Functional Genomic Analysis of Aspergillus flavus Interacting with Resistant and Susceptible Peanut

Mitigating fungal contamination of cereals: The efficacy of microplasma-based far-UVC lamps against Aspergillus flavus and Fusarium graminearum

Fungal contaminations of cereal grains are a profound food-safety and food-security concern worldwide, threatening consumers' and animals' health and causing enormous economic burdens. Because far-ultraviolet C (far-UVC) light at 222 nm has recently been shown to be human-safe, we investigated its efficacy as an alternative to thermal, chemical, and conventional 254 nm UVC anti-fungal treatments. Our microplasma-based far-UVC lamp system achieved a 5.21-log reduction in the conidia of Aspergillus flavus suspended in buffer with a dose of 1032.0 mJ/cm2, and a 5.11-log reduction of Fusarium graminearum conidia in suspension with a dose of 619.2 mJ/cm2. We further observed that far-UVC treatments could induce fungal-cell apoptosis, alter mitochondrial membrane potential, lead to the accumulation of intracellular reactive oxygen species, cause lipid peroxidation, and result in cell-membrane damage. The lamp system also exhibited a potent ability to inhibit the mycelial growth of both A. flavus and F. graminearum. On potato dextrose agar plates, such growth was completely inhibited after doses of 576.0 mJ/cm2 and 460.8 mJ/cm2, respectively. To test our approach's efficacy at decontaminating actual cereal grains, we designed a cubical 3D treatment chamber fitted with six lamps. At a dose of 780.0 mJ/cm2 on each side, the chamber achieved a 1.88-log reduction of A. flavus on dried yellow corn kernels and a 1.11-log reduction of F. graminearum on wheat grains, without significant moisture loss to either cereal type (p > 0.05). The treatment did not cause significant changes in the propensity of wheat grains to germinate in the week following treatment (p > 0.05). However, it increased the germination propensity of corn kernels by more than 71% in the same timeframe (p < 0.05). Collectively, our results demonstrate that 222 nm far-UVC radiation can effectively inactivate fungal growth in liquid, on solid surfaces, and on cereal grains. If scalable, its emergence as a safe, cost-effective alternative tool for reducing fungi-related post-harvest cereal losses could have important positive implications for the fight against world hunger and food insecurity.

The KdmB-EcoA-RpdA-SntB (KERS) chromatin regulatory complex controls development, secondary metabolism and pathogenicity in Aspergillus flavus

The filamentous fungus Aspergillus flavus is a plant and human pathogen predominantly found in the soil as spores or sclerotia and is capable of producing various secondary metabolites (SM) such as the carcinogenic mycotoxin aflatoxin. Recently, we have discovered a novel nuclear chromatin binding complex (KERS) that contains the JARID1-type histone demethylase KdmB, a putative cohesion acetyl transferase EcoA, a class I type histone deacetylase RpdA and the PHD ring finger reader protein SntB in the model filamentous fungus Aspergillus nidulans. Here, we show the presence of the KERS complex in A. flavus by immunoprecipitation-coupled mass spectrometry and constructed kdmBΔ and rpdAΔ strains to study their roles in fungal development, SM production and histone post-translational modifications (HPTMs). We found that KdmB and RpdA couple the regulation of SM gene clusters with fungal light-responses and HPTMs. KdmB and RpdA have opposing roles in light-induced asexual conidiation, while both factors are positive regulators of sclerotia development through the nsdC and nsdD pathway. KdmB and RpdA are essential for the productions of aflatoxin (similar to findings for SntB) as well as cyclopiazonic acid, ditryptophenaline and leporin B through controlling the respective SM biosynthetic gene clusters. We further show that both KdmB and RpdA regulate H3K4me3 and H3K9me3 levels, while RpdA also acts on H3K14ac levels in nuclear extracts. Therefore, the chromatin modifiers KdmB and RpdA of the KERS complex are key regulators for fungal development and SM metabolism in A. flavus.

