Thermodependence of growth and enzymatic activities implicated in pathogenicity of two Erwinia carotovora subspecies (Pectobacterium spp.)

Phyloproteomic study by MALDI-TOF MS in view of intraspecies variation in a significant homogenous phytopathogen Dickeya solani

Dickeya solani is an economically significant pectinolytic phytopathogen belonging to the Pectobacteriaceae family, which causes soft rot and blackleg diseases. Despite its notable impact on global potato production, there are no effective methods to control this pest. Here, we undertook a phyloproteomic study on 20 D. solani strains, of various origin and year of isolation, with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) supported by an in-depth characterization of the strains in terms of the virulence-associated phenotype. In spite of high homogeneity in this species, we herein revealed for the first time intraspecies variation in the MALDI-TOF MS protein profiles among the studied D. solani isolates. Finally, representative mass spectra for the four delineated clades are presented. A majority of the analysed D. solani strains showed high virulence potential, while two strains stood out in their growth dynamics, virulence factors production and ability to macerate plant tissue. Nonetheless, the metabolic profiles of D. solani strains turned out to be uniform, except for gelatinase activity. Given that all D. solani isolates distinctly grouped from the other Dickeya species in the MALDI-TOF MS analysis, there is strong evidence supporting the potential routine use of this method for fast and reliable to-species identification of D. solani isolates of environmental origin.

Pseudomonas fluorescens MFE01 uses 1-undecene as aerial communication molecule

Bacterial communication is a fundamental process used to synchronize gene expression and collective behavior among the bacterial population. The most studied bacterial communication system is quorum sensing, a cell density system, in which the concentration of inductors increases to a threshold level allowing detection by specific receptors. As a result, bacteria can change their behavior in a coordinated way. While in Pseudomonas quorum sensing based on the synthesis of N-acyl homoserine lactone molecules is well studied, volatile organic compounds, although considered to be communication signals in the rhizosphere, are understudied. The Pseudomonas fluorescens MFE01 strain has a very active type six secretion system that can kill some competitive bacteria. Furthermore, MFE01 emits numerous volatile organic compounds, including 1-undecene, which contributes to the aerial inhibition of Legionella pneumophila growth. Finally, MFE01 appears to be deprived of N-acyl homoserine lactone synthase. The main objective of this study was to explore the role of 1-undecene in the communication of MFE01. We constructed a mutant affected in undA gene encoding the enzyme responsible for 1-undecene synthesis to provide further insight into the role of 1-undecene in MFE01. First, we studied the impacts of this mutation both on volatile organic compounds emission, using headspace solid-phase microextraction combined with gas chromatography-mass spectrometry and on L. pneumophila long-range inhibition. Then, we analyzed influence of 1-undecene on MFE01 coordinated phenotypes, including type six secretion system activity and biofilm formation. Next, to test the ability of MFE01 to synthesize N-acyl homoserine lactones in our conditions, we investigated in silico the presence of corresponding genes across the MFE01 genome and we exposed its biofilms to an N-acyl homoserine lactone-degrading enzyme. Finally, we examined the effects of 1-undecene emission on MFE01 biofilm maturation and aerial communication using an original experimental set-up. This study demonstrated that the ΔundA mutant is impaired in biofilm maturation. An exposure of the ΔundA mutant to the volatile compounds emitted by MFE01 during the biofilm development restored the biofilm maturation process. These findings indicate that P. fluorescens MFE01 uses 1-undecene emission for aerial communication, reporting for the first time this volatile organic compound as bacterial intraspecific communication signal.

