Bioactive bis-naphtho-γ-pyrones from rice false Smut pathogen Ustilaginoidea virens

Selective biotic stressors' action on seed germination: A review

In the realm of plant biology and agriculture, seed germination serves as a fundamental process with far-reaching implications for crop production and environmental health. This comprehensive review seeks to unravel the intricate web of interactions between some biotic stressors and seed germination, addressing the pertinent issue of how these stressors influence seed germination. Different chemicals produced by interacting plants (different parts), fungi, bacteria, or insects can either promote or inhibit seed germination. Releasing chemicals that modulate signaling pathways and cellular processes significantly disrupt essential cellular functions. This disruption leads to diverse germination outcomes, introducing additional layers of complexity to this regulatory landscape. The chemicals perturb enzyme activity and membrane integrity, imposing unique challenges on the germination process. Understanding the mechanisms- how allelochemicals, mycotoxins, or bacterial toxins affect seed germination or the modes of action holds promise for more sustainable agricultural practices, enhanced pest control, and improved environmental outcomes. In sum, this review contributes to a fundamental exposition of the pivotal role of biotic stressors in shaping the germination of seeds.

Co-infection of Four Novel Mycoviruses from Three Lineages Confers Hypovirulence on Phytopathogenic Fungus Ustilaginoidea virens

Rice false smut caused by Ustilaginoidea virens has become one of the most important diseases of rice. Mycoviruses are viruses that can infect fungi with the potential to control fungal diseases. However, little is known about the biocontrol role of hypoviruses in U. virens. In this study, we revealed that the hypovirulence-associated U. virens strain Uv325 was co-infected by four novel mycoviruses from three lineages, designated Ustilaginoidea virens RNA virus 16 (UvRV16), Ustilaginoidea virens botourmiavirus virus 8 (UvBV8), Ustilaginoidea virens botourmiavirus virus 9 (UvBV9), and Ustilaginoidea virens narnavirus virus 13 (UvNV13), respectively. The U. virens strain co-infected by four mycoviruses showed slower growth rates, reduced conidial yield, and attenuated pigmentation. We demonstrated that UvRV16 was not only the major factor responsible for the hypovirulent phenotype in U. vriens, but also able to prevent U. virens to accumulate more mycotoxin, thereby weakening the inhibitory effects on rice seed germination and seedling growth. Additionally, we indicated that UvRV16 can disrupt the antiviral response of U. virens by suppressing the transcriptional expression of multiple genes involved in autophagy and RNA silencing. In conclusion, our study provided new insights into the biological control of rice false smut.

Rosellichalasins A-H, cytotoxic cytochalasans from the endophytic fungus Rosellinia sp. Glinf021

Eight new cytochalasans rosellichalasins A-H (1-8), as well as two new shunt metabolites rosellinins A (9) and B (10) before intramolecular Diels-Alder cycloaddition reaction in cytochalasan biosynthesis, along with nine known cytochalsans (11-19) were isolated from the endophytic fungus Rosellinia sp. Glinf021, which was derived from the medicinal plant Glycyrrhiza inflata. Their structures were characterized by extensive analysis of 1D and 2D NMR as well as HRESIMS spectra and quantum chemical ECD calculations. The cytotoxic activities of these compounds were evaluated against four human cancer cell lines including HCT116, MDA-MB-231, BGC823, and PANC-1 with IC50 values ranging from 0.5 to 58.2 μM.

Rice false smut pathogen: implications for mycotoxin contamination, current status, and future perspectives

Rice serves as a staple food across various continents worldwide. The rice plant faces significant threats from a range of fungal, bacterial, and viral pathogens. Among these, rice false smut disease (RFS) caused by Villosiclava virens is one of the devastating diseases in rice fields. This disease is widespread in major rice-growing regions such as China, Pakistan, Bangladesh, India, and others, leading to significant losses in rice plantations. Various toxins are produced during the infection of this disease in rice plants, impacting the fertilization process as well. This review paper lightens the disease cycle, plant immunity, and infection process during RFS. Mycotoxin production in RFS affects rice plants in multiple ways, although the exact phenomena are still unknown.

UvHOS3-mediated histone deacetylation is essential for virulence and negatively regulates ustilaginoidin biosynthesis in Ustilaginoidea virens

Ustilaginoidea virens is the causal agent of rice false smut, which has recently become one of the most important rice diseases worldwide. Ustilaginoidins, a major type of mycotoxins produced in false smut balls, greatly deteriorates grain quality. Histone acetylation and deacetylation are involved in regulating secondary metabolism in fungi. However, little is yet known on the functions of histone deacetylases (HDACs) in virulence and mycotoxin biosynthesis in U. virens. Here, we characterized the functions of the HDAC UvHOS3 in U. virens. The ΔUvhos3 deletion mutant exhibited the phenotypes of retarded growth, increased mycelial branches and reduced conidiation and virulence. The ΔUvhos3 mutants were more sensitive to sorbitol, sodium dodecyl sulphate and oxidative stress/H2 O2 . ΔUvhos3 generated significantly more ustilaginoidins. RNA-Seq and metabolomics analyses also revealed that UvHOS3 is a key negative player in regulating secondary metabolism, especially mycotoxin biosynthesis. Notably, UvHOS3 mediates deacetylation of H3 and H4 at H3K9, H3K18, H3K27 and H4K8 residues. Chromatin immunoprecipitation assays indicated that UvHOS3 regulates mycotoxin biosynthesis, particularly for ustilaginoidin and sorbicillinoid production, by modulating the acetylation level of H3K18. Collectively, this study deepens the understanding of molecular mechanisms of the HDAC UvHOS3 in regulating virulence and mycotoxin biosynthesis in phytopathogenic fungi.

