Genome-driven investigation of compatible solute biosynthesis pathways of Pseudomonas syringae pv. syringae and their contribution to water stress tolerance

AlgU mediates hyperosmotic tolerance in Pseudomonas protegens SN15-2 by regulating membrane stability, ROS scavenging, and osmolyte synthesis

Pseudomonas protegens can serve as an agricultural biocontrol agent. P. protegens often encounters hyperosmotic stress during industrial production and field application. The ability of P. protegens to withstand hyperosmotic stress is important for its application as a biocontrol agent. AlgU is a global regulator responsible for stress response and biocontrol ability. However, the specific regulatory role of AlgU in the hyperosmotic adaptation of P. protegens is poorly understood. In this study, we found that the AlgU mutation disrupted the hyperosmotic tolerance of P. protegens. Many genes and metabolites related to cell envelope formation were significantly downregulated in ΔalgU compared with that in the wild-type (WT) strain under hyperosmotic conditions, and we found that the algU mutation caused membrane integrity to be compromised and increased membrane permeability. Further experiments revealed that the cell envelope integrity protein TolA, which is regulated by AlgU, contributes to cell membrane stability and osmotic tolerance in P. protegens. In addition, several genes related to oxidative stress response were significantly downregulated in ΔalgU, and higher levels of intracellular reactive oxygen species were found in ΔalgU. Furthermore, we found that the synthesis of N-acetyl glutaminyl glutamine amide is directly regulated by AlgU and contributes to the hyperosmotic adaptation of P. protegens. This study revealed the mechanisms of AlgU's participation in osmotic tolerance in P. protegens, and it provides potential molecular targets for research on the hyperosmotic adaptation of P. protegens.IMPORTANCEIn this study, we found that the extracytoplasmic function sigma factor AlgU is essential for the survival of P. protegens under hyperosmotic conditions. We provided evidence supporting the roles of AlgU in influencing cell membrane stability, intracellular reactive oxygen species (ROS) accumulation, and dipeptide N-acetylglutaminylglutamine amide (NAGGN) synthesis in P. protegens under hyperosmotic conditions. Our findings revealed the mechanisms of AlgU's participation in hyperosmotic stress tolerance in P. protegens, and they provide potential molecular targets for research on the hyperosmotic adaptation of P. protegens, which is of value in improving the biocontrol ability of P. protegens.

Enhanced catabolism of glycine betaine and derivatives provides improved osmotic stress protection in Methylorubrum extorquens PA1

Integration of metabolites into the overall metabolic network of a cell requires careful coordination dependent upon the ultimate usage of the metabolite. Different stoichiometric needs, and thus pathway fluxes, must exist for compounds destined for diverse uses, such as carbon sources, nitrogen sources, or stress-protective agents. Herein, we expand upon our previous work that highlighted the nature of glycine betaine (GB) metabolism in Methylobacteria to examine the utilization of GB-derivative compounds dimethylglycine (DMG) and sarcosine into Methylorubrum extorquens in different metabolic capacities, including as sole nitrogen and/or carbon sources. We isolated gain-of-function mutations that allowed M. extorquens PA1 to utilize dimethylglycine as a carbon source and dimethylglycine and sarcosine as nitrogen source. Characterization of mutants demonstrated selection for variants of the AraC-like regulator Mext_3735 that confer constitutive expression of the GB metabolic gene cluster, allowing direct utilization of the downstream GB derivatives. Finally, among the distinct isolates examined, we found that catabolism of the osmoprotectant used for selection (GB or dimethylglycine) enhanced osmotic stress resistance provided in the presence of that particular osmolyte. Thus, access to the carbon and nitrogen and osmoprotective effects of GB and DMG are made readily accessible through adaptive mutations. In M. extorquens PA1, the limitations to exploiting this group of compounds appear to exist predominantly at the levels of gene regulation and functional activity, rather than being constrained by transport or toxicity.IMPORTANCEOsmotic stress is a common challenge for bacteria colonizing the phyllosphere, where glycine betaine (GB) can be found as a prevalent osmoprotectant. Though Methylorubrum extorquens PA1 cannot use GB or its demethylation products, dimethylglycine (DMG) and sarcosine, as a sole carbon source, utilization is highly selectable via single nucleotide changes for both GB and DMG growth. The innate inability to use these compounds is due to limited flux through steps in the pathway and regulatory constraints. Herein, the characterization of the transcriptional regulator, Mext_3735 (GbdR), expands our understanding of the various roles in which GB derivatives can be used in M. extorquens PA1. Interestingly, increased catabolism of GB and derivatives does not interfere with, but rather improves, the ability of cells to thrive under increased salt stress conditions, suggesting that metabolic flux improves stress tolerance rather than providing a distinct tension between uses.

The undiscovered biosynthetic potential of the Greenland Ice Sheet microbiome

The Greenland Ice Sheet is a biome which is mainly microbially driven. Several different niches can be found within the glacial biome for those microbes able to withstand the harsh conditions, e.g., low temperatures, low nutrient conditions, high UV radiation in summer, and contrasting long and dark winters. Eukaryotic algae can form blooms during the summer on the ice surface, interacting with communities of bacteria, fungi, and viruses. Cryoconite holes and snow are also habitats with their own microbial community. Nevertheless, the microbiome of supraglacial habitats remains poorly studied, leading to a lack of representative genomes from these environments. Under-investigated extremophiles, like those living on the Greenland Ice Sheet, may provide an untapped reservoir of chemical diversity that is yet to be discovered. In this study, an inventory of the biosynthetic potential of these organisms is made, through cataloging the presence of biosynthetic gene clusters in their genomes. There were 133 high-quality metagenome-assembled genomes (MAGs) and 28 whole genomes of bacteria obtained from samples of the ice sheet surface, cryoconite, biofilm, and snow using culturing-dependent and -independent approaches. AntiSMASH and BiG-SCAPE were used to mine these genomes and subsequently analyze the resulting predicted gene clusters. Extensive sets of predicted Biosynthetic Gene Clusters (BGCs) were collected from the genome collection, with limited overlap between isolates and MAGs. Additionally, little overlap was found in the biosynthetic potential among different environments, suggesting specialization of organisms in specific habitats. The median number of BGCs per genome was significantly higher for the isolates compared to the MAGs. The most talented producers were found among Proteobacteria. We found evidence for the capacity of these microbes to produce antimicrobials, carotenoid pigments, siderophores, and osmoprotectants, indicating potential survival mechanisms to cope with extreme conditions. The majority of identified BGCs, including those in the most prevalent gene cluster families, have unknown functions, presenting a substantial potential for bioprospecting. This study underscores the diverse biosynthetic potential in Greenland Ice Sheet genomes, revealing insights into survival strategies and highlighting the need for further exploration and characterization of these untapped resources.

