The phytopathogenic fungus Alternaria brassicicola: phytotoxin production and phytoalexin elicitation

Conserved copper regulation of the antimicrobial isocyanide brassicicolin A in Alternaria brassicicola

Phytopathogenic Alternaria species are renown for production of toxins that contribute to virulence on host plants. Typically, these toxins belong to well-known secondary metabolite chemical classes including polyketides, non-ribosomal peptides and terpenes. However, the purported host toxin brassicicolin A produced by A. brassicicola is an isocyanide, a chemical class whose genetics and encoding gene structure is largely unknown. The chemical structure of brassicicolin A shows it to have similarity to the recently characterized fumicicolins derived from the Aspergillus fumigatus isocyanide synthase CrmA. Examination of the A. brassicicola genome identified AbcrmA, a putative homolog with 64% identity to A. fumigatus CrmA. Deletion of AbcrmA resulted in loss of production of brassicicolin A. Contrary to reports that brassicicolin A is a host-specific toxin, the ΔAbcrmA mutants were equally virulent as the wildtype on Brassica hosts. However, in line with results of A. fumigatus CrmA generated metabolites, we find that brassicicolin A increased 360-fold under copper limited conditions. Also, like A. fumigatus CrmA derived metabolites, we find brassicicolin A to be a broad-spectrum antimicrobial. We speculate that CrmA-like isocyanide synthase products provide the producing fungi a fitness advantage in copper depleted environments.

Structural diversification of fungal natural products by oxidative enzymes

Ascomycota and basidiomycota fungi are prolific producers of biologically active natural products. Fungal natural products exhibit remarkable structural diversity and complexity, which are generated by the enzymes involved in their biosynthesis. After the formation of core skeletons, oxidative enzymes play a critical role in converting them into mature natural products. Besides simple oxidations, more complex transformations, such as multiple oxidations by single enzymes, oxidative cyclization, and skeletal rearrangement, are often observed. Those oxidative enzymes are of significant interest for the identification of new enzyme chemistry and have the potential to be biocatalysts for the synthesis of complex molecules. This review presents selected examples of unique oxidative transformations that have been found in the biosynthesis of fungal natural products. The development of strategies for refactoring the fungal biosynthetic pathways with an efficient genome-editing method is also introduced.

Untargeted profiling of secondary metabolites and phytotoxins associated with stemphylium blight of lentil

Stemphylium botryosum alters lentil secondary metabolism and differentially affects resistant and susceptible genotypes. Untargeted metabolomics identifies metabolites and their potential biosynthetic pathways that play a crucial role in resistance to S. botryosum. The molecular and metabolic processes that mediate resistance to stemphylium blight caused by Stemphylium botryosum Wallr. in lentil are largely unknown. Identifying metabolites and pathways associated with Stemphylium infection may provide valuable insights and novel targets to breed for enhanced resistance. The metabolic changes following infection of four lentil genotypes by S. botryosum were investigated by comprehensive untargeted metabolic profiling employing reversed-phase or hydrophilic interaction liquid chromatography (HILIC) coupled to a Q-Exactive mass spectrometer. At the pre-flowering stage, plants were inoculated with S. botryosum isolate SB19 spore suspension and leaf samples were collected at 24, 96 and 144 h post-inoculation (hpi). Mock-inoculated plants were used as negative controls. After analyte separation, high-resolution mass spectrometry data was acquired in positive and negative ionization modes. Multivariate modeling revealed significant treatment, genotype and hpi effects on metabolic profile changes that reflect lentil response to Stemphylium infection. In addition, univariate analyses highlighted numerous differentially accumulated metabolites. By contrasting the metabolic profiles of SB19-inoculated and mock-inoculated plants and among lentil genotypes, 840 pathogenesis-related metabolites were detected including seven S. botryosum phytotoxins. These metabolites included amino acids, sugars, fatty acids and flavonoids in primary and secondary metabolism. Metabolic pathway analysis revealed 11 significant pathways including flavonoid and phenylpropanoid biosynthesis, which were affected upon S. botryosum infection. This research contributes to ongoing efforts toward a comprehensive understanding of the regulation and reprogramming of lentil metabolism under biotic stress, which will provide targets for potential applications in breeding for enhanced disease resistance.

Interactions of fungi with non-isothiocyanate products of the plant glucosinolate pathway: A review on product formation, antifungal activity, mode of action and biotransformation

The glucosinolate pathway, which is present in the order Brassicales, is one of the most researched defensive natural product biosynthesis pathways. Its core molecules, the glucosinolates are broken down upon pathogen challenge or tissue damage to yield an array of natural products that may help plants defend against the stressor. Though the most widely known glucosinolate decomposition products are the antimicrobial isothiocyanates, there is a wide range of other volatile and non-volatile natural products that arise from this biosynthetic pathway. This review summarizes our current knowledge on the interaction of these much less examined, non-isothiocyanate products with fungi. It deals with compounds including (1) glucosinolates and their biosynthesis precursors; (2) glucosinolate-derived nitriles (e.g. derivatives of 1H-indole-3-acetonitrile), thiocyanates, epithionitriles and oxazolidine-2-thiones; (3) putative isothiocyanate downstream products such as raphanusamic acid, 1H-indole-3-methanol (= indole-3-carbinol) and its oligomers, 1H-indol-3-ylmethanamine and ascorbigen; (4) 1H-indole-3-acetonitrile downstream products such as 1H-indole-3-carbaldehyde (indole-3-carboxaldehyde), 1H-indole-3-carboxylic acid and their derivatives; and (5) indole phytoalexins including brassinin, cyclobrassinin and brassilexin. Herein, a literature review on the following aspects is provided: their direct antifungal activity and the proposed mechanisms of antifungal action, increased biosynthesis after fungal challenge, as well as data on their biotransformation/detoxification by fungi, including but not limited to fungal myrosinase activity.

