil. The EC50 values ranged from 0.082 to 0.539 μg/mL with a mean of 0.207 ± 0.0053 μg/mL. Laboratory-created mutants with a high resistance factor to fludioxonil were genetically stable after 10 transfers and showed positive cross-resistance to iprodione and procymidone but not to azoxystrobin, carbendazim, pydiflumetofen, and prochloraz. There was no significant difference in mycelial growth and temperature adaptation between the mutant s and the sensitive isolates, except for pathogenicity and sporulation. The resistant isolates accumulated less glycerol than their parental isolates and were more sensitive to osmotic stress. The histidine kinase activity of the sensitive isolates was significantly inhibited compared to that of the resistant mutants. Sequence alignment of the histidine kinase gene CCos revealed that the mutants RTL4, RXM5, and RFS102 had point mutations at different sites that resulted in amino acid changes at G934E, S739F, and A825P in the CCos protein. The mutant RFS102 had an alanine deletion at site 824. After fludioxonil treatment, CCos expression by RFS20 was significantly lower than that of the parental isolate. Our findings demonstrate that C. cassiicola exhibits moderate resistance to fludioxonil.

Overexpression of FgPtp3 Is Involved in Fludioxonil Resistance in Fusarium graminearum by Inhibiting the Phosphorylation of FgHog1

Fusarium graminearum is the main causal agent of Fusarium head blight (FHB), a destructive disease in cereal crops worldwide. Resistance to fludioxonil has been reported in F. graminearum in the field, but its underlying mechanisms remain elusive. In this study, 152 fludioxonil-resistant (FR) mutants of F. graminearum were obtained by selection in vitro. The FR strains exhibited dramatically impaired fitness, but only 7 of the 13 analyzed strains possessed mutations in genes previously reported to underlie fludioxonil resistance. Comparison between the FR-132 strain and its parental strain PH-1 using whole genome sequencing revealed no mutations between them, but transcriptome analysis, after the strains were treated with 0.5 μg/mL fludioxonil, revealed 2778 differently expressed genes (DEGs) mapped to 96 KEGG pathways. Investigation of DEGs in the MAPK pathway showed that overexpression of the tyrosine protein phosphatase FgPtp3, but not FgPtp2, enhanced fludioxonil resistance. Further analysis found that FgPtp3 interacted directly with FgHog1 to regulate the phosphorylation of Hog1, and overexpressed FgPtp3 in PH-1 could significantly suppress the phosphorylation of FgHog1 and hinder signal transmission of the HOG-MAPK pathway. Overall, FgPtp3 plays a significant role in regulating fludioxonil resistance in F. graminearum.

Venturicidin A Is a Potential Fungicide for Controlling Fusarium Head Blight by Affecting Deoxynivalenol Biosynthesis, Toxisome Formation, and Mitochondrial Structure

Fusarium graminearum, which causes Fusarium head blight (FHB) in cereals, is one of the most devastating fungal diseases by causing great yield losses and mycotoxin contamination. A major bioactive ingredient, venturicidin A (VentA), was isolated from Streptomyces pratensis S10 mycelial extract with an activity-guided approach. No report is available on antifungal activity of VentA against F. graminearum and effects on deoxynivalenol (DON) biosynthesis. Here, VentA showed a high antagonistic activity toward F. graminearum with an EC50 value of 3.69 μg/mL. As observed by scanning electron microscopy, after exposure to VentA, F. graminearum conidia and mycelia appeared abnormal. Different dyes staining revealed that VentA increased cell membrane permeability. In growth chamber and field trials, VentA effectively reduced disease severity of FHB. Moreover, VentA inhibited DON biosynthesis by reducing pyruvic acid, acetyl-CoA production, and accumulation of reactive oxygen species (ROS) and then inhibiting trichothecene (TRI) genes expression and toxisome formation. These results suggest that VentA is a potential fungicide for controlling FHB.