Biochemical and Biotechnological Insights into Fungus-Plant Interactions for Enhanced Sustainable Agricultural and Industrial Processes

The literature is full of studies reporting environmental and health issues related to using traditional pesticides in food production and storage. Fortunately, alternatives have arisen in the last few decades, showing that organic agriculture is possible and economically feasible. And in this scenario, fungi may be helpful. In the natural environment, when associated with plants, these microorganisms offer plant-growth-promoting molecules, facilitate plant nutrient uptake, and antagonize phytopathogens. It is true that fungi can also be phytopathogenic, but even they can benefit agriculture in some way-since pathogenicity is species-specific, these fungi are shown to be useful against weeds (as bioherbicides). Finally, plant-associated yeasts and molds are natural biofactories, and the metabolites they produce while dwelling in leaves, flowers, roots, or the rhizosphere have the potential to be employed in different industrial activities. By addressing all these subjects, this manuscript comprehensively reviews the biotechnological uses of plant-associated fungi and, in addition, aims to sensitize academics, researchers, and investors to new alternatives for healthier and more environmentally friendly production processes.

A broadly conserved fungal alcohol oxidase (AOX) facilitates fungal invasion of plants

Alcohol oxidases (AOXs) are ecologically important enzymes that facilitate a number of plant-fungal interactions. Within Ascomycota they are primarily associated with methylotrophy, as a peroxisomal AOX catalysing the conversion of methanol to formaldehyde in methylotrophic yeast. In this study we demonstrate that AOX orthologues are phylogenetically conserved proteins that are common in the genomes of nonmethylotrophic, plant-associating fungi. Additionally, AOX orthologues are highly expressed during infection in a range of diverse pathosystems. To study the role of AOX in plant colonization, AOX knockout mutants were generated in the broad host range pathogen Sclerotinia sclerotiorum. Disease assays in soybean showed that these mutants had a significant virulence defect as evidenced by markedly reduced stem lesions and mortality rates. Chemical genomics suggested that SsAOX may function as an aromatic AOX, and growth assays demonstrated that ΔSsAOX is incapable of properly utilizing plant extract as a nutrient source. Profiling of known aromatic alcohols pointed towards the monolignol coniferyl alcohol (CA) as a possible substrate for SsAOX. As CA and other monolignols are ubiquitous among land plants, the presence of highly conserved AOX orthologues throughout Ascomycota implies that this is a broadly conserved protein used by ascomycete fungi during plant colonization.

Biocontrol and Action Mechanism of Bacillus subtilis Lipopeptides' Fengycins Against Alternaria solani in Potato as Assessed by a Transcriptome Analysis

Alternaria solani is an airborne fungus and the primary causal agent of potato early blight worldwide. No available fungicides that are both effective and environmentally friendly are usable to control this fungus. Therefore, biological control is a potential approach for its suppression. In this study, Bacillus subtilis strain ZD01's fermentation broth strongly reduced A. solani pathogenicity under greenhouse conditions. The effects of strain ZD01's secondary metabolites on A. solani were investigated. The exposure of A. solani hyphae to the supernatant resulted in swelling and swollen sacs, and the ZD01 supernatant reduced A. solani conidial germination significantly. Matrix-assisted laser desorption/ionization time of flight mass spectrometry and pure product tests revealed that fengycins were the main antifungal lipopeptide substances. To elucidate the molecular mechanism of the fengycins' biological control, RNA sequencing analyses were performed. A transcriptome analysis revealed that 304 and 522 genes in A. solani were differentially expressed after 2-h and 6-h fengycin treatments, respectively. These genes were respectively mapped to 53 and 57 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. In addition, the most enriched KEGG pathway analysis indicated that the inhibitory mechanisms of fengycins against A. solani regulated the expression of genes related to cell wall, cell membrane, transport, energy process, protein synthesis and genetic information. In particular, cell wall and cell membrane metabolism were the main processes affected by fengycin stress. Scanning and transmission electron microscope results revealed hyphal enlargement and a wide range of abnormalities in A. solani cells after exposure to fengycins. Furthermore, fengycins induced chitin synthesis in treated cells, and also caused the capture of cellular fluorescent green labeling and the release of adenosine triphosphate (ATP) from outer membranes of A. solani cells, which may enhance the fengycins ability to alter cell membrane permeability. Thus, this study increases the transcriptome data resources available and supplies a molecular framework for B. subtilis ZD01 inhibition of A. solani HWC-168 through various mechanisms, especially damaging A. solani cell walls and membranes. The transcriptomic insights may lead to an effective control strategy for potato early blight.