New PCR-Based Assay for the Identification of Pectobacterium carotovorum Causing Potato Soft Rot

Soft rot on potato tuber is a destructive disease caused by pathogenic bacterial species of the genera Pectobacterium and Dickeya. Accurate identification of the causal agent is necessary to ensure adequate disease management because different species may have distinct levels of aggressiveness and host range. One of the most important potato pathogens is Pectobacterium carotovorum, a highly heterogeneous species capable of infecting multiple hosts. The complexity of this species, until recently divided into several subspecies, has made it difficult to develop precise diagnostic tests. This study proposes a PCR assay based on the new pair of primers Pcar1F/R to facilitate the identification of potato isolates of P. carotovorum according to the most recent taxonomic description of this species. The new primers were designed on a variable segment of the 16S rRNA gene and the intergenic spacer region of available DNA sequences from classical and recently established species in the genus Pectobacterium. The results of the PCR analysis of genomic DNA from 32 Pectobacterium and Dickeya strains confirmed that the Pcar1F/R primers have sufficient nucleotide differences to discriminate between P. carotovorum and other Pectobacterium species associated with damage to potato crops, with the exception of Pectobacterium versatile, which improves the specificity of the currently available primers. The proposed assay was originally developed as a conventional PCR but was later adapted to the real-time PCR format for application in combination with the existing real-time PCR test for the potato-specific pathogen Pectobacterium parmentieri. This should be useful for the routine diagnosis of potato soft rot.

Genome-Wide Analyses of the Temperature-Responsive Genetic Loci of the Pectinolytic Plant Pathogenic Pectobacterium atrosepticum

Temperature is one of the critical factors affecting gene expression in bacteria. Despite the general interest in the link between bacterial phenotypes and environmental temperature, little is known about temperature-dependent gene expression in plant pathogenic Pectobacterium atrosepticum, a causative agent of potato blackleg and tuber soft rot worldwide. In this study, twenty-nine P. atrosepticum SCRI1043 thermoregulated genes were identified using Tn5-based transposon mutagenesis coupled with an inducible promotorless gusA gene as a reporter. From the pool of 29 genes, 14 were up-regulated at 18 °C, whereas 15 other genes were up-regulated at 28 °C. Among the thermoregulated loci, genes involved in primary bacterial metabolism, membrane-related proteins, fitness-corresponding factors, and several hypothetical proteins were found. The Tn5 mutants were tested for their pathogenicity in planta and for features that are likely to remain important for the pathogen to succeed in the (plant) environment. Five Tn5 mutants expressed visible phenotypes differentiating these mutants from the phenotype of the SCRI1043 wild-type strain. The gene disruptions in the Tn5 transposon mutants caused alterations in bacterial generation time, ability to form a biofilm, production of lipopolysaccharides, and virulence on potato tuber slices. The consequences of environmental temperature on the ability of P. atrosepticum to cause disease symptoms in potato are discussed.

Biosensors Used for Epifluorescence and Confocal Laser Scanning Microscopies to Study Dickeya and Pectobacterium Virulence and Biocontrol

Promoter-probe vectors carrying fluorescent protein-reporter genes are powerful tools used to study microbial ecology, epidemiology, and etiology. In addition, they provide direct visual evidence of molecular interactions related to cell physiology and metabolism. Knowledge and advances carried out thanks to the construction of soft-rot Pectobacteriaceae biosensors, often inoculated in potato Solanum tuberosum, are discussed in this review. Under epifluorescence and confocal laser scanning microscopies, Dickeya and Pectobacterium-tagged strains managed to monitor in situ bacterial viability, microcolony and biofilm formation, and colonization of infected plant organs, as well as disease symptoms, such as cell-wall lysis and their suppression by biocontrol antagonists. The use of dual-colored reporters encoding the first fluorophore expressed from a constitutive promoter as a cell tag, while a second was used as a regulator-based reporter system, was also used to simultaneously visualize bacterial spread and activity. This revealed the chronology of events leading to tuber maceration and quorum-sensing communication, in addition to the disruption of the latter by biocontrol agents. The promising potential of these fluorescent biosensors should make it possible to apprehend other activities, such as subcellular localization of key proteins involved in bacterial virulence in planta, in the near future.

