Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte

Transcriptional profile of Pseudomonas syringae pv. phaseolicola NPS3121 in response to tissue extracts from a susceptible Phaseolus vulgaris L. cultivar

BACKGROUND

Pseudomonas syringae pv. phaseolicola is a Gram-negative plant-pathogenic bacterium that causes "halo blight" disease of beans (Phaseolus vulgaris L.). This disease affects both foliage and pods, and is a major problem in temperate areas of the world. Although several bacterial genes have been determined as participants in pathogenesis, the overall process still remains poorly understood, mainly because the identity and function of many of the genes are largely unknown. In this work, a genomic library of P. syringae pv. phaseolicola NPS3121 was constructed and PCR amplification of individual fragments was carried out in order to print a DNA microarray. This microarray was used to identify genes that are differentially expressed when bean leaf extracts, pod extracts or apoplastic fluid were added to the growth medium.

RESULTS

Transcription profiles show that 224 genes were differentially expressed, the majority under the effect of bean leaf extract and apoplastic fluid. Some of the induced genes were previously known to be involved in the first stages of the bacterial-plant interaction and virulence. These include genes encoding type III secretion system proteins and genes involved in cell-wall degradation, phaseolotoxin synthesis and aerobic metabolism. On the other hand, most repressed genes were found to be involved in the uptake and metabolism of iron.

CONCLUSION

This study furthers the understanding of the mechanisms involved, responses and the metabolic adaptation that occurs during the interaction of P. syringae pv. phaseolicola with a susceptible host plant.

The evolution of Pseudomonas syringae host specificity and type III effector repertoires

The discovery 45 years ago that many Pseudomonas syringae pathovars elicit the hypersensitive response in plant species other than their hosts fostered the use of these bacteria as experimental models. However, the basis for host specificity and the corresponding resistance of nonhosts remain unclear. Pseudomonas syringae is now known to inject into the host cytoplasm, via the type III secretion system, effector proteins that suppress basal innate immunity, but may be recognized by cognate resistance (R) proteins in a second level of defence. The identification and manipulation of complete repertoires of type III effectors have revealed the highly polymorphic nature of effector repertoires and their potential to limit the host range. However, the maintenance of compatible effector repertoires may be driven by adaptations to life in a given plant species involving many factors. Tools are now available to test several hypotheses for the nature and evolution of P. syringae host specificity and nonhost resistance.

Pseudomonas syringae pv. syringae B728a hydrolyses indole-3-acetonitrile to the plant hormone indole-3-acetic acid

Nitrilase enzymes catalyse the hydrolysis of nitrile compounds to the corresponding carboxylic acid and ammonia, and have been identified in plants, bacteria and fungi. There is mounting evidence to support a role for nitrilases in plant-microbe interactions, but the activity of these enzymes in plant pathogenic bacteria remains unexplored. The genomes of the plant pathogenic bacteria Pseudomonas syringae pv. syringae B728a and Pseudomonas syringae pv. tomato DC3000 contain nitrilase genes with high similarity to characterized bacterial arylacetonitrilases. In this study, we show that the nitrilase of P. syringae pv. syringae B728a is an arylacetonitrilase, which is capable of hydrolysing indole-3-acetonitrile to the plant hormone indole-3-acetic acid, and allows P. syringae pv. syringae B728a to use indole-3-acetonitrile as a nitrogen source. This enzyme may represent an additional mechanism for indole-3-acetic acid biosynthesis by P. syringae pv. syringae B728a, or may be used to degrade and assimilate aldoximes and nitriles produced during plant secondary metabolism. Nitrilase activity was not detected in P. syringae pv. tomato DC3000, despite the presence of a homologous nitrilase gene. This raises the interesting question of why nitrilase activity has been retained in P. syringae pv. syringae B728a and not in P. syringae pv. tomato DC3000.

Community proteogenomics reveals insights into the physiology of phyllosphere bacteria

Aerial plant surfaces represent the largest biological interface on Earth and provide essential services as sites of carbon dioxide fixation, molecular oxygen release, and primary biomass production. Rather than existing as axenic organisms, plants are colonized by microorganisms that affect both their health and growth. To gain insight into the physiology of phyllosphere bacteria under in situ conditions, we performed a culture-independent analysis of the microbiota associated with leaves of soybean, clover, and Arabidopsis thaliana plants using a metaproteogenomic approach. We found a high consistency of the communities on the 3 different plant species, both with respect to the predominant community members (including the alphaproteobacterial genera Sphingomonas and Methylo bacterium) and with respect to their proteomes. Observed known proteins of Methylobacterium were to a large extent related to the ability of these bacteria to use methanol as a source of carbon and energy. A remarkably high expression of various TonB-dependent receptors was observed for Sphingomonas. Because these outer membrane proteins are involved in transport processes of various carbohydrates, a particularly large substrate utilization pattern for Sphingomonads can be assumed to occur in the phyllosphere. These adaptations at the genus level can be expected to contribute to the success and coexistence of these 2 taxa on plant leaves. We anticipate that our results will form the basis for the identification of unique traits of phyllosphere bacteria, and for uncovering previously unrecorded mechanisms of bacteria-plant and bacteria-bacteria relationships.

Nanoparticles affect the survival of bacteria on leaf surfaces

The increasing presence of nanomaterial and nanoproducts makes it imperative to learn more about the associated impacts of these materials on human health and the environment. In this study, the effect of the nanomaterial TiO2 on the phyllosphere microbial community was investigated. Analysis results by PCR-denaturing gradient gel electrophoresis revealed a TiO2-induced change in the community structure of microorganisms. An epiphytic bacterium, Bacillus cereus 905, was chosen to study the role of the superoxide dismutase-encoding genes, sodA-1 and sodA-2, and its survival from TiO2 photocatalysis. Our results showed that the expression of sodA-1 and sodA-2 was induced by photocatalytic oxidation, with a higher induction observed in sodA-2. In addition, compared with wild-type B. cereus 905, a reduced bacterial population was observed in a sodA-1 and sodA-2 double deletion mutant strain KOS on a cucumber leaf surface sprayed with TiO2. Because the phyllosphere is considered as one of the major habitats for microorganisms, and substantial areas of the earth are covered with leaves, the results of this work provides information of the potential impact of photocatalytic nanomaterial in the environment.

Populations of Xanthomonas citri pv. mangiferaeindicae from asymptomatic mango leaves are primarily endophytic

Epiphytic survival of several Xanthomonas pathovars has been reported, but most studies failed to determine whether such populations were resident epiphytes, resulting from latent infections, or casual epiphytes. This study aimed at understanding the nature of Xanthomonas citri pv. mangiferaeindicae populations associated with asymptomatic leaves. When spray-inoculated on mango leaves cv. Maison Rouge, the pathogen multiplied markedly in association with juvenile leaves, but was most often detected as low population sizes (<1 x 10(3) cfu g(-1)) in association with mature leaves. Our results suggest a very low biological significance of biofilm-associated populations of X. citri pv. mangiferaeindicae, while saprophytic microbiota associated with mango leaves survived frequently as biofilms. A chloroform vapor-based disinfestation assay which kills cells specifically located on the leaf surface and not those located within the leaf mesophyll was developed. When applied to spray-inoculated leaves maintained under controlled environmental conditions, 155 out of the 168 analyzed datasets collected over three assessment dates for seven bacterial strains representative of the genetic diversity of the pathogen failed to demonstrate a significant X. citri pv. mangiferaeindicae population decrease on chloroform treated leaves up to 13 days after inoculation. We conclude that an efficient survival of X. citri pv. mangiferaeindicae present on mango leaf surfaces following a limited dissemination event is largely dependent on the availability of juvenile plant tissues. The bacterium gains access to protected sites (e.g., mesophyll) through stomata where it becomes endophytic and eventually causes disease. Chloroform vapor-based disinfestation assays should be useful for further studies aiming at evaluating survival sites of bacteria associated with the phyllosphere.