Functions and biosynthesis of plant signaling metabolites mediating plant-microbe interactions

Covering: 2015-2022Plants and microbes have coevolved since their appearance, and their interactions, to some extent, define plant health. A reasonable fraction of small molecules plants produced are involved in mediating plant-microbe interactions, yet their functions and biosynthesis remain fragmented. The identification of these compounds and their biosynthetic genes will open up avenues for plant fitness improvement by manipulating metabolite-mediated plant-microbe interactions. Herein, we integrate the current knowledge on their chemical structures, bioactivities, and biosynthesis with the view of providing a high-level overview on their biosynthetic origins and evolutionary trajectory, and pinpointing the yet unknown and key enzymatic steps in diverse biosynthetic pathways. We further discuss the theoretical basis and prospects for directing plant signaling metabolite biosynthesis for microbe-aided plant health improvement in the future.

Camalexin accumulation as a component of plant immunity during interactions with pathogens and beneficial microbes

This review provides an overview on the role of camalexin in plant immunity taking into account various plant-pathogen and beneficial microbe interactions, regulation mechanisms and the contribution in basal and induced plant resistance. In a hostile environment, plants evolve complex and sophisticated defense mechanisms to counteract invading pathogens and herbivores. Several lines of evidence support the assumption that secondary metabolites like phytoalexins which are synthesized de novo, play an important role in plant defenses and contribute to pathogens' resistance in a wide variety of plant species. Phytoalexins are synthesized and accumulated in plants upon pathogen challenge, root colonization by beneficial microbes, following treatment with chemical elicitors or in response to abiotic stresses. Their protective properties against pathogens have been reported in various plant species as well as their contribution to human health. Phytoalexins are synthesized through activation of particular sets of genes encoding specific pathways. Camalexin (3'-thiazol-2'-yl-indole) is the primary phytoalexin produced by Arabidopsis thaliana after microbial infection or abiotic elicitation and an iconic representative of the indole phytoalexin family. The synthesis of camalexin is an integral part of cruciferous plant defense mechanisms. Although the pathway leading to camalexin has been largely elucidated, the regulatory networks that control the induction of its biosynthetic steps by pathogens with different lifestyles or by beneficial microbes remain mostly unknown. This review thus presents current knowledge regarding camalexin biosynthesis induction during plant-pathogen and beneficial microbe interactions as well as in response to microbial compounds and provides an overview on its regulation and interplay with signaling pathways. The contribution of camalexin to basal and induced plant resistance and its detoxification by some pathogens to overcome host resistance are also discussed.

Pathogenic potential and phytotoxic effects of Coniolariella gamsii Iran 2506C on Iranian knapweed (Centaurea depressa)

Application of fungal phytotoxins is one of the possible solutions to reduce the use of chemical products in agricultural fields. Phytotoxic compounds isolated from phytopathogenic fungi provide a promising source of environmentally friendly herbicides. This paper focuses on the phytotoxic fungus Coniolariella gamsii Iran 2506C as a fungal pathogen against Iranian knapweed (Centaurea depressa) in the western Iran and investigate its phytotoxic constituents. The fungal pathogen was identified on the basis of morphological characteristics and confirmed by sequencing of the internal transcribed spacer (ITS) region and partial LSU rDNA gene. Pathogenicity tests were conducted on the weed seedlings and C. gamsii Iran 2506C isolate with high disease severity was selected for phytotoxin studies. Phytotoxic activity of the isolate was checked by screening the production of phytotoxins, which interestingly inhibited seed germination and seedling growth of Iranian knapweed as compared to wheat in the bioassays. The active metabolites were extracted from cell-free culture filtrate (CFCF) by ethyl acetate and separated by thin layer chromatography (TLC). The results indicated that two out of four spots had phytotoxicity with R_f values of 0.43 and 0.82 on the weed, whereas wheat was not sensitive in the bioassays. Using Gas Chromatography-Mass spectrometry (GC-MS), 3-Carene and Oleic acid were identified as the main constituents and quantified by a gas chromatographic method with 12.7 ± 0.03 and 2.9 ± 0.01 μg ml^-1 respectively. This is the first finding presented on the phytotoxic effects of the active metabolites of C. gamsii Iran 2506C and highlights its herbicidal potential which can be used as a biocontrol agent of C. depressa.

Diterpenes Specially Produced by Fungi: Structures, Biological Activities, and Biosynthesis (2010–2020)

Fungi have traditionally been a very rewarding source of biologically active natural products, while diterpenoids from fungi, such as the cyathane-type diterpenoids from Cyathus and Hericium sp., the fusicoccane-type diterpenoids from Fusicoccum and Alternaria sp., the guanacastane-type diterpenoids from Coprinus and Cercospora sp., and the harziene-type diterpenoids from Trichoderma sp., often represent unique carbon skeletons as well as diverse biological functions. The abundances of novel skeletons, biological activities, and biosynthetic pathways present new opportunities for drug discovery, genome mining, and enzymology. In addition, diterpenoids peculiar to fungi also reveal the possibility of differing biological evolution, although they have similar biosynthetic pathways. In this review, we provide an overview about the structures, biological activities, evolution, organic synthesis, and biosynthesis of diterpenoids that have been specially produced by fungi from 2010 to 2020. We hope this review provides timely illumination and beneficial guidance for future research works of scholars who are interested in this area.

New α-pyrone derivatives with herbicidal activity from the endophytic fungus Alternaria brassicicola

Three pairs of undescribed enantiomeric α-pyrone derivatives (1a/1b-3a/3b) and six undescribed congeners (4-9), were obtained from the fungus Alternaria brassicicola that was isolated from the fresh leaves of Siegesbeckia pubescens Makino (Compositae). The structures of these new compounds were characterized by extensive NMR spectroscopic and HRESIMS data, and their absolute configurations were further elucidated by a modified Mosher's method, chemical conversion, single-crystal X-ray diffraction analysis, and ECD calculations. This is the first report of three pairs of enantiomeric α-pyrone derivatives from the fungus A. brassicicola, and these enantiomers were successfully acquired from scalemic mixtures via chiral HPLC. Compounds 1a/1b-3a/3b and 4-9 were evaluated for the herbicidal activity against Echinochloa crusgalli, Setaria viridis, Portulaca oleracea, and Taraxacum mongolicum. At a concentration of 100 μg/mL, compounds 1a and 1b could significantly inhibit the germination of monocotyledon weed seeds (E. crusgalli and S. viridis) with inhibitory ratios ranging from 68.6 ± 6.4% to 84.2 ± 5.1%, which was equivalent to that of the positive control (glyphosate). The potential structure-herbicidal activity relationship of these compounds was also discussed. To a certain extent, the results of this study will attract great interest for the potential practical application of promising fungal metabolites, α-pyrone derivatives, as ecofriendly herbicides.

