Indole-3-acetic acid improves Escherichia coli’s defences to stress

Bisphenol A removal by a Pseudomonas aeruginosa immobilized on granular activated carbon and operating in a fluidized bed reactor

Serratia rubidiae, Pseudomonas aeruginosa and Escherichia coli K12 have been studied for their ability of Bisphenol A removal from aqueous systems and biofilm formation on activated granule carbon. Mathematical equations for biodegradation process have been elaborated and discussed. P. aeruginosa was found the best strain to be employed in the process of Bisphenol A removal. The yield in BPA removal of a P. aeruginosa biofilm grown on GAC and operating in a fluidized bed reactor has been evaluated. The results confirm the usefulness in using biological activated carbon (BAC process) to remove phenol compounds from aqueous systems.

Indole: a signaling molecule or a mere metabolic byproduct that alters bacterial physiology at a high concentration?

Indole is an organic compound that is widespread in microbial communities inhabiting diverse habitats, like the soil environment and human intestines. Measurement of indole production is a traditional method for the identification of microbial species. Escherichia coli can produce millimolar concentrations of indole in the stationary growth phase under nutrient-rich conditions. Indole has received considerable attention because of its remarkable effects on various biological functions of the microbial communities, for example, biofilm formation, motility, virulence, plasmid stability, and antibiotic resistance. Indole may function as an intercellular signaling molecule, like a quorum-sensing signal. Nevertheless, a receptor system for indole and the function of this compound in coordinated behavior of a microbial population (which are requirements for a true signaling molecule) have not yet been confirmed. Recent findings suggest that a long-known quorum-sensing regulator, E. coli's SdiA, cannot recognize indole and that this compound may simply cause membrane disruption and energy reduction, which can lead to various changes in bacterial physiology including unstable folding of a quorum-sensing regulator. Indole appears to be responsible for acquisition of antibiotic resistance via the formation of persister cells and activation of an exporter. This review highlights and summarizes the current knowledge about indole as a multitrophic molecule among bacteria, together with recently identified new avenues of research.

Molecular characterization and identification of target protein of an important vesicle trafficking gene AlRab7 from a salt excreting halophyte Aeluropus lagopoides

The endomembrane system plays an important role during cellular adaptation of the plants with the extracellular environment. The small GTP-binding protein Rab7 located at the vacuolar membrane regulates the vesicle fusion with the vacuole and thereby helps in recycling of the molecules. This is the first report on isolation and characterization of AlRab7 gene from the halophyte plant, Aeluropus that extrudes NaCl through salt glands and grows luxuriantly throughout the year at the Gujarat coast, India. The AlRab7 encodes a protein with 206 amino acids, and a highly conserved effector-binding domain and four nucleotide-binding domains. The in silico analysis predicts the presence of the prenylation site for Rab geranylgeranyltransferase 2 and the Rab escort protein site. The C-terminal two cysteine residues in -XCC sequence are present for membrane attachment. Transcript expression of the AlRab7 gene was differentially regulated by different environmental stimuli such as dehydration, salinity, and hormone abscisic acid (ABA). The recombinant Escherichia coli cells showed improved growth in Luria Bertani medium supplemented with NaCl, KCl, mannitol, ABA, and indole-3-acetic acid. A novel Rab7 interacting partner AlRabring7 was identified by yeast two-hybrid screening.

Controlling bacterial behavior with indole-containing natural products and derivatives

Indole has recently been implicated as an important small molecule signal utilized by many bacteria to coordinate various forms of behavior. Indole plays a role in numerous bacterial processes, including: biofilm formation and maintenance, virulence factor production, antibiotic resistance and persister cell formation. Intercepting indole-signaling pathways with appropriately designed small molecules provides a n opportunity to control unwanted bacterial behaviors, and is an attractive anti-virulence therapeutic strategy. In this review, we give an overview of the process controlled by indole signaling, and summarize current efforts to design indole-containing small molecules to intercept these pathways, and detail the synthetic efforts towards accessing indole derived bioactive small molecules.