Adhesion and fitness in the bean phyllosphere and transmission to seed of Xanthomonas fuscans subsp. fuscans

Deciphering the mechanisms enabling plant-pathogenic bacteria to disperse, colonize, and survive on their hosts provides the necessary basis to set up new control methods. We evaluated the role of bacterial attachment and biofilm formation in host colonization processes for Xanthomonas fuscans subsp. fuscans on its host. This bacterium is responsible for the common bacterial blight of bean (Phaseolus vulgaris), a seedborne disease. The five adhesin genes (pilA, fhab, xadA1, xadA2, and yapH) identified in X. fuscans subsp. fuscans CFBP4834-R strain were mutated. All mutants were altered in their abilities to adhere to polypropylene or seed. PilA was involved in adhesion and transmission to seed, and mutation of pilA led to lower pathogenicity on bean. YapH was required for adhesion to seed, leaves, and abiotic surfaces but not for in planta transmission to seed or aggressiveness on leaves. Transmission to seed through floral structures did not require any of the known adhesins. Conversely, all mutants tested, except in yapH, were altered in their vascular transmission to seed. In conclusion, we showed that adhesins are implicated in the various processes leading to host phyllosphere colonization and transmission to seed by plant-pathogenic bacteria.

Nonculturable response of animal enteropathogens in the agricultural environment and implications for food safety

Concerns about animal enteropathogen contamination of fresh horticultural products have,increased worldwide and are mainly due to the ability of bacteria to survive under stress conditions in the agricultural environment and during raw-vegetable processing. This review challenges the idea that the viable but nonculturable phenomenon that has been proven to occur in plant-associated environments contributes to human pathogen survival and might be correlated with foodborne infection. Factors associated with the nonculturable response of bacteria in the field and during postharvest processing and distribution are discussed, specifically for the most common animal enteropathogens linked with the consumption of raw products: Escherichia coli O157:H7, Salmonella, Listeria monocytogenes, and Shigella spp. The accurate detection of live bacterial populations is essential for pathogen screening in food and environmental safety control and in epidemiological analysis and may have to be considered for identification of critical control points at the time of food inspection.

Visual evidence of horizontal gene transfer between plants and bacteria in the phytosphere of transplastomic tobacco

Plant surfaces, colonized by numerous and diverse bacterial species, are often considered hot spots for horizontal gene transfer (HGT) between plants and bacteria. Plant DNA released during the degradation of plant tissues can persist and remain biologically active for significant periods of time, suggesting that soil or plant-associated bacteria could be in direct contact with plant DNA. In addition, nutrients released during the decaying process may provide a copiotrophic environment conducive for opportunistic microbial growth. Using Acinetobacter baylyi strain BD413 and transplastomic tobacco plants harboring the aadA gene as models, the objective of this study was to determine whether specific niches could be shown to foster bacterial growth on intact or decaying plant tissues, to develop a competence state, and to possibly acquire exogenous plant DNA by natural transformation. Visualization of HGT in situ was performed using A. baylyi strain BD413(rbcL-DeltaPaadA::gfp) carrying a promoterless aadA::gfp fusion. Both antibiotic resistance and green fluorescence phenotypes were restored in recombinant bacterial cells after homologous recombination with transgenic plant DNA. Opportunistic growth occurred on decaying plant tissues, and a significant proportion of the bacteria developed a competence state. Quantification of transformants clearly supported the idea that the phytosphere constitutes a hot spot for HGT between plants and bacteria. The nondisruptive approach used to visualize transformants in situ provides new insights into environmental factors influencing HGT for plant tissues.

Acyl-homoserine lactone-mediated cross talk among epiphytic bacteria modulates behavior of Pseudomonas syringae on leaves

The leaf surface harbors a host of bacterial epiphytes that are capable of influencing the quorum sensing (QS) system of the plant pathogen Pseudomonas syringae pv. syringae (Pss). Pss uses QS to regulate expression of genes conferring extracellular polysaccharide production, motility and factors contributing to virulence to plants. About 7% of bacterial epiphytes isolated in this study produce the Pss cognate signal, 3-oxohexanoyl-homoserine lactone (3OC6HSL), often in amounts more than 10-fold higher than Pss. Premature induction of QS in Pss by these 3OC6HSL-producing epiphytes suppressed swarming motility and subsequent disease of the leaf. Co-inoculation of 3OC6HSL-producing strains with Pss reduced the number of lesions when inoculated together onto leaves compared with that of plants inoculated with Pss alone. Strains in which 3OC6HSL accumulation was quenched by expression of an N-acyl-homoserine lactonase did not decrease disease when co-inoculated with Pss. Disease incidence caused by a nonmotile mutant of Pss was not affected by 3OC6HSL-producing bacteria, suggesting that exogenous 3OC6HSL signal that altered the motility of Pss was responsible for reducing the apparent virulence of this pathogen. Thus, considerable cross talk involving exogenous 3OC6HSL occurs on leaves and this process can be exploited for disease control.

Pea aphid as both host and vector for the phytopathogenic bacterium Pseudomonas syringae

Aphids are widespread agricultural pests that are capable of disseminating plant viral diseases; however, despite coming into frequent contact with epiphytic bacteria, aphids are considered to have no role in bacterial transmission. Here, we demonstrate the ability of pea aphids to vector the phytopathogen Pseudomonas syringae pv. syringae B728a (PsyB728a). While feeding on plants colonized by epiphytic bacteria, aphids acquire the bacteria, which colonize the digestive tract, multiply, and are excreted in the aphid honeydew, resulting in inoculation of the phyllosphere with up to 10(7) phytopathogenic bacteria per cm(2). Within days of ingesting bacteria, aphids succumb to bacterial sepsis, indicating that aphids serve as an alternative, nonplant host for PsyB728a. The related strain Pseudomonas syringae pv. tomato DC3000 is >1,000-fold less virulent than PsyB728a in the pea aphid, suggesting that PsyB728a possesses strain-specific pathogenicity factors that allow it to exploit aphids as hosts. To identify these factors, we performed a mutagenesis screen and recovered PsyB728a mutants that were hypovirulent, including one defective in a gene required for flagellum formation and motility. These interactions illustrate that aphids can also vector bacterial pathogens and that even seemingly host-restricted pathogens can have alternative host specificities and lifestyles.

