"Lupinosis"-like lesions in mice caused by ustiloxin, produced by Ustilaginoieda virens: a morphological study

The TOR Signaling Pathway Governs Fungal Development, Virulence and Ustiloxin Biosynthesis in Ustilaginoidea virens

Ustilaginoidea virens is an economically important plant pathogen that causes rice false smut, which causes yield reduction and produces mycotoxins in infected grains that pose a serious threat to human and animal health. The target of rapamycin (TOR) signaling pathway acts as a master regular in regulating cell growth and secondary metabolism in fungi. However, little is known about the function of the TOR pathway in regulating fungal development, pathogenicity and mycotoxin biosynthesis in U. virens. Here, we demonstrate that the TOR signaling pathway positively regulates the cell growth, conidiation and pathogenicity in U. virens through the biochemical inhibition of TOR kinases. The inhibition of TOR in U. virens (UvTOR) by rapamycin significantly induces the expression of genes related to mycotoxin biosynthesis, especially that of ustiloxins. Transcriptome analysis under TOR inhibition revealed that the TOR signaling pathway is a regulatory hub that governs U. virens growth and metabolism. A total of 275 differentially expressed genes (DEGs), consisting of 109 up-regulated DEGs and 166 down-regulated DEGs, were identified after rapamycin treatment. The up-regulated DEGs were enriched in amino acid- and acetyl-CoA-related metabolism pathways and the down-regulated DEGs were enriched in carbohydrate- and fatty acid-related metabolism pathways. Collectively, our results provide the first in-depth insight into the TOR signaling pathway in regulating vegetable growth, virulence and mycotoxin biosynthesis in U. virens.

Ustiloxin A impairs oocyte quality by disrupting organelles function

Oocyte quality is pivotal for fertilization and early embryonic development. Ustiloxin A (UA), is an emerging mycotoxin that has been frequently detected in rice and paddy. Because UA has been reported to be phytotoxic and cytotoxic, it poses a potential hazard to human and animal health. However, the effects of UA on oocyte maturation remain unknown. Here, we investigated the effects of acute UA exposure on mouse oocyte maturation. First, UA exposure inhibited oocyte maturation in a concentration-dependent manner and induced meiotic arrest by disrupting spindle assembly and reducing actin density. Moreover, mitochondrial function was substantially disrupted in oocytes upon UA exposure. Aberrant mitochondrial distribution, substantial downregulation of mitochondrial dynamics-associated genes Mfn1, Mfn2 and Fis1, decreased membrane potential and TOM20 expression were observed in UA-exposed oocytes; these effects further led to oxidative stress and DNA damage. Furthermore, UA induced ER and Golgi dysfunction and triggered ER stress by increasing GRP78 expression, which ultimately resulted in autophagy and early apoptosis in oocytes. Therefore, these results demonstrate that UA impairs oocyte quality by disrupting organelles function, providing new insight into the influence of UA on female reproduction in mammals.

Detoxification of Ustiloxin A Through Oxidative Deamination and Decarboxylation by Endophytic Fungus Petriella setifera

Ustiloxins are a group of cyclopeptide mycotoxins produced by rice false smut pathogen Villosiclava virens (anamorph: Ustilaginoidea virens) which seriously threaten the safety production of rice and the health of humans and livestock. Ustiloxin A, accounting for 60% of the total ustiloxins, is the main toxic component. Biotransformation, a process of modifying the functional groups of compounds by means of regio- or stereo-specific reactions catalyzed by the enzymes produced by organisms, has been considered as an efficient way to detoxify mycotoxins. In this study, the endophytic fungus Petriella setifera Nitaf10 was found to be able to detoxify ustiloxin A through biotransformation. Two transformed products were obtained by using the cell-free extract (CFE) containing intracellular enzymes of P. setifera Nitaf10. They were structurally characterized as novel ustiloxin analogs named ustiloxins A1 (1) and A2 (2) by analysis of the 1D and 2D NMR and HRESIMS spectra as well as by comparison with known ustiloxins. The cytotoxic activity of ustiloxins A1 (1) and A2 (2) was much weaker than that of ustiloxin A. The biotransformation of ustiloxin A was found to proceed via oxidative deamination and decarboxylation and was possibly catalyzed by the intracellular amine oxidase and oxidative decarboxylase in the CFE. An appropriate bioconversion was achieved by incubating ustiloxin A with the CFE prepared in 0.5 mol/L phosphate buffer (pH 7.0) for 24 to 48 h. The optimum initial pH values for the bioconversion of ustiloxin A were 7-9. Among eight metal ions (Co2+, Cu2+, Fe3+, Zn2+, Ba2+, Ca2+, Mg2+ and Mn2+) tested at 5 mmol/L, Cu2+, Fe3+ and Zn2+ totally inhibited the conversion of ustiloxin A. In conclusion, detoxification of ustiloxin A through oxidative deamination and decarboxylation is an efficient strategy.

Resistance risk and mechanism of Ustilaginoidea virens to pydiflumetofen

Rice false smut, caused by Ustilaginoidea virens, is a devastating fungal disease in rice that not only leads to yield reduction but also poses a serious threat to food safety and human health due to the production of numerous mycotoxins. Pydiflumetofen, one of the most promising SDHI fungicides widely used for controlling various plant diseases, lacks available information regarding its antifungal activity against U. virens and the potential risk of resistance development in this pathogen. In this study, we evaluated the sensitivity of 33 field-isolated strains of U. virens to pydiflumetofen using mycelial growth inhibition method and assessed the potential for resistance development. The EC50 values for pydiflumetofen against the tested strains ranged from 0.0032 to 0.0123 μg/mL, with an average EC50 value of 0.0056 ± 0.0025 μg/mL. In addition, four strains of U. virens were randomly selected for chemical taming to evaluate their resistance risk to pydiflumetofen, resulting in the successful generation of eight stable and inheritable resistant mutants at a frequency of 1 %. These mutants exhibited significant differences in biological fitness compared to their respective parental strains. Cross-resistance tests revealed a correlation between pydiflumetofen and fluxapyroxad as well as fluopyram, but no evidence of cross-resistance was observed between pydiflumetofen and boscalid or tebuconazole. Therefore, we can conclude that the risk of resistance development in U. virens to pydiflumetofen is moderate. Finally, the target genes SDHB, SDHC, and SDHD in U. virens were initially identified, cloned, and sequenced to elucidate the mechanism underlying U. virens resistance to pydiflumetofen. Three mutation genotypes were found in the mutants: SDHB-H239Y, SDHB-H239L, and SDHC-A77V. The mutants carrying SDHB-H239Y exhibited low resistance, while SDHC-A77V showed moderate resistance, but the mutants with SDHB-H239L demonstrated high resistance. These findings contribute significantly to our comprehensive understanding of molecular mechanisms involved in the resistance of U. virens to pydiflumetofen, and provide an important reference for chemical control strategies against rice false smut in the field.