Medicinal Chemistry of Isocyanides

In eons of evolution, isocyanides carved out a niche in the ecological systems probably thanks to their metal coordinating properties. In 1859 the first isocyanide was synthesized by humans and in 1950 the first natural isocyanide was discovered. Now, at the beginning of XXI century, hundreds of isocyanides have been isolated both in prokaryotes and eukaryotes and thousands have been synthesized in the laboratory. For some of them their ecological role is known, and their potent biological activity as antibacterial, antifungal, antimalarial, antifouling, and antitumoral compounds has been described. Notwithstanding, the isocyanides have not gained a good reputation among medicinal chemists who have erroneously considered them either too reactive or metabolically unstable, and this has restricted their main use to technical applications as ligands in coordination chemistry. The aim of this review is therefore to show the richness in biological activity of the isocyanide-containing molecules, to support the idea of using the isocyanide functional group as an unconventional pharmacophore especially useful as a metal coordinating warhead. The unhidden hope is to convince the skeptical medicinal chemists of the isocyanide potential in many areas of drug discovery and considering them in the design of future drugs.

Minimizing the risk of deducing wrong natural product structures from NMR data

There continues to be a disturbing number of natural products reported in the literature whose structures are incorrect. At least in part, this reflects the fact that many natural product chemists have limited formal nuclear magnetic resonance training. Gaps in training and lack of awareness regarding the challenges and ambiguities associated with two-dimensional nuclear magnetic resonance data interpretation can easily lead to errors in structure elucidation. The purpose of this tutorial is to point out some of these issues, highlight the kinds of errors that have been made and provide specific advice on how to avoid these missteps such that the risk of reporting a wrong structure is minimized.

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.

Large-scale culture as a complementary and practical method for discovering natural products with novel skeletons

Covering: up to July 2020Fungal metabolites with diverse and novel scaffolds can be assembled from well-known biosynthetic precursors through various mechanisms. Recent examples of novel alkaloids (e.g., cytochalasans and diketopiperazine derivatives), terpenes (e.g., sesterterpenes and diterpenes) and polyketides produced by fungi are presented through case studies. We show that large-scale culture is a complementary and practical method for genome mining and OSMAC approaches to discover natural products of unprecedented skeletal classes from fungi. We also summarize the discovery strategies and challenges for characterizing these compounds.

Complexity of Brassica oleracea-Alternaria brassicicola Susceptible Interaction Reveals Downregulation of Photosynthesis at Ultrastructural, Transcriptional, and Physiological Levels

Black spot disease, caused by Alternaria brassicicola in Brassica species, is one of the most devastating diseases all over the world, especially since there is no known fully resistant Brassica cultivar. In this study, the visualization of black spot disease development on Brassica oleracea var. capitata f. alba (white cabbage) leaves and subsequent ultrastructural, molecular and physiological investigations were conducted. Inter- and intracellular hyphae growth within leaf tissues led to the loss of host cell integrity and various levels of organelle disintegration. Severe symptoms of chloroplast damage included the degeneration of chloroplast envelope and grana, and the loss of electron denseness by stroma at the advanced stage of infection. Transcriptional profiling of infected leaves revealed that photosynthesis was the most negatively regulated biological process. However, in infected leaves, chlorophyll and carotenoid content did not decrease until 48 hpi, and several chlorophyll a fluorescence parameters, such as photosystem II quantum yield (F_v/F_m), non-photochemical quenching (NPQ), or plant vitality parameter (Rdf) decreased significantly at 24 and 48 hpi compared to control leaves. Our results indicate that the initial stages of interaction between B. oleracea and A. brassicicola are not uniform within an inoculation site and show a complexity of host responses and fungal attempts to overcome host cell defense mechanisms. The downregulation of photosynthesis at the early stage of this susceptible interaction suggests that it may be a part of a host defense strategy, or, alternatively, that chloroplasts are targets for the unknown virulence factor(s) of A. brassicicola. However, the observed decrease of photosynthetic efficiency at the later stages of infection is a result of the fungus-induced necrotic lesion expansion.

Modified Fusicoccane-Type Diterpenoids from Alternaria brassicicola

Seven new modified fusicoccane-type diterpenoids (1-7), together with two known congeners (8 and 9), were obtained from Alternaria brassicicola. Their structures were elucidated from a combination of NMR and HRESIMS data and ¹³C NMR calculation as well as DP4+ probability analyses, and the absolute configurations of 1-5 were determined by ECD calculation and single-crystal X-ray diffraction (Cu Kα). Compounds 1-3 belong to a rare class of 16-nor-dicyclopenta[a,d]cyclooctane diterpenoids, and compounds 2 and 4 represent the first examples of fusicoccane-type diterpenoids featuring two previously undescribed tetracyclic 5/6/6/5 ring systems, while compound 5 features a previously undescribed tetracyclic 5/8/5/3 ring system. Compound 7 was moderately anti-inflammatory, and compounds 2, 3, 5, and 7 were weakly cytotoxic.

Psathyrins: Antibacterial Diterpenoids from Psathyrella candolleana

Two skeletally novel tetracyclic diterpenoids, psathyrins A (1) and B (2), have been characterized from cultures of the basidiomycete Psathyrella candolleana. Their structures including absolute configurations were established by means of spectroscopic methods, as well as ECD calculations. They possess a novel 5/5/4/6-fuesd ring system, for which the biosynthetic pathway is proposed. Compounds 1 and 2 inhibited the growth of Staphylococcus aureus and Salmonella enterica.