Global Map of Specialized Metabolites Encoded in Prokaryotic Plasmids

Plasmids are the main mobile elements responsible for horizontal gene transfer (HGT) in microorganisms. These replicons extend the metabolic spectrum of their host cells by carrying functional genes. However, it is still unknown to what extent plasmids carry biosynthetic gene clusters (BGCs) related to the production of secondary or specialized metabolites (SMs). Here, we analyzed 9,183 microbial plasmids to unveil their potential to produce SMs, finding a large diversity of cryptic BGCs in a few varieties of prokaryotic host taxa. Some of these plasmids harbored 15 or more BGCs, and many others were exclusively dedicated to mobilizing BGCs. We found an occurrence pattern of BGCs within groups of homologous plasmids shared by a common taxon, mainly in host-associated microbes (e.g., Rhizobiales, Enterobacteriaceae members). Our results add to the knowledge of the ecological functions and potential industrial uses of plasmids and shed light on the dynamics and evolution of SMs in prokaryotes. IMPORTANCE Plasmids are mobile DNA elements that can be shared among microbial cells, and they are useful for bringing to fruition some microbial ecological traits. However, it is not known to what extent plasmids harbor genes related to the production of specialized/secondary metabolites (SMs). In microbes, these metabolites are frequently useful for defense purposes, signaling, etc. In addition, these molecules usually have biotechnological and clinical applications. Here, we analyzed the content, dynamics, and evolution of genes related to the production of SMs in >9,000 microbial plasmids. Our results confirm that some plasmids act as a reservoir of SMs. We also found that some families of biosynthetic gene clusters are exclusively present in some groups of plasmids shared among closely related microbes. Host-associated bacteria (e.g., plant and human microbes) harbor the majority of specialized metabolites encoded in plasmids. These results provide new knowledge about microbial ecological traits and might enable the discovery of novel metabolites.

Agronomic efficiency and genome mining analysis of the wheat-biostimulant rhizospheric bacterium Pseudomonas pergaminensis sp. nov. strain 1008T

Pseudomonas sp. strain 1008 was isolated from the rhizosphere of field grown wheat plants at the tillering stage in an agricultural plot near Pergamino city, Argentina. Based on its in vitro phosphate solubilizing capacity and the production of IAA, strain 1008 was formulated as an inoculant for bacterization of wheat seeds and subjected to multiple field assays within the period 2010-2017. Pseudomonas sp. strain 1008 showed a robust positive impact on the grain yield (+8% on average) across a number of campaigns, soil properties, seed genotypes, and with no significant influence of the simultaneous seed treatment with a fungicide, strongly supporting the use of this biostimulant bacterium as an agricultural input for promoting the yield of wheat. Full genome sequencing revealed that strain 1008 has the capacity to access a number of sources of inorganic and organic phosphorus, to compete for iron scavenging, to produce auxin, 2,3-butanediol and acetoin, and to metabolize GABA. Additionally, the genome of strain 1008 harbors several loci related to rhizosphere competitiveness, but it is devoid of biosynthetic gene clusters for production of typical secondary metabolites of biocontrol representatives of the Pseudomonas genus. Finally, the phylogenomic, phenotypic, and chemotaxonomic comparative analysis of strain 1008 with related taxa strongly suggests that this wheat rhizospheric biostimulant isolate is a representative of a novel species within the genus Pseudomonas, for which the name Pseudomonas pergaminensis sp. nov. (type strain 1008T = DSM 113453T = ATCC TSD-287T) is proposed.

Genomic insights into the biosynthesis and physiology of the cyanobacterial neurotoxin 2,4-diaminobutanoic acid (2,4-DAB)

Cyanobacteria are an ancient clade of photosynthetic prokaryotes, whose worldwide occurrence, especially in water, presents health hazards to humans and animals due to the production of a range of toxins (cyanotoxins). These include the sometimes co-occurring, non-encoded diaminoacid neurotoxins 2,4-diaminobutanoic acid (2,4-DAB) and its structural analogue β-N-methylaminoalanine (BMAA). Knowledge of the biosynthetic pathway for 2,4-DAB, and its role in cyanobacteria, is lacking. The aspartate 4-phosphate pathway is a known route of 2,4-DAB biosynthesis in other bacteria and in some plant species. Another pathway to 2,4-DAB has been described in Lathyrus species. Here, we use bioinformatics analyses to investigate hypotheses concerning 2,4-DAB biosynthesis in cyanobacteria. We assessed the presence or absence of each enzyme in candidate biosynthesis routes, the aspartate 4-phosphate pathway and a pathway to 2,4-DAB derived from S-adenosyl-L-methionine (SAM), in 130 cyanobacterial genomes using sequence alignment, profile hidden Markov models, substrate specificity/active site identification and the reconstruction of gene phylogenies. In the aspartate 4-phosphate pathway, for the 18 species encoding diaminobutanoate-2-oxo-glutarate transaminase, the co-localisation of genes encoding the transaminase with the downstream decarboxylase or ectoine synthase - often within hybrid non-ribosomal peptide synthetase (NRPS)-polyketide synthases (PKS) clusters, NRPS-independent siderophore (NIS) clusters and incomplete ectoine clusters - is compatible with the hypothesis that some cyanobacteria use the aspartate 4-phosphate pathway for 2,4-DAB production. Through this route, in cyanobacteria, 2,4-DAB may be functionally associated with environmental iron-scavenging, via the production of siderophores of the schizokinen/synechobactin type and of some polyamines. In the pathway to 2,4-DAB derived from SAM, eight cyanobacterial species encode homologs of SAM-dependent 3-amino-3-carboxypropyl transferases. Other enzymes in this pathway have not yet been purified or sequenced. Ultimately, the biosynthesis of 2,4-DAB appears to be either restricted to some cyanobacterial species, or there may be multiple and additional routes, and roles, for the synthesis of this neurotoxin.

