Lophiostomin A–D: new 3,4-dihydroisocoumarin derivatives from the endophytic fungus Lophiostoma sp. Sigrf10

Endophytic Fungi: A Treasure Trove of Antifungal Metabolites

Emerging and reemerging fungal infections are very common in nosocomial and non-nosocomial settings in people having poor immunogenic profiles either due to hematopoietic stem cell transplants or are using immunomodulators to treat chronic inflammatory disease or autoimmune disorders, undergoing cancer therapy or suffering from an immune weakening disease like HIV. The refractory behavior of opportunistic fungi has necessitated the discovery of unconventional antifungals. The emergence of black fungus infection during COVID-19 also triggered the antifungal discovery program. Natural products are one of the alternative sources of antifungals. Endophytic fungi reside and co-evolve within their host plants and, therefore, offer a unique bioresource of novel chemical scaffolds with an array of bioactivities. Hence, immense possibilities exist that these unique chemical scaffolds expressed by the endophytic fungi may play a crucial role in overcoming the burgeoning antimicrobial resistance. These chemical scaffolds so expressed by these endophytic fungi comprise an array of chemical classes beginning from cyclic peptides, sesquiterpenoids, phenols, anthraquinones, coumarins, etc. In this study, endophytic fungi reported in the last six years (2018-2023) have been explored to document the antifungal entities they produce. Approximately 244 antifungal metabolites have been documented in this period by different groups of fungi existing as endophytes. Various aspects of these antifungal metabolites, such as antifungal potential and their chemical structures, have been presented. Yet another unique aspect of this review is the exploration of volatile antifungal compounds produced by these endophytic fungi. Further strategies like epigenetic modifications by chemical as well as biological methods and OSMAC to induce the silent gene clusters have also been presented to generate unprecedented bioactive compounds from these endophytic fungi.

Fungal endophytes of Taxus species and regulatory effect of two strains on taxol synthesis

BACKGROUND

Taxol, derived from Taxus trees, is a valuable natural resource for the development of anticancer drugs. Endophytic fungi from Taxus trees are a promising alternative source of Taxol. However, the impact of plant-endophytic microbial interaction on the host's Taxol biosynthesis is largely unknown.

RESULTS

In the current study, the diversity of endophytic fungi in three different Taxus species was analyzed using Internal Transcribed Spacer sequencing. A total of 271 Operational Taxonomic Units (OTUs) were identified, grouping into 2 phyla, 8 classes, 16 orders, 19 families, and 19 genera. Alpha and beta diversity analysis indicated significant differences in endophytic fungal communities among the various Taxus trees. At the genus level, Alternaria and Davidiella were predominantly found in T. mairei and T. media, respectively. By utilizing a previously published dataset, a Pearson correlation analysis was conducted to predict the taxol biosynthesis-related fungal genera. Following screening, two isolates of Alternaria (L7 and M14) were obtained. Effect of inoculation with Alternaria isolates on the gene expression and metabolite accumulation of T. mairei was determined by transcriptomic and untargeted metabolomic studies. The co-inoculation assay suggests that the two Alternaria isolates may have a negative regulatory effect on taxol biosynthesis by influencing hormone signaling pathways.

CONCLUSION

Our findings will serve as a foundation for advancing the production and utilization of Taxus and will also aid in screening endophytic fungi related to taxol production.

Improvement in the Phytochemical Content and Biological Properties of Stevia rebaudiana (Bertoni) Bertoni Plant Using Endophytic Fungi Fusarium fujikuroi

This study aimed to increase the therapeutic potential of medicinal plants through inoculation with endophytic fungi. As endophytes influence medicinal plants' biological properties, twenty fungal strains were isolated from the medicinal plant Ocimum tenuiflorum. Among all fungal isolates, the R2 strain showed the highest antagonistic activity towards plant pathogenic fungi Rosellinia necatrix and Fusarium oxysporum. The partial ITS region of the R2 strain was deposited in the GenBank nucleotide sequence databases under accession number ON652311 as Fusarium fujikuroi isolate R2 OS. To ascertain the impact of an endophytic fungus on the biological functions of medicinal plants, Stevia rebaudiana seeds were inoculated with Fusarium fujikuroi (ON652311). In the DPPH assay, the IC₅₀ value of the inoculated Stevia plant extracts (methanol, chloroform, and positive control) was 72.082 µg/mL, 85.78 µg/mL, and 18.86 µg/mL, respectively. In the FRAP assay, the IC₅₀ value of the inoculated Stevia extracts (methanol, chloroform extract, and positive control) was 97.064 µM Fe²⁺ equivalents, 117.662 µM Fe²⁺ equivalents, and 53.384 µM Fe²⁺ equivalents, respectively. In the extracts of the plant inoculated with endophytic fungus, rutin and syringic acid (polyphenols) concentrations were 20.8793 mg/L and 5.4389 mg/L, respectively, which were higher than in the control plant extracts. This approach can be further utilized for other medicinal plants to increase their phytochemical content and hence medicinal potential in a sustainable way.

Fungal Endophytes: A Potential Source of Antibacterial Compounds

Antibiotic resistance is becoming a burning issue due to the frequent use of antibiotics for curing common bacterial infections, indicating that we are running out of effective antibiotics. This has been more obvious during recent corona pandemics. Similarly, enhancement of antimicrobial resistance (AMR) is strengthening the pathogenicity and virulence of infectious microbes. Endophytes have shown expression of various new many bioactive compounds with significant biological activities. Specifically, in endophytic fungi, bioactive metabolites with unique skeletons have been identified which could be helpful in the prevention of increasing antimicrobial resistance. The major classes of metabolites reported include anthraquinone, sesquiterpenoid, chromone, xanthone, phenols, quinones, quinolone, piperazine, coumarins and cyclic peptides. In the present review, we reported 451 bioactive metabolites isolated from various groups of endophytic fungi from January 2015 to April 2021 along with their antibacterial profiling, chemical structures and mode of action. In addition, we also discussed various methods including epigenetic modifications, co-culture, and OSMAC to induce silent gene clusters for the production of noble bioactive compounds in endophytic fungi.

