Effect of Priestia endophytica on the metabolites accumulation in chicory and lettuce plants cultivated in vitro

The influence of the culture medium without bacterial cells, obtained after the cultivation of endophytic bacteria Priestia endophytica UCM B-5715, on the growth and synthesis of some metabolites in lettuce and chicory seedlings under in vitro conditions was studied. Bacteria were cultivated in liquid LB medium at 37 ºC for 24 h with periodic stirring. The culture fluid was separated from the cell biomass. For preparing the test solution, the supernatant was sterilized by filtration through a filter with a pore diameter of 0.2 µm (Sartorius, Minisart) and diluted with sterile distilled water. The 20% culture fluid (30 µl/plant) was applied to 3-day-old seedlings. In 28 days root and shoot weights of treated chicory plants were 54.3 ± 6.9 and 260.0 ± 20.2 mg, respectively (8.0 ± 0.7 and 91.4 ± 7.0 mg for the control plants). Total flavonoid content and antioxidant activity increased only in chicory plants after the addition of the test solution. Significant changes in the metabolism of treated plants were detected. In the treated lettuce plants asparagine content increased compared to the control (90 vs 22 µg/g, p < 0.1). The median content of fructose was also higher in treated lettuce and chicory plants (1469 vs 73 µg/g and 2278 vs 1051 µg/g). Therefore, the use of culture fluid obtained after the cultivation of P. endophytica UСM B-5715 stimulated the growth of lettuce and chicory plants, affecting the synthesis of some compounds in single-treated plants. These results indicate the potential of compounds excreted during bacterial growth to create natural growth stimulators.

The nucleoid-associated protein IHF acts as a 'transcriptional domainin' protein coordinating the bacterial virulence traits with global transcription

Bacterial pathogenic growth requires a swift coordination of pathogenicity function with various kinds of environmental stress encountered in the course of host infection. Among the factors critical for bacterial adaptation are changes of DNA topology and binding effects of nucleoid-associated proteins transducing the environmental signals to the chromosome and coordinating the global transcriptional response to stress. In this study, we use the model phytopathogen Dickeya dadantii to analyse the organisation of transcription by the nucleoid-associated heterodimeric protein IHF. We inactivated the IHFα subunit of IHF thus precluding the IHFαβ heterodimer formation and determined both phenotypic effects of ihfA mutation on D. dadantii virulence and the transcriptional response under various conditions of growth. We show that ihfA mutation reorganises the genomic expression by modulating the distribution of chromosomal DNA supercoils at different length scales, thus affecting many virulence genes involved in both symptomatic and asymptomatic phases of infection, including those required for pectin catabolism. Altogether, we propose that IHF heterodimer is a 'transcriptional domainin' protein, the lack of which impairs the spatiotemporal organisation of transcriptional stress-response domains harbouring various virulence traits, thus abrogating the pathogenicity of D. dadantii.

Two herbicides, two fungicides and spore-associated bacteria affect Funneliformis mosseae extraradical mycelium structural traits and viability

The extraradical mycelium (ERM) produced by arbuscular mycorrhizal fungi is fundamental for the maintenance of biological fertility in agricultural soils, representing an important inoculum source, together with spores and mycorrhizal root fragments. Its viability and structural traits, such as density, extent and interconnectedness, which are positively correlated with the growth and nutrition of host plants, may be affected by different agronomic practices, including the use of pesticides and by different mycorrhizospheric communities. This work, carried out using a whole-plant experimental model system, showed that structural traits of ERM, such as length and density, were strongly decreased by the herbicides dicamba and glufosinolate and the fungicides benomyl and fenhexamid, while anastomosis frequency and hyphal branching were differentially modulated by singly inoculated mycorrhizospheric bacteria, depending on their identity.

Efficient Genome Editing Using CRISPR/Cas9 Technology in Chicory

CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated with protein CAS9) is a genome-editing tool that has been extensively used in the last five years because of its novelty, affordability, and feasibility. This technology has been developed in many plant species for gene function analysis and crop improvement but has never been used in chicory (Cichorium intybus L.). In this study, we successfully applied CRISPR/Cas9-mediated targeted mutagenesis to chicory using Agrobacterium rhizogenes-mediated transformation and protoplast transfection methods. A U6 promoter (CiU6-1p) among eight predicted U6 promoters in chicory was selected to drive sgRNA expression. A binary vector designed to induce targeted mutations in the fifth exon of the chicory phytoene desaturase gene (CiPDS) was then constructed and used to transform chicory. The mutation frequency was 4.5% with the protoplast transient expression system and 31.25% with A. rhizogenes-mediated stable transformation. Biallelic mutations were detected in all the mutant plants. The use of A. rhizogenes-mediated transformation seems preferable as the regeneration of plants is faster and the mutation frequency was shown to be higher. With both transformation methods, foreign DNA was integrated in the plant genome. Hence, selection of vector (transgene)-free segregants is required. Our results showed that genome editing with CRISPR/Cas9 system can be efficiently used with chicory, which should facilitate and accelerate genetic improvement and functional biology.