The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

Ectoine and its derivative 5-hydroxyectoine are compatible solutes and chemical chaperones widely synthesized by Bacteria and some Archaea as cytoprotectants during osmotic stress and high- or low-growth temperature extremes. The function-preserving attributes of ectoines led to numerous biotechnological and biomedical applications and fostered the development of an industrial scale production process. Synthesis of ectoines requires the expenditure of considerable energetic and biosynthetic resources. Hence, microorganisms have developed ways to exploit ectoines as nutrients when they are no longer needed as stress protectants. Here, we summarize our current knowledge on the phylogenomic distribution of ectoine producing and consuming microorganisms. We emphasize the structural enzymology of the pathways underlying ectoine biosynthesis and consumption, an understanding that has been achieved only recently. The synthesis and degradation pathways critically differ in the isomeric form of the key metabolite N-acetyldiaminobutyric acid (ADABA). γ-ADABA serves as preferred substrate for the ectoine synthase, while the α-ADABA isomer is produced by the ectoine hydrolase as an intermediate in catabolism. It can serve as internal inducer for the genetic control of ectoine catabolic genes via the GabR/MocR-type regulator EnuR. Our review highlights the importance of structural enzymology to inspire the mechanistic understanding of metabolic networks at the biological scale.

Leveraging Pseudomonas Stress Response Mechanisms for Industrial Applications

Members of the genus Pseudomonas are metabolically versatile and capable of adapting to a wide variety of environments. Stress physiology of Pseudomonas strains has been extensively studied because of their biotechnological potential in agriculture as well as their medical importance with regards to pathogenicity and antibiotic resistance. This versatility and scientific relevance led to a substantial amount of information regarding the stress response of a diverse set of species such as Pseudomonas chlororaphis, P. fluorescens, P. putida, P. aeruginosa, and P. syringae. In this review, environmental and industrial stressors including desiccation, heat, and cold stress, are cataloged along with their corresponding mechanisms of survival in Pseudomonas. Mechanisms of survival are grouped by the type of inducing stress with a focus on adaptations such as synthesis of protective substances, biofilm formation, entering a non-culturable state, enlisting chaperones, transcription and translation regulation, and altering membrane composition. The strategies Pseudomonas strains utilize for survival can be leveraged during the development of beneficial strains to increase viability and product efficacy.

Rhizosphere plant-microbe interactions under water stress

Climate change, with its extreme temperature, weather and precipitation patterns, is a major global concern of dryland farmers, who currently meet the challenges of climate change agronomically and with growth of drought-tolerant crops. Plants themselves compensate for water stress by modifying aerial surfaces to control transpiration and altering root hydraulic conductance to increase water uptake. These responses are complemented by metabolic changes involving phytohormone network-mediated activation of stress response pathways, resulting in decreased photosynthetic activity and the accumulation of metabolites to maintain osmotic and redox homeostasis. Phylogenetically diverse microbial communities sustained by plants contribute to host drought tolerance by modulating phytohormone levels in the rhizosphere and producing water-sequestering biofilms. Drylands of the Inland Pacific Northwest, USA, illustrate the interdependence of dryland crops and their associated microbiota. Indigenous Pseudomonas spp. selected there by long-term wheat monoculture suppress root diseases via the production of antibiotics, with soil moisture a critical determinant of the bacterial distribution, dynamics and activity. Those pseudomonads producing phenazine antibiotics on wheat had more abundant rhizosphere biofilms and provided improved tolerance to drought, suggesting a role of the antibiotic in alleviation of drought stress. The transcriptome and metabolome studies suggest the importance of wheat root exudate-derived osmoprotectants for the adaptation of these pseudomonads to the rhizosphere lifestyle and support the idea that the exchange of metabolites between plant roots and microorganisms profoundly affects and shapes the belowground plant microbiome under water stress.

AlgU, a Conserved Sigma Factor Regulating Abiotic Stress Tolerance and Promoting Virulence in Pseudomonas syringae

Pseudomonas syringae can rapidly deploy specialized functions to deal with abiotic and biotic stresses. Host niches pose specific sets of environmental challenges driven, in part, by immune defenses. Bacteria use a "just-in-time" strategy of gene regulation, meaning that they only produce the functions necessary for survival as needed. Extracytoplasmic function (ECF) sigma factors transduce a specific set of environmental signals and change gene expression patterns by altering RNA polymerase promoter specificity, to adjust bacterial physiology, structure, or behavior, singly or in combination, to improve chances of survival. The broadly conserved ECF sigma factor AlgU affects virulence in both animal and plant pathogens. Pseudomonas syringae AlgU controls expression of more than 800 genes, some of which contribute to suppression of plant immunity and bacterial fitness in plants. This review discusses AlgU activation mechanisms, functions controlled by AlgU, and how these functions contribute to P. syringae survival in plants.[Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law. 2021.

Nuclear Magnetic Resonance Aerosolomics: A Tool for Analysis of Polar Compounds in Atmospheric Aerosols

Nuclear magnetic resonance aerosolomics was proposed as a new approach to the analysis of the water-soluble organic compound fraction in aerosol particulate matter. The identification of individual compounds is based on a comparison of precise chemical shifts in the 1H NMR spectrum with the signals in the standards library. For this purpose, Chenomx metabolomics software and a comprehensive spectra library of 150 compounds known from chemistry of aerosols were used. This approach enabled the identification of 60 compounds in real aerosol samples collected at a suburban site in Prague. Using the metabolomic spectra library, three new compounds were identified in aerosols for the first time, and an association of four other compounds to the atmospheric particulate matter was confirmed. The obtained concentration profiles of all identified chemical individuals were subsequently subjected to advanced statistical analysis. NMR aerosolomics clearly differentiates between summer and winter aerosol samples via multivariate statistical analysis and revealed some interesting trends in composition, according to aerosol particle size. Furthermore, the univariate statistical analysis was applied to highlight compounds responsible for the group separation, and possible sources of these compounds were suggested.

Influence of rehydration on transcriptome during resuscitation of desiccated Pseudomonas putida KT2440

Purpose

Pseudomonas putida KT2440 is a desiccation-sensitive bacterium that loses culturability after 15 days of air desiccation. We have previously shown that P. putida KT2440 can develop a viable but nonculturable (VBNC) state after being exposed to desiccation stress and eventually recover when desiccated cells are rehydrated for at least 24 h.

Methods

To determine which genes of transport, oxidation-reduction, and transcription processes could be involved in the return of P. putida KT2440 to the culturable state, a transcriptome analysis was carried out comparing the gene expression of non-desiccated samples with samples subjected to desiccation followed by 20 min of rehydration or desiccation followed by 24 h of rehydration.