Full-length transcriptomics study of Ustiloxins-induced hepatocyte injury

Ustiloxins is a mycotoxin produced by the metabolism of Rice false smut. Studies have shown that Ustiloxins may be toxic to animals, but there is still a lack of toxicological evidence. The liver, as the main organ for the biotransformation of foreign chemicals, may be the direct target organ of Ustiloxins toxicity. In this study, we found that cell viability decreased in a dose- and time-dependent manner when BNL CL.2 cells were treated with different concentrations of Ustiloxins (0, 5, 10, 20, 30, 40, 60, 80, 100, 150 and 200 μg/mL) for 24 and 48 h. In addition, scanning electron microscope observation showed that the cell membrane of the experimental group was damaged, with the appearance of apoptotic bodies. Moreover, the ROS and GSH levels were significantly increased in cells exposed to Ustiloxins. We analyzed the key action targets of Ustiloxins on hepatocyte injury using full-length transcriptomics. A total of 1099 differentially expressed genes were screened, of which 473 genes were up-regulated, and 626 genes were down-regulated. Besides, we also found that the expression of MCM7 and CDC45 in BNL CL.2 cells treated with Ustiloxins decreased, and the expression of CCl-2, CYP1b1, CYP4f13, and GSTM1 increased according to qRT-PCR. Ustiloxins might change CYP450 and GST-related genes, affect DNA replication and cell cycle, and lead to oxidative stress and liver cell injury.

An Ustilaginoidea virens glycoside hydrolase 42 protein is an essential virulence factor and elicits plant immunity as a PAMP

Rice false smut, caused by the ascomycete fungus Ustilaginoidea virens, which infects rice florets before heading, severely threatens rice grain yield and quality worldwide. The U. virens genome encodes a number of glycoside hydrolase (GH) proteins. So far, the functions of these GHs in U. virens are largely unknown. In this study, we identified a GH42 protein secreted by U. virens, named UvGHF1, that exhibits β-galactosidase activity. UvGHF1 not only functions as an essential virulence factor during U. virens infection, but also serves as a pathogen-associated molecular pattern (PAMP) in Nicotiana benthamiana and rice. The PAMP activity of UvGHF1 is independent of its β-galactosidase activity. Moreover, UvGHF1 triggers cell death in N. benthamiana in a BAK1-dependent manner. Ectopic expression of UvGHF1 in rice induces pattern-triggered immunity and enhances rice resistance to fungal and bacterial diseases. RNA-seq analysis revealed that UvGHF1 expression in rice not only activates expression of many defence-related genes encoding leucine-rich repeat receptor-like kinases and WRKY and ERF transcription factors, but also induces diterpenoid biosynthesis and phenylpropanoid biosynthesis pathways. Therefore, UvGHF1 contributes to U. virens virulence, but is also recognized by the rice surveillance system to trigger plant immunity.

A Genome-Wide Comparison of Rice False Smut Fungus Villosiclava virens Albino Strain LN02 Reveals the Genetic Diversity of Secondary Metabolites and the Cause of Albinism

Rice false smut (RFS) caused by Villosiclava virens (anamorph: Ustilaginoidea virens) has become one of the most destructive fungal diseases to decrease the yield and quality of rice grains. An albino strain LN02 was isolated from the white RFS balls collected in the Liaoning Province of China in 2019. The strain LN02 was considered as a natural albino mutant of V. virens by analyzing its phenotypes, internal transcribed spacer (ITS) conserved sequence, and biosynthesis gene clusters (BGCs) for secondary metabolites. The total assembled genome of strain LN02 was 38.81 Mb, which was comprised of seven nuclear chromosomes and one mitochondrial genome with an N50 value of 6,326,845 bp and 9339 protein-encoding genes. In addition, the genome of strain LN02 encoded 19 gene clusters for biosynthesis of secondary metabolites mainly including polyketides, terpenoids and non-ribosomal peptides (NRPs). Four sorbicillinoid metabolites were isolated from the cultures of strain LN02. It was found that the polyketide synthase (PKS)-encoding gene uspks1 for ustilaginoidin biosynthesis in strain LN02 was inactivated due to the deletion of four bases in the promoter sequence of uvpks1. The normal uvpks1 complementary mutant of strain LN02 could restore the ability to synthesize ustilaginoidins. It demonstrated that deficiency of ustilaginoidin biosynthesis is the cause of albinism for RFS albino strain LN02, and V. virens should be a non-melanin-producing fungus. This study further confirmed strain LN02 as a white phenotype mutant of V. virens. The albino strain LN02 will have a great potential in the development and application of secondary metabolites. The physiological and ecological functions of ustilaginoidins in RFS fungus are needed for further investigation.

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.

Sensitivity Baselines, Resistance Monitoring, and Molecular Mechanisms of the Rice False Smut Pathogen Ustilaginoidea virens to Prochloraz and Azoxystrobin in Four Regions of Southern China

Rice false smut caused by Ustilaginoidea virens is one of the most devastating fungal diseases of rice (Oryza sativa) worldwide. Prochloraz and azoxystrobin belong to the groups of demethylation inhibitors and quinone outside inhibitors, respectively, and are commonly used for controlling this disease. In this study, we analyzed the sensitivities of 100 U. virens isolates from Yunnan, Sichuan, Chongqing, and Zhejiang in Southern China to prochloraz and azoxystrobin. The ranges of EC50 for prochloraz and azoxystrobin were 0.004-0.536 and 0.020-0.510 μg/mL, with means and standard errors of 0.062 ± 0.008 and 0.120 ± 0.007 μg/mL, respectively. However, the sensitivity frequency distributions of U. virens to prochloraz and azoxystrobin indicated the emergence of subpopulations with decreased sensitivity. Therefore, the mean EC50 values of 74% and 68% of the isolates at the main peak, 0.031 ± 0.001 and 0.078 ± 0.004 μg/mL, were used as the sensitivity baselines of U. virens to prochloraz and azoxystrobin, respectively. We found significant sensitivity differences to azoxystrobin among different geographical populations and no correlation between the sensitivities of U. virens to prochloraz and azoxystrobin. Among 887 U. virens isolates, the isolate 5-3-1 from Zhejiang showed moderate resistance to prochloraz, with a resistance factor of 22.45, while no nucleotide variation in the 1986-bp upstream or 1827-bp gene regions of CYP51 from 5-3-1 was detected. Overexpression of CYP51 is probably responsible for its resistance to prochloraz. Finally, artificial inoculation showed that 5-3-1 was highly pathogenic to rice, suggesting that the resistance of U. virens to prochloraz must be monitored and managed in Zhejiang.

