Sporogenic effect of polyunsaturated fatty acids on development of Aspergillus spp

Deciphering Aphanomyces euteiches-pea-biocontrol bacterium interactions through untargeted metabolomics

Aphanomyces euteiches causes root rot in pea, leading to significant yield losses. However, the metabolites involved in this pathosystem have not been thoroughly studied. This study aimed to fill this gap and explore mechanisms of bacterial suppression of A. euteiches via untargeted metabolomics using pea grown in a controlled environment. Chemical isotope labeling (CIL), followed by liquid chromatography-mass spectrometry (LC-MS), was used for metabolite separation and detection. Univariate and multivariate analyses showed clear separation of metabolites from pathogen-treated pea roots and roots from other treatments. A three-tier approach positively or putatively identified 5249 peak pairs or metabolites. Of these, 403 were positively identified in tier 1; 940 were putatively identified with high confidence in tier 2. There were substantial changes in amino acid pool, and fatty acid and phenylpropanoid pathway products. More metabolites, including salicylic and jasmonic acids, were upregulated than downregulated in A. euteiches-infected roots. 1-aminocyclopropane-1-carboxylic acid and 12-oxophytodienoic acid were upregulated in A. euteiches + bacterium-treated roots compared to A. euteiches-infected roots. A great number of metabolites were up- or down-regulated in response to A. euteiches infection compared with the control and A. euteiches + bacterium-treated plants. The results of this study could facilitate improved disease management.

FgFAD12 Regulates Vegetative Growth, Pathogenicity and Linoleic Acid Biosynthesis in Fusarium graminearum

Polyunsaturated fatty acids (PUFAs), as important components of lipids, play indispensable roles in the development of all organisms. ∆12 fatty acid desaturase (FAD12) is a speed-determining step in the biosynthesis of PUFAs. Here, we report the characterization of FAD12 in Fusarium graminearum, which is the prevalent agent of Fusarium head blight, a destructive plant disease worldwide. The results demonstrated that deletion of the FgFAD12 gene resulted in defects in vegetative growth, conidial germination and plant pathogenesis but not sexual reproduction. A fatty acid analysis further proved that the deletion of FgFAD12 restrained the reaction of oleic acid to linoleic acid, and a large amount of oleic acid was detected in the cells. Moreover, the ∆Fgfad12 mutant showed increased resistance to osmotic stress and reduced tolerance to oxidative stress. The expression of FgFAD12 did show a temperature-dependent manner, which was not affected at a low temperature of 10 °C when compared to 25 °C. RNA-seq analysis further demonstrated that most genes enriched in fatty acid metabolism, the biosynthesis of unsaturated fatty acids, fatty acid biosynthesis, fatty acid degradation, steroid biosynthesis and fatty acid elongation pathways were significantly up-regulated in the ∆Fgfad12 mutants. Overall, our results indicate that FgFAD12 is essential for linoleic acid biosynthesis and plays an important role in the infection process of F. graminearum.

Comparative metabolomic study of fungal foliar endophytes and their long-lived host Astrocaryum sciophilum: a model for exploring the chemodiversity of host-microbe interactions

Introduction

In contrast to the dynamics observed in plant/pathogen interactions, endophytic fungi have the capacity to establish enduring associations within their hosts, leading to the development of a mutually beneficial relationship that relies on specialized chemical interactions. Research indicates that the presence of endophytic fungi has the ability to significantly modify the chemical makeup of the host organism. Our hypothesis proposes the existence of a reciprocal exchange of chemical signals between plants and fungi, facilitated by specialized chemical processes that could potentially manifest within the tissues of the host. This research aimed to precisely quantify the portion of the cumulative fungal endophytic community's metabolome detectable within host leaves, and tentatively evaluate its relevance to the host-endophyte interplay. The understory palm Astrocaryum sciophilum (Miq.) Pulle was used as a interesting host plant because of its notable resilience and prolonged life cycle, in a tropical ecosystem.

Method

Using advanced metabolome characterization, including UHPLC-HRMS/MS and molecular networking, the study explored enriched metabolomes of both host leaves and 15 endophytic fungi. The intention was to capture a metabolomic "snapshot" of both host and endophytic community, to achieve a thorough and detailed analysis.

Results and discussion

This approach yielded an extended MS-based molecular network, integrating diverse metadata for identifying host- and endophyte-derived metabolites. The exploration of such data (>24000 features in positive ionization mode) enabled effective metabolome comparison, yielding insights into cultivable endophyte chemodiversity and occurrence of common metabolites between the holobiont and its fungal communities. Surprisingly, a minor subset of features overlapped between host leaf and fungal samples despite significant plant metabolome enrichment. This indicated that fungal metabolic signatures produced in vitro remain sparingly detectable in the leaf. Several classes of primary metabolites were possibly shared. Specific fungal metabolites and/or compounds of their chemical classes were only occasionally discernible in the leaf, highlighting endophytes partial contribution to the overall holobiont metabolome. To our knowledge, the metabolomic study of a plant host and its microbiome has rarely been performed in such a comprehensive manner. The general analytical strategy proposed in this paper seems well-adapted for any study in the field of microbial- or microbiome-related MS and can be applied to most host-microbe interactions.

LOX1- and PLP1-dependent transcriptional reprogramming is essential for injury-induced conidiophore development in a filamentous fungus

IMPORTANCE

In addition to being considered a biocontrol agent, the fungus Trichoderma atroviride is a relevant model for studying mechanisms of response to injury conserved in plants and animals that opens a new landscape in relation to regeneration and cell differentiation mechanisms. Here, we reveal the co-functionality of a lipoxygenase and a patatin-like phospholipase co-expressed in response to wounding in fungi. This pair of enzymes produces oxidized lipids that can function as signaling molecules or oxidative stress signals that, in ascomycetes, induce asexual development. Furthermore, we determined that both genes participate in the regulation of the synthesis of 13-HODE and the establishment of the physiological responses necessary for the formation of reproductive aerial mycelium ultimately leading to asexual development. Our results suggest an injury-induced pathway to produce oxylipins and uncovered physiological mechanisms regulated by LOX1 and PLP1 to induce conidiation, opening new hypotheses for the novo regeneration mechanisms of filamentous fungi.

Fungi under Modified Atmosphere-The Effects of CO2 Stress on Cell Membranes and Description of New Yeast Stenotrophomyces fumitolerans gen. nov., sp. nov

High levels of carbon dioxide are known to inhibit the growth of microorganisms. A total of twenty strains of filamentous fungi and yeasts were isolated from habitats with enriched carbon dioxide concentration. Most strains were derived from modified atmosphere packed (MAP) food products or mofettes and were cultivated under an atmosphere of 20% CO2 and 80% O2. The influence of CO2 on fungal cell membrane fatty acid profiles was examined in this study. Major changes were the increase in linolenic acid (C18:3 cis 9, 12, 15) and, additionally in most strains, linoleic acid (C18:2 cis 9, 12) with a maximum of 24.8%, at the expense of oleic (C18:1 cis 9), palmitic (C16:0), palmitoleic (C16:1 cis 9) and stearic acid (C18:0). The degree of fatty acid unsaturation increased for all of the strains in the study, which consequently led to lower melting temperatures of the cell membranes after incubation with elevated levels of CO2, indicating fluidization of the membrane and a potential membrane malfunction. Growth was reduced in 18 out of 20 strains in laboratory experiments and a change in pigmentation was observed in several strains. Two of the isolated strains, strain WT5 and strain WR1, were found to represent a hitherto undescribed yeast for which the new genus and species Stenotrophomyces fumitolerans (MB# 849906) is proposed.