Bacterial diversity analysis of Huanglongbing pathogen-infected citrus, using PhyloChip arrays and 16S rRNA gene clone library sequencing

The bacterial diversity associated with citrus leaf midribs was characterized for citrus groves that contained the Huanglongbing (HLB) pathogen, which has yet to be cultivated in vitro. We employed a combination of high-density phylogenetic 16S rRNA gene microarrays and 16S rRNA gene clone library sequencing to determine the microbial community composition for symptomatic and asymptomatic citrus midribs. Our results revealed that citrus leaf midribs can support a diversity of microbes. PhyloChip analysis indicated that 47 orders of bacteria in 15 phyla were present in the citrus leaf midribs, while 20 orders in 8 phyla were observed with the cloning and sequencing method. PhyloChip arrays indicated that nine taxa were significantly more abundant in symptomatic midribs than in asymptomatic midribs. "Candidatus Liberibacter asiaticus" was detected at a very low level in asymptomatic plants but was over 200 times more abundant in symptomatic plants. The PhyloChip analysis results were further verified by sequencing 16S rRNA gene clone libraries, which indicated the dominance of "Candidatus Liberibacter asiaticus" in symptomatic leaves. These data implicate "Candidatus Liberibacter asiaticus" as the pathogen responsible for HLB disease.

The impacts of cypermethrin pesticide application on the non-target microbial community of the pepper plant phyllosphere

Although pesticides have been extensively used for controlling insects and disease pathogens of plants, little is known regarding the impacts of applying these pesticides on the microbial community in the plant phyllosphere. Here, we report the effects of cypermethrin pesticide application upon the microbial community of the pepper plant phyllosphere. Assessments were made using culture-independent techniques including phospholipid fatty acid analysis (PLFA) and 16S rRNA gene directed Polymerase Chain Reaction with Denaturing Gradient Gel Electrophoresis (PCR-DGGE). During the 21 day greenhouse study, PLFA results indicated that both total and bacterial biomass increased after application of the pesticide. PLFA profiles also indicated that Gram-negative bacteria became predominant. DGGE analysis confirmed a significant change in bacterial community structure within the phyllosphere following the pesticide application where different dendrogram clusters were observed between control and treated samples. Phylogenetic analysis also suggested a change in bacterial phyla following treatment, where bands sequenced within control cultures were predominantly of the Firmicutes phylum, but those bands sequenced in the treated samples were predominantly members of the Bacteroidetes and gamma-Proteobacteria phyla. In conclusion, this study revealed an increase in bacterial abundance and a shift in community composition within the pepper plant phyllosphere following the pesticide application, and highlighted the effective use of PLFA and PCR-DGGE for studying the effect of pesticides upon indigenous phyllosphere microbes.

Environmental control in tea fields to reduce infection by Pseudomonas syringae pv. theae

Bacterial shoot blight (BSB) disease, caused by Pseudomonas syringae pv. theae, is a major bacterial disease of tea plants in Japan. BSB mainly occurs in the low-temperature season, and lesion formation by P. syringae pv. theae is enhanced by both low temperature and the presence of ice nucleation-active Xanthomonas campestris (INAX), which catalyzes ice formation at -2 to -4 degrees C and is frequently co-isolated with P. syringae pv. theae from tea plants. Low temperature is thus the most important environmental factor influencing the incidence of BSB; however, the effects of low temperature on infection of the host by P. syringae pv. theae and of environmental controls in fields on the occurrence of the disease are poorly understood. In this study, we show that ice formation on tea leaves by INAX enhanced P. syringae pv. theae invasion into leaf tissue. The natural incidence of BSB in the field was closely related to early autumn frost. Frost protection in late autumn, which prevented ice formation on tea plants, significantly decreased the incidence of BSB, and frost protection combined with bactericide application held the incidence under the economic threshold level. Our data indicate that environmental control in the field based on microbial interactions in the host offers a new strategy for plant disease control.

A conserved mechanism for nitrile metabolism in bacteria and plants

Pseudomonas fluorescens SBW25 is a plant growth-promoting bacterium that efficiently colonizes the leaf surfaces and rhizosphere of a range of plants. Previous studies have identified a putative plant-induced nitrilase gene (pinA) in P. fluorescens SBW25 that is expressed in the rhizosphere of sugar beet plants. Nitrilase enzymes have been characterised in plants, bacteria and fungi and are thought to be important in detoxification of nitriles, utilisation of nitrogen and synthesis of plant hormones. We reveal that pinA is a NIT4-type nitrilase that catalyses the hydrolysis of beta-cyano-L-alanine, a nitrile common in the plant environment and an intermediate in the cyanide detoxification pathway in plants. In plants cyanide is converted to beta-cyano-L-alanine, which is subsequently detoxified to aspartic acid and ammonia by NIT4. In P. fluorescens SBW25 pinA is induced in the presence of beta-cyano-L-alanine, and the beta-cyano-L-alanine precursors cyanide and cysteine. pinA allows P. fluorescens SBW25 to use beta-cyano-L-alanine as a nitrogen source and to tolerate toxic concentrations of this nitrile. In addition, pinA is shown to complement a NIT4 mutation in Arabidopsis thaliana, enabling plants to grow in concentrations of beta-cyano-L-alanine that would otherwise prove lethal. Interestingly, over-expression of pinA in wild-type A. thaliana not only resulted in increased growth in high concentrations of beta-cyano-L-alanine, but also resulted in increased root elongation in the absence of exogenous beta-cyano-L-alanine, demonstrating that beta-cyano-L-alanine nitrilase activity can have a significant effect on root physiology and root development.

Two dissimilar N-acyl-homoserine lactone acylases of Pseudomonas syringae influence colony and biofilm morphology

Plant aerial surfaces comprise a complex habitat for microorganisms, and many plant-associated bacteria, such as the pathogen Pseudomonas syringae, exhibit density-dependent survival on leaves by utilizing quorum sensing (QS). QS is often mediated by diffusible signals called N-acyl-homoserine lactones (AHLs), and P. syringae utilizes N-3-oxo-hexanoyl-dl-homoserine lactone (3OC6HSL) to control traits influencing epiphytic fitness and virulence. The P. syringae pathovar syringae B728a genome sequence revealed two putative AHL acylases, termed HacA (Psyr_1971) and HacB (Psyr_4858), which are N-terminal nucleophile hydrolases that inactivate AHLs by cleaving their amide bonds. HacA is a secreted AHL acylase that degrades only long-chain (C > or = 8) AHLs, while HacB is not secreted and degrades all tested AHLs. Targeted disruptions of hacA, hacB, and hacA and hacB together do not alter endogenous 3OC6HSL levels under the tested conditions. Surprisingly, targeted disruptions of hacA alone and hacA and hacB together confer complementable phenotypes that are very similar to autoaggregative phenotypes seen in other species. While AHL acylases might enable P. syringae B728a to degrade signals of competing species and block expression of their QS-dependent traits, these enzymes also play fundamental roles in biofilm formation.