Kernel Smut and False Smut: The Old-Emerging Diseases of Rice-A Review

Kernel smut, caused by Tilletia horrida, is a disease characterized by the replacement of rice grains with black sooty masses of teliospores or chlamydospores. Kernel smut differs from rice false smut, caused by Ustilaginoidea virens, in the color of chlamydospores. False smut is characterized by globose, velvety spore balls ranging from orangish yellow to greenish black in color. Both kernel smut and false smut have been persistent but are considered minor diseases in many countries since they were discovered in the late 1870s to the 1980s due to their sporadic outbreaks and limited economic impacts. In recent years, however, kernel smut and false smut have emerged as two of the most economically important diseases in rice, including organic rice, in many countries, especially in the United States. The increased use of susceptible rice cultivars, especially hybrids, excessive use of nitrogen fertilizer, and short crop rotations have resulted in an increase in kernel smut and false smut, causing significant losses in grain yield and quality. In this article, we provide a review of the distribution and economic importance of kernel smut; our current understanding of the taxonomy, biology, and epidemiology of kernel smut; and the genomics of the kernel smut fungus as compared with false smut and its causal agent. We also provide an update on the current management strategies of pathogen exclusion, cultivar resistance, fungicides, biological control, and cultural practices for kernel smut and false smut of rice.

SPR9 encodes a 60 S ribosomal protein that modulates panicle spreading and affects resistance to false smut in rice (Oryza sativa. L)

BACKGROUND

The architecture of inflorescence in crops is a key agronomic feature determining grain yield and thus has been a major target trait of cereal domestication.

RESULTS

In this study, we show that a simple spreading panicle change in rice panicle shape, controlled by the Spreading Panicle 9 (SPR9) locus, also has a significant impact on the resistance to rice false smut (RFS). Meanwhile, we mapped a novel spr9 mutant gene between markers Indel5-18 and Indel5-22 encompassing a genomic region of 43-kb with six candidate genes. Through gene prediction and cDNA sequencing, we confirmed that LOC_Os05g38520 is the target gene in the spr9 mutant, which encodes 60 S ribosomal protein L36-2. Further analysis showed that the spr9 mutant is caused by a 1 bp deletion in the first exon that resulted in premature termination. Knockout experiments showed that the SPR9 gene is responsible for the spreading panicle phenotype of the spr9 mutant. Interestingly, the spr9 mutant was found to improve resistance to RFS without affecting major agronomic traits. Taken together, our results revealed that the spr9 allele has good application prospects in rice breeding for disease resistance and panicle improvement.

CONCLUSIONS

We report the map-based cloning and functional characterization of SPR9, which encodes a 60 S ribosomal protein that regulates spreading panicles and affects the resistance to false smut in rice.

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.

The Genetic Mechanism of the Immune Response to the Rice False Smut (RFS) Fungus Ustilaginoidea virens

Rice false smut (RFS), which is caused by Ustilaginoidea virens (U. virens), has become one of the most devastating diseases in rice-growing regions worldwide. The disease results in a significant yield loss and poses health threats to humans and animals due to producing mycotoxins. In this review, we update the understanding of the symptoms and resistance genes of RFS, as well as the genomics and effectors in U. virens. We also highlight the genetic mechanism of the immune response to RFS. Finally, we analyse and explore the identification method for RFS, breeding for resistance against the disease, and interactions between the effector proteins and resistance (R) proteins, which would be involved in the development of rice disease resistance materials for breeding programmes.

SnRK1A-mediated phosphorylation of a cytosolic ATPase positively regulates rice innate immunity and is inhibited by Ustilaginoidea virens effector SCRE1

Rice false smut caused by Ustilaginoidea virens is becoming one of the most recalcitrant rice diseases worldwide. However, the molecular mechanisms underlying rice immunity against U. virens remain unknown. Using genetic, biochemical and disease resistance assays, we demonstrated that the xb24 knockout lines generated in non-Xa21 rice background exhibit an enhanced susceptibility to the fungal pathogens U. virens and Magnaporthe oryzae. Consistently, flg22- and chitin-induced oxidative burst and expression of pathogenesis-related genes in the xb24 knockout lines were greatly attenuated. As a central mediator of energy signaling, SnRK1A interacts with and phosphorylates XB24 at Thr83 residue to promote ATPase activity. SnRK1A is activated by pathogen-associated molecular patterns and positively regulates plant immune responses and disease resistance. Furthermore, the virulence effector SCRE1 in U. virens targets host ATPase XB24. The interaction inhibits ATPase activity of XB24 by blocking ATP binding to XB24. Meanwhile, SCRE1 outcompetes SnRK1A for XB24 binding, and thereby suppresses SnRK1A-mediated phosphorylation and ATPase activity of XB24. Our results indicate that the conserved SnRK1A-XB24 module in multiple crop plants positively contributes to plant immunity and uncover an unidentified molecular strategy to promote infection in U. virens and a novel host target in fungal pathogenesis.

UvSorA and UvSorB Involved in Sorbicillinoid Biosynthesis Contribute to Fungal Development, Stress Response and Phytotoxicity in Ustilaginoidea virens

Ustilaginoidea virens (teleomorph: Villosiclava virens) is an important fungal pathogen that causes a devastating rice disease. It can produce mycotoxins including sorbicillinoids. The biosynthesis and biological functions of sorbicillinoids have not been reported in U. virens. In this study, we identified a sorbicillinoid biosynthetic gene cluster in which two polyketide synthase genes UvSorA and UvSorB were responsible for sorbicillinoid biosynthesis in U. virens. In ∆UvSorA and ∆UvSorB mutants, the mycelial growth, sporulation and hyphal hydrophobicity were increased dramatically, while the resistances to osmotic pressure, metal cations, and fungicides were reduced. Both phytotoxic activity of rice germinated seeds and cell wall integrity were also reduced. Furthermore, mycelia and cell walls of ∆UvSorA and ∆UvSorB mutants showed alterations of microscopic and submicroscopic structures. In addition, feeding experiment showed that sorbicillinoids could restore mycelial growth, sporulation, and cell wall integrity in ∆UvSorA and ∆UvSorB mutants. The results demonstrated that both UvSorA and UvSorB were responsible for sorbicillinoid biosynthesis in U. virens, and contributed to development (mycelial growth, sporulation, and cell wall integrity), stress responses, and phytotoxicity through sorbicillinoid mediation. It provides an insight into further investigation of biological functions and biosynthesis of sorbicillinoids.

