Bioactive seco-Sativene Sesquiterpenoids from an Artemisia desertorum Endophytic Fungus, Cochliobolus sativus

Biological activities and mass fragmentation pathways of meroterpenoid cochlioquinones from plant pathogenic fungus Bipolaris sorokiniana

Cochlioquinones are a member of meroterpenoids that partially possessed phenolic hydroxyls with potential antioxidant activities. This study investigated the mass fragmentation pathways, antioxidant, cytotoxic, and phytotoxic activities of cochlioquinone analogs. The mass fragmentation pathways of cochlioquinones (1-7) were firstly analyzed using UPLC-Q-TOF-MS/MS, in which Retro Diels-Alder reaction, neutral loss, and McLafferty rearrangement were the main cleavage patterns. Compound 8 and 9 (a unique new analog) were then isolated in target. Cochlioquinones (4-6, 9) displayed strong antioxidant activities for DPPH radical scavenging assay as the first antioxidant effects report. In addition, 1-9 exhibited cytotoxic activities against B16 cells (IC50 from 1.91 to 12.33 μM) and Hep G2 cells (IC50 from 3.21 to 77.15 μM), and 5, 7, and 8 showed phytotoxic activities against foxtail leaves. These biological activities imply that cochlioquinones can be as antioxidant agents for food additives or bioactive molecules for cancer drugs and pesticides.

Sativene Sesquiterpenoids from the Plant Endophytic Fungus Bipolaris victoriae S27 and Their Potential as Plant-Growth Regulators

Thirteen new sativene sesquiterpenoids (1 and 3-14), one new natural product (2), and 16 known compounds (15-30) were isolated from the endophytic fungus Bipolaris victoriae S27. Their structures were elucidated by extensive spectroscopic analysis, NMR and ECD calculations, and X-ray crystal diffractions. Compound 1 represented the first example of sativene sesquiterpenoids with a 6/5/3/5-caged tetracyclic ring system. All obtained compounds were evaluated for their plant-growth regulatory activity. The results showed that 1, 3, 4, 6, 8, 11, 12, 17, 19, 26, and 27 could suppress the growth of Arabidopsis thaliana, while 2, 5, 13, 15, 18, and 25 showed promoting effects. Among them, compound 3 showed the most potent plant-growth inhibitory activity, which is obviously superior to that of the marked herbicide glyphosate.

Genome Mining Reveals the Biosynthesis of Sativene and Its Oxidative Conversion to seco-Sativene

Sativene (1) and seco-sativene are an important family of fungal sesquiterpenoids that feature unique tricyclo[4.4.0.01,7]decane and bicyclo[3.2.1]octane skeletons, respectively. Herein, we identify a three-enzyme cassette: SatA cyclizes farnesyl diphosphate (FPP) to form compound 1; CYP450 SatB catalyzes C14-C15 dihydroxylations and subsequent bond cleavage; and reductase SatC regioselectively reduces C14 aldehyde and mediates hemiacetal ring closure to generate prehelminthosporol (2). Our findings clarify the synthetic step of sativene and its oxidative transformation processes into seco-sativene.

Identification and characterization of a novel sativene synthase from Fischerella thermalis

Sesquiterpene synthases (TPS) determine the structural diversity of terpenoids, which are species specific. In this study, we report a TPS from Fischerella thermalis (named as FtTPS), recombinantly expressed as a soluble protein in Escherichia coli BL21(DE3) strain. The FtTPS protein could catalyze the conversion of farnesyl pyrophosphate (FPP) to sativene, a kind of tricyclic sesquiterpene. The optimal pH and temperature of FtTPS were 7.5 and 30 °C, respectively. The KM and Vmax values of FtTPS for FPP were 1.846 μM and 0.372 μM/min, respectively. By constructing an engineered E. coli strain carrying the FtTPS and the heterologous mevalonate (MVA) pathway genes, sativene could be detected and its yield reached 24 mg/L after 96 h cultivation. The highest yield of sativene was obtained when E.coli BL21 Star was used as the host with SBMSN medium. These results exhibited the biosynthesis of sativene for the first time.

The antifungal properties of terpenoids from the endophytic fungus Bipolaris eleusines

A series of terpenoids (1-17), comprising six new compounds designated bipolariterpenes A-F (1-6) and eleven recognized compounds (7-17), were isolated from the wheat culture of the potato endophytic fungus Bipolaris eleusines. Their structures and stereochemistry were clarified by HRESIMS, NMR, DP4 + probability analyses, and computations for electronic circular dichroism (ECD). All compounds are made up of six meroterpenoids, four sesterterpenes and seven sesquiterpenes. Among them, four sesterterpenes (4, 5, 10, 11) were investigated for their antifungal, antibacterial and cytotoxic properties, and six meroterpenoids (1-3, 7-9) were evaluated for their antifungal properties. The compounds 7, 9, and 10 had substantial antifungal activity against Epidermophyton floccosum at a concentration of 100 µM. No antibacterial and cytotoxic activities were observed.

Bioactive secondary metabolites from endophytic strains of Neocamarosporium betae collected from desert plants

Endophytic fungi from desert plants belong to a unique microbial community that has been scarcely investigated chemically and could be a new resource for bioactive natural products. In this study, 13 secondary metabolites (1-13) with diverse carbon skeletons, including a novel polyketide (1) with a unique 5,6-dihydro-4H,7H-2,6-methanopyrano[4,3-d][1,3]dioxocin-7-one ring system and three undescribed polyketides (2, 7, and 11), were obtained from the endophytic fungus Neocamarosporium betae isolated from two desert plant species. Different approaches, including HR-ESI-MS, UV spectroscopy, IR spectroscopy, NMR, and CD, were used to determine the planar and absolute configurations of the compounds. The possible biosynthetic pathways were proposed based on the structural characteristics of compounds 1-13. Compounds 1, 3, 4, and 9 exhibited strong cytotoxicity toward HepG2 cells compared with the positive control. Several metabolites (2, 4-5, 7-9, and 11-13) were phytotoxic to foxtail leaves. The results support the hypothesis that endophytic fungi from special environments, such as desert areas, produce novel bioactive secondary metabolites.

Undescribed diphenyl ethers betaethrins A-I from a desert plant endophytic strain of the fungus Phoma betae A.B. Frank (Didymellaceae)

Ten diphenyl ethers (DPEs), including nine undescribed analogs named betaethrins A-I, were isolated from the desert plant endophytic fungus Phoma betae A.B. Frank (Didymellaceae). Their structures were determined mainly by NMR, HR-ESI-MS spectral and X-ray diffraction experiments. Betaethrins D-I possessed different fatty acid chains connected with the B-ring, which was the first report in all DPEs. The shielding effect of the B-ring on H-6 (A-ring) in methyl barceloneate, betaethrin A and betaethrins D-F (asterric acid analogs) was first observed and analyzed, which could differentiate the ¹H-NMR chemical shift values of H-4/H-6 without the assistance of 3-OH. An empirical rule was then suggested: the steric hindrance between the A- and B-rings in asterric acid analogs might prevent these two aromatic rings from rotating freely, which led to the ¹H-NMR chemical shift value of H-6 being in the high field zone due to the shielding effect of the B-ring on H-6. Based on the empirical rule, the chemical shift values of the A-ring in methyl barceloneate were revised. The possible biosynthesis of these isolates was postulated. Betaethrin H showed moderate cytotoxicity against MCF-7 and HepG2 cancer cell lines. Betaethrins A-F, H and I displayed strong antioxidant activities. These results further implied that endophytic fungi from unique environments, such as desert plants, with few chemical studies are an important resource of undescribed and bioactive metabolites.

Bioactive terpenoids derived from plant endophytic fungi: An updated review (2011-2020)

Plant endophytes have been considered as novel sources of naturally occurring compounds with various biological activities, including cytotoxic, antimicrobial, anti-inflammatory, anticancer, herbicides, antileishmanial and antioxidant. A variety of specialised products, comprising terpenoids, alkaloids, polyketides, phenolic compounds, coumarins, and quinone derivatives have been reported from various strains. An increasing number of products, especially terpenoids, are being isolated from endophytes. Herein, the isolated new terpenoids from plant endophytic fungi, their hosts, as well as biological activities, from January 2011 until the end of 2020 are reviewed. In this period, 516 terpenoids are classified into monoterpenes (5), sesquiterpenes (299), diterpenes (76), sesterterpens (22), meroterpenes (83), triterpenes (29), and other terpenoids (2), were isolated from different plant endophytic fungi species.

UPLC-Q-TOF-MS/MS Analysis of Seco-Sativene Sesquiterpenoids to Detect New and Bioactive Analogues From Plant Pathogen Bipolaris sorokiniana

Seco-sativene sesquiterpenoids are an important member of phytotoxins and plant growth regulators isolated from a narrow spectrum of fungi. In this report, eight seco-sativene sesquiterpenoids (1-8) were first analyzed using the UPLC-Q-TOF-MS/MS technique in positive mode, from which their mass fragmentation pathways were suggested. McLafferty rearrangement, 1,3-rearrangement, and neutral losses were considered to be the main fragmentation patterns for the [M+1]+ ions of 1-8. According to the structural features (of different substitutes at C-1, C-2, and C-13) in compounds 1-8, five subtypes (A-E) of seco-sativene were suggested, from which subtypes A, B/D, and E possessed the diagnostic daughter ions at m/z 175, 189, and 203, respectively, whereas subtype C had the characteristic daughter ion at m/z 187 in the UPLC-Q-TOF-MS/MS profiles. Based on the fragmentation patterns of 1-8, several known compounds (1-8) and two new analogues (9 and 10) were detected in the extract of plant pathogen fungus Bipolaris sorokiniana based on UPLC-Q-TOF-MS/MS analysis, of which 1, 2, 9, and 10 were then isolated and elucidated by NMR spectra. The UPLC-Q-TOF-MS/MS spectra of these two new compounds (9 and 10) were consistent with the fragmentation mechanisms of 1-8. Compound 1 displayed moderate antioxidant activities with IC50 of 0.90 and 1.97 mM for DPPH and ABTS+ scavenging capacity, respectively. The results demonstrated that seco-sativene sesquiterpenoids with the same subtypes possessed the same diagnostic daughter ions in the UPLC-Q-TOF-MS/MS profiles, which could contribute to structural characterization of seco-sativene sesquiterpenoids. Our results also further supported that UPLC-Q-TOF-MS/MS is a powerful and sensitive tool for dereplication and detection of new analogues from crude extracts of different biological origins.

Phaeosphspirone (1/1'), a pair of unique polyketide enantiomers with an unusual 6/5/5/6 tetracyclic ring from the desert plant endophytic fungus Phaeosphaeriaceae sp

Phaeosphspirone, an undescribed polyketide with a unique 6/5/5/6-fused tetracyclic system, and two known analogues, herbarin and O-methylherbarin, were purified from the endophytic fungus Phaeosphaeriaceae sp. isolated from the desert plant Bassia dasyphylla. The connectivity and relative configuration of phaeosphspirone was elucidated by comprehensive HR-ESI-MS and NMR analysis together with a computer-assisted structure elucidation (CASE) method. A pair of enantiomers existing in phaeosphspirone were separated by HPLC chromatography after reacting with chiral reagents, from which the absolute configuration of phaeosphspirone was simultaneously determined based on Mosher's rule. This tandem strategy provides a useful approach for the separation and stereochemical determination of enantiomers possessing secondary hydroxyl groups. The structural feature of phaeosphspirone, herbarin and O-methylherbarin together with gene cluster analysis suggested their polyketide biosynthetic origin. Herbarin and O-methylherbarin exhibited moderate cytotoxicity against three cancer cell lines.

Phytotoxic compounds from endophytic fungi

Weeds represent one of the most challenging biotic factors for the agricultural sector, responsible for causing significant losses in important agricultural crops. Traditional herbicides have managed to keep weeds at bay, but overuse has resulted in negative environmental and toxicological impacts, including the increase of herbicide-resistant species. Within this context, the use of biologically derived (bio-)herbicides represents a promising solution because they are able to provide the desired phytotoxic effects while causing less toxic environmental damage. In recent years, bioactive secondary metabolites, in particular those bio-synthesized by endophytic fungi, have been shown to be promising sources of novel compounds that can be exploited in agriculture, including their use in weed control. Endophytic fungi have the ability to produce volatile and nonvolatile compounds with broad phytotoxic activity. In addition, as a result of the beneficial relationships they establish with their host plants, they are part of the colonization mechanism and can provide protection for their hosts. As such, endophytic fungi can be exploited as bioherbicides and as research tools. In this review, we cover 100 nonvolatile secondary metabolites with phytotoxic activity and more than 20 volatile organic compounds in a mixture, produced by 28 isolates of endophytic fungi from 21 host plant families, collected in 8 countries. This information can form the basis for the application of endophytic fungal compounds in weed control. KEY POINTS: • Endophytic fungi produce a wide variety of secondary metabolites with unique and complex structures. • Fungal endophytes produce volatile and nonvolatile compounds with promising phytotoxic activity. • Endophytic fungi are a promising source of useful bioherbicides.

Carotane sesquiterpenoids A–G from the desert endophytic fungus Fusarium sp. HM 166

Seven undescribed carotane sesquiterpenoids named fusanoids A–G (1–7), along with one known analog (8) and two known sesterterpenes (9 and 10), were isolated from the fermentation broth of the desert endophytic fungi Fusarium sp. HM166. The structures of the compounds, including their absolute configurations, were determined by spectroscopic data, single-crystal X-ray diffraction analysis, and ECD calculations. Compound 10 showed cytotoxic activities against human hepatoma carcinoma cell line (Huh-7) and human breast cell lines (MCF-7 and MDA-MB-231), and compound 2 showed cytotoxic activity against MCF-7, while compounds 4–9 were inactive against all the tested cell lines. Compounds 4 and 10 showed potent inhibitory activities against the IDH1^R132h mutant.

Seven undescribed carotane sesquiterpenoids were isolated from the endophytic fungi Fusarium sp. HM166. Single-crystal X-ray diffraction and ECD defined absolute configurations. Cytotoxicity for Huh-7, MCF-7, and MDA-MB-231 cancer cell lines and IDH1^R132h mutant were studied.

Accurate Identification of Degraded Products of Aflatoxin B1 Under UV Irradiation Based on UPLC-Q-TOF-MS/MS and NMR Analysis

Analysis, purification, and characterization of AFB₁ degraded products are vital steps for elucidation of the photocatalytic mechanism. In this report, the UPLC-Q-TOF-MS/MS technique was first coupled with purification and NMR spectral approaches to analyze and characterize degraded products of AFB₁ photocatalyzed under UV irradiation. A total of seventeen degraded products were characterized based on the UPLC-Q-TOF-MS/MS analysis, in which seven ones (1-7) including four (stereo) isomers (1,2, 5, and 6) were purified and elucidated by NMR experiments. According to the structural features of AFB₁ and degraded products (1-7), the possible photocatalytic mechanisms were suggested. Furthermore, AFB₁ and degraded products (1-7) were evaluated against different cell lines. The results indicated that the UPLC-Q-TOF-MS/MS technique combined with purification, NMR spectral experiments, and biological tests was an applicable integrated approach for analysis, characterization, and toxic evaluation of degraded products of AFB₁, which could be used to evaluate other mycotoxin degradation processes.

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.

Bioactive metabolites from the desert plant-associated endophytic fungus Chaetomium globosum (Chaetomiaceae)

Globosumin, an undescribed chromene-4,7(4aH)-dione-tetramic acid PKS-PKS-NRPS hybrid, and globosumone, an undescribed azaphilone, together with ten known metabolites, were isolated from the desert plant-associated endophytic fungus Chaetomium globosum (Chaetomiaceae). The planar structures and relative configurations of globosumin and globosumone were determined by high-resolution ESI-MS and NMR data, and the absolute configurations of these two metabolites were determined by electronic circular dichroism (ECD) and circular dichroism (CD) combined with time-dependent density functional theory (TDDFT)-based quantum-chemical calculations. Chaetoglobosin A displayed biological effects against the seedling growth of Arabidopsis thaliana (Brassicaceae) in a dose-dependent manner, and this compound also exhibited biological activity against two cancer cell lines, A549 and HepG2, with IC₅₀ values of 6.82 ± 2.34 and 38.62 ± 7.44 μM, respectively.

Phytotoxic Secondary Metabolites from Fungi

Fungal phytotoxic secondary metabolites are poisonous substances to plants produced by fungi through naturally occurring biochemical reactions. These metabolites exhibit a high level of diversity in their properties, such as structures, phytotoxic activities, and modes of toxicity. They are mainly isolated from phytopathogenic fungal species in the genera of Alternaria, Botrytis, Colletotrichum, Fusarium, Helminthosporium, and Phoma. Phytotoxins are either host specific or non-host specific phytotoxins. Up to now, at least 545 fungal phytotoxic secondary metabolites, including 207 polyketides, 46 phenols and phenolic acids, 135 terpenoids, 146 nitrogen-containing metabolites, and 11 others, have been reported. Among them, aromatic polyketides and sesquiterpenoids are the main phytotoxic compounds. This review summarizes their chemical structures, sources, and phytotoxic activities. We also discuss their phytotoxic mechanisms and structure-activity relationships to lay the foundation for the future development and application of these promising metabolites as herbicides.

Naturally Occurring seco-Sativene Sesquiterpenoid: Chemistry and Biology

seco-Sativenes are a small group of sesquiterpenoids with a unique bicyclo[3.2.1]octane core carbon skeleton, which implies the unusual biosynthetic pathway. Up to date, there are 40 seco-sativene analogues with diverse post-modifications isolated from different fungi. Interestingly, some seco-sativene analogues display strong phytotoxic effects, whereas others possess plant-growth-promoting biological activities. The possible mechanism of actions about phytotoxic or growth-promoting activities are partly elucidated, but structure-activity relationships are still not clear. This review provides a comprehensive overview on the structures, ¹H nuclear magnetic resonance features, bioactivities, and biosynthesis of seco-sativene sesquiterpenoids from 1956 to 2020.