In vitro analysis of the interaction of Pseudomonas savastanoi pvs. savastanoi and nerii with micropropagated olive plants

This study assessed the use of in vitro olive plants to evaluate the virulence of Pseudomonas savastanoi pv. savastanoi strains isolated from olive and P. savastanoi pv. nerii strains isolated from oleander knots. First, different olive isolates were inoculated into stem wounds and differences in knot formation and weight of overgrowths were observed for the selected strains. Tissue proliferation was clearly visible in all inoculated plants 30 days after inoculation. Virulence of P. savastanoi pv. nerii mutants with defects in regard to biosynthesis of indole-3-acetic acid and/or cytokinins was tested using this system. In agreement with data previously reported, all mutant strains multiplied in olive but induced attenuated symptoms. To analyze the virulence of P. savastanoi pv. savastanoi affected in their ability to grow in olive tissue, a trpE tryptophan auxotroph mutant was generated using a collection of signature tagged mutagenesis transposons. Virulence of this mutant was clearly reduced as evidenced by swelling of the olive tissue that evolved into attenuated knots. Furthermore, mixed infections with its parental strain revealed that the wild-type strain completely out-competed the trpE mutant. Results shown here demonstrate the usefulness of in vitro olive plants for the analysis of P. savastanoi pvs. savastanoi and nerii virulence. In addition, this system offers the possibility of quantifying virulence differences as weight of overgrowths. Moreover, we established the basis for the use of mixed infections in combination with signature tagged mutagenesis for high-throughput functional genomic analysis of this bacterial pathogen.

Roadmap to new virulence determinants in Pseudomonas syringae: insights from comparative genomics and genome organization

Systematic comparison of the current repertoire of virulence-associated genes for three Pseudomonas syringae strains with complete genome sequences, P. syringae pv. tomato DC3,000, P. syringae pv. phaseolicola 1448A, and P. syringae pv. syringae B728a, is prompted by recent advances in virulence factor identification in P. syringae and other bacteria. Among these are genes linked to epiphytic fitness, plant- and insect-active toxins, secretion pathways, and virulence regulators, all reflected in the recently updated DC3,000 genome annotation. Distribution of virulence genes in relation to P. syringae genome organization was analyzed to distinguish patterns of conservation among genomes and association between genes and mobile genetic elements. Variable regions were identified on the basis of deviation in sequence composition and gaps in syntenic alignment among the three genomes. Mapping gene location relative to the genome structure revealed strong segregation of the HrpL regulon with variable genome regions (VR), divergent distribution patterns for toxin genes depending on association with plant or insect pathogenesis, and patterns of distribution for other virulence genes that highlight potential sources of strain-to-strain differences in host interaction. Distribution of VR among other sequenced bacterial genomes was analyzed and future plans for characterization of this potential reservoir of virulence genes are discussed.

Characterization of the RND-type multidrug efflux pump MexAB-OprM of the plant pathogen Pseudomonas syringae

In gram-negative bacteria, transporters belonging to the RND family are the transporters most relevant for resistance to antimicrobial compounds. In Pseudomonas aeruginosa, a clinically important pathogen, the RND-type pump MexAB-OprM has been recognized as one of the major multidrug efflux systems. Here, homologues of MexAB-OprM in the plant pathogens Pseudomonas syringae pv. phaseolicola 1448A, P. syringae pv. syringae B728a, and P. syringae pv. tomato DC3000 were identified, and mexAB-oprM-deficient mutants were generated. Determination of MICs revealed that mutation of MexAB-OprM dramatically reduced the tolerance to a broad range of antimicrobials. Moreover, the ability of the mexAB-oprM-deficient mutants to multiply in planta was reduced. RNA dot blot hybridization revealed growth-dependent regulation of the mexAB-oprM operon in P. syringae; the expression of this operon was maximal in early exponential phase and decreased gradually during further growth.

Early detection of bean infection by Pseudomonas syringae in asymptomatic leaf areas using chlorophyll fluorescence imaging

Chlorophyll fluorescence imaging has been used to analyse the response elicited in Phaseolus vulgaris after inoculation with Pseudomonas syringae pv. phaseolicola 1448A (compatible interaction) and P. syringae pv. tomato DC3000 (incompatible interaction). With the aim of modulating timing of symptom development, different cell densities were used to inoculate bean plants and the population dynamics of both bacterial strains was followed within the leaf tissue. Fluorescence quenching analysis was carried out and images of the different chlorophyll fluorescence parameters were obtained for infected as well as control plants at different timepoints post-infection. Among the different parameters analysed, we observed that non-photochemical quenching maximised the differences between the compatible and the incompatible interaction before the appearance of visual symptom. A decrease in non-photochemical quenching, evident in both infiltrated and non-infiltrated leaf areas, was observed in P. syringae pv. phaseolicola-infected plants as compared with corresponding values from controls and P. syringae pv. tomato-infected plants. No photoinhibitory damage was detected, as the maximum photosystem II quantum yield remained stable during the infection period analysed.

Naturally occurring nonpathogenic isolates of the plant pathogen Pseudomonas syringae lack a type III secretion system and effector gene orthologues

Pseudomonas syringae causes plant diseases, and the main virulence mechanism is a type III secretion system (T3SS) that translocates dozens of effector proteins into plant cells. Here we report the existence of a subgroup of P. syringae isolates that do not cause disease on any plant species tested. This group is monophyletic and most likely evolved from a pathogenic P. syringae ancestor through loss of the T3SS. In the nonpathogenic isolate P. syringae 508 the genomic region that in pathogenic P. syringae strains contains the hrp-hrc cluster coding for the T3SS and flanking effector genes is absent. P. syringae 508 was also surveyed for the presence of effector orthologues from the closely related pathogenic strain P. syringae pv. syringae B728a, but none were detected. The absence of the hrp-hrc cluster and effector orthologues was confirmed for other nonpathogenic isolates. Using the AvrRpt2 effector as reporter revealed the inability of P. syringae 508 to translocate effectors into plant cells. Adding a plasmid-encoded T3SS and the P. syringae pv. syringae 61 effector gene hopA1 increased in planta growth almost 10-fold. This suggests that P. syringae 508 supplemented with a T3SS could be used to determine functions of individual effectors in the context of a plant infection, avoiding the confounding effect of other effectors with similar functions present in effector mutants of pathogenic isolates.

Leaf age as a risk factor in contamination of lettuce with Escherichia coli O157:H7 and Salmonella enterica

Outbreaks of Escherichia coli O157:H7 infections have been linked increasingly to leafy greens, particularly to lettuce. We present here the first evidence that this enteric pathogen can multiply on the leaves of romaine lettuce plants. The increases in population size of E. coli O157:H7 in the phyllosphere of young lettuce plants ranged from 16- to 100-fold under conditions of warm temperature and the presence of free water on the leaves and varied significantly with leaf age. The population size was consistently ca. 10-fold higher on the young (inner) leaves than on the middle leaves. The growth rates of Salmonella enterica and of the natural bacterial microflora were similarly leaf age dependent. Both enteric pathogens also achieved higher population sizes on young leaves than on middle leaves harvested from mature lettuce heads, suggesting that leaf age affects preharvest as well as postharvest colonization. Elemental analysis of the exudates collected from the surfaces of leaves of different ages revealed that young-leaf exudates were 2.9 and 1.5 times richer in total nitrogen and carbon, respectively, than middle-leaf exudates. This trend mirrored the nitrogen and carbon content of the leaf tissue. Application of ammonium nitrate, but not glucose, to middle leaves enhanced the growth of E. coli O157:H7 significantly, suggesting that low nitrogen limits its growth on these leaves. Our results indicate that leaf age and nitrogen content contribute to shaping the bacterial communities of preharvest and postharvest lettuce and that young lettuce leaves may be associated with a greater risk of contamination with E. coli O157:H7.