Combined transcriptome and metabolome analyses reveal the potential mechanism for the inhibition of Penicillium digitatum by X33 antimicrobial oligopeptide

Penicillium digitatum is the primary spoilage fungus that causes green mold during postharvest in citrus. To reduce economic losses, developing more efficient and less toxic natural antimicrobial agents is urgently required. We previously found that the X33 antimicrobial oligopeptide (X33 AMOP), produced by Streptomyces lavendulae X33, exhibited a sterilization effect on P. digitatum. In this study, the effects, and physiological mechanisms of X33 AMOP as an inhibitor of P. digitatum were investigated. The transcriptional and metabolome profiling of P. digitatum exposed to X33 AMOP revealed 3648 genes and 190 metabolites that were prominently changed. The omics analyses suggested that X33 AMOP mainly inhibited P. digitatum growth by affecting cell integrity, genetic information delivery, oxidative stress tolerance, and energy metabolism. These findings provide helpful information regarding the antimicrobial mechanism of X33 AMOP against P. digitatum at the molecular level and indicate that X33 AMOP is a potential candidate to control P. digitatum.

High-Density Genetic Linkage Map Construction Using Whole-Genome Resequencing for Mapping QTLs of Resistance to Aspergillus flavus Infection in Peanut

The cultivated peanut (Arachis hypogaea L.), which is rich in edible oil and protein, is widely planted around the world as an oil and cash crop. However, aflatoxin contamination seriously affects the quality safety of peanuts, hindering the development of the peanut industry and threatening the health of consumers. Breeding peanut varieties with resistance to Aspergillus flavus infection is important for the control of aflatoxin contamination, and understanding the genetic basis of resistance is vital to its genetic enhancement. In this study, we reported the quantitative trait locus (QTL) mapping of resistance to A. flavus infection of a well-known resistant variety, J11. A mapping population consisting of 200 recombinant inbred lines (RILs) was constructed by crossing a susceptible variety, Zhonghua 16, with J11. Through whole-genome resequencing, a genetic linkage map was constructed with 2,802 recombination bins and an average inter-bin distance of 0.58 cM. Combined with phenotypic data of an infection index in 4 consecutive years, six novel resistant QTLs with 5.03-10.87% phenotypic variances explained (PVE) were identified on chromosomes A05, A08, B01, B03, and B10. The favorable alleles of five QTLs were from J11, while that of one QTL was from Zhonghua 16. The combination of these favorable alleles significantly improved resistance to A. flavus infection. These results could contribute greatly to the understanding of the genetic basis of A. flavus resistance and could be meaningful in the improvement of further resistance in peanuts.

Quantitative Proteomic Analysis for High- and Low-Aflatoxin-Yield Aspergillus flavus Strains Isolated From Natural Environments