Results

Desiccation stress triggered a VBNC state of P. putida. The major response was detected after 24 h of rehydration with 148 upregulated and 42 downregulated genes. During the VBNC state, P. putida activated transmembrane transport processes like that of siderophores through a TonB-dependent transporter and putative polyhydric alcohol transport systems. Prolonged rehydration with distilled water resuscitated P. putida KT2440 cells activating the catabolism of phenylalanine/tyrosine to provide energy and carbon for ubiquinone biosynthesis while maintaining a reduced protein synthesis. On the other hand, the interruption of the TonB-dependent receptor gene (PP_1446) increased desiccation survival of the mutant strain.

Conclusion

The activation of the iron transport system (TonB-dependent siderophore receptor) and alcohol transport can be helping the VBNC state of P. putida. Activation of catabolism of phenylalanine/tyrosine and reduced protein synthesis was needed for resuscitation from the VBNC state.

Degradation of the microbial stress protectants and chemical chaperones ectoine and hydroxyectoine by a bacterial hydrolase-deacetylase complex

When faced with increased osmolarity in the environment, many bacterial cells accumulate the compatible solute ectoine and its derivative 5-hydroxyectoine. Both compounds are not only potent osmostress protectants, but also serve as effective chemical chaperones stabilizing protein functionality. Ectoines are energy-rich nitrogen and carbon sources that have an ecological impact that shapes microbial communities. Although the biochemistry of ectoine and 5-hydroxyectoine biosynthesis is well understood, our understanding of their catabolism is only rudimentary. Here, we combined biochemical and structural approaches to unravel the core of ectoine and 5-hydroxy-ectoine catabolisms. We show that a conserved enzyme bimodule consisting of the EutD ectoine/5-hydroxyectoine hydrolase and the EutE deacetylase degrades both ectoines. We determined the high-resolution crystal structures of both enzymes, derived from the salt-tolerant bacteria Ruegeria pomeroyi and Halomonas elongata These structures, either in their apo-forms or in forms capturing substrates or intermediates, provided detailed insights into the catalytic cores of the EutD and EutE enzymes. The combined biochemical and structural results indicate that the EutD homodimer opens the pyrimidine ring of ectoine through an unusual covalent intermediate, N-α-2 acetyl-l-2,4-diaminobutyrate (α-ADABA). We found that α-ADABA is then deacetylated by the zinc-dependent EutE monomer into diaminobutyric acid (DABA), which is further catabolized to l-aspartate. We observed that the EutD-EutE bimodule synthesizes exclusively the α-, but not the γ-isomers of ADABA or hydroxy-ADABA. Of note, α-ADABA is known to induce the MocR/GabR-type repressor EnuR, which controls the expression of many ectoine catabolic genes clusters. We conclude that hydroxy-α-ADABA might serve a similar function.

Draft Genome Sequence of Lipopeptide-Producing Strain Pseudomonas fluorescens DSM 11579 and Comparative Genomics with Pseudomonas sp. Strain SH-C52, a Closely Related Lipopeptide-Producing Strain

Pseudomonas fluorescens DSM 11579 is known to be a producer of the lipopeptides brabantamide and thanamycin. Its draft genome gives insight into the complete secondary metabolite production capacity of the strain and builds the basis for a comparative study with Pseudomonas sp. strain SH-C52, a lipopeptide-producing strain involved in natural disease-suppressive soils.

Pseudomonas fluorescens DSM 11579 is known to be a producer of the lipopeptides brabantamide and thanamycin. Its draft genome gives insight into the complete secondary metabolite production capacity of the strain and builds the basis for a comparative study with Pseudomonas sp. strain SH-C52, a lipopeptide-producing strain involved in natural disease-suppressive soils.

Draft Genome Sequence of Pseudomonas chlororaphis subsp. aurantiaca ARS-38, a Bacterial Strain with Plant Growth Promotion Potential, Isolated from the Rhizosphere of Cotton in Pakistan

Strain ARS-38 is a potential plant growth-promoting rhizobacterium that exhibits antifungal properties. Here, we report a 6.6-Mb draft genome, which gives insight into the complete secondary metabolite production capacity and reveals genes putatively responsible for its antifungal activity, as well as genes which contribute to plant growth promotion.

Strain ARS-38 is a potential plant growth-promoting rhizobacterium that exhibits antifungal properties. Here, we report a 6.6-Mb draft genome, which gives insight into the complete secondary metabolite production capacity and reveals genes putatively responsible for its antifungal activity, as well as genes which contribute to plant growth promotion.

Ethanol Stimulates Trehalose Production through a SpoT-DksA-AlgU-Dependent Pathway in Pseudomonas aeruginosa

Pseudomonas aeruginosa frequently resides among ethanol-producing microbes, making its response to the microbially produced concentrations of ethanol relevant to understanding its biology. Our transcriptome analysis found that genes involved in trehalose metabolism were induced by low concentrations of ethanol, and biochemical assays showed that levels of intracellular trehalose increased significantly upon growth with ethanol. The increase in trehalose was dependent on the TreYZ pathway but not other trehalose-metabolic enzymes (TreS or TreA). The sigma factor AlgU (AlgT), a homolog of RpoE in other species, was required for increased expression of the treZ gene and trehalose levels, but induction was not controlled by the well-characterized proteolysis of its anti-sigma factor, MucA. Growth with ethanol led to increased SpoT-dependent (p)ppGpp accumulation, which stimulates AlgU-dependent transcription of treZ and other AlgU-regulated genes through DksA, a (p)ppGpp and RNA polymerase binding protein. Ethanol stimulation of trehalose also required acylhomoserine lactone (AHL)-mediated quorum sensing (QS), as induction was not observed in a ΔlasR ΔrhlR strain. A network analysis using a model, eADAGE, built from publicly available P. aeruginosa transcriptome data sets (J. Tan, G. Doing, K. A. Lewis, C. E. Price, et al., Cell Syst 5:63-71, 2017, https://doi.org/10.1016/j.cels.2017.06.003) provided strong support for our model in which treZ and coregulated genes are controlled by both AlgU- and AHL-mediated QS. Consistent with (p)ppGpp- and AHL-mediated quorum-sensing regulation, ethanol, even when added at the time of culture inoculation, stimulated treZ transcript levels and trehalose production in cells from post-exponential-phase cultures but not in cells from exponential-phase cultures. These data highlight the integration of growth and cell density cues in the P. aeruginosa transcriptional response to ethanol.IMPORTANCE Pseudomonas aeruginosa is often found with bacteria and fungi that produce fermentation products, including ethanol. At concentrations similar to those produced by environmental microbes, we found that ethanol stimulated expression of trehalose-biosynthetic genes and cellular levels of trehalose, a disaccharide that protects against environmental stresses. The induction of trehalose by ethanol required the alternative sigma factor AlgU through DksA- and SpoT-dependent (p)ppGpp. Trehalose accumulation also required AHL quorum sensing and occurred only in post-exponential-phase cultures. This work highlights how cells integrate cell density and growth cues in their responses to products made by other microbes and reveals a new role for (p)ppGpp in the regulation of AlgU activity.

