Strain identification and metabolites isolation of Aspergillus capensis CanS-34A from Brassica napus

Naturally Occurring Asterric Acid Analogs: Chemistry and Biology

Asterric acid and its analogs belong to diphenyl ethers (DPEs) with multiple substitutions on A/B aromatic rings. This member of DPEs originates from the polyketide pathway and displays a wide range of biological effects. Though the structures of asterric acid analogs are not complex, there were only more than 50 asterric acid analogs found in nature from 1960 to 2023. In this review, the structures, bioactivities, and biosynthesis of asterric acid analogs are summarized. More importantly, the empirical rule about the shielding effect of B-ring on H-6 is suggested, and this provides a convenient and useful way to analyze the NMR spectral data of asterric acid analogs, based on which the chemical shift values of the A-ring in some asterric acid analogs are revised.

The Identity, Virulence, and Antifungal Effects of the Didymellacesous Fungi Associated with the Rapeseed Blackleg Pathogen Leptosphaeria biglobosa

Eight fungal strains (P1 to P8) were isolated from rapeseed stems (Brassica napus) infected with the blackleg pathogen Leptosphaeria biglobosa (Lb). They formed pycnidia with similar morphology to those of Lb, and thus were considered as Lb relatives (LbRs). The species-level identification of these strains was performed. Their virulence on rapeseed and efficacy in the suppression of Lb infection were determined, and the biocontrol potential and biocontrol mechanisms of strain P2 were investigated. The results showed that the LbRs belong to two teleomorphic genera in the family Didymellaceae, Didymella for P1 to P7 and Boeremia for P8. Pathogenicity tests on rapeseed cotyledons and stems indicated the LbRs were weakly virulent compared to L. biglobosa. Co-inoculation assays on rapeseed cotyledons demonstrated that P1 to P7 (especially P1 to P4) had a suppressive effect on Lb infection, whereas P8 had a marginal effect on infection by L. biglobosa. Moreover, D. macrostoma P2 displayed a more aggressive behavior than L. biglobosa in the endophytic colonization of healthy rapeseed cotyledons. Cultures of P2 in potato dextrose broth (PDB) and pycnidiospore mucilages exuded from P2 pycnidia showed antifungal activity to L. biglobosa. Further leaf assays revealed that antifungal metabolites (AM) of strain P2 from PDB cultures effectively suppressed infection by L. biglobosa, Botrytis cinerea (gray mold), and Sclerotinia sclerotiorum (white mold). An antifungal metabolite, namely penicillither, was purified and identified from PDB cultures and detected in pycnidiospore mucilages of strain P2. This study suggests that the LbRs are a repertoire for screening biocontrol agents (BCAs) against rapeseed diseases, and D. macrostoma P2 is a multi-functional BCA, a penicillither producer, and an endophyte.

Extraction and characterization of anti-virus anthraquinones from Nicotiana tabacum-derived Aspergillus oryzae YNCA1220

In this study, seven novel anthraquinones (1-7) and four described anthraquinones (8-11) were purified from Nicotiana tabacum-derived Aspergillus oryzae YNCA1220. It is worth noting that only analogs of 4 and 5 have been reported as natural products to date, while the nuclei of compounds 1-3, 6 and 7 were isolated for the first time in nature. Among them, compounds 1-3 bear an unusual anthra[2,3-b]furan-9,10-dione nucleus, 4 and 5 possess a rare 3-methyl-1H-pyrrol-2-yl substituent, and 6 and 7 are new framework anthraquinones bearing a 6-methyl-1,7-dihydro-2H-azepin-2-one ring. Interestingly, the in vivo assays indicated that 1, 4 and 5 had inactivation effects against tobacco mosaic virus (TMV) with inhibition rates of 41.6%, 55.4% and 38.6%, respectively, at a concentration of 50 μg/mL, which were better than that of the positive control agent, ningnanmycin (33.8%). Compounds 1, 4 and 5 also had protective effects with inhibition rates of 48.7%, 60.2% and 43.5% at the same concentration, while 4 had a better curative effect than ningnanmycin at a concentration of 100 μg/mL. In addition, mechanistic studies also revealed that a potent direct effect on TMV, the induction of SAR in tobacco plants, and the effective regulation of defense enzymes, defense genes, and defense hormones may be the reasons for the significant effects of 4 against TMV. At the same time, downregulation of the expression of total NtHsp70 protein by inhibiting the related Hsp70 genes may also be involved in tobacco resistance to TMV. To evaluate whether compounds have broader antiviral activities, the antirotavirus activities of new isolates were also evaluated and found to be highly effective with a therapeutic index (TI) value ranging from 11.6 to 17.7. This study suggests that the above anthraquinone compounds, particularly 4, have broad spectrum antiviral activities. The successful isolation and structure identification of the above anthraquinones provide new materials for the screening of anti-TMV agents and contribute to the improved utilization of N. tabacum-derived fungi.

