Structure elucidation and biosynthetic locus of trinickiabactin from the plant pathogenic bacterium Trinickia caryophylli

Functional biosynthetic stereodivergence in a gene cluster via a dihydrosydnone N-oxide

Chirality plays a critical role in the biochemistry of life and often only one enantiomeric series is observed (homochirality). Only a few natural products have been obtained as racemates, e.g. the signalling molecule valdiazen produced by Burkholderia cenocepacia H111. In this study, we investigated the ham biosynthetic gene cluster and discovered that both the enantiomerically pure (R)-fragin and the racemic valdiazen result from the same pathway. This stereodivergence is based on the unusual heterocyclic intermediate dihydrosydnone N-oxide, as evident from gene knockout, stable isotope feeding experiments, and mass spectrometry experiments. Both non-enzymatic racemisation via keto-enol tautomerisation and enzyme-mediated dynamic kinetic resolution were found to be crucial to this stereodivergent pathway. This novel mechanism underpins the production of configurationally and biologically distinct metabolites from a single gene cluster. Our findings highlight the intricate design of an intertwined biosynthetic pathway and provide a deeper understanding of microbial secondary metabolism related to microbial communication.

Discovery, isolation, and characterization of diazeniumdiolate siderophores

The C-diazeniumdiolate (N-nitrosohydroxylamine) group in the amino acid graminine (Gra) is a newly discovered Fe(III) ligand in microbial siderophores. Graminine was first identified in the siderophore gramibactin, and since this discovery, other Gra-containing siderophores have been identified, including megapolibactins, plantaribactin, gladiobactin, trinickiabactin (gramibactin B), and tistrellabactins. The C-diazeniumdiolate is photoreactive in UV light which provides a convenient characterization tool for this type of siderophore. This report details the process of genomics-driven identification of bacteria producing Gra-containing siderophores based on selected biosynthetic enzymes, as well as bacterial culturing, isolation and characterization of the C-diazeniumdiolate siderophores containing Gra.

Polyyne-producing Burkholderia suppress Globisporangium ultimum damping-off disease of Pisum sativum (pea)

Extensive crop losses are caused by oomycete and fungal damping-off diseases. Agriculture relies heavily on chemical pesticides to control disease, but due to safety concerns multiple agents have been withdrawn. Burkholderia were successfully used as commercial biopesticides because of their fungicidal activity and plant protective traits. However, their potential for opportunistic pathogenicity led to a moratorium on their registration as biopesticides. Subsequently, Burkholderia were shown to produce multiple specialised metabolites including potent antimicrobial polyynes. Cepacin A, a polyyne produced by Burkholderia ambifaria biopesticide strains was shown to be an important metabolite for the protection of germinating peas against Globisporangium ultimum (formerly Pythium) damping-off disease. Recently, there has been an expansion in bacterial polyyne discovery, with the metabolites and their biosynthetic gene pathways found in several bacterial genera including Burkholderia, Collimonas, Trinickia, and Pseudomonas. To define the efficacy of these bacterial polyyne producers as biopesticidal agents, we systematically evaluated metabolite production, in vitro microbial antagonism, and G. ultimum biocontrol across a panel of 30 strains representing four bacterial genera. In vitro polyyne production and antimicrobial activity was demonstrated for most strains, but only Burkholderia polyyne producers were protective within the in vivo G. ultimum damping-off pea protection model. B. ambifaria was the most effective cepacin-expressing biopesticide, and despite their known potential for plant pathogenicity Burkholderia gladioli and Burkholderia plantarii were uniquely shown to be protective as caryoynencin-producing biopesticides. In summary, Burkholderia are effective biopesticides due to their suite of antimicrobials, but the ability to deploy polyyne metabolites, caryoynencin and cepacin, is strain and species dependent. Graphical Abstract.

Fermentation in Minimal Media and Fungal Elicitation Enhance Violacein and Deoxyviolacein Production in Two Janthinobacterium Strains

Violacein and its biosynthesis by-product deoxyviolacein are valuable natural pigments with different biological activities. Various efforts have been made to enhance violacein and deoxyviolacein production in microbes. However, the effect of different culture media, agitation, and fungal elicitation on biosynthesis in Janthinobacterium has not been evaluated. In this study, the effect of eight different culture media, agitation, and fungal elicitation by Agaricus bisporus on violacein and deoxviolacein production in Janthinobacterium agaricidamnosum DSM 9628 and Janthinobacterium lividum DSM 1552 were examined. The results showed that violacein and deoxviolacein are produced at high-levels when Janthinobacterium is cultivated in minimal media such as Davis minimal broth with glycerol (DMBgly), shipworm basal medium (SBM), and MM9 media. A 50-fold increase was observed in violacein production when Janthinobacterium was cultivated in these media compared to cultivation in Luria–Bertani (LB), nutrient broth (NB), and King’s B (KB). Agitation reduces violacein and deoxyviolacein production, while fungal elicitation decreases violacein but increases deoxyviolacein when Janthinobacterium is cultured in KB media, SBM, and modified SBM (MSBM). An antibacterial assay using various pathogenic bacteria showed that violacein and deoxyviolacein extracted from Janthinobacterium are effective against both Gram-positive and Gram-negative pathogens, confirming their functionality as antibacterial agents. The findings suggest that cultivation in minimal media and fungal elicitation might invoke a stress response, enhancing the production of violacein and deoxviolacein in Janthinobacterium.

Reduced chemodiversity suppresses rhizosphere microbiome functioning in the mono-cropped agroecosystems

BACKGROUND

Rhizodeposits regulate rhizosphere interactions, processes, nutrient and energy flow, and plant-microbe communication and thus play a vital role in maintaining soil and plant health. However, it remains unclear whether and how alteration in belowground carbon allocation and chemodiversity of rhizodeposits influences microbiome functioning in the rhizosphere ecosystems. To address this research gap, we investigated the relationship of rhizosphere carbon allocation and chemodiversity with microbiome biodiversity and functioning during peanut (Arachis hypogaea) continuous mono-cropping. After continuously labeling plants with ¹³CO₂, we studied the chemodiversity and composition of rhizodeposits, along with the composition and diversity of active rhizosphere microbiome using metabolomic, amplicon, and shotgun metagenomic sequencing approaches based on DNA stable-isotope probing (DNA-SIP).

RESULTS

Our results indicated that enrichment and depletion of rhizodeposits and active microbial taxa varied across plant growth stages and cropping durations. Specifically, a gradual decrease in the rhizosphere carbon allocation, chemodiversity, biodiversity and abundance of plant-beneficial taxa (such as Gemmatimonas, Streptomyces, Ramlibacter, and Lysobacter), and functional gene pathways (such as quorum sensing and biosynthesis of antibiotics) was observed with years of mono-cropping. We detected significant and strong correlations between rhizodeposits and rhizosphere microbiome biodiversity and functioning, though these were regulated by different ecological processes. For instance, rhizodeposits and active bacterial communities were mainly governed by deterministic and stochastic processes, respectively. Overall, the reduction in carbon deposition and chemodiversity during peanut continuous mono-cropping tended to suppress microbial biodiversity and its functions in the rhizosphere ecosystem.

CONCLUSIONS

Our results, for the first time, provide the evidence underlying the mechanism of rhizosphere microbiome malfunctioning in mono-cropped systems. Our study opens new avenues to deeply disentangle the complex plant-microbe interactions from the perspective of rhizodeposits chemodiversity and composition and will serve to guide future microbiome research for improving the functioning and services of soil ecosystems. Video abstract.

Mitigation of Pseudomonas syringae virulence by signal inactivation

Pseudomonas syringae is an important plant pathogen of many valuable crops worldwide, with more than 60 identified pathovars. The phytotoxins produced by these organisms were related to the severity of the damage caused to the plant. An emerging strategy to treat bacterial infections relies on interference with their signaling systems. In this study, we investigated P. syringae pv. syringae, which produces the virulence factor mangotoxin that causes bacterial apical necrosis on mango leaves. A previously unknown signaling molecule named leudiazen was identified, determined to be unstable and volatile, and responsible for mangotoxin production. A strategy using potassium permanganate, compatible with organic farming, was developed to degrade leudiazen and thus to attenuate the pathogenicity of P. syringae pv. syringae.

Burkholderia in the genomic era: from taxonomy to the discovery of new antimicrobial secondary metabolites

Species of Burkholderia are highly versatile being found not only abundantly in soil, but also as plants and animals' commensals or pathogens. Their complex multireplicon genomes harbour an impressive number of polyketide synthase (PKS) and nonribosomal peptide-synthetase (NRPS) genes coding for the production of antimicrobial secondary metabolites (SMs), which have been successfully deciphered by genome-guided tools. Moreover, genome metrics supported the split of this genus into Burkholderia sensu stricto (s.s.) and five new other genera. Here, we show that the successful antimicrobial SMs producers belong to Burkholderia s.s. Additionally, we reviewed the occurrence, bioactivities, modes of action, structural, and biosynthetic information of thirty-eight Burkholderia antimicrobial SMs shedding light on their diversity, complexity, and uniqueness as well as the importance of genome-guided strategies to facilitate their discovery. Several Burkholderia NRPS and PKS display unusual features, which are reflected in their structural diversity, important bioactivities, and varied modes of action. Up to now, it is possible to observe a general tendency of Burkholderia SMs being more active against fungi. Although the modes of action and biosynthetic gene clusters of many SMs remain unknown, we highlight the potential of Burkholderia SMs as alternatives to fight against new diseases and antibiotic resistance.

Iron Coordination Properties of Gramibactin as Model for the New Class of Diazeniumdiolate Based Siderophores

Gramibactin (GBT) is an archetype for the new class of diazeniumdiolate siderophores, produced by Paraburkholderia graminis, a cereal-associated rhizosphere bacterium, for which a detailed solution thermodynamic study exploring the iron coordination properties is reported. The acid-base behavior of gramibactin as well as its complexing ability toward Fe3+ was studied over a wide range of pH values (2≤pH≤11). For the latter the ligand-competition method employing EDTA was used. Only two species are formed: [Fe(GBT)]- (pH 2 to 9) and [Fe(GBT)(OH)2 ]3- (pH≥9). The formation of [Fe(GBT)]- and its occurrence in real systems was confirmed by LC-HRESIMS analysis of the bacteria culture broth extract. The sequestering ability of gramibactin was also evaluated in terms of the parameters pFe and pL0.5 . Gramibactin exhibits a higher sequestering ability toward Fe3+ than EDTA and of the same order of magnitude as hydroxamate-type microbial siderophores, but smaller than most of the catecholate-type siderophores and much higher than the most known phytosiderophores.

Draft Genome Sequence of Ochrobactrum sp. Strain MC-1LL, a Bacterial Strain with Antimicrobial Properties, Isolated from Marine Sediments in Nigeria

Here, we report a 4.3-Mb draft genome sequence of a potential new Ochrobactrum species, which clarified its taxonomic position and gave insight into the complete secondary metabolite production capacity of the strain.

Draft Genome Sequences of Six Type Strains of the Genus Massilia

High-quality draft genome sequences were determined for 6 Massilia sp. type strains. The genomes of these strains show considerable biosynthetic potential for producing secondary metabolites.

High-quality draft genome sequences were determined for 6 Massilia sp. type strains. The genomes of these strains show considerable biosynthetic potential for producing secondary metabolites.