Acute exposure to ustiloxin A affects growth and development of early life zebrafish, Danio rerio

S-palmitoylation of MAP kinase is essential for fungal virulence

S-palmitoylation is an important reversible protein post-translational modification in organisms. However, its role in fungi is uncertain. Here, we found the treatment of the rice false fungus Ustilaginoidea virens with S-palmitoylation inhibitor 2 BP resulted in a significant decrease in fungal virulence. Comprehensive identification of S-palmitoylation sites and proteins in U. virens revealed a total of 4,089 S-palmitoylation sites identified among 2,192 proteins and that S-palmitoylated proteins were involved in diverse biological processes. Among the five palmitoyltransferases, UvPfa3 and UvPfa4 were found to regulate the pathogenicity of U. virens. We then performed quantitative proteomic analysis of ∆UvPfa3 and ∆UvPfa4 mutants. Interestingly, S-palmitoylated proteins were significantly enriched in the mitogen-activated protein kinase and autophagy pathways, and MAP kinase UvSlt2 was confirmed to be an S-palmitoylated protein which was palmitoylated by UvPfa4. Mutations of S-palmitoylation sites in UvSlt2 resulted in significantly reduced fungal virulence and decreased kinase enzymatic activity and phosphorylation levels. Simulations of molecular dynamics demonstrated mutation of S-palmitoylation sites in UvSlt2 causing decreased hydrophobic solvent-accessible surface area, thereby weakening the bonding force with its substrate UvRlm1. Taken together, S-palmitoylation promotes U. virens virulence through palmitoylation of MAP kinase UvSlt2 by palmitoyltransferase UvPfa4. This enhances the enzymatic phosphorylation activity of the kinase, thereby increasing hydrophobic solvent-accessible surface area and binding activity between the UvSlt2 enzyme and its substrate UvRlm1. Our studies provide a framework for dissecting the biological functions of S-palmitoylation and reveal an important role for S-palmitoylation in regulating the virulence of the pathogen.IMPORTANCES-palmitoylation is an important post-translational lipid modification of proteins. However, its role in fungi is uncertain. In this study, we found that S-palmitoylation promotes virulence of rice false smut fungus U. virens through palmitoylation of MAP kinase UvSlt2 by palmitoyltransferase UvPfa4. This enhances the enzymatic phosphorylation activity of the kinase, thereby increasing hydrophobic solvent-accessible surface area and binding activity between the UvSlt2 enzyme and its substrate UvRlm1. Our studies provide a framework for dissecting the biological functions of S-palmitoylation and reveal an important role for S-palmitoylation in regulating the virulence of the pathogen. This is the first functional study to reveal the role of S-palmitoylation in fungal virulence.

Biological variability hampers the use of skeletal staining methods in zebrafish embryo developmental toxicity assays

Zebrafish embryo assays are used by pharmaceutical and chemical companies as new approach methodologies (NAMs) in developmental toxicity screening. Despite an overall high concordance of zebrafish embryo assays with in vivo mammalian studies, false negative and false positive results have been reported. False negative results in risk assessment models are of particular concern for human safety, as developmental anomalies may be missed. Interestingly, for several chemicals and drugs that were reported to be false negative in zebrafish, skeletal findings were noted in the in vivo studies. As the number of skeletal endpoints assessed in zebrafish is very limited compared to the in vivo mammalian studies, the aim of this study was to investigate whether the sensitivity could be increased by including a skeletal staining method. Three staining methods were tested on zebrafish embryos that were exposed to four teratogens that caused skeletal anomalies in rats and/or rabbits and were false negative in zebrafish embryo assays. These methods included a fixed alizarin red-alcian blue staining, a calcein staining, and a live alizarin red staining. The results showed a high variability in staining intensity of larvae exposed to mammalian skeletal teratogens, as well as variability between control larvae originating from the same clutch of zebrafish. Hence, biological variability in (onset of) bone development in zebrafish hampers the detection of (subtle) treatment-related bone effects that are not picked-up by gross morphology. In conclusion, the used skeletal staining methods did not increase the sensitivity of zebrafish embryo developmental toxicity assays.