Quorum Sensing-Mediated Lipid Oxidation Further Regulating the Environmental Adaptability of Aspergillus ochraceus

Quorum sensing (QS) is a cellular strategy of communication between intra- and inter-specific microorganisms, characterized by the release of quorum sensing molecules (QSMs) that achieve coordination to adaptation to the environment. In Aspergillus, lipids carry population density-mediated stresses, and their oxidative metabolite oxylipins act as signaling to transmit information inside cells to regulate fungal development in a synchronized way. In this study, we investigated the regulation of density-dependent lipid metabolism in the toxigenic fungi Aspergillus ochraceus by the oxidative lipid metabolomics in conjunction with transcriptomics. In addition to proven hydroxyoctadecadienoic acids (HODEs), prostaglandins (PGs) also appear to have the properties of QSM. As a class of signaling molecule, oxylipins regulate the fungal morphology, secondary metabolism, and host infection through the G protein signaling pathway. The results of combined omics lay a foundation for further verification of oxylipin function, which is expected to elucidate the complex adaptability mechanism in Aspergillus and realize fungal utilization and damage control.

Variation with In Vitro Analysis of Volatile Profiles among Aspergillus flavus Strains from Louisiana

Volatile organic compounds (VOCs) produced by A. flavus strains were first captured and identified to discern between non-aflatoxigenic and toxigenic phenotypes, and more recently to help with detecting fungal infection, but not with the goal of using VOCs produced by non-aflatoxigenic strains to inhibit growth and/or production of one or more mycotoxins (e.g., aflatoxin and cyclopiazonic acid) by toxigenic aspergilli. In this study, four Aspergillus strains from Louisiana (one non-aflatoxigenic and three toxigenic) were grown on various substrates and had their headspaces captured and analyzed by solid-phase microextraction/gas chromatography/mass spectroscopy (SPME/GC/MS), to find biocontrol and biomarker compounds. Here, we present a collection of nearly 100 fungus-related VOCs, many of which were substrate dependent. Thirty-one were produced across multiple replicates and the rest were observed in a single replicate. At least three VOCs unique to non-aflatoxigenic strain LA1 can be tested for biocontrol properties (e.g., euparone, 4-nonyne), and at least four VOCs unique to toxigenic strains LA2-LA4 can be explored as biomarkers (e.g., 2-heptanone, glycocyamidine) to detect their presence while infecting crops in the field or in storage.

Genomic and Phenotypic Trait Variation of the Opportunistic Human Pathogen Aspergillus flavus and Its Close Relatives

Fungal diseases affect millions of humans annually, yet fungal pathogens remain understudied. The mold Aspergillus flavus can cause both aspergillosis and fungal keratitis infections, but closely related species are not considered clinically relevant. To study the evolution of A. flavus pathogenicity, we examined genomic and phenotypic traits of two strains of A. flavus and three closely related species, Aspergillus arachidicola (two strains), Aspergillus parasiticus (two strains), and Aspergillus nomiae (one strain). We identified >3,000 orthologous proteins unique to A. flavus, including seven biosynthetic gene clusters present in A. flavus strains and absent in the three nonpathogens. We characterized secondary metabolite production for all seven strains under two clinically relevant conditions, temperature and salt concentration. Temperature impacted metabolite production in all species, whereas salinity did not affect production of any species. Strains of the same species produced different metabolites. Growth under stress conditions revealed additional heterogeneity within species. Using the invertebrate fungal disease model Galleria mellonella, we found virulence of strains of the same species varied widely; A. flavus strains were not more virulent than strains of the nonpathogens. In a murine model of fungal keratitis, we observed significantly lower disease severity and corneal thickness for A. arachidicola compared to other species at 48 h postinfection, but not at 72 h. Our work identifies variations in key phenotypic, chemical, and genomic attributes between A. flavus and its nonpathogenic relatives and reveals extensive strain heterogeneity in virulence that does not correspond to the currently established clinical relevance of these species. IMPORTANCE Aspergillus flavus is a filamentous fungus that causes opportunistic human infections, such as aspergillosis and fungal keratitis, but its close relatives are considered nonpathogenic. To begin understanding how this difference in pathogenicity evolved, we characterized variation in infection-relevant genomic, chemical, and phenotypic traits between strains of A. flavus and its relatives. We found extensive variation (or strain heterogeneity) within the pathogenic A. flavus as well as within its close relatives, suggesting that strain-level differences may play a major role in the ability of these fungi to cause disease. Surprisingly, we also found that the virulence of strains from species not considered to be pathogens was similar to that of A. flavus in both invertebrate and murine models of disease. These results contrast with previous studies on Aspergillus fumigatus, another major pathogen in the genus, for which significant differences in infection-relevant chemical and phenotypic traits are observed between closely related pathogenic and nonpathogenic species.

Antifungal Activity and Biochemical Profiling of Exudates from Germinating Maize Nostrano di Storo Local Variety

Plant pathogens are responsible for important damages to valuable crops causing important economic losses. Agrobiodiversity protection is crucial for the valorization of local varieties that could possess higher resistance to biotic and abiotic stress. At the beginning of germination, seeds are susceptible to pathogens attacks, thus they can release endogenous antimicrobial compounds of different natures in the spermosphere, to contrast proliferation of microorganisms. The work aimed at characterizing the maize of local variety Nostrano di Storo seed exudates secreted during the first phases of germination, to identify compounds active in the defense towards pathogens. Storo seed exudates were proven to inhibit F. verticilloides germination. In order to investigate the cause of the described effect, compositional profiling of the exudates was performed through NMR, lipidomic, and proteomic analyses. This study suggests an important role of microbial endophytic communities in the protection of the seed during the early phases of the germination process and their interplay with fatty acids released by the seeds, rather than a specific antifungal compound. The valorization of agronomically acceptable maize lines with pre-harvest enhanced resistances to pathogens contamination could lead, in the near future, to commercially available varieties potentially requiring more limited chemical protective treatments.

Fungal and bacterial oxylipins are signals for intra- and inter-cellular communication within plant disease

Lipids are central at various stages of host-pathogen interactions in determining virulence and modulating plant defense. Free fatty acids may act as substrates for oxidizing enzymes [e.g., lipoxygenases (LOXs) and dioxygenases (DOXs)] that synthesize oxylipins. Fatty acids and oxylipins function as modulators of several pathways in cell-to-cell communication; their structural similarity among plant, fungal, and bacterial taxa suggests potential in cross-kingdom communication. We provide a prospect of the known role of fatty acids and oxylipins in fungi and bacteria during plant-pathogen interactions. In the pathogens, oxylipin-mediated signaling pathways are crucial both in development and host infection. Here, we report on case studies suggesting that oxylipins derived from oleic, linoleic, and linolenic acids are crucial in modulating the pathogenic lifestyle in the host plant. Intriguingly, overlapping (fungi-plant/bacteria-plant) results suggest that different inter-kingdom pathosystems use similar lipid signals to reshape the lifestyle of the contenders and occasionally determine the outcome of the challenge.

The Potential of Fatty Acids and Their Derivatives as Antifungal Agents: A Review

Fungal contamination presents several problems: in humans, health issues arise from infections with opportunistic filamentous fungi and yeast, while in food, fungi cause spoilage and, in particular, in the case of mycotoxigenic fungi, can cause serious health issues. Several types of fatty acids and their derivatives, oxylipins, have been found to have inhibitory effect towards fungal growth and the production of mycotoxins. The use of fatty acids as antifungals could fulfil consumer’s requests of more natural and environmentally friendly compounds, while being less likely to promote fungal resistance. In addition, due to their nature, fatty acids are easily used as food additives. In this work, we review the most relevant and recent studies on the antifungal ability of fatty acids. We focused on saturated fatty acids, unsaturated fatty acids, and oxylipins, their different impact on fungal inhibition, their proposed modes of action, and their ability to impair mycotoxin production. Applications of fatty acids as antifungals and their limitations are also addressed.