Illuminating the catalytic core of ectoine synthase through structural and biochemical analysis

Ectoine synthase (EctC) is the signature enzyme for the production of ectoine, a compatible solute and chemical chaperone widely synthesized by bacteria as a cellular defense against the detrimental effects of osmotic stress. EctC catalyzes the last step in ectoine synthesis through cyclo-condensation of the EctA-formed substrate N-gamma-acetyl-L-2,4-diaminobutyric acid via a water elimination reaction. We have biochemically and structurally characterized the EctC enzyme from the thermo-tolerant bacterium Paenibacillus lautus (Pl). EctC is a member of the cupin superfamily and forms dimers, both in solution and in crystals. We obtained high-resolution crystal structures of the (Pl)EctC protein in forms that contain (i) the catalytically important iron, (ii) iron and the substrate N-gamma-acetyl-L-2,4-diaminobutyric acid, and (iii) iron and the enzyme reaction product ectoine. These crystal structures lay the framework for a proposal for the EctC-mediated water-elimination reaction mechanism. Residues involved in coordinating the metal, the substrate, or the product within the active site of ectoine synthase are highly conserved among a large group of EctC-type proteins. Collectively, the biochemical, mutational, and structural data reported here yielded detailed insight into the structure-function relationship of the (Pl)EctC enzyme and are relevant for a deeper understanding of the ectoine synthase family as a whole.

Diffusible signal factor (DSF)-mediated quorum sensing modulates expression of diverse traits in Xanthomonas citri and responses of citrus plants to promote disease

Background

The gram-negative Xanthomonas genus contains a large group of economically important plant pathogens, which cause severe diseases on many crops worldwide. The diffusible signal factor (DSF) - mediated quorum sensing (QS) system coordinates expression of virulence factors in plant pathogenic Xanthomonas spp. However, the regulatory effects of this system during the Xanthomonas- plant interactions remain unclear from both the pathogen and host aspects.

Results

In this study, we investigated the in planta DSF- mediated QS regulon of X. citri subsp. citri (Xac), the causal agent of citrus canker. We also characterized the transcriptional responses of citrus plants to DSF-mediated Xac infection via comparing the gene expression patterns of citrus trigged by wild type Xac strain 306 with those trigged by its DSF- deficient (∆rpfF) mutant using the dual RNA-seq approach. Comparative global transcript profiles of Xac strain 306 and the ∆rpfF mutant during host infection revealed that DSF- mediated QS specifically modulates bacterial adaptation, nutrition uptake and metabolisms, stress tolerance, virulence, and signal transduction to favor host infection. The transcriptional responses of citrus to DSF-mediated Xac infection are characterized by downregulation of photosynthesis genes and plant defense related genes, suggesting photosynthetically inactive reactions and repression of defense responses. Alterations of phytohormone metabolism and signaling pathways were also triggered by DSF-mediated Xac infection to benefit the pathogen.

Conclusions

Collectively, our findings provide new insight into the DSF- mediated QS regulation during plant-pathogen interactions and advance the understanding of traits used by Xanthomonas to promote infection on host plants.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5384-4) contains supplementary material, which is available to authorized users.

Engineering the Salt-Inducible Ectoine Promoter Region of Halomonas elongata for Protein Expression in a Unique Stabilizing Environment

It has been firmly established that organic osmolytes (compatible solutes) of halophilic Bacteria and Archaea have positive effects on conformation and activity of proteins, and may therefore improve their functional production. In particular, the amino acid derivative ectoine is known for its conformational stabilization, aggregation suppression, and radical protection properties. The natural producer and industrial production strain Halomonas elongata accumulates ectoine in the cytoplasm, and as a result offers a unique stabilizing environment for recombinant proteins. For the construction of broad hoast range vector systems with fluorescent reporter proteins, we chose the salt-inducible promoter region of the ectoine gene cluster (promA). A closer inspection of the genetic background revealed that its combination of sigma 38 (σ³⁸) and sigma 70 (σ⁷⁰) promoters was followed by a weak ribosomal binding site (RBS). This inspired a systematic approach for the construction of a promA-based vector series with a synthetic RBS region using the RBS Calculator v2.0, which resulted in a greatly improved salt-dependent expression-even in a deletion construct lacking the σ³⁸ promoter. To expand the application range of this expression system, we looked further into the possible export of recombinant proteins into the periplasm. Both sec and tat leader sequences from H. elongata proved to be suitable for directed periplasmic transport into an extreme environment of freely selectable ionic strength.

Role of the Extremolytes Ectoine and Hydroxyectoine as Stress Protectants and Nutrients: Genetics, Phylogenomics, Biochemistry, and Structural Analysis

Fluctuations in environmental osmolarity are ubiquitous stress factors in many natural habitats of microorganisms, as they inevitably trigger osmotically instigated fluxes of water across the semi-permeable cytoplasmic membrane. Under hyperosmotic conditions, many microorganisms fend off the detrimental effects of water efflux and the ensuing dehydration of the cytoplasm and drop in turgor through the accumulation of a restricted class of organic osmolytes, the compatible solutes. Ectoine and its derivative 5-hydroxyectoine are prominent members of these compounds and are synthesized widely by members of the Bacteria and a few Archaea and Eukarya in response to high salinity/osmolarity and/or growth temperature extremes. Ectoines have excellent function-preserving properties, attributes that have led to their description as chemical chaperones and fostered the development of an industrial-scale biotechnological production process for their exploitation in biotechnology, skin care, and medicine. We review, here, the current knowledge on the biochemistry of the ectoine/hydroxyectoine biosynthetic enzymes and the available crystal structures of some of them, explore the genetics of the underlying biosynthetic genes and their transcriptional regulation, and present an extensive phylogenomic analysis of the ectoine/hydroxyectoine biosynthetic genes. In addition, we address the biochemistry, phylogenomics, and genetic regulation for the alternative use of ectoines as nutrients.

Metabolomic study of the response to cold shock in a strain of Pseudomonas syringae isolated from cloud water

INTRODUCTION

Active microorganisms have been recently discovered in clouds, thus demonstrating the capacity of microorganisms to exist in harsh environments, including exposure to UV and oxidants, osmotic and cold shocks, etc. It is important to understand how microorganisms respond to and survive such stresses at the metabolic level.