Sensitivity and resistance risk of Botryosphaeria dothidea causing Chinese hickory trunk canker to fludioxonil

Hickory trunk canker (HTC), primarily caused by Botryosphaeria dothidea, is an aggravating disease that threatens an important regional economic tree species of Chinese hickory and few information is available in the control of this disease. Here, the sensitivity of 93 isolates to fludioxonil and the resistance risk were investigated. All the isolates tested were sensitive to fludioxonil and the EC50 ranged from 0.0028 to 0.0569 μg/mL. The tamed fludioxonil-resistant mutants remained highly resistant to fludioxonil even after 10 consecutive transfers to fludioxonil-free PDA plates. As for fitness penalty, the fludioxonil-resistant mutants demonstrated a reduction in conidia production and virulence as well as increased sensitivity to high osmotic stress. While, variations in mycelial growth and responses to SDS and H2O2 were not detected in all the resistant mutants. In addition, the resistant mutants demonstrated positive cross-resistance to iprodione but not to fungicides of other modes of action. Sequential analysis of BdNik1 showed that premature stop codon occurred in all the resistant mutants despite of point mutation (BD16-22R9 and BD16-22R20) or frameshift mutation (BD16-22R8, BD16-22R11 and BD16-22R18). Our study suggested that fludioxonil exhibited excellent inhibition activity on mycelial growth of B. dothidea in vitro, the resistance risk of B. dothidea to fludioxonil should be low to moderate and fludioxonil would be a nice candidate in controlling HTC caused by B. dothidea.

Streptomyces pratensis S10 Controls Fusarium Head Blight by Suppressing Different Stages of the Life Cycle and ATP Production

Fusarium head blight (FHB) of wheat, predominately caused by Fusarium graminearum, is an economically important plant disease worldwide. With increased fungicide resistance, controlling this filamentous fungal disease has become an enormous challenge. Biocontrol agents alone or integrated with other methods could better manage FHB. Streptomyces pratensis S10 has strong antagonistic activity against FHB as reported in our previous study. We now have investigated S10 controls of FHB in more detail by combining microscope observations, biological assays, and transcriptome profiling. S10 culture filtrates (SCF) significantly inhibited essential stages of the life cycle of F. graminearum in the laboratory and under simulated natural conditions. SCF at different concentrations inhibited conidiation of F. graminearum with an inhibition of 57.49 to 83.83% in the medium and 64.04 to 85.89% in plants. Different concentrations of SCF reduced conidia germination by 47.33 to 67.67%. Two percent (vol/vol) SCF suppressed perithecia formation of F. graminearum by 84 and 81% in the laboratory and under simulated natural conditions, respectively. The S10 also reduced the pathogenicity and penetration ability of F. graminearum by suppressing ATP production. Collectively, these findings indicate that S. pratensis S10 should be explored further for efficacy at controlling FHB.

Exploring Potential Mechanisms of Fludioxonil Resistance in Fusarium oxysporum f. sp. melonis

Melon Fusarium wilt (MFW), which is caused by Fusarium oxysporum f. sp. melonis (FOM), is a soil-borne disease that commonly impacts melon cultivation worldwide. In the absence of any disease-resistant melon cultivars, the control of MFW relies heavily on the application of chemical fungicides. Fludioxonil, a phenylpyrrole fungicide, has been shown to have broad-spectrum activity against many crop pathogens. Sensitivity analysis experiments suggest that fludioxonil has a strong inhibitory effect on the mycelial growth of FOM isolates. Five fludioxonil-resistant FOM mutants were successfully generated by repeated exposure to fludioxonil under laboratory conditions. Although the mutants exhibited significantly reduced mycelial growth in the presence of the fungicide, there initially appeared to be little fitness cost, with no significant difference (p < 0.05) in the growth rates of the mutants and wild-type isolates. However, further investigation revealed that the sporulation of the fludioxonil-resistant mutants was affected, and mutants exhibited significantly (p < 0.05) reduced growth rates in response to KCl, NaCl, glucose, and mannitol. Meanwhile, molecular analysis of the mutants strongly suggested that the observed fludioxonil resistance was related to changes in the sequence and expression of the FoOs1 gene. In addition, the current study found no evidence of cross-resistance between fludioxonil and any of the other fungicides tested. These results indicate that fludioxonil has great potential as an alternative method of control for FOM in melon crops.