Comparison of growth and aflatoxin B1 production profiles of Aspergillus flavus strains on conventional and isogenic GM-maize-based nutritional matrices

Maize grown in both North and South America are now predominantly genetically modified (GM) cultivars with some resistance to herbicide, pesticide, or both. There is little information on the relative colonisation and aflatoxin B₁ (AFB₁) production with maize meal-based nutritional matrices based on kernels of non-GM maize and isogenic GM-ones by strains of Aspergillus flavus. The objectives were to examine the effect of interacting conditions of temperature (25-35 °C) and water availability (0.99-0.90 water activity, a_w) on (a) mycelial growth, (b) AFB₁ production and (c) develop contour maps of optimum and marginal conditions of these parameters for four strains of A. flavus on three different non-GM and isogenic GM-maize based nutritional media. The growth of the four strains of A. flavus (three aflatoxigenic; one non-aflatoxigenic) was relatively similar in relation to the temperature × a_w conditions examined on both non-GM and GM-based matrices. Optimum growth overall was at 30-35 °C and 0.99 a_w for all four strains. Under water stress (0.90 a_w) growth was optimum at 35 °C. Statistically: non-GM, GM cultivars, temperature and a_w all significantly affected growth rates. For AFB₁ production, all single and interacting factors were statistically significant except for non-GM × GM cultivar. In conclusion, colonisation of GM- and non-GM nutritional sources was similar for the different A. flavus strains examined. The contour maps will be very useful for understanding the ecological niches for both toxigenic and non-toxigenic strains in the context of the competitive exclusion of those producing aflatoxins.

Efficacy of Bottle Gourd Seeds' Extracts in Chemical Hazard Reduction Secreted as Toxigenic Fungi Metabolites

Bottle gourd seeds are surrounded by innumerable bioactive components of phytochemicals. This work aimed to evaluate the effectiveness of bottle gourd extracts as antimicrobial and an-ti-mycotoxigenic against toxigenic fungi and mycotoxins. Polar and nonpolar extracts were made from the seeds. The polar eco-friendly extract was prepared by an ultrasonication-assisted technique utilizing aqueous isopropanol (80%), whereas the non-polar extract was obtained using petroleum ether (40–60). The antioxidant efficacy, total phenolic content, and flavonoid content of the extracts were all measured. The fatty acid profile was measured using GC equipment, and the influence on toxigenic fungus and mycotoxin release was also investigated. The antioxidant efficacy of the polar extract is reflected. The total phenolic values of the oil and polar extract were 15.5 and 267 mg of GAE/g, respectively. The total flavonoid content of the oil was 2.95 mg catechol/g, whereas the isopropyl extract of seeds contained 14.86 mg catechol/g. The polar extract inhibited the DPPH more effectively than oil. When compared to other seed oils, the fatty acid composition differed. The pathogens were distinguished by the MIC and MFC for the polar extract. Three sterols were found in the oil, with a high concentration of B-sitosterols. The oil’s valuable -carotene content and tocopherol content were recorded. When compared to traditional antibiotics, the polar extract has shown promising antimicrobial activity against infections and toxigenic fungi. Bottle gourd extracts, as a non-traditional bioactive source, are viewed as a potentially promising alternative that might contribute to increased food safety, shelf-life, and security.

Unfolded Protein Response and Scaffold Independent Pheromone MAP Kinase Signaling Control Verticillium dahliae Growth, Development, and Plant Pathogenesis

Differentiation, growth, and virulence of the vascular plant pathogen Verticillium dahliae depend on a network of interconnected cellular signaling cascades. The transcription factor Hac1 of the endoplasmic reticulum-associated unfolded protein response (UPR) is required for initial root colonization, fungal growth, and vascular propagation by conidiation. Hac1 is essential for the formation of microsclerotia as long-time survival resting structures in the field. Single endoplasmic reticulum-associated enzymes for linoleic acid production as precursors for oxylipin signal molecules support fungal growth but not pathogenicity. Microsclerotia development, growth, and virulence further require the pheromone response mitogen-activated protein kinase (MAPK) pathway, but without the Ham5 scaffold function. The MAPK phosphatase Rok1 limits resting structure development of V.dahliae, but promotes growth, conidiation, and virulence. The interplay between UPR and MAPK signaling cascades includes several potential targets for fungal growth control for supporting disease management of the vascular pathogen V.dahliae.

Brachypodium Phenylalanine Ammonia Lyase (PAL) Promotes Antiviral Defenses against Panicum mosaic virus and Its Satellites

Brachypodium distachyon has recently emerged as a premier model plant for monocot biology, akin to Arabidopsis thaliana We previously reported genome-wide transcriptomic and alternative splicing changes occurring in Brachypodium during compatible infections with Panicum mosaic virus (PMV) and its satellite virus (SPMV). Here, we dissected the role of Brachypodium phenylalanine ammonia lyase 1 (PAL1), a key enzyme for phenylpropanoid and salicylic acid (SA) biosynthesis and the induction of plant defenses. Targeted metabolomics profiling of PMV-infected and PMV- plus SPMV-infected (PMV/SPMV) Brachypodium plants revealed enhanced levels of multiple defense-related hormones and metabolites such as cinnamic acid, SA, and fatty acids and lignin precursors during disease progression. The virus-induced accumulation of SA and lignin was significantly suppressed upon knockdown of B. distachyon PAL1 (BdPAL1) using RNA interference (RNAi). The compromised SA accumulation in PMV/SPMV-infected BdPAL1 RNAi plants correlated with weaker induction of multiple SA-related defense gene markers (pathogenesis related 1 [PR-1], PR-3, PR-5, and WRKY75) and enhanced susceptibility to PMV/SPMV compared to that of wild-type (WT) plants. Furthermore, exogenous application of SA alleviated the PMV/SPMV necrotic disease phenotypes and delayed plant death caused by single and mixed infections. Together, our results support an antiviral role for BdPAL1 during compatible host-virus interaction, perhaps as a last resort attempt to rescue the infected plant.IMPORTANCE Although the role of plant defense mechanisms against viruses are relatively well studied in dicots and in incompatible plant-microbe interactions, studies of their roles in compatible interactions and in grasses are lagging behind. In this study, we leveraged the emerging grass model Brachypodium and genetic resources to dissect Panicum mosaic virus (PMV)- and its satellite virus (SPMV)-compatible grass-virus interactions. We found a significant role for PAL1 in the production of salicylic acid (SA) in response to PMV/SPMV infections and that SA is an essential component of the defense response preventing the plant from succumbing to viral infection. Our results suggest a convergent role for the SA defense pathway in both compatible and incompatible plant-virus interactions and underscore the utility of Brachypodium for grass-virus biology.

Fungal G-Protein-Coupled Receptors: A Promising Mediator of the Impact of Extracellular Signals on Biosynthesis of Ochratoxin A

G-protein-coupled receptors (GPCRs) are transmembrane receptors involved in transducing signals from the external environment inside the cell, which enables fungi to coordinate cell transport, metabolism, and growth to promote their survival, reproduction, and virulence. There are 14 classes of GPCRs in fungi involved in sensing various ligands. In this paper, the synthesis of mycotoxins that are GPCR-mediated is discussed with respect to ligands, environmental stimuli, and intra-/interspecific communication. Despite their apparent importance in fungal biology, very little is known about the role of ochratoxin A (OTA) biosynthesis by Aspergillus ochraceus and the ligands that are involved. Fortunately, increasing evidence shows that the GPCR that involves the AF/ST (sterigmatocystin) pathway in fungi belongs to the same genus. Therefore, we speculate that GPCRs play an important role in a variety of environmental signals and downstream pathways in OTA biosynthesis. The verification of this inference will result in a more controllable GPCR target for control of fungal contamination in the future.