Interactions between food-borne pathogens and protozoa isolated from lettuce and spinach

The survival of Salmonella enterica was recently shown to increase when the bacteria were sequestered in expelled food vacuoles (vesicles) of Tetrahymena. Because fresh produce is increasingly linked to outbreaks of enteric illness, the present investigation aimed to determine the prevalence of protozoa on spinach and lettuce and to examine their interactions with S. enterica, Escherichia coli O157:H7, and Listeria monocytogenes. Glaucoma sp., Colpoda steinii, and Acanthamoeba palestinensis were cultured from store-bought spinach and lettuce and used in our study. A strain of Tetrahymena pyriformis previously isolated from spinach and a soil-borne Tetrahymena sp. were also used. Washed protozoa were allowed to graze on green fluorescent protein- or red fluorescent protein-labeled enteric pathogens. Significant differences in interactions among the various protist-enteric pathogen combinations were observed. Vesicles were produced by Glaucoma with all of the bacterial strains, although L. monocytogenes resulted in the smallest number per ciliate. Vesicle production was observed also during grazing of Tetrahymena on E. coli O157:H7 and S. enterica but not during grazing on L. monocytogenes, in vitro and on leaves. All vesicles contained intact fluorescing bacteria. In contrast, C. steinii and the amoeba did not produce vesicles from any of the enteric pathogens, nor were pathogens trapped within their cysts. Studies of the fate of E. coli O157:H7 in expelled vesicles revealed that by 4 h after addition of spinach extract, the bacteria multiplied and escaped the vesicles. The presence of protozoa on leafy vegetables and their sequestration of enteric bacteria in vesicles indicate that they may play an important role in the ecology of human pathogens on produce.

A novel gene, phcA from Pseudomonas syringae induces programmed cell death in the filamentous fungus Neurospora crassa

The phytopathogen Pseudomonas syringae competes with other epiphytic organisms, such as filamentous fungi, for resources. Here we characterize a gene in P. syringae pv. syringae B728a and P. syringae pv. tomato DC3000, termed phcA, that has homology to a filamentous fungal gene called het-c. phcA is conserved in many P. syringae strains, but is absent in one of the major clades, which includes the P. syringae pathovar phaseolicola. In the filamentous fungus Neurospora crassa, HET-C regulates a conserved programmed cell death pathway called heterokaryon incompatibility (HI). Ectopic expression of phcA in N. crassa induced HI and cell death that was dependent on the presence of a functional het-c pin-c haplotype. Further, by co-immunoprecipitation experiments, a heterocomplex between N. crassa HET-C1 and PhcA was associated with phcA-induced HI. P. syringae was able to attach and extensively colonize N. crassa hyphae, while an Escherichia coli control showed no association with the fungus. We further show that the P. syringae is able to use N. crassa as a sole nutrient source. Our results suggest that P. syringae has the potential to utilize phcA to acquire nutrients from fungi in nutrient-limited environments like the phyllosphere by the novel mechanism of HI induction.

The Type III secretion system of Xanthomonas fuscans subsp. fuscans is involved in the phyllosphere colonization process and in transmission to seeds of susceptible beans

Understanding the survival, multiplication, and transmission to seeds of plant pathogenic bacteria is central to study their pathogenesis. We hypothesized that the type III secretion system (T3SS), encoded by hrp genes, could have a role in host colonization by plant pathogenic bacteria. The seed-borne pathogen Xanthomonas fuscans subsp. fuscans causes common bacterial blight of bean (Phaseolus vulgaris). Directed mutagenesis in strain CFBP4834-R of X. fuscans subsp. fuscans and bacterial population density monitoring on bean leaves showed that strains with mutations in the hrp regulatory genes, hrpG and hrpX, were impaired in their phyllospheric growth, as in the null interaction with Escherichia coli C600 and bean. In the compatible interaction, CFBP4834-R reached high phyllospheric population densities and was transmitted to seeds at high frequencies with high densities. Strains with mutations in structural hrp genes maintained the same constant epiphytic population densities (1 x 10(5) CFU g(-1) of fresh weight) as in the incompatible interaction with Xanthomonas campestris pv. campestris ATCC 33913 and the bean. Low frequencies of transmission to seeds and low bacterial concentrations were recorded for CFBP4834-R hrp mutants and for ATCC 33913, whereas E. coli C600 was not transmitted. Moreover, unlike the wild-type strain, strains with mutations in hrp genes were not transmitted to seeds by vascular pathway. Transmission to seeds by floral structures remained possible for both. This study revealed the involvement of the X. fuscans subsp. fuscans T3SS in phyllospheric multiplication and systemic colonization of bean, leading to transmission to seeds. Our findings suggest a major contribution of hrp regulatory genes in host colonization processes.

The life history of the plant pathogen Pseudomonas syringae is linked to the water cycle

Pseudomonas syringae is a plant pathogen well known for its capacity to grow epiphytically on diverse plants and for its ice-nucleation activity. The ensemble of its known biology and ecology led us to postulate that this bacterium is also present in non-agricultural habitats, particularly those associated with water. Here, we report the abundance of P. syringae in rain, snow, alpine streams and lakes and in wild plants, in addition to the previously reported abundance in epilithic biofilms. Each of these substrates harbored strains that corresponded to P. syringae in terms of biochemical traits, pathogenicity and pathogenicity-related factors and that were ice-nucleation active. Phylogenetic comparisons of sequences of four housekeeping genes of the non-agricultural strains with strains of P. syringae from disease epidemics confirmed their identity as P. syringae. Moreover, strains belonging to the same clonal lineage were isolated from snow, irrigation water and a diseased crop plant. Our data suggest that the different substrates harboring P. syringae modify the structure of the associated populations. Here, we propose a comprehensive life cycle for P. syringae--in agricultural and non-agricultural habitats--driven by the environmental cycle of water. This cycle opens the opportunity to evaluate the importance of non-agricultural habitats in the evolution of a plant pathogen and the emergence of virulence. The ice-nucleation activity of all strains from snow, unlike from other substrates, strongly suggests that P. syringae plays an active role in the water cycle as an ice nucleus in clouds.

Role of soil, crop debris, and a plant pathogen in Salmonella enterica contamination of tomato plants

BACKGROUND

In the U.S., tomatoes have become the most implicated vehicle for produce-associated Salmonellosis with 12 outbreaks since 1998. Although unconfirmed, trace backs suggest pre-harvest contamination with Salmonella enterica. Routes of tomato crop contamination by S. enterica in the absence of direct artificial inoculation have not been investigated.