Baseline Sensitivity and Potential Resistance Mechanisms for Fusarium pseudograminearum to Fludioxonil

Fusarium crown rot (FCR), which is caused by Fusarium pseudograminearum, is one of the most important diseases affecting wheat production in the Huanghuai wheat-growing region of China. Although the phenylpyrrole fungicide fludioxonil is known to have a broad-spectrum activity against a wide range of plant pathogens, including F. pseudograminearum, it has not yet been registered for the control of FCR in China, and further research is needed to assess the biological characteristics and molecular mechanisms associated with fludioxonil resistance, and especially the potential for highly resistant isolates to emerge. The current study demonstrated that the baseline fludioxonil sensitivity of 61 F. pseudograminearum isolates collected from the Henan province of China during the summers of 2019 to 2021 conformed to a unimodal distribution with a mean effective concentration for 50% inhibition (EC₅₀) value of 0.021 ± 0.003 μg/ml, which indicated that none of the isolates exhibited natural resistance to fludioxonil. Nevertheless, four fludioxonil-resistant mutants were attained after repeated exposure to fludioxonil under laboratory conditions. All resistant mutants exhibited significantly lower growth rates on potato dextrose agar (PDA) and lower levels of sporulation and pathogenicity in wheat seedlings. In addition, the resistant mutants also exhibited less growth on PDA amended with either 0.5 M mannitol, 0.5 M glucose, 0.5 M MgCl₂, or 0.5 M NaCl, which indicated that they had greater sensitivity to osmotic stress. Molecular analysis of the proposed fludioxonil target protein FpOs1 indicated that the predicted sequences of the resistant mutants contained none of the characteristic amino acid changes previously associated with fludioxonil resistance in other species. Further investigation via quantitative real-time PCR analysis revealed that expression of the FpOs1 gene was significantly altered in the resistant mutants in both the absence and presence of fludioxonil. Meanwhile, plate assays found evidence of cross-resistance between fludioxonil and cyprodinil, as well as with the triazole fungicides tebuconazole and difenoconazole, but not with other commonly used fungicides including prochloraz, fluazinam, and carbendazim. Taken together, these results provide new insights into the mechanism and biological characteristics associated with fludioxonil resistance in F. pseudograminearum and indicate that fludioxonil could provide effective and sustained control of FCR during wheat production.

Toxicity of procymidone to Bombyx mori based on physiological and transcriptomic analysis

Procymidone is widely used in vegetables and fruits because of its broad-spectrum and high efficiency. However, it is unclear whether procymidone can affect silkworm (Bombyx mori) growth and cocoon production. This study investigated the effects of procymidone on the growth and cocoon production of silkworms. We analyzed the growth, and cocoon quality of fifth instar larvae fed on mulberry leaves saturated with different concentrations (2.5, 5, and 10 mg/ml) of procymidone and the control. Results showed that procymidone supplementation decreased the larval growth and cocoon quality compared to the control group, suggesting that procymidone had toxicity to silkworms. Additionally, after transcriptomic analysis, we identified 396 significantly differentially expressed genes (DEGs) in the presence of procymidone. Gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) illustrated that these DEGs were closely related to metabolism. Taken together, these results confirmed that procymidone could cause toxicity by affecting metabolism in silkworm larvae. We believed that these results could provide important materials for the effect of procymidone on silkworms and gave us some clues for pesticides used in the mulberry garden.