Comparisons of the Sexual Cycles for the Coccidian Parasites Eimeria and Toxoplasma

Toxoplasma gondii and Eimeria spp. are widely prevalent Coccidian parasites that undergo sexual reproduction during their life cycle. T. gondii can infect any warm-blooded animal in its asexual cycle; however, its sexual cycle is restricted to felines. Eimeria spp. are usually restricted to one host species, and their whole life cycle is completed within this same host. The literature reviewed in this article comprises the recent findings regarding the unique biology of the sexual development of T. gondii and Eimeria spp. The molecular basis of sex in these pathogens has been significantly unraveled by new findings in parasite differentiation along with transcriptional analysis of T. gondii and Eimeria spp. pre-sexual and sexual stages. Focusing on the metabolic networks, analysis of these transcriptome datasets shows enrichment for several different metabolic pathways. Transcripts for glycolysis enzymes are consistently more abundant in T. gondii cat infection stages than the asexual tachyzoite stage and Eimeria spp. merozoite and gamete stages compared to sporozoites. Recent breakthroughs in host-pathogen interaction and host restriction have significantly expanded the understating of the unique biology of these pathogens. This review aims to critically explore advances in the sexual cycle of Coccidia parasites with the ultimate goal of comparing and analyzing the sexual cycle of Eimeria spp. and T. gondii.

The Aspergilli and Their Mycotoxins: Metabolic Interactions With Plants and the Soil Biota

Species of the highly diverse fungal genus Aspergillus are well-known agricultural pests, and, most importantly, producers of various mycotoxins threatening food safety worldwide. Mycotoxins are studied predominantly from the perspectives of human and livestock health. Meanwhile, their roles are far less known in nature. However, to understand the factors behind mycotoxin production, the roles of the toxins of Aspergilli must be understood from a complex ecological perspective, taking mold-plant, mold-microbe, and mold-animal interactions into account. The Aspergilli may switch between saprophytic and pathogenic lifestyles, and the production of secondary metabolites, such as mycotoxins, may vary according to these fungal ways of life. Recent studies highlighted the complex ecological network of soil microbiotas determining the niches that Aspergilli can fill in. Interactions with the soil microbiota and soil macro-organisms determine the role of secondary metabolite production to a great extent. While, upon infection of plants, metabolic communication including fungal secondary metabolites like aflatoxins, gliotoxin, patulin, cyclopiazonic acid, and ochratoxin, influences the fate of both the invader and the host. In this review, the role of mycotoxin producing Aspergillus species and their interactions in the ecosystem are discussed. We intend to highlight the complexity of the roles of the main toxic secondary metabolites as well as their fate in natural environments and agriculture, a field that still has important knowledge gaps.

Exposure of Aspergillus flavus NRRL 3357 to the Environmental Toxin, 2,3,7,8-Tetrachlorinated Dibenzo-p-Dioxin, Results in a Hyper Aflatoxicogenic Phenotype: A Possible Role for Caleosin/Peroxygenase (AfPXG)

Aflatoxins (AFs) as potent food contaminants are highly detrimental to human and animal health. The production of such biological toxins is influenced by environmental factors including pollutants, such as dioxins. Here, we report the biological feedback of an active AF-producer strain of A. flavus upon in vitro exposure to the most toxic congener of dioxins, the 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD). The phenotype of TCDD-exposed A. flavus was typified by a severe limitation in vegetative growth, activation of conidia formation and a significant boost in AF production. Furthermore, the level of reactive oxygen species (ROS) in fungal protoplast was increased (3.1- to 3.8-fold) in response to TCDD exposure at 10 and 50 ng mL-1, respectively. In parallel, superoxide dismutase (SOD) and catalase (CAT) activities were, respectively, increased by a factor of 2 and 3. In contrast to controls, transcript, protein and enzymatic activity of caleosin/peroxygenase (AfPXG) was also significantly induced in TCDD-exposed fungi. Subsequently, fungal cells accumulated fivefold more lipid droplets (LDs) than controls. Moreover, the TCDD-exposed fungi exhibited twofold higher levels of AFB1. Interestingly, TCDD-induced hyperaflatoxicogenicity was drastically abolished in the AfPXG-silencing strain of A. flavus, suggesting a role for AfPXG in fungal response to TCDD. Finally, TCDD-exposed fungi showed an increased in vitro virulence in terms of sporulation and AF production. The data highlight the possible effects of dioxin on aflatoxicogenicity of A. flavus and suggest therefore that attention should be paid in particular to the potential consequences of climate change on global food safety.

Carbohydrate, glutathione, and polyamine metabolism are central to Aspergillus flavus oxidative stress responses over time

Background

The primary and secondary metabolites of fungi are critical for adaptation to environmental stresses, host pathogenicity, competition with other microbes, and reproductive fitness. Drought-derived reactive oxygen species (ROS) have been shown to stimulate aflatoxin production and regulate in Aspergillus flavus, and may function in signaling with host plants. Here, we have performed global, untargeted metabolomics to better understand the role of aflatoxin production in oxidative stress responses, and also explore isolate-specific oxidative stress responses over time.

Results

Two field isolates of A. flavus, AF13 and NRRL3357, possessing high and moderate aflatoxin production, respectively, were cultured in medium with and without supplementation with 15 mM H₂O₂, and mycelia were collected following 4 and 7 days in culture for global metabolomics. Overall, 389 compounds were described in the analysis which encompassed 9 biological super-pathways and 47 sub-pathways. These metabolites were examined for differential accumulation. Significant differences were observed in both isolates in response to oxidative stress and when comparing sampling time points.

Conclusions

The moderately high aflatoxin-producing isolate, NRRL3357, showed extensive stimulation of antioxidant mechanisms and pathways including polyamines metabolism, glutathione metabolism, TCA cycle, and lipid metabolism while the highly aflatoxigenic isolate, AF13, showed a less vigorous response to stress. Carbohydrate pathway levels also imply that carbohydrate repression and starvation may influence metabolite accumulation at the later timepoint. Higher conidial oxidative stress tolerance and antioxidant capacity in AF13 compared to NRRL3357, inferred from their metabolomic profiles and growth curves over time, may be connected to aflatoxin production capability and aflatoxin-related antioxidant accumulation. The coincidence of several of the detected metabolites in H₂O₂-stressed A. flavus and drought-stressed hosts also suggests their potential role in the interaction between these organisms and their use as markers/targets to enhance host resistance through biomarker selection or genetic engineering.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1580-x) contains supplementary material, which is available to authorized users.