The molecular mechanisms underlying aflatoxin production have been well-studied in strains of the fungus Aspergillus flavus (A. flavus) under artificial conditions. However, aflatoxin biosynthesis has rarely been studied in A. flavus strains isolated from field conditions with different aflatoxin-producing ability. In the present study, tandem mass tag (TMT) labeling and high-performance liquid chromatography (HPLC) coupled with tandem-mass spectrometry analysis were used for proteomic quantification in natural isolates of high- and low-aflatoxin-yield A. flavus strains. Additionally, findings obtained using the TMT-labeling method were validated using the high-resolution multiple reaction monitoring (MRM-HR) method. In total, 4,363 proteins were quantified, among which 1,045 proteins were differentially expressed between the high- and low-aflatoxin-yield A. flavus strains. Bioinformatics analysis showed that the up-regulated proteins were significantly enriched in carbon-related metabolism and the biosynthesis of secondary metabolites, whereas the down-regulated proteins were enriched in oxidative phosphorylation. Moreover, GST proteins were found to be significantly down-regulated in high-yield A. flavus strains; this result contradicted previous findings obtained from A. flavus strains grown under artificial conditions. In summary, our study provides novel insights into aflatoxin regulation in A. flavus under field conditions and could facilitate the development of various strategies for the effective control of aflatoxin contamination in food crops.

Effects of Peptide C12-OOWW-NH2 on Transcriptome and Cell Wall of the Postharvest Fungal Pathogen Penicillium digitatum

In this study, the transcriptional profiling of Penicillium digitatum after C₁₂O₃TR treatment was analyzed by RNA-Seq technology. A total of 2562 and 667 genes in P. digitatum were differentially expressed after 2 and 12 h treatment, respectively. These genes were respectively mapped to 91 and 79 KEGG pathways. The expression patterns of differentially expressed genes (DEGs) at 2 and 12 h were similar, mainly were the metabolic processes in cell wall, cell membrane, genetic information and energy. Particularly, the main metabolic process which was affected by C₁₂O₃TR stress for 2 and 12 h was cell integrity, including cell wall and cell membrane. The changes of chitin in cell wall was observed by Calcofluor White (CFW) staining assay. The weaker blue fluorescence in the cell wall septa, the decrease of β-1, 3-glucan synthase activity and the increase of chitinase and AKP activity showed that C₁₂O₃TR could damage the cell wall integrity. In conclusion, these results suggested that C₁₂O₃TR could inhibit the growth of P. digitatum through various mechanisms at transcriptional level, and could influence the cell wall permeability and integrity.

Transcriptomic analyses reveal the expression and regulation of genes associated with resistance to early leaf spot in peanut

Objective

Early leaf spot (ELS) caused by Cercospora arachidicola (Hori) is a serious foliar disease in peanut worldwide, which causes considerable reduction of yield. Identification of resistance genes is important for both conventional and molecular breeding. Few resistance genes have been identified and the mechanism of defense responses to this pathogen remains unknown.

Results

We detected several genes involved in disease resistance to ELS through transcriptome analysis. Using RNA-seq technology, one hundred thirty-three differentially expressed genes (DEGs) were identified between resistant and susceptible lines. Among these DEGs, coiled coil-nucleotide binding-leucine rich repeat (NLR) type resistance genes were identified as duplicated R genes on the chromosome B2. Peanut phytoalexin deficient 4 (PAD4) regulator of effector-triggered immunity mediated by NLR resistance proteins and polyphenol oxidase (PPO) genes play important roles in early leaf spot resistance. Our study provides the useful information on plant response to C. arachidicola infection in peanut. The results suggest that a few major genes and several factors mediate the resistance to ELS disease, showing the characteristics of quantitative trait in defense responses.

Integrated small RNA and mRNA expression profiles reveal miRNAs and their target genes in response to Aspergillus flavus growth in peanut seeds

BACKGROUND

MicroRNAs are important gene expression regulators in plants immune system. Aspergillus flavus is the most common causal agents of aflatoxin contamination in peanuts, but information on the function of miRNA in peanut-A. flavus interaction is lacking. In this study, the resistant cultivar (GT-C20) and susceptible cultivar (Tifrunner) were used to investigate regulatory roles of miRNAs in response to A. flavus growth.