Culturable Endophytic Fungi from Glycyrrhiza inflata Distributed in Xinjiang, China with Antifungal Activity

A total of 99 endophytic fungal isolates were obtained from the roots of Glycyrrhiza inflata, which was a traditional medicinal plant mainly distributed in Xinjiang, China. Twenty-two distinct isolates were selected for further taxonomical identification by morphological traits and internal transcribed spacer (ITS) rRNA gene sequence analysis. Eleven genera were identified, among which Aspergillus, Alternaria and Fusarium were dominant. The crude extracts of 22 distinct identified fungi were successively evaluated for their antifungal activities on three rice fungal pathogens using the method of hyphal radial growth rate. Among them, the crude extract of Alternaria an-gustiovoidea Glinf007 showed the significantly mycelial growth inhibitory activity. The results demonstrated that G. inflata contained a diversity of culturable endophytic fungi, which could produce natural antimicrobial compounds that might be of great value to the agriculture and pharmaceutical industries.

Diversity and antibacterial activity of fungal endophytes from Eucalyptus exserta

BACKGROUND

Eucalyptus bacterial wilt caused by Ralstonia solanacearum is an important eucalyptus disease. Endophytic fungi, an important source of natural active substances, provide a new breakthrough for the control of plant diseases.

RESULTS

In the present study, 80 endophytic fungal isolates were obtained from the healthy branches and fruits of Eucalyptus exserta. Fifteen distinct isolates (MK120854-MK120868) were selected for further taxonomic identification through morphological trait assessments and internal transcribed spacer (ITS) region-rRNA gene sequence analysis. Thirteen genera, namely, Phyllosticta, Penicillium, Eutypella, Purpureocillium, Talaromyces, Lophiostoma, Cladosporium, Pestalotiopsis, Chaetomium, Fusarium, Gongronella, Scedosporium and Pseudallescheria, were identified on the basis of their morphological characteristics. Members of the genus Phyllosticta were the primary isolates, with a colonization frequency (CF) of 27.5 %. Most of the fungal isolates displayed antibacterial activity. The crude extracts obtained from Lophiostoma sp. Eef-7, Pestalotiopsis sp. Eef-9 and Chaetomium sp. Eef-10 exhibited strong inhibition on the test bacteria, and Lophiostoma sp. Eef-7 was further cultured on a large scale. Three known compounds, scorpinone (1), 5-deoxybostrycoidin (2) and 4-methyl-5,6-dihydro-2 H-pyran-2-one (3), were isolated from the endophytic fungus Lophiostoma sp. Eef-7 associated with E. exserta. The structures of these compounds were elucidated by analysis of 1D and 2D NMR and HR-ESI-MS spectra and a comparison of their spectral data with published values. Compounds 1 and 2 showed weak antimicrobial activity against Ralstonia solanacearum.

CONCLUSIONS

Endophytic fungi from Eucalyptus exserta may represent alternative sources of antimicrobial agents. Lophiostoma sp. Eef-7 can produce 2-azaanthraquinone derivatives and shows weak antibacterial activity against Ralstonia solanacearum.

Extracellular metabolites of endophytic fungi from Azadirachta indica inhibit multidrug-resistant bacteria and phytopathogens

Aim: To evaluate antimicrobial activity of extracellular metabolites (EMs) of endophytic fungal isolates (EFIs) from Azadirachta indica. Materials & methods: EFIs were identified by internal transcribed spacer (ITS) sequencing. Antimicrobial activity, and minimum inhibitor concentration (MIC) and minimum bactericidal concentration (MBC) were determined using agar diffusion and microdilution method, respectively. Results: Seventeen EFIs were isolated from different organs of A. indica. Eight of them were identified based on ITS sequencing. The EMs of EFIs inhibited the growth of six multidrug-resistant (MDR) bacterial superbugs and three phytopathogenic fungi. The MDR bacterial superbugs are resistant to six commercial antibiotics of different generations but susceptible to EMs of EFIs. The MIC (0.125-1.0 μg/μl), MBC (0.5-4.0 μg/μl) and minimum fungicidal concentration (1.0-4.0 μg/μl) of the EMs from EFIs are lower enough. Conclusion: The EMs of the EFIs have promising antimicrobial activity against MDR bacteria and phytopathogenic fungi.

Natural isocoumarins: Structural styles and biological activities, the revelations carry on …

Isocoumarins and dihydroisocoumarins are lactonic phytochemicals plentiful in microbes and higher plants. These are an amazing small scaffolds consecrated with all types of pharmacological applications. Our previous review covered the period 2000-2016, documenting the then known natural products of this class; the current article is a critical account of discovery of known as well as undescribed structural types and pharmacological activities reported in the course of 2016-2020. The classification revealed in our previous review based on the biogenetic origin is further buttressed by discovery of new members of each class and some new structural types hitherto unknown have also been identified. Similarly, the biological activities and SAR conclusions identified were found to be valid as well, nonetheless with new accompaniments. The most recent available literature on the structural diversity and biological activity of these natural products has been included. The information documented in this article are collected from scientific journals, books, electronic search engines and scientific databases.