Sensing and transduction of nutritional and chemical signals in filamentous fungi: Impact on cell development and secondary metabolites biosynthesis

Filamentous fungi respond to hundreds of nutritional, chemical and environmental signals that affect expression of primary metabolism and biosynthesis of secondary metabolites. These signals are sensed at the membrane level by G protein coupled receptors (GPCRs). GPCRs contain usually seven transmembrane domains, an external amino terminal fragment that interacts with the ligand, and an internal carboxy terminal end interacting with the intracellular G protein. There is a great variety of GPCRs in filamentous fungi involved in sensing of sugars, amino acids, cellulose, cell-wall components, sex pheromones, oxylipins, calcium ions and other ligands. Mechanisms of signal transduction at the membrane level by GPCRs are discussed, including the internalization and compartmentalisation of these sensor proteins. We have identified and analysed the GPCRs in the genome of Penicillium chrysogenum and compared them with GPCRs of several other filamentous fungi. We have found 66 GPCRs classified into 14 classes, depending on the ligand recognized by these proteins, including most previously proposed classes of GPCRs. We have found 66 putative GPCRs, representatives of twelve of the fourteen previously proposed classes of GPCRs, depending on the ligand recognized by these proteins. A staggering fortytwo putative members of the new GPCR class XIV, the so-called Pth11 sensors of cellulosic material as reported for Neurospora crassa and some other fungi, were identified. Several GPCRs sensing sex pheromones, known in yeast and in several fungi, were also identified in P. chrysogenum, confirming the recent unravelling of the hidden sexual capacity of this species. Other sensing mechanisms do not involve GPCRs, including the two-component systems (HKRR), the HOG signalling system and the PalH mediated pH transduction sensor. GPCR sensor proteins transmit their signals by interacting with intracellular heterotrimeric G proteins, that are well known in several fungi, including P. chrysogenum. These G proteins are inactive in the GDP containing heterotrimeric state, and become active by nucleotide exchange, allowing the separation of the heterotrimeric protein in active Gα and Gβγ dimer subunits. The conversion of GTP in GDP is mediated by the endogenous GTPase activity of the G proteins. Downstream of the ligand interaction, the activated Gα protein and also the Gβ/Gγ dimer, transduce the signals through at least three different cascades: adenylate cyclase/cAMP, MAPK kinase, and phospholipase C mediated pathways.

Fathoming Aspergillus oryzae metabolomes in formulated growth matrices

The stochasticity of Aspergillus oryzae (Trivially: the koji mold) pan-metabolomes commensurate with its ubiquitously distributed landscapes, i.e. growth matrices have been seemed uncharted since its food fermentative systems are mostly being investigated. In this review, we explicitly have discussed the likely tendencies of A. oryzae metabolomes pertaining to its growth milieu formulated with substrate matrices of varying nature, composition, texture, and associated physicochemical parameters. We envisaged typical food matrices, namely, meju, koji, and moromi as the semi-natural cultivation models toward delineating the metabolomic patterns of the koji mold, which synergistically influences the organoleptic and functional properties of the end products. Further, we highlighted how tailored conditions in sub-natural growth matrices, i.e. synthetic cultivation media blends, inducers, and growth surfaces, may influence A. oryzae metabolomes and targeted phenotypes. In general, the sequential or synchronous growth of A. oryzae on formulated matrices results in a number of metabolic tradeoffs with its immediate microenvironment influencing its adaptive and regulatory metabolomes. In broader context, evaluating the metabolic plasticity of A. oryzae relative to the tractable variables in formulated growth matrices might help approximate its growth and metabolism in the more complex natural matrices and environs. These approaches may considerably help in the design and manipulation of hybrid cultivation systems towards the efficient harnessing of commercial molds.

Disturbance in biosynthesis of arachidonic acid impairs the sexual development of the onion blight pathogen Stemphylium eturmiunum

The formation of sexual fruiting bodies for plant pathogenic fungi is a key strategy to propagate their progenies upon environmental stresses. Stemphylium eturmiunum is an opportunistic plant pathogen fungus causing blight in onion. This self-fertilizing filamentous ascomycete persists in the soil by forming pseudothecia, the sexual fruiting body which helps the fungus survive in harsh environments. However, the regulatory mechanism of pseudothecial formation remains unknown. To uncover the mechanism for pseudothecial formation so as to find a practical measure to control the propagation of this onion pathogen, we tentatively used DNA methyltransferase inhibitor 5-azacytidine (5-AC) to treat S. eturmiunum. 5-AC treatment silenced the gene-encoding monoacylglycerol lipase (magl) concomitant with the presence of the inheritable fluffy phenotype and defectiveness in pseudothecial development. Moreover, the silence of magl also resulted in a reduction of arachidonic acid (AA) formation from 27 ± 3.1 µg/g to 9.5 ± 1.5 µg/g. To correlate the biosynthesis of AA and pseudothecial formation, we created magl knockdown and overexpression strains. Knockdown of magl reduced AA to 11 ± 2.4 µg/g, which subsequently disabled pseudothecial formation. In parallel, overexpression of magl increased AA to 37 ± 3.4 µg/g, which also impaired pseudothecial formation. Furthermore, exogenous addition of AA to the culture of magl-silenced or magl knockdown strains rescued the pseudothecial formation but failed in the gpr1 knockdown strain of S. eturmiunum, which implicates the involvement of AA in signal transduction via a putative G protein-coupled receptor 1. Thus, AA at a cellular level of 27 ± 3.1 µg/g is essential for sexual development of S. eturmiunum. Disturbance in the biosynthesis of AA by up- and down-regulating the expression of magl disables the pseudothecial development. The specific requirement for AA in pseudothecial development by S. eturmiunum provides a hint to curb this onion pathogen: to impede pseudothecial formation by application of AA.

Identification of an Asexual Reproduction Inducer of Phytopathogenic and Toxigenic Fusarium

Asexual and sexual reproduction are the most important biological events in the life cycle of phytopathogenic and toxigenic Fusarium and are responsible for disease epidemics. However, the signaling molecules which induce the asexual reproduction of Fusarium are unknown. Herein we describe the structure elucidation, including the absolute configuration, of Fusarium asexual reproduction inducer (FARI), a new sesquiterpene derivative, by spectroscopic analysis, total synthesis, and conidium-inducing assays of synthetic isomers. We have also uncovered the universality of FARI among Fusarium species. Moreover, a mechanism-of-action study suggested that the Gpmk1 and LaeA signaling pathways are required for conidium formation induced by FARI; conversely, the Mgv1 of mitogen-activated protein kinase is not involved in conidium formation. FARI exhibited conidium-inducing activity at an extremely low dose and high stereoselectivity, which may suggest the presence of a stereospecific target.

Volatiles Mediated Interactions Between Aspergillus oryzae Strains Modulate Morphological Transition and Exometabolomes

Notwithstanding its mitosporic nature, an improbable morpho-transformation state i. e., sclerotial development (SD), is vaguely known in Aspergillus oryzae. Nevertheless an intriguing phenomenon governing mold's development and stress response, the effects of exogenous factors engendering SD, especially the volatile organic compounds (VOCs) mediated interactions (VMI) pervasive in microbial niches have largely remained unexplored. Herein, we examined the effects of intra-species VMI on SD in A. oryzae RIB 40, followed by comprehensive analyses of associated growth rates, pH alterations, biochemical phenotypes, and exometabolomes. We cultivated A. oryzae RIB 40 (S1_VMI: KACC 44967) opposite a non-SD partner strain, A. oryzae (S2: KCCM 60345), conditioning VMI in a specially designed “twin plate assembly.” Notably, SD in S1_VMI was delayed relative to its non-conditioned control (S1) cultivated without partner strain (S2) in twin plate. Selectively evaluating A. oryzae RIB 40 (S1_VMI vs. S1) for altered phenotypes concomitant to SD, we observed a marked disparity for corresponding growth rates (S1_VMI < S1)_7days, media pH (S1_VMI > S1)_7days, and biochemical characteristics viz., protease (S1_VMI > S1)_7days, amylase (S1_VMI > nS1)_3–7days, and antioxidants (S1_VMI > S1)_7days levels. The partial least squares—discriminant analysis (PLS-DA) of gas chromatography—time of flight—mass spectrometry (GC-TOF-MS) datasets for primary metabolites exhibited a clustered pattern (PLS1, 22.04%; PLS2, 11.36%), with 7 days incubated S1_VMI extracts showed higher abundance of amino acids, sugars, and sugar alcohols with lower organic acids and fatty acids levels, relative to S1. Intriguingly, the higher amino acid and sugar alcohol levels were positively correlated with antioxidant activity, likely impeding SD in S1_VMI. Further, the PLS-DA (PLS1, 18.11%; PLS2, 15.02%) based on liquid chromatography—mass spectrometry (LC-MS) datasets exhibited a notable disparity for post-SD (9–11 days) sample extracts with higher oxylipins and 13-desoxypaxilline levels in S1_VMI relative to S1, intertwining Aspergillus morphogenesis and secondary metabolism. The analysis of VOCs for the 7 days incubated samples displayed considerably higher accumulation of C-8 compounds in the headspace of twin-plate experimental sets (S1_VMI:S2) compared to those in non-conditioned controls (S1 and S2—without respective partner strains), potentially triggering altered morpho-transformation and concurring biochemical as well as metabolic states in molds.