The Role of Endophytes in Combating Fungal- and Bacterial-Induced Stress in Plants

Plants are subjected to multifaceted stresses that significantly jeopardize crop production. Pathogenic microbes influence biotic stress in plants, which ultimately causes annual crop loss worldwide. Although the use of pesticides and fungicides can curb the proliferation of pathogens in plants and enhance crop production, they pollute the environment and cause several health issues in humans and animals. Hence, there is a need for alternative biocontrol agents that offer an eco-friendly mode of controlling plant diseases. This review discusses fungal- and bacterial-induced stress in plants, which causes various plant diseases, and the role of biocontrol defense mechanisms, for example, the production of hydrolytic enzymes, secondary metabolites, and siderophores by stress-tolerant fungi and bacteria to combat plant pathogens. It is observed that beneficial endophytes could sustain crop production and resolve the issues regarding crop yield caused by bacterial and fungal pathogens. The collated literature review indicates that future research is necessary to identify potential biocontrol agents that can minimize the utility of synthetic pesticides and increase the tenable agricultural production.

Seven Undescribed Aspergillus Species from Different Niches in Korea

An investigation of species of the genus Aspergillus present in arthropod, freshwater, and soil led to the discovery of seven undescribed species in Korea. Based on their morphological characteristics and molecular phylogeny analyses using a combined data set of β-tubulin (BenA) and calmodulin (CaM) sequences, the isolated strains CNUFC IGS2-5, CNUFC YJ1-19, CNUFC WD27, CNUFC U8-70, CNUFC AS2-24, CNUFC S32-1, and CNUFC U7-48, were identified as Aspergillus brunneoviolaceus, A. capensis, A. floccosus, A. inflatus, A. parvulus, A. polyporicola, and A. spelaeus, respectively. In the present study, the detailed morphological descriptions and phylogenetic relationships of these species are provided.

Polyphasic Characterization of Four Aspergillus Species as Potential Biocontrol Agents for White Mold Disease of Bean

The genus Aspergillus comprises several species that play pivotal roles in agriculture. Herein, we morphologically and physiologically characterized four genetically distinct Aspergillus spp., namely A. japonicus, A. niger, A. flavus, and A. pseudoelegans, and examined their ability to suppress the white mold disease of bean caused by Sclerotinia sclerotiorum in vitro and under greenhouse conditions. Seriation type of Aspergillus spp. correlates with conidiospores discharge as detected on the Petri glass lid. Members of Nigri section cover their conidial heads with hard shells after prolonged incubation. In addition, sporulation of the tested Aspergillus isolates is temperature sensitive as it becomes inhibited at low temperatures and the colonies become white. Examined Aspergillus spp. were neither infectious to legumes nor aflatoxigenic as confirmed by HPLC except for A. flavus and A. pseudoelegans which, secreted 5 and 1 ppm of aflatoxin B1, respectively. Co-inoculations of Sclerotinia’s mycelium or sclerotia with a spore suspension of Aspergillus spp. inhibited their germination on PDA at 18 °C and 28 °C, and halted disease onset on detached common bean and soybean leaves. Similarly, plants treated with A. japonicus and A. niger showed the highest survival rates compared to untreated plants. In conclusion, black Aspergillus spp. are efficient biocides and safe alternatives for the management of plant diseases, particularly in organic farms.