Innovative application of CRISPR for eliminating Ustiloxin in Cordyceps militaris: Enhancing food safety and quality

Cordyceps militaris (L.) Fr. Has long been recognized as a valuable functional food consumed in numerous countries. However, biosynthetic gene clusters of this species and safety regarding mycotoxin production remain largely unexplored. In this study, a ribosomally synthesized and post-translationally modified peptide (RiPP) cluster responsible for the production of cyclopeptide mycotoxins in Cordyceps was unveiled via genome mining. Ustiloxin B and a novel, predominant and Cordyceps specific ustiloxin I were confirmed by extraction and structural analysis. The difference between Ustiloxins I and B lied in the side chain at C19, where an additional methyl substituent in Ustiloxin I resulted in an alanine moiety substitution for glycine of Ustiloxin B. The simultaneous deletion of the two adjacent core genes, CmustYb and CmustYa, using a single guide RNA designed in the intergenic region, and subsequent in-situ complementation via AMA-mediated CRISPR/Cas9 system confirmed the RiPP cluster's responsibility for ustiloxin production. The cultivation of the edited strain yielded ustiloxin-free fruiting bodies without affecting agronomic characters. PCR and genome resequencing confirmed the absence of any off-target events or foreign sequence remnants. This study marks a significant advancement in utilizing CRISPR technology to control ustiloxins in food, underscoring its broader implications for food safety and quality improvement.

Knockout of lws1 in zebrafish (Danio rerio) reveals its role in regulating feeding and vision-guided behavior

Long-wave sensitive (LWS) is a G protein-coupled receptor expressed in the retina, and zebrafish is a better model organism for studying vision, but the role of LWS1 in vision-guided behavior of larvae fish has rarely been reported. In this study, we found that zebrafish lws1 and lws2 are tandemly replicated genes, both with six exons, with lws1 being more evolutionarily conserved. The presence of Y277F in the amino acid sequence of lws2 may have contributed to the shift of λmax to green light. We established a lws1 knockout zebrafish model using CRISPR/Cas9 technology. Lws1-/- larvae showed significantly higher levels of feeding and appetite gene (agrp) expression than WT, and significantly lower levels of anorexia gene (pomc, cart) expression. In addition, green light gene compensation was observed in lws1-/- larvae with significantly increased expression levels of rh2-1. The light-dark movement test showed that lws1-/- larvae were more active under light-dark transitions or vibrational stimuli, and the expression of phototransduction-related genes was significantly up-regulated. This study reveals the important role of lws1 gene in the regulation of vision-guided behavior in larvae.

Ustilaginoidea virens-secreted effector Uv1809 suppresses rice immunity by enhancing OsSRT2-mediated histone deacetylation

Rice false smut caused by Ustilaginoidea virens is a devastating rice (Oryza sativa) disease worldwide. However, the molecular mechanisms underlying U. virens-rice interactions are largely unknown. In this study, we identified a secreted protein, Uv1809, as a key virulence factor. Heterologous expression of Uv1809 in rice enhanced susceptibility to rice false smut and bacterial blight. Host-induced gene silencing of Uv1809 in rice enhanced resistance to U. virens, suggesting that Uv1809 inhibits rice immunity and promotes infection by U. virens. Uv1809 suppresses rice immunity by targeting and enhancing rice histone deacetylase OsSRT2-mediated histone deacetylation, thereby reducing H4K5ac and H4K8ac levels and interfering with the transcriptional activation of defence genes. CRISPR-Cas9 edited ossrt2 mutants showed no adverse effects in terms of growth and yield but displayed broad-spectrum resistance to rice pathogens, revealing a potentially valuable genetic resource for breeding disease resistance. Our study provides insight into defence mechanisms against plant pathogens that inactivate plant immunity at the epigenetic level.

Natural occurrence of ustiloxins in rice from five provinces in China and the removal efficiencies of different milling steps

BACKGROUND

The widespread incidence of "false smut" disease in rice has caused extensive ustiloxin contamination around the world. Until now there has been a lack of knowledge regarding the natural occurrence of ustiloxins in paddy. The development of efficient removal methods is also still a challenge that remains unexplored.