Occurrence and translocation of ustiloxins in rice false smut-occurred paddy fields, Hubei, China

Ustiloxin A (UA) and ustiloxin B (UB), two major mycotoxins produced by the pathogen of rice false smut (RFS) during rice cultivation, have attracted increasing attentions due to their potential health risks. However, limited data are available about their occurrence and fate in paddy fields and contamination profiles in rice. In this study, a field study was performed to investigate the occurrence and translocation of UA and UB in RFS-occurred paddies. For the first time to our knowledge, we reported a ubiquitous occurrence of the two ustiloxins in the paddy water (range: 0.01-3.46 μg/L for UA and <0.02-1.15 μg/L for UB) and brown rice (range: 0.09-154.08 μg/kg for UA and <0.09-23.57 μg/kg for UB). A significant positive correlation was observed between ustiloxin levels in paddy water and brown rice (r_s = 0.48-0.79, p < 0.01). The occurrence of ustiloxin uptake in water-rice system was also evidenced by the rice exposure experiment, suggesting paddy water might be an important source for ustiloxin accumulation in rice. These results suggested that the contamination of ustiloxins in rice might occur widely, which was supported by the significantly high detection frequencies of UA (96.6%) and UB (62.4%) in polished rice (149 samples) from Hubei Province, China. The total concentrations of ustiloxins in the polished rice samples collected from Hubei Province ranged from <20.7 ng/kg (LOD) to 55.1 μg/kg (dry weight). Further studies are needed to evaluate the potential risks of ustiloxin exposure in the environment and humans.

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.

Transcription Profiling of Rice Panicle in Response to Crude Toxin Extract of Ustilaginoidea virens

Ustilaginoidea virens infects rice, causing rice false smut disease and reduced yields. During its growth, U. virens can also produce some toxins but less is known about the response mechanisms of the plant to U. virens toxins. U. virens toxins can inhibit the accumulation of total sugar in rice panicles. We used RNA sequencing to analyze the differential expression profile induced by infiltrating crude toxins into early growth-stage rice panicles. We compared the transcriptomes of the control and crude toxin-treated rice panicles and determined variable transcriptional responses under the action of the crude toxins. A total of 6,127 differentially expressed genes (DEGs) were identified. Among these genes, 3,150 were upregulated and 2,977 were downregulated. Gene Ontology (GO) and metabolic pathway enrichment analyses indicated that U. virens toxins mainly influenced glycometabolism, amino acid metabolism, and secondary metabolism of rice panicles. DEG analysis showed that the gene expression levels of 10 transcription factor families were significantly changed. Genes involved in phenylpropanoid biosynthesis, flavonoid biosynthesis, sugar transporters, and starch synthesis-related were significantly downregulated, including cytochrome P450, beta-glucosidase, CHS1, sucrose transporters, SWEETs, starch-branching enzymes, and UDP-glucose pyrophosphorylase. However, genes involved in programmed cell death (PCD) were significantly upregulated and contained cytochrome c, metacaspase, and protein kinase genes. The results indicate that U. virens toxins may act as the pathogenic factors to reduce stress resistance, disrupt total sugar accumulation and starch formation, and induce PCD.

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.

High-resolution mapping of the quantitative trait locus (QTLs) conferring resistance to false smut disease in rice

Rice false smut (RFS), an emerging major fungal disease worldwide caused by Ustilaginoidea virens, affects rice grain quality and yield. RFS cause 2.8-49% global yield loss depending upon disease severity and cultivars. In India, the yield loss due to RFS ranged from 2 to 75%. Identification of the genes or quantitative trait loci (QTLs) governing disease resistance would be of utmost importance towards mitigating the economic losses incurred due to RFS. Here, we report mapping of RFS resistance QTLs from a resistant breeding line RYT2668. The mapping population was evaluated for RFS resistance under the field condition in three cropping seasons 2013, 2015, and 2016. A positive correlation among infected panicle/plant, total smut ball/panicle, and disease score was observed in the years 2013, 2015, and the mean data. A total of seven QTLs were mapped on rice chromosomes 2, 4, 5, 7, and 9 using 2326 single nucleotide polymorphism markers. Of these, two QTLs, qRFSr5.3 and qRFSr7.1a, were associated with the infected panicle per plant, one QTL qRFsr9.1 with total smut ball per panicle, and four QTLs qRFSr2.2, qRFSr4.3, qRFSr5.4, and qRFSr7.1b with disease score. Among them, a novel QTL qRFSr9.1 on chromosome 9 exhibits the largest phenotypic effect. The prediction of putative candidate genes within the qRFSr9.1 revealed four nucleotide-binding sites-leucine-rich repeat (NBS-LRR) domain-containing disease resistance proteins. In summary, our findings mark the hotspot region of rice chromosomes carrying genes/QTLs for resistance to the RFS disease.

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.

ATAC-Seq Data for Genome-Wide Profiling of Transcription Factor Binding Sites in the Rice False Smut Fungus Ustilaginoidea virens

Identification of transcription factor binding sites is one of the most important steps in understanding the function of transcription factors and regulatory networks in organisms. The assay for transposase accessible chromatin sequencing (ATAC-seq) is a simple protocol for detection of open chromatin that could be a powerful tool to advance studies of protein-DNA interactions. Although ATAC-seq has been used in systematic identification of cis-regulatory regions in animal and plant genomes, this method has been rarely applied in fungi. Here, we describe a valuable ATAC-seq resource in the genome of an economically important phytopathogen, the rice false smut fungus Ustilaginoidea virens. The ATAC-seq data of U. virens mycelia collected from potato sucrose broth (PSB) and PSB supplied with rice spikelet extract were both generated. This is the first genome-wide profiling of open chromatin and transcription factor binding sites in U. virens.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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 pheromone response module, a mitogen-activated protein kinase pathway implicated in the regulation of fungal development, secondary metabolism and pathogenicity

Mitogen-activated protein kinase (MAPK) pathways are highly conserved from yeast to human and are required for the regulation of a multitude of biological processes in eukaryotes. A pentameric MAPK pathway known as the Fus3 pheromone module was initially characterised in Saccharomyces cerevisiae and was shown to regulate cell fusion and sexual development. Individual orthologous pheromone module genes have since been found to be highly conserved in fungal genomes and have been shown to regulate a diverse array of cellular responses, such as cell growth, asexual and sexual development, secondary metabolite production and pathogenicity. However, information regarding the assembly and structure of orthologous pheromone modules, as well as the mechanisms of signalling and their biological significance is limited, specifically in filamentous fungal species. Recent studies have provided insight on the utilization of the pheromone module as a central signalling hub for the co-ordinated regulation of fungal development and secondary metabolite production. Various proteins of this pathway are also known to regulate reproduction and virulence in a range of plant pathogenic fungi. In this review, we discuss recent findings that help elucidate the structure of the pheromone module pathway in a myriad of fungal species and its implications in the control of fungal growth, development, secondary metabolism and pathogenicity.