Identification by Tn-seq of Dickeya dadantii genes required for survival in chicory plants

The identification of the virulence factors of plant-pathogenic bacteria has relied on the testing of individual mutants on plants, a time-consuming process. Transposon sequencing (Tn-seq) is a very powerful method for the identification of the genes required for bacterial growth in their host. We used this method in a soft-rot pathogenic bacterium to identify the genes required for the multiplication of Dickeya dadantii in chicory. About 100 genes were identified showing decreased or increased fitness in the plant. Most had no previously attributed role in plant-bacterium interactions. Following our screening, in planta competition assays confirmed that the uridine monophosphate biosynthesis pathway and the purine biosynthesis pathway were essential to the survival of D. dadantii in the plant, as the mutants ∆carA, ∆purF, ∆purL, ∆guaB and ∆pyrE were unable to survive in the plant in contrast with the wild-type (WT) bacterium. This study also demonstrated that the biosynthetic pathways of leucine, cysteine and lysine were essential for bacterial survival in the plant and that RsmC and GcpA were important in the regulation of the infection process, as the mutants ∆rsmC and ∆gcpA were hypervirulent. Finally, our study showed that D. dadantii flagellin was glycosylated and that this modification conferred fitness to the bacterium during plant infection. Assay by this method of the large collections of environmental pathogenic strains now available will allow an easy and rapid identification of new virulence factors.

Shelf life evaluation of fresh-cut red chicory subjected to different minimal processes

Microbiological, chemical and physical parameters of minimally processed red chicory (Cichorium intybus L.) subjected to two different transformation processes were investigated. A classic ready-to-eat (RTE) process (P1) and a production without cutting (P2) were monitored during refrigerated (4 °C) storage (15 d). Total mesophilic microorganisms, total psychrotrophic microorganisms and pseudomonads were detected at the highest cell densities in all samples. Presumptive Pseudomonas population dominated the cultivable microbial community of RTE red chicory and were characterized genetically. Twenty-two randomly amplified polymorphic DNA (RAPD) types were investigated by 16S rRNA gene sequencing, resulting in members of Rahnella and Pseudomonas. The identification of Pseudomonas species was further determined by sequencing of gyrB, rpoB and rpoD genes resulting in 16 species. A highest visual quality and a lower weight loss and colour variation were registered for P2, while soluble solid, nitrate and ascorbic acid contents were not affected by processing and storage. The integrated microbiological, chemical and physical approach applied in this study demonstrated the longer shelf-life of P2 red chicory.

Interplay of classic Exp and specific Vfm quorum sensing systems on the phenotypic features of Dickeya solani strains exhibiting different virulence levels

Bacteria from the genus Dickeya cause severe symptoms on numerous economically important plants. Dickeya solani is the Dickeya species most frequently found on infected potato plants in Europe. D. solani strains from different countries show high genetic homogeneity, but significant differences in their virulence level. Dickeya species possess two quorum sensing (QS) mechanisms: the Exp system based on classic N-acyl-homoserine lactone (AHL) signals and a specific system depending on the production and perception of a molecule of unknown structure, Virulence Factor Modulating (VFM). To study the interplay between these two QS systems, five D. solani strains exhibiting different virulence levels were selected. Mutants were constructed by inactivating genes coding for each QS system. Double mutants were obtained by simultaneous inactivation of genes coding for both QS systems. Most of the D. solani mutants showed an attenuation of chicory maceration and a decreased production of plant cell wall-degrading enzymes (PCWDEs) and motility, but to different degrees depending on the strain. The VFM-QS system seems to regulate virulence in both D. solani and Dickeya dadantii, but the AHL-QS system has greater effects in D. solani than in D. dadantii. The inactivation of both QS systems in D. solani did not reveal any additive effect on the tested features. The inactivation of vfm genes generally has a more dominant effect relative to that of exp genes. Thus, VFM- and AHL-QS systems do not work in synergy to modulate the production of diverse virulence factors and the ability to macerate plant tissue.