Advances in molecular and genomic research to safeguard food and feed supply from aflatoxin contamination

Worldwide recognition that aflatoxin contamination of agricultural commodities by the fungus Aspergillus flavus is a global problem has significantly benefitted from global collaboration for understanding the contaminating fungus, as well as for developing and implementing solutions against the contamination. The effort to address this serious food and feed safety issue has led to a detailed understanding of the taxonomy, ecology, physiology, genomics and evolution of A. flavus, as well as strategies to reduce or control pre-harvest aflatoxin contamination, including (1) biological control, using atoxigenic aspergilli, (2) proteomic and genomic analyses for identifying resistance factors in maize as potential breeding markers to enable development of resistant maize lines, and (3) enhancing host-resistance by bioengineering of susceptible crops, such as cotton, maize, peanut and tree nuts. A post-harvest measure to prevent the occurrence of aflatoxin contamination in storage is also an important component for reducing exposure of populations worldwide to aflatoxins in food and feed supplies. The effect of environmental changes on aflatoxin contamination levels has recently become an important aspect for study to anticipate future contamination levels. The ability of A. flavus to produce dozens of secondary metabolites, in addition to aflatoxins, has created a new avenue of research for understanding the role these metabolites play in the survival and biodiversity of this fungus. The understanding of A. flavus, the aflatoxin contamination problem, and control measures to prevent the contamination has become a unique example for an integrated approach to safeguard global food and feed safety.

Deciphering drought-induced metabolic responses and regulation in developing maize kernels

Drought stress conditions decrease maize growth and yield, and aggravate preharvest aflatoxin contamination. While several studies have been performed on mature kernels responding to drought stress, the metabolic profiles of developing kernels are not as well characterized, particularly in germplasm with contrasting resistance to both drought and mycotoxin contamination. Here, following screening for drought tolerance, a drought-sensitive line, B73, and a drought-tolerant line, Lo964, were selected and stressed beginning at 14 days after pollination. Developing kernels were sampled 7 and 14 days after drought induction (DAI) from both stressed and irrigated plants. Comparative biochemical and metabolomic analyses profiled 409 differentially accumulated metabolites. Multivariate statistics and pathway analyses showed that drought stress induced an accumulation of simple sugars and polyunsaturated fatty acids and a decrease in amines, polyamines and dipeptides in B73. Conversely, sphingolipid, sterol, phenylpropanoid and dipeptide metabolites accumulated in Lo964 under drought stress. Drought stress also resulted in the greater accumulation of reactive oxygen species (ROS) and aflatoxin in kernels of B73 in comparison with Lo964 implying a correlation in their production. Overall, field drought treatments disordered a cascade of normal metabolic programming during development of maize kernels and subsequently caused oxidative stress. The glutathione and urea cycles along with the metabolism of carbohydrates and lipids for osmoprotection, membrane maintenance and antioxidant protection were central among the drought stress responses observed in developing kernels. These results also provide novel targets to enhance host drought tolerance and disease resistance through the use of biotechnologies such as transgenics and genome editing.

Functional analysis of diacylglycerol O-acyl transferase 2 gene to decipher its role in virulence of Botrytis cinerea

Gray mold disease inflicted by Botrytis cinerea is a serious menace responsible for significant economic loss worldwide. Due to its polyphagous nature, the pathogen has enthused inquisitiveness in researchers to unravel its complexity. Agrobacterium tumefaciens-mediated transformation was used to generate insertional mutants of Botrytis cinerea. A mutant (BCM-55) with disruption in a gene (BcDGAT2) that encodes for diacylglycerol O-acyl transferase 2 (DGAT2), showed enervated virulence on various hosts' tissues. Enzyme DGAT2 is crucial in the final step of synthesis of triacylglycerol (TAG) that plays an important role in homeostasis of membrane and cellular processes. However, the role of DGAT2 has never been reported in a phytopathogenic fungus. In this study, BCM-55 was characterized to ascertain the role of DGAT2 in virulence of B. cinerea. The insertional mutant was defective in spore production and lacked sclerotia formation as a consequence of lower accumulation of TAG. A significant delay in spore germination in BCM-55 was accompanied with a low penetration potential. Hyphae of the mutant formed swollen endings with considerable impairment in penetration. Deletion of BcDGAT2 also led to increased sensitivity towards cell wall and membrane-disturbing agents. Furthermore, BCM-55 was deficient in the production of oxalic acid and showed lower activity of a cell wall-degrading enzyme, polygalacturonase. The role of BcDGAT2 in virulence was further confirmed by targeted deletion and complementation of the gene. The results insinuate a crucial role of BcDGAT2 in penetration and consequently virulence of B. cinerea. The study provides novel insights into plant-pathogen interactions that can be exploited to develop suitable disease management strategies.

Metabolites contributing to Rhizoctonia solani AG-1-IA maturation and sclerotial differentiation revealed by UPLC-QTOF-MS metabolomics

Rhizoctonia solani is a causative agent of sheath blight, which results in huge economic losses every year. During its life cycle, the formation of sclerotia helps Rhizoctonia solani withstand a variety of unfavorable factors. Oxidative stress is a key factor that induces sclerotium formation. The differentiated and undifferentiated phenotypes of R. solani AG-1-IA were obtained by controlling aerial conditions. Metabolomics based on the mass spectrometry technique combined with multivariate and univariate analyses was used to investigate the metabolic variation in vegetative, differentiated and undifferentiated mycelia. Our results revealed that during maturation, the metabolic levels of N2-acetyl-L-ornithine, 3,1'-(OH)2-Gamma-carotene, (5Z,7E)-(1S,3R)-24,24-difluoro-24a-homo-9,10-seco-5,7,10(19)-cholestatrien-1,3,25-triol, stoloniferone O, PA(O-18:0/12:0), PA(P-16:0/14:0), PA(P-16:0/16:(19Z)) and PA(P-16:0/17:2(9Z,12Z)) were suppressed in both differentiated and undifferentiated mycelia. The concentrations of PE(20:1(11Z)/14:1(9Z)), PE(P-16:0/20:4(5Z,8Z,11Z,13E)(15OH[S])) and PS(12:0/18:1(9Z)) were increased in the differentiated group, while increased levels of N(gamma)-nitro-L-arginine, tenuazonic acid and 9S,10S,11R-trihydroxy-12Z,15Z-octadecadienoic acid were found in the undifferentiated group. Our results suggest that different levels of these metabolites may act as biomarkers for the developmental stages of R. solani AG-1-IA. Moreover, the mechanisms of sclerotium formation and mycelium differentiation were elucidated at the metabolic level.