Biological and molecular characterizations of field fludioxonil-resistant isolates of Fusarium graminearum

Fusarium head blight (FHB) predominately caused by F. graminearum, is an economical devastating disease for grain cereal crops especially on wheat. The phenylpyrrole fungicide fludioxonil exhibits excellent activity against F. graminearum and has been registered to control FHB in China. In this study, 6 fludioxonil-resistant (FludR) isolates of F. graminearum were identified from 2910 isolates collected from wheat cultivated field in Jiang Su, An Hui and Henan province of China in 2020. The sensitivity assay showed that resistance factor (RF) of FludR isolates ranges from 170.73 to >1000. In comparison with fludioxonil-sensitive (FludS) isolates, all of FludR isolates showed fitness defects in terms of mycelial growth, conidiation and virulence. Under fludioxonil treatment condition, the glycerol accumulation was obviously increased in FludS isolates, but was slightly increased in FludR isolates. Four FludR isolates exhibited increased sensitivity to osmotic stresses. Moreover, there is no positive cross-resistance between fludioxonil and other fungicides including phenamacril, carbendazim and tebuconazole. When treated with fludioxonil, the phosphorylation level of Hog1 was significantly decreased in the four FludR isolates, which was in contrast to the observation in the FludS and two FludR isolates where phosphorylation level of Hog1 was increased. Sequencing assay showed that the mutations were identified in different domains in FgOS1, FgOS2 or FgOS4 in FludR isolates. This was first reported that biological and molecular characterizations of field isolates of F. graminearum resistant to fludioxonil. The results can provide scientific directions for controlling FHB using fludioxonil.

The Endoplasmic Reticulum Cargo Receptor FgErv14 Regulates DON Production, Growth and Virulence in Fusarium graminearum

Fusarium graminearum is a plant filamentous pathogenic fungi and the predominant causal agent of Fusarium head blight (FHB) in cereals worldwide. The regulators of the secretory pathway contribute significantly to fungal mycotoxin synthesis, development, and virulence. However, their roles in these processes in F. graminearum remain poorly understood. Here, we identified and functionally characterized the endoplasmic reticulum (ER) cargo receptor FgErv14 in F. graminearum. Firstly, it was observed that FgErv14 is mainly localized in the ER. Then, we constructed the FgErv14 deletion mutant (ΔFgerv14) and found that the absence of the FgErv14 caused a serious reduction in vegetative growth, significant defects in asexual and sexual reproduction, and severely impaired virulence. Furthermore, we found that the ΔFgerv14 mutant exhibited a reduced expression of TRI genes and defective toxisome generation, both of which are critical for deoxynivalenol (DON) biosynthesis. Importantly, we found the green fluorescent protein (GFP)-tagged FgRud3 was dispersed in the cytoplasm, whereas GFP-FgSnc1-PEM was partially trapped in the late Golgi in ΔFgerv14 mutant. These results demonstrate that FgErv14 mediates anterograde ER-to-Golgi transport as well as late secretory Golgi-to-Plasma membrane transport and is necessary for DON biosynthesis, asexual and sexual reproduction, vegetative growth, and pathogenicity in F. graminearum.

Developmental toxicity of procymidone to larval zebrafish based on physiological and transcriptomic analysis

As a broad-spectrum with low toxicity, procymidone (PCM), is widely used in agriculture and frequently observed in aquatic system, which may cause some impacts on aquatic organisms. Here, to determine the developmental toxicity of PCM, embryonic and larval zebrafish were exposed to PCM at 0, 1, 10, 100 μg/L in dehydrogenated natural water containing 0.01% acetone for 7 days. The results showed that high concentration of PCM could cause the pericardial edema and increase the heart rates in larval zebrafish, suggesting that PCM had developmental toxicity to zebrafish. We also observed that PCM exposure not only changed the physiological parameters including TBA, GLU and pyruvic acid, but also changed the transcriptional levels of glycolipid metabolism related genes. In addition, after transcriptomics analysis, a total of 1065 differentially expressed genes, including 456 up-regulated genes and 609 down-regulated genes, changed significantly in 100 μg/L PCM treated larval zebrafish. Interestingly, after GO (Gene Ontology) analysis, the different expression genes (DEGs) were mainly enriched to the three different biology processes including GABA-nervous, lipid Metabolism and response to drug. We also observed that the levels of GABA receptor related genes including gabrg2, gabbr1α, gabbr1 and gabra6α were inhibited by PCM exposure. Interestingly, the swimming distance of larval zebrafish had the tendency to decrease after PCM exposure, indicating that the nervous system was affected by PCM. Taken together, the results confirmed that the fungicide PCM could cause developmental toxicity by influencing the lipid metabolism and GABA mediated nervous system and behavior in larval zebrafish. We believed that the results could provide an important data for the influence of PCM on aquatic animals.