Molecular identification and characterization of the new 16SrIX-J and cpn60 UT IX-J phytoplasma subgroup associated with chicory bushy stunt disease in Saudi Arabia

Chicory (Cichorium intybus) is a perennial plant (Asteraceae) that grows wild in pasture fields in Saudi Arabia. Chicory plants displaying symptoms typically induced by phytoplasmas, such as bushy phenotype and stunt, were observed in the Mulayda region, Qassim governorate, Saudi Arabia. In this study we examined samples taken from three symptomatic chicory plants and confirmed the presence of phytoplasma DNA. Analysis of the 16S rRNA-encoding sequences showed that the plants were infected with a phytoplasma from the pigeon pea witches'-broom group (16SrIX). Sequencing of the 16S rRNA-encoding gene and the partial cpn60 sequence, computer-simulated RFLP analysis, and phylogenetic analysis of both markers revealed that the phytoplasma identified was representative of a new 16SrIX-J and cpn60 UT IX-IJ subgroup. The present study identified chicory plants as a novel host for phytoplasma strains within the pigeon pea witches'-broom phytoplasma group, and expanded the known diversity of this group.

An in vivo whole-plant experimental system for the analysis of gene expression in extraradical mycorrhizal mycelium

Arbuscular mycorrhizal fungi (AMF) establish beneficial mutualistic symbioses with land plants, receiving carbon in exchange for mineral nutrients absorbed by the extraradical mycelium (ERM). With the aim of obtaining in vivo produced ERM for gene expression analyses, a whole-plant bi-dimensional experimental system was devised and tested with three host plants and three fungal symbionts. In such a system, Funneliformis mosseae in symbiosis with Cichorium intybus var. foliosum, Lactuca sativa, and Medicago sativa produced ERM whose lengths ranged from 9.8 ± 0.8 to 20.8 ± 1.2 m per plant. Since ERM produced in symbiosis with C. intybus showed the highest values for the different structural parameters assessed, this host was used to test the whole-plant system with F. mosseae, Rhizoglomus irregulare, and Funneliformis coronatus. The whole-plant system yielded 1-7 mg of ERM fresh biomass per plant per harvest, and continued producing new ERM for 6 months. Variable amounts of high-quality and intact total RNA, ranging from 15 to 65 μg RNA/mg ERM fresh weight, were extracted from the ERM of the three AMF isolates. Ammonium transporter gene expression was successfully determined in the cDNAs obtained from ERM of the three fungal symbionts by RT-qPCR using gene-specific primers designed on available (R. irregulare) and new (F. mosseae and F. coronatus) ammonium transporter gene sequences. The whole-plant experimental system represents a useful research tool for large production and easy collection of ERM for morphological, physiological, and biochemical analyses, suitable for a wide variety of AMF species, for a virtually limitless range of host plants and for studies involving diverse symbiotic interactions.

Scout-MRM: Multiplexed Targeted Mass Spectrometry-Based Assay without Retention Time Scheduling Exemplified by Dickeya dadantii Proteomic Analysis during Plant Infection

Targeted mass spectrometry of a surrogate peptide panel is a powerful method to study the dynamics of protein networks, but chromatographic time scheduling remains a major limitation for dissemination and implementation of robust and large multiplexed assays. We unveil a Multiple Reaction Monitoring method (Scout-MRM) where the use of spiked scout peptides triggers complex transition lists, regardless of the retention time of targeted surrogate peptides. The interest of Scout-MRM method regarding the retention time independency, multiplexing capability, reproducibility, and putative interest in facilitating method transfer was illustrated by a 782-peptide-plex relative assay targeting 445 proteins of the phytopathogen Dickeya dadantii during plant infection.