Air-borne genotype by genotype indirect genetic effects are substantial in the filamentous fungus Aspergillus nidulans

Genotype by genotype indirect genetic effects (G × G IGEs) occur when the phenotype of an individual is influenced by an interaction between its own genotype and those of neighbour individuals. Little is known regarding the relative importance of G × G IGEs compared with other forms of direct and indirect genetic effects. We quantified the relative importance of IGEs in the filamentous fungus Aspergillus nidulans, a species in which IGEs are likely to be important as air-borne social interactions are known to affect growth. We used a collection of distantly related wild isolates, lab strains and a set of closely related mutation accumulation lines to estimate the contribution of direct and indirect genetic effects on mycelium growth rate, a key fitness component. We found that indirect genetic effects were dominated by G × G IGEs that occurred primarily between a focal genotype and its immediate neighbour within a vertical stack, and these accounted for 11% of phenotypic variation. These results indicate that G × G IGEs may be substantial, at least in some systems, and that the evolutionary importance of these interactions may be underappreciated, especially in microbes. We advocate for a wider use of the IGE framework in both applied (for example, choice of varietal mixtures in plant breeding) and evolutionary genetics (kin selection/kin competition studies).

Specific Caleosin/Peroxygenase and Lipoxygenase Activities Are Tissue-Differentially Expressed in Date Palm (Phoenix dactylifera L.) Seedlings and Are Further Induced Following Exposure to the Toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin

Two caleosin/peroxygenase isoforms from date palm, Phoenix dactylifera L., PdCLO2 and PdCLO4, were characterized with respect to their tissue expression, subcellular localization, and oxylipin pathway substrate specificities in developing seedlings. Both PdCLO2 and PdCLO4 had peroxygenase activities that peaked at the mid-stage (radicle length of 2.5 cm) of seedling growth and were associated with the lipid droplet (LD) and microsomal fractions. Recombinant PdCLO2 and PdCLO4 proteins heterologously expressed in yeast cells were localized in both LD and microsomal fractions. Each of the purified recombinant proteins exhibited peroxygenase activity but they were catalytically distinct with respect to their specificity and product formation from fatty acid epoxide and hydroxide substrates. We recently showed that date palm CLO genes were upregulated following exposure to the potent toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Hanano et al., 2016), and we show here that transcripts of 9- and 13-lipoxygenase (LOX) genes were also induced by TCDD exposure. At the enzyme level, 9-LOX and 13-LOX activities were present in a range of seedling tissues and responded differently to TCDD exposure, as did the 9- and 13-fatty acid hydroperoxide reductase activities. This demonstrates that at least two branches of the oxylipin pathway are involved in responses to the environmental organic toxin, TCDD in date palm.

How climate change and regulations can affect the economics of mycotoxins

In the decades to come, the one factor that will likely have the greatest effect on the economics of the mycotoxin problem is climate change. This article reviews the current state of known science on how the global climate has been changing in recent decades, as well as likely climate change trends in the near future. The article focuses in depth on how climatic variables affect fungal infection and production of specific mycotoxins in food crops, and how near-future climatic changes will shape the prevalence of these mycotoxins in crops in different parts of the world. Because of regulatory limits set on maximum allowable levels of mycotoxins in food and feed, growers will experience economic losses if climatic factors cause certain mycotoxins to become more prevalent. A case study is presented of how maize growers in the United States will experience increased economic losses due to slightly higher aflatoxin levels in maize, even if those levels may still be below regulatory limits. We discuss the overall expected economic impacts of climate change-induced mycotoxin contamination worldwide – not just market-related losses, but also losses to human and animal health and risks to food security. Aflatoxin is the mycotoxin that is most likely to increase under near-future climate scenarios; and thus is likely to pose the greatest amount of economic risk of all the mycotoxins.

The Fungus Trichoderma Regulates Submerged Conidiation Using the Steroid Pregnenolone

In previous work, we evolved a population of Trichoderma citrinoviride in liquid cultures to speed up its asexual development cycle. The evolved population, called T-6, formed conidia 3 times sooner and in >1000-fold greater numbers. Here, we identify the steroid pregnenolone as a molecular signal for this different behavior. Media in which the ancestral T. citrinoviride population was grown (called ancestral spent media) contained a submerged conidiation inhibitor. Growing the evolved population T-6 in ancestral spent media eliminated the abundant formation of conidia. This inhibition depended on the amount and age of the ancestral spent medium and the time that the ancestral spent medium was added to the T-6 culture. Fractionation of the ancestral spent medium identified a hydrophobic inhibiting compound with a molecular weight less than 2000 g/mol. A combination of GC-MS, ELISA, and reaction with cholesterol oxidase identified it as pregnenolone. The addition of pregnenolone to cultures of T-6 inhibited submerged conidiation by inhibiting formation of conidiophores, while 10 other analogous steroids did not. Pregnenolone also inhibited submerged conidiation of Fusarium graminearum PH-1, a plant pathogen that causes head blight in wheat and barley. This identification of steroids as signal molecules in fungi creates opportunities to disrupt this signaling to control fungal behavior.

Interactive Effects of Elevated [CO2] and Drought on the Maize Phytochemical Defense Response against Mycotoxigenic Fusarium verticillioides

Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. We recently demonstrated that elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels remain unaffected. In this study we show that maize simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Despite the increase in fumonisin levels, the amount of fumonisin produced in relation to pathogen biomass remained lower than corresponding plants grown at ambient [CO2]. Therefore, the increase in fumonisin contamination was likely due to even greater pathogen biomass rather than an increase in host-derived stimulants. Drought did not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. However, since elevated [CO2] does not influence the drought-induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the effects elevated [CO2] are negated belowground, but the stifled defense response aboveground may be a consequence of resource redirection to the roots.

De novo assembly and transcriptome analysis of sclerotial development in Wolfiporia cocos

Wolfiporia cocos Ryvarden et Gilbertson, a well-known medicinal fungus in the Basidiomycetes, is widely distributed in East Asia. Its dried sclerotium, which is known as Fuling in China, has been used as a traditional crude drug in Chinese traditional medicine for thousand years. However, little is known about how the sclerotium is developed at the genetic level. In this study, the de novo sequencing of sclerotia of W. cocos (S1_initial stage; S2_developmental stage and S3_mature stage) was carried out by illumina HiSeq 2000 technology. 27,438 unigenes were assembled from ~30Gbp raw data, and 12,093 unigenes were significantly annotated. The analysis of expression profiles during development returned 304 differentially expressed genes (DEGs), which were clustered into four different groups according to their expression trends. Especially for the maturation stage (S3), the sclerotium exhibited a markedly different expression profile from other stages. We further showed that peroxisome, unsaturation of fatty acids and degradation pathway were respectively prevalent in S1, S2 and S3 stages as evidenced by enrichment analysis. To our knowledge, this study represents the first report of sclerotial development transcriptomics in W. cocos. The obtained results provide novel insights into the developmental biology of the sclerotia, which is helpful for future studies about cultivation and breeding of W. cocos.

Friends or foes? Emerging insights from fungal interactions with plants

Fungi interact with plants in various ways, with each interaction giving rise to different alterations in both partners. While fungal pathogens have detrimental effects on plant physiology, mutualistic fungi augment host defence responses to pathogens and/or improve plant nutrient uptake. Tropic growth towards plant roots or stomata, mediated by chemical and topographical signals, has been described for several fungi, with evidence of species-specific signals and sensing mechanisms. Fungal partners secrete bioactive molecules such as small peptide effectors, enzymes and secondary metabolites which facilitate colonization and contribute to both symbiotic and pathogenic relationships. There has been tremendous advancement in fungal molecular biology, omics sciences and microscopy in recent years, opening up new possibilities for the identification of key molecular mechanisms in plant-fungal interactions, the power of which is often borne out in their combination. Our fragmentary knowledge on the interactions between plants and fungi must be made whole to understand the potential of fungi in preventing plant diseases, improving plant productivity and understanding ecosystem stability. Here, we review innovative methods and the associated new insights into plant-fungal interactions.