Genome Sequence Analysis of the Fungal Pathogen Fusarium graminearum Using Oxford Nanopore Technology

Fusarium graminearum is a plant pathogen of global importance which causes not only significant yield loss but also crop spoilage due to mycotoxins that render grain unsafe for human or livestock consumption. Although the full genome of several F. graminearum isolates from different parts of the world have been sequenced, there are no similar studies of isolates originating from China. The current study sought to address this by sequencing the F. graminearum isolate FG-12, which was isolated from the roots of maize seedlings exhibiting typical symptoms of blight growing in the Gansu province, China, using Oxford Nanopore Technology (ONT). The FG-12 isolate was found to have a 35.9 Mb genome comprised of five scaffolds corresponding to the four chromosomes and mitochondrial DNA of the F. graminearum type strain, PH-1. The genome was found to contain an approximately 2.23% repetitive sequence and encode 12,470 predicted genes. Additional bioinformatic analysis identified 437 genes that were predicted to be secreted effectors, one of which was confirmed to trigger a hypersensitive responses (HR) in the leaves of Nicotiana benthamiana during transient expression experiments utilizing agro-infiltration. The F. graminearum FG-12 genome sequence and annotation data produced in the current study provide an extremely useful resource for both intra- and inter-species comparative analyses as well as for gene functional studies, and could greatly advance our understanding of this important plant pathogen.

Exploring the Biological and Molecular Characteristics of Resistance to Fludioxonil in Sclerotinia sclerotiorum From Soybean in China

Sclerotinia sclerotiorum is one of the most damaging and economically important necrotrophic plant pathogens, infecting more than 400 plant species globally. Although the phenylpyrrole fungicide fludioxonil has high activity against S. sclerotiorum, reports indicate that there is also substantial potential for the development of fungicide resistance. However, the current study investigating five fludioxonil-resistant laboratory mutants found a significant fitness cost associated with fludioxonil resistance resulting in significantly (P < 0.05) reduced mycelial growth and sclerotia formation on potato dextrose agar as well as significantly (P < 0.05) lower pathogenicity on detached tomato leaves, with one mutant, LK-1R, completely losing the capacity to cause infection. In addition, all of the fludioxonil-resistant mutants had significantly (P < 0.05) increased sensitivity to osmotic stress (0.5 M of potassium chloride and 1.0 M of glucose), which is consistent with the proposed fludioxonil target sites within the high osmolarity glycerol stress response mitogen-activated protein kinase (HOG1-MAPK) signaling transduction pathway. Sequence analysis of six genes from this two-component pathway, including SsHk, SsYpd, SsSk1, SsSk2, SsPbs, and SsHog, revealed several mutations that may be associated with fludioxonil resistance. For example, six separate point mutations were found in SsHk that led to changes in the predicted amino acid sequence, including A136G, F249V, G353A, E560K, M610K, and K727R. Similarly, SsPbs had three mutations (D34G, S46L, and L337E), SsSk1 and SsYpd had two (S53G and A795V for SsSk1, and E67G and Y141H for SsYpd), and SsHog and SsSk2 had one each (V220A and S763P, respectively). To our knowledge, these constitute the first reports of amino acid changes in proteins of the HOG1-MAPK pathway being associated with fludioxonil resistance in S. sclerotiorum. This study also showed a positive cross-resistance between fludioxonil and dimethachlone and procymidone, but none with tebuconazole or carbendazim, indicating that the inclusion of tebuconazole within an integrated pest management program could reduce the risk of fludioxonil resistance developing in field populations of S. sclerotiorum and ensure the sustainable production of soybeans in China into the future.