Antimicrobial Peptide Resistance Genes in the Plant Pathogen Dickeya dadantii

Modification of teichoic acid through the incorporation of d-alanine confers resistance in Gram-positive bacteria to antimicrobial peptides (AMPs). This process involves the products of the dltXABCD genes. These genes are widespread in Gram-positive bacteria, and they are also found in a few Gram-negative bacteria. Notably, these genes are present in all soft-rot enterobacteria (Pectobacterium and Dickeya) whose dltDXBAC operons have been sequenced. We studied the function and regulation of these genes in Dickeya dadantii dltB expression was induced in the presence of the AMP polymyxin. It was not regulated by PhoP, which controls the expression of some genes involved in AMP resistance, but was regulated by ArcA, which has been identified as an activator of genes involved in AMP resistance. However, arcA was not the regulator responsible for polymyxin induction of these genes in this bacterium, which underlines the complexity of the mechanisms controlling AMP resistance in D. dadantii Two other genes involved in resistance to AMPs have also been characterized, phoS and phoH dltB, phoS, phoH, and arcA but not dltD mutants were more sensitive to polymyxin than the wild-type strain. Decreased fitness of the dltB, phoS, and phoH mutants in chicory leaves indicates that their products are important for resistance to plant AMPs.

IMPORTANCE

Gram-negative bacteria can modify their lipopolysaccharides (LPSs) to resist antimicrobial peptides (AMPs). Soft-rot enterobacteria (Dickeya and Pectobacterium spp.) possess homologues of the dlt genes in their genomes which, in Gram-positive bacteria, are involved in resistance to AMPs. In this study, we show that these genes confer resistance to AMPs, probably by modifying LPSs, and that they are required for the fitness of the bacteria during plant infection. Two other new genes involved in resistance were also analyzed. These results show that bacterial resistance to AMPs can occur in bacteria through many different mechanisms that need to be characterized.

Phenotype and toxicity of the recently discovered exlA-positive Pseudomonas aeruginosa strains collected worldwide

We recently identified a hypervirulent strain of Pseudomonas aeruginosa, differing significantly from the classical strains in that it lacks the type 3 secretion system (T3SS), a major determinant of P. aeruginosa virulence. This new strain secretes a novel toxin, called ExlA, which induces plasma membrane rupture in host cells. For this study, we collected 18 other exlA-positive T3SS-negative strains, analyzed their main virulence factors and tested their toxicity in various models. Phylogenetic analysis revealed two groups. The strains were isolated on five continents from patients with various pathologies or in the environment. Their proteolytic activity and their motion abilities were highly different, as well as their capacity to infect epithelial, endothelial, fibroblastic and immune cells, which correlated directly with ExlA secretion levels. In contrast, their toxicity towards human erythrocytes was limited. Some strains were hypervirulent in a mouse pneumonia model and others on chicory leaves. We conclude that (i) exlA-positive strains can colonize different habitats and may induce various infection types, (ii) the strains secreting significant amounts of ExlA are cytotoxic for most cell types but are poorly hemolytic, (iii) toxicity in planta does not correlate with ExlA secretion.

Control of spoiler Pseudomonas spp. on fresh cut vegetables by neutral electrolyzed water

In the present study, we evaluated the antimicrobial activity of neutral electrolyzed water (NEW) against 14 strains of spoilage Pseudomonas of fresh cut vegetables under cold storage. The NEW, produced from solutions of potassium and sodium chloride, and sodium bicarbonate developed up to 4000 mg/L of free chlorine, depending on the salt and relative concentration used. The antimicrobial effect of the NEW was evaluated against different bacterial strains at 10(5) cells/ml, with different combinations of free chlorine concentration/contact time; all concentrations above 100 mg/L, regardless of the salt used, were found to be bactericidal already after 2 min. When catalogna chicory and lettuce leaves were dipped for 5 min in diluted NEW, microbial loads of mesophilic bacteria and Enterobacteriaceae were reduced on average of 1.7 log cfu/g. In addition, when lettuce leaves were dipped in a cellular suspension of the spoiler Pseudomonas chicorii I3C strain, diluted NEW was able to reduce Pseudomonas population of about 1.0 log cfu/g. Thanks to its high antimicrobial activity against spoilage microorganisms, and low cost of operation, the application of cycles of electrolysis to the washing water looks as an effective tool in controlling fresh cut vegetable microbial spoilage contamination occurring during washing steps.