Ustilaginoidea virens: Insights into an Emerging Rice Pathogen

False smut of rice, caused by Ustilaginoidea virens, has become one of the most important diseases in rice-growing regions worldwide. The disease causes a significant yield loss and imposes health threats to humans and animals by producing mycotoxins. In this review, we update our understanding of the pathogen, including the disease cycle and infection strategies, the decoding of the U. virens genome, comparative/functional genomics, and effector biology. Whereas the decoding of the U. virens genome unveils specific adaptations of the pathogen in successfully occupying rice flowers, progresses in comparative/functional genomics and effector biology have begun to uncover the molecular mechanisms underlying U. virens virulence and pathogenicity. We highlight the identification and characterization of the produced mycotoxins and their biosynthetic pathways in U. virens.The management strategies for this disease are also discussed. The flower-specific infection strategy makes the pathogen a unique tool to unveil novel mechanisms for the interactions between nonobligate biotrophic pathogens and their hosts.

Peptides derived from Kex2-processed repeat proteins are widely distributed and highly diverse in the Fungi kingdom

BACKGROUND

Recently, a gene cluster responsible for biosynthesis of ustiloxin in Aspergillus flavus was identified as the first case of a ribosomally synthesized and post-translationally modified peptide (RiPP) synthetic pathway in Ascomycota. RiPPs are biosynthesized from precursor peptides, which are processed to produce the RiPP backbone (core peptides) for further modifications such as methylation and cyclization. Ustiloxin precursor peptide has two distinctive features: a signal peptide for translocation into the endoplasmic reticulum and highly repeated core sequences cleaved by Kex2 protease in the Golgi apparatus. On the basis of these characteristics, the ustiloxin-type RiPP precursor peptides or Kex2-processed repeat proteins (KEPs) in strains belonging to the Fungi kingdom were computationally surveyed, in order to investigate the distribution and putative functions of KEPs in fungal ecology.

RESULTS

In total, 7878 KEPs were detected in 1345 of 1461 strains belonging to 8 phyla. The average number of KEPs per strain was 5.25 in Ascomycota and 5.30 in Basidiomycota, but only 1.35 in the class Saccharomycetes (Ascomycota) and 1.00 in the class Tremellomycetes (Basidiomycota). The KEPs were classified into 838 types and 2560 stand-alone ones, which had no homologs. Nearly 200 types were distributed in more than one genus, and 14 types in more than one phylum. These types included yeast α-mating factors and fungal pheromones. Genes for 22% KEPs were accompanied by genes for DUF3328-domain-containing proteins, which are indispensable for cyclization of the core peptides. DUF3328-domain-containing protein genes were located at an average distance of 3.09 genes from KEP genes. Genes for almost all (with three exceptions) KEPs annotated as yeast α-mating factors or fungal pheromones were not accompanied by DUF3328-domain-containing protein genes.

CONCLUSION

KEPs are widely distributed in the Fungi kingdom, but their repeated sequences are highly diverse. From these results and some examples, a hypothesis was raised that KEPs initially evolved as unmodified linear peptides (e.g., mating factors), and then those that adopted a modified cyclic form emerged (e.g., toxins) to utilize their strong bioactivity against predators and competitive microorganisms.

Development of a Low-Cost Narrow Band Multispectral Imaging System Coupled with Chemometric Analysis for Rapid Detection of Rice False Smut in Rice Seed

Spectral imaging is a promising technique for detecting the quality of rice seeds. However, the high cost of the system has limited it to more practical applications. The study was aimed to develop a low-cost narrow band multispectral imaging system for detecting rice false smut (RFS) in rice seeds. Two different cultivars of rice seeds were artificially inoculated with RFS. Results have demonstrated that spectral features at 460, 520, 660, 740, 850, and 940 nm were well linked to the RFS. It achieved an overall accuracy of 98.7% with a false negative rate of 3.2% for Zheliang, and 91.4% with 6.7% for Xiushui, respectively, using the least squares-support vector machine. Moreover, the robustness of the model was validated through transferring the model of Zheliang to Xiushui with the overall accuracy of 90.3% and false negative rate of 7.8%. These results demonstrate the feasibility of the developed system for RFS identification with a low detecting cost.

The false smut pathogen Ustilaginoidea virens requires rice stamens for false smut ball formation

Rice false smut has emerged as a serious grain disease in rice production worldwide. The disease is characterized by the transformation of individual rice florets into false smut balls, which is caused by the fungal pathogen Ustilaginoidea virens. To date, little is known about the host factors required for false smut ball formation by U. virens. In this study, we identified histological determinants for the formation of false smut balls by inoculating U. virens into rice floral mutants defective with respect to individual floral parts. The results showed that U. virens could form mature false smut balls in rice floral mutants with defective pistils, but failed to develop false smut balls in the superwoman mutant lacking stamens, identifying that U. virens requires rice stamens to complete its infection cycle. Comparative transcriptome analysis indicated a list of candidate host genes that may facilitate nutrient acquisition by U. virens from the rice stamens, such as SWEET11, SWEET14 and SUT5, and genes involved in the biosynthesis of trehalose and raffinose family sugars. These data pinpoint rice stamens as the key target organ of U. virens infection and provide a valuable starting point for dissecting the molecular mechanism of false smut ball formation.

UvCom1 Is an Important Regulator Required for Development and Infection in the Rice False Smut Fungus Ustilaginoidea virens

Ustilaginoidea virens is an economically important biotrophic fungal pathogen that causes rice false smut worldwide. However, the regulatory mechanisms of smut ball formation under U. virens infection remain unclear. Here, we identified an important transcription factor, UvCom1, from this pathogen, which regulates the formation of smut balls on rice spikelets. UvCom1 contains two conserved internal repeat 1 (RPT) domains and is found only in fungi, with specific conservation in species of Basidiomycetes and Ascomycetes. UvCom1 protein N- or C-terminal fusion vectors both showed transactivation activity in yeast. Deletion of UvCom1 significantly affected the vegetative growth and conidiation of U. virens. UvCom1 negatively regulated the responses to oxidative, osmotic, and cell wall stresses. Remarkably, UvCom1 was found to be essential for the formation of rice smut balls, and UvCom1 deletion mutants lost the ability to stably utilize nutrients from the rice host. UvCom1 was also highly expressed at the mycelial expansion stage. Transcriptomic analysis and quantitative real-time PCR revealed that UvCom1 could affect the expression of genes significantly enriched in transmembrane transport. This study demonstrates that UvCom1 is a key transcription factor governing smut ball formation of this biotrophic fungus.