Volatile Metabolic Markers for Monitoring Pectobacterium carotovorum subsp. carotovorum Using Headspace Solid-Phase Microextraction Coupled with Gas Chromatography-Mass Spectrometry

Identifying the extracellular metabolites of microorganisms in fresh vegetables is industrially useful for assessing the quality of processed foods. Pectobacterium carotovorum subsp. carotovorum (PCC) is a plant pathogenic bacterium that causes soft rot disease in cabbages. This microbial species in plant tissues can emit specific volatile molecules with odors that are characteristic of the host cell tissues and PCC species. In this study, we used headspace solid-phase microextraction followed by gas chromatography coupled with mass spectrometry (HS-SPME-GC-MS) to identify volatile compounds (VCs) in PCC-inoculated cabbage at different storage temperatures. HS-SPME-GC-MS allowed for recognition of extracellular metabolites in PCC-infected cabbages by identifying specific volatile metabolic markers. We identified 4-ethyl-5-methylthiazole and 3-butenyl isothiocyanate as markers of fresh cabbages, whereas 2,3-butanediol and ethyl acetate were identified as markers of soft rot in PCC-infected cabbages. These analytical results demonstrate a suitable approach for establishing non-destructive plant pathogen-diagnosis techniques as alternatives to standard methods, within the framework of developing rapid and efficient analytical techniques for monitoring plant-borne bacterial pathogens. Moreover, our techniques could have promising applications in managing the freshness and quality control of cabbages.

Risk of Spreading Soft Rot Through Cutting Dips Against Whiteflies in Greenhouse-Grown Poinsettia

Dipping is a quick and cost-effective technique to reduce pest infestations on unrooted cuttings of greenhouse ornamental crops. Large numbers of cuttings are immersed in an insecticidal treatment, e.g., biopesticides and/or insecticidal soap, before they are stuck in the growing medium and rooted. This research investigated the risk of cross-contamination of poinsettia cuttings with Pectobacterium carotovorum subsp. carotovorum, a potentially devastating pathogen causing soft rot, through the dipping process. Sampling at four commercial greenhouses showed that P. carotovorum subsp. carotovorum was present in all dip suspensions during and after processing poinsettia cuttings; concentrations up to 1 × 10⁵ CFU/ml were detected. A laboratory experiment determined that P. carotovorum subsp. carotovorum-infected cuttings could contaminate clean dip suspensions to similar levels. These results indicated that there is potential for disease transfer during dipping. The potential for cross-contamination of healthy cuttings was evaluated by immersing poinsettia cuttings in dip suspensions artificially inoculated with P. carotovorum subsp. carotovorum (from 1 × 10³ to 1 × 10⁷ CFU/ml). Disease incidence increased as P. carotovorum subsp. carotovorum concentrations in the dip suspension increased and the variety 'Prestige Red' was more susceptible than 'Freedom White.' However, even at the highest P. carotovorum subsp. carotovorum concentration of 1 × 10⁷ CFU/ml, the proportion of diseased cuttings was low at 6% for var. 'Freedom White,' but higher at 21% for var. 'Prestige Red.' We conclude that P. carotovorum subsp. carotovorum transfer among unrooted poinsettia cuttings through the dipping process is relatively low although some varieties are sensitive to high levels of inoculum. Even so, strict sanitation practices are still important to prevent build-up of inoculum in the dip treatment.

Biocontrol of Soft Rot: Confocal Microscopy Highlights Virulent Pectobacterial Communication and Its Jamming by Rhodococcal Quorum-Quenching

Confocal laser-scanning microscopy was chosen to observe the colonization and damage caused by the soft rot Pectobacterium atrosepticum and the protection mediated by the biocontrol agent Rhodococcus erythropolis. We developed dual-color reporter strains suited for monitoring quorum-sensing and quorum-quenching activities leading to maceration or biocontrol, respectively. A constitutively expressed cyan or red fluorescent protein served as a cell tag for plant colonization, while an inducible expression reporter system based on the green fluorescent protein gene enabled the simultaneous recording of signaling molecule production, detection, or degradation. The dual-colored pathogen and biocontrol strains were used to coinoculate potato tubers. At cellular quorum, images revealed a strong pectobacterial quorum-sensing activity, especially at the plant cell walls, as well as a concomitant rhodococcal quorum-quenching response, at both the single-cell and microcolony levels. The generated biosensors appear to be promising and complementary tools useful for molecular and cellular studies of bacterial communication and interference.