Effects of Escherichia coli on mixotrophic growth of Chlorella minutissima and production of biofuel precursors

Chlorella minutissima was co-cultured with Escherichia coli in airlift reactors under mixotrophic conditions (glucose, glycerol, and acetate substrates) to determine possible effects of bacterial contamination on algal biofuel production. It was hypothesized that E. coli would compete with C. minutissima for nutrients, displacing algal biomass. However, C. minutissima grew more rapidly and to higher densities in the presence of E. coli, suggesting a symbiotic relationship between the organisms. At an initial 1% substrate concentration, the co-culture produced 200-587% more algal biomass than the axenic C. minutissima cultures. Co-cultures grown on 1% substrate consumed 23–737% more of the available carbon substrate than the sum of substrate consumed by E. coli and C. minutissima alone. At 1% substrate, total lipid and starch productivity were elevated in co-cultures compared to axenic cultures indicating that bacterial contamination was not detrimental to the production of biofuel precursors in this specific case. Bio-fouling of the reactors observed in co-cultures and acid formation in all mixotrophic cultures, however, could present challenges for scale-up.

Indole-3-acetic acid in plant-microbe interactions

Indole-3-acetic acid (IAA) is an important phytohormone with the capacity to control plant development in both beneficial and deleterious ways. The ability to synthesize IAA is an attribute that many bacteria including both plant growth-promoters and phytopathogens possess. There are three main pathways through which IAA is synthesized; the indole-3-pyruvic acid, indole-3-acetamide and indole-3-acetonitrile pathways. This chapter reviews the factors that effect the production of this phytohormone, the role of IAA in bacterial physiology and in plant-microbe interactions including phytostimulation and phytopathogenesis.

Stress resistance of Escherichia coli and Bacillus subtilis is modulated by auxins

Two bacterial species, Gram-negative Escherichia coli and Gram-positive Bacillus subtilis, were exposed to different auxins to examine possible effects of these substances on bacterial stress tolerance. Bacterial resistance to UV irradiation, heat shock, and streptomycin was assessed with and without previous exposure to the following auxins: indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and 1-naphthalene acetic acid (NAA). Escherichia coli and B. subtilis cultures pretreated with any of the 3 auxins survived UV irradiation better than the untreated cultures. Also, B. subtilis cultures pretreated with IBA or NAA survived prolonged heat exposure better than the untreated cultures, while IAA pretreatment had no effect on heat shock survival. In contrast, auxin pretreatment rendered E. coli more sensitive to heat shock. Escherichia coli cultures pretreated with auxins were also more sensitive to streptomycin, while auxin pretreatment had no effect on sensitivity of B. subtilis to streptomycin. These results show that auxins may either enhance or reduce bacterial tolerance to different stressors, depending on the bacterial species and the type and level of the stress. Auxins usually had similar effects on the same bacterial species in cases when the same type and level of stress were applied.

Effects of indole-3-acetic acid on the transcriptional activities and stress tolerance of Bradyrhizobium japonicum

A genome-wide transcriptional profile of Bradyrhizobium japonicum, the nitrogen-fixing endosymbiont of the soybean plant, revealed differential expression of approximately 15% of the genome after a 1 mM treatment with the phytohormone indole-3-acetic acid (IAA). A total of 1,323 genes were differentially expressed (619 up-regulated and 704 down-regulated) at a two-fold cut off with q value ≤ 0.05. General stress response genes were induced, such as those involved in response to heat, cold, oxidative, osmotic, and desiccation stresses and in exopolysaccharide (EPS) biosynthesis. This suggests that IAA is effective in activating a generalized stress response in B. japonicum. The transcriptional data were corroborated by the finding that stress tolerance of B. japonicum in cell viability assays was enhanced when pre-treated with 1 mM IAA compared to controls. The IAA treatment also stimulated biofilm formation and EPS production by B. japonicum, especially acidic sugar components in the total EPS. The IAA pre-treatment did not influence the nodulation ability of B. japonicum. The data provide a comprehensive overview of the potential transcriptional responses of the symbiotic bacterium when exposed to the ubiquitous hormone of its plant host.

Indole-3-butyric acid induces lateral root formation via peroxisome-derived indole-3-acetic acid and nitric oxide

Controlled plant growth requires regulation through a variety of signaling molecules, including steroids, peptides, radicals of oxygen and nitrogen, as well as the 'classical' phytohormone groups. Auxin is critical for the control of plant growth and also orchestrates many developmental processes, such as the formation of new roots. It modulates root architecture both slowly, through actions at the transcriptional level and, more rapidly, by mechanisms targeting primarily plasma membrane sensory systems and intracellular signaling pathways. The latter reactions use several second messengers, including Ca(2+) , nitric oxide (NO) and reactive oxygen species (ROS). Here, we investigated the different roles of two auxins, the major auxin indole-3-acetic acid (IAA) and another endogenous auxin indole-3-butyric acid (IBA), in the lateral root formation process of Arabidopsis and maize. This was mainly analyzed by different types of fluorescence microscopy and inhibitors of NO production. This study revealed that peroxisomal IBA to IAA conversion is followed by peroxisomal NO, which is important for IBA-induced lateral root formation. We conclude that peroxisomal NO emerges as a new player in auxin-induced root organogenesis. In particular, the spatially and temporally coordinated release of NO and IAA from peroxisomes is behind the strong promotion of lateral root formation via IBA.