METHODOLOGY/PRINCIPAL FINDINGS

This work examined the role of contaminated soil, the potential for crop debris to act as inoculum from one crop to the next, and any interaction between the seedbourne plant pathogen Xanthomonas campestris pv. vesicatoria and S. enterica on tomato plants. Our results show S. enterica can survive for up to six weeks in fallow soil with the ability to contaminate tomato plants. We found S. enterica can contaminate a subsequent crop via crop debris; however a fallow period between crop incorporation and subsequent seeding can affect contamination patterns. Throughout these studies, populations of S. enterica declined over time and there was no bacterial growth in either the phyllosphere or rhizoplane. The presence of X. campestris pv. vesicatoria on co-colonized tomato plants had no effect on the incidence of S. enterica tomato phyllosphere contamination. However, growth of S. enterica in the tomato phyllosphere occurred on co-colonized plants in the absence of plant disease.

CONCLUSIONS/SIGNIFICANCE

S. enterica contaminated soil can lead to contamination of the tomato phyllosphere. A six week lag period between soil contamination and tomato seeding did not deter subsequent crop contamination. In the absence of plant disease, presence of the bacterial plant pathogen, X. campestris pv. vesicatoria was beneficial to S. enterica allowing multiplication of the human pathogen population. Any event leading to soil contamination with S. enterica could pose a public health risk with subsequent tomato production, especially in areas prone to bacterial spot disease.

Quorum size of Pseudomonas syringae is small and dictated by water availability on the leaf surface

The paradigm of bacterial quorum sensing (QS), which mediates cell-density-dependent gene expression, usually has been studied in high-cell-density planktonic liquid cultures or in biofilms in which signal concentrations accumulate to sufficiently high levels to induce QS. Presumably under conditions with restricted diffusion of the signal molecule, smaller population sizes could achieve such a state of QS induction. The plant-pathogenic bacterium Pseudomonas syringae, in which QS controls traits involved in epiphytic fitness and virulence, occurs on leaf surfaces in aggregates of various sizes. Because leaves often harbor limited surface water, we investigated the size of aggregates that would permit QS in a nonsaturated environment. QS induction was visualized via dual fluorescence of P. syringae cells harboring a transcriptional fusion of mRFP1 with ahlI, which exhibits N-acyl homoserine lactone-dependent transcriptional activity, and a constitutive GFP marker to account for all P. syringae cells on a leaf. Confocal microscopy revealed that, on wet leaves, no QS induction was evident within 2 days after inoculation, but it increased rapidly with increasing aggregate sizes >40 and 22 cells per aggregate by 3 and 4 days, respectively. In contrast, QS induction was common in aggregates >33 cells by 2 days after inoculation on dry leaves and increased rapidly with increasing aggregate sizes >35 and 13 cells after 3 and 4 days, respectively. These observations demonstrate that small groups of cells experience QS conditions on dry leaves where signal diffusion is restricted. Quorum size of bacteria in non-water-saturated environments such as on leaves is small, and QS induction may be commonly operative.

Drop-size soda lakes: transient microbial habitats on a salt-secreting desert tree

We describe a hitherto unrecognized bacterial community, inhabiting the leaf surfaces of the salt-excreting desert tree Tamarix. High temperatures, strong radiation, and very low humidity dictate a daytime existence in complete desiccation, but damp nights allow the microbial population to proliferate in a sugar-rich, alkaline, and hypersaline solution, before drying up again after sunrise. The exclusively bacterial population contains many undescribed species and genera, but nevertheless appears to be characterized by relatively limited species diversity. Sequences of 16S rRNA genes from either isolates or total community DNA place the identified members of the community in five bacterial groups (Actinobacteria, Bacteroidetes, Firmicutes, alpha-, and gamma-Proteobacteria); in each of these, they concentrate in a very narrow branch that in most cases harbors organisms isolated from unrelated halophilic environments.

A blue light inducible two-component signal transduction system in the plant pathogen Pseudomonas syringae pv. tomato

The open reading frame PSPTO2896 from the plant pathogen Pseudomonas syringae pv. tomato encodes a protein of 534 amino acids showing all salient features of a blue light-driven two-component system. The N-terminal LOV (light, oxygen, voltage) domain, potentially binding a flavin chromophore, is followed by a histidine kinase (HK) motif and a response regulator (RR). The full-length protein (PST-LOV) and, separately, the RR and the LOV+HK part (PST-LOV(DeltaRR)) were heterologously expressed and functionally characterized. The two LOV proteins showed typical LOV-like spectra and photochemical reactions, with the blue light-driven, reversible formation of a covalent flavin-cysteine bond. The fluorescence changes in the lit state of full-length PST-LOV, but not in PST-LOV(DeltaRR), indicating a direct interaction between the LOV core and the RR module. Experiments performed with radioactive ATP uncover the light-driven kinase activity. For both PST-LOV and PST-LOV(DeltaRR), much more radioactivity is incorporated when the protein is in the lit state. Furthermore, addition of the RR domain to the fully phosphorylated PST-LOV(DeltaRR) leads to a very fast transfer of radioactivity, indicating a highly efficient HK activity and a tight interaction between PST-LOV(DeltaRR) and RR, possibly facilitated by the LOV core itself.

Pseudomonas syringae pv. tomato DC3000 uses constitutive and apoplast-induced nutrient assimilation pathways to catabolize nutrients that are abundant in the tomato apoplast

The plant apoplast is the intercellular space that surrounds plant cells, in which metabolic and physiological processes relating to cell wall biosynthesis, nutrient transport, and stress responses occur. The apoplast is also the primary site of infection for hemibiotrophic pathogens such as P. syringae, which obtain nutrients directly from apoplastic fluid. We have used apoplastic fluid extracted from healthy tomato leaves as a growth medium for Pseudomonas spp. in order to investigate the role of apoplastic nutrients in plant colonization by Pseudomonas syringae. We have confirmed that apoplast extracts mimic some of the environmental and nutritional conditions that bacteria encounter during apoplast colonization by demonstrating that expression of the plant-induced type III protein secretion pathway is upregulated during bacterial growth in apoplast extracts. We used a modified phenoarray technique to show that apoplast-adapted P. syringae pv. tomato DC3000 expresses nutrient utilization pathways that allow it to use sugars, organic acids, and amino acids that are highly abundant in the tomato apoplast. Comparative analyses of the nutrient utilization profiles of the genome-sequenced strains P. syringae pv. tomato DC3000, P. syringae pv. syringae B728a, P. syringae pv. phaseolicola 1448A, and the unsequenced strain P. syringae pv. tabaci 11528 with nine other genome-sequenced strains of Pseudomonas provide further evidence that P. syringae strains are adapted to use nutrients that are abundant in the leaf apoplast. Interestingly, P. syringae pv. phaseolicola 1448A lacks many of the nutrient utilization abilities that are present in three other P. syringae strains tested, which can be directly linked to differences in the P. syringae pv. phaseolicola 1448A genome.

The alternative sigma factor AlgT, but not alginate synthesis, promotes in planta multiplication of Pseudomonas syringae pv. glycinea

The phytopathogen Pseudomonas syringae pv. glycinea produces the exopolysaccharide (EPS) alginate, which is thought to function in epiphytic fitness and virulence. A key regulator for alginate biosynthesis in Pseudomonas aeruginosa and P. syringae is the alternative sigma factor AlgT (sigma(22)). In this study, the contribution of alginate synthesis and AlgT to in planta epiphytic fitness and virulence of P. syringae was examined. Alginate biosynthesis mutants were generated for the P. syringae pv. glycinea strains PG4180 and PG4180.muc, representing a comprehensive set of alginate- and AlgT-positive or -negative derivatives. Analysis of in vitro and in planta phenotypes revealed that AlgT strongly promoted in planta growth, survival and symptom development, but decreased the ability to grow in vitro. In contrast, alginate biosynthesis had only marginal impact. Quantitative in vitro and in planta gene expression analyses for alginate biosynthesis and algT were carried out at two temperatures in AlgT-negative and -positive backgrounds. algT as well as algD gene expression was AlgT-dependent, plant-inducible and temperature-dependent, with higher expression at 18 compared to 28 degrees C; however, no temperature dependence was observed in vitro. Our data suggest that AlgT may act as a global regulator for virulence and in planta fitness traits of P. syringae independent of its role in EPS biosynthesis.

The influence of exogenous nutrients on the abundance of yeasts on the phylloplane of turfgrass

Four experiments were conducted to assess the effect of foliar applications of various nutrient solutions on the phylloplane yeast community of tall fescue (Festuca arundinacea Schreb.). In the first three experiments, increasing concentrations of sucrose (2-16%), yeast extract (0.5-2.5%), and sucrose plus yeast extract (2.5-18.5% total) were applied and the yeast colony forming units (cfu) enumerated 14 h later by dilution plating. Significant positive linear relationships were observed between the number of yeast cfu and applications of both yeast extract and sucrose plus yeast extract. Foliar applications of sucrose alone had no significant effect on yeast community abundance, indicating that phylloplane yeasts of turfgrass are not limited by the amount or availability of carbohydrates. In the fourth experiment, five different solutions were applied to tall fescue to investigate the response of the yeast community to organic and inorganic nitrogen sources. Tryptone or yeast extract, both with considerable amino acid composition, significantly increased the yeast population, while yeast nitrogen base (with or without amino acids) and ammonium sulfate had no affect on yeast abundance. These results suggest that organic nitrogen stimulate yeast community growth and development on the phylloplane of tall fescue, while carbohydrates, inorganic nitrogen, and non-nitrogenous nutrients have little positive effect.

Effect of cypermethrin insecticide on the microbial community in cucumber phyllosphere

Cucumber (Cucumis sativus) is one of the most widely used vegetable in the world, and different pesticides have been extensively used for controlling the insects and disease pathogens of this plant. However, little is known about how the pesticides affect the microbial community in cucumber phyllosphere. This study was the first attempt to assess the impact of pyrethroid insecticide cyperemethrin on the microbial communities of cucumber phyllosphere using biochemical and genetic approaches. Phospholipid fatty acid (PLFA) assay indicated that cyperemethrin insecticide treatment led to a significant increase in both total and bacterial biomass and a decrease in fungal biomass and the ratio of Gram-positive (GP) bacteria to Gram-negative (GN) bacteria within the cucumber phyllosphere. Principal-component analyses (PCA) suggested that the number of unsaturated and cyclopropane PLFAs (16:1 omega 9t, 18:1 omega 7, cy17:0, cy19:0) increased with the insecticide treatment, whereas the saturated PLFA i16:0, i17:0 decreased. The increase of GN bacteria implied that the cypermethrin insecticide might be a nutrient for the growth of these phyllosphere microbes. Terminal restriction fragment length polymorphism (T-RFLP) reinforced the PLFA results. A significant change of bacterial community structure was observed in the separate dendrogram cluster between control and treated samples with the cucumber phyllosphere following cypermethrin insecticide treatment. Moreover, the increased terminal restriction fragments (T-RFs) (58, 62, 89, 99, 119, 195, 239, 311, 340, and 473 bp) indicated that some bacteria might play a significant role in the insecticide degradation within the cucumber phylosphere, whereas the disappeared T-RFs (44, 51, 96, 223, 306, and 338 bp) implied that some other bacteria might potentially serve as microbial indicator of cyperemethrin insecticide exposure.

Global genomic analysis of Pseudomonas savastanoi pv. savastanoi plasmids

Pseudomonas savastanoi pv. savastanoi strains harbor native plasmids belonging to the pPT23A plasmid family (PFPs) which are detected in all pathovars of the related species Pseudomonas syringae examined and contribute to the ecological and pathogenic fitness of their host. However, there is a general lack of information about the gene content of P. savastanoi pv. savastanoi plasmids and their role in the interaction of this pathogen with olive plants. We designed a DNA macroarray containing 135 plasmid-borne P. syringae genes to conduct a global genetic analysis of 32 plasmids obtained from 10 P. savastanoi pv. savastanoi strains. Hybridization results revealed that the number of PFPs per strain varied from one to four. Additionally, most strains contained at least one plasmid (designated non-PFP) that did not hybridize to the repA gene of pPT23A. Only three PFPs contained genes involved in the biosynthesis of the virulence factor indole-3-acetic acid (iaaM, iaaH, and iaaL). In contrast, ptz, a gene involved in the biosynthesis of cytokinins, was found in five PFPs and one non-PFP. Genes encoding a type IV secretion system (T4SS), type IVA, were found in both PFPs and non-PFPs; however, type IVB genes were found only on PFPs. Nine plasmids encoded both T4SSs, whereas seven other plasmids carried none of these genes. Most PFPs and non-PFPs hybridized to at least one putative type III secretion system effector gene and to a variety of additional genes encoding known P. syringae virulence factors and one or more insertion sequence transposase genes. These results indicate that non-PFPs may contribute to the virulence and fitness of the P. savastanoi pv. savastanoi host. The overall gene content of P. savastanoi pv. savastanoi plasmids, with their repeated information, mosaic arrangement, and insertion sequences, suggests a possible role in adaptation to a changing environment.

Role of stomata in plant innate immunity and foliar bacterial diseases

Pathogen entry into host tissue is a critical first step in causing infection. For foliar bacterial plant pathogens, natural surface openings, such as stomata, are important entry sites. Historically, these surface openings have been considered as passive portals of entry for plant pathogenic bacteria. However, recent studies have shown that stomata can play an active role in limiting bacterial invasion as part of the plant innate immune system. As a counter-defense, the plant pathogen Pseudomonas syringae pv. tomato DC3000 uses the virulence factor coronatine to actively open stomata. In nature, many foliar bacterial disease outbreaks require high humidity, rain, or storms, which could favor stomatal opening and/or bypass stomatal defense by creating wounds as alternative entry sites. Further studies on microbial and environmental regulation of stomatal closure and opening could fill gaps in our understanding of bacterial pathogenesis, disease epidemiology, and microbiology of the phyllosphere.

Salicylic acid and jasmonic acid signaling defense pathways reduce natural bacterial diversity on Arabidopsis thaliana

Terrestrial plants serve as large and diverse habitats for a wide range of pathogenic and nonpathogenic microbes, yet these communities are not well described and little is known about the effects of plant defense on microbial communities in nature. We designed a field experiment to determine how variation in two plant defense signaling pathways affects the size, diversity, and composition of the natural endophytic and epiphytic bacterial communities of Arabidopsis thaliana. To do this, we provide an initial characterization of these bacterial communities in one population in southwestern Michigan, United States, and we compare these two communities among A. thaliana mutants deficient in salicylic acid (SA) and jasmonic acid (JA) signaling defense pathways, controls, and plants with artificially elevated levels of defense. We identified 30 distinct bacterial groups on A. thaliana that differ in colony morphology and 16S rRNA sequence. We show that induction of SA-mediated defenses reduced endophytic bacterial community diversity, whereas plants deficient in JA-mediated defenses experienced greater epiphytic bacterial diversity. Furthermore, there was a positive relationship between total community size and diversity, indicating that relatively susceptible plants should, in general, harbor higher bacterial diversity. This experiment provides novel information about the ecology of bacteria on A. thaliana and demonstrates that variation in two specific plant-signaling defense pathways can influence bacterial diversity on plants.

Glycine betaine improves Listeria monocytogenes tolerance to desiccation on parsley leaves independent of the osmolyte transporters BetL, Gbu and OpuC

AIMS

To investigate the effect of glycine betaine (GB) on the survival of Listeria monocytogenes on leaf surfaces under low relative humidity (RH).

METHODS AND RESULTS

The addition of GB (> or = 25 mmol l(-1)) improved the survival of L. monocytogenes under low RH on parsley leaves, thus suggesting that GB can improve the tolerance of L. monocytogenes to desiccation. Ten times less GB was needed to improve L. monocytogenes survival under low RH on nonbiological surfaces compared with parsley leaves, suggesting that, on the leaf surface, L. monocytogenes may have to compete for the available GB with autochthonous bacteria and/or the plant itself. Wild type and mutants carrying deletions in the three GB uptake systems, BetL, Gbu and OpuC, behaved similarly with and without added GB on parsley leaves (P > 0.05). In addition, preaccumulation of GB, triggered by osmotic stress prior to inoculation, failed to improve survival under low RH compared with osmotic stress without GB accumulation.

CONCLUSIONS

Exogenous GB had a protective effect on L. monocytogenes cells from desiccation during survival on parsley leaves. This effect was independent of intracellular GB accumulation by the known uptake systems.

SIGNIFICANCE AND IMPACT OF THE STUDY

Presence of GB could improve the survival of L. monocytogenes to desiccation on leaf surfaces and nonbiological surfaces.

Comparative genomic analysis of two-component regulatory proteins in Pseudomonas syringae

Background

Pseudomonas syringae is a widespread bacterial plant pathogen, and strains of P. syringae may be assigned to different pathovars based on host specificity among different plant species. The genomes of P. syringae pv. syringae (Psy) B728a, pv. tomato (Pto) DC3000 and pv. phaseolicola (Pph) 1448A have been recently sequenced providing a major resource for comparative genomic analysis. A mechanism commonly found in bacteria for signal transduction is the two-component system (TCS), which typically consists of a sensor histidine kinase (HK) and a response regulator (RR). P. syringae requires a complex array of TCS proteins to cope with diverse plant hosts, host responses, and environmental conditions.

Results

Based on the genomic data, pattern searches with Hidden Markov Model (HMM) profiles have been used to identify putative HKs and RRs. The genomes of Psy B728a, Pto DC3000 and Pph 1448A were found to contain a large number of genes encoding TCS proteins, and a core of complete TCS proteins were shared between these genomes: 30 putative TCS clusters, 11 orphan HKs, 33 orphan RRs, and 16 hybrid HKs. A close analysis of the distribution of genes encoding TCS proteins revealed important differences in TCS proteins among the three P. syringae pathovars.

Conclusion

In this article we present a thorough analysis of the identification and distribution of TCS proteins among the sequenced genomes of P. syringae. We have identified differences in TCS proteins among the three P. syringae pathovars that may contribute to their diverse host ranges and association with plant hosts. The identification and analysis of the repertoire of TCS proteins in the genomes of P. syringae pathovars constitute a basis for future functional genomic studies of the signal transduction pathways in this important bacterial phytopathogen.

Viable but non-culturableListeria monocytogeneson parsley leaves and absence of recovery to a culturable state

AIMS

To investigate the presence of viable but non-culturable Listeria monocytogenes during survival on parsley leaves under low relative humidity (RH) and to evaluate the ability of L. monocytogenes to recover from VBNC to culturable state under satured humidity.

METHODS AND RESULTS

Under low RH (47-69%) on parsley leaves, the initial number of L. monocytogenes populations counted on non selective media (10(9) L. monocytogenes per leaf on TSA) was reduced by 6 log10 scales in 15 days, whereas number of viable L. monocytogenes counted under the microscope was reduced by 3-4 log10 scales, indicating the presence of VBNC cells. This was demonstrated on three L. monocytogenes strains (EGDe, Bug 1995 and LmP60). Changing from low to 100% RH permitted an increase of the culturable counts of L. monocytogenes and this growth was observed only when residual culturable cells were present. Moreover, VBNC L. monocytogenes inoculated on parsley leaves did not become culturable after incubation under 100% RH.

CONCLUSIONS

Dry conditions induced VBNC L. monocytogenes on parsley leaves but these VBNC were likely unable to recover culturability after transfer to satured humidity.

SIGNIFICANCE AND IMPACT OF STUDY

Enumeration on culture media presumably under-estimates the number of viable L. monocytogenes on fresh produce after exposure to low RH.

Salicylic acid, yersiniabactin, and pyoverdin production by the model phytopathogen Pseudomonas syringae pv. tomato DC3000: synthesis, regulation, and impact on tomato and Arabidopsis host plants

A genetically tractable model plant pathosystem, Pseudomonas syringae pv. tomato DC3000 on tomato and Arabidopsis thaliana hosts, was used to investigate the role of salicylic acid (SA) and iron acquisition via siderophores in bacterial virulence. Pathogen-induced SA accumulation mediates defense in these plants, and DC3000 contains the genes required for the synthesis of SA, the SA-incorporated siderophore yersiniabactin (Ybt), and the fluorescent siderophore pyoverdin (Pvd). We found that DC3000 synthesizes SA, Ybt, and Pvd under iron-limiting conditions in culture. Synthesis of SA and Ybt by DC3000 requires pchA, an isochorismate synthase gene in the Ybt genomic cluster, and exogenous SA can restore Ybt production by the pchA mutant. Ybt was also produced by DC3000 in planta, suggesting that Ybt plays a role in DC3000 pathogenesis. However, the pchA mutant did not exhibit any growth defect or altered virulence in plants. This lack of phenotype was not attributable to plant-produced SA restoring Ybt production, as the pchA mutant grew similarly to DC3000 in an Arabidopsis SA biosynthetic mutant, and in planta Ybt was not detected in pchA-infected wild-type plants. In culture, no growth defect was observed for the pchA mutant versus DC3000 for any condition tested. Instead, enhanced growth of the pchA mutant was observed under stringent iron limitation and additional stresses. This suggests that SA and Ybt production by DC3000 is costly and that Pvd is sufficient for iron acquisition. Further exploration of the comparative synthesis and utility of Ybt versus Pvd production by DC3000 found siderophore-dependent amplification of ybt gene expression to be absent, suggesting that Ybt may play a yet unknown role in DC3000 pathogenesis.