A Rhodococcal Transcriptional Regulatory Mechanism Detects the Common Lactone Ring of AHL Quorum-Sensing Signals and Triggers the Quorum-Quenching Response

The biocontrol agent Rhodococcus erythropolis disrupts virulence of plant and human Gram-negative pathogens by catabolizing their N-acyl-homoserine lactones. This quorum-quenching activity requires the expression of the qsd (quorum-sensing signal degradation) operon, which encodes the lactonase QsdA and the fatty acyl-CoA ligase QsdC, involved in the catabolism of lactone ring and acyl chain moieties of signaling molecules, respectively. Here, we demonstrate the regulation of qsd operon expression by a TetR-like family repressor, QsdR. This repression was lifted by adding the pathogen quorum signal or by deleting the qsdR gene, resulting in enhanced lactone degrading activity. Using interactomic approaches and transcriptional fusion strategy, the qsd operon derepression was elucidated: it is operated by the binding of the common part of signaling molecules, the homoserine lactone ring, to the effector-receiving domain of QsdR, preventing a physical binding of QsdR to the qsd promoter region. To our knowledge, this is the first evidence revealing quorum signals as inducers of the suitable quorum-quenching pathway, confirming this TetR-like protein as a lactone sensor. This regulatory mechanism designates the qsd operon as encoding a global disrupting pathway for degrading a wide range of signal substrates, allowing a broad spectrum anti-virulence activity mediated by the rhodococcal biocontrol agent. Understanding the regulation mechanisms of qsd operon expression led also to the development of biosensors useful to monitor in situ the presence of exogenous signals and quorum-quenching activity.

Threat of establishment of non-indigenous potato blackleg and tuber soft rot pathogens in Great Britain under climate change

Potato blackleg and soft rot caused by Pectobacterium and Dickeya species are among the most significant bacterial diseases affecting potato production globally. In this study we estimate the impact of future temperatures on establishment of non-indigenous but confirmed Pectobacterium and Dickeya species in Great Britain (GB). The calculations are based on probabilistic climate change data and a model fitted to disease severity data from a controlled environment tuber assay with the dominant potato blackleg and soft rot-causing species in GB (P. atrosepticum), and three of the main causative agents in Europe (P. carotovorum subsp. brasiliense, P. parmentieri, Dickeya solani). Our aim was to investigate if the European strains could become stronger competitors in the GB potato ecosystem as the climate warms, on the basis of their aggressiveness in tubers at different temperatures. Principally, we found that the tissue macerating capacity of all four pathogens will increase in GB under all emissions scenarios. The predominant Pectobacterium and Dickeya species in Europe are able to cause disease in tubers under field conditions currently seen in GB but are not expected to become widely established in the future, at least on the basis of their aggressiveness in tubers relative to P. atrosepticum under GB conditions. Our key take-home messages are that the GB potato industry is well positioned to continue to thrive via current best management practices and continued reinforcement of existing legislation.

Root exudate of Solanum tuberosum is enriched in galactose-containing molecules and impacts the growth of Pectobacterium atrosepticum

Background and aims Potato (Solanum tuberosum) is an important food crop and is grown worldwide. It is, however, significantly sensitive to a number of soil-borne pathogens that affect roots and tubers, causing considerable economic losses. So far, most research on potato has been dedicated to tubers and hence little attention has been paid to root structure and function. Methods In the present study we characterized root border cells using histochemical staining, immunofluorescence labelling of cell wall polysaccharides epitopes and observation using laser confocal microscopy. The monosaccharide composition of the secreted exudates was determined by gas chromatography of trimethylsilyl methylglycoside derivatives. The effects of root exudates and secreted arabinogalactan proteins on bacterial growth were investigated using in vitro bioassays. Key Results Root exudate from S. tuberosum was highly enriched in galactose-containing molecules including arabinogalactan proteins as major components. Treatment of the root with an elicitor derived from Pectobacterium atrosepticum, a soil-borne pathogen of potato, altered the composition of the exudates and arabinogalactan proteins. We found that the growth of the bacterium in vitro was differentially affected by exudates from elicited and non-elicited roots (i.e. inhibition versus stimulation). Conclusions Taken together, these findings indicate that galactose-containing polymers of potato root exudates play a central role in root-microbe interactions.