Stress responses in the opportunistic pathogen Acinetobacter baumannii

Acinetobacter baumannii causes a wide range of severe infections among compromised and injured patients worldwide. The relevance of these infections are, in part, due to the ability of this pathogen to sense and react to environmental and host stress signals, allowing it to persist and disseminate in medical settings and the human host. This review summarizes current knowledge on the roles that environmental and cellular stressors play in the ability of A. baumannii to resist nutrient deprivation, oxidative and nitrosative injury, and even the presence of the commonly used antiseptic ethanol, which could serve as a nutrient- and virulence-enhancing signal rather than just being a convenient disinfectant. Emerging experimental evidence supports the role of some of these responses in the pathogenesis of the infections A. baumannii causes in humans and its capacity to resist antibiotics and host response effectors.

Carbon catabolite repression-independent and pH-dependent production of indoles by Rubrivivax benzoatilyticus JA2

Rubrivivax benzoatilyticus JA2 produces indole derivatives (indoles) from aniline, anthranilate or L-tryptophan. Glucose repressed indole production in R. benzoatilyticus JA2, while malate had no effect. Growth of R. benzoatilyticus JA2 on glucose resulted in decrease in culture pH (6.4) compared with malate (8.4). Growth of R. benzoatilyticus JA2 on sugar carbon sources decreased culture pH (6.4-6.6) and indole production. Further, culture pH of 6.4 repressed the indole production, and pH 8.4 promoted the production irrespective of carbon sources used for growth. Moreover, correlation between indole production and culture pH was observed, where acidic pH inhibited indole production, while alkaline pH promoted the production, suggesting the role of pH in indole production. Tryptophan-catabolizing enzyme activities are significantly high in malate-grown cultures (pH 8.4) compared with that of the glucose (pH 6.4)-grown cultures and corroborated well with indole production, indicating their role in indole production. These results confirm that indole production in R. benzoatilyticus JA2 is pH dependent rather than carbon catabolite repression.

Quorum sensing and indole-3-acetic acid degradation play a role in colonization and plant growth promotion of Arabidopsis thaliana by Burkholderia phytofirmans PsJN

Although not fully understood, molecular communication in the rhizosphere plays an important role regulating traits involved in plant-bacteria association. Burkholderia phytofirmans PsJN is a well-known plant-growth-promoting bacterium, which establishes rhizospheric and endophytic colonization in different plants. A competent colonization is essential for plant-growth-promoting effects produced by bacteria. Using appropriate mutant strains of B. phytofirmans, we obtained evidence for the importance of N-acyl homoserine lactone-mediated (quorum sensing) cell-to-cell communication in efficient colonization of Arabidopsis thaliana plants and the establishment of a beneficial interaction. We also observed that bacterial degradation of the auxin indole-3-acetic acid (IAA) plays a key role in plant-growth-promoting traits and is necessary for efficient rhizosphere colonization. Wildtype B. phytofirmans but not the iacC mutant in IAA mineralization is able to restore promotion effects in roots of A. thaliana in the presence of exogenously added IAA, indicating the importance of this trait for promoting primary root length. Using a transgenic A. thaliana line with suppressed auxin signaling (miR393) and analyzing the expression of auxin receptors in wild-type inoculated plants, we provide evidence that auxin signaling in plants is necessary for the growth promotion effects produced by B. phytofirmans. The interplay between ethylene and auxin signaling was also confirmed by the response of the plant to a 1-aminocyclopropane-1-carboxylate deaminase bacterial mutant strain.

Indole toxicity involves the inhibition of adenosine triphosphate production and protein folding in Pseudomonas putida