RESULTS

In the current study, three main ustiloxins - ustiloxin A (UA), ustiloxin B (UB), and ustiloxin G (UG) - were determined simultaneously by ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) in 206 paddy samples collected in 2021 from five rice-producing provinces in China. The predominant ustiloxin was UA with an occurrence of 46.1% and an average concentration of 49.71 μg kg-1 . This was followed by UB (31.1%, 13.31 μg kg-1 ) and UG (18.4%, 9.19 μg kg-1 ). No targeted ustiloxins were detected in white rice samples randomly collected from supermarkets in Shanghai. To reveal the causes, two approaches were tested for the removal of the ustiloxins: most of the targeted ustiloxins (>93%) were removed in brown rice by husking and, subsequently, all targeted ustiloxins (100%) were removed by whitening.

CONCLUSION

A wide distribution of ustiloxins was discovered in paddy samples in this study. The UA contaminations were significantly different depending on their origin, with the highest occurrence in paddy from Shanghai and Jiangsu, southeast coast provinces in China. Contamination by UG was also found in paddy for the first time and was strongly correlated with those of UA and UB. A combination of husking and whitening has been verified to be a practicable and promising way to ensure efficient removal and food safety. © 2023 Society of Chemical Industry.

Histone Deacetylase UvHST2 Is a Global Regulator of Secondary Metabolism in Ustilaginoidea virens

Ustilaginoidea virens, the causal agent of rice false smut, produces a large amount of mycotoxins, including ustilaginoidins and sorbicillinoids. However, little is known about the regulatory mechanism of mycotoxin biosynthesis inU. virens. Here, we demonstrate that the NAD+-dependent histone deacetylase UvHST2 negatively regulates ustilaginoidin biosynthesis. UvHst2 knockout caused retarded hypha growth and reduced conidiation and pathogenicity inU. virens. Transcriptome analysis revealed that the transcription factor genes, transporter genes, and other tailoring genes in eight biosynthetic gene clusters (BGCs) including ustilaginoidin and sorbicillinoid BGCs were upregulated in ΔUvhst2. Interestingly, the UvHst2 deletion affects alternative splicing. Metabolomics revealed that UvHST2 negatively regulates the biosynthesis of various mycotoxins including ustilaginoidins, sorbicillin, ochratoxin B, zearalenone, and O-M-sterigmatocystin. Combined transcriptome and metabolome analyses uncover that UvHST2 positively regulates pathogenicity but negatively modulates the expression of BGCs involved in secondary metabolism. Collectively, UvHST2 functions as a global regulator of secondary metabolism inU. virens.

Post-Translational Modification β-Hydroxybutyrylation Regulates Ustilaginoidea virens Virulence

Lysine β-hydroxybutyrylation (Kbhb) is an evolutionarily conserved and widespread post-translational modification that is associated with active gene transcription and cellular proliferation. However, its role in phytopathogenic fungi remains unknown. Here, we characterized Kbhb in the rice false smut fungus Ustilaginoidea virens. We identified 2204 Kbhb sites in 852 proteins, which are involved in diverse biological processes. The mitogen-activated protein kinase UvSlt2 is a Kbhb protein, and a strain harboring a point mutation at K72, the Kbhb site of this protein, had decreased UvSlt2 activity and reduced fungal virulence. Molecular dynamic simulations revealed that K72bhb increases the hydrophobic solvent-accessible surface area of UvSlt2, thereby affecting its binding to its substrates. The mutation of K298bhb in the septin UvCdc10 resulted in reduced virulence and altered the subcellular localization of this protein. Moreover, we confirmed that the NAD+-dependent histone deacetylases UvSirt2 and UvSirt5 are the major enzymes that remove Kbhb in U. virens. Collectively, our findings identify regulatory elements of the Kbhb pathway and reveal important roles for Kbhb in regulating protein localization and enzymatic activity. These findings provide insight into the regulation of virulence in phytopathogenic fungi via post-translational modifications.