OBJECTIVES

The objective of this work is to assess metabolome modulation in a strain of Pseudomonas syringae isolated from cloud water and facing temperature downshift from 17 to 5 °C by identifying key molecules and pathways of the response/adaptation to cold shock.

METHODS

Bacterial extracts from suspensions of cells grown at 17 °C and further incubated in microcosms at 5 and 17 °C to mimic cloud conditions were analysed by combining LC-MS and NMR; the results were evaluated in comparison to similar suspensions kept at constant temperature. The differences in the metabolome profiles were deciphered using multivariate statistics (PLS-DA).

RESULTS

Key cold shock biomarkers were observed, including cryoprotectants (trehalose, glucose, glycerol, carnitine, glutamate), antioxidants (glutathione and carnitine) and their precursors, alkaloids (bellendine and slaframine) and metabolites involved in energy metabolism (ATP, carbohydrates). Furthermore, new short peptides (nine dipeptides and a tetrapeptide) were found that have no known function.

CONCLUSIONS

This study shows that in response to cold temperatures, Pseudomonas syringae PDD-32b-74 demonstrates numerous metabolism modifications to counteract the impacts of low temperatures.

Strangers in the archaeal world: osmostress-responsive biosynthesis of ectoine and hydroxyectoine by the marine thaumarchaeon Nitrosopumilus maritimus

Ectoine and hydroxyectoine are compatible solutes widely synthesized by members of the Bacteria to cope with high osmolarity surroundings. Inspection of 557 archaeal genomes revealed that only 12 strains affiliated with the Nitrosopumilus, Methanothrix or Methanobacterium genera harbour ectoine/hydroxyectoine gene clusters. Phylogenetic considerations suggest that these Archaea have acquired these genes through horizontal gene transfer events. Using the Thaumarchaeon 'Candidatus Nitrosopumilus maritimus' as an example, we demonstrate that the transcription of its ectABCD genes is osmotically induced and functional since it leads to the production of both ectoine and hydroxyectoine. The ectoine synthase and the ectoine hydroxylase were biochemically characterized, and their properties resemble those of their counterparts from Bacteria. Transcriptional analysis of osmotically stressed 'Ca. N. maritimus' cells demonstrated that they possess an ectoine/hydroxyectoine gene cluster (hyp-ectABCD-mscS) different from those recognized previously since it contains a gene for an MscS-type mechanosensitive channel. Complementation experiments with an Escherichia coli mutant lacking all known mechanosensitive channel proteins demonstrated that the (Nm)MscS protein is functional. Hence, 'Ca. N. maritimus' cells cope with high salinity not only through enhanced synthesis of osmostress-protective ectoines but they already prepare themselves simultaneously for an eventually occurring osmotic down-shock by enhancing the production of a safety-valve (NmMscS).

Comparative genomics of Pseudomonas syringae pv. syringae strains B301D and HS191 and insights into intrapathovar traits associated with plant pathogenesis

Pseudomonas syringae pv. syringae is a common plant-associated bacterium that causes diseases of both monocot and dicot plants worldwide. To help delineate traits critical to adaptation and survival in the plant environment, we generated complete genome sequences of P. syringae pv. syringae strains B301D and HS191, which represent dicot and monocot strains with distinct host specificities. Intrapathovar comparisons of the B301D (6.09 Mb) and HS191 (5.95 Mb plus a 52 kb pCG131 plasmid) genomes to the previously sequenced B728a genome demonstrated that the shared genes encompass about 83% of each genome, and include genes for siderophore biosynthesis, osmotolerance, and extracellular polysaccharide production. Between 7% and 12% of the genes are unique among the genomes, and most of the unique gene regions carry transposons, phage elements, or IS elements associated with horizontal gene transfer. Differences are observed in the type III effector composition for the three strains that likely influences host range. The HS191 genome had the largest number at 25 of effector genes, and seven effector genes are specific to this monocot strain. Toxin production is another major trait associated with virulence of P. syringae pv. syringae, and HS191 is distinguished by genes for production of syringopeptin SP25 and mangotoxin.

Biochemical properties of ectoine hydroxylases from extremophiles and their wider taxonomic distribution among microorganisms

Ectoine and hydroxyectoine are well-recognized members of the compatible solutes and are widely employed by microorganisms as osmostress protectants. The EctABC enzymes catalyze the synthesis of ectoine from the precursor L-aspartate-β-semialdehyde. A subgroup of the ectoine producers can convert ectoine into 5-hydroxyectoine through a region-selective and stereospecific hydroxylation reaction. This compatible solute possesses stress-protective and function-preserving properties different from those of ectoine. Hydroxylation of ectoine is carried out by the EctD protein, a member of the non-heme-containing iron (II) and 2-oxoglutarate-dependent dioxygenase superfamily. We used the signature enzymes for ectoine (EctC) and hydroxyectoine (EctD) synthesis in database searches to assess the taxonomic distribution of potential ectoine and hydroxyectoine producers. Among 6428 microbial genomes inspected, 440 species are predicted to produce ectoine and of these, 272 are predicted to synthesize hydroxyectoine as well. Ectoine and hydroxyectoine genes are found almost exclusively in Bacteria. The genome context of the ect genes was explored to identify proteins that are functionally associated with the synthesis of ectoines; the specialized aspartokinase Ask_Ect and the regulatory protein EctR. This comprehensive in silico analysis was coupled with the biochemical characterization of ectoine hydroxylases from microorganisms that can colonize habitats with extremes in salinity (Halomonas elongata), pH (Alkalilimnicola ehrlichii, Acidiphilium cryptum), or temperature (Sphingopyxis alaskensis, Paenibacillus lautus) or that produce hydroxyectoine very efficiently over ectoine (Pseudomonas stutzeri). These six ectoine hydroxylases all possess similar kinetic parameters for their substrates but exhibit different temperature stabilities and differ in their tolerance to salts. We also report the crystal structure of the Virgibacillus salexigens EctD protein in its apo-form, thereby revealing that the iron-free structure exists already in a pre-set configuration to incorporate the iron catalyst. Collectively, our work defines the taxonomic distribution and salient biochemical properties of the ectoine hydroxylase protein family and contributes to the understanding of its structure.

