New Alkylitaconic Acid Derivatives from Nodulisporium sp. A21 and Their Auxin Herbicidal Activities on Weed Seeds

Colonization of root endophytic fungus Serendipita indica improves drought tolerance of Pinus taeda seedlings by regulating metabolome and proteome

Pinus taeda is an important forest tree species for plantations because of its rapid growth and high yield of oleoresins. Although P. taeda plantations distribute in warm and wet southern China, drought, sometime serious and long time, often occurs in the region. To explore drought tolerance of P. taeda and usage of beneficial microorganisms, P. taeda seedlings were planted in pots and were inoculated with root endophytic fungus Serendipita indica and finally were treated with drought stress for 53 d. Metabolome and proteome of their needles were analyzed. The results showed that S. indica inoculation of P. taeda seedlings under drought stress caused great changes in levels of some metabolites in their needles, especially some flavonoids and organic acids. Among them, the levels of eriocitrin, trans-aconitic acid, vitamin C, uric acid, alpha-ketoglutaric acid, vitamin A, stachydrine, coumalic acid, itaconic acid, calceolarioside B, 2-oxoglutaric acid, and citric acid were upregulated more than three times in inoculated seedlings under drought stress, compared to those of non-inoculated seedlings under drought stress. KEGG analysis showed that some pathways were enriched in inoculated seedlings under drought stress, such as flavonoid biosynthesis, ascorbate and aldarate metabolism, C5-branched dibasic acid metabolism. Proteome analysis revealed some specific differential proteins. Two proteins, namely, H9X056 and H9VDW5, only appeared in the needles of inoculated seedlings under drought stress. The protein H9VNE7 was upregulated more than 11.0 times as that of non-inoculated seedlings under drought stress. In addition, S. indica inoculation increased enrichment of water deficient-inducible proteins (such as LP3-1, LP3-2, LP3-3, and dehydrins) and those involved in ribosomal structures (such as A0A385JF23). Meanwhile, under drought stress, the inoculation caused great changes in biosynthesis and metabolism pathways, mainly including phenylpropanoid biosynthesis, cutin, suberine and wax biosynthesis, and 2-oxocarboxylic acid metabolism. In addition, there were positive relationships between accumulation of some metabolites and enrichment of proteins in P. taeda under drought stress. Altogether, our results showed great changes in metabolome and proteome in inoculated seedlings under drought stress and provided a guideline to further study functions of metabolites and proteins, especially those related to drought stress.

Seco-pimarane diterpenoids and androstane steroids from an endophytic Nodulisporium fungus derived from Cyclosorus parasiticus

Five previously undescribed specialized metabolites, including three 9,11-seco-pimarane diterpenoids, nodulisporenones A-C, and two androstane steroids, nodulisporisterones A and B, together with previously described ergosterol derivatives, dankasterone A and demethylincisterol A₃, were isolated from solid cultures of the endophytic fungus Nodulisporium sp. SC-J597. Their structures including absolute configurations were elucidated by extensive spectroscopic analysis and theoretical calculations of electronic circular dichroism spectra. Among them, nodulisporenones A and B are the first examples of seco-pimarane diterpenoids that is cyclized to form an unprecedented diterpenoid lactone scaffold and nodulisporisterones A and B represent the first normal C₁₉ androstane steroids of fungal origin. Nodulisporisterone B exhibited potent inhibitory effect on the production of NO in LPS-stimulated RAW264.7 macrophages (IC₅₀ = 2.95 μM). This compound, together with the two known ergosterol derivatives, also displayed cytotoxicity against A549, HeLa, HepG2 and MCF-7 cancer cell lines with IC₅₀ values of 5.2-16.9 μM.

Novel metabolites from the Cercis chinensis derived endophytic fungus Alternaria alternata ZHJG5 and their antibacterial activities

BACKGROUND

Phytopathogenic bacteria, such as Xanthomonas oryzae pv. oryzae (Xoo) and Ralstonia solanacearum (Rs), seriously threaten crop production and are the cause of enormous yield losses. Endophytic fungi are abundant sources of bioactive metabolites that may be potential candidates in the development of new agrochemicals. This work emphasizes the discovery of bioactive polyketides from endophytic Alternaria alternata ZHJG5 and reports their structural elucidation and antibacterial activities in detail.

RESULTS

Five novel polyketide derivatives, isotalaroflavone (2), (+/-)-5'-dehydroxytalaroflavone (3a/3b), (+)-talaroflavone (4b), and bialternacin G (7), along with five known compounds (1, 4a, 5, 6, and 8), were obtained from the Cercis chinensis-derived fungus A. alternata ZHJG5. The compounds' structures were characterized using spectroscopic methods and X-ray diffraction. Chiral high-performance liquid chromatography was used to separate the racemates 3 and 4, whose absolute configurations were unambiguously confirmed by comparing their experimental and calculated electron circular dichroism data. All isolated compounds were tested for antibacterial activity against the phytopathogenic bacteria Xoo, Xanthomonas oryzae pv. oryzicola (Xoc) and Rs. Compounds 1, 2 and 8 showed pronounced antibacterial activity against all tested bacteria, with minimal inhibitory concentrations ranging from 0.5 to 64 μg/ml. In addition, compound 1 showed a potent protective effect against rice bacterial leaf blight caused by Xoo with a protective efficacy of 75.1% at a concentration of 200 μg/ml.

CONCLUSION

These findings highlight the practical potential of antibacterial compounds as candidates for the discovery of novel bactericides. © 2021 Society of Chemical Industry.

Discovery of Novel 3,4-Dichloroisothiazole-Containing Coumarins as Fungicidal Leads

Natural products are one of the resources for discovering novel fungicidal leads. As a natural fungicide, osthole was used as a coumarin-based lead compound for the development of novel fungicides. Here, a series of 3,4-dichloroisothiazole-containing 7-hydroxycoumarins were rationally designed, synthesized, and characterized by introducing a bioactive substructure, 3,4-dichloroisothiazole, into the coumarin skeleton. In vitro bioassay indicated that compound 7g displayed good activity against Rhizoctonia solani, Physalospora piricola, Sclerotinia sclerotiorum, and Botrytis cinerea. Its median effective concentration (EC₅₀) value against each of these fungi fell between 0.88 and 2.50 μg/mL, which was much lower than that of osthole against the corresponding pathogen (between 7.38 and 74.59 μg/mL). In vivo screening validated that 7k exhibited 100%, 60%, and 20% efficacy against R. solani Kühn at 200, 100, and 50 μg/mL, respectively. RNA sequence analysis implied that growth inhibition of R. solani by 7k might result from potential disruptions of fungal membrane formation and intracellular metabolism. Furthermore, a field experiment with cucumber plants indicated that 7b showed 62.73% and 74.03% efficacy against Pseudoperonospora cubensis (Berk. & Curt.) Rostov. at rates of 12.5 g a.i./ha and 25 g a.i./ha, respectively, which showed no significant difference between 7b and osthole at 30 g a.i./ha. Our studies suggested that 7b, 7g, and 7k might be used as fungicidal leads for further optimization.

Eremophilane Sesquiterpenes and Benzene Derivatives from the Endophyte Microdiplodia sp. WGHS5

Three new eremophilane sesquiterpenes phomadecalins G-I (1-3) and two new benzene derivatives microdiplzenes A and B (12 and 13), together with nine known eremophilane sesquiterpenes (4-11 and 14) were isolated from an endophytic fungus, Microdiplodia sp. WGHS5. Their structures were elucidated by the interpretation of HR-ESI-MS and NMR data; meanwhile, the absolute configurations of new compounds were determined on the base of ECD calculations. All compounds were evaluated for the antimicrobial activities and antiproliferative effect on human gastric cancer cell lines (BGC-823).

Enantiomeric Dibenzo-α-Pyrone Derivatives from Alternaria alternata ZHJG5 and Their Potential as Agrochemicals

Three pairs of enantiomeric dibenzo-α-pyrone derivatives (1-3) including two pairs of new racemates (±)-alternaone A (1) and (±)-alternaone B (2) and one new enantiomer (-)-alternatiol (3), together with five known compounds (4-8) were isolated from the fungus Alternaria alternata ZHJG5. Their structures were confirmed by spectroscopic data and single-crystal X-ray diffraction analysis. All enantiomers were separated via chiral high-performance liquid chromatography, with their configurations determined by electronic circular dichroism calculation. Biogenetically, a key epoxy-rearrangement step was proposed for the formation of skeletons in 1-3; (+) 1, (-)-1, and 5 presented moderate antibacterial inhibition on phytopathogenic bacteria Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola. In the antifungal test, compounds 7 and 8 showed a moderate protective effect against Botrytis cinerea in vivo.

Antifungal meroterpenes and dioxolanone derivatives from plant-associated endophytic fungus Phyllosticta sp. WGHL2

Four new meroterpenes named as guignardones U-X (1-4), along with eleven known meroterpenes (5-15) and three known dioxolanone derivatives (16-18), were obtained from the endophytic fungus Phyllosticta sp. WGHL2. The structural elucidation was conducted by HRESIMS, NMR, single crystal X-ray diffraction, along with ECD calculations and comparison. In antifungal tests, compound 16 possessed broad-spectrum antifungal activities against Rhizoctonia solani, Fusarium graminearum and Botrytis cinerea with inhibition ratio of 48.43%, 40.98%, and 49.53% at 50 μg/mL, respectively. Moreover, compound 16 showed moderate protective effect against B. cinerea in vivo at 200 μg/mL and exhibited effective inhibition on the spore germination of B. cinerea.

Caprylic acid enhances hydroxyhexylitaconic acid production in Aspergillus niger S17-5

AIM

Aspergillus niger S17-5 produces two alkylitaconic acids, 9-hydroxyhexylitaconic acid (9-HHIA) and 10-hydroxyhexylitaconic acid (10-HHIA), which have cytotoxic and polymer building block properties. In this study, we characterized the production of 9-HHIA and 10-HHIA by addition of their expected precursor, caprylic acid, to a culture of A. niger S17-5, and demonstrated batch fermentation of 9-HHIA and 10-HHIA in a jar fermenter with DO-stat.

METHODS AND RESULTS

Production titres of 9-HHIA and 10-HHIA from 3% glucose in a flask after 25 days cultivation were 0·35 and 1·01 g l^-1 respectively. Addition of 0·22 g l^-1 of caprylic acid to a suspension of resting cells of A. niger S17-5 led to 32% enhancement of total 9-HHIA and 10-HHIA production compared to no addition. No enhancement of the production of 9-HHIA or 10-HHIA by the addition of oxaloacetic acid was observed. Addition of caprylic acid to the culture at mid-growth phase was more suitable for 9-HHIA and 10-HHIA production due to less cell growth inhibition by caprylic acid. DO-stat batch fermentation with 3% glucose and 14·4 g l^-1 of caprylic acid in a 1·5 l jar fermenter resulted in the production titres of 9-HHIA and 10-HHIA being 0·48 and 1·54 g l^-1 respectively after 10 days of cultivation.

CONCLUSIONS

Addition of caprylic acid to the culture of A. niger S17-5 enhances 9-HHIA and 10-HHIA production.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results suggest that 9-HHIA and 10-HHIA are synthesized with octanoyl-CoA derived from caprylic acid, and that the supply of octanoyl-CoA is a rate-limiting step in 9-HHIA and 10-HHIA production. To the best of our knowledge, this is the first report regarding the fermentation of naturally occurring itaconic acid derivatives in a jar fermenter.

Turpentine Derived Secondary Amines for Sustainable Crop Protection: Synthesis, Activity Evaluation and QSAR Study

In this study, we will report on the synthesis and application of efficient botanical agrochemicals from turpentine for sustainable crop protection. Two series of turpentine derived secondary amines were synthesized and identified by FT-IR, 1H NMR, 13C NMR, and HRMS. The herbicidal activities against Echinochloa crus-galli were evaluated. The potential toxicity of the synthesized compounds was tested by MTT cytotoxicity analysis. The effect of structure of the synthesized secondary amines and corresponding Schiff base compounds on their activities was investigated by quantitative structure-activity relationship (QSAR) study. All target products were found to be low toxicity, with similar or higher herbicidal activities than commercial herbicides diuron and Glyphosate. Results of QSAR study showed that a best four-descriptor QSAR model with R2 of 0.880 and Rloo2 of 0.818 was obtained. The four descriptors most relevant to the herbicidal activities are the min valency of a N atom, the max total interaction for a C-H bond, the relative number of aromatic bonds, and the min partial charge (Qmin).

Itaconic acid derivatives: structure, function, biosynthesis, and perspectives

Itaconic acid possessing a vinylidene group, which is mainly produced by fungi, is used as a biobased platform chemical and shows distinctive bioactivities. On the other hand, some fungi and lichens produce itaconic acid derivatives possessing itaconic acid skeleton, and the number of the derivatives is currently more than seventy. Based on the molecular structures, they can be categorized into two groups, alkylitaconic acids and α-methylene-γ-butyrolactones. Interestingly, some itaconic acid derivatives show versatile functions such as antimicrobial, anti-inflammatory, antitumor, and plant growth-regulating activities. The vinylidene group of itaconic acid derivatives likely participates in these functions. It is suggested that α-methylene-γ-butyrolactones are biosynthesized from alkylitaconic acids which are first biosynthesized from acyl-CoA and oxaloacetic acid. Some modifying enzymes such as hydroxylase and dehydratase are likely involved in the further modification after biosynthesis of their precursors. This contributes to the diversity of itaconic acid derivatives. In this review, we summarize their structures, functions, and biosynthetic pathways together with a discussion of a strategy for the industrial use. KEY POINTS: • Itaconic acid derivatives can be categorized into alkylitaconic acids and α-methylene-γ-butyrolactones. • The vinylidene group of itaconic acid derivatives likely participates in their versatile function. • It is suggested that α-methylene-γ-butyrolactones are biosynthesized from alkylitaconic acids which are first synthesized from acyl-CoA and oxaloacetic acid.

Nematicidal metabolites from endophytic fungus Chaetomium globosum YSC5

Management of nematodes is a very hectic job due to a highly diverse group of organisms. To find lead compounds for new nematicide development, five metabolites (1-5) were isolated from the culture broth of Chaetomium globosum YSC5 and tested for nematicidal activities against the second stage juveniles (J2s) of Meloidogyne javanica. The results revealed that chaetoglobosin A (1), chaetoglobosin B (2) and flavipin (3) exhibited strong adverse effects (91.6, 83.8 and 87.4%, respectively) on J2 mortality at 200 μg/mL with LC50 values of 88.4, 107.7 and 99.2 μg/mL after 72 h, respectively, while 3-methoxyepicoccone (4) and 4,5,6-trihydroxy-7-methylphthalide (5) showed moderate effects (78.0 and 75.5%, respectively) with LC50 values of 124.0 and 131.6 μg/mL, respectively. Furthermore, in pot assay compounds 1 and 2 appeared to be promising metabolites at 200 μg/mL that significantly reduced nematode reproduction and showed a positive influence on plant growth. Our findings could be helpful for development of new potential bio-based pesticides for integrated management of plant-parasitic nematode.

Microbial Screening Based on the Mizoroki-Heck Reaction Permits Exploration of Hydroxyhexylitaconic-Acid-Producing Fungi in Soils

Recently, we developed a unique microbial screening method based on the Mizoroki–Heck reaction for itaconic acid (IA)-producing fungi. This method revealed that 37 out of 240 fungal strains isolated from soils produce vinyl compounds, including IA. In this study, we further characterized these compounds in order to verify that the screening method permits the isolation of fungi that produce other vinyl compounds, excluding IA. HPLC analysis showed that 11 out of 37 isolated strains produced IA, similar to Aspergillus terreus S12-1. Surprisingly, the other 8 isolated strains produced two vinyl compounds with HPLC retention times different from that of IA. From these strains, the vinyl compounds of Aspergillus niger S17-5 were characterized. Mass spectrometric and NMR analyses showed that they were identical to 8-hydroxyhexylitaconic acid (8-HHIA) and 9-HHIA. This finding showed that 8-HHIA- and 9-HHIA-producing fungi, as well as IA-producing fungi, are ubiquitously found in soils. Neither 8-HHIA nor 9-HHIA showed antibacterial or anti-inflammatory activities. Interestingly, 8-HHIA and 9-HHIA showed cytotoxicity against the human cervical cancer cell line (HeLa) and human diploid cell line (MRC-5), and MRC-5 only, respectively, compared to IA at the same concentration. This study indicates that the screening method could easily discover fungi producing 8-HHIA and 9-HHIA in soils.

Endophytic fungal extracts: evaluation as photosynthesis and weed growth inhibitors

A central pillar of modern weed control is the discovery of new herbicides which are nontoxic to humans and the environment and which have low application dosage. The natural products found in plants and microorganisms are well suited in this context because they are generally nontoxic and have a wide variety of biological activities. In this work, Diaporthe phaseolorum (Dp), Penicillium simplicissimum (Ps) and Trichoderma spirale (Ts) (methanolic extracts) were evaluated as photosynthesis and plant growth inhibitors in Senna occidentalis and Ipomoea grandifolia. The most significant results were observed for Ts and Dp in S. occidentalis and I. grandifolia, respectively. Ts reduced PI_(abs), ET₀/CS₀, PHI(E₀) and PSI₀ parameters by 64, 28, 40 and 38%, respectively, indicating a reduction on electron transport efficiency. Additionally, Ts decreased shoot length by 9%, affecting the plant growth. Dp reduced PI_(abs), ET₀/CS₀ and PHI(E₀) parameters by 50, 20, 26 and 22%, respectively, revealing the inhibition competency on PSII acceptor site. Furthermore, Dp decreased by 50% the shoot length on germination assay. Thus, the phytotoxic behaviors based on endophytic fungal extracts may serve as a valuable tool in the further development of a bioherbicide since natural products represent an interesting alternative to replace commercial herbicides.

Herbicidal Polyketides and Diketopiperazine Derivatives from Penicillium viridicatum

Herbicidal activity-guided isolation from the fermentation extract of Penicillium viridicatum had obtained two herbicidal series of polyketides (1-7) and diketopiperazine derivatives (8-11), especially including three novel polyketides (1-3). The structures and absolute configurations of new polyketides 1-3 were elucidated by extensive spectroscopic analyses, as well as comparisons between measured and calculated ECD spectra. Novel polyketides 1-3 and known 4, all bearing the heptaketide skeleton with a trans-fused decalin ring of 8-CH₃ substitution, could significantly inhibit the radicle growth of Echinochloa crusgalli seedlings with a dose-dependent relationship. Especially at the concentration of 10 μg/mL, 1-4 exhibited the inhibition rates with 81.5% ± 2.0, 76.4% ± 0.8, 79.6% ± 1.1, and 80.0 ± 1.8%, respectively, even better than the commonly used synthetic herbicide of acetochlor with 76.1 ± 1.4%. Further greenhouse bioassay revealed that 4 showed pre-emergence herbicidal activity against E. crusgalli with the fresh-weight inhibition rate of 74.1% at a dosage of 400 g ai/ha, also better than acetochlor, while the other isolated metabolites (5-11) exhibited moderate herbicidal activities. The structure-activity differences of isolated polyketides indicated that the heptaketide skeleton, characterized by a trans-fused decalin ring with 8-CH₃ substitution, should be the key factor of their herbicidal activities, which could give new insights for the bioherbicide developments.

Angularly Prenylated Indole Alkaloids with Antimicrobial and Insecticidal Activities from an Endophytic Fungus Fusarium sambucinum TE-6L

Bioactivity-guided isolation of the endophytic fungus Fusarium sambucinum TE-6L residing in Nicotiana tabacum L. led to the discovery of two new angularly prenylated indole alkaloids (PIAs) with pyrano[2,3-g]indole moieties, amoenamide C (1) and sclerotiamide B (2), and four known biosynthetic congeners (3-6). Their structures were determined by comprehensive spectroscopic techniques, electronic circular dichroism (ECD), and X-ray diffraction. Compound 1 containing the bicyclo[2.2.2]diazaoctane core and indoxyl unit is rarely reported. All the compounds were evaluated for their antimicrobial and insecticidal activities. Notably, compounds 1-3 showed potent inhibitory effects against three human- and one plant-pathogenic bacterium, and seven plant-pathogenic fungi. Compounds 2-4 also exhibited remarkable larvicidal activity against first instar larvae of the cotton bollworm Helicoverpa armigera with mortality rates of 70.2%, 83.2%, and 70.5%, respectively. Further toxicity tests on zebrafish embryos were performed to evaluate the potential toxicity of PIAs. Of significance was that compound 3 in particular exhibited the highest activities but the lowest effects on the hatching of embryos among all the compounds. This study provides a basis for understanding developmental toxicity of PIAs exposure to zebrafish embryos, and also indicates the potential environmental risks of other natural compounds exposure in the aquatic ecosystem.