Production of cross-kingdom oxylipins by pathogenic fungi: An update on their role in development and pathogenicity

Oxylipins are a class of molecules derived from the incorporation of oxygen into polyunsaturated fatty acid substrates through the action of oxygenases. While extensively investigated in the context of mammalian immune responses, over the last decade it has become apparent that oxylipins are a common means of communication among and between plants, animals, and fungi to control development and alter host-microbe interactions. In fungi, some oxylipins are derived nonenzymatically while others are produced by lipoxygenases, cyclooxygenases, and monooxygenases with homology to plant and human enzymes. Recent investigations of numerous plant and human fungal pathogens have revealed oxylipins to be involved in the establishment and progression of disease. This review highlights oxylipin production by pathogenic fungi and their role in fungal development and pathogen/host interactions.

Specificity of acyl-CoA binding protein to acyl-CoAs: influence on the lipid metabolism in Aspergillus oryzae

Acyl-CoA binding protein (ACBP) is involved in lipid metabolism and regulation of gene expression in eukaryotic cells, however, the specific functional roles of this important class of proteins remain to be elucidated.

Deep sequencing analysis of transcriptomes in Aspergillus flavus in response to resveratrol

Background

Resveratrol has been reported as a natural phytoalexin that inhibits infection or the growth of certain fungi including Aspergillus flavus. Our previous research revealed that aflatoxin production in A. flavus was reduced in medium with resveratrol. To understand the molecular mechanism of the A. flavus response to resveratrol treatment, the high-throughput paired-end RNA-Seq was applied to analyze the transcriptomic profiles of A. flavus.

Results

In total, 366 and 87 genes of A. flavus were significantly up- and down- regulated, respectively, when the fungus was treated with resveratrol. Gene Ontology (GO) functional enrichment analysis revealed that 48 significantly differentially expressed genes were involved in 6 different terms. Most genes in the aflatoxin biosynthetic pathway genes cluster (#54) did not show a significant change when A. flavus was treated with resveratrol, but 23 of the 30 genes in the #54 cluster were down-regulated. The transcription of aflA and aflB was significantly suppressed under resveratrol treatment, resulting in an insufficient amount of the starter unit hexanoate for aflatoxin biosynthesis. In addition, resveratrol significantly increased the activity of antioxidative enzymes that destroy radicals, leading to decreased aflatoxin production. Moreover, stuA, fluG, flbC, and others genes involved in mycelial and conidial development were down-regulated, which disrupted the cell’s orderly differentiation and blocked conidia formation and mycelia development. The transcripts of laeA and veA were slightly inhibited by resveratrol, which may partly decrease aflatoxin production and depress conidia formation.

Conclusions

Resveratrol can affect the expression of A. flavus genes that are related to developmental and secondary metabolic processes, resulting in decreased aflatoxin production and conidia formation and could also cause abnormal mycelia development. These results provide insight into the transcriptome of A. flavus in response to resveratrol and a new clew for further study in regulation of aflatoxin biosynthesis in A. flavus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0513-6) contains supplementary material, which is available to authorized users.

Using genome-wide associations to identify metabolic pathways involved in maize aflatoxin accumulation resistance

BACKGROUND

Aflatoxin is a potent carcinogen that can contaminate grain infected with the fungus Aspergillus flavus. However, resistance to aflatoxin accumulation in maize is a complex trait with low heritability. Here, two complementary analyses were performed to better understand the mechanisms involved. The first coupled results of a genome-wide association study (GWAS) that accounted for linkage disequilibrium among single nucleotide polymorphisms (SNPs) with gene-set enrichment for a pathway-based approach. The rationale was that the cumulative effects of genes in a pathway would give insight into genetic differences that distinguish resistant from susceptible lines of maize. The second involved finding non-pathway genes close to the most significant SNP-trait associations with the greatest effect on reducing aflatoxin in multiple environments. Unlike conventional GWAS, the latter analysis emphasized multiple aspects of SNP-trait associations rather than just significance and was performed because of the high genotype x environment variability exhibited by this trait.

RESULTS

The most significant metabolic pathway identified was jasmonic acid (JA) biosynthesis. Specifically, there was at least one allelic variant for each step in the JA biosynthesis pathway that conferred an incremental decrease to the level of aflatoxin observed among the inbred lines in the GWAS panel. Several non-pathway genes were also consistently associated with lowered aflatoxin levels. Those with predicted functions related to defense were: leucine-rich repeat protein kinase, expansin B3, reversion-to-ethylene sensitivity1, adaptor protein complex2, and a multidrug and toxic compound extrusion protein.

CONCLUSIONS

Our genetic analysis provided strong evidence for several genes that were associated with aflatoxin resistance. Inbred lines that exhibited lower levels of aflatoxin accumulation tended to share similar haplotypes for genes specifically in the pathway of JA biosynthesis, along with several non-pathway genes with putative defense-related functions. Knowledge gained from these two complementary analyses has improved our understanding of population differences in aflatoxin resistance.

Secondary metabolites in fungus-plant interactions

Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

A Caleosin-Like Protein with Peroxygenase Activity Mediates Aspergillus flavus Development, Aflatoxin Accumulation, and Seed Infection

Caleosins are a small family of calcium-binding proteins endowed with peroxygenase activity in plants. Caleosin-like genes are present in fungi; however, their functions have not been reported yet. In this work, we identify a plant caleosin-like protein in Aspergillus flavus that is highly expressed during the early stages of spore germination. A recombinant purified 32-kDa caleosin-like protein supported peroxygenase activities, including co-oxidation reactions and reduction of polyunsaturated fatty acid hydroperoxides. Deletion of the caleosin gene prevented fungal development. Alternatively, silencing of the gene led to the increased accumulation of endogenous polyunsaturated fatty acid hydroperoxides and antioxidant activities but to a reduction of fungal growth and conidium formation. Two key genes of the aflatoxin biosynthesis pathway, aflR and aflD, were downregulated in the strains in which A. flavus PXG (AfPXG) was silenced, leading to reduced aflatoxin B1 production in vitro. Application of caleosin/peroxygenase-derived oxylipins restored the wild-type phenotype in the strains in which AfPXG was silenced. PXG-deficient A. flavus strains were severely compromised in their capacity to infect maize seeds and to produce aflatoxin. Our results uncover a new branch of the fungal oxylipin pathway and may lead to the development of novel targets for controlling fungal disease.

Biotechnological advances for combating Aspergillus flavus and aflatoxin contamination in crops

Aflatoxins are toxic, carcinogenic, mutagenic, teratogenic and immunosuppressive byproducts of Aspergillus spp. that contaminate a wide range of crops such as maize, peanut, and cotton. Aflatoxin not only affects crop production but renders the produce unfit for consumption and harmful to human and livestock health, with stringent threshold limits of acceptability. In many crops, breeding for resistance is not a reliable option because of the limited availability of genotypes with durable resistance to Aspergillus. Understanding the fungal/crop/environment interactions involved in aflatoxin contamination is therefore essential in designing measures for its prevention and control. For a sustainable solution to aflatoxin contamination, research must be focused on identifying and improving knowledge of host-plant resistance factors to aflatoxin accumulation. Current advances in genetic transformation, proteomics, RNAi technology, and marker-assisted selection offer great potential in minimizing pre-harvest aflatoxin contamination in cultivated crop species. Moreover, developing effective phenotyping strategies for transgenic as well as precision breeding of resistance genes into commercial varieties is critical. While appropriate storage practices can generally minimize post-harvest aflatoxin contamination in crops, the use of biotechnology to interrupt the probability of pre-harvest infection and contamination has the potential to provide sustainable solution.