Antibacterial activity of carob (Ceratonia siliqua L.) extracts against phytopathogenic bacteria Pectobacterium atrosepticum

Acetone and ethanol extracts of carob (Ceratonia siliqua L.) leaf and pods were evaluated for their in vitro inhibitory ability against the pectinolytic Gram negative Pectobacterium atrosepticum (Pca, CFBP-5384) bacteria, the causal agent of potato soft rot. Potato (Solanum tuberosum, var nicola) tuber rot tissues obtained after 5 day bacterial inoculation was analyzed by LC-MS and GC-MS to study Pca pathogenicity. Trans/cis N-feruloylputrescine was identified in potato tuber after 5-day inoculation with Pca in a dark moist chamber. Although glycoalkoloid (α-chaconine and α-solanine) production increased due to Pca soft rot infection, it was not a resistance-determining factor. Many secondary metabolites were identified including the phytoalexins solavetivone and fatty acids responsible for plant defence responses. Acetone extract of carob leaf (FCA) exhibited the strongest inhibitory effect (IC50 = 1.5 mg/ml) and displayed synergistic antimicrobial effect in the presence of infected potato tuber extract (Pdt-Pca extract) against Pca. This synergy could be used in an integrated control program against potato soft rot pathogens, thereby reducing chemical treatments.

Genomic overview of the phytopathogen Pectobacterium wasabiae strain RNS 08.42.1A suggests horizontal acquisition of quorum-sensing genes

The blackleg and soft-rot diseases caused by pectinolytic enterobacteria such as Pectobacterium and Dickeya are major causes of losses affecting potato crop in the field and upon storage. In this work, we report the isolation, characterization and genome analysis of the Pectobacterium wasabiae (formerly identified as Pectobacterium carotovorum subsp. carotovorum) strain RNS 08.42.1A, that has been isolated from a Solanum tuberosum host plant in France. Comparative genomics with 3 other P. wasabiae strains isolated from potato plants in different areas in North America and Europe, highlighted both a strong similarity at the whole genome level (ANI > 99 %) and a conserved synteny of the virulence genes. In addition, our analyses evidenced a robust separation between these four P. wasabiae strains and the type strain P. wasabiae CFBP 3304(T), isolated from horseradish in Japan. In P. wasabiae RNS 08.42.1A, the expI and expR nucleotidic sequences are more related to those of some Pectobacterium atrosepticum and P. carotovorum strains (90 % of identity) than to those of the other potato P. wasabiae strains (70 to 74 % of identity). This could suggest a recruitment of these genes in the P. wasabiae strain RNS 08.42.1A by an horizontal transfer between pathogens infecting the same potato host plant.

In Planta Biocontrol of Pectobacterium atrosepticum by Rhodococcus erythropolis Involves Silencing of Pathogen Communication by the Rhodococcal Gamma-Lactone Catabolic Pathway

The virulence of numerous Gram-negative bacteria is under the control of a quorum sensing process based on synthesis and perception of N-acyl homoserine lactones. Rhodococcus erythropolis, a Gram-positive bacterium, has recently been proposed as a biocontrol agent for plant protection against soft-rot bacteria, including Pectobacterium. Here, we show that the γ-lactone catabolic pathway of R. erythropolis disrupts Pectobacterium communication and prevents plant soft-rot. We report the first characterization and demonstration of N-acyl homoserine lactone quenching in planta. In particular, we describe the transcription of the R. erythropolis lactonase gene, encoding the key enzyme of this pathway, and the subsequent lactone breakdown. The role of this catabolic pathway in biocontrol activity was confirmed by deletion of the lactonase gene from R. erythropolis and also its heterologous expression in Escherichia coli. The γ-lactone catabolic pathway is induced by pathogen communication rather than by pathogen invasion. This is thus a novel and unusual biocontrol pathway, differing from those previously described as protecting plants from phytopathogens. These findings also suggest the existence of an additional pathway contributing to plant protection.

Quorum sensing signaling molecules produced by reference and emerging soft-rot bacteria (Dickeya and Pectobacterium spp.)