Metabolites Produced by Fungi against Fungal Phytopathogens: Review, Implementation and Perspectives

Many fungi, especially endophytes, have been found to produce multiple benefits in their plant hosts, with many of these benefits associated with the protection of plants against fungal diseases. This fact could be used in the development of new bio-products that could gradually reduce the need for chemical fungicides, which have been associated with multiple health and environmental problems. However, the utilization of the living organism may present several issues, such as an inconsistency in the results obtained and more complicated management and application, as fungal species are highly influenced by environmental conditions, the type of relationship with the plant host and interaction with other microorganisms. These issues could be addressed by using the bioactive compounds produced by the fungus, in cases where they were responsible for positive effects, instead of the living organism. Multiple bioactive compounds produced by fungal species, especially endophytes, with antifungal properties have been previously reported in the literature. However, despite the large amount of these metabolites and their potential, extensive in-field application on a large scale has not yet been implemented. In the present review, the main aspects explaining this limited implementation are analyzed, and the present and future perspectives for its development are discussed.

Antifungal Secondary Metabolites Produced by the Fungal Endophytes: Chemical Diversity and Potential Use in the Development of Biopesticides

Plant diseases caused by phytopathogenic fungi can lead to huge losses in the agricultural fields and therefore remain a continuous threat to the global food security. Chemical-based fungicides contributed significantly in securing crop production. However, indiscriminate application of fungicides has led to increased chemical resistance and potential risks to human health and environment. Thus, there is an urgent need for searching for new bioactive natural products and developing them into new biopesticides. Fungal endophytes, microorganisms that reside in the fresh tissues of living plants, are regarded as untapped sources of novel natural products for exploitation in agriculture and/or medicine. Chemical examination of endophytic fungi has yielded enormous antifungal natural products with potential use in the development of biopesticides. This review summarizes a total of 132 antifungal metabolites isolated from fungal endophytes in the past two decades. The emphasis is on the unique chemical diversity of these metabolic products, together with their relevant antifungal properties. Moreover, some "star molecules," such as griseofulvin and trichothecene, as well as their synthetic derivatives that possess high potential as candidates of new natural fungicides, are also presented herein.

Re-examination of species limits in Aspergillus section Flavipedes using advanced species delimitation methods and description of four new species

Since the last revision in 2015, the taxonomy of section Flavipedes evolved rapidly along with the availability of new species delimitation techniques. This study aims to re-evaluate the species boundaries of section Flavipedes members using modern delimitation methods applied to an extended set of strains (n = 90) collected from various environments. The analysis used DNA sequences of three house-keeping genes (benA, CaM, RPB2) and consisted of two steps: application of several single-locus (GMYC, bGMYC, PTP, bPTP) and multi-locus (STACEY) species delimitation methods to sort the isolates into putative species, which were subsequently validated using DELINEATE software that was applied for the first time in fungal taxonomy. As a result, four new species are introduced, i.e. A. alboluteus, A. alboviridis, A. inusitatus and A. lanuginosus, and A. capensis is synonymized with A. iizukae. Phenotypic analyses were performed for the new species and their relatives, and the results showed that the growth parameters at different temperatures and colonies characteristics were useful for differentiation of these taxa. The revised section harbors 18 species, most of them are known from soil. However, the most common species from the section are ecologically diverse, occurring in the indoor environment (six species), clinical samples (five species), food and feed (four species), droppings (four species) and other less common substrates/environments. Due to the occurrence of section Flavipedes species in the clinical material/hospital environment, we also evaluated the susceptibility of 67 strains to six antifungals (amphotericin B, itraconazole, posaconazole, voriconazole, isavuconazole, terbinafine) using the reference EUCAST method. These results showed some potentially clinically relevant differences in susceptibility between species. For example, MICs higher than those observed for A. fumigatus wild-type were found for both triazoles and amphotericin B for A. ardalensis, A. iizukae, and A. spelaeus whereas A. lanuginosus, A. luppiae, A. movilensis, A. neoflavipes, A. olivimuriae and A. suttoniae were comparable to or more susceptible as A. fumigatus. Finally, terbinafine was in vitro active against all species except A. alboviridis.