Antibacterial activity of caffeine against plant pathogenic bacteria

The objective of the present study was to evaluate the antibacterial properties of a plant secondary metabolite - caffeine. Caffeine is present in over 100 plant species. Antibacterial activity of caffeine was examined against the following plant-pathogenic bacteria: Ralstonia solanacearum (Rsol), Clavibacter michiganesis subsp. sepedonicus (Cms), Dickeya solani (Dsol), Pectobacterium atrosepticum (Pba), Pectobacterium carotovorum subsp. carotovorum (Pcc), Pseudomonas syringae pv. tomato (Pst), and Xanthomonas campestris subsp. campestris (Xcc). MIC and MBC values ranged from 5 to 20 mM and from 43 to 100 mM, respectively. Caffeine increased the bacterial generation time of all tested species and caused changes in cell morphology. The influence of caffeine on the synthesis of DNA, RNA and proteins was investigated in cultures of plant pathogenic bacteria with labelled precursors: [(3)H]thymidine, [(3)H]uridine or (14)C leucine, respectively. RNA biosynthesis was more affected than DNA or protein biosynthesis in bacterial cells treated with caffeine. Treatment of Pba with caffeine for 336 h did not induce resistance to this compound. Caffeine application reduced disease symptoms caused by Dsol on chicory leaves, potato slices, and whole potato tubers. The data presented indicate caffeine as a potential tool for the control of diseases caused by plant-pathogenic bacteria, especially under storage conditions.

Mycorrhizal fungi modulate phytochemical production and antioxidant activity of Cichorium intybus L. (Asteraceae) under metal toxicity

Cichorium intybus (common chicory), a perennial plant, common in anthropogenic sites, has been the object of a multitude of studies in recent years due to its high content of antioxidants utilized in pharmacy and food industry. Here, the role of arbuscular mycorrhizal fungi (AMF) in the biosynthesis of plant secondary metabolites and the activity of enzymatic antioxidants under toxic metal stress was studied. Plants inoculated with Rhizophagus irregularis and non-inoculated were grown on non-polluted and toxic metal enriched substrata. The results presented here indicate that AMF improves chicory fitness. Fresh and dry weight was found to be severely affected by the fungi and heavy metals. The concentration of hydroxycinnamates was increased in the shoots of mycorrhizal plants cultivated on non-polluted substrata, but no differences were found in plants cultivated on metal enriched substrata. The activity of SOD and H2O2 removing enzymes CAT and POX was elevated in the shoots of mycorrhizal plants regardless of the cultivation environment. Photochemical efficiency of inoculated chicory was significantly improved. Our results indicate that R. irregularis inoculation had a beneficial role in sustaining the plants ability to cope with the deleterious effects of metal toxicity.

Regulators involved in Dickeya solani virulence, genetic conservation, and functional variability

Bacteria from the genus Dickeya (formerly Erwinia chrysanthemi) are plant pathogens causing severe diseases in many economically important crops. A majority of the strains responsible for potato disease in Europe belong to a newly identified Dickeya solani species. Although some ecological and epidemiological studies have been carried out, little is known about the regulation of D. solani virulence. The characterization of four D. solani strains indicates significant differences in their virulence on potato, although they are genetically similar based on genomic fingerprinting profiles. A phenotypic examination included an analysis of virulence on potato; growth rate in culture; motility; Fe3+ chelation; and pectate lyase, cellulase, protease, biosurfactant, and blue pigment production. Mutants of four D. solani strains were constructed by inactivating the genes coding either for one of the main negative regulators of D. dadantii virulence (kdgR, pecS, and pecT) or for the synthesis and perception of signaling molecules (expI and expR). Analysis of these mutants indicated that PecS, PecT, and KdgR play a similar role in both species, repressing, to different degrees, the synthesis of virulence factors. The thermoregulator PecT seems to be a major regulator of D. solani virulence. This work also reveals the role of quorum sensing mediated by ExpI and ExpR in D. solani virulence on potato.

Condensed tannin accumulation and nitrogen fixation potential of Onobrychis viciifolia Scop. grown in a Mediterranean environment

BACKGROUND

Sainfoin (Onobrychis viciifolia Scop.) is a forage legume found in temperate areas but is less widespread in Mediterranean environments. Compared with other perennial legumes, it has the advantage of containing condensed tannins (CT) that can be important for their implications on ruminant nutrition and health. Data on nitrogen (N) fixation by sainfoin in the literature originate from very different environments and only a few field data are available, so it is important to improve knowledge on the N fixation potential of this species, particularly under a Mediterranean climate. Here the accumulation pattern of polyphenolic compounds (total, non-tannic polyphenols and CT) and the N fixation potential of sainfoin were studied in order to contribute to its valorisation for sustainable farming management in Mediterranean environments.

RESULTS

CT concentrations were always in the range considered beneficial for animals, not exceeding 50 g delphinidin equivalent kg⁻¹ dry matter (DM). The regression of aerial fixed N on aerial DM showed a relationship of 22 kg fixed N t⁻¹ aerial DM in a Mediterranean environment.