RESULTS

A total of 30 miRNAs, 447 genes and 21 potential miRNA/mRNA pairs were differentially expressed significantly when treated with A. flavus. A total of 62 miRNAs, 451 genes and 44 potential miRNA/mRNA pairs exhibited differential expression profiles between two peanut varieties. Gene Ontology (GO) analysis showed that metabolic-process related GO terms were enriched. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses further supported the GO results, in which many enriched pathways were related with biosynthesis and metabolism, such as biosynthesis of secondary metabolites and metabolic pathways. Correlation analysis of small RNA, transcriptome and degradome indicated that miR156/SPL pairs might regulate the accumulation of flavonoids in resistant and susceptible genotypes. The miR482/2118 family might regulate NBS-LRR gene which had the higher expression level in resistant genotype. These results provided useful information for further understanding the roles of miR156/157/SPL and miR482/2118/NBS-LRR pairs.

CONCLUSIONS

Integration analysis of the transcriptome, miRNAome and degradome of resistant and susceptible peanut varieties were performed in this study. The knowledge gained will help to understand the roles of miRNAs of peanut in response to A. flavus.

Genomic Insights Into Sugar Adaptation in an Extremophile Yeast Zygosaccharomyces rouxii

The osmotolerant Zygosaccharomyces rouxii is known for its trait to survive in extreme high sugar environments. This ability determines its role in the fermentation process and leads to yeast spoilage in the food industry. However, our knowledge of the gene expression in response to high sugar stress remains limited. Here, we conducted RNA-sequencing (RNA-seq) under different sugar concentrations of the spoilage yeast, Z. rouxii, which exhibit extremely high tolerance to sugar stress. The obtained differentially expressed genes (DEGs) are significantly different to that of the Saccharomyces cerevisiae, which is sensitive to extreme high sugar stress. Most of the DEGs participated in the “glucan synthesis,” “transmembrane transport,” “ribosome,” etc. In this work, we also demonstrated that the gene ZYRO0B03476g (ZrKAR2) encoding Kar2p can significantly affect the growth of Z. rouxii under high sugar stress. In addition, we combined with a previous study on the genome sequence of Z. rouxii, indicating that several gene families contain significantly more gene copies in the Z. rouxii lineage, which involved in tolerance to sugar stress. Our results provide a gene insight for understanding the high sugar tolerance trait, which may impact food and biotechnological industries and improve the osmotolerance in other organisms.

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.

Transcriptomic Analysis of the Molecular Mechanisms Underlying the Antibacterial Activity of IONPs@pDA-Nisin Composites toward Alicyclobacillus acidoterrestris

A simple and no-drug resistance antibacterial method was developed by the synthesis of heat-stable and pH-tolerant nisin-loaded iron oxide nanoparticles polydopamine (IONPs@pDA) composites. The composites had a crystal structure and diameters of 25 ± 3 nm, with a saturation magnetization ( M_s) of 43.7995 emu g^-1. Nisin was successfully conjugated onto the IONPs@pDA nanoparticles, as evinced by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses. The novel synthesized material showed good performance in reducing Alicyclobacillus acidoterrestris, a common food spoilage bacterium that represents a significant problem for the food industry. Treatment of A. acidoterrestris cells with composites resulted in membrane damage, as observed by live/dead staining and scanning electron microscopy and transmission electron microscopy analyses. Further, the composites exhibited highly efficient antibacterial activity against cells in only 5 min. Transcriptomic sequencing of culture RNA pools after exposure to composites resulted in a total of 334 differentially expressed genes that were primarily associated with transcriptional regulation, energy metabolism, membrane transporters, membrane and cell wall syntheses, and cell motility. Thus, these results suggested that changes in transcriptional regulation caused by aggregated composites on target cells led to major changes in homeostasis that manifested by decreased energy metabolism, pore formation in the membrane, and repressed cell wall synthesis. Concomitantly, cell motility and sporulation activities were both repressed, and finally, intracellular substances flowed out of leaky cells. The proposed biocontrol method represents a novel means to control microorganisms without inducing drug resistance. Further, these results provide novel insights into the molecular mechanisms underlying the antibacterial activity of composites against microorganisms.