High concentrations of indole are known to be toxic to cells due to perturbations in membrane potential. Here, we report for the first time a transcriptome analysis of a soil model bacterium, Pseudomonas putida KT2440, under indole treatment. We demonstrated that 47 genes are differentially expressed, including 11 genes involved in the tricarboxylic acid cycle (TCA cycle) and 12 genes involved in chaperone and protease functions (hslV, hslU, htpG, grpE, dnaK, ibpA, groEL, groES, clpB, lon-1, lon-2, and hflk). Mutant analysis supported the observation that protease genes including hslU are essential for the indole resistance of Pseudomonas strains. Subsequent biochemical analyses have shown that indole increases the NADH/NAD(+) ratio and decreases the adenosine triphosphate (ATP) concentration inside cells, due to membrane perturbation and higher expression of TCA cycle genes in the presence of indole. This energy reduction leads to a reduction in cell size and an enhancement of biofilm formation in P. putida. The observed upregulation in many chaperones and proteases led us to speculate that protein folding might be inhibited by indole treatment. Interestingly, our in vitro protein-refolding assay using malate dehydrogenase with purified GroEL/GroES demonstrated that indole interferes with protein folding. Taken together, our data provide new evidence that indole causes toxicity to P. putida by inhibiting cellular energy production and protein folding.

Effect of ethanol on differential protein production and expression of potential virulence functions in the opportunistic pathogen Acinetobacter baumannii

Acinetobacter baumannii persists in the medical environment and causes severe human nosocomial infections. Previous studies showed that low-level ethanol exposure increases the virulence of A. baumannii ATCC 17978. To better understand the mechanisms involved in this response, 2-D gel electrophoresis combined with mass spectrometry was used to investigate differential protein production in bacteria cultured in the presence or absence of ethanol. This approach showed that the presence of ethanol significantly induces and represses the production of 22 and 12 proteins, respectively. Although over 25% of the ethanol-induced proteins were stress-response related, the overall bacterial viability was uncompromised when cultured under these conditions. Production of proteins involved in lipid and carbohydrate anabolism was increased in the presence of ethanol, a response that correlates with increased carbohydrate biofilm content, enhanced biofilm formation on abiotic surfaces and decrease bacterial motility on semi-solid surfaces. The presence of ethanol also induced the acidification of bacterial cultures and the production of indole-3-acetic acid (IAA), a ubiquitous plant hormone that signals bacterial stress-tolerance and promotes plant-bacteria interactions. These responses could be responsible for the significantly enhanced virulence of A. baumannii ATCC 17978 cells cultured in the presence of ethanol when tested with the Galleria mellonella experimental infection model. Taken together, these observations provide new insights into the effect of ethanol in bacterial virulence. This alcohol predisposes the human host to infections by A. baumannii and could favor the survival and adaptation of this pathogen to medical settings and adverse host environments.

Activity, distribution and function of indole-3-acetic acid biosynthetic pathways in bacteria

The capacity to produce the phytohormone indole-3-acetic acid (IAA) is widespread among bacteria that inhabit diverse environments such as soils, fresh and marine waters, and plant and animal hosts. Three major pathways for bacterial IAA synthesis have been characterized that remove the amino and carboxyl groups from the α-carbon of tryptophan via the intermediates indolepyruvate, indoleacetamide, or indoleacetonitrile; the oxidized end product IAA is typically secreted. The enzymes in these pathways often catabolize a broad range of substrates including aromatic amino acids and in some cases the branched chain amino acids. Moreover, expression of some of the genes encoding key IAA biosynthetic enzymes is induced by all three aromatic amino acids. The broad distribution and substrate specificity of the enzymes suggests a role for these pathways beyond plant-microbe interactions in which bacterial IAA has been best studied.

Non-genetic mechanisms communicating antibiotic resistance: rethinking strategies for antimicrobial drug design

INTRODUCTION

Infections by multidrug-resistant bacteria are of great concern worldwide. In many cases, resistance is not due to the presence of specific antibiotic-modifying enzymes, but rather associated with a general impermeability of the bacterial cell envelope. The molecular bases of this intrinsic resistance are not completely understood. Moreover, horizontal gene transfers cannot solely explain the spread of intrinsic resistance among bacterial strains.

AREAS COVERED

This review focuses on the increased intrinsic antibiotic resistance mediated by small molecules. These small molecules can either be secreted from bacterial cells of the same or different species (e.g., indole, polyamines, ammonia, and the Pseudomonas quinolone signal) or be present in the bacterial cell milieu, whether in the environment, such as indole acetic acid and other plant hormones, or in human tissues and body fluids, such as polyamines. These molecules are metabolic byproducts that act as infochemicals and modulate bacterial responses toward antibiotics leading to increasing or decreasing resistance levels.

EXPERT OPINION

The non-genetic mechanisms of antibiotic response modulation and communication discussed in this review should reorient our thinking of the mechanisms of intrinsic resistance to antibiotics and its spread across bacterial cell populations. The identification of chemical signals mediating increased intrinsic antibiotic resistance will expose novel critical targets for the development of new antimicrobial strategies.