Brassica oleracea var. acephala (kale) improvement by biological activity of root endophytic fungi

Brassica oleracea var. acephala (kale) is a cruciferous vegetable widely cultivated for its leaves and flower buds in Atlantic Europe and the Mediterranean area, being a food of great interest as a "superfood" today. Little has been studied about the diversity of endophytic fungi in the Brassica genus, and there are no studies regarding kale. In this study, we made a survey of the diversity of endophytic fungi present in the roots of six different Galician kale local populations. In addition, we investigated whether the presence of endophytes in the roots was beneficial to the plants in terms of growth, cold tolerance, or resistance to bacteria and insects. The fungal isolates obtained belonged to 33 different taxa. Among those, a Fusarium sp. and Pleosporales sp. A between Setophoma and Edenia (called as Setophoma/Edenia) were present in many plants of all five local populations, being possible components of a core kale microbiome. For the first time, several interactions between endophytic fungus and Brassica plants are described and is proved how different interactions are beneficial for the plant. Fusarium sp. and Pleosporales sp. B close to Pyrenophora (called as Pyrenophora) promoted plant growth and increased cold tolerance. On the other hand, isolates of Trichoderma sp., Pleosporales sp. C close to Phialocephala (called as Phialocephala), Fusarium sp., Curvularia sp., Setophoma/Edenia and Acrocalymma sp. were able to activate plant systemic resistance against the bacterial pathogen Xanthomonas campestris. We also observed that Fusarium sp., Curvularia sp. and Setophoma/Edenia confered resistance against Mamestra brassicae larvae.

Aspergillus spp. eliminate Sclerotinia sclerotiorum by imbalancing the ambient oxalic acid concentration and parasitizing its sclerotia

Sclerotinia sclerotiorum, a pathogen of more than 600 host plants, secretes oxalic acid to regulate the ambient acidity and provide conducive environment for pathogenicity and reproduction. Few Aspergillus spp. were previously proposed as potential biocontrol agents for S. sclerotiorum as they deteriorate sclerotia and prevent pathogen's overwintering and initial infections. We studied the nature of physical and biochemical interactions between Aspergillus and Sclerotinia. Aspergillus species inhibited sclerotial germination as they colonized its rind layer. However, Aspergillus-infested sclerotia remain solid and viable for vegetative and carpogenic germination, indicating that Aspergillus infestation is superficial. Aspergillus spp. of section Nigri (Aspergillus japonicus and Aspergillus niger) were also capable of suppressing sclerotial formation by S. sclerotiorum on agar plates. Their culture filtrate contained high levels of oxalic, citric and glutaric acids comparing to the other Aspergillus spp. tested. Exogenous supplementation of oxalic acid altered growth and reproduction of S. sclerotiorum at low concentrations. Inhibitory concentrations of oxalic acid displayed lower pH values comparing to their parallel concentrations of other organic acids. Thus, S. sclerotiorum growth and reproduction are sensitive to the ambient oxalic acid fluctuations and the environmental acidity. Together, Aspergillus species parasitize colonies of Sclerotinia and prevent sclerotial formation through their acidic secretions.

Natural product diversity from the endophytic fungi of the genus Aspergillus

The endophytic fungus Aspergillus is considered as an enormous source of chemical leads with promising biological activities. Different Aspergillus species have proved their ability to produce plenty of secondary metabolites including butenolides, alkaloids, terpenoids, cytochalasins, phenalenones, ρ-terphenyls, xanthones, sterols, diphenyl ether and anthraquinone derivatives with diverse biological activities, such as anti-cancer, antifungal, anti-bacterial, anti-viral, anti-inflammatory, antitrypanosomal and antileishmanial activities. From January 2015 until December 2019, three hundred and sixty-one secondary metabolites were reported from different endophytic Aspergillus species. This review discusses the isolated secondary metabolites from different endophytic Aspergillus species reported from January 2015 to December 2019 along with their reported biological activities and structural aspects whenever applicable.