CONCLUSION

A wider exploitation of sainfoin is suggested for production under rain-fed conditions, thus enlarging the limited set of available perennial legumes suitable for Mediterranean environments.

A family 3 glycosyl hydrolase of Dickeya dadantii 3937 is involved in the cleavage of aromatic glucosides

Dickeya dadantii is a phytopathogenic bacterium secreting a large array of plant-cell-wall-degrading enzymes that participate in the infection and maceration of the host plant tissue. Sequencing of the D. dadantii 3937 genome predicted several genes encoding potential glycosidases. One of these genes, bgxA, encodes a protein classified in family 3 of glycosyl hydrolases. Inactivation of bgxA and the use of a gene fusion revealed that this gene is not essential for D. dadantii pathogenicity but that it is expressed during plant infection. The bgxA expression is induced in the presence of glycosidic or non-glycosidic aromatic compounds, notably ferulic acid, cinnamic acid, vanillic acid and salicin. The BgxA enzyme has a principal β-d-glucopyranosidase activity and a secondary β-d-xylopyranosidase activity (ratio 70 : 1). This enzyme activity is inhibited by different aromatic glycosides or phenolic compounds, in particular salicin, arbutin, ferulic acid and vanillic acid. Together, the induction effects and the enzyme inhibition suggest that BgxA is mostly involved in the cleavage of aromatic β-glucosides. There is evidence of functional redundancy in the D. dadantii β-glucoside assimilation pathway. In contrast to other β-glucoside assimilation systems, involving cytoplasmic phospho-β-glucosidases, the cleavage of aromatic glucosides in the periplasmic space by BgxA may avoid the release of a toxic phenolic aglycone into the cytoplasm while still allowing for catabolism of the glucose moiety.

The nucleoid-associated protein Fis directly modulates the synthesis of cellulose, an essential component of pellicle-biofilms in the phytopathogenic bacterium Dickeya dadantii

Bacteria use biofilm structures to colonize surfaces and to survive in hostile conditions, and numerous bacteria produce cellulose as a biofilm matrix polymer. Hence, expression of the bcs operon, responsible for cellulose biosynthesis, must be finely regulated in order to allow bacteria to adopt the proper surface-associated behaviours. Here we show that in the phytopathogenic bacterium, Dickeya dadantii, production of cellulose is required for pellicle-biofilm formation and resistance to chlorine treatments. Expression of the bcs operon is growth phase-regulated and is stimulated in biofilms. Furthermore, we unexpectedly found that the nucleoid-associated protein and global regulator of virulence functions, Fis, directly represses bcs operon expression by interacting with an operator that is absent from the bcs operon of animal pathogenic bacteria and the plant pathogenic bacterium Pectobacterium. Moreover, production of cellulose enhances plant surface colonization by D. dadantii. Overall, these data suggest that cellulose production and biofilm formation may be important factors for surface colonization by D. dadantii and its subsequent survival in hostile environments. This report also presents a new example of how bacteria can modulate the action of a global regulator to co-ordinate basic metabolism, virulence and modifications of lifestyle.

The mitochondrial genome of the arbuscular mycorrhizal fungus Gigaspora margarita reveals two unsuspected trans-splicing events of group I introns

• Arbuscular mycorrhizal fungi (AMF) are ubiquitous organisms that benefit ecosystems through the establishment of an association with the roots of most plants: the mycorrhizal symbiosis. Despite their ecological importance, however, these fungi have been poorly studied at the genome level. • In this study, total DNA from the AMF Gigaspora margarita was subjected to a combination of 454 and Illumina sequencing, and the resulting reads were used to assemble its mitochondrial genome de novo. This genome was annotated and compared with those of other relatives to better comprehend the evolution of the AMF lineage. • The mitochondrial genome of G. margarita is unique in many ways, exhibiting a large size (97 kbp) and elevated GC content (45%). This genome also harbors molecular events that were previously unknown to occur in fungal mitochondrial genomes, including trans-splicing of group I introns from two different genes coding for the first subunit of the cytochrome oxidase and for the small subunit of the rRNA. • This study reports the second published genome from an AMF organelle, resulting in relevant DNA sequence information from this poorly studied fungal group, and providing new insights into the frequency, origin and evolution of trans-spliced group I introns found across the mitochondrial genomes of distantly related organisms.