Ustiloxin A inhibits proliferation of renal tubular epithelial cells in vitro and induces renal injury in mice by disrupting structure and respiratory function of mitochondria

Recently, we found that Ustiloxin A (UA, a mycotoxin) was widely detected in paddy environment and rice samples from several countries, and was also detected in human urine samples from China. However, the current knowledge about the health risks of UA are limited. In this research, the cytotoxicity of UA in mice renal tubular epithelial cells (mRTECs) was evaluated, and the results indicated that UA arrested cell cycle in G2/M phase via altering cellular morphology and microtubule, and inhibited the proliferation and division of mRTECs. Furthermore, UA could inhibit mitochondrial respiration via binding to the CoQ-binding site in dihydro-orotate dehydrogenase (DHODH) protein, and resulted in mitochondrial damage. These adverse effects of UA on mitochondria might be responsible for the cytotoxicity observed in vitro. In vivo, UA at concentrations that were comparable to the realistic concentrations of human exposure induced renal insufficiency in mice, and this might be associated with the renal mitochondrial damage in mice. However, exposure to UA at those realistic concentrations did not promote the progression from renal insufficiency to renal fibrosis and chronic kidney disease was not observed in mice.

Prevalence of Carbendazin Resistance in Field Populations of the Rice False Smut Pathogen Ustilaginoidea virens from Jiangsu, China, Molecular Mechanisms, and Fitness Stability

Rice false smut (RFS), caused by Ustilaginoidea virens, is an important fungal disease of rice. In China, Methyl Benzimidazole Carbamates (MBCs), including carbendazim, are common fungicides used to control RFS and other rice diseases. In this study, resistance of U. virens to carbendazim was monitored for three consecutive years during 2018 to 2020. A total of 321 U. virens isolates collected from Jiangsu Province of China were tested for their sensitivity to carbendazim on PSA. The concentration at which mycelial growth is inhibited by 50% (EC50) of the carbendazim-sensitive isolates was 0.11 to 1.38 µg/mL, with a mean EC50 value of 0.66 μg/mL. High level of resistance to carbendazim was detected in 14 out of 321 isolates. The resistance was stable but associated with a fitness penalty. There was a statistically significant and moderate negative correlation (r= −0.74, p < 0.001) in sensitivity between carbendazim and diethofencarb. Analysis of the U. virens genome revealed two potential MBC targets, Uvβ1Tub and Uvβ2Tub, that putatively encode β-tubulin gene. The two β-tubulin genes in U. virens share 78% amino acid sequence identity, but their function in MBC sensitivity has been unclear. Both genes were identified and sequenced from U. virens sensitive and resistant isolates. It is known that mutations in the β2-tubulin gene have been shown to confer resistance to carbendazim in other fungi. However, no mutation was found in the Uvβ2Tub gene in either resistant or sensitive isolates. Variations including point mutations, non-sense mutations, codon mutations, and frameshift mutations were found in the Uvβ1Tub gene from the 14 carbendazim-resistant isolates, which have not been reported in other fungi before. Thus, these results indicated that variations of Uvβ1Tub result in the resistance to carbendazim in field isolates of Ustilaginoidea virens.

The Toxicity of Salicylhydroxamic Acid and Its Effect on the Sensitivity of Ustilaginoidea virens to Azoxystrobin and Pyraclostrobin

Rice false smut (RFS) caused by Ustilaginoidea virens has been one of the most severe rice diseases. Fungicide-based chemical control is a significant measure to control RFS. In the sensitivity determination of quinone outside inhibitor (QoI) fungicide in vitro, salicylhydroxamic acid (SHAM) has been commonly added to artificial culture media in order to inhibit alternative oxidase of phytopathogenic fungi. However, some studies showed that artificial media should not include SHAM due to its toxicity. Whether SHAM should be added in the assay of U. virens sensitivity to QoI fungicide remains unknown. In this study, two appropriate media, potato sucrose agar (PSA) and minimal medium (MM), were selected to test SHAM toxicity and sensitivity of U. virens to azoxystrobin and pyraclostrobin. The mycelial growth and sensitivity to azoxystrobin and pyraclostrobin had no significant difference between on PSA and MM. SHAM could significantly inhibit mycelial growth, conidial germination, peroxidase (POD) and esterase activity of U. virens. Average effective concentration for inhibiting 50% (EC₅₀) values of SHAM against mycelial growth of ten U. virens were 27.41 and 12.75 μg/mL on PSA and MM, respectively. The EC₅₀ values of SHAM against conidial germination of isolates HWD and JS60 were 70.36 and 44.69 μg/mL, respectively. SHAM at 30 μg/mL significantly inhibited POD and esterase activity of isolates HWD and JS60, and even SHAM at 10 μg/mL significantly inhibited POD activity of isolate HWD. In addition, SHAM significantly reduced EC₅₀ values and EC₉₀ values of azoxystrobin and pyraclostrobin on both PSA and MM. Even in the presence of SHAM at 10 μg/mL, average EC₅₀ values of ten U. virens isolates for azoxystrobin decreased 1.7-fold on PSA and 4.8-fold on MM, and for pyraclostrobin that decreased 2.8-fold on PSA and 4.8-fold on MM. Therefore, these results suggest that SHAM should not be included in artificial media in the assay of U. virens sensitivity to QoI fungicides.