Activation of Ustilaginoidin Biosynthesis Gene uvpks1 in Villosiclava virens Albino Strain LN02 Influences Development, Stress Responses, and Inhibition of Rice Seed Germination

Villosiclava virens (anamorph: Ustilaginoidea virens) is the pathogen of rice false smut (RFS), which is a destructive rice fungal disease. The albino strain LN02 is a natural white-phenotype mutant of V. virens due to its incapability to produce toxic ustilaginoidins. In this study, three strains including the normal strain P1, albino strain LN02, and complemented strain uvpks1C-1 of the LN02 strain were employed to investigate the activation of the ustilaginoidin biosynthesis gene uvpks1 in the albino strain LN02 to influence sporulation, conidia germination, pigment production, stress responses, and the inhibition of rice seed germination. The activation of the ustilaginoidin biosynthesis gene uvpks1 increased fungal tolerances to NaCl-induced osmotic stress, Congo-red-induced cell wall stress, SDS-induced cell membrane stress, and H2O2-induced oxidative stress. The activation of uvpks1 also increased sporulation, conidia germination, pigment production, and the inhibition of rice seed germination. In addition, the activation of uvpks1 was able to increase the mycelial growth of the V. virens albino strain LN02 at 23 °C and a pH from 5.5 to 7.5. The findings help in understanding the effects of the activation of uvpks1 in albino strain LN02 on development, pigment production, stress responses, and the inhibition of rice seed germination by controlling ustilaginoidin biosynthesis.

Design, Synthesis, Structure-Activity Relationship, and Three-Dimensional Quantitative Structure-Activity Relationship of Fusarium Acid Derivatives and Analogues as Potential Fungicides

In research related to fungicides, the development of compounds from natural products with high antifungal activity has attracted considerable attention. Fusaric acid (FA), an alkaloid isolated from the metabolites of Fusarium oxysporum, is an important precursor for developing pharmacologically active herbicides. In our previous work, we reported that FA has a wide range of inhibitory activities against 14 plant pathogenic fungi. In particular, it exhibited excellent antifugal effects on Colletotrichum higginsianum (EC50 = 31.7 μg/mL). Herein, to explore the practical application in the agricultural field, the design and synthesis of three series of FA derivatives and their inhibitory activities against plant pathogenic fungi were examined. Results demonstrated that the optimized FA derivatives had excellent inhibitory activities against C. higginsianum, Helminthosporium (Harpophora maydis), and Pyricularia grisea. In particular, the inhibitory activities were considerably improved when the 5-butyl groups of FA were substituted. The EC50 of C. higginsianum and P. grisea was only 1.2 and 12.0 μg/mL when 5-butylalkyl groups were substituted with 5-([1,1'-biphenyl]-4-yl) and 5-phenyl, respectively. Moreover, the safety index of target compounds, which was obtained from the treatment index of medicines, on rice seeds was evaluated. Finally, 16 leading compounds (H4, H22-H24, H27, H29, H30-H34, H37, H45, H50, H52, and H53) were obtained; they had considerable potential for additional modification and optimization as agricultural fungicides. Moreover, three-dimensional quantitative structure-activity relationship models were developed for obtaining a systematic structure-activity relationship profile to explore the possibility of more potent FA derivatives as novel fungicides.

Rosellosides A and B, two phenyloxazole glycosides from Glycyrrhiza inflata-derived fungus Rosellinia sp. Glinf021

Two new chlorinated phenyloxazole glycosides, named rosellosides A (1) and B (2), were isolated from the endophytic fungus Rosellinia sp. Glinf021, which was derived from the medicinal plant Glycyrrhiza inflata (Leguminosae). Both compounds were rare chlorinated polyketide glycosides bearing an oxazole moiety. Their structures were elucidated by analysis of the NMR and HRESIMS data, and their absolute configurations were determined by quantum chemical ECD calculations and X-ray crystallography.

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.

Diphenyl Ether Derivative Rhexocerins and Rhexocercosporins from the Endophytic Fungus Rhexocercosporidium sp. Dzf14 Active against Gram-Positive Bacteria with Multidrug-Resistance

Ten new diphenyl ether polyketides, including rhexocerins A-D (1-4) and rhexocercosporins A-F (5-10), together with three known congeners (11-13), were isolated from the endophytic fungus Rhexocercosporidium sp. Dzf14 obtained from Dioscorea zingiberensis. Their structures were elucidated by analysis of NMR and HRESIMS data, and their absolute configurations were determined by quantum chemical ECD calculations and X-ray crystallography. Compounds 1-4 featured an unprecedented tetracyclic carbon skeleton (6/7/5/6). Among them, compounds 1 and 5-9 showed antibacterial activities against methicillin-resistant S. aureus T144 and vancomycin-resistant E. faecalis 10.

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.

Cytotoxic cytochalasans from cultures of the fungus Metarhizium brunneum TBRC-BCC 79240

Fourteen new cytochalasans, brunnesins A-N (1-14), along with eleven known compounds, were isolated from the culture extracts of the insect pathogenic fungus Metarhizium brunneum strain TBRC-BCC 79240. The compound structures were established by spectroscopy, X-ray diffraction analysis, and electronic circular dichroism. Compound 4 exhibited antiproliferative activity against all cell lines tested (mammalian), with 50% inhibition concentration (IC₅₀) values ranging from 2.09 to 16.8 μg mL^-1. Compounds 6 and 16 were shown to be bioactive only against non-cancerous Vero cells (IC₅₀ 4.03 and 0.637 μg mL^-1, respectively) whereas compounds 9 and 12 were bioactive only against NCI-H187 small-cell lung cancer cells (IC₅₀ 18.59 and 18.54 μg mL^-1, respectively). Compounds 7, 13, and 14 showed cytotoxicity against NCI-H187 and Vero cell lines with IC₅₀ values ranging from 3.98-44.81 μg mL^-1.

Effects of Carbon, Nitrogen, Ambient pH and Light on Mycelial Growth, Sporulation, Sorbicillinoid Biosynthesis and Related Gene Expression in Ustilaginoidea virens

Sorbicillinoids are a class of hexaketide metabolites produced by Ustilaginoidea virens (teleomorph: Villosiclava virens), an important fungal pathogen that causes a devastating rice disease. In this study, we investigated the effects of environmental factors, including carbon and nitrogen sources, ambient pH and light exposure, on mycelial growth, sporulation, as well as the accumulation of sorbicillinoids, and the expression of related genes involved in sorbicillinoid biosynthesis. It was found that the environmental factors had great influences on mycelial growth and sporulation of U. virens. Fructose and glucose, complex nitrogen sources, acidic conditions and light exposure were favorable for sorbicillinoid production. The relative transcript levels of sorbicillinoid biosynthesis genes were up-regulated when U. virens was separately treated with those environmental factors that favored sorbicillinoid production, indicating that sorbicillinoid biosynthesis was mainly regulated at the transcriptional level by different environmental factors. Two pathway-specific transcription factor genes, UvSorR1 and UvSorR2, were found to participate in the regulation of sorbicillinoid biosynthesis. These results will provide useful information to better understand the regulation mechanisms of sorbicillinoid biosynthesis, and be conducive to develop effective means for controlling sorbicillinoid production in U. virens.

Genome Mining and Heterologous Expression Guided the Discovery of Antimicrobial Naphthocyclinones from Streptomyces eurocidicus CGMCC 4.1086

A type II polyketide synthase biosynthetic gene cluster (nap) was identified in Streptomyces eurocidicus CGMCC 4.1086 via genome mining. The heterologous expression of the cryptic nap gene cluster in Streptomyces albus J1074 generated dimerized aromatic polyketide naphthocyclinones (1-3), whose structures were determined via extensive analysis using nuclear magnetic resonance and high-resolution electrospray ionization mass spectroscopy. The biological pathway of naphthocyclinone synthesis was revealed via in vivo gene deletion, in vitro biochemical reactions, and comparative genomics. Remarkably, 3 played a crucial role in inhibiting Phytophthora capsici and Phytophthora sojae, with EC₅₀ values of 6.1 and 20.2 μg/mL, respectively. Furthermore, 3 exhibited a potent protective effect against P. capsici and P. sojae in greenhouse tests.

Chemical Investigation of Endophytic Diaporthe unshiuensis YSP3 Reveals New Antibacterial and Cytotoxic Agents

Chemical investigation of the plant-derived endophytic fungus Diaporthe unshiuensis YSP3 led to the isolation of four new compounds (1-4), including two new xanthones (phomopthane A and B, 1 and 2), one new alternariol methyl ether derivative (3) and one α-pyrone derivative (phomopyrone B, 4), together with eight known compounds (5-12). The structures of new compounds were interpreted on the basis of spectroscopic data and single-crystal X-ray diffraction analysis. All new compounds were assessed for their antimicrobial and cytotoxic potential. Compound 1 showed cytotoxic activity against HeLa and MCF-7 cells with IC₅₀ values of 5.92 µM and 7.50 µM, respectively, while compound 3 has an antibacterial effect on Bacillus subtilis (MIC value 16 μg/mL).

Antifungal mechanisms of silver nanoparticles on mycotoxin producing rice false smut fungus

Ustilaginoidea virens, which causes rice false smut disease, is a destructive filamentous fungal pathogen, attracting more attention to search for effective fungicides against U. virens. Here, the results showed that the inhibition of 2 nm AgNPs on U. virens growth and virulence displayed concentration-dependent manner. Abnormalities of fungal morphology were observed upon exposure to AgNPs. RNA-sequencing (RNA-seq) analysis revealed that AgNPs treatment up-regulated 1185 genes and down-regulated 937 genes, which significantly overlapped with the methyltransferase UvKmt6-regulated genes. Furthermore, we found that AgNPs reduced the UvKmt6-mediated H3K27me3 modification, resulting in the up-regulation of ustilaginoidin biosynthetic genes The decrease of H3K27me3 level was associated with the inhibition of mycelial growth by AgNPs treatment. These results suggested that AgNPs are an effective nano-fungicide for the control of rice false smut disease, but when using AgNPs, it needs to be combined with mycotoxin-reducing fungicides to reduce the risk of toxin pollution.

A Nanobody-Based Immunoassay for Detection of Ustilaginoidins in Rice Samples

Ustilaginoidins are a class of bis-naphtho-γ-pyrone mycotoxins produced by the pathogen Villosiclava virens of rice false smut, which has recently become one of the most devastating diseases in rice-growing regions worldwide. In this research, the nanobody phage display library was established after an alpaca was immunized with the hemiustilaginoidin F-hapten coupled with bovine serum albumin (BSA). Heterologous antigen selection and combing trypsin with competition alternant elution methods were performed for nanobody screening. Two nanobodies, namely, Nb-B15 and Nb–C21, were selected for the establishment of indirect competitive enzyme-linked immunosorbent assays (ic-ELISAs). For Nb–B15 and Nb-C21, their IC₅₀ values were 11.86 μg/mL and 11.22 μg/mL, and the detection ranges were at 3.41–19.98 μg/mL and 1.17–32.13 μg/mL, respectively. Two nanobodies had a broad spectrum to quantify the contents of total ustilaginoidins in rice samples according to cross-reactivity. The recognition mechanisms of Nb-B15 and Nb-C21 against ustilaginoidin A were elucidated by molecular modeling and docking. The key amino acid sites for the binding of Nb–B15 or Nb–C21 to ustilaginoidin A were mainly located in the FR1 and CDR1 regions. As Nb-B15 was superior to Nb–C21 in the aspects of protein expression, ELISA titer, and tolerance to organic solvents, it was selected for application in the detection of actual contaminated rice samples. The total ustilaginoidin contents of rice samples were analyzed by Nb–B15-based ic–ELISA and HPLC-DAD, between which the results were found to be consistent. The developed immunoassay based on the nanobody from the alpaca can be employed as a rapid and effective method for detection of total utilaginoidins in contaminated rice samples.

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.

Orchestrated Biosynthesis of the Secondary Metabolite Cocktails Enables the Producing Fungus to Combat Diverse Bacteria

Fungal secondary metabolites with antibiotic activities can promote fungal adaptation to diverse environments. Besides the global regulator, individual biosynthetic gene clusters (BGCs) usually contain a pathway-specific transcription factor for the tight regulation of fungal secondary metabolism. Here, we report the chemical biology mediated by a supercluster containing three BGCs in the entomopathogenic fungus Metarhiziumrobertsii. These clusters are jointly controlled by an embedded transcription factor that orchestrates the collective production of four classes of chemicals: ustilaginoidin, indigotide, pseurotin, and hydroxyl-ovalicin. The ustilaginoidin BGC is implicated as a late-acquired cluster in Metarhizium to produce both the bis-naphtho-γ-pyrones and the monomeric naphtho-γ-pyrone glycosides (i.e., indigotides). We found that the biosynthesis of indigotides additionally requires the functions of paired methylglucosylation genes located outside the supercluster. The pseurotin/ovalicin BGCs are blended and mesosyntenically conserved to the intertwined pseurotin/fumagillin BGCs of Aspergillus fumigatus. However, the former have lost a few genes, including a polyketide synthase gene responsible for the production of a pentaene chain used for assembly with ovalicin to form fumagillin, as observed in A. fumigatus. The collective production of chemical cocktails by this supercluster was dispensable for fungal virulence against insects and could enable the fungus to combat different bacteria better than the metabolite(s) produced by an individual BGC could. Thus, our results unveil a novel strategy employed by fungi to manage chemical ecology against diverse bacteria.

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.

Antifungal Mechanism of MTE-1, a Novel Oligosaccharide Ester, against Ustilaginoidea virens

Ustilaginoidea virens is a pathogenic fungus that causes false smut disease in rice during the flowering stage through stamen filaments. Currently, there is a need to develop safe and effective antifungal agents for the control of this disease. In our preliminary experiments, we found that MTE-1, a new trisaccharide ester, exhibits significant inhibitory activity against U. virens. Hence, the effects and inhibitory mechanism of MTE-1 in U. virens were investigated. Results showed that the MTE-1 inhibited the hyphae growth of U. virens with an IC₅₀ of 5.67 μg/mL. Similarly, MTE-1 disrupted the endomembrane system in U. virens, especially the plasma membrane, mitochondria, and lipidosome. Moreover, transcriptome and proteome analysis indicated that MTE-1 inhibited the growth of U. virens by inhibiting the synthesis of lipids, altering the primary metabolic pathways including carbohydrates and amino acid metabolism, and affecting the intracellular redox dyshomeostasis, thus leading to the disorder of active oxygen metabolism. These findings lay the foundation for the future application of MTE-1-derived agents in the management of antifungal diseases.

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.

Induction of Isochromanones by Co-Cultivation of the Marine Fungus Cosmospora sp. and the Phytopathogen Magnaporthe&nbsp;oryzae

Microbial co-cultivation is a promising approach for the activation of biosynthetic gene clusters (BGCs) that remain transcriptionally silent under artificial culture conditions. As part of our project aiming at the discovery of marine-derived fungal agrochemicals, we previously used four phytopathogens as model competitors in the co-cultivation of 21 marine fungal strains. Based on comparative untargeted metabolomics analyses and anti-phytopathogenic activities of the co-cultures, we selected the co-culture of marine Cosmospora sp. with the phytopathogen Magnaporthe oryzae for in-depth chemical studies. UPLC-MS/MS-based molecular networking (MN) of the co-culture extract revealed an enhanced diversity of compounds in several molecular families, including isochromanones, specifically induced in the co-culture. Large scale co-cultivation of Cosmospora sp. and M. oryzae resulted in the isolation of five isochromanones from the whole co-culture extract, namely the known soudanones A, E, D (1-3) and their two new derivatives, soudanones H-I (4-5), the known isochromans, pseudoanguillosporins A and B (6, 7), naphtho-γ-pyrones, cephalochromin and ustilaginoidin G (8, 9), and ergosterol (10). Their structures were established by NMR, HR-ESIMS, FT-IR, electronic circular dichroism (ECD) spectroscopy, polarimetry ([α]D), and Mosher's ester reaction. Bioactivity assays revealed antimicrobial activity of compounds 2 and 3 against the phytopathogens M. oryzae and Phytophthora infestans, while pseudoanguillosporin A (6) showed the broadest and strongest anti-phytopathogenic activity against Pseudomonas syringae, Xanthomonas campestris, M. oryzae and P. infestans. This is the first study assessing the anti-phytopathogenic activities of soudanones.

Recent Advances in Sorbicillinoids from Fungi and Their Bioactivities (Covering 2016–2021)

Sorbicillinoids are a family of hexaketide metabolites with a characteristic sorbyl side chain residue. Sixty-nine sorbicillinoids from fungi, newly identified from 2016 to 2021, are summarized in this review, including their structures and bioactivities. They are classified into monomeric, dimeric, trimeric, and hybrid sorbicillinoids according to their basic structural features, with the main groups comprising both monomeric and dimeric sorbicillinoids. Some of the identified sorbicillinoids have special structures such as ustilobisorbicillinol A, and sorbicillasins A and B. The majority of sorbicillinoids have been reported from fungi genera such as Acremonium, Penicillium, Trichoderma, and Ustilaginoidea, with some sorbicillinoids exhibiting cytotoxic, antimicrobial, anti-inflammatory, phytotoxic, and α-glucosidase inhibitory activities. In recent years, marine-derived, extremophilic, plant endophytic, and phytopathogenic fungi have emerged as important resources for diverse sorbicillinoids with unique skeletons. The recently revealed biological activities of sorbicillinoids discovered before 2016 are also described in this review.

Elucidation of ustilaginoidin biosynthesis reveals a previously unrecognised class of ene-reductases

Ustilaginoidins are a type of mycotoxin featuring a dimeric naphtho-γ-pyrone skeleton, produced by the rice false smut pathogen Ustilaginoidea virens. Here we used gene disruption, heterologous expression in Aspergillus oryzae, feeding experiments, and in vitro experiments to fully elucidate the biosynthesis of ustilaginoidins. A new route to dimeric 2,3-unsaturated naphtho-γ-pyrones via dimerization of YWA1 (and 3-methyl YWA1) followed by dehydration was discovered. Intriguingly, the reduction of the 2,3-double bond of the pyrenone ring was catalyzed by a phospholipid methyltransferase-like enzyme (UsgR). The reductase was specific for reduction of monomeric, linear naphtho-γ-pyrenones, but not for the dimers. Atroposelective coupling of various monomers by the laccase (UsgL) led to diverse ustilaginoidins. Moreover, 3-epimerism of the 3-methyl-2,3-dihydro-naphtho-γ-pyrones adds additional complexity to the biosynthesis.

The 2,3-double bond reduction of the pyrenone ring in linear naphtho-γ-pyrenones was catalyzed by a phospholipid methyltransferase-like enzyme, namely UsgR, which is a previously unrecognised class of ene-reductases.

Novel metabolites from the Cercis chinensis derived endophytic fungus Alternaria alternata ZHJG5 and their antibacterial activities

BACKGROUND

Phytopathogenic bacteria, such as Xanthomonas oryzae pv. oryzae (Xoo) and Ralstonia solanacearum (Rs), seriously threaten crop production and are the cause of enormous yield losses. Endophytic fungi are abundant sources of bioactive metabolites that may be potential candidates in the development of new agrochemicals. This work emphasizes the discovery of bioactive polyketides from endophytic Alternaria alternata ZHJG5 and reports their structural elucidation and antibacterial activities in detail.

RESULTS

Five novel polyketide derivatives, isotalaroflavone (2), (+/-)-5'-dehydroxytalaroflavone (3a/3b), (+)-talaroflavone (4b), and bialternacin G (7), along with five known compounds (1, 4a, 5, 6, and 8), were obtained from the Cercis chinensis-derived fungus A. alternata ZHJG5. The compounds' structures were characterized using spectroscopic methods and X-ray diffraction. Chiral high-performance liquid chromatography was used to separate the racemates 3 and 4, whose absolute configurations were unambiguously confirmed by comparing their experimental and calculated electron circular dichroism data. All isolated compounds were tested for antibacterial activity against the phytopathogenic bacteria Xoo, Xanthomonas oryzae pv. oryzicola (Xoc) and Rs. Compounds 1, 2 and 8 showed pronounced antibacterial activity against all tested bacteria, with minimal inhibitory concentrations ranging from 0.5 to 64 μg/ml. In addition, compound 1 showed a potent protective effect against rice bacterial leaf blight caused by Xoo with a protective efficacy of 75.1% at a concentration of 200 μg/ml.

CONCLUSION

These findings highlight the practical potential of antibacterial compounds as candidates for the discovery of novel bactericides. © 2021 Society of Chemical Industry.

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.

Quantitative Proteomics Analysis Reveals the Function of the Putative Ester Cyclase UvEC1 in the Pathogenicity of the Rice False Smut Fungus Ustilaginoidea virens

Rice false smut is a fungal disease distributed worldwide and caused by Ustilaginoidea virens. In this study, we identified a putative ester cyclase (named as UvEC1) as being significantly upregulated during U. virens infection. UvEC1 contained a SnoaL-like polyketide cyclase domain, but the functions of ketone cyclases such as SnoaL in plant fungal pathogens remain unclear. Deletion of UvEC1 caused defects in vegetative growth and conidiation. UvEC1 was also required for response to hyperosmotic and oxidative stresses and for maintenance of cell wall integrity. Importantly, ΔUvEC1 mutants exhibited reduced virulence. We performed a tandem mass tag (TMT)-based quantitative proteomic analysis to identify differentially accumulating proteins (DAPs) between the ΔUvEC1-1 mutant and the wild-type isolate HWD-2. Proteomics data revealed that UvEC1 has a variety of effects on metabolism, protein localization, catalytic activity, binding, toxin biosynthesis and the spliceosome. Taken together, our findings suggest that UvEC1 is critical for the development and virulence of U. virens.

Phytotoxic Secondary Metabolites from Fungi

Fungal phytotoxic secondary metabolites are poisonous substances to plants produced by fungi through naturally occurring biochemical reactions. These metabolites exhibit a high level of diversity in their properties, such as structures, phytotoxic activities, and modes of toxicity. They are mainly isolated from phytopathogenic fungal species in the genera of Alternaria, Botrytis, Colletotrichum, Fusarium, Helminthosporium, and Phoma. Phytotoxins are either host specific or non-host specific phytotoxins. Up to now, at least 545 fungal phytotoxic secondary metabolites, including 207 polyketides, 46 phenols and phenolic acids, 135 terpenoids, 146 nitrogen-containing metabolites, and 11 others, have been reported. Among them, aromatic polyketides and sesquiterpenoids are the main phytotoxic compounds. This review summarizes their chemical structures, sources, and phytotoxic activities. We also discuss their phytotoxic mechanisms and structure-activity relationships to lay the foundation for the future development and application of these promising metabolites as herbicides.

Enantiomeric Dibenzo-α-Pyrone Derivatives from Alternaria alternata ZHJG5 and Their Potential as Agrochemicals

Three pairs of enantiomeric dibenzo-α-pyrone derivatives (1-3) including two pairs of new racemates (±)-alternaone A (1) and (±)-alternaone B (2) and one new enantiomer (-)-alternatiol (3), together with five known compounds (4-8) were isolated from the fungus Alternaria alternata ZHJG5. Their structures were confirmed by spectroscopic data and single-crystal X-ray diffraction analysis. All enantiomers were separated via chiral high-performance liquid chromatography, with their configurations determined by electronic circular dichroism calculation. Biogenetically, a key epoxy-rearrangement step was proposed for the formation of skeletons in 1-3; (+) 1, (-)-1, and 5 presented moderate antibacterial inhibition on phytopathogenic bacteria Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola. In the antifungal test, compounds 7 and 8 showed a moderate protective effect against Botrytis cinerea in vivo.

Discovery of Natural FabH Inhibitors Using an Immobilized Enzyme Column and Their Antibacterial Activity against Xanthomonas oryzae pv. oryzae

β-Ketoacyl-acyl carrier protein synthase III (KAS III, FabH) is essential for bacterial fatty acid biosynthesis. Recent studies indicate that FabH can be a potential target for bactericide development. In the present study, an immobilized FabH column was developed and used to screen FabH inhibitors from complex natural product extracts. Combined with HPLC, four secondary metabolites, alternariol (1), altenuisol (2), alterlactone (3), and dehydroaltenusin (4), were site-directed, isolated, and identified from the crude extract of Alternaria alternata ZHJG5. These compounds showed inhibitory activities on FabH of Xanthomonas oryzae pv. oryzae (Xoo) with IC50 values from 29.5 to 74.1 μM and also displayed a varying degree of antibacterial activities against Xoo with minimal inhibitory concentration values from 4 to 64 μg/mL. Molecular modeling was then used to picture how the compounds interact with XooFabH. Two inhibitors, compounds 1 and 3, exhibited significant bactericidal activity against rice bacterial leaf blight with a protective efficiency of 66.2 and 82.5% at the concentration of 200 μg/mL, respectively, suggesting that they could be lead candidates to develop novel bactericides.

Eremophilane-Type Sesquiterpenoids From the Endophytic Fungus Rhizopycnis vagum and Their Antibacterial, Cytotoxic, and Phytotoxic Activities

Fourteen new eremophilane-type sesquiterpenoids, named rhizoperemophilanes A~N (1~14), together with eight known congeners, were isolated from the culture of the endophytic fungus Rhizopycnis vagum. The structures of the new compounds were elucidated by extensive spectroscopic analyses, as well as ECD calculations and the modified Mosher's method for the assignment of the absolute configurations. Rhizoperemophilane J (10) contains an uncommon C-4/C-11 epoxy ring, while rhizoperemophilane N (14) features an unprecedented 3-nor-eremophilane lactone-lactam skeleton. These metabolites were evaluated for their antibacterial, cytotoxic, and phytotoxic activities. Among them, compounds 11, 16, and 20 displayed antibacterial activities, while 14 showed selective cytotoxicity against NCI-H1650 and BGC823 tumor cells. Moreover, compounds 5, 6, 12, 13, 16, and 19 exhibited strong phytotoxic activities against the radicle elongation of rice seedlings.

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.

UvAtg8-Mediated Autophagy Regulates Fungal Growth, Stress Responses, Conidiation, and Pathogenesis in Ustilaginoidea virens

BACKGROUND

Ustilaginoidea virens has become one of the most devastating rice pathogens in China, as well as other rice-growing areas. Autophagy is an important process in normal cell differentiation and development among various organisms. To date, there has been no optimized experimental system introduced for the study of autophagy in U. virens. In addition, the function of autophagy in pathogenesis remains unknown in U. virens. Therefore, the functional analyses of UvAtg8 may potentially shed some light on the regulatory mechanism and function of autophagy in U. virens.

RESULTS

In this study, we characterized the functions of UvAtg8, which is a homolog of Saccharomyces cerevisiae ScAtg8, in the rice false smut fungus U. virens. The results showed that UvATG8 is essential for autophagy in U. virens. Also, the GFP-UvATG8 strain, which could serve as an appropriate marker for monitoring autophagy in U. virens, was generated. Furthermore, this study found that the ΔUvatg8 mutant was defective in the vegetative growth, conidiation, adaption to oxidative, hyperosmotic, cell wall stresses, and production of toxic compounds. Pathogenicity assays indicated that deletion of UvATG8 resulted in significant reduction in virulence of U. virens. Further microscopic examinations of the infection processes revealed that the severe virulence defects in the ∆Uvatg8 were mainly caused by the highly reduced conidiation and secondary spore formation.

CONCLUSIONS

Our results indicated that the UvAtg8 is necessary for the fungal growth, stresses responses, conidiation, secondary spore formation, and pathogenicity of U. virens. Moreover, our research finding will potentially assist in further clarifying the molecular mechanism of U. virens infection, as well as provide a good marker for autophagy in U. virens and a good reference value for the further development of effective fungicides based on gene targeting.

Bioactive Metabolites and Potential Mycotoxins Produced by Cordyceps Fungi: A Review of Safety

Ascomycete Cordyceps fungi such as C. militaris, C. cicadae, and C. guangdongensis have been mass produced on artificial media either as food supplements or health additives while the byproducts of culture substrates are largely used as animal feed. The safety concerns associated with the daily consumption of Cordyceps fungi or related products are still being debated. On the one hand, the known compounds from these fungi such as adenosine analogs cordycepin and pentostatin have demonstrated different beneficial or pharmaceutical activities but also dose-dependent cytotoxicities, neurological toxicities and or toxicological effects in humans and animals. On the other hand, the possibility of mycotoxin production by Cordyceps fungi has not been completely ruled out. In contrast to a few metabolites identified, an array of biosynthetic gene clusters (BGCs) are encoded in each genome of these fungi with the potential to produce a plethora of as yet unknown secondary metabolites. Conservation analysis of BGCs suggests that mycotoxin analogs of PR-toxin and trichothecenes might be produced by Cordyceps fungi. Future elucidation of the compounds produced by these functionally unknown BGCs, and in-depth assessments of metabolite bioactivity and chemical safety, will not only facilitate the safe use of Cordyceps fungi as human food or alternative medicine, but will also benefit the use of mass production byproducts as animal feed. To corroborate the long record of use as a traditional medicine, future efforts will also benefit the exploration of Cordyceps fungi for pharmaceutical purposes.

Detoxification of Mycotoxins through Biotransformation

Mycotoxins are toxic fungal secondary metabolites that pose a major threat to the safety of food and feed. Mycotoxins are usually converted into less toxic or non-toxic metabolites through biotransformation that are often made by living organisms as well as the isolated enzymes. The conversions mainly include hydroxylation, oxidation, hydrogenation, de-epoxidation, methylation, glycosylation and glucuronidation, esterification, hydrolysis, sulfation, demethylation and deamination. Biotransformations of some notorious mycotoxins such as alfatoxins, alternariol, citrinin, fomannoxin, ochratoxins, patulin, trichothecenes and zearalenone analogues are reviewed in detail. The recent development and applications of mycotoxins detoxification through biotransformation are also discussed.

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.

Lophiostomin A–D: new 3,4-dihydroisocoumarin derivatives from the endophytic fungus Lophiostoma sp. Sigrf10

Four new 3,4-dihydroisocoumarin congeners, named lophiostomin A–D (1–4), together with two known α-pyridones (5 and 6) were isolated from cultures of the endophytic fungus Lophiostoma sp. Sigrf10 obtained from Siraitia grosvenorii. The structures of the new compounds were determined via combined analysis involving 1D and 2D NMR, high-resolution electrospray ionization mass spectrometry (HRESIMS), and electronic circular dichroism (ECD) spectra, as well as quantum chemical ECD computations for assigning the absolute configurations. All the compounds were evaluated for their antibacterial and antifungal activities. Compounds 1 and 2 displayed moderate inhibitory activities against the spore germination of Magnaporthe oryzae, whereas 5 and 6 were active against the following tested pathogenic bacteria: Bacillus subtilis, Agrobacterium tumefaciens, Ralstonia solanacearum, and Xanthomonas vesicatoria.

Four new 3,4-dihydroisocoumarin congeners, named lophiostomin A–D (1–4), together with two known α-pyridones (5 and 6) were isolated from cultures of the endophytic fungus Lophiostoma sp. Sigrf10 obtained from Siraitia grosvenorii.

Naphtho-Gamma-Pyrones Produced by Aspergillus tubingensis G131: New Source of Natural Nontoxic Antioxidants

Seven naphtho-gamma-pyrones (NγPs), including asperpyrone E, aurasperone A, dianhydroaurasperone C, fonsecin, fonsecinone A, fonsecin B, and ustilaginoidin A, were isolated from Aspergillus tubingensis G131, a non-toxigenic strain. The radical scavenging activity of these NγPs was evaluated using ABTS assay. The Trolox equivalent antioxidant capacity on the seven isolated NγPs ranged from 2.4 to 14.6 μmol L^-1. The toxicity and ability of the NγPs to prevent H₂O₂-mediated cell death were evaluated using normal/not cancerous cells (CHO cells). This cell-based assay showed that NγPs: (1) Are not toxic or weakly toxic towards cells and (2) are able to protect cells from oxidant injuries with an IC₅₀ on H₂O₂-mediated cell death ranging from 2.25 to 1800 μmol mL^-1. Our data show that A. tubingensis G131 strain is able to produce various NγPs possessing strong antioxidant activities and low toxicities, making this strain a good candidate for antioxidant applications in food and cosmetic industries.

Ustilaginoidin M1, a new bis-naphtho-γ-pyrone from the fungus Villosiclava virens

Ustilaginoidin M₁ (1), a novel bis-naphtho-γ-pyrone, was isolated from the cultures of the fungus Villosiclava virens which was the pathogen of rice false smut disease. Its structure was elucidated by spectroscopic analysis and by comparison of its physical and spectroscopic data with the literature. Compound 1 was tested for its cytotoxicity against five human cancer cell lines.

Sorbicillinoids From the Fungus Ustilaginoidea virens and Their Phytotoxic, Cytotoxic, and Antimicrobial Activities

Ustilaginoidea virens, the causal fungus of rice false smut, was found in previous studies to produce two types of metabolites, ustiloxins and ustilaginoidins. However, genome sequencing of U. virens revealed a plethora of secondary-metabolites-biosynthetic core genes that were capable to biosynthesize unreported metabolites. A large-scale fermentation of U. virens was thus performed, and the fungal extract was chemically re-investigated. After removing the known metabolites, we found a fraction containing unknown phytotoxic substances. Fractionation of this part has led to the isolation of six new sorbicillinoids, namely ustisorbicillinols A~F (1~6), and two new sorbicillinoid-related pyrones, named ustilopyrones A (7) and B (8), together with nine known cogeners (9~17). The structures of the new compounds were elucidated by analysis of their NMR, HRMS, and CD spectra, while ECD, ¹³C NMR and optical rotation calculations were additionally used for configurational assignments. Plausible biosynthetic pathways for the new compounds were proposed. Phytotoxicity assays revealed that the major sorbicillinoids (12~14, and 16) showed strong inhibition against the radicle and germ elongation of rice and lettuce seeds, with compound 12 displaying the strongest inhibition. The isolated compounds were also evaluated for their cytotoxic, antibacterial, and antifungal activities. Compounds 10, and 12~14 showed moderate cytotoxicities against the tested cell lines with IC₅₀s of 8.83~74.7 μM. Compounds 2, and 10~13 were active against the tested bacteria (MICs 4~128 μg/mL), while compounds 11~13 displayed moderate antifungal activities.

New nitrogen-containing metabolites from cultures of rice false smut pathogen Villosiclava virens

Two new nitrogen-containing metabolites methyl N-acetyl-O-(4-acetylphenyl)-L-homoserinate (1), dimethyl (1H-indole-3-carbonyl)-D-glutamate (2), and two new natural products, 1,2-O-isopropylidene-D-mannitol (3), N-acetyl-β-methyl-L-phenylalanine (4), along with five known compounds (5-9) were isolated from the rice false smut pathogen Villosiclava virens UV-8b cultured in the solid rice medium. The structures were elucidated by spectroscopic analysis and by comparison of their physical and spectroscopic data with the literature. These metabolites were evaluated for their antibacterial and phytotoxic activities. Compounds 5-7 showed weak inhibition against the tested bacteria, while compounds 4-6 and 9 displayed inhibitory activity against the radicle elongation of rice seeds.

New Naphtho-γ-Pyrones Isolated from Marine-Derived Fungus Penicillium sp. HK1-22 and Their Antimicrobial Activities

Three novel monomeric naphtho-γ-pyrones, peninaphones A–C (compounds 1–3), along with two known bis-naphtho-γ-pyrones (compounds 4 and 5) were isolated from mangrove rhizosphere soil-derived fungus Penicillium sp. HK1-22. The absolute configurations of compounds 1 and 2 were determined by electronic circular dichroism (ECD) spectra, and the structure of compound 3 was confirmed by single-crystal X-ray diffraction analysis. Compounds 4 and 5 are a pair of hindered rotation isomers. A hypothetical biosynthetic pathway for the isolated monomeric and dimeric naphtho-γ-pyrones is also discussed in this study. Compounds 1–3 showed antibacterial activity against Staphylococcus aureus (ATCC 43300, 33591, 29213, and 25923) with minimum inhibitory concentration (MIC) values in the range of 12.5–50 μg/mL. Compound 3 exhibited significant activity against the rice sheath blight pathogen Rhizoctonia solani.

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.

Towards understanding the biosynthetic pathway for ustilaginoidin mycotoxins in Ustilaginoidea virens

Ustilaginoidins, toxic to plants, animals and human, are one of major types of mycotoxins produced by Ustilaginoidea virens. In this study, a gene cluster containing the polyketide synthase gene UvPKS1 was analysed via gene replacement and biochemical studies to determine ustilaginoidin biosynthetic pathway in U. virens. UvPKS1 was first proven to be responsible for the first step of ustilaginoidin biosynthesis, since neither ustilaginoidin derivatives nor intermediates were produced when UvPKS1 was deleted. Replacement of ugsO greatly reduced ustilaginoidin production but increased the ratios of dehydrogenated/hydrogenated ustilagioidin derivatives. The enhanced growth rate of the ΔugsO mutant indicates that accumulation of certain ustilaginoidin derivatives may adversely affect mycelial growth in U. virens. Deletion of ugsT encoding a putative MFS transporter disrupted the ability to generate ustilaginoidins. The ustilaginoidin derivatives produced in the ΔugsJ mutant all lack C3-methyl, indicating that UgsJ is responsible for C3-methylation. Only monomeric intermediates, such as 3-methyl-dihydro-nor-rubrofusarin, but no ustilaginoidin derivatives were generated in the ΔugsL mutant, indicating that UgsL is responsible for the dimerization of nor-rubrofusarin derivatives to produce ustilaginoidins. However, ugsR2 deletion had no dramatic effect on ustilaginoidin biosynthesis. Together, biochemical analyses with bioinformatics and chemoinformatics uncover a multiple-step enzyme-catalysed pathway for ustilaginoidin biosynthesis in U. virens.

Ustilobisorbicillinol A, a Cytotoxic Sorbyl-Containing Aromatic Polyketide from Ustilaginoidea virens

Ustilobisorbicillinol A (1), which is a novel bisorbicillinoid featuring a unique cage structure that incorporates one sorbicillinol and one sorbyl-containing phenanthrenone unit, was isolated from the culture of Ustilaginoidea virens. Three biogenetically related new metabolites (2-4) were also isolated. Their structures were elucidated by extensive spectroscopic analyses, including the ¹³C NMR and electronic circular dichroism (ECD) calculations for the configurational assignment. The biosynthetic pathway for these sorbyl-containing polyketides was proposed. Compound 1 showed pronounced cytotoxicity, and it induced significant cell cycle arrest and apoptosis.