Mycotoxins are secondary metabolites produced by several fungal species. The biosynthesis and regulation of mycotoxins depend on genetic characteristics of mycotoxin-producing fungi, hosts and environmental factors. This paper summarizes the previous research advances involving mycotoxin biosynthesis and the factors that influence mycotoxin production, such as temperature, water activity, light, osmotic pressure, substrate, pH, plant damage and host resistance. Prevention and control of mycotoxins was also introduced.
Aflatoxin (AF), a kind of toxic and carcinogenic secondary metabolite mainly produced by Aspergillus flavus and Aspergillus parasiticus, not only causes serious threat to human and animal health but also incurs huge economic losses to food and feed. Since the finding of AF in 1960s, a lot of research progress on biosynthesis pathway, degradation, synthetic and pathogenic mechanisms of AF have been achieved. In this article the study progress on the genetic regulation mechanism of AF biosynthesis in A. flavus, including transcriptional regulation, posttranslational protein modification, signal transduction pathway, the proteins involved in development and morphogenesis and other functional enzymes are summarized. This review provides reference for the control of A. flavus and AF in China.
Aspergillus flavus, an aerobic saprophytic fungus producing aflatoxin, seriously endangers food safety and human and animal health. In recent years, omics research has developed rapidly, and the basic data platform for biological research has been established gradually. Genomics, transcriptomics, proteomics and metabolomics have been widely used in the study of secondary metabolism of A. flavus. In this paper, the important research progress on genome, transcriptome, proteome and metabolome in A. flavus secondary metabolism was summarized, and the development trend of related research and the problems to be solved were proposed.
Fusarium graminearum species complex (FGSC) is the major causal agent of Fusarium head blight (FHB) on wheat and many other small grain cereals worldwide, causing yield losses, reducing grain quality due to contamination of mycotoxin, and seriously threatening food safety. Over the last two decades, with the publication of the F. graminearum genome sequence as well as the establishment of genetic transformation system of the fungus, significant progresses for gene function studies and a more comprehensive understanding for pathogenesis and mycotoxin biosynthesis regulatory mechanism of the fungs have been achieved. In this paper, recent advances on Fusarium mycotoxin biosynthesis and its regulatory mechanisms, including biosynthetic gene cluster, regulatory elements, and epigenetic modification for mycotoxin regulation are reviewed and summarized.
Fungi are highly diverse organisms producing secondary metabolites with many important functions such as UV protection, self-development and defense against external aggression. The biosynthesis of these secondary metabolites requires participation of multiple genes. In this paper the medicinal value of secondary metabolites of filamentous fungi, the ecological functions during fungal development and the mechanisms of synthetic regulation are reviewed.
LaeA was first characterized as a global regulator of secondary metabolism in Aspergillus nidulan. Its orthologs are highly conserved and have been identified in several filamentous ascomycetes. LaeA and its orthologs contain an S-adenosyl methionine binding domain and have similarities to methyltransferases. The function of LaeA and its orthologs on regulating gene expression has been linked to histone modification and changes in chromatin structure. A large number of studies have shown that LaeA and its orthologs are involved in regulating the biosynthesis of various secondary metabolism, morphological development and differentiation of filamentous fungi. Recently, LaeA orthologs were reported also to play a key role in the production of organic acids and some industrial enzymes. In this paper, the mechanism of action of LaeA orthologs and their effect on secondary metabolism, growth and development, as well as other important biological processes in filamentous fungi were reviewed. The existing problems and the application prospects of LaeA orthologs were discussed.
Ochratoxin A (OTA) is classified as a renal carcinogen of animals and a possible (group 2B) human carcinogen by the International Agency for Research on Cancer. Aspergillus niger has been generally recognized as safety (GRAS) by the US Food and Drug Administration (FDA). However, there have been increasingly reported that some isolates of A. niger are able to produce OTA, arousing people’s great concern. Elucidating the key genes required for the biosynthesis of OTA is helpful to understand the mechanism of OTA production in A. niger, which is of great significance for the development of effective preventive and control strategies to reduce the risk of OTA contamination in foods and other products. In this study, the non-ribosomal peptide synthase (NRPS) coding gene (An15g07910) of A. niger was cloned and analyzed by bioinformatics. The gene was knocked out by homologous recombination, and a stable knockout mutant, Δnrps, was obtained. No statistically significant differences in growth, colony morphology and conidial morphology were observed in the Δnrps mutant in comparison with the wild-type strain of A. niger on the Czapek yeast autolysate (CYA) agar plates. Disruption of the gene An15g07910 completely eliminated the production of ochratoxin α (OTα) and OTA during 7 days of culture, but it does not affect the production of ochratoxin β (OTβ). The highest expression level of the gene An15g07910 was detected on day 4 after inoculation, and it decreased and tended to be stable afterward, being basically consistent with the change of OTA content. The results showed that the nrps gene (An15g07910) was involved in OTA biosynthesis, and its encoded NRPS may be responsible for the cross-linking of phenylalanine and dihydroisocoumarin sectors.
The homeobox transcription factors (HTFs), that are conserved in eukaryotic organisms, have been shown to be involved in the regulations of embryonic development, cell differentiation, cell proliferation and carcinogenesis. In this study, eight HTF genes were identified by bioinformatic method in Aspergillus flavus, a co-pathogenic fungus both for animals and plants. To study their functions in A. flavus, these eight genes were knocked out from the genome of A. flavus by homologous recombination strategies. It was found that the deletion of htf1, htf2, htf4 or htf8 genes resulted in significant reduction in hyphal growth of A. flavus on YES medium, and the growth defect of htf2 deletion mutant was the most obvious. It was also found that Δhtf1 and Δhtf2 mutants failed to form normal conidiophores, leading to decrease of conidia. Δhtf8 mutant caused shortening and decreasing conidiophores. Further analysis demonstrated that the deletion of htf1 resulted in a significant reduction of sclerotial formation, and the Δhtf2 mutant failed to produce sclerotia. It was worthy of note, the production of aflatoxin was found to be significantly reduced in Δhtf1 mutant, and the biosynthesis of aflatoxin in the Δhtf2 mutant was completely inhibited. This study provides valuable information for preventing from contamination and control of A. flavus.
Endophytic fungi from desert plants are a member of specially bioenvironmental microbe with poorly chemical investigated, being one of new resources of bioactive natural products. Chemical investigation of an endophytic fungus Stagonospora sp. inhabiting in desert plant Bassia dasyphylla led to isolate four usnic acid analogs including a new compound stagonone (1) and three known ones, usnic acid (2), cercosporamide (3) and usnic acid amide (4). The structure of the new compound was determined by HRESI-MS, NMR, and circular dichroism (CD) experiments. The X-ray diffraction and CD data of 2, and the cytotoxic activities of 1-4 were also determined. In addition, according to the structural features of 1-4, the possible biosynthetic pathway was postulated.
Four resorcylic acid lactones (1-4) including two new analogs rhinoclactones E (2) and F (1), and two known ones 8,9-dihyrogreensporone D (3) and 8,9-dihydrogreensporone A (4) were isolated from the endophytic fungus Rhinocladiella similis collected from desert plant Agriophyllum squarrosum. The structures of the two new compounds (1 and 2) were determined on the basis of HRESI-MS and NMR data in comparison with the data of literatures. Compounds 1 and 2 are a pair of stereo-isomers with a unique furan ring fused in the macrolide ring, which are rare in nature. Compounds 1-4 showed no inhibitory effects against three cancer cell lines and plant pathogens. The results imply that endophytic fungi inhabiting desert plants have potential to produce secondary metabolites with novel structures, and are one of vital resources for natural product discovery. Based on the structural features and biological effects, the possible ecological role of these compounds (1-4) was suggested.
Ochratoxin A (OTA), a secondary metabolite produced by species of Aspergillus and Penicillium with multiple toxicity such as nephrotoxicity, teratogenicity and immunotoxicity, is gaining more and more attention worldwide. A detection technique possessing high sensitivity that would be operable with ease is preferred for determination of OTA levels in foods and feeds. In this study, a rapid and sensitive magnetic nanoparticle- biotin-streptavidin-Au nanoparticles-enzyme linked immunosorbent assay (MNPs-bs-AuNPs-ELISA) was prototyped and used for detection of OTA. The result proved that biotin-streptavidin enhanced the detection signal and thereby improved the sensitivity of the assay. The lower limit of detection by using MNPs-bs-AuNPs-ELISA was 0.01ng/mL while the IC50 was 0.13ng/mL. The detectable range was 0.02-0.73ng/mL. Cross-reactivities to OTA analogues, ochratoxin B and ochratoxin C, were 4.3% and 8.1%, respectively. No cross-reactivity (<0.01%) was observed to other co-occurring mycotoxins (AFB1, ZEN, FB1, DON, CIT and PAT). The recovery rates in spiked samples of corn, wheat and soybean ranged 85.6%-115.7%, and the intra-day and inter-day relative standard deviations were <10%. Simultaneous analysis of commercially obtained samples (corn, wheat, and feedstuff) showed a good correlation between MNPs-bs-AuNPs-ELISA and liquid chromatography tandem mass spectrometry (LC/MS-MS). This new method can be used as a sensitive, simple and cost-effective assay to quantify the levels of OTA in cereal and feed samples.