A secreted fungal effector suppresses rice immunity through host histone hypoacetylation

Histone acetylation is a critical epigenetic modification that regulates plant immunity. Fungal pathogens secrete effectors that modulate host immunity and facilitate infection, but whether fungal pathogens have evolved effectors that directly target plant histone acetylation remains unknown. Here, we identified a secreted protein, UvSec117, from the rice false smut fungus, Ustilaginoidea virens, as a key effector that can target the rice histone deacetylase OsHDA701 and negatively regulates rice broad-spectrum resistance against rice pathogens. UvSec117 disrupts host immunity by recruiting OsHDA701 to the nucleus and enhancing OsHDA701-modulated deacetylation, thereby reducing histone H3K9 acetylation levels in rice plants and interfering with defense gene activation. Host-induced gene silencing of UvSec117 promotes rice resistance to U. virens, thus providing an alternative way for developing rice false smut-resistant plants. This is the first direct evidence demonstrating that a fungal effector targets a histone deacetylase to suppress plant immunity. Our data provided insight into a counter-defense mechanism in a plant pathogen that inactivates host defense responses at the epigenetic level.

Dynamic Genome-Wide Transcription Profiling and Direct Target Genes of CmWC-1 Reveal Hierarchical Light Signal Transduction in Cordyceps militaris

Light is necessary for primordium differentiation and fruiting body development for most edible fungi; however, light perception and signal transduction have only been well studied in model fungi. In this study, a hierarchical network of transcriptional response to light in Cordyceps militaris, one of the edible fungi, has been described on a genome-wide scale using dynamic transcriptome analysis. It was shown that light regulated the transcript of 1722 genes, making up 18% of the whole genome of C. militaris. Analysis of light-responsive genes in C. militaris identified 4 categories: immediate-early, early, late, and continuous light-responsive genes, and the gene number increased distinctly with prolonged light exposure. Light-responsive genes with distinct functional categories showed specific time-dependent regulation. The target genes of CmWC-1, the most important photoreceptor, were revealed by ChIP-seq. A total of 270 significant peaks corresponding to 427 genes were identified to be directly regulated by CmWC-1, among which 143 genes respond to light. Based on 270 ChIP-seq peaks, the binding site for CmWC-1 was identified as AAATCAGACCAC/GTGGTCTGATTT, differing from the binding site by the homolog in Neurospora crassa. Elucidating the mechanisms of light perception and signal transduction will be helpful for further research on the fruiting body development in edible fungi.

Global distribution of ustiloxins in rice and their male-biased hepatotoxicity

Ustiloxins, a group of bioactive metabolites produced by the pathogen of rice false smut (RFS), have emerged as ubiquitous contaminants in RFS-occurred paddy fields and could accumulate in rice. Nevertheless, the prevalence of ustiloxins in rice and exposure risks of humans are limited. In this study, concentrations of ustiloxin A (UA) and ustiloxins B (UB), which are two predominant ustiloxins, were measured in 240 rice samples from China and 72 rice samples from 12 other counties. The detection rates (DRs) of UA and UB were 82.1% and 49.3%, respectively, and their concentrations in rice ranged from below detection limit (LOD: 0.22 μg/kg) to 85.96 μg/kg dw. Furthermore, for the first time, we reported the occurrence of UA (DR = 22.8%) in urine collected from residues of Enshi city, China. Urinary UA were significantly correlated with the activities of alanine aminotransferase in male, and this male-biased hepatotoxicity was further confirmed in mice exposure experiment. This study for the first time reported the widespread geographical distribution of ustiloxins in rice, as well as emphasized the occurrence of internal exposure and potential health risk in humans.