Complete Genome Sequence of the Sugar Cane Endophyte Pseudomonas aurantiaca PB-St2, a Disease-Suppressive Bacterium with Antifungal Activity toward the Plant Pathogen Colletotrichum falcatum

The endophytic bacterium Pseudomonas aurantiaca PB-St2 exhibits antifungal activity and represents a biocontrol agent to suppress red rot disease of sugar cane. Here, we report the completely sequenced 6.6-Mb genome of P. aurantiaca PB-St2. The sequence contains a repertoire of biosynthetic genes for secondary metabolites that putatively contribute to its antagonistic activity and its plant-microbe interactions.

The widespread plant-colonizing bacterial species Pseudomonas syringae detects and exploits an extracellular pool of choline in hosts

The quaternary ammonium compound (QAC) choline is a major component of membrane lipids in eukaryotes and, if available to microbial colonists of plants, could provide benefits for growth and protection from stress. Free choline is found in homogenized plant tissues, but its subcellular location and availability to plant microbes are not known. Whole-cell bacterial bioreporters of the phytopathogen Pseudomonas syringae were constructed that couple a QAC-responsive transcriptional fusion with well-characterized bacterial QAC transporters. These bioreporters demonstrated the presence of abundant free choline compounds released from germinating seeds and seedlings of the bean Phaseolus vulgaris, and a smaller but consistently detectable amount of QACs, probably choline, from leaves. The localization of P. syringae bioreporter cells to the surface and intercellular sites of plant tissues demonstrated the extracellular location of these QAC pools. Moreover, P. syringae mutants that were deficient in the uptake of choline compounds exhibited reduced fitness on leaves, highlighting the importance of extracellular choline to P. syringae on leaves. Our data support a model in which this choline pool is derived from the phospholipid phosphatidylcholine through plant-encoded phospholipases that release choline into the intercellular spaces of plant tissues, such as for membrane lipid recycling. The consequent extracellular release of choline compounds enables their interception and exploitation by plant-associated microbes, and thus provides a selective advantage for microbes such as P. syringae that are adapted to maximally exploit choline.

Physiological and transcriptional responses to osmotic stress of two Pseudomonas syringae strains that differ in epiphytic fitness and osmotolerance

The foliar pathogen Pseudomonas syringae is a useful model for understanding the role of stress adaptation in leaf colonization. We investigated the mechanistic basis of differences in the osmotolerance of two P. syringae strains, B728a and DC3000. Consistent with its higher survival rates following inoculation onto leaves, B728a exhibited superior osmotolerance over DC3000 and higher rates of uptake of plant-derived osmoprotective compounds. A global transcriptome analysis of B728a and DC3000 following an osmotic upshift demonstrated markedly distinct responses between the strains; B728a showed primarily upregulation of genes, including components of the type VI secretion system (T6SS) and alginate biosynthetic pathways, whereas DC3000 showed no change or repression of orthologous genes, including downregulation of the T3SS. DC3000 uniquely exhibited improved growth upon deletion of the biosynthetic genes for the compatible solute N-acetylglutaminylglutamine amide (NAGGN) in a minimal medium, due possibly to NAGGN synthesis depleting the cellular glutamine pool. Both strains showed osmoreduction of glnA1 expression, suggesting that decreased glutamine synthetase activity contributes to glutamate accumulation as a compatible solute, and both strains showed osmoinduction of 5 of 12 predicted hydrophilins. Collectively, our results demonstrate that the superior epiphytic competence of B728a is consistent with its strong osmotolerance, a proactive response to an osmotic upshift, osmoinduction of alginate synthesis and the T6SS, and resiliency of the T3SS to water limitation, suggesting sustained T3SS expression under the water-limited conditions encountered during leaf colonization.

Homeostasis and catabolism of choline and glycine betaine: lessons from Pseudomonas aeruginosa

Most sequenced bacteria possess mechanisms to import choline and glycine betaine (GB) into the cytoplasm. The primary role of choline in bacteria appears to be as the precursor to GB, and GB is thought to primarily act as a potent osmoprotectant. Choline and GB may play accessory roles in shaping microbial communities, based on their limited availability and ability to enhance survival under stress conditions. Choline and GB enrichment near eukaryotes suggests a role in the chemical relationships between these two kingdoms, and some of these interactions have been experimentally demonstrated. While many bacteria can convert choline to GB for osmoprotection, a variety of soil- and water-dwelling bacteria have catabolic pathways for the multistep conversion of choline, via GB, to glycine and can thereby use choline and GB as sole sources of carbon and nitrogen. In these choline catabolizers, the GB intermediate represents a metabolic decision point to determine whether GB is catabolized or stored as an osmo- and stress protectant. This minireview focuses on this decision point in Pseudomonas aeruginosa, which aerobically catabolizes choline and can use GB as an osmoprotectant and a nutrient source. P. aeruginosa is an experimentally tractable and ecologically relevant model to study the regulatory pathways controlling choline and GB homeostasis in choline-catabolizing bacteria. The study of P. aeruginosa associations with eukaryotes and other bacteria also makes this a powerful model to study the impact of choline and GB, and their associated regulatory and catabolic pathways, on host-microbe and microbe-microbe relationships.

Glycine betaine catabolism contributes to Pseudomonas syringae tolerance to hyperosmotic stress by relieving betaine-mediated suppression of compatible solute synthesis

Many bacteria can accumulate glycine betaine for osmoprotection and catabolize it as a growth substrate, but how they regulate these opposing roles is poorly understood. In Pseudomonas syringae B728a, expression of the betaine catabolism genes was reduced by an osmotic upshift to an intermediate stress level, consistent with betaine accumulation, but was increased by an upshift to a high stress level, as confirmed by an accompanying increase in degradation of radiolabeled betaine. Deletion of the gbcAB betaine catabolism genes reduced osmotolerance at a high osmolarity, and this reduction was due to the relief of betaine-mediated suppression of compatible solute synthesis. This conclusion was supported by the findings that, at high osmolarity, the ΔgbcAB mutant accumulated high betaine levels and low endogenous solutes and exhibited reduced expression of the solute synthesis genes. Moreover, the ΔgbcAB mutant and a mutant deficient in the synthesis of the compatible solutes NAGGN and trehalose exhibited similar reductions in osmotolerance and also in fitness on bean leaves. Activation of betaine catabolism at high osmotic stress resulted, in part, from induction of gbdR, which encodes the transcriptional activator GbdR. Betaine catabolism was subject to partial repression by succinate under hyperosmotic stress conditions, in contrast to strong repression in the absence of stress, suggesting that betaine functions both in nutrition and as an intracellular signal modulating solute synthesis under hyperosmotic stress conditions. Collectively, these results begin to provide a detailed mechanistic understanding of how P. syringae transitions from reliance on exogenously derived betaine to the use of endogenous solutes during adaptation to hyperosmotic conditions.