Identification of genomic regions and diagnostic markers for resistance to aflatoxin contamination in peanut (Arachis hypogaea L.)

Background

Aflatoxin contamination caused by Aspergillus flavus is a major constraint to peanut industry worldwide due to its toxicological effects to human and animals. Developing peanut varieties with resistance to seed infection and/or aflatoxin accumulation is the most effective and economic strategy for reducing aflatoxin risk in food chain. Breeding for resistance to aflatoxin in peanut is a challenging task for breeders because the genetic basis is still poorly understood. To identify the quantitative trait loci (QTLs) for resistance to aflatoxin contamination in peanut, a recombinant inbred line (RIL) population was developed from crossing Zhonghua 10 (susceptible) with ICG 12625 (resistant). The percent seed infection index (PSII), the contents of aflatoxin B₁ (AFB₁) and aflatoxin B₂ (AFB₂) of RILs were evaluated by a laboratory kernel inoculation assay.

Results

Two QTLs were identified for PSII including one major QTL with 11.32–13.00% phenotypic variance explained (PVE). A total of 12 QTLs for aflatoxin accumulation were detected by unconditional analysis, and four of them (qAFB1A07 and qAFB1B06.1 for AFB₁, qAFB2A07 and qAFB2B06 for AFB₂) exhibited major and stable effects across multiple environments with 9.32–21.02% PVE. Furthermore, not only qAFB1A07 and qAFB2A07 were co-localized in the same genetic interval on LG A07, but qAFB1B06.1 was also co-localized with qAFB2B06 on LG B06. Conditional QTL mapping also confirmed that there was a strong interaction between resistance to AFB₁ and AFB₂ accumulation. Genotyping of RILs revealed that qAFB1A07 and qAFB1B06.1 interacted additively to improve the resistance to both AFB₁ and AFB₂ accumulation. Additionally, validation of the two markers was performed in diversified germplasm collection and four accessions with resistance to aflatoxin accumulation were identified.

Conclusions

Single major QTL for resistance to PSII and two important co-localized intervals associated with major QTLs for resistance to AFB₁ and AFB₂. Combination of these intervals could improve the resistance to aflatoxin accumulation in peanut. SSR markers linked to these intervals were identified and validated. The identified QTLs and associated markers exhibit potential to be applied in improvement of resistance to aflatoxin contamination.

Electronic supplementary material

The online version of this article (10.1186/s12863-019-0734-z) contains supplementary material, which is available to authorized users.

Quercetin Inhibits the Proliferation and Aflatoxins Biosynthesis of Aspergillus flavus

In this work of quercetin’s anti-proliferation action on A. flavus, we revealed that quercetin can effectively hamper the proliferation of A. flavus in dose-effect and time-effect relationships. We tested whether quercetin induced apoptosis in A. flavus via various detection methods, such as phosphatidylserine externalization and Hoechst 33342 staining. The results showed that quercetin had no effect on phosphatidylserine externalization and cell nucleus in A. flavus. Simultaneously, quercetin reduced the levels of reactive oxygen species (ROS). For a better understanding of the molecular mechanism of the A. flavus response to quercetin, the RNA-Seq was used to explore the transcriptomic profiles of A. flavus. According to transcriptome sequencing data, quercetin inhibits the proliferation and aflatoxin biosynthesis by regulating the expression of development-related genes and aflatoxin production-related genes. These results will provide some theoretical basis for quercetin as an anti-mildew agent resource.