Development of Generic Immuno-Magnetic Bead-Based Enzyme-Linked Immunoassay for Ustiloxins in Rice Coupled with Enrichment

Ustiloxins are a group of mycotoxins produced by rice false smut pathogen. Previous studies have shown that the false smut balls contain six types of ustiloxins, and these toxins are toxic to living organisms. Thus, immunoassay for on-site monitoring of ustiloxins in rice is urgently required. The current immunoassays are only for detecting single ustiloxin, and they cannot meet the demand for synchronous and rapid detection of the group toxins. Therefore, this study designed and synthesized a generic antigen with ustiloxin G as material based on the common structure of the mycotoxins. Ustiloxin G was conjugated to two carrier proteins including bovine serum albumin (BSA) and ovalbvmin (OVA) by carbon diimide method. The mice were immunized with ustiloxin-G-BSA to generate the antibody serum, which was further purified to obtain the generic antibody against ustiloxins. The conjugated ustiloxin G-OVA and generic antibodies were used for establishing the enzyme-linked immunosorbent assay (ELISA) for ustiloxin detection and optimizing experiment conditions. The characterization of the antibody showed that the semi-inhibitory concentrations (IC50) of ustiloxin A, B, and G were 0.53, 0.34, and 0.06 µg/mL, respectively, and that their corresponding cross-reactivities were 11.9%, 18.4%, and 100%, respectively. To increase ELISA detection efficiency, generic antibody was combined with magnetic beads to obtain sensitive and class-specific immune-magnetic beads. Based on these immuno-magnetic beads, a high-efficiency enzyme-linked immunoassay method was developed for ustiloxin detection, whose sensitivity to ustiloxin A, B, and G was improved to 0.15 µg/mL, 0.14 µg/mL, and 0.04 µg/mL, respectively. The method accuracy was evaluated by spiking ustiloxin G as standard, and the spiked samples were tested by the immune-magnetic bead-based ELISA. The result showed the ustiloxin G recoveries ranged from 101.9% to 116.4% and were accepted by a standard HPLC method, indicating that our developed method would be promising for on-site monitoring of ustiloxins in rice.

Ustilaginoidea virens modulates lysine 2-hydroxyisobutyrylation in rice flowers during infection

The post-translational modification lysine 2-hydroxyisobutyrylation (K_hib ) plays an important role in gene transcription, metabolism, and enzymatic activity. K_hib sites have been identified in rice (Oryza sativa). However, the K_hib status of proteins in rice flowers during pathogen infection remains unclear. Here, we report a comprehensive identification of K_hib -modified proteins in rice flowers, and the changes in these proteins during infection with the fungal pathogen Ustilaginoidea virens. By using a tandem mass tag-based quantitative proteomics approach, we identified 2,891 K_hib sites on 964 proteins in rice flowers. Our data demonstrated that 2-hydroxyisobutyrylated proteins are involved in diverse biological processes. K_hib levels were substantially reduced upon infection with U. virens. Chromatin immunoprecipitation polymerase chain reaction (PCR) and reverse transcription quantitative PCR analyses revealed that histone K_hib is involved in the expression of disease-resistance genes. More importantly, most quantified sites on core histones H3 were downregulated upon U. virens infection. In addition, the histone deacetylases HDA705, HDA716, SRT1, and SRT2 are involved in the removal of K_hib marks in rice. HDA705 was further confirmed to negatively regulate rice disease resistance to pathogens U. virens, Magnaporthe oryzae, and Xanthomonas oryzae pv. oryzae (Xoo). Our data suggest that U. virens could modulate K_hib in rice flowers during infection.

Detection of Ustiloxin A in urine by ultra-high-performance liquid chromatography-tandem mass spectrometry coupled with two-step solid-phase extraction

Due to global outbreak of rice false smut disease, ustiloxin A (UA) was detected in rice. However, accurate methods for monitoring UA in human body fluids were lacking. In this context, a UPLC-MS/MS method based on two-step SPE was constructed for measuring UA in urine. The limits of UA quantification in human and mice urine were 58.3 and 108.7 ng/L, respectively. The proposed method was applied to detect UA in urine samples collected from human and mice. After dietary exposure, the contents of UA in mice urine were from 6.03 to 16.76 μg/g of creatine, accounting for approximate 14% of daily intake dose. Furthermore, due to the trace residues in rice (78-109 ng/kg), no detectable UA was observed in the urine of 20 volunteers. To the best of our knowledge, it is the first time to report the occurrence of UA in mammal urine.

