The isolation, structural determination, and total synthesis of terfestatin A, a novel auxin signaling inhibitor from Streptomyces sp

Exploring the specialized metabolome of the plant pathogen Streptomyces sp. 11-1-2

Streptomyces bacteria are notable for producing chemically diverse specialized metabolites that exhibit various bioactivities and mediate interactions with different organisms. Streptomyces sp. 11-1-2 is a plant pathogen that produces nigericin and geldanamycin, both of which display toxic effects against various plants. Here, the 'One Strain Many Compounds' approach was used to characterize the metabolic potential of Streptomyces sp. 11-1-2. Organic extracts were prepared from 11-1-2 cultures grown on six different agar media, and the extracts were tested in antimicrobial and plant bioassays and were subjected to untargeted metabolomics and molecular networking. Most extracts displayed strong bioactivity against Gram-positive bacteria and yeast, and they exhibited phytotoxic activity against potato tuber tissue and radish seedlings. Several known specialized metabolites, including musacin D, galbonolide B, guanidylfungin A, meridamycins and elaiophylin, were predicted to be present in the extracts along with closely related compounds with unknown structure and bioactivity. Targeted detection confirmed the presence of elaiophylin in the extracts, and bioassays using pure elaiophylin revealed that it enhances the phytotoxic effects of geldanamycin and nigericin on potato tuber tissue. Overall, this study reveals novel insights into the specialized metabolites that may mediate interactions between Streptomyces sp. 11-1-2 and other bacteria and eukaryotic organisms.

p-Terphenyl glucosides from the culture broth of Phlebiopsis castanea

A new p-terphenyl derivative, hydroxystrepantibin D (1), was isolated along with two known p-terphenyls (2 and 3) from the culture broth of Phlebiopsis castanea. These compounds were isolated using silica gel column chromatography, reversed-phase medium-pressure liquid chromatography, Sephadex LH-20 column chromatography, and preparative HPLC. Their structures were determined based on spectroscopic methods. These compounds exhibited free radical scavenging activities with IC₅₀ values in the range from 22.2 to 158.4 μM against 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical and in the range from 161.1 to 356.1 μM against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical.

Structural diversity and biological activity of natural p-terphenyls

p-Terphenyls are aromatic compounds consisting of a central benzene ring substituted with two phenyl groups, and they are mainly isolated from terrestrial and marine organisms. The central ring of p-Terphenyls is usually modified into more oxidized forms, e.g., para quinone and phenols. In some cases, additional ring systems were observed on the terphenyl-type core structure or between two benzene moieties. p-Terphenyls have been reported to have cytotoxic, antimicrobial, antioxidant and α-glucosidase inhibitory effects. In this review, we will mainly summarize the structural diversity and biological activity of naturally occurring p-Terphenyls referring to the research works published before.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00117-8.

Antibacterial p-Terphenyl with a Rare 2,2'-Bithiazole Substructure and Related Compounds Isolated from the Marine-Derived Actinomycete Nocardiopsis sp. HDN154086

Assisted by MS/MS-based molecular networking and X-ray diffraction analysis, five new p-terphenyl derivatives, namely, nocarterphenyls D-H (1-5), were obtained and characterized from the cultures of the marine sediment-derived actinomycete Nocardiopsis sp. HDN154086. The skeleton of nocarterphenyl D (1) was defined to possess a rare 2,2'-bithiazole scaffold, naturally occurring for the first time, and nocarterphenyls E-H (2-5) are p-terphenylquinones with unusual thioether linked fatty acid methyl ester substitutions. Compound 1 showed promising activity against multiple bacteria with MIC values ranging from 1.5 to 6.2 μM, and 2 exhibited notable antibacterial activity against MRSA which surpassed the positive control ciprofloxacin.

Polyhydroxy p-Terphenyls from a Mangrove Endophytic Fungus Aspergillus candidus LDJ-5

Six undescribed polyhydroxy p-terphenyls, namely asperterphenyllins A–F, were isolated from an endophytic fungus Aspergillus candidus LDJ-5. Their structures were determined by NMR and MS data. Differing from the previously reported p-terphenyls, asperterphenyllin A represents the first p-terphenyl dimer connected by a C-C bond. Asperterphenyllin A displayed anti-influenza virus A (H1N1) activity and protein tyrosine phosphatase 1B (PTP1B) inhibitory activity with IC₅₀ values of 53 μM and 21 μM, respectively. The anti-influenza virus A (H1N1) activity and protein tyrosine phosphatase 1B (PTP1B) inhibitory activity of p-terphenyls are reported for the first time. Asperterphenyllin G exhibited cytotoxicity against nine cell lines with IC₅₀ values ranging from 0.4 to 1.7 μM. Asperterphenyllin C showed antimicrobial activity against Proteus species with a MIC value of 19 μg/mL.

p-Terphenyls and actinomycins from a Streptomyces sp. associated with the larva of mud dauber wasp

In the course of searching for cytotoxic metabolites from insects associated actinomyces, two new natural p-terphenyl glycosides, strepantibin D (1) and strepantibin E (2), along with terferol (3), actinomycin D (4), actinomycin V (5) and actinomycin V_0β (6), were identified from the fermentation medium of a Streptomyces sp. which was obtained from the larva body of mud dauber wasp. Strepantibin D (1), previously reported as a synthetic derivative of terfestatin A, is firstly isolated as a natural p-terphenyl in this research. Strepantibin D (1) and terferol (3) showed medium cytotoxic activity against breast cancer cells MCF-7, MDA-MB-231 and BT-474. Actinomycins (4-6), especially actinomycin V (5), displayed remarkable cytotoxicity against breast cancer cells, with IC₅₀ values ranging from 0.83 nM to 369.90 nM.

Hawaiienols A-D, Highly Oxygenated p-Terphenyls from an Insect-Associated Fungus, Paraconiothyrium hawaiiense

Four new highly oxygenated p-terphenyls, hawaiienols A-D (1-4), have been isolated from cultures of Paraconiothyrium hawaiiense, a fungus associated with the Septobasidium-infected insect Diaspidiotus sp.; their structures were elucidated primarily by NMR experiments. The absolute configurations of 1 and 2-4 were assigned by single-crystal X-ray diffraction analysis using Cu Kα radiation and via electronic circular dichroism calculations, respectively. Compound 1 incorporated the first naturally occurring 4,7-dioxatricyclo[3.2.1.0^3,6]octane unit in its p-terphenyl skeleton and showed cytotoxicity toward six human tumor cell lines.

Secondary Metabolites from the Root Rot Biocontrol Fungus Phlebiopsis gigantea

Three cyclopentanoids (phlebiopsin A⁻C), one glycosylated p-terphenyl (methyl-terfestatin A), and o-orsellinaldehyde were isolated from the biocontrol fungus Phlebiopsis gigantea, and their structures were elucidated by 1D and 2D NMR spectroscopic analysis, as well as by LC-HRMS. The biological activity of the compounds against the root rot fungus Heterobasidion occidentale, as well as against Fusarium oxysporum and Penicillium canescens, was also investigated, but only o-orsellinaldehyde was found to have any antifungal activity in the concentration range tested.

Total Synthesis of Kehokorins A-E, Cytotoxic p-Terphenyls

This paper describes a general method for the synthesis of kehokorins A-E, novel cytotoxic p-terphenyls. 2,4,6-Trihydroxybenzaldehyde served as a common building block for preparation of the central aromatic ring. Construction of their p-terphenyl skeletons was achieved by a stepwise Suzuki-Miyaura coupling, whereas the phenyldibenzofuran moiety was built up by an intramolecular Ullmann reaction. Introduction of an l-rhamnose residue into partly protected kehokorin B was performed by the trichloroacetimidate method.

Terfestatins B and C, New p-Terphenyl Glycosides Produced by Streptomyces sp. RM-5-8

Terfestatins B (1) and C (2), new p-terphenyls bearing a novel unsaturated hexuronic acid (4-deoxy-α-L-threo-hex-4-enopyranuronate), a unique β-D-glycosyl ester of 5-isoprenylindole-3-carboxylate (3) and the same rare sugar, and two new hygromycin precursors, were characterized as metabolites of the coal mine fire isolate Streptomyces sp. RM-5-8. EtOH damage neuroprotection assays using rat hippocampal-derived primary cell cultures with 1, 2, 3 and echoside B (a terfestatin C-3'-β-D-glucuronide from Streptomyces sp. RM-5-8) revealed 1 as potently neuroprotective, highlighting a new potential application of the terfestatin scaffold.

Unraveling plant hormone signaling through the use of small molecules

Plants have acquired the capacity to grow continuously and adjust their morphology in response to endogenous and external signals, leading to a high architectural plasticity. The dynamic and differential distribution of phytohormones is an essential factor in these developmental changes. Phytohormone perception is a fast but complex process modulating specific developmental reprogramming. In recent years, chemical genomics or the use of small molecules to modulate target protein function has emerged as a powerful strategy to study complex biological processes in plants such as hormone signaling. Small molecules can be applied in a conditional, dose-dependent and reversible manner, with the advantage of circumventing the limitations of lethality and functional redundancy inherent to traditional mutant screens. High-throughput screening of diverse chemical libraries has led to the identification of bioactive molecules able to induce plant hormone-related phenotypes. Characterization of the cognate targets and pathways of those molecules has allowed the identification of novel regulatory components, providing new insights into the molecular mechanisms of plant hormone signaling. An extensive structure-activity relationship (SAR) analysis of the natural phytohormones, their designed synthetic analogs and newly identified bioactive molecules has led to the determination of the structural requirements essential for their bioactivity. In this review, we will summarize the so far identified small molecules and their structural variants targeting specific phytohormone signaling pathways. We will highlight how the SAR analyses have enabled better interrogation of the molecular mechanisms of phytohormone responses. Finally, we will discuss how labeled/tagged hormone analogs can be exploited, as compelling tools to better understand hormone signaling and transport mechanisms.

Strategies and future trends to identify the mode of action of phytotoxic compounds

Small molecules affecting plant processes have been widely used as probes to study basic physiology. In agricultural practices some of these molecules have served as herbicides or plant growth regulators. Historically, most of the compounds were identified in large screens by the agrochemical industry, but also as phytoactive natural products. More recently, novel phytoactive compounds originated from academic research by chemical screens performed to induce specific phenotypes of interest. In the present review different approaches were evaluated for the identification of the mode of action (MoA) of phytoactive compounds. Based on the methodologies used for MoA identification, three approaches are differentiated: a phenotyping approach, an approach based on a genetic screen and a biochemical screening approach. Target sites of compounds targeting primary or secondary metabolism were identified most successfully with a phenotyping approach. Target sites for compounds that influence cell structure, such as cell wall biosynthesis or the cytoskeleton, or compounds that interact with the hormone system, were in most cases discovered by using a genetic approach. Examples showing the strengths and weaknesses of the different approaches are discussed in detail. Additionally, new techniques that could contribute to future MoA identification projects are reviewed. In particular, next-generation sequencing techniques may be used for the fast-forward mapping of mutants identified in genetic screens. Finally, a revised three-tiered approach for the MoA identification of phytoactive compounds is proposed. The approach consists of a 1st tier, which addresses compound stability, uniformity of effects in different species, general cytotoxicity and the effect on common processes such as transcription and translation. Advanced studies based on these findings initiate the 2nd tier MoA characterization, either with further phenotypic characterization, starting a genetic screen or establishing a biochemical screen. At the 3rd tier, enzyme assays or protein affinity studies should show the activity of the compound on the hypothesized target and should associate the in vitro effects with the in vivo profile of the compound.

Selective formation of glycosidic linkages of N-unsubstituted 4-hydroxyquinolin-2-(1H)-ones

A comparative study for selective glucosylation of N-unsubstituted 4-hydroxyquinolin-2(1H)-ones into 4-(tetra-O-acetyl-beta-D-glucopyranosyloxy)quinolin-2(1H)-ones is reported. Four glycosyl donors including tetra-O-acetyl-alpha-D-glucopyranosyl bromide, beta-D-glucose pentaacetate, glucose tetraacetate and tetra-O-acetyl-alpha-D-glucopyranosyl trichloroacetimidate were tested, along with different promoters and reaction conditions. The best results were obtained with tetra-O-acetyl-alpha-D-glucopyranosyl bromide with Cs(2)CO(3) in CH(3)CN. In some cases the 4-O-glucosylation of the quinolinone ring was accompanied by 2-O-glucosylation yielding the corresponding 2,4-bis(tetra-O-acetyl-beta-D-glucopyranosyloxy)quinoline. Next, 4-(tetra-O-acetyl-beta-D-glucopyranosyloxy)quinolin-2(1H)-ones were deacetylated into 4-(beta-D-glucopyranosyloxy)quinolin-2(1H)-ones with Et(3)N in MeOH. In some instances the deacetylation was accompanied by the sugar-aglycone bond cleavage. Structure elucidation, complete assignment of proton and carbon resonances as well as assignment of anomeric configuration for all the products under investigation were performed by 1D and 2D NMR spectroscopy.

Synergy effect of sodium acetate and glycosidically bound volatiles on the release of volatile compounds from the unscented cut flower (Delphinium elatum L. "Blue Bird")

Many modern floricultural varieties have lost their scent during traditional breeding programs. The factors that result in the nonscent emission of some cut flowers remain unclear. The objective of this study was to investigate whether the nonscent emission is due to one of the factors, the lack of suitable substrates (or precursors of scent compounds). Using solid-phase microextraction and dynamic headspace volatile sampling techniques, the supplement of nonvolatile compounds such as 2-coumaric acid glucoside to the unscented cut flower such as Delphinium elatum L. "Blue Bird" enhanced the emission of scent from the flower, which was sufficient for detection by the human olfaction. Interestingly, compared with feeding with each compound, the combination of sodium acetate and 2-coumaric acid glucoside showed the synergy effect on enhancement of coumarin, a cherry leaf-like scent emission from the flower, which is due to one of factors that sodium acetate enhanced the activity of beta-glucosidase being involved in the formation of the scent compound. These results suggest that some enzymes responsible for the formation of floral scents indeed occur in the unscented flowers such as Delphinium elatum L. "Blue Bird", and the non- or low-scent emission of the flowers are due to the lack of suitable substrates.

Bioactive p-terphenyl derivatives from a Cordyceps-colonizing isolate of Gliocladium sp

Gliocladinins A (1) and B (2), two new p-terphenyl derivatives, have been isolated from solid cultures of an isolate of Gliocladium sp. that colonizes Cordyceps sinensis. The structures of these compounds were determined mainly by analysis of their NMR spectroscopic data. In standard disk assays, compounds 1 and 2 showed modest antimicrobial activity against Staphylococcus aureus (ATCC 6538). In addition, these compounds displayed inhibitory effects on the growth of two human tumor cell lines, HeLa and HCT116.

Characterization of terfestatin A, a new specific inhibitor for auxin signaling

Terfestatin A (TrfA), terphenyl-beta-glucoside, was isolated from Streptomyces sp. F40 in a forward screen for compounds that inhibit the expression of auxin-inducible genes in Arabidopsis (Arabidopsis thaliana). TrfA specifically and competitively inhibited the expression of primary auxin-inducible genes in Arabidopsis roots, but did not affect the expression of genes regulated by other plant hormones such as abscisic acid and cytokinin. TrfA also blocked the auxin-enhanced degradation of auxin/indole-3-acetic acid (Aux/IAA) repressor proteins without affecting the auxin-stimulated interaction between Aux/IAAs and the F-box protein TIR1. TrfA treatment antagonized auxin responses in roots, including primary root inhibition, lateral root initiation, root hair promotion, and root gravitropism, but had only limited effects on shoot auxin responses. Taken together, these results indicate that TrfA acts as a modulator of Aux/IAA stability and thus provides a new tool for dissecting auxin signaling.