Dissection of mechanisms of resistance to Aspergillus flavus and aflatoxin using tropical maize germplasm

Aspergillus flavus induced ear rots and subsequent contamination of maize (Zea mays L.) by aflatoxin is a serious food safety issue, especially in developing countries where the crop is mostly cultivated by smallholder famers for own consumption and income generation. A better understanding of the mechanisms of resistance could help breeders to develop resistant maize varieties. In this study, a set of six tropical maize inbred lines previously identified as resistant or susceptible under natural field conditions were evaluated for response to A. flavus colonisation and aflatoxin contamination. Fungal biomass was significantly higher (P<0.05) in susceptible than resistant maize inbred lines, and this was highly correlated (P=0.001) to aflatoxin levels. Maize inbred lines MRI, MR2 and MR3 had low fungal biomass and low aflatoxin levels, suggesting that resistance in these lines was mediated through restricted fungal colonisation and establishment. Among the three putatively resistant inbred lines mentioned above, MR2 had a relatively high colonisation compared to the other two lines, revealing that A. flavus could establish and colonise kernels that were injured during inoculation, but did not contain high levels of aflatoxin. This could signify the presence of host genes that interfere with the aflatoxin biosynthetic pathway.

Aspergillus flavus Secondary Metabolites: More than Just Aflatoxins

Aspergillus flavus is best known for producing the family of potent carcinogenic secondary metabolites known as aflatoxins. However, this opportunistic plant and animal pathogen also produces numerous other secondary metabolites, many of which have also been shown to be toxic. While about forty of these secondary metabolites have been identified from A. flavus cultures, analysis of the genome has predicted the existence of at least 56 secondary metabolite gene clusters. Many of these gene clusters are not expressed during growth of the fungus on standard laboratory media. This presents researchers with a major challenge of devising novel strategies to manipulate the fungus and its genome so as to activate secondary metabolite gene expression and allow identification of associated cluster metabolites. In this review, we discuss the genetic, biochemical and bioinformatic methods that are being used to identify previously uncharacterized secondary metabolite gene clusters and their associated metabolites. It is important to identify as many of these compounds as possible to determine their bioactivity with respect to fungal development, survival, virulence and especially with respect to any potential synergistic toxic effects with aflatoxin.

Effect of Cinnamaldehyde and Citral Combination on Transcriptional Profile, Growth, Oxidative Damage and Patulin Biosynthesis of Penicillium expansum

Penicillium expansum, as a main postharvest pathogen of fruits, can secrete patulin (PAT), causing fruit decay and health problems. In this study, the antifungal test, SEM (scanning electron microscope) observation, transcriptional profile, PAT biosynthesis, and physiological characters of P. expansum exposed to cinnamaldehyde and citral combination (Cin/Cit) were evaluated. Cin/Cit could inhibit the mycelial growth and spore germination of P. expansum in a dose-dependent manner. Besides, Cin/Cit caused spores and mycelia wrinkled and depressed by SEM observation. Gene expression profiles of P. expansum were conducted by RNA sequencing (RNA-seq) in the presence or absence of Cin/Cit treatment. A total of 1713 differentially expressed genes (DEGs) were obtained, including 793 down-regulated and 920 up-regulated genes. Most of the DEGs participated in the biosynthesis of secondary metabolites, amino acid metabolism, and oxidation-reduction process, etc. Cin/Cit induced the dysfunction of the mitochondrial membrane, causing the potential influence on energy metabolism and reactive oxidative species production. The changes of superoxide dismutase (SOD) and catalase (CAT) activities combing with the increase of hydrogen peroxide content indicated the oxidative stress on P. expansum induced by Cin/Cit, which corresponded well with the transcriptional results. Moreover, both the RNA-seq data and the qRT-PCR showed the remarkable down-regulation of genes included in the PAT biosynthetic pathway under the Cin/Cit treatment. These findings provided more useful information about the antifungal mechanism of Cin/Cit against P. expansum at molecular and gene levels and suggested that Cin/Cit is a potential candidate to control P. expansum.

The Pathogenesis-Related Maize Seed (PRms) Gene Plays a Role in Resistance to Aspergillus flavus Infection and Aflatoxin Contamination

Aspergillus flavus is an opportunistic plant pathogen that colonizes and produces the toxic and carcinogenic secondary metabolites, aflatoxins, in oil-rich crops such as maize (Zea mays ssp. mays L.). Pathogenesis-related (PR) proteins serve as an important defense mechanism against invading pathogens by conferring systemic acquired resistance in plants. Among these, production of the PR maize seed protein, ZmPRms (AC205274.3_FG001), has been speculated to be involved in resistance to infection by A. flavus and other pathogens. To better understand the relative contribution of ZmPRms to A. flavus resistance and aflatoxin production, a seed-specific RNA interference (RNAi)-based gene silencing approach was used to develop transgenic maize lines expressing hairpin RNAs to target ZmPRms. Downregulation of ZmPRms in transgenic kernels resulted in a ∼250-350% increase in A. flavus infection accompanied by a ∼4.5-7.5-fold higher accumulation of aflatoxins than control plants. Gene co-expression network analysis of RNA-seq data during the A. flavus-maize interaction identified ZmPRms as a network hub possibly responsible for regulating several downstream candidate genes associated with disease resistance and other biochemical functions. Expression analysis of these candidate genes in the ZmPRms-RNAi lines demonstrated downregulation (vs. control) of a majority of these ZmPRms-regulated genes during A. flavus infection. These results are consistent with a key role of ZmPRms in resistance to A. flavus infection and aflatoxin accumulation in maize kernels.

Proteome-wide profiling of protein lysine acetylation in Aspergillus flavus

Protein lysine acetylation is a prevalent post-translational modification that plays pivotal roles in various biological processes in both prokaryotes and eukaryotes. Aspergillus flavus, as an aflatoxin-producing fungus, has attracted tremendous attention due to its health impact on agricultural commodities. Here, we performed the first lysine-acetylome mapping in this filamentous fungus using immune-affinity-based purification integrated with high-resolution mass spectrometry. Overall, we identified 1383 lysine-acetylation sites in 652 acetylated proteins, which account for 5.18% of the total proteins in A. flavus. According to bioinformatics analysis, the acetylated proteins are involved in various cellular processes involving the ribosome, carbon metabolism, antibiotic biosynthesis, secondary metabolites, and the citrate cycle and are distributed in diverse subcellular locations. Additionally, we demonstrated for the first time the acetylation of fatty acid synthase α and β encoded by aflA and aflB involved in the aflatoxin-biosynthesis pathway (cluster 54), as well as backbone enzymes from secondary metabolite clusters 20 and 21 encoded by AFLA_062860 and AFLA_064240, suggesting important roles for acetylation associated with these processes. Our findings illustrating abundant lysine acetylation in A. flavus expand our understanding of the fungal acetylome and provided insight into the regulatory roles of acetylation in secondary metabolism.

Histone Methyltransferase aflrmtA gene is involved in the morphogenesis, mycotoxin biosynthesis, and pathogenicity of Aspergillus flavus

Arginine methyltransferases catalyze the posttranslational methylation of arginine, which is involved in a range of important biological processes. aflrmtA gene, an arginine methyltransferase was deleted from Aspergillus flavus in this study by homologous recombination. In morphogenesis assay, aflrmtA was found to down-regulate conidiation by regulating the activity of brlA and abaA genes. It was also found to increase sclerotia formation by up-regulating the expression of nsdC and nsdD genes. In mycotoxin biosynthesis, aflrmtA gene was found to significantly up-regulate the biosynthesis of AFB1 in PDA and PDB media by improving the expression of aflR, aflC and aflK, but it was of no effect in YES medium. aflrmtA was further found to be an important regulator of response to plasma membrane lesion, osmotic, and H₂O₂ - induced oxidative stresses. In pathogenicity analysis, aflrmtA was found to repress conidiation and up-regulate the AFB1 biosynthesis of A. flavus on peanut and corn seeds and also the activities of protease and lipase, but the activity of amylase was down-regulated. It was concluded that aflrmtA gene played important roles in the morphogenesis, mycotoxin biosynthesis and pathogenicity of A. flavus, and it could be a potential target in the prevention and control of crop contamination by A. flavus.