Ustilaginoidin D induces hepatotoxicity and behaviour aberrations in zebrafish larvae

Ustilaginoidins, a group of bis-naphtho-γ-pyrones, are one of the major mycotoxins produced by Ustilaginoidea virens. This group of bis-naphtho-γ-pyrone mycotoxins has been demonstrated to have antibacterial and immunological inhibitory activities and strong cytotoxicity to human oral epidermoid carcinoma. However, little is yet known about the toxicity of ustilaginoidins to animals or toxicity mechanisms. In this study, toxicity assays to zebrafish larvae show that ustilaginoidin D is highly toxic to zebrafish with an LC₅₀ of ∼7.50 μM. Ustilaginoidin D causes an obvious yolk sac absorption delay and liver damage in zebrafish, which is indicated by liver atrophy and the increased alanine and aspartate transaminase activities. Interestingly, different doses of ustilaginoidin D can alter zebrafish movement behavior in a distinct manner. Transcriptome analyses show that global gene expression profiling in zebrafish is significantly changed in response to ustilaginoidin D exposure. KEGG pathway analyses reveal that differentially expressed genes are enriched in the pathways related to lipid metabolism and hyperbilirubinemia, which are indicators of severe liver injury. Consistently, the expression of the marker genes for hepatotoxic responses is significantly induced by ustilaginoidin D. The findings indicate that ustilaginoidin D induces lipid metabolism disorders and hepatotoxicity in zebrafish larvae and poses a potential risk to food safety.

Comprehensive identification of lysine 2-hydroxyisobutyrylated proteins in Ustilaginoidea virens reveals the involvement of lysine 2-hydroxyisobutyrylation in fungal virulence

Lysine 2-hydroxyisobutyrylation (K_hib ) is a newly identified post-translational modification (PTM) that plays important roles in transcription and cell proliferation in eukaryotes. However, its function remains unknown in phytopathogenic fungi. Here, we performed a comprehensive assessment of K_hib in the rice false smut fungus Ustilaginoidea virens, using Tandem Mass Tag (TMT)-based quantitative proteomics approach. A total of 3 426 K_hib sites were identified in 977 proteins, suggesting that K_hib is a common and complex PTM in U. virens. Our data demonstrated that the 2-hydroxyisobutyrylated proteins are involved in diverse biological processes. Network analysis of the modified proteins revealed a highly interconnected protein network that included many well-studied virulence factors. We confirmed that the Zn-binding reduced potassium dependency3-type histone deacetylase (UvRpd3) is a major enzyme that removes 2-hydroxyisobutyrylation and acetylation in U. virens. Notably, mutations of K_hib sites in the mitogen-activated protein kinase (MAPK) UvSlt2 significantly reduced fungal virulence and decreased the enzymatic activity of UvSlt2. Molecular dynamics simulations demonstrated that 2-hydroxyisobutyrylation in UvSlt2 increased the hydrophobic solvent-accessible surface area and thereby affected binding between the UvSlt2 enzyme and its substrates. Our findings thus establish K_hib as a major post-translational modification in U. virens and point to an important role for K_hib in the virulence of this phytopathogenic fungus.

Extraction and purification of ustiloxin A from rice false smut balls by a combination of macroporous resin and high-speed countercurrent chromatography

Abstract

Rice false smut is an emerging plant disease worldwide. Ustiloxin A (UstA) is the major mycotoxin found in rice false smut balls, which are fungal colonies in rice florets. In this study, a new method consisting of macroporous resin column chromatography and high-speed countercurrent chromatography (HSCCC) was developed for UstA separation. UstA was extracted by a 3.81% HCOOH solution and adsorbed by XAD-4 resin. UstA was then eluted by a 40% methanol solution supplemented with 0.1% trifluoroacetic acid (TFA). Further purification was achieved by HSCCC using a two-phase solvent system consisting of n-butanol/TFA/H2O (1/0.05/1, v/v/v). Under the optimized conditions, 225 mg of UstA was obtained with a purity of 97.39% in a single run, with a final recovery of 65.2%. An inhibitory effect on seed germination of wheat and maize caused by UstA was observed in a preliminary phytotoxicity assay.