Background

Several small diffusible molecules are involved in bacterial quorum sensing and virulence. The production of autoinducers-1 and -2, quinolone, indole and γ-amino butyrate signaling molecules was investigated in a set of soft-rot bacteria belonging to six Dickeya or Pectobacterium species including recent or emerging potato isolates.

Methodology/Principal Findings

Using bacterial biosensors, immunoassay, and chromatographic analysis, we showed that soft-rot bacteria have the common ability to produce transiently during their exponential phase of growth the N-3-oxo-hexanoyl- or the N-3-oxo-octanoyl-l-homoserine lactones and a molecule of the autoinducer-2 family. Dickeya spp. produced in addition the indole-3-acetic acid in tryptophan-rich conditions. All these signaling molecules have been identified for the first time in the novel Dickeya solani species. In contrast, quinolone and γ-amino butyrate signals were not identified and the corresponding synthases are not present in the available genomes of soft-rot bacteria. To determine if the variations of signal production according to growth phase could result from expression modifications of the corresponding synthase gene, the respective mRNA levels were estimated by reverse transcriptase-PCR. While the N-acyl-homoserine lactone production is systematically correlated to the synthase expression, that of the autoinducer-2 follows the expression of an enzyme upstream in the activated methyl cycle and providing its precursor, rather than the expression of its own synthase.

Conclusions/Significance

Despite sharing the S-adenosylmethionine precursor, no strong link was detected between the production kinetics or metabolic pathways of autoinducers-1 and -2. In contrast, the signaling pathway of autoinducer-2 seems to be switched off by the indole-3-acetic acid pathway under tryptophan control. It therefore appears that the two genera of soft-rot bacteria have similarities but also differences in the mechanisms of communication via the diffusible molecules. Our results designate autoinducer-1 lactones as the main targets for a global biocontrol of soft-rot bacteria communications, including those of emerging isolates.

N-acyl homoserine lactones in diverse Pectobacterium and Dickeya plant pathogens: diversity, abundance, and involvement in virulence

Soft-rot bacteria Pectobacterium and Dickeya use N-acyl homoserine lactones (NAHSLs) as diffusible signals for coordinating quorum sensing communication. The production of NAHSLs was investigated in a set of reference strains and recently-collected isolates, which belong to six species and share the ability to infect the potato host plant. All the pathogens produced different NAHSLs, among which the 3-oxo-hexanoyl- and the 3-oxo-octanoyl-l-homoserine lactones represent at least 90% of total produced NAHSL-amounts. The level of NAHSLs varied from 0.6 to 2 pg/cfu. The involvement of NAHSLs in tuber maceration was investigated by electroporating a quorum quenching vector in each of the bacterial pathogen strains. All the NAHSL-lactonase expressing strains produced a lower amount of NAHSLs as compared to those harboring the empty vector. Moreover, all except Dickeya dadantii 3937 induced a lower level of symptoms in potato tuber assay. Noticeably, aggressiveness appeared to be independent of both nature and amount of produced signals. This work highlights that quorum sensing similarly contributed to virulence in most of the tested Pectobacterium and Dickeya, even the strains had been isolated recently or during the past decades. Thus, these key regulatory-molecules appear as credible targets for developing anti-virulence strategies against these plant pathogens.

Binding sequences for RdgB, a DNA damage-responsive transcriptional activator, and temperature-dependent expression of bacteriocin and pectin lyase genes in Pectobacterium carotovorum subsp. carotovorum