Propiconazole inhibits the sterol 14α-demethylase in Glomus irregulare like in phytopathogenic fungi

The increasing concentrations impact (0.02, 0.2 and 2 mg L(-1)) of a Sterol Biosynthesis Inhibitor (SBI) fungicide, propiconazole, was evaluated on development and sterol metabolism of two non-target organisms: mycorrhizal or non-mycorrhizal transformed chicory roots and the arbuscular mycorrhizal fungus (AMF) Glomus irregulare using monoxenic cultures. In this work, we provide the first evidence of a direct impact of propiconazole on the AMF by disturbing its sterol metabolism. A significant decrease in end-products sterols contents (24-methylcholesterol and in 24-ethylcholesterol) was observed concomitantly to a 24-methylenedihydrolanosterol accumulation indicating the inhibition of a key enzyme in sterol biosynthesis pathway, the sterol 14α-demethylase like in phytopathogenic fungi. A decrease in end-product sterol contents in propiconazole-treated roots was also observed suggesting a slowing down of the sterol metabolism in plant. Taken together, our findings suggest that the inhibition of the both AM symbiotic partners development by propiconazole results from their sterol metabolism alterations.

Calcareous impact on arbuscular mycorrhizal fungus development and on lipid peroxidation in monoxenic roots

The present work underlined the negative effects of increasing CaCO(3) concentrations (5, 10 and 20 mM) both on the chicory root growth and the arbuscular mycorrhizal fungus (AMF) Glomus irregulare development in monoxenic system. CaCO(3) was found to reduce drastically the main stages of G. irregulare life cycle (spore germination, germinative hyphae elongation, root colonization, extraradical hyphae development and sporulation) but not to inhibit it completely. The root colonization drop was confirmed by the decrease in the arbuscular mycorrhizal fungal marker C16:1ω5 amounts in the mycorrhizal chicory roots grown in the presence of CaCO(3). Oxidative damage evaluated by lipid peroxidation increase measured by (i) malondialdehyde (MDA) production and (ii) the antioxidant enzyme peroxidase (POD) activities, was highlighted in chicory roots grown in the presence of CaCO(3). However, MDA formation was significantly higher in non-mycorrhizal roots as compared to mycorrhizal ones. This study pointed out the ability of arbuscular mycorrhizal symbiosis to enhance plant tolerance to high levels of CaCO(3) by preventing lipid peroxidation and so less cell membrane damage.

[Regeneration of transgenic plants from Cichorium intybus L. var. foliosum Hegi hairy roots]

Transgenic plants were regenerated from Cichorium intybus L. hairy roots transformed with genes of tuberculosis antigenes ESAT6 and Ag85B or human interferon alpha2b. The plant regeneration was light-dependent and occurred on the media without growth regulators. The DNA PCR and RT-PCR analyses have shown the presence and expression both selective and target genes in all root lines and regenerated plants.

Identification of two feruloyl esterases in Dickeya dadantii 3937 and induction of the major feruloyl esterase and of pectate lyases by ferulic acid

The plant-pathogenic bacterium Dickeya dadantii (formerly Erwinia chrysanthemi) produces a large array of plant cell wall-degrading enzymes. Using an in situ detection test, we showed that it produces two feruloyl esterases, FaeD and FaeT. These enzymes cleave the ester link between ferulate and the pectic or xylan chains. FaeD and FaeT belong to the carbohydrate esterase family CE10, and they are the first two feruloyl esterases to be identified in this family. Cleavage of synthetic substrates revealed strong activation of FaeD and FaeT by ferulic acid. The gene faeT appeared to be weakly expressed, and its product, FaeT, is a cytoplasmic protein. In contrast, the gene faeD is strongly induced in the presence of ferulic acid, and FaeD is an extracellular protein secreted by the Out system, responsible for pectinase secretion. The product of the adjacent gene faeR is involved in the positive control of faeD in response to ferulic acid. Moreover, ferulic acid acts in synergy with polygalacturonate to induce pectate lyases, the main virulence determinant of soft rot disease. Feruloyl esterases dissociate internal cross-links in the polysaccharide network of the plant cell wall, suppress the polysaccharide esterifications, and liberate ferulic acid, which contributes to the induction of pectate lyases. Together, these effects of feruloyl esterases could facilitate soft rot disease caused by pectinolytic bacteria.

Chemical and biological control of Sclerotinia sclerotiorum in witloof chicory culