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PubMed2006

Detection and characterization of bacteria from the potato rhizosphere degrading N-acyl-homoserine lactone.

Quorum sensing plays a role in the regulation of soft rot diseases caused by the plant pathogenic bacterium Pectobacterium carotovorum subsp. carotovorum. The signal molecules involved in quorum sensing in P. carotovorum subsp. carotovorum belong to the group of N-acyl homoserine lactones (AHLs). In our study, we screened bacteria isolated from the potato rhizosphere for the ability to degrade AHLs produced by P. carotovorum subsp. carotovorum. Six isolates able to degrade AHLs were selected for further studies. According to 16S rDNA sequence analysis and fatty acid methyl ester profiling, the isolates belonged to the genera Ochrobactrum, Rhodococcus, Pseudomonas, Bacillus, and Delftia. For the genera Ochrobactrum and Delftia, for the first time AHL-degrading isolates were found. Data presented in this study revealed for the first time that Ochrobactrum sp. strain A44 showed the capacity to inactivate various synthetic AHL molecules; the substituted AHLs were inactivated with a lower efficiency than the unsubstituted AHLs. Compared with the other isolates, A44 was very effective in the degradation of AHLs produced by P. carotovorum subsp. carotovorum. It was verified by polymerase chain reaction, DNA-DNA hybridization, and a lactone ring reconstruction assay that Ochrobactrum sp. strain A44 did not possess AHL lactonase activity. AHL degradation in Ochrobactrum sp. strain A44 occurred intracellularly; it was not found in the culture supernatant. AHL-degrading activity of A44 was thermo sensitive. Experiments in planta revealed that Ochrobactrum sp. strain A44 significantly inhibited the maceration of potato tuber tissue. Since A44 did not produce antibiotics, the attenuation of the decay might be due to the quenching of quorum- sensing-regulated production of pectinolytic enzymes. The strain can potentially serve to control P. carotovorum subsp. carotovorum in potato.

4-Butyrolactone↗
PubMed1998

Characterization of the Erwinia chrysanthemi expI-expR locus directing the synthesis of two N-acyl-homoserine lactone signal molecules.

The plant pathogen Erwinia chrysanthemi produces three acyl-homoserine lactones (acyl-HSLs). One has been identified as N-(3-oxohexanoyl)-homoserine lactone (OHHL), and the two others were supposed to be N (hexanoyl)-homoserine lactone (HHL) and N-(decanoyl)-homoserine lactone (DHL). The genes for a quorum-sensing signal generator (expI) and a response regulator (expR) were cloned. These genes are convergently transcribed and display high similarity to the expI-expR genes of Erwinia carotovora. ExpI is responsible for both OHHL and HHL production. Inactivation of expl had little effect on pectinase synthesis in E. chrysanthemi, as expression of only two of the pectate lyase genes, pelA and pelB, was decreased. E. chrysanthemi expR mutants still produced acyl-HSL and pectinases. However, gel shift and DNAse I footprinting experiments showed that the purified E. chrysanthemi ExpR protein binds specifically to the promoter regions of the five major pel genes. Addition of OHHL modified the ExpR-DNA bandshift profiles, indicating that ExpR interacts with OHHL and binds to DNA in different ways, depending on the OHHL concentration. Localization of the ExpR binding sites just upstream of promoter regions suggests that ExpR functions as an activator of pel expression in the presence of OHHL. The absence of a phenotype in expR mutants strongly suggests that at least an additional interchangeable ExpR homologue exists in E. chrysanthemi. Finally, transcription of expI::uidA and expR::uidA fusions is dependent on the population density, suggesting the existence of a quorum-sensing hierarchy in E. chrysanthemi. These results suggest that the expI-expR locus is part of a complex autoregulatory system that controls quorum sensing in E. chrysanthemi.

4-Butyrolactone↗
PubMed2003

Acyl-homoserine lactone acylase from Ralstonia strain XJ12B represents a novel and potent class of quorum-quenching enzymes.

N-acylhomoserine lactones (AHLs) are used as signal molecules by many quorum-sensing Proteobacteria. Diverse plant and animal pathogens use AHLs to regulate infection and virulence functions. These signals are subject to biological inactivation by AHL-lactonases and AHL-acylases. Previously, little was known about the molecular details underlying the latter mechanism. An AHL signal-inactivating bacterium, identified as a Ralstonia sp., was isolated from a mixed-species biofilm. The signal inactivation encoding gene from this organism, which we call aiiD, was cloned and successfully expressed in Escherichia coli and inactivated three AHLs tested. The predicted 794-amino-acid polypeptide was most similar to the aculeacin A acylase (AAC) from Actinoplanes utahensis and also shared significant similarities with cephalosporin acylases and other N-terminal (Ntn) hydrolases. However, the most similar homologues of AiiD are deduced proteins of undemonstrated function from available Ralstonia, Deinococcus and Pseudomonas genomes. LC-MS analyses demonstrated that AiiD hydrolyses the AHL amide, releasing homoserine lactone and the corresponding fatty acid. Expression of AiiD in Pseudomonas aeruginosa PAO1 quenched quorum sensing by this bacterium, decreasing its ability to swarm, produce elastase and pyocyanin and to paralyze nematodes. Thus, AHL-acylases have fundamental implications and hold biotechnological promise in quenching quorum sensing.

4-Butyrolactone↗
PubMed2001

Inhibition of biofilm formation and swarming of Escherichia coli by (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)-furanone.

The quorum-sensing disrupter (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)-furanone (furanone) of the alga Delisea pulchra was found to inhibit the swarming motility of Escherichia coli completely at 13 microg cm-2 (also at 20 microg ml-1) but did not inhibit its growth rate at 13-52 microg cm-2 or from 20 to 100 microg ml-1. Swimming was not inhibited by the furanone at 20-40 microg ml-1. In addition, confocal scanning laser microscopy revealed that this furanone at 60 microg ml-1 inhibited the biofilm formation of E. coli, as it decreased its thickness by 55%, reduced the number of water channels and decreased the percentage of live cells by 87%. This suggests that natural furanone may be used as a new method to control bacterial biofilms that does not involve toxicity. Furanone at 10 microg ml-1 also inhibited by 3300-fold the quorum sensing of Vibrio harveyi via autoinducer 1 (AI-1) and inhibited by 5500-fold that of V. harveyi via of autoinducer 2 (AI-2) as well as inhibited by 26-600-fold the quorum sensing of E. coli via AI-2; hence, this furanone is a non-specific intercellular signal antagonist.

Biofilms↗
PubMed2004

Biofilm mode of growth of Streptococcus intermedius favored by a competence-stimulating signaling peptide.

Gram-positive and gram-negative bacteria use quorum sensing to coordinate population behavior. In several streptococci, quorum sensing mediated by competence-stimulating peptides (CSP) is associated with development of competence for transformation. We show here that a synthetic CSP favored the biofilm mode of growth of Streptococcus intermedius without affecting the rate of culture growth.

Adaptation, Physiological↗
PubMed2001

The global posttranscriptional regulator RsmA modulates production of virulence determinants and N-acylhomoserine lactones in Pseudomonas aeruginosa.

Posttranscriptional control is known to contribute to the regulation of secondary metabolism and virulence determinants in certain gram-negative bacteria. Here we report the isolation of a Pseudomonas aeruginosa gene which encodes a global translational regulatory protein, RsmA (regulator of secondary metabolites). Overexpression of rsmA resulted in a substantial reduction in the levels of extracellular products, including protease, elastase, and staphylolytic (LasA protease) activity as well as the PA-IL lectin, hydrogen cyanide (HCN), and the phenazine pigment pyocyanin. While inactivation of rsmA in P. aeruginosa had only minor effects on the extracellular enzymes and the PA-IL lectin, the production of HCN and pyocyanin was enhanced during the exponential phase. The influence of RsmA on N-acylhomoserine lactone-mediated quorum sensing was determined by assaying the levels of N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL) and N-butanoylhomoserine lactone (C4-HSL) produced by the rsmA mutant and the rsmA-overexpressing strain. RsmA exerted a negative effect on the synthesis of both 3-oxo-C12-HSL and C4-HSL, which was confirmed by using lasI and rhlI translational fusions. These data also highlighted the temporal expression control of the lasI gene, which was induced much earlier and to a higher level during the exponential growth phase in an rsmA mutant. To investigate whether RsmA modulates HCN production solely via quorum-sensing control, hcn translational fusions were employed to monitor the regulation of the cyanide biosynthesis genes (hcnABC). RsmA was shown to exert an additional negative effect on cyanogenesis posttranscriptionally by acting on a region surrounding the hcnA ribosome-binding site. This suggests that, in P. aeruginosa, RsmA functions as a pleiotropic posttranscriptional regulator of secondary metabolites directly and also indirectly by modulating the quorum-sensing circuitry.

4-Butyrolactone↗
PubMed2002

Characterization of the Sinorhizobium meliloti sinR/sinI locus and the production of novel N-acyl homoserine lactones.

Sinorhizobium meliloti is a soil bacterium which can establish a nitrogen-fixing symbiosis with the legume Medicago sativa. Recent work has identified a pair of genes, sinR and sinI, which represent a potential quorum-sensing system and are responsible for the production of N-acyl homoserine lactones (AHLs) in two S. meliloti strains, Rm1021 and Rm41. In this work, we characterize the sinRI locus and show that these genes are responsible for the synthesis of several long-chain AHLs ranging from 12 to 18 carbons in length. Four of these, 3-oxotetradecanoyl HL, 3-oxohexadecenoyl HL, hexadecenoyl HL, and octadecanoyl HL, have novel structures. This is the first report of AHLs having acyl chains longer than 14 carbons. We show that a disruption in sinI eliminates these AHLs and that a sinR disruption results in only basal levels of the AHLs. Moreover, the same sinI and sinR mutations also lead to a decrease in the number of pink nodules during nodulation assays, as well as a slight delay in the appearance of pink nodules, indicating a role for quorum sensing in symbiosis. We also show that sinI and sinR mutants are still capable of producing several short-chain AHLs, one of which was identified as octanoyl HL. We believe that these short-chain AHLs are evidence of a second quorum-sensing system in Rm1021, which we refer to here as the mel system, for "S. meliloti."

4-Butyrolactone↗
PubMed2002

MetR and CRP bind to the Vibrio harveyi lux promoters and regulate luminescence.

The induction of luminescence in Vibrio harveyi at the later stages of growth is controlled by a quorum-sensing mechanism in addition to nutritional signals. However, the mechanism of transmission of these signals directly to the lux promoters is unknown and only one regulatory protein, LuxR, has been shown to bind directly to lux promoter DNA. In this report, we have cloned and sequenced two genes, crp and metR, coding for the nutritional regulators, CRP (cAMP receptor protein) and MetR (a LysR homologue), involved in catabolite repression and methionine biosynthesis respectively. The metR gene was cloned based on a general strategy to detect lux DNA-binding proteins expressed from a genomic library, whereas the crp gene was cloned based on its complementation of an Escherichia coli crp mutant. Both CRP and MetR were shown to bind to lux promoter DNA, with CRP being dependent on the presence of cAMP. Expression studies indicated that the two regulators had opposite effects on luminescence: CRP was an activator and MetR a repressor. Disruption of crp decreased luminescence by about 1,000-fold showing that CRP is a major activator of luminescence the same as LuxR, whereas disruption of MetR resulted in activation of luminescence over 10-fold, confirming its function as a repressor. Comparison of the levels of the autoinducers involved in quorum sensing excreted by V. harveyi, and the crp and metR mutants, showed that autoinducer production was not significantly different, thus indicating that the nutritional signals do not affect luminescence by changing the levels of the signals required for quorum sensing. Indeed, the large effects of these nutritional sensors show that luminescence is controlled by multiple signals related to the environment and the cell density which must be integrated at the molecular level to control expression at the lux promoters.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed2003

Utilization of acyl-homoserine lactone quorum signals for growth by a soil pseudomonad and Pseudomonas aeruginosa PAO1.

Acyl-homoserine lactones (AHLs) are employed by several Proteobacteria as quorum-sensing signals. Past studies have established that these compounds are subject to biochemical decay and can be used as growth nutrients. Here we describe the isolation of a soil bacterium, Pseudomonas strain PAI-A, that degrades 3-oxododecanoyl-homoserine lactone (3OC12HSL) and other long-acyl, but not short-acyl, AHLs as sole energy sources for growth. The small-subunit rRNA gene from strain PAI-A was 98.4% identical to that of Pseudomonas aeruginosa, but the soil isolate did not produce obvious pigments or AHLs or grow under denitrifying conditions or at 42 degrees C. The quorum-sensing bacterium P. aeruginosa, which produces both 3OC12HSL and C4HSL, was examined for the ability to utilize AHLs for growth. It did so with a specificity similar to that of strain PAI-A, i.e., degrading long-acyl but not short-acyl AHLs. In contrast to the growth observed with strain PAI-A, P. aeruginosa strain PAO1 growth on AHLs commenced only after extremely long lag phases. Liquid-chromatography-atmospheric pressure chemical ionization-mass spectrometry analyses indicate that strain PAO1 degrades long-acyl AHLs via an AHL acylase and a homoserine-generating HSL lactonase. A P. aeruginosa gene, pvdQ (PA2385), has previously been identified as being a homologue of the AHL acylase described as occurring in a Ralstonia species. Escherichia coli expressing pvdQ catalyzed the rapid inactivation of long-acyl AHLs and the release of HSL. P. aeruginosa engineered to constitutively express pvdQ did not accumulate its 3OC12HSL quorum signal when grown in rich media. However, pvdQ knockout mutants of P. aeruginosa were still able to grow by utilizing 3OC12HSL. To our knowledge, this is the first report of the degradation of AHLs by pseudomonads or other gamma-Proteobacteria, of AHL acylase activity in a quorum-sensing bacterium, of HSL lactonase activity in any bacterium, and of AHL degradation with specificity only towards AHLs with long side chains.

4-Butyrolactone↗
PubMed2002

Differential regulation of twitching motility and elastase production by Vfr in Pseudomonas aeruginosa.

Vfr, a homolog of Escherichia coli cyclic AMP (cAMP) receptor protein, has been shown to regulate quorum sensing, exotoxin A production, and regA transcription in Pseudomonas aeruginosa. We identified a twitching motility-defective mutant that carries a transposon insertion in vfr and confirmed that vfr is required for twitching motility by construction of an independent allelic deletion-replacement mutant of vfr that exhibited the same phenotype, as well as by the restoration of normal twitching motility by complementation of these mutants with wild-type vfr. Vfr-null mutants exhibited severely reduced twitching motility with barely detectable levels of type IV pili, as well as loss of elastase production and altered pyocyanin production. We also identified reduced-twitching variants of quorum-sensing mutants (PAK lasI::Tc) with a spontaneous deletion in vfr (S. A. Beatson, C. B. Whitchurch, A. B. T. Semmler, and J. S. Mattick, J. Bacteriol., 184:3598-3604, 2002), the net result of which was the loss of five residues (EQERS) from the putative cAMP-binding pocket of Vfr. This allele (VfrDeltaEQERS) was capable of restoring elastase and pyocyanin production to wild-type levels in vfr-null mutants but not their defects in twitching motility. Furthermore, structural analysis of Vfr and VfrDeltaEQERS in relation to E. coli CRP suggests that Vfr is capable of binding both cAMP and cyclic GMP whereas VfrDeltaEQERS is only capable of responding to cAMP. We suggest that Vfr controls twitching motility and quorum sensing via independent pathways in response to these different signals, bound by the same cyclic nucleotide monophosphate-binding pocket.

Bacterial Proteins↗
PubMed2025

Analysis of gene expression within individual cells reveals spatiotemporal patterns underlying Vibrio cholerae biofilm development.

Bacteria commonly exist in multicellular, surface-attached communities called biofilms. Biofilms are central to ecology, medicine, and industry. The Vibrio cholerae pathogen forms biofilms from single founder cells that, via cell division, mature into three-dimensional structures with distinct, yet reproducible, regional architectures. To define mechanisms underlying biofilm developmental transitions, we establish a single-molecule fluorescence in situ hybridization (smFISH) approach that enables accurate quantitation of spatiotemporal gene-expression patterns in biofilms at cell-scale resolution. smFISH analyses of V. cholerae biofilm regulatory and structural genes demonstrate that, as biofilms mature, overall matrix gene expression decreases, and simultaneously, a pattern emerges in which matrix gene expression becomes largely confined to peripheral biofilm cells. Both quorum sensing and c-di-GMP-signaling are required to generate the proper temporal pattern of matrix gene expression. Quorum sensing signaling is uniform across the biofilm, and thus, c-di-GMP-signaling alone sets the regional matrix gene expression pattern. The smFISH strategy provides insight into mechanisms conferring particular fates to individual biofilm cells.

Biofilms↗
PubMed2004

Specificity and enzyme kinetics of the quorum-quenching N-Acyl homoserine lactone lactonase (AHL-lactonase).

N-Acyl homoserine lactone (AHL) quorum-sensing signals are the vital elements of bacterial quorum-sensing systems, which regulate diverse biological functions, including virulence. The AHL-lactonase, a quorumquenching enzyme encoded by aiiA from Bacillus sp., inactivates AHLs by hydrolyzing the lactone bond to produce corresponding N-acyl homoserines. To characterize the enzyme, the recombinant AHL-lactonase and its four variants were purified. Kinetic and substrate specificity analysis showed that AHL-lactonase had no or little residue activity to non-acyl lactones and noncyclic esters, but displayed strong enzyme activity toward all tested AHLs, varying in length and nature of the substitution at the C3 position of the acyl chain. The data also indicate that the amide group and the ketone at the C1 position of the acyl chain of AHLs could be important structural features in enzyme-substrate interaction. Surprisingly, although carrying a (104)HX- HXDH(109) short sequence identical to the zinc-binding motif of several groups of metallohydrolytic enzymes, AHL-lactonase does not contain or require zinc or other metal ions for enzyme activity. Except for the amino acid residue His-104, which was shown previously to not be required for catalysis, kinetic study and conformational analysis using circular dichroism spectrometry showed that substitution of the other key residues in the motif (His-106, Asp-108, and His-109), as well as His-169 with serine, respectively, caused conformational changes and significant loss of enzyme activity. We conclude that AHL-lactonase is a highly specific enzyme and that the (106)HXDH(109) approximately H(169) of AHL-lactonase represents a novel catalytic motif, which does not rely on zinc or other metal ions for activity.

Bacillus↗
PubMed2006

Occurrence of proteolytic activity and N-acyl-homoserine lactone signals in the spoilage of aerobically chill-stored proteinaceous raw foods.

Proteolytic pseudomonads dominate the spoilage flora of aerobically chill-stored proteinaceous raw foods. Proteolysis during spoilage of these food systems affects both food quality and the dynamics of the bacterial community because it increases the availability of nutrients to the community as a whole. Quorum sensing, or cell-cell signaling, is associated closely with ecological interactions among bacteria in mixed communities. The potential role of quorum sensing in proteolytic food spoilage was examined, based on the evaluation of N-acyl-homoserine lactone (AHL) signal molecules. The occurrence of proteolytic activity and AHL signals was studied during spoilage of aerobically chill-stored ground beef, fish, chicken, and raw milk. Pseudomonads dominated the psychrotrophic flora, followed distantly by members of the Enterobacteriaceae. The growth of pseudomonads was correlated with the occurrence of proteolytic activity in all food systems. AHL concentration began increasing significantly only after the onset of proteolytic activity. Widely divergent AHL profiles were revealed by thin-layer chromatography analysis of the different food samples, and these profiles were likely determined by the undefined bacterial flora in these systems and by the characterized pseudomonads and Enterobacteriaceae. Although Hafnia alvei was a major component of the Enterobacteriaceae flora in all foods tested and a strong AHL producer, the signal molecules produced by H. alvei strain EB1 did not influence protease production by Pseudomonas fluorescens strain 395 in vitro. These results do not indicate any clear correlation between the overall detectable AHL signal molecules accumulated in the food samples and proteolytic activity.

4-Butyrolactone↗
PubMed2006

Staphylococcal cutaneous infections: invasion, evasion and aggression.

Staphylococcal infections cause a variety of cutaneous and systemic infections, including impetigo, furuncle, subcutaneous abscess, staphylococcal scalded skin syndrome (SSSS), toxic shock syndrome (TSS) and neonatal toxic shock syndrome-like exanthematous disease (NTED), in association with microbial virulence factors. The virulence factors produced by Staphylococcus aureus have a wide array of biological properties, including disruption of the epithelial barrier, inhibition of opsonization by antibody and complement, interference with neutrophil chemotaxis, cytolysis of neutrophils, and inactivation of antimicrobial peptides. Exfoliative toxins (ETs) induce the 'acantholytic' infection of S. aureus due to the disruption of cell-to-cell cohesion, which allows the pathogenic organisms to spread within the epithelium. Furthermore, S. aureus expresses exotoxins with biological properties of superantigens that induce T-cell activation with subsequent anergy and immunosuppression. Of the S. aureus leukotoxins, Panton-Valentine leukocidin (PVL) is involved in the development of multiple furuncles with more intense erythema, particularly in healthy young adults. TSS is an acute life-threatening illness caused by TSS toxin-1 (TSST-1) and is usually classified into two categories; menstrual TSS, originally described in association with tampon use, and nonmenstrual TSS with a variety of clinical settings. NTED is a neonatal disease induced by TSST-1 although clinical symptoms are much milder than those of TSS. In TSS and NTED, the expansion of TSST-1-reactive Vbeta2-positive T cells is observed. The production of pathogenic S. aureus exotoxins and biofilm formation is regulated by the accessory gene regulator (agr) locus in the quorum-sensing signaling pathway. There is no doubt that targeting the quorum-sensing signaling pathway or anti-toxin therapy is a promising therapeutic approach supportive of primary antibiotic therapy.

Biofilms↗
PubMed2004

Azithromycin inhibits MUC5AC production induced by the Pseudomonas aeruginosa autoinducer N-(3-Oxododecanoyl) homoserine lactone in NCI-H292 Cells.

The features of chronic airway diseases, including chronic bronchitis, cystic fibrosis, bronchiectasis, and diffuse panbronchiolitis, include chronic bacterial infection and airway obstruction by mucus. Pseudomonas aeruginosa is one of the most common pathogens in chronic lung infection, and quorum-sensing systems contribute to the pathogenesis of this disease. The quorum-sensing signal molecule [N-(3-oxododecanoyl) homoserine lactone (3O-C(12)-HSL)] not only regulates bacterial virulence but also is associated with the immune response. In this study, we investigated whether 3O-C(12)-HSL could stimulate the production of a major mucin core protein, MUC5AC. The effect of a macrolide on MUC5AC production was also studied. 3O-C(12)-HSL induced NCI-H292 cells to express MUC5AC at both the mRNA and the protein levels in time- and dose-dependent manners. A 15-membered macrolide, azithromycin, inhibited MUC5AC production that was activated by 3O-C(12)-HSL. 3O-C(12)-HSL induced extracellular signal-regulated kinase (ERK) 1/2 and I-kappa B phosphorylation in cells, and this induction was suppressed by azithromycin. 3O-C(12)-HSL-induced MUC5AC production was blocked by the ERK pathway inhibitor PD98059. Our findings suggest that the P. aeruginosa autoinducer 3O-C(12)-HSL contributes to excessive mucin production in chronic bacterial infection. Azithromycin seems to reduce this mucin production by interfering with intracellular signal transduction.

4-Butyrolactone↗
PubMed2003

Quorum quenching and proactive host defense.

Both plants and humans have inducible defense mechanisms. This passive defense strategy leaves the host unprotected for a period of time until resistance is activated. Moreover, many bacterial pathogens have evolved cell-cell communication (quorum-sensing) mechanisms to mount population-density-dependent attacks to overwhelm the host's defense responses. Several chemicals and enzymes have been investigated for years for their potential to target the key components of bacterial quorum-sensing systems. These quorum-quenching reagents, which block bacterial cell-cell communications, can disintegrate a bacterial population-density-dependent attack. It has now been shown that a quorum-quenching mechanism can be engineered in plants and might be used as a strategy in controlling bacterial pathogens and to build up a proactive defense barrier.

Bacteria↗
PubMed2026

Synthetic community derived from the root core microbes of a desert shrub Caragana korshinskii enhances wheat drought tolerance.

BACKGROUND: Drought, intensified by climate change, poses a mounting threat to global food security by severely constraining crop productivity. While microbial inoculants offer promise for drought tolerance, their poor adaptability remains insufficient for extremely water-deficient environments. Desert plants host unique drought-adapted microbiomes that remain largely unexplored for agricultural applications. RESULTS: Here, we investigated the microbial community of the desert shrub Caragana korshinskii and identified a core set of drought-responsive strains. A synthetic microbial community (SynCom) derived from these strains significantly improved wheat growth under drought stress. Metagenomic analyses revealed that microbial functions related to biofilm formation, quorum sensing, and carbon metabolism were enriched, with Pseudomonas identified as a key functional taxon. Guided by inter-strain interactions in biofilm assembly, we streamlined the consortium into a five-member synthetic community, where quorum-sensing signals promoted community-wide biofilm formation. Community biofilm production improved strain colonization and conferred greater drought tolerance compared to monocultures. In plants, mechanistic investigations indicated that the simplified SynCom inoculation universally upregulated MAPK and jasmonic acid signaling pathways. Furthermore, carbohydrate metabolic pathways such as starch and sucrose metabolism were specifically activated, suggesting a multi-level mechanism underlying SynCom-mediated drought tolerance. CONCLUSIONS: These findings demonstrate that SynCom constructed on the endophytic flora of desert plants can significantly enhance crop drought tolerance. Our work highlights the pivotal role of community biofilm synthesis in facilitating root colonization and activating a multidimensional drought tolerance network in plants. This study not only gives an ecological perspective on desert microbiome adaptations but also offers a strategic framework for developing effective microbial inoculants for arid-region agriculture. Video Abstract.

Caragana↗
PubMed2004

The Aeromonas hydrophila LuxR homologue AhyR regulates the N-acyl homoserine lactone synthase, AhyI positively and negatively in a growth phase-dependent manner.

Aeromonas hydrophila is a pathogen of fish, amphibians and humans which produces N-acylhomoserine lactone quorum sensing signal molecules and possesses homologues of the Vibrio fischeri luxI and luxR quorum sensing genes termed ahyI and ahyR, respectively. The ahyI and ahyR genes of A. hydrophila comprise a divergon with a 62 bp intergenic region and control biofilm maturation and extracellular protease production. Stationary phase culture supernatants from an ahyR but not an ahyI mutant contain N-butanoylhomoserine lactone (C4-HSL) which is shown to be required for maximal ahyI expression. To determine whether AhyR regulates ahyI, the expression of AhyI was followed throughout growth by Western blot analysis. This revealed that AhyI can be detected in the exponential phase but appears to be degraded in stationary phase in the parent A. hydrophila strain. In an ahyR mutant however, the AhyI protein is only produced in stationary phase but production is sustained suggesting that AhyR controls the timing of AhyI production and turnover. By using RT-PCR, we mapped the transcriptional start site of ahyI which revealed that the 12 bp symmetrical lux-box like sequence present in the 62 bp ahyRI intergenic region overlaps with the -10 region of the ahyI promoter. To determine whether AhyR could bind to the ahyRI intergenic region, the ahyR gene was expressed and purified as a maltose binding protein (MalE) fusion. Electrophoretic mobility shift assays demonstrated that MalE-AhyR specifically bound to this sequence in both the presence and absence of N-butanoylhomoserine lactone (C4-HSL). Taken together, these data suggest that AhyR acts as both a negative and a positive regulator of ahyI and hence C4-HSL production in a growth phase dependent manner.

4-Butyrolactone↗