Occurrence of the fungus mycotoxin, ustiloxin A, in surface waters of paddy fields in Enshi, Hubei, China, and toxicity in Tetrahymena thermophila

There has been an increasing incidence rate of rice false smut in global rice cultivation areas. However, there is a dearth of studies on the environmental concentrations and hazards of ustiloxin A (UA), which is the major mycotoxin produced by a pathogenic fungus of the rice false smut. Here, the concentrations of UA in the surface waters of two paddy fields located in Enshi city, Hubei province, China, were measured, and its toxicity in T. Thermophila was evaluated. This is the first study to detect UA in the surface waters of the two paddy fields, and the measured mean concentrations were 2.82 and 0.26 μg/L, respectively. Exposure to 2.19, 19.01 or 187.13 μg/L UA for 5 days significantly reduced the theoretical population and cell size of T. thermophila. Furthermore, treatment with 187.13 μg/L UA changed the percentages of T. thermophila cells in different cell-cycle stages, and with an increased malformation rate compared with the control, suggesting the disruption of the cell cycle. The expressions of 30 genes involved in the enriched proteasome pathway, 7 cyclin genes (cyc9, cyc10, cyc16, cyc22, cyc23, cyc26, cyc33) and 2 histone genes (mlh1 and hho1) were significantly down-regulated, which might be the modes of action responsible for the disruption of cell cycling due to UA exposure.

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

Ustiloxin A is a cyclopeptide mycotoxin originally isolated from rice false smut balls (FSBs) that formed in rice spikelets infected by the fungal pathogen Ustilaginoidea virens. Studies have shown that ustiloxin A was toxic to animals, but the toxicological evidence is still lacking. To reveal the negative influence of ustiloxin A on model organism, zebrafish were selected and exposed to ustiloxin A at concentrations of 0, 0.25, 2.5 or 25 μM from 2 h post-fertilization (hpf) to 144 hpf. The hatching rates of embryos in the 25 μM exposure group was 12.85% less than the control group at 96 hpf. Meanwhile, exposure to 0.25, 2.5 or 25 μM ustiloxin A resulted in a distinct dose-dependent increase in mortality rate of embryos at 96 hpf. We also found that exposed to ustiloxin A could cause some other damages on zebrafish larvae, such as growth delay and increased heart rate. In addition, the athletic behavior of zebrafish larvae exposed to ustiloxin A at 25 μM was dramatically different with that of control. Transcriptome sequencing showed that abundances of 339 transcripts (125 up-regulated and 214 down-regulated) were significantly altered in larvae exposed to 25 μM of ustiloxin A. Several of the crucial genes were validated by RT-qPCR. This is the first report on the toxicologic study of ustiloxins against model organism zebrafish. Results suggested that ustiloxins have become a potential danger for food security.

Fumigation and contact activities of 18 plant essential oils on Villosiclava virens, the pathogenic fungus of rice false smut

Rice false smut (RFS), caused by Villosiclava virens, is an emerging devastating disease of rice panicles worldwide and produces yield loss and mycotoxin residues in rice. In this study, 18 plant essential oils (PEOs) were selected to evaluate antifungal activity via fumigation and contact methods against the mycelial growth and conidial germination of V. virens. The primary compositions of PEOs with stronger fungistatic activity were analyzed using gas chromatography (GC)-mass spectrometry (MS), and the changes in the mycelial morphology were observed using scanning electron microscopy (SEM). Antifungal tests showed that cinnamon bark oil and cinnamon oil had stronger fumigation and contact effects on V. virens than the other oils tested. The primary active composition in both cinnamon bark oil and cinnamon oil was trans-cinnamaldehyde, which exhibited contact activities with EC50 values of 2.13 and 35.9 μg/mL against mycelial growth and conidial germination, respectively. The hyphae surface morphological alterations caused by cinnamon bark oil, cinnamon oil and trans-cinnamaldehyde included shriveling, vacuolation and exfoliation. In conclusion, cinnamon bark oil and cinnamon oil have the potential to prevent and control RFS, and trans-cinnamaldehyde is a promising natural lead compound for new fungicide discoveries to control RFS contamination and mycotoxin residues in rice.

A Novel Effector Gene SCRE2 Contributes to Full Virulence of Ustilaginoidea virens to Rice

Ustilaginoidea virens, the causal agent of rice false smut (RFS), has become one of the most devastating rice pathogens worldwide. As a group of essential virulence factors, the effectors in the filamentous fungus might play central roles in the interaction between plants and pathogens. However, little is known about the roles of individual effectors in U. virens virulence. In this study, we identified and characterized a small secreted cysteine-rich effector, SCRE2, in U. virens. SCRE2 was first confirmed as an effector through yeast secretion, protein localization and translocation assays, as well as its expression pattern during U. virens infection. Transient expression of SCRE2 in Nicotiana benthamiana suppressed necrosis-like defense symptoms triggered by the mammalian BAX and oomycete elicitin INF1 proteins. The ability of SCRE2 to inhibit immunity-associated responses in N. benthamiana, including elicitor-triggered cell death and oxidative burst, is further defined to a small peptide region SCRE2^68-85 through expressing a series of truncated proteins. Convincingly, ectopic expression of SCRE2 in the transgenic rice cells significantly inhibited pathogen-associated molecular pattern-triggered immunity including flg22- and chitin-induced defense gene expression and oxidative burst. Furthermore, the scre2 knockout mutant generated by the CRISPR/Cas9 system greatly attenuated in U. virens virulence to rice. Collectively, this study indicates that the effector SCRE2 is able to inhibit plant immunity and is required for full virulence of U. virens.

Development of Colloidal Gold-Based Lateral Flow Immunoassay for Rapid Qualitative and SemiQuantitative Analysis of Ustiloxins A and B in Rice Samples

Rice false smut is a worldwide devastating rice disease infected by the fungal pathogen Villosiclava virens. Ustiloxin A (UA) and ustiloxin B (UB), cyclopeptide mycotoxins, were the major ustiloxins isolated from the rice false smut balls (FSBs) that formed in the pathogen-infected rice spikelets. Based on the specific monoclonal antibodies (mAbs) 2D3G5 and 1B5A10, respectively, against UA and UB, the lateral flow immunoassays (LFIAs) were developed, and the indicator ranges for UA and UB both were 50-100 ng/mL. The cross-reactivities of UB for UA LFIA, and UA for UB LFIA were 5% and 20%, respectively, which were consistent with the icELISA results reported previously. Even at 50,000 ng/mL, none of other commonly existent metabolites in rice samples caused noticeable inhibition. The LFIAs were used for determination of UA and UB contents in rice FSBs and rice grains, and the results were agreeable with those by HPLC and icELISA. There was no change in the sensitivity of either dipstick stored at 4 °C) after at least three months. The developed LFIA has specificity and sensitivity for detecting UA and UB as well as simplicity to use. It will be a potential point-of-care device for rapid evaluation of the rice samples contaminated by UA and UB.