Rhizosphere indole-3-acetic acid as a mediator in the Sorghum bicolor-phenanthrene-Sinorhizobium meliloti interactions

We studied a model system consisting of Sorghum bicolor, phenanthrene, and an auxin-producing polycyclic aromatic hydrocarbon-degrading Sinorhizobium meliloti strain to clarify whether rhizosphere indole-3-acetic acid (IAA) takes part in the plant-pollutant-bacteria interactions. Phenanthrene and S. meliloti treatments of sorghum contributed to a decrease in the rhizosphere IAA concentration and to phytohormone accumulation, respectively. Regression analysis showed significant correlations between alteration in root-zone IAA content and alterations in the root-surface area, exudation, and rhizosphere effects for culturable heterotrophic bacteria, the S. meliloti strain, and other phenanthrene degraders. According to the data obtained, phenanthrene degraders get an advantage over nondegradative rhizobacteria from IAA for rhizosphere colonization. An IAA-dependent increase in the root-surface area leads to improved sorghum growth under pollutant stress. The carbon flux from the roots is corrected by the auxin because of its influence on the exuding-surface area and on the intensity of secretion by the root cells. On the other hand, the rhizosphere IAA pool may be plant-regulated by means of alteration in carboxylate exudation and its influence on bacterial auxin production. A scenario for the IAA-mediated S. bicolor-phenanthrene-S. meliloti interactions is proposed.

Transcriptome analysis of the rhizosphere bacterium Azospirillum brasilense reveals an extensive auxin response

The rhizosphere bacterium Azospirillum brasilense produces the auxin indole-3-acetic acid (IAA) through the indole-3-pyruvate pathway. As we previously demonstrated that transcription of the indole-3-pyruvate decarboxylase (ipdC) gene is positively regulated by IAA, produced by A. brasilense itself or added exogenously, we performed a microarray analysis to study the overall effects of IAA on the transcriptome of A. brasilense. The transcriptomes of A. brasilense wild-type and the ipdC knockout mutant, both cultured in the absence and presence of exogenously added IAA, were compared.Interfering with the IAA biosynthesis/homeostasis in A. brasilense through inactivation of the ipdC gene or IAA addition results in much broader transcriptional changes than anticipated. Based on the multitude of changes observed by comparing the different transcriptomes, we can conclude that IAA is a signaling molecule in A. brasilense. It appears that the bacterium, when exposed to IAA, adapts itself to the plant rhizosphere, by changing its arsenal of transport proteins and cell surface proteins. A striking example of adaptation to IAA exposure, as happens in the rhizosphere, is the upregulation of a type VI secretion system (T6SS) in the presence of IAA. The T6SS is described as specifically involved in bacterium-eukaryotic host interactions. Additionally, many transcription factors show an altered regulation as well, indicating that the regulatory machinery of the bacterium is changing.

Auxin and plant-microbe interactions

Microbial synthesis of the phytohormone auxin has been known for a long time. This property is best documented for bacteria that interact with plants because bacterial auxin can cause interference with the many plant developmental processes regulated by auxin. Auxin biosynthesis in bacteria can occur via multiple pathways as has been observed in plants. There is also increasing evidence that indole-3-acetic acid (IAA), the major naturally occurring auxin, is a signaling molecule in microorganisms because IAA affects gene expression in some microorganisms. Therefore, IAA can act as a reciprocal signaling molecule in microbe-plant interactions. Interest in microbial synthesis of auxin is also increasing in yet another recently discovered property of auxin in Arabidopsis. Down-regulation of auxin signaling is part of the plant defense system against phytopathogenic bacteria. Exogenous application of auxin, e.g., produced by the pathogen, enhances susceptibility to the bacterial pathogen.

Indole affects biofilm formation in bacteria

Biofilm is bacterial population adherent to each other and to surfaces or interfaces, often enclosed by a matrix. Various biomolecules contribute to the establishment of biofilms, yet the process of building a biofilm is still under active investigation. Indole is known as a metabolite of amino acid tryptophan, which, however, has recently been proved to participate in various aspects of bacterial life including virulence induction, cell cycle regulation, acid resistance, and especially, signaling biofilm formation. Moreover, indole is also proposed to be a novel signal involved in quorum sensing, a bacterial cooperation behavior sometimes concerning the biofilm formation. Here the signaling role and molecular mechanism of indole on bacterial biofilm formation are reviewed, as well discussed is its relation to bacterial living adaptivity.

Transcriptome analysis of parallel-evolved Escherichia coli strains under ethanol stress

Background

Understanding ethanol tolerance in microorganisms is important for the improvement of bioethanol production. Hence, we performed parallel-evolution experiments using Escherichia coli cells under ethanol stress to determine the phenotypic changes necessary for ethanol tolerance.

Results

After cultivation of 1,000 generations under 5% ethanol stress, we obtained 6 ethanol-tolerant strains that showed an approximately 2-fold increase in their specific growth rate in comparison with their ancestor. Expression analysis using microarrays revealed that common expression changes occurred during the adaptive evolution to the ethanol stress environment. Biosynthetic pathways of amino acids, including tryptophan, histidine, and branched-chain amino acids, were commonly up-regulated in the tolerant strains, suggesting that activating these pathways is involved in the development of ethanol tolerance. In support of this hypothesis, supplementation of isoleucine, tryptophan, and histidine to the culture medium increased the specific growth rate under ethanol stress. Furthermore, genes related to iron ion metabolism were commonly up-regulated in the tolerant strains, which suggests the change in intracellular redox state during adaptive evolution.

Conclusions

The common phenotypic changes in the ethanol-tolerant strains we identified could provide a fundamental basis for designing ethanol-tolerant strains for industrial purposes.

Improvement of phosphate solubilization and Medicago plant yield by an indole-3-acetic acid-overproducing strain of Sinorhizobium meliloti

Nitrogen (N) and phosphorus (P) are the most limiting factors for plant growth. Some microorganisms improve the uptake and availability of N and P, minimizing chemical fertilizer dependence. It has been published that the RD64 strain, a Sinorhizobium meliloti 1021 strain engineered to overproduce indole-3-acetic acid (IAA), showed improved nitrogen fixation ability compared to the wild-type 1021 strain. Here, we present data showing that RD64 is also highly effective in mobilizing P from insoluble sources, such as phosphate rock (PR). Under P-limiting conditions, the higher level of P-mobilizing activity of RD64 than of the 1021 wild-type strain is connected with the upregulation of genes coding for the high-affinity P transport system, the induction of acid phosphatase activity, and the increased secretion into the growth medium of malic, succinic, and fumaric acids. Medicago truncatula plants nodulated by RD64 (Mt-RD64), when grown under P-deficient conditions, released larger amounts of another P-solubilizing organic acid, 2-hydroxyglutaric acid, than plants nodulated by the wild-type strain (Mt-1021). It has already been shown that Mt-RD64 plants exhibited higher levels of dry-weight production than Mt-1021 plants. Here, we also report that P-starved Mt-RD64 plants show significant increases in both shoot and root fresh weights when compared to P-starved Mt-1021 plants. We discuss how, in a Rhizobium-legume model system, a balanced interplay of different factors linked to bacterial IAA overproduction rather than IAA production per se stimulates plant growth under stressful environmental conditions and, in particular, under P starvation.

Production of indole by wine-associated microorganisms under oenological conditions

A high concentration of indole has been linked to 'plastic-like' off-flavour in wines, predominantly in wines produced under sluggish fermentation conditions. The purpose of this study was to determine the ability of yeast and bacteria to form indole and whether tryptophan was required for indole accumulation during winemaking. Wine-associated yeast and bacteria species (Saccharomyces cerevisiae, Saccharomyces bayanus, Candida stellata, Hanseniaspora uvarum, Kluyveromyces thermoloterans, Oenococcus oeni, Lactobacillus lindneri, Pediococcus cerevisiae and Pediococcus parvulus) were screened for their potential to generate indole during alcoholic or malolactic fermentation. Tryptophan was required for the accumulation of indole in chemically defined medium, and all yeast and bacteria fermentations were able to accumulate indole. C. stellata showed the greatest potential for indole formation (1033 microg/L) and among the bacteria, the highest concentration was generated by L. lindneri (370 microg/L). Whether primary fermentation is the principle cause of indole formation remains to be determined. We hypothesise that during an efficient fermentation, indole is removed through catabolic metabolism, but, when a sluggish fermentation arises, non-Saccharomyces species might produce excess indole that is still present by end of fermentation.

Auxin-treatment induces recovery of phytoplasma-infected periwinkle

AIMS

To test the effect of auxin-treatment on plant pathogenic phytoplasmas and phytoplasma-infected host.

METHODS AND RESULTS

In vitro grown periwinkle shoots infected with different 'Candidatus Phytoplasma' species were treated with indole-3-acetic acid (IAA) or indole-3-butyric acid (IBA). Both auxins induced recovery of phytoplasma-infected periwinkle shoots, but IBA was more effective. The time period and concentration of the auxin needed to induce recovery was dependent on the 'Candidatus Phytoplasma' species and the type of auxin. Two 'Candidatus Phytoplasma' species, 'Ca. P. pruni' (strain KVI, clover phyllody from Italy) and 'Ca. P. asteris' (strain HYDB, hydrangea phyllody), were susceptible to auxin-treatment and undetected by nested PCR or detected only in the second nested PCR in the host tissue. 'Ca. P. solani' (strain SA-I, grapevine yellows) persisted in the host tissue despite the obvious recovery of the host plant and was always detected in the direct PCR.

CONCLUSIONS

Both auxins induced recovery of phytoplasma-infected plants and affected tested 'Candidatus Phytoplasma' species in the same manner, implying that the mechanism involved in phytoplasma elimination/survival is common to both, IAA and IBA.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results imply that in the case of some 'Candidatus Phytoplasma' species, IBA-treatment could be used to eliminate phytoplasmas from in vitro grown Catharanthus roseus shoots.

Legumes like more IAA

The improvement of the effectiveness and survival of rhizobia in the rhizosphere of legume host plants is a common practice in agricultural legume production. We have recently reported that the overexpression of IAA in S. meliloti 1021 played a positive role in the adaptation to various stress conditions both in free-living bacteria and in nodulated plants. We show here that IAA triggers the coordinate enhancement of various cellular defense systems and that IAA-treated bacteria are more resistant to desiccation. In addition, Medicago plants nodulated by the IAA-overproducing strain RD64 (Mt-RD64), restore auxin/cytokinin balance by increasing the transcription of cytokinin signaling genes. Finally, we bring here that RD64 is less competitive in comparison to the wild type strain under normal conditions, but it works better under salt stress conditions.

Effects of indole-3-acetic acid on Sinorhizobium meliloti survival and on symbiotic nitrogen fixation and stem dry weight production

We evaluated the effects of the main auxin phytohormone, indole-3-acetic acid (IAA), on the central metabolism of Sinorhizobium meliloti 1021. We either treated S. meliloti 1021 wild-type cells with 0.5 mM IAA, 1021+, or use a derivative, RD64, of the same strain harboring an additional pathway for IAA biosynthesis (converting tryptophan into IAA via indoleacetamide). We assayed the activity of tricarboxylic acid cycle (TCA) key enzymes and found that activity of citrate synthase and alpha-ketoglutarate dehydrogenase were increased in both 1021+ and RD64 as compared to the wild-type strain. We also showed that the intracellular acetyl-CoA content was enhanced in both RD64 and 1021+ strains when compared to the control strain. The activity of key enzymes, utilizing acetyl-CoA for poly-beta-hydroxybutyrate (PHB) biosynthesis, was also induced. The PHB level measured in these cells were significantly higher than that found in control cells. Moreover, 4-week-long survival experiments showed that 80% of 1021 cells died, whereas 50% of RD64 cells were viable. Medicago truncatula plants nodulated by RD64 (Mt-RD64) showed an induction of both acetylene reduction activity and stem dry weight production.

Medicago truncatula improves salt tolerance when nodulated by an indole-3-acetic acid-overproducing Sinorhizobium meliloti strain

The abiotic stress resistance of wild-type Sinorhizobium meliloti 1021 was compared with that of RD64, a derivative of the 1021 strain harbouring an additional pathway for the synthesis of indole-3-acetic acid (IAA), expressed in both free-living bacteria and bacteroids. It is shown here that the IAA-overproducing RD64 strain accumulated a higher level of trehalose as its endogenous osmolyte and showed an increased tolerance to several stress conditions (55 degrees C, 4 degrees C, UV-irradiation, 0.5 M NaCl, and pH 3). Medicago truncatula plants nodulated by RD64 (Mt-RD64) showed re-modulation of phytohormones, with a higher IAA content in nodules and roots and a decreased IAA level in shoots as compared with plants nodulated by the wild-type strain 1021 (Mt-1021). The response of nodulated M. truncatula plants to salt stress, when 0.3 M NaCl was applied, was analysed. For Mt-RD64 plants higher internal proline contents, almost unchanged hydrogen peroxide levels, and enhanced activity of antioxidant enzymes (superoxide dismutase, total peroxidase, glutathione reductase, and ascorbate peroxidase) were found compared with Mt-1021 plants. These results were positively correlated with reduced symptoms of senescence, lower expression of ethylene signalling genes, lower reduction of shoot dry weight, and better nitrogen-fixing capacity observed for these plants. Upon re-watering, after 0.3 M NaCl treatment, Mt-1021 plants almost die whereas Mt-RD64 plants showed visual signs of recovery. Finally, the shoot dry weight of Mt-RD64 plants treated with 0.15 M NaCl was not statistically different from that of Mt-1021 plants grown under non-stressed conditions.

D'orenone blocks polarized tip growth of root hairs by interfering with the PIN2-mediated auxin transport network in the root apex

The C(18) ketone (5E,7E)-6-methyl-8-(2,6,6-trimethylcyclohex-1-enyl)octa-5,7-dien-2-one (D'orenone) has been postulated to be an early cleavage product of beta-carotene en route to trisporic acids; these act as morphogenetic factors during the sexual reproduction of zygomycetes. Here we report that D'orenone blocks the highly polarized tip growth of root hairs, causing tip growth to stop completely within a few minutes. Importantly, external auxin reverses the effects of D'orenone on root hairs. Further analysis revealed that D'orenone lowers the auxin concentration in trichoblasts via PIN2-mediated auxin efflux to below the critical levels essential for root hair growth. D'orenone specifically increases PIN2 protein abundance without affecting PIN2 transcripts, and the PIN2 expression domain enlarges and shifts basipetally, resulting in more active auxin transport. The observation that D'orenone does not interfere with the root hair growth in roots of null mutant lines provides additional evidence that PIN2 is its specific target.

Escherichia coli biofilms

Escherichia coli is a predominant species among facultative anaerobic bacteria of the gastrointestinal tract. Both its frequent community lifestyle and the availability of a wide array of genetic tools contributed to establish E. coli as a relevant model organism for the study of surface colonization. Several key factors, including different extracellular appendages, are implicated in E. coli surface colonization and their expression and activity are finely regulated, both in space and time, to ensure productive events leading to mature biofilm formation. This chapter will present known molecular mechanisms underlying biofilm development in both commensal and pathogenic E. coli.

Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis thaliana auxin physiology

The Pseudomonas syringae type III effector AvrRpt2 promotes bacterial virulence on Arabidopsis thaliana plants lacking a functional RPS2 gene (rps2 mutant plants). To investigate the mechanisms underlying the virulence activity of AvrRpt2, we examined the phenotypes of transgenic A. thaliana rps2 seedlings constitutively expressing AvrRpt2. These seedlings exhibited phenotypes reminiscent of A. thaliana mutants with altered auxin physiology, including longer primary roots, increased number of lateral roots, and increased sensitivity to exogenous auxin. They also had increased levels of free indole acetic acid (IAA). The presence of AvrRpt2 also was correlated with a further increase in free IAA levels during infection with P. syringae pv. tomato strain DC3000 (PstDC3000). These results indicate that AvrRpt2 alters A. thaliana auxin physiology. Application of the auxin analog 1-naphthaleneacetic acid promoted disease symptom development in PstDC3000-infected plants, suggesting that elevated auxin levels within host tissue promote PstDC3000 virulence. Thus, AvrRpt2 may be among the virulence factors of P. syringae that modulate host auxin physiology to promote disease.

Indole-3-acetic acid in microbial and microorganism-plant signaling

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phyto-stimulation. Reviewing the role of bacterial IAA in different microorganism-plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phyto-stimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.

YcfR (BhsA) influences Escherichia coli biofilm formation through stress response and surface hydrophobicity

DNA microarrays revealed that expression of ycfR, which encodes a putative outer membrane protein, is significantly induced in Escherichia coli biofilms and is also induced by several stress conditions. We show that deletion of ycfR increased biofilm formation fivefold in the presence of glucose; the glucose effect was corroborated by showing binding of the cyclic AMP receptor protein to the ycfR promoter. It appears that YcfR is a multiple stress resistance protein, since deleting ycfR also rendered the cell more sensitive to acid, heat treatment, hydrogen peroxide, and cadmium. Increased biofilm formation through YcfR due to stress appears to be the result of decreasing indole synthesis, since a mutation in the tnaA gene encoding tryptophanase prevented enhanced biofilm formation upon stress and adding indole prevented enhanced biofilm formation upon stress. Deleting ycfR also affected outer membrane proteins and converted the cell from hydrophilic to hydrophobic, as well as increased cell aggregation fourfold. YcfR seems to be involved in the regulation of E. coli K-12 biofilm formation by decreasing cell aggregation and cell surface adhesion, by influencing the concentration of signal molecules, and by interfering with stress responses. Based on our findings, we propose that this locus be named bhsA, for influencing biofilm through hydrophobicity and stress response.