Temperature- and salinity-decoupled overproduction of hydroxyectoine by Chromohalobacter salexigens

Hydroxyectoine overproduction by the natural producer Chromohalobacter salexigens is presented in this study. Genetically engineered strains were constructed that at low salinity coexpressed, in a vector derived from a native plasmid, the ectoine (ectABC) and hydroxyectoine (ectD) genes under the control of the ectA promoter, in a temperature-independent manner. Hydroxyectoine production was further improved by increasing the copies of ectD and using a C. salexigens genetic background unable to synthesize ectoines.

Transcriptional responses of Pseudomonas syringae to growth in epiphytic versus apoplastic leaf sites

Some strains of the foliar pathogen Pseudomonas syringae are adapted for growth and survival on leaf surfaces and in the leaf interior. Global transcriptome profiling was used to evaluate if these two habitats offer distinct environments for bacteria and thus present distinct driving forces for adaptation. The transcript profiles of Pseudomonas syringae pv. syringae B728a support a model in which leaf surface, or epiphytic, sites specifically favor flagellar motility, swarming motility based on 3-(3-hydroxyalkanoyloxy) alkanoic acid surfactant production, chemosensing, and chemotaxis,indicating active relocation primarily on the leaf surface. Epiphytic sites also promote high transcript levels for phenylalanine degradation, which may help counteract phenylpropanoid-based defenses before leaf entry. In contrast, intercellular, or apoplastic,sites favor the high-level expression of genes for GABA metabolism (degradation of these genes would attenuate GABA repression of virulence) and the synthesis of phytotoxins, two additional secondary metabolites, and syringolin A. These findings support roles for these compounds in virulence, including a role for syringolin A in suppressing defense responses beyond stomatal closure. A comparison of the transcriptomes from in planta cells and from cells exposed to osmotic stress, oxidative stress, and iron and nitrogen limitation indicated that water availability, in particular,was limited in both leaf habitats but was more severely limited in the apoplast than on the leaf surface under the conditions tested. These findings contribute to a coherent model of the adaptations of this widespread bacterial phytopathogen to distinct habitats within its host.

Genome sequence of the moderately halotolerant, arsenite-oxidizing bacterium Pseudomonas stutzeri TS44

We present the draft genome sequence of Pseudomonas stutzeri TS44, a moderately halotolerant, arsenite-oxidizing bacterium isolated from arsenic-contaminated soil. The genome contains genes for arsenite oxidation, arsenic resistance, and ectoine/hydroxyectoine biosynthesis. The genome information will be useful for exploring adaptation of P. stutzeri TS44 to an arsenic-contaminated environment.

A specialized aspartokinase enhances the biosynthesis of the osmoprotectants ectoine and hydroxyectoine in Pseudomonas stutzeri A1501

The compatible solutes ectoine and hydroxyectoine are widely produced by bacteria as protectants against osmotic and temperature stress. l-Aspartate-beta-semialdehyde is used as the precursor molecule for ectoine/hydroxyectoine biosynthesis that is catalyzed by the EctABCD enzymes. l-Aspartate-beta-semialdehyde is a central intermediate in different biosynthetic pathways and is produced from l-aspartate by aspartokinase (Ask) and aspartate-semialdehyde-dehydrogenase (Asd). Ask activity is typically stringently regulated by allosteric control to avoid gratuitous synthesis of aspartylphosphate. Many organisms have evolved multiple forms of aspartokinase, and feedback regulation of these specialized Ask enzymes is often adapted to the cognate biochemical pathways. The ectoine/hydroxyectoine biosynthetic genes (ectABCD) are followed in a considerable number of microorganisms by an askgene (ask_ect), suggesting that Ask_Ect is a specialized enzyme for this osmoadaptive biosynthetic pathway. However, none of these Ask_Ect enzymes have been functionally characterized. Pseudomonas stutzeri A1501 synthesizes both ectoine and hydroxyectoine in response to increased salinity, and it possesses two Ask enzymes: Ask_Lys and Ask_Ect. We purified both Ask enzymes and found significant differences with regard to their allosteric control: Ask_LysC was inhibited by threonine and in a concerted fashion by threonine and lysine, whereas Ask_Ect showed inhibition only by threonine. The ectABCD_askgenes from P. stutzeri A1501 were cloned and functionally expressed in Escherichia coli, and this led to osmostress protection. An E. colistrain carrying the plasmid-based ectABCD_askgene cluster produced significantly more ectoine/hydroxyectoine than a strain expressing the ectABCDgene cluster alone. This finding suggests a specialized role for Ask_Ect in ectoine/hydroxyectoine biosynthesis.

Water relations in the interaction of foliar bacterial pathogens with plants

This review examines the many ways in which water influences the relations between foliar bacterial pathogens and plants. As a limited resource in aerial plant tissues, water is subject to manipulation by both plants and pathogens. A model is emerging that suggests that plants actively promote localized desiccation at the infection site and thus restrict pathogen growth as one component of defense. Similarly, many foliar pathogens manipulate water relations as one component of pathogenesis. Nonvascular pathogens do this using effectors and other molecules to alter hormonal responses and enhance intercellular watersoaking, whereas vascular pathogens use many mechanisms to cause wilt. Because of water limitations on phyllosphere surfaces, bacterial colonists, including pathogens, benefit from the protective effects of cellular aggregation, synthesis of hygroscopic polymers, and uptake and production of osmoprotective compounds. Moreover, these bacteria employ tactics for scavenging and distributing water to overcome water-driven barriers to nutrient acquisition, movement, and signal exchange on plant surfaces.

Natural and engineered hydroxyectoine production based on the Pseudomonas stutzeri ectABCD-ask gene cluster

We report on the presence of a functional hydroxyectoine biosynthesis gene cluster, ectABCD-ask, in Pseudomonas stutzeri DSM5190(T) and evaluate the suitability of P. stutzeri DSM5190(T) for hydroxyectoine production. Furthermore, we present information on heterologous de novo production of the compatible solute hydroxyectoine in Escherichia coli. In this host, the P. stutzeri gene cluster remained under the control of its salt-induced native promoters. We also noted the absence of trehalose when hydroxyectoine genes were expressed, as well as a remarkable inhibitory effect of externally applied betaine on hydroxyectoine synthesis. The specific heterologous production rate in E. coli under the conditions employed exceeded that of the natural producer Pseudomonas stutzeri and, for the first time, enabled effective hydroxyectoine production at low salinity (2%), with the added advantage of simple product processing due to the absence of other cosolutes.