Ustiloxin G, a New Cyclopeptide Mycotoxin from Rice False Smut Balls

Ustiloxins were cyclopeptide mycotoxins from rice false smut balls (FSBs) that formed in rice spikelets infected by the fungal pathogen Ustilaginoidea virens. To investigate the chemical diversity of these metabolites and their bioactivities, one new cyclopeptide, ustiloxin G (1), together with four known congeners—ustiloxins A (2), B (3), D (4), and F (5)—were isolated from water extract of rice FSBs. Their structures were elucidated by analyses of their physical and spectroscopic data, including ultraviolet spectrometry (UV), infrared spectroscopy (IR), 1D and 2D nuclear magnetic resonance (NMR), and high-resolution electrospray ionization-mass spectrometry (HR-ESI-MS). All the compounds were evaluated for their cytotoxic as well as radicle and germ elongation inhibitory activities. Ustiloxin B (3) showed the best activity against the cell line BGC-823 with an IC₅₀ value of 1.03 µM, while ustiloxin G (1) showed moderate activity against the cell lines A549 and A375 with IC₅₀ values of 36.5 µM and 22.5 µM, respectively. Ustiloxins A (2), B (3), and G (1) showed strong inhibition of radicle and germ elongation of rice seeds. When their concentrations were at 200 µg/mL, the inhibitory ratios of radicle and germ elongation were more than 90% and 50%, respectively, the same effect as that of positive control (glyphosate). They also induced abnormal swelling of the roots and germs of rice seedlings.

Current understanding on Villosiclava virens, a unique flower-infecting fungus causing rice false smut disease

Villosiclava virens (Vv) is an ascomycete fungal pathogen that causes false smut disease in rice. Recent reports have revealed some interesting aspects of the enigmatic pathogen to address the question of why it specifically infects rice flowers and converts a grain into a false smut ball. Comparative and functional genomics have suggested specific adaptation of Vv in the colonization of rice flowers. Anatomical studies have disclosed that Vv specifically infects rice stamen filaments before heading and intercepts seed formation. In addition, Vv can occupy the whole inner space of a spikelet embracing all floral organs and activate the rice grain-filling network, presumably for nutrient acquisition to support the development of the false smut ball. This profile provides a general overview of the rice false smut pathogen, and summarizes advances in the Vv life cycle, genomics and genetics, and the molecular Vv-rice interaction. Current understandings of the Vv-rice pathosystem indicate that it is a unique and interesting system which can enrich the study of plant-pathogen interactions. Taxonomy: Ustilaginoidea virens is the anamorph form of the pathogen (Kingdom Fungi; Phylum Ascomycota; Class Ascomycetes; Subclass Incertae sedis; Order Incertae sedis; Family Incertae sedis; Genus Ustilaginoidea). The teleomorph form is Villosiclava virens (Kingdom Fungi; Phylum Ascomycota; Class Ascomycetes; Subclass Sordariomycetes; Order Hypocreales; Family Clavicipitaceae; Genus Villosiclava). Disease symptoms: The only visible symptom is the replacement of rice grains by ball-shaped fungal mycelia, namely false smut balls. When maturing, the false smut ball is covered with powdery chlamydospores, and the colour changes to yellowish, yellowish orange, green, olive green and, finally, to greenish black. Sclerotia are often formed on the false smut balls in autumn. Identification and detection: Vv conidia are round to elliptical, measuring 3-5 μm in diameter. Chlamydospores are ornamented with prominent irregularly curved spines, which are 200-500 nm in length. The sclerotia are black, horseshoe-shaped and irregular oblong or flat, ranging from 2 to 20 mm. Nested polymerase chain reaction (PCR) and quantitative PCR have been developed to specifically detect Vv presence in rice tissues and other biotic and abiotic samples in fields. Host range: Rice is the primary host for Vv. Natural infection by Vv has been found on several paddy field weeds, including Digitaria marginata, Panicum trypheron, Echinochloa crusgalli and Imperata cylindrica. However, the occurrence of infection in these potential alternative hosts is very rare. Life cycle: Vv infects rice spikelets at the late rice booting stage, and produces false smut balls covered with dark-green chlamydospores. Occasionally, sclerotia form on the surface of false smut balls in late autumn when the temperature fluctuates greatly between day and night. Both chlamydospores and sclerotia may serve as primary infection sources. Rainfall at the rice booting stage is a major environmental factor resulting in epidemics of rice false smut disease. Disease control: The use of fungicides is the major approach for the control of Vv. Several fungicides, such as cuproxat SC, copper oxychloride, tebuconazole, propiconazole, difenoconazole and validamycin, are often applied. However, the employment of resistant rice cultivars and genes has been limited, because of the poor understanding of rice resistance to Vv. Useful websites: Villosiclava virens genome sequence: http://www.ncbi.nlm.nih.gov/Traces/wgs/?val=JHTR01#contigs.

The Contents of Ustiloxins A and B along with Their Distribution in Rice False Smut Balls

Ustiloxins are cyclopeptide mycotoxins isolated from rice false smut balls (FSBs), the ball-like colonies transformed from the individual grains through the filament infection by the fungal pathogen Villosiclava virens. There were no obvious relations between ustiloxin content and any of the collection areas, collection times, or average weight of each FSB. The rice false smut balls at early, middle, and late maturity stages were respectively divided into different parts (glume, chlamydospores, mycelia, and pseudoparenchyma). The highest content of ustiloxins A and B of rice FSBs was found at the early maturity stage. Both ustiloxins A and B were mainly distributed in the middle layer containing mycelia and immature chlamydospores of the FSBs. When the rice FSBs were at the early maturity stage, the total yield of ustiloxins A and B in the middle layer of each ball was 48.3 µg, which was 3.20-fold of the yield (15.1 µg) of the inner part of the ball. The rice FSBs at the early maturity stage are the appropriate materials for the production of ustiloxins A and B.

Genome Sequence of Ustilaginoidea virens IPU010, a Rice Pathogenic Fungus Causing False Smut

Ustilaginoidea virens is a rice pathogenic fungus that causes false smut disease, a disease that seriously damages the yield and quality of the grain. Analysis of the U. virens IPU010 33.6-Mb genome sequence will aid in the understanding of the pathogenicity of the strain, particularly in regard to effector proteins and secondary metabolic genes.

Main Ustilaginoidins and Their Distribution in Rice False Smut Balls

Rice false smut has become an increasingly serious fungal disease in rice (Oryza sativa L.) production worldwide. Ustilaginoidins are bis-naphtho-γ-pyrone mycotoxins previously isolated from the rice false smut balls (FSBs) infected by the pathogen Villosiclava virens in rice spikelets on panicles. To investigate the main ustilaginoidins and their distribution in rice FSBs, five main bis-naphtho-γ-pyrones, namely ustilaginoidins A (1), G (2), B (3), I (4) and C (5), were isolated and identified by NMR and high-resolution mass spectrometry as well as by comparison with the data in the literature. The rice FSBs at early, middle and late maturity stages were divided into their different parts and the contents of five main ustilaginoidins for each part were determined by HPLC analysis. The results revealed that the highest levels of ustilaginoidins were in late stage rice FSBs, followed by those at middle stage. Most ustilaginoidins, 96.4% of the total quantity, were distributed in the middle layer at early stage. However, ustilaginoidins were mainly distributed in the outer and middle layers at middle and late stages. Small amounts of ustilaginoidins A (1) and G (2) were found in the inner part of rice FSBs at each maturity stage. The contents of ustilaginoidins A (1) and G (2) without hydroxymethyl groups at C-2 and C-2’ of the γ-pyrone rings in rice FSBs were relatively high at early stage, while the contents of ustilaginoidins B (3), I (4), and C (5) with hydroxymethyl groups at C-2 or C-2’ were relatively high at late stage.