Graphical abstract

The 'pears and lemons' protein UvPal1 regulates development and virulence of Ustilaginoidea virens

Ustilaginoidea virens is an economically important fungus causing a devastating grain disease, rice false smut. An insertional mutagenesis screen was used to explore biological mechanisms underlying infection process of U. virens. T184, a new mutant was identified, with abnormal conidial morphology and deficient virulence. Analysis of the T-DNA inserted gene UvPal1 in the mutant confirmed it as a putative homologue of a cellular morphogenetic protein in yeast, Pal1, whose function has not been well characterized. Deletion of UvPal1 affected hyphal growth, cell morphology, stress adaptation and virulence. UvPal1 could interact with the endocytic proteins, UvEde1 and UvSla2, but was not required for receptor-mediated endocytosis. A yeast two-hybrid (Y2H) analysis was further carried out to screen the UvPal1-interacting proteins, resulting in the identification of 16 putative interacting proteins. Interestingly, UvPal1 interacted with a septin protein, UvCdc11 in vivo and in vitro, and also affected subcellular localization of UvCdc11 protein. Deletion of the four core septins impaired the growth, morphogenesis, stress response and virulence. Collectively, effects on cell morphology, oxidative stress response and virulence are similar to those of UvPal1, suggesting that UvPal1 physically interacts with UvCdc11 to mediate the septin complex to maintain the cellular morphology and virulence of U. virens.

Ustiloxin A is Produced Early in Experimental Ustilaginoidea virens Infection and Affects Transcription in Rice

Ustiloxin is a kind of 13-membered cyclic peptides found in mature rice false smut generated by Ustilaginoidea virens infecting rice spikelet. So far, six kinds of ustiloxins have been identified from false smut balls (FSBs) in which ustiloxin A is the main component. The toxins can not only inhibit the growth of rice, wheat, and corn, but also poison people and animals. However, so far, there have been few studies of the content of ustiloxin except that in mature FSB. The effect of ustiloxins on the process of infection has not been clarified. In this study, the technique of artificial inoculation coupled with UPLC-ESI-MS was introduced to investigate the content of ustiloxins in the course of infection. The initial formation time of ustiloxin A, B, C, D, F, and G was no later than 5, 5, 9, 7, 7, and 9 days post inoculation (dpi) prior to FSB's formation, respectively. The content of ustiloxin A per spikelet was increased rapidly from 6.0 ng at 5 dpi to 14,157.1 ng at 25 dpi. Meanwhile, the content of ustiloxin A per dry weight (DW) of the FSBs also peaked at 1321.2 μg/g at 25 dpi. Interestingly, both the contents of ustiloxin A per dry weight and per spikelet were significantly reduced from 25 to 30 dpi. Transcriptome sequencing revealed that a total of 146 transcripts (103 upregulated and 43 downregulated) were significantly changed in rice spikelets after 3-h acute exposure to 100 ng ustiloxin A. In addition, several of the significantly altered genes were validated by RT-qPCR.

Exploring the mechanisms of graphene oxide behavioral and morphological changes in zebrafish

The presence of natural organic matter such as humic acid (HA) can influence the behavior of graphene oxide (GO) in the aquatic environment. In this study, zebrafish embryos were analyzed after 5 and 7 days of exposure to GO (100 mg L^-1) and HA (20 mg L^-1) alone or together. The results indicated that, regardless of the presence of HA, larvae exposed to GO for 5 days showed an increase in locomotor activity, reduction in the yolk sac size, and total length and inhibition of AChE activity, but there was no difference in enzyme expression. The statistical analysis indicated that the reductions in total larval length, yolk sac size, and AChE activity in larvae exposed to GO persisted in relation to the control group, but there was a recovery of these parameters in groups also exposed to HA. Larvae exposed to GO for 7 days did not show significant differences in locomotor activity, but the RT-PCR gene expression analysis evidenced an increase in the AChE expression. Since the embryos exposed to GO showed a reduction in overall length, they were submitted to confocal microscopy and their muscle tissue configuration investigated. No changes were observed in the muscle tissue. The results indicated that HA is associated with the toxicity risk modulation by GO and that some compensatory homeostasis mechanisms may be involved in the developmental effects observed in zebrafish.