Pectobacterium carotovorum subsp. carotovorum strain Er simultaneously produces the phage tail-like bacteriocin carotovoricin (Ctv) and pectin lyase (Pnl) in response to DNA-damaging agents. The regulatory protein RdgB of the Mor/C family of proteins activates transcription of pnl through binding to the promoter. However, the optimal temperature for the synthesis of Ctv (23 degrees C) differs from that for synthesis of Pnl (30 degrees C), raising the question of whether RdgB directly activates ctv transcription. Here we report that RdgB directly regulates Ctv synthesis. Gel mobility shift assays demonstrated RdgB binding to the P(0), P(1), and P(2) promoters of the ctv operons, and DNase I footprinting determined RdgB-binding sequences (RdgB boxes) on these and on the pnl promoters. The RdgB box of the pnl promoter included a perfect 7-bp inverted repeat with high binding affinity to the regulator (K(d) [dissociation constant] = 150 nM). In contrast, RdgB boxes of the ctv promoters contained an imperfect inverted repeat with two or three mismatches that consequently reduced binding affinity (K(d) = 250 to 350 nM). Transcription of the rdgB and ctv genes was about doubled at 23 degrees C compared with that at 30 degrees C. In contrast, the amount of pnl transcription tripled at 30 degrees C. Thus, the inverse synthesis of Ctv and Pnl as a function of temperature is apparently controlled at the transcriptional level, and reduced rdgB expression at 30 degrees C obviously affected transcription from the ctv promoters with low-affinity RdgB boxes. Pathogenicity toward potato tubers was reduced in an rdgB knockout mutant, suggesting that the RdgAB system contributes to the pathogenicity of this bacterium, probably by activating pnl expression.

Evaluation of reference genes for real-time RT-PCR expression studies in the plant pathogen Pectobacterium atrosepticum

BACKGROUND

Real-time RT-PCR has become a powerful technique to monitor low-abundance mRNA expression and is a useful tool when examining bacterial gene expression inside infected host tissues. However, correct evaluation of data requires accurate and reliable normalisation against internal standards. Thus, the identification of reference genes whose expression does not change during the course of the experiment is of paramount importance. Here, we present a study where manipulation of cultural growth conditions and in planta experiments have been used to validate the expression stability of reference gene candidates for the plant pathogen Pectobacterium atrosepticum, belonging to the family Enterobacteriaceae.

RESULTS

Of twelve reference gene candidates tested, four proved to be stably expressed both in six different cultural growth conditions and in planta. Two of these genes (recA and ffh), encoding recombinase A and signal recognition particle protein, respectively, proved to be the most stable set of reference genes under the experimental conditions used. In addition, genes proC and gyrA, encoding pyrroline-5-carboxylate reductase and DNA gyrase, respectively, also displayed relatively stable mRNA expression levels.

CONCLUSION

Based on these results, we suggest recA and ffh as suitable candidates for accurate normalisation of real-time RT-PCR data for experiments investigating the plant pathogen P. atrosepticum and potentially other related pathogens.

Thermoregulation of N-acyl homoserine lactone-based quorum sensing in the soft rot bacterium Pectobacterium atrosepticum

The psychrotolerant bacterium Pectobacterium atrosepticum produces four N-acyl homoserine lactones under a wide range of temperatures. Their thermoregulation differs from that of the exoenzyme production, described as being under quorum-sensing control. A mechanism involved in this thermoregulation consists of controlling N-acyl homoserine lactones synthase production at a transcriptional level.

Involvement of N-acylhomoserine lactones throughout plant infection by Erwinia carotovora subsp. atroseptica (Pectobacterium atrosepticum)

Erwinia carotovora subsp. atroseptica is responsible for potato blackleg disease in the field and tuber soft rot during crop storage. The process leading to the disease occurs in two phases: a primary invasion step followed by a maceration step. Bacteria-to-bacteria communication is associated with a quorum-sensing (QS) process based on the production of N-acylhomoserine lactones (HSL). The role of HSL throughout plant infection was analyzed. To this purpose, HSL produced by a specific E. carotovora subsp. atroseptica wild-type strain, which was particularly virulent on potato, were identified. A derivative of this strain that expressed an HSL lactonase gene and produced low amounts of HSL was generated. The comparison of these strains allowed the evaluation of the role of HSL and QS in disease establishment and development. Bacterial growth and motility; activity of proteins secreted by type I, II, and III systems; and hypersensitive and maceration reactions were evaluated. Results indicated that HSL production and QS regulate only those traits involved in the second stage of the host plant infection (i.e., tissue maceration) and hypersensitive response in nonhost tobacco plants. Therefore, the use of QS quenching strategies for biological control in E. carotovora subsp. atroseptica cannot prevent initial infection and multiplication of this pathogen.