Development of a Monoclonal Antibody-Based icELISA for the Detection of Ustiloxin B in Rice False Smut Balls and Rice Grains

Rice false smut is an emerging and economically-important rice disease caused by infection by the fungal pathogen Villosiclava virens. Ustiloxin B is an antimitotic cyclopeptide mycotoxin isolated from the rice false smut balls that formed in the pathogen-infected rice spikelets. A monoclonal antibody (mAb) designated as mAb 1B5A10 was generated with ustiloxin B—ovalbumin conjugate. A highly-sensitive and specific indirect competitive enzyme-linked immunosorbent assay (icELISA) was then developed. The median inhibitory concentration (IC₅₀) of the icELISA was 18.0 ng/mL for the detection of ustiloxin B; the limit of detection was 0.6 ng/mL, and the calibration range was from 2.5 to 107.4 ng/mL. The LOD/LOQ values of the developed ELISA used for the determination of ustiloxin B in rice false smut balls and rice grains were 12/50 μg/g and 30/125 ng/g, respectively. The mAb 1B5A10 cross-reacted with ustiloxin A at 13.9% relative to ustiloxin B. Average recoveries of ustiloxin B ranged from 91.3% to 105.1% for rice false smut balls at spiking levels of 0.2 to 3.2 mg/g and from 92.6% to 103.5% for rice grains at spiking levels of 100 to 5000 ng/g. Comparison of ustiloxin B content in rice false smut balls and rice grains detected by both icELISA and high performance liquid chromatography (HPLC) demonstrated that the developed icELISA can be employed as an effective and accurate method for the detection of ustiloxin B in rice false smut balls, as well as rice food and feed samples.

Infection of Ustilaginoidea virens intercepts rice seed formation but activates grain-filling-related genes

Rice false smut has become an increasingly serious disease in rice (Oryza sativa L.) production worldwide. The typical feature of this disease is that the fungal pathogen Ustilaginoidea virens (Uv) specifically infects rice flower and forms false smut ball, the ustiloxin-containing ball-like fungal colony, of which the size is usually several times larger than that of a mature rice seed. However, the underlying mechanisms of Uv-rice interaction are poorly understood. Here, we applied time-course microscopic and transcriptional approaches to investigate rice responses to Uv infection. The results demonstrated that the flower-opening process and expression of associated transcription factors, including ARF6 and ARF8, were inhibited in Uv-infected spikelets. The ovaries in infected spikelets were interrupted in fertilization and thus were unable to set seeds. However, a number of grain-filling-related genes, including seed storage protein genes, starch anabolism genes and endosperm-specific transcription factors (RISBZ1 and RPBF), were highly transcribed as if the ovaries were fertilized. In addition, critical defense-related genes like NPR1 and PR1 were downregulated by Uv infection. Our data imply that Uv may hijack host nutrient reservoir by activation of the grain-filling network because of growth and formation of false smut balls.

Genetic diversity analysis reveals that geographical environment plays a more important role than rice cultivar in Villosiclava virens population selection

Rice false smut caused by Villosiclava virens is an economically important disease of grains worldwide. The genetic diversity of 153 isolates from six fields located in Wuhan (WH), Yichang Wangjia (YCW), Yichang Yaohe (YCY), Huanggang (HG), Yangxin (YX), and Jingzhou (JZ) in Hubei province of China were phylogenetically analyzed to evaluate the influence of environments and rice cultivars on the V. virens populations. Isolates (43) from Wuhan were from two rice cultivars, Wanxian 98 and Huajing 952, while most of the other isolates from fields YCW, YCY, HG, YX, and JZ originated from different rice cultivars with different genetic backgrounds. Genetic diversity of isolates was analyzed using random amplified polymorphic DNA (RAPD) and single-nucleotide polymorphisms (SNP). The isolates from the same cultivars in Wuhan tended to group together, indicating that the cultivars had an important impact on the fungal population. The 110 isolates from individual fields tended to cluster according to geographical origin. The values of Nei's gene diversity (H) and Shannon's information index (I) showed that the genetic diversity among isolates was higher between than within geographical populations. Furthermore, mean genetic distance between groups (0.006) was higher than mean genetic distance within groups (0.0048) according to MEGA 5.2. The pairwise population fixation index (FST) values also showed significant genetic differentiation between most populations. Higher genetic similarity of isolates from individual fields but different rice cultivars suggested that the geographical factor played a more important role in the selection of V. virens isolates than rice cultivars. This information could be used to improve the management strategy for rice false smut by adjusting the cultivation measures, such as controlling fertilizer, water, and planting density, in the rice field to change the microenvironment.

Determination and analysis of ustiloxins A and B by LC-ESI-MS and HPLC in false smut balls of rice

Ustiloxins are cyclopeptide mycotoxins produced by the pathogenic fungus Villosiclava virens of rice false smut. Ustiloxins A and B as two main mycotoxins were determined conveniently by LC-ESI-MS in the water extract from rice false smut balls which were mostly composed of the chlamydospores and mycelia of the pathogen. Both ustiloxins A and B in the water extract were also quantitatively analyzed by HPLC. This is the first report on the determination and analysis of ustiloxins A and B simultaneously by LC-ESI-MS and HPLC in false smut balls of rice.

Interactions of antimitotic peptides and depsipeptides with tubulin

Tubulin is the target for an ever increasing number of structurally unusual peptides and depsipeptides isolated from a wide range of organisms. Since tubulin is the subunit protein of microtubules, the compounds are usually potently toxic to mammalian cells. Without exception, these (depsi)peptides disrupt cellular microtubules and prevent spindle formation. This causes cells to accumulate at the G2/M phase of the cell cycle through inhibition of mitosis. In biochemical assays, the compounds inhibit microtubule assembly from tubulin and suppress microtubule dynamics at low concentrations. Most of the (depsi)peptides inhibit the binding of Catharanthus alkaloids to tubulin in a noncompetitive manner, GTP hydrolysis by tubulin, and nucleotide turnover at the exchangeable GTP site on beta-tubulin. In general, the (depsi)peptides induce the formation of tubulin oligomers of aberrant morphology. In all cases tubulin rings appear to be formed, but these rings differ in diameter, depending on the (depsi)peptide present during their formation.