Ralfuranone Thioether Production by the Plant PathogenRalstonia solanacearum

Rh-Catalyzed Carbonylative Cyclization of Propargylic Alcohols with Aryl Boronic Acids

2(3H)-Furanones are tremendously important not only because of their wide occurrence in bioactive compounds but also due to their versatility in organic synthesis. Here, a straightforward approach to 2(3H)-furanones from readily available tertiary propargylic alcohols with arylboronic acids in the presence of CO using rhodium as a catalyst has been established. The method exhibits a broad substrate scope tolerating useful functional groups with a moderate to high stereoselectivity.

The phc Quorum-Sensing System in Ralstonia solanacearum Species Complex

Ralstonia solanacearum species complex (RSSC) strains are devastating plant pathogens distributed worldwide. The primary cell density-dependent gene expression system in RSSC strains is phc quorum sensing (QS). It regulates the expression of about 30% of all genes, including those related to cellular activity, primary and secondary metabolism, pathogenicity, and more. The phc regulatory elements encoded by the phcBSRQ operon and phcA gene play vital roles. RSSC strains use methyl 3-hydroxymyristate (3-OH MAME) or methyl 3-hydroxypalmitate (3-OH PAME) as the QS signal. Each type of RSSC strain has specificity in generating and receiving its QS signal, but their signaling pathways might not differ significantly. In this review, I describe the genetic and biochemical factors involved in QS signal input and the regulatory network and summarize control of the phc QS system, new cell-cell communications, and QS-dependent interactions with soil fungi.

The behavior of Ralstonia pseudosolanacearum strain OE1-1 and morphological changes of cells in tomato roots

The soil-borne Gram-negative β-proteobacterium Ralstonia solanacearum species complex (RSSC) infects tomato roots through the wounds where secondary roots emerge, infecting xylem vessels. Because it is difficult to observe the behavior of RSSC by a fluorescence-based microscopic approach at high magnification, we have little information on its behavior at the root apexes in tomato roots. To analyze the infection route of a strain of phylotype I of RSSC, R. pseudosolanacearum strain OE1-1, which invades tomato roots through the root apexes, we first developed an in vitro pathosystem using 4 day-old-tomato seedlings without secondary roots co-incubated with the strain OE1-1. The microscopic observation of toluidine blue-stained longitudinal semi-thin resin sections of tomato roots allowed to detect attachment of the strain OE1-1 to surfaces of the meristematic and elongation zones in tomato roots. We then observed colonization of OE1-1 in intercellular spaces between epidermis and cortex in the elongation zone, and a detached epidermis in the elongation zone. Furthermore, we observed cortical and endodermal cells without a nucleus and with the cell membrane pulling away from the cell wall. The strain OE1-1 next invaded cell wall-degenerated cortical cells and formed mushroom-shaped biofilms to progress through intercellular spaces of the cortex and endodermis, infecting pericycle cells and xylem vessels. The deletion of egl encoding β-1,4-endoglucanase, which is one of quorum sensing (QS)-inducible plant cell wall-degrading enzymes (PCDWEs) secreted via the type II secretion system (T2SS) led to a reduced infectivity in cortical cells. Furthermore, the QS-deficient and T2SS-deficient mutants lost their infectivity in cortical cells and the following infection in xylem vessels. Taking together, infection of OE1-1, which attaches to surfaces of the meristematic and elongation zones, in cortical cells of the elongation zone in tomato roots, dependently on QS-inducible PCDWEs secreted via the T2SS, leads to its subsequent infection in xylem vessels.

PhcQ mainly contributes to the regulation of quorum sensing-dependent genes, in which PhcR is partially involved, in Ralstonia pseudosolanacearum strain OE1-1

The gram-negative plant-pathogenic β-proteobacterium Ralstonia pseudosolanacearum strain OE1-1 produces methyl 3-hydroxymyristate as a quorum sensing (QS) signal via the methyltransferase PhcB and senses the chemical through the sensor histidine kinase PhcS. This leads to functionalization of the LysR family transcriptional regulator PhcA, regulating QS-dependent genes responsible for the QS-dependent phenotypes including virulence. The phc operon consists of phcB, phcS, phcR, and phcQ, with the latter two encoding regulator proteins with a receiver domain and a histidine kinase domain and with a receiver domain, respectively. To elucidate the function of PhcR and PhcQ in the regulation of QS-dependent genes, we generated phcR-deletion and phcQ-deletion mutants. Though the QS-dependent phenotypes of the phcR-deletion mutant were largely unchanged, deletion of phcQ led to a significant change in the QS-dependent phenotypes. Transcriptome analysis coupled with quantitative reverse transcription-PCR and RNA-sequencing revealed that phcB, phcK, and phcA in the phcR-deletion and phcQ-deletion mutants were expressed at similar levels as in strain OE1-1. Compared with strain OE1-1, expression of 22.9% and 26.4% of positively and negatively QS-dependent genes, respectively, was significantly altered in the phcR-deletion mutant. However, expression of 96.8% and 66.9% of positively and negatively QS-dependent genes, respectively, was significantly altered in the phcQ-deletion mutant. Furthermore, a strong positive correlation of expression of these QS-dependent genes was observed between the phcQ-deletion and phcA-deletion mutants. Our results indicate that PhcQ mainly contributes to the regulation of QS-dependent genes, in which PhcR is partially involved.

Cryptic Sulfur Incorporation in Thioangucycline Biosynthesis

Sulfur incorporation into natural products is a critical area of biosynthetic studies. Recently, a subset of sulfur-containing angucyclines has been discovered, and yet, the sulfur incorporation step is poorly understood. In this work, a series of thioether-bridged angucyclines were discovered, and a cryptic epoxide Michael acceptor intermediate was revealed en route to thioangucyclines (TACs) A and B. However, systematic gene deletion of the biosynthetic gene cluster (BGC) by CRISPR/Cas9 could not identify any gene responsible for the conversion of the epoxide intermediate to TACs. Instead, a series of in vitro and in vivo experiments conclusively showed that the conversion is the result of two non-enzymatic steps, possibly mediated by endogenous hydrogen sulfide. Therefore, the TACs are proposed to derive from a detoxification process. These results are expected to contribute to the study of both angucyclines and the utilization of inorganic sulfur in natural product biosynthesis.

Major exopolysaccharide, EPS I, is associated with the feedback loop in the quorum sensing of Ralstonia solanacearum strain OE1-1

The Gram-negative soil-borne bacterium Ralstonia solanacearum first infects roots of host plants and then invades xylem vessels. In xylem vessels, the bacteria grow vigorously and produce exopolysaccharides (EPSs) to cause a wilt symptom on host plants. The EPSs are thus the main virulence factors of R. solanacearum. The strain OE1-1 of R. solanacearum produces methyl 3-hydroxymyristate as a quorum-sensing (QS) signal, and senses this QS signal, activating QS. The QS-activated LysR-type transcriptional regulator PhcA induces the production of virulence-related metabolites including ralfuranone and the major EPS, EPS I. To elucidate the function of EPS I, the transcriptomes of R. solanacearum strains were analysed using RNA sequencing technology. The expression of 97.2% of the positively QS-regulated genes was down-regulated in the epsB-deleted mutant ΔepsB, which lost its EPS I productivity. Furthermore, expression of 98.0% of the negatively QS-regulated genes was up-regulated in ΔepsB. The deficiency to produce EPS I led to a significantly suppressed ralfuranone productivity and significantly enhanced swimming motility, which are suppressed by QS, but did not affect the expression levels of phcA and phcB, which encode a methyltransferase required for methyl 3-hydroxymyristate production. Overall, QS-dependently produced EPS I may be associated with the feedback loop of QS.

Activation of Ralfuranone/Ralstonin Production by Plant Sugars Functions in the Virulence of Ralstonia solanacearum

Plant pathogenic bacteria possess sophisticated mechanisms to detect the presence of host plants by sensing host-derived compounds. Ralstonia solanacearum, the causative agent of bacterial wilt on solanaceous plants, employs quorum sensing to control the production of the secondary metabolite ralfuranones/ralstonins, which have been suggested to be involved in virulence. Here, we report that d-galactose and d-glucose, plant sugars, activate the production of ralfuranones/ralstonins in R. solanacearum. As a result, two new derivatives, ralfuranone M (1) and ralstonin C (2), were found in the culture extracts, and their structures were elucidated by spectroscopic and chemical methods. Ralstonin C (2) is a cyclic lipopeptide containing a unique fatty acid, (2S,3S,Z)-3-amino-2-hydroxyicos-13-enoic acid, whereas ralfuranone M (1) has a common aryl-furanone structure with other ralfuranones. d-Galactose and d-glucose activated the expression of the biosynthetic ralfuranone/ralstonin genes and in part became the biosynthetic source of ralfuranones/ralstonins. Ralfuranones and ralstonins were detected from the xylem fluid of the infected tomato plants, and their production-deficient mutants exhibited reduced virulence on tomato and tobacco plants. Taken together, these results suggest that activation of ralfuranone/ralstonin production by host sugars functions in R. solanacearum virulence.

Contribution of a lectin, LecM, to the quorum sensing signalling pathway of Ralstonia solanacearum strain OE1-1

The soil-borne bacterium Ralstonia solanacearum invades the roots and colonizes the intercellular spaces and then the xylem. The expression of lecM, encoding a lectin LecM, is induced by an OmpR family response regulator HrpG in R. solanacearum strain OE1-1. LecM contributes to the attachment of strain OE1-1 to the host cells of intercellular spaces. OE1-1 produces methyl 3-hydroxymyristate (3-OH MAME) through a methyltransferase (PhcB) and extracellularly secretes the chemical as a quorum sensing (QS) signal, which activates QS. The expression of lecM is also induced by the PhcA virulence regulator functioning through QS, and the resulting LecM is implicated in the QS-dependent production of major exopolysaccharide EPS I and the aggregation of OE1-1 cells. To investigate the function of LecM in QS, we analysed the transcriptome of R. solanacearum strains generated by RNA sequencing technology. In the lecM mutant, the expression of positively QS-regulated genes and negatively QS-regulated genes was down-regulated (by >90%) and up-regulated (by ~60%), respectively. However, phcB and phcA in the lecM mutant were expressed at levels similar to those in strain OE1-1. The lecM mutant produced significantly less ralfuranone and exhibited a significantly greater swimming motility, which were positively and negatively regulated by QS, respectively. In addition, the extracellular 3-OH MAME content of the lecM mutant was significantly lower than that of OE1-1. The application of 3-OH MAME more strongly increased EPS I production in the phcB-deleted mutant and strain OE1-1 than in the lecM mutant. Thus, the QS-dependent production of LecM contributes to the QS signalling pathway.

Design, Synthesis, and Herbicidal Activity Evaluation of Novel Aryl-Naphthyl Methanone Derivatives

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is one of the most vital targets for herbicides discovery. In search for HPPD inhibitors with novel scaffolds, a series of aryl-naphthyl methanone derivatives have been designed and synthesized through alkylation and Friedel-Crafts acylation reactions. The bioassay indicated some of these compounds displayed preferable herbicidal activity at the rate of 0.75 mmol/m² by post-emergence application, in which compound 3h displayed the best herbicidal activity. The molecular docking showed that compound 3h could bind well to the active site of the AtHPPD. This study shows that aryl-naphthyl methanone derivatives could be a potential lead structure for further development of novel herbicides.

A genomics perspective on natural product biosynthesis in plant pathogenic bacteria

This review summarizes findings from genomics-inspired natural product research in plant pathogenic bacteria and discusses emerging trends in this field.

Ralfuranones contribute to mushroom-type biofilm formation by Ralstonia solanacearum strain OE1-1

After invasion into intercellular spaces of tomato plants, the soil-borne, plant-pathogenic Ralstonia solanacearum strain OE1-1 forms mushroom-shaped biofilms (mushroom-type biofilms, mBFs) on tomato cells, leading to its virulence. The strain OE1-1 produces aryl-furanone secondary metabolites, ralfuranones (A, B, J, K and L), dependent on the quorum sensing (QS) system, with methyl 3-hydroxymyristate (3-OH MAME) synthesized by PhcB as a QS signal. Ralfuranones are associated with the feedback loop of the QS system. A ralfuranone productivity-deficient mutant (ΔralA) exhibited significantly reduced growth in intercellular spaces compared with strain OE1-1, losing its virulence. To analyse the function of ralfuranones in mBF formation by OE1-1 cells, we observed cell aggregates of R. solanacearum strains statically incubated in tomato apoplast fluids on filters under a scanning electron microscope. The ΔralA strain formed significantly fewer microcolonies and mBFs than strain OE1-1. Supplementation of ralfuranones A, B, J and K, but not L, significantly enhanced the development of mBF formation by ΔralA. Furthermore, a phcB- and ralA-deleted mutant (ΔphcB/ralA) exhibited less formation of mBFs than OE1-1, although a QS-deficient, phcB-deleted mutant formed mBFs similar to OE1-1. Supplementation with 3-OH MAME significantly reduced the formation of mBFs by ΔphcB/ralA. The application of each ralfuranone significantly increased the formation of mBFs by ΔphcB/ralA supplied with 3-OH MAME. Together, our findings indicate that ralfuranones are implicated not only in the development of mBFs by strain OE1-1, but also in the suppression of QS-mediated negative regulation of mBF formation.

Bacterial quorum sensing in symbiotic and pathogenic relationships with hosts

Gram-negative bacteria communicate with each other by producing and sensing diffusible signaling molecules. This mechanism is called quorum sensing (QS) and regulates many bacterial activities from gene expression to symbiotic/pathogenic interactions with hosts. Therefore, the elucidation and control of bacterial QS systems have been attracted increasing attention over the past two decades. The most common QS signals in Gram-negative bacteria are N-acyl homoserine lactones (AHLs). There are also bacteria that employ different QS systems, for example, the plant pathogen Ralstonia solanacearum utilizes 3-hydroxy fatty acid methyl esters as its QS signals. The QS system found in the endosymbiotic bacterium associated with the fungus Mortierella alpina, the development of an affinity pull-down method for AHL synthases, and the elucidation of a unique QS circuit in R. solanacearum are discussed herein.

Involvement of ralfuranones in the quorum sensing signalling pathway and virulence of Ralstonia solanacearum strain OE1-1

The soil-borne, plant-pathogenic Ralstonia solanacearum strain OE1-1 produces and secretes methyl 3-hydroxymyristate (3-OH MAME) as a quorum sensing (QS) signal, which contributes to its virulence. A global virulence regulator, PhcA, functioning through the QS system, positively regulates the expression of ralA, which encodes furanone synthase, to produce aryl-furanone secondary metabolites, ralfuranones. A ralfuranone-deficient mutant (ΔralA) is weakly virulent when directly inoculated into tomato xylem vessels. To investigate the functions of ralfuranones, we analysed R. solanacearum transcriptome data generated by RNA sequencing technology. ΔralA expressed phcB, which is associated with 3-OH MAME production, and phcA at levels similar to those in strain OE1-1. In addition, ΔralA exhibited down-regulated expression of more than 90% of the QS positively regulated genes, and up-regulated expression of more than 75% of the QS negatively regulated genes. These results suggest that ralfuranones affect the QS feedback loop. Ralfuranone supplementation restored the ability of ΔralA cells to aggregate. In addition, ralfuranones A and B restored the swimming motility of ΔralA to wild-type levels. However, the application of exogenous ralfuranones did not affect the production of the major exopolysaccharide, EPS I, in ΔralA. Quantitative real-time polymerase chain reaction assays revealed that the deletion of ralA results in the down-regulated expression of vsrAD and vsrBC, which encode a sensor kinase and a response regulator, respectively, in the two-component regulatory systems that influence EPS I production. The application of ralfuranone B restored the expression of these two genes. Overall, our findings indicate that integrated signalling via ralfuranones influences the QS and virulence of R. solanacearum.

Stereoselective synthesis of E-3-(arylmethylidene)-5-(alkyl/aryl)-2(3H)-furanones by sequential hydroacyloxylation-Mizoroki–Heck reactions of iodoalkynes

Stereoselective hydroacyloxylation of iodoalkynes with β-aryl cinnamic acids and subsequent Mizoroki–Heck reaction provides an efficient route to substituted 2(3H)-furanones.

Structure of Ralsolamycin, the Interkingdom Morphogen from the Crop Plant Pathogen Ralstonia solanacearum

Ralsolamycin, an inducer of chlamydospore formation in fungi, was recently reported from the plant pathogenic bacterium Ralstonia solanacearum. Although interpretation of tandem mass data and bioinformatics enabled a preliminary chemical characterization, the full structure of ralsolamycin was not resolved. We now report the recovery of this secondary metabolite from an engineered R. solanacearum strain. The structure of ralsolamycin was elucidated by extensive spectroscopic analyses. Chemical derivatization as well as bioinformatics were used to assign the absolute stereochemistry. Our results identified an erroneous genome sequence, thereby emphasizing the value of chemical methods to complement bioinformatics-based procedures in natural product research.

Antibiotics from Gram-negative bacteria: a comprehensive overview and selected biosynthetic highlights

Covering: up to 2017The overwhelming majority of antibiotics in clinical use originate from Gram-positive Actinobacteria. In recent years, however, Gram-negative bacteria have become increasingly recognised as a rich yet underexplored source of novel antimicrobials, with the potential to combat the looming health threat posed by antibiotic resistance. In this article, we have compiled a comprehensive list of natural products with antimicrobial activity from Gram-negative bacteria, including information on their biosynthetic origin(s) and molecular target(s), where known. We also provide a detailed discussion of several unusual pathways for antibiotic biosynthesis in Gram-negative bacteria, serving to highlight the exceptional biocatalytic repertoire of this group of microorganisms.

Regulation Involved in Colonization of Intercellular Spaces of Host Plants in Ralstonia solanacearum

A soil-borne bacterium Ralstonia solanacearum invading plant roots first colonizes the intercellular spaces of the root, and eventually enters xylem vessels, where it replicates at high levels leading to wilting symptoms. After invasion into intercellular spaces, R. solanacearum strain OE1-1 attaches to host cells and expression of the hrp genes encoding components of the type III secretion system (T3SS). OE1-1 then constructs T3SS and secrets effectors into host cells, inducing expression of the host gene encoding phosphatidic acid phosphatase. This leads to suppressing plant innate immunity. Then, OE1-1 grows on host cells, inducing quorum sensing (QS). The QS contributes to regulation of OE1-1 colonization of intercellular spaces including mushroom-type biofilm formation on host cells, leading to its virulence. R. solanacearum strains AW1 and K60 produce methyl 3-hydroxypalmitate (3-OH PAME) as a QS signal. The methyltransferase PhcB synthesizes 3-OH PAME. When 3-OH PAME reaches a threshold level, it increases the ability of the histidine kinase PhcS to phosphorylate the response regulator PhcR. This results in elevated levels of functional PhcA, the global virulence regulator. On the other hand, strains OE1-1 and GMI1000 produce methyl 3-hydroxymyristate (3-OH MAME) as a QS signal. Among R. solanacearum strains, the deduced PhcB and PhcS amino acid sequences are related to the production of QS signals. R. solanacearum produces aryl-furanone secondary metabolites, ralfuranones, which are extracellularly secreted and required for its virulence, dependent on the QS. Interestingly, ralfuranones affect the QS feedback loop. Taken together, integrated signaling via ralfuranones influences the QS, contributing to pathogen virulence.

Enzymatic Carbon-Sulfur Bond Formation in Natural Product Biosynthesis

Sulfur plays a critical role for the development and maintenance of life on earth, which is reflected by the wealth of primary metabolites, macromolecules, and cofactors bearing this element. Whereas a large body of knowledge has existed for sulfur trafficking in primary metabolism, the secondary metabolism involving sulfur has long been neglected. Yet, diverse sulfur functionalities have a major impact on the biological activities of natural products. Recent research at the genetic, biochemical, and chemical levels has unearthed a broad range of enzymes, sulfur shuttles, and chemical mechanisms for generating carbon-sulfur bonds. This Review will give the first systematic overview on enzymes catalyzing the formation of organosulfur natural products.

Studies on the biosynthesis of ralfuranones in Ralstonia solanacearum

Ralfuranones, aryl-furanone secondary metabolites, are involved in the virulence of Ralstonia solanacearum in solanaceous plants. Ralfuranone I (6) has been suggested as a biosynthetic precursor for other ralfuranones; however, this conversion has not been confirmed. We herein investigate the biosynthesis of ralfuranones using feeding experiments with ralfuranone I (6) and its putative metabolite, ralfuranone B (2). The results obtained demonstrated that the biosynthesis of ralfuranones proceeded in enzymatic and non-enzymatic manners.

Synthesis and Herbicidal Activity of Triketone-Quinoline Hybrids as Novel 4-Hydroxyphenylpyruvate Dioxygenase Inhibitors

4-Hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27, HPPD) is one of the most important targets for herbicide discovery. In the search for new HPPD inhibitors with novel scaffolds, triketone-quinoline hybrids were designed and subsequently optimized on the basis of the structure-activity relationship (SAR) studies. Most of the synthesized compounds displayed potent inhibition of Arabidopsis thaliana HPPD (AtHPPD), and some of them exhibited broad-spectrum and promising herbicidal activity at the rate of 150 g ai/ha by postemergence application. Most promisingly, compound III-l, 3-hydroxy-2-(2-methoxy-7-(methylthio)quinoline-3-carbonyl)cyclohex-2-enone (Ki = 0.009 μM, AtHPPD), had broader spectrum of weed control than mesotrione. Furthermore, compound III-l was much safer to maize at the rate of 150 g ai/ha than mesotrione, demonstrating its great potential as herbicide for weed control in maize fields. Therefore, triketone-quinoline hybrids may serve as new lead structures for novel herbicide discovery.

Ralfuranone Is Produced by an Alternative Aryl-Substituted γ-Lactone Biosynthetic Route in Ralstonia solanacearum

The aryl-substituted γ-lactones ralfuranones A and B were isolated after feeding L-[1-(13)C]-phenylalanine to a liquid culture of the plant pathogenic bacterium Ralstonia solanacearum. (13)C NMR analysis demonstrated specific enrichment of the label at position 2 of the γ-lactone. This labeling pattern is consistent with a biosynthetic mechanism that includes direct cyclization of two monomeric phenylpyruvate precursors into an α,β-substituted lactone, but incompatible with a terphenylquinone intermediate. As the latter was shown as an intermediate in allantofuranone biosynthesis, we conclude that aryl-substituted γ-lactones can be assembled via divergent biosynthetic routes.