Leaf position-dependent effect of Alternaria brassicicola development on host cell death, photosynthesis and secondary metabolites in Brassica juncea

During the first 24 hours of infection, Alternaria brassicicola developmental parameters such as conidial germination, germ tubes and appressoria formation on each of the five mature Brassica juncea leaves, correlated with a leaf position showing stronger development of the pathogen on older leaves than on young ones. As a consequence of fungal development, the black spot disease was observed during 96 hours of infection on a macroscopic scale, as well as via confocal microscopy. Degradation of the chloroplast thylakoids and plastoglobule appearance during infection, followed by the decrease in chlorophyll a fluorescence parameters i.e. maximum quantum yield of PSII (F_v /F_m ), non-photochemical quenching (NPQ) and chlorophyll a:b ratio, have been observed. Also, after an initial increase of carbohydrates (glucose, fructose and sucrose), content far below the respective control values was found. The content of secondary metabolites such as flavonoids and glucosinolates increased in a leaf position-dependent manner in infected leaves, with a lower level in older leaves than in younger ones. Although, the total phenolic compounds (TPCs) content did not differ significantly in infected leaves compared to control leaves, TPCs level in both control and infected leaves was leaf position-dependent. To the best of our knowledge, this is the first report on leaf position-dependent effect on the B. juncea biochemical response to A. brassicicola infection.

Baseline Sensitivity and Action Mechanism of Propamidine Against Alternaria brassicicola, the Causal Agent of Dark Leaf Spot on Cabbage

In the current study, a total of 53 isolates of Alternaria brassicicola collected from Shaanxi Province of China were characterized for their sensitivity to propamidine. The EC₅₀ (50% effective concentration) values for propamidine inhibiting mycelial growth and spore germination ranged from 0.515 to 3.247 µg/ml and 0.393 to 2.982 µg/ml, with average EC₅₀ values of 1.327 ± 0.198 µg/ml and 1.106 ± 0.113 µg/ml, respectively. In greenhouse experiments, propamidine at 100 µg/ml provided >90% efficacy against dark leaf spot on cabbage, which was higher than the efficacy obtained by azoxystrobin at the same concentration. After treatment with propamidine, fungal growth distortions were observed in the form of excess mycelial branching, thickened cell walls, decreased cell membrane permeability, and increased chitin content. Interestingly, colony color faded after treatment with propamidine compared with that of the untreated parental isolates. Importantly, the expressions of melanin biosynthesis-associated genes Amr1, Scd1, Brn1, and Brn2 were downregulated at different levels. The obtained baseline sensitivity and control efficacy data suggested that propamidine inhibited not only growth of A. brassicicola but also melanin biosynthesis, which could reduce the biocompatibility of A. brassicicola in the field. These biological characteristics encourage further investigation of the mechanism of action of propamidine against A. brassicicola.

Phytotoxic Diterpenoids from Plants and Microorganisms

Phytotoxic natural products with either unique or various structures are one of the most abundant sources for the discovery of potential allelochemicals, natural herbicides, and plant growth regulators. Phytotoxic diterpenoids, a relatively large class of natural products, play an important role in the plant-plant or plant-microorganism interactions. This article argues that the phytotoxic diterpenoids isolated from the plants and microorganisms can either inhibit the seed germination and the growth of plant seedlings or lead to some disease symptoms on the tested plant tissues and plant seedlings.

Recent trends in the structural revision of natural products

Covering: 2012 to 2017 This article reviews recent reports on the structural revision of natural products. Through a critical assessment of the original and revised published structures, the article addresses why each structure was targeted for revision, discusses the techniques and key discrepancies that led to the proposal of the revised structure, and offers measures that may have been taken during the original structure determination to prevent error. With the revised structures in hand, weaknesses of original proposals are assessed, providing a better understanding on the logic behind structure determination.

The MAP Kinase Kinase Gene AbSte7 Regulates Multiple Aspects of Alternaria brassicicola Pathogenesis

Mitogen-activated protein kinase (MAPK) cascades in fungi are ubiquitously conserved signaling pathways that regulate stress responses, vegetative growth, pathogenicity, and many other developmental processes. Previously, we reported that the AbSte7 gene, which encodes a mitogen-activated protein kinase kinase (MAPKK) in Alternaria brassicicola, plays a central role in pathogenicity against host cabbage plants. In this research, we further characterized the role of AbSte7 in the pathogenicity of this fungus using ΔAbSte7 mutants. Disruption of the AbSte7 gene of A. brassicicola reduced accumulation of metabolites toxic to the host plant in liquid culture media. The ΔAbSte7 mutants could not efficiently detoxify cruciferous phytoalexin brassinin, possibly due to reduced expression of the brassinin hydrolase gene involved in detoxifying brassinin. Disruption of the AbSte7 gene also severely impaired fungal detoxification of reactive oxygen species. AbSte7 gene disruption reduced the enzymatic activity of cell wall-degrading enzymes, including cellulase, β-glucosidase, pectin methylesterase, polymethyl-galacturonase, and polygalacturonic acid transeliminase, during host plant infection. Altogether, the data strongly suggest the MAPKK gene AbSte7 plays a pivotal role in A. brassicicola during host infection by regulating multiple steps, and thus increasing pathogenicity and inhibiting host defenses.

Cyclodepsipeptides and Sesquiterpenes from Marine-Derived Fungus Trichothecium roseum and Their Biological Functions

On the basis of the 'one strain, many compounds' (OSMAC) strategy, chemical investigation of the marine-derived fungus Trichothecium roseum resulted in the isolation of trichomide cyclodepsipeptides (compounds 1⁻4) from PDB medium, and destruxin cyclodepsipeptides (compounds 5⁻7) and cyclonerodiol sesquiterpenes (compounds 8⁻10) from rice medium. The structures and absolute configurations of novel (compounds 1, 8, and 9) and known compounds were elucidated by extensive spectroscopic analyses, X-ray crystallographic analysis, and ECD calculations. All isolated compounds were evaluated for cytotoxic, nematicidal, and antifungal activities, as well as brine shrimp lethality. The novel compound 1 exhibited significant cytotoxic activities against the human cancer cell lines MCF-7, SW480, and HL-60, with IC50 values of 0.079, 0.107, and 0.149 μM, respectively. In addition, it also showed significant brine shrimp lethality, with an LD50 value of 0.48 μM, and moderate nematicidal activity against Heterodera avenae, with an LC50 value of 94.9 μg/mL. This study constitutes the first report on the cytotoxic and nematicidal potential of trichomide cyclodepsipeptides.

Allelopathy of Wild Mushrooms—An Important Factor for Assessing Forest Ecosystems in Japan

Research Highlights: Some organisms such as plants and fungi release certain secondary metabolites, generally called allelochemicals, which can influence the organisms around them. Some of the secondary metabolites released by mushrooms may have certain effects on the growth and development of neighboring plants. Background Objectives: The purpose of the present study was to investigate the allelopathic potential of mushrooms in a forest ecosystem. To this end, 289 Japanese mushroom species were collected from the wild and tested using a modified sandwich method, which is a quick and effective bioassay technique. Materials and Methods: The collected specimens were prepared for bioassay as dried samples, and 10 mg/well (10 cm2) was added to a 6-well multidish according to the mycelia biomass, which was estimated at 700−900 kg ha−1 year−1 (7–9 mg 10 cm−2) in coniferous forests. Results: Of the screened mushroom species, 74% inhibited more than 50% of the radicle elongation in lettuce (Lactuca sativa var. Great Lakes 366) seedlings, while the average of all species was 41.1%. This result suggests that wild mushrooms have a significant regulatory effect on lettuce growth. According to our standard deviation variance analysis, 54 out of 289 species showed significant allelopathic activity. Among these species, Xeromphalina tenuipes, Cortinarius violaceus, and Clavaria miyabeana exhibited the strongest growth inhibitory activity, with radicle elongation of 5.1%, 4.3%, and 7.6% of the control, respectively. In contrast, Ischnoderma resinosum stimulated the length of radicle and hypocotyl growth by 30.6% and 42.0%, respectively. These results suggest that these species may play important roles in ecosystems. In addition, the wide range of allelopathic activities observed in mushrooms indicates that various amounts of diverse secondary metabolites from these species are involved in mushroom allelopathy. Conclusions: Our study reveals the importance of evaluating mushroom allelopathy to understand the wider ecological structures within complex ecosystems.

Total Biosynthesis of Brassicicenes: Identification of a Key Enzyme for Skeletal Diversification

The biosynthetic pathway of brassicicenes, derived from the phytopathogen Pseudocercospora fijiensis, was fully reconstituted. Heterologous expression of the eight genes highly expressed in infected leaf tissues generated a new brassicicene derivative as a final product. Together with the characterization of P450 from Alternaria brassicicola, a late stage of the biosynthetic pathway, which generates remarkable structural diversity, has been proposed. Notably, a unique P450 that converts 3 to the structurally distinct 4 and 6 was identified.

Anti-inflammatory fusicoccane-type diterpenoids from the phytopathogenic fungus Alternaria brassicicola

Eight undescribed fusicoccane-type diterpenoids (1–8), wherein 1 had a rare 16-nor-dicyclopenta[a,d]cyclooctane skeleton, were isolated from Alternaria brassicicola.

Occurrence, biochemistry and biological effects of host-selective plant mycotoxins

Host-selective mycotoxins (HSTs) are various secondary metabolites or proteinaceous compounds secreted by pathogenic necrotrophic fungi that feed off on dead tissues of certain plants. Research on the HSTs has not only fundamental but also practical importance. On one hand they are implicated in the onset of devastating crop diseases. On the other hand, they have been studied as a good model for revealing the intricate mechanisms of plant-pathogen interactions. At the cellular level, HSTs target different compartments and in most instances induce programmed cell death (PCD) by a wide range of mechanisms. Often the responses provoked by HSTs resemble the effector-triggered immunity used by plant cells to combat biotrophic pathogens, which suggests that HST-producing fungi exploit the plants' own defensive systems to derive benefits. Although by definition HSTs are active only in tissues of susceptible plant genotypes, it has been demonstrated that some of them are able to influence animal cells as well. The possible effects, like cytotoxicity or cytostasis, can be harmful or beneficial and thus HSTs may either pose a health risk for humans and livestock, or be of prospective use in the fields of pharmacology, medicine and agriculture.

Cytotoxic Fusicoccane-Type Diterpenoids from Streptomyces violascens Isolated from Ailuropoda melanoleuca Feces

Six new fusicoccane-type diterpenoids (2-7) were isolated from the fermentation broth of Streptomyces violascens, which was isolated from Ailuropoda melanoleuca (giant panda) feces. The structures of these new compounds were elucidated by a detailed spectroscopic data and X-ray crystallographic analysis. Compounds 5-7 demonstrated cytotoxicity against five human cancer cell lines, with IC₅₀ values ranging from 3.5 ± 0.7 to 14.1 ± 0.8 μM. Cell adhesion, migration, and invasion assays showed that 6 inhibited the migration and invasion of human hepatocellular carcinoma SMMC7721 cells in a dose-dependent manner. Through further investigation, it was revealed that 6 inhibited the enzymatic activity of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9), in addition to down-regulating the expressions of MMP-2 and MMP-9 at both the protein and mRNA levels to influence the migration and invasion of cancer cells.

In Silico Identification of Mimicking Molecules as Defense Inducers Triggering Jasmonic Acid Mediated Immunity against Alternaria Blight Disease in Brassica Species

Alternaria brassicae and Alternaria brassicicola are two major phytopathogenic fungi which cause Alternaria blight, a recalcitrant disease on Brassica crops throughout the world, which is highly destructive and responsible for significant yield losses. Since no resistant source is available against Alternaria blight, therefore, efforts have been made in the present study to identify defense inducer molecules which can induce jasmonic acid (JA) mediated defense against the disease. It is believed that JA triggered defense response will prevent necrotrophic mode of colonization of Alternaria brassicae fungus. The JA receptor, COI1 is one of the potential targets for triggering JA mediated immunity through interaction with JA signal. In the present study, few mimicking compounds more efficient than naturally occurring JA in terms of interaction with COI1 were identified through virtual screening and molecular dynamics simulation studies. A high quality structural model of COI1 was developed using the protein sequence of Brassica rapa. This was followed by virtual screening of 767 analogs of JA from ZINC database for interaction with COI1. Two analogs viz. ZINC27640214 and ZINC43772052 showed more binding affinity with COI1 as compared to naturally occurring JA. Molecular dynamics simulation of COI1 and COI1-JA complex, as well as best screened interacting structural analogs of JA with COI1 was done for 50 ns to validate the stability of system. It was found that ZINC27640214 possesses efficient, stable, and good cell permeability properties. Based on the obtained results and its physicochemical properties, it is capable of mimicking JA signaling and may be used as defense inducers for triggering JA mediated resistance against Alternaria blight, only after further validation through field trials.

Pathogen inactivation of cruciferous phytoalexins: detoxification reactions, enzymes and inhibitors

This review covers the detoxification pathways of cruciferous phytoalexins, the corresponding detoxifying enzymes and their natural and synthetic inhibitors. Paldoxins are examined as a potentially sustainable strategy to control plant pathogenic fungi.

The biosynthesis of brassicicolin A in the phytopathogen Alternaria brassicicola

Alternaria brassicicola (Schwein.) Wiltshire is a phytopathogenic fungus that together with A. brassicae causes Alternaria black spot disease in Brassica species. Brassicicolin A is the major host-selective phytotoxin produced in cultures of A. brassicicola. Biosynthetic studies to establish the metabolic precursors of brassicicolin A were carried out with isotopically labeled compounds. Incorporation of D-[¹³C₆]glucose, L-[¹⁵N]valine, or L-[²H₈]valine into brassicicolin A was established using ¹H, ¹³C, ¹⁵N NMR and INADEQUATE spectroscopy and HPLC-ESI-MS spectrometry. Based on analyses of the spectroscopic data, the labeling patterns of brassicicolin A isolated from cultures incubated with the labeled precursors are found to be consistent with both the glycolytic and the valine pathways. That is, the carbons of mannitol and acetyl units and the isocyanide carbon atoms are derived from D-[¹³C₆]glucose whereas the hydroxyisopentanoyl and isocyanoisopentanoyl units are derived from L-valine, including the nitrogen atoms of both isocyanide groups.

Biotransformation of rutabaga phytoalexins by the fungus Alternaria brassicicola: Unveiling the first hybrid metabolite derived from a phytoalexin and a fungal polyketide

The biotransformations of the rutabaga phytoalexins rutalexin, brassicanate A, isalexin and rapalexin A by the plant pathogenic fungus Alternaria brassicicola are reported. While the biotransformations of rutalexin, brassicanate A, and isalexin are fast, rapalexin A is resistant to fungal transformation. Unexpectedly, biotransformation of rutalexin yields a hybrid metabolite named rutapyrone, derived from rutalexin metabolism and phomapyrone G, a fungal metabolite produced by A. brassicicola. These fungal transformations are detoxification reactions likely carried out by different enzymes. The discovery of rapalexin A resistance to detoxification suggests that this phytoalexin in combination with additional phytoalexins could protect crucifers against this pathogen. Phytoalexins resistant to degradation by A. brassicicola are expected to provide the producing plants with higher disease resistance levels.

Transcriptome Analysis Reveals the Genetic Basis of the Resveratrol Biosynthesis Pathway in an Endophytic Fungus (Alternaria sp. MG1) Isolated from Vitis vinifera

Alternaria sp. MG1, an endophytic fungus previously isolated from Merlot grape, produces resveratrol from glucose, showing similar metabolic flux to the phenylpropanoid biosynthesis pathway, currently found solely in plants. In order to identify the resveratrol biosynthesis pathway in this strain at the gene level, de novo transcriptome sequencing was conducted using Illumina paired-end sequencing. A total of 22,954,434 high-quality reads were assembled into contigs and 18,570 unigenes were identified. Among these unigenes, 14,153 were annotated in the NCBI non-redundant protein database and 5341 were annotated in the Swiss-Prot database. After KEGG mapping, 2701 unigenes were mapped onto 115 pathways. Eighty-four unigenes were annotated in major pathways from glucose to resveratrol, coding 20 enzymes for glycolysis, 10 for phenylalanine biosynthesis, 4 for phenylpropanoid biosynthesis, and 4 for stilbenoid biosynthesis. Chalcone synthase was identified for resveratrol biosynthesis in this strain, due to the absence of stilbene synthase. All the identified enzymes indicated a reasonable biosynthesis pathway from glucose to resveratrol via glycolysis, phenylalanine biosynthesis, phenylpropanoid biosynthesis, and stilbenoid pathways. These results provide essential evidence for the occurrence of resveratrol biosynthesis in Alternaria sp. MG1 at the gene level, facilitating further elucidation of the molecular mechanisms involved in this strain's secondary metabolism.

Unveiling the first indole-fused thiazepine: structure, synthesis and biosynthesis of cyclonasturlexin, a remarkable cruciferous phytoalexin

The structure, synthesis, biosynthesis and antifungal activity of cyclonasturlexin, the most intriguing indolyl phytoalexin isolated from watercress plants, are reported.

A γ-lactamase from cereal infecting Fusarium spp. catalyses the first step in the degradation of the benzoxazolinone class of phytoalexins

The benzoxazolinone class of phytoalexins are released by wheat, maize, rye and other agriculturally important species in the Poaceae family upon pathogen attack. Benzoxazolinones show antimicrobial effects on plant pathogens, but certain fungi have evolved mechanisms to actively detoxify these compounds which may contribute to the virulence of the pathogens. In many Fusarium spp. a cluster of genes is thought to be involved in the detoxification of benzoxazolinones. However, only one enzyme encoded in the cluster has been unequivocally assigned a role in this process. The first step in the detoxification of benzoxazolinones in Fusarium spp. involves the hydrolysis of a cyclic ester bond. This reaction is encoded by the FDB1 locus in F. verticillioides but the underlying gene is yet to be cloned. We previously proposed that FDB1 encodes a γ-lactamase, and here direct evidence for this is presented. Expression analyses in the important wheat pathogen F. pseudograminearum demonstrated that amongst the three predicted γ-lactamase genes only the one designated as FDB1, part of the proposed benzoxazolinone cluster in F. pseudograminearum, was strongly responsive to exogenous benzoxazolinone application. Analysis of independent F. pseudograminearum and F. graminearum FDB1 gene deletion mutants, as well as biochemical assays, demonstrated that the γ-lactamase enzyme, encoded by FDB1, catalyses the first step in detoxification of benzoxazolinones. Overall, our results support the notion that Fusarium pathogens that cause crown rot and head blight on wheat have adopted strategies to overcome host-derived chemical defences.

A Pectate Lyase-Coding Gene Abundantly Expressed during Early Stages of Infection Is Required for Full Virulence in Alternaria brassicicola

Alternaria brassicicola causes black spot disease of Brassica species. The functional importance of pectin digestion enzymes and unidentified phytotoxins in fungal pathogenesis has been suspected but not verified in A. brassicicola. The fungal transcription factor AbPf2 is essential for pathogenicity and induces 106 genes during early pathogenesis, including the pectate lyase-coding gene, PL1332. The aim of this study was to test the importance and roles of PL1332 in pathogenesis. We generated deletion strains of the PL1332 gene, produced heterologous PL1332 proteins, and evaluated their association with virulence. Deletion strains of the PL1332 gene were approximately 30% less virulent than wild-type A. brassicicola, without showing differences in colony expansion on solid media and mycelial growth in nutrient-rich liquid media or minimal media with pectins as a major carbon source. Heterologous PL1332 expressed as fusion proteins digested polygalacturons in vitro. When the fusion proteins were injected into the apoplast between leaf veins of host plants the tissues turned dark brown and soft, resembling necrotic leaf tissue. The PL1332 gene was the first example identified as a general toxin-coding gene and virulence factor among the 106 genes regulated by the transcription factor, AbPf2. It was also the first gene to have its functions investigated among the 19 pectate lyase genes and several hundred putative cell-wall degrading enzymes in A. brassicicola. These results further support the importance of the AbPf2 gene as a key pathogenesis regulator and possible target for agrochemical development.

The Alternaria genomes database: a comprehensive resource for a fungal genus comprised of saprophytes, plant pathogens, and allergenic species

Background

Alternaria is considered one of the most common saprophytic fungal genera on the planet. It is comprised of many species that exhibit a necrotrophic phytopathogenic lifestyle. Several species are clinically associated with allergic respiratory disorders although rarely found to cause invasive infections in humans. Finally, Alternaria spp. are among the most well known producers of diverse fungal secondary metabolites, especially toxins.

Description

We have recently sequenced and annotated the genomes of 25 Alternaria spp. including but not limited to many necrotrophic plant pathogens such as A. brassicicola (a pathogen of Brassicaceous crops like cabbage and canola) and A. solani (a major pathogen of Solanaceous plants like potato and tomato), and several saprophytes that cause allergy in human such as A. alternata isolates. These genomes were annotated and compared. Multiple genetic differences were found in the context of plant and human pathogenicity, notably the pro-inflammatory potential of A. alternata. The Alternaria genomes database was built to provide a public platform to access the whole genome sequences, genome annotations, and comparative genomics data of these species. Genome annotation and comparison were performed using a pipeline that integrated multiple computational and comparative genomics tools. Alternaria genome sequences together with their annotation and comparison data were ported to Ensembl database schemas using a self-developed tool (EnsImport). Collectively, data are currently hosted using a customized installation of the Ensembl genome browser platform.

Conclusion

Recent efforts in fungal genome sequencing have facilitated the studies of the molecular basis of fungal pathogenicity as a whole system. The Alternaria genomes database provides a comprehensive resource of genomics and comparative data of an important saprophytic and plant/human pathogenic fungal genus. The database will be updated regularly with new genomes when they become available. The Alternaria genomes database is freely available for non-profit use at http://alternaria.vbi.vt.edu.

The Alternaria genomes database: a comprehensive resource for a fungal genus comprised of saprophytes, plant pathogens, and allergenic species

BACKGROUND

Alternaria is considered one of the most common saprophytic fungal genera on the planet. It is comprised of many species that exhibit a necrotrophic phytopathogenic lifestyle. Several species are clinically associated with allergic respiratory disorders although rarely found to cause invasive infections in humans. Finally, Alternaria spp. are among the most well known producers of diverse fungal secondary metabolites, especially toxins.

DESCRIPTION

We have recently sequenced and annotated the genomes of 25 Alternaria spp. including but not limited to many necrotrophic plant pathogens such as A. brassicicola (a pathogen of Brassicaceous crops like cabbage and canola) and A. solani (a major pathogen of Solanaceous plants like potato and tomato), and several saprophytes that cause allergy in human such as A. alternata isolates. These genomes were annotated and compared. Multiple genetic differences were found in the context of plant and human pathogenicity, notably the pro-inflammatory potential of A. alternata. The Alternaria genomes database was built to provide a public platform to access the whole genome sequences, genome annotations, and comparative genomics data of these species. Genome annotation and comparison were performed using a pipeline that integrated multiple computational and comparative genomics tools. Alternaria genome sequences together with their annotation and comparison data were ported to Ensembl database schemas using a self-developed tool (EnsImport). Collectively, data are currently hosted using a customized installation of the Ensembl genome browser platform.

CONCLUSION

Recent efforts in fungal genome sequencing have facilitated the studies of the molecular basis of fungal pathogenicity as a whole system. The Alternaria genomes database provides a comprehensive resource of genomics and comparative data of an important saprophytic and plant/human pathogenic fungal genus. The database will be updated regularly with new genomes when they become available. The Alternaria genomes database is freely available for non-profit use at http://alternaria.vbi.vt.edu .

How the necrotrophic fungus Alternaria brassicicola kills plant cells remains an enigma

Alternaria species are mainly saprophytic fungi, but some are plant pathogens. Seven pathotypes of Alternaria alternata use secondary metabolites of host-specific toxins as pathogenicity factors. These toxins kill host cells prior to colonization. Genes associated with toxin synthesis reside on conditionally dispensable chromosomes, supporting the notion that pathogenicity might have been acquired several times by A. alternata. Alternaria brassicicola, however, seems to employ a different mechanism. Evidence on the use of host-specific toxins as pathogenicity factors remains tenuous, even after a diligent search aided by full-genome sequencing and efficient reverse-genetics approaches. Similarly, no individual genes encoding lipases or cell wall-degrading enzymes have been identified as strong virulence factors, although these enzymes have been considered important for fungal pathogenesis. This review describes our current understanding of toxins, lipases, and cell wall-degrading enzymes and their roles in the pathogenesis of A. brassicicola compared to those of other pathogenic fungi. It also describes a set of genes that affect pathogenesis in A. brassicicola. They are involved in various cellular functions that are likely important in most organisms and probably indirectly associated with pathogenesis. Deletion or disruption of these genes results in weakly virulent strains that appear to be sensitive to the defense mechanisms of host plants. Finally, this review discusses the implications of a recent discovery of three important transcription factors associated with pathogenesis and the putative downstream genes that they regulate.

Transcriptional responses of the Bdtf1-deletion mutant to the phytoalexin brassinin in the necrotrophic fungus Alternaria brassicicola

Brassica species produce the antifungal indolyl compounds brassinin and its derivatives, during microbial infection. The fungal pathogen Alternaria brassicicola detoxifies brassinin and possibly its derivatives. This ability is an important property for the successful infection of brassicaceous plants. Previously, we identified a transcription factor, Bdtf1, essential for the detoxification of brassinin and full virulence. To discover genes that encode putative brassinin-digesting enzymes, we compared gene expression profiles between a mutant strain of the transcription factor and wild-type A. brassicicola under two different experimental conditions. A total of 170 and 388 genes were expressed at higher levels in the mutants than the wild type during the infection of host plants and saprophytic growth in the presence of brassinin, respectively. In contrast, 93 and 560 genes were expressed, respectively, at lower levels in the mutant than the wild type under the two conditions. Fifteen of these genes were expressed at lower levels in the mutant than in the wild type under both conditions. These genes were assumed to be important for the detoxification of brassinin and included Bdtf1 and 10 putative enzymes. This list of genes provides a resource for the discovery of enzyme-coding genes important in the chemical modification of brassinin.

Alt a 1 from Alternaria interacts with PR5 thaumatin-like proteins

Alt a 1 is a protein found in Alternaria alternata spores related to virulence and pathogenicity and considered to be responsible for chronic asthma in children. We found that spores of Alternaria inoculated on the outer surface of kiwifruits did not develop hyphae. Nevertheless, the expression of Alt a 1 gene was upregulated, and the protein was detected in the pulp where it co-localized with kiwi PR5. Pull-down assays demonstrated experimentally that the two proteins interact in such a way that Alt a 1 inhibits the enzymatic activity of PR5. These results are relevant not only for plant defense, but also for human health as patients with chronic asthma could suffer from an allergic reaction when they eat fruit contaminated with Alternaria.

The Bdtf1 gene in Alternaria brassicicola is important in detoxifying brassinin and maintaining virulence on Brassica species

Brassinin is an antifungal compound induced in Brassica plants after microbial infection. Molecular evidence is incomplete, however, in supporting the importance of brassinin in plant resistance to pathogens. To test the importance of brassinin in plant defense, we studied the functions of the gene Bdtf1 in the necrotrophic fungus Alternaria brassicicola. Several strains of mutants of this gene were weakly virulent on Brassica species, causing lesions 70% smaller in diameter than the wild type on three Brassica species. These mutants, however, were as virulent as the wild type on Arabidopsis thaliana. They were similar to the wild type in spore germination, colony morphology, and mycelial growth in nutrient-rich media, both with and without stress-inducing chemicals. Unlike wild-type A. brassicicola, however, the mutants failed to germinate and their hyphal growth was arrested in the presence of 200 μM brassinin. When grown in a medium containing 100 μM brassinin, wild-type mycelium entirely converted the brassinin into a nontoxic derivative, of which the precise chemical nature was not established. Mutants of the Bdtf1 gene were unable to perform this conversion. Our results support the hypothesis that the ability of A. brassicicola to detoxify brassinin is necessary for successful infection of Brassica species.

Metabolism of the phytoalexins camalexins, their bioisosteres and analogues in the plant pathogenic fungus Alternaria brassicicola

The metabolism of the phytoalexins camalexin (1), 1-methylcamalexin (10) and 6-methoxycamalexin (11) by Alternaria brassicicola and their antifungal activity is reported. This work establishes that camalexins are slowly biotransformed (ca. six days) to the corresponding indole-3-thiocarboxamides, which are further transformed to the indole-3-carboxylic acids. These metabolites are substantially less inhibitory to A. brassicicola than the parent camalexins, indicating that these enzyme-mediated transformations are detoxifications. In addition, analyses of the metabolism of synthetic isomers and bioisosteres of camalexin (1) indicate that isomers of camalexin in the thiazole ring are not metabolized. Based on these results, the potential intermediates that lead to formation of indole-3-thiocarboxamides are proposed.

Metabolites from Alternaria fungi and their bioactivities

Alternaria is a cosmopolitan fungal genus widely distributing in soil and organic matter. It includes saprophytic, endophytic and pathogenic species. At least 268 metabolites from Alternaria fungi have been reported in the past few decades. They mainly include nitrogen-containing metabolites, steroids, terpenoids, pyranones, quinones, and phenolics. This review aims to briefly summarize the structurally different metabolites produced by Alternaria fungi, as well as their occurrences, biological activities and functions. Some considerations related to synthesis, biosynthesis, production and applications of the metabolites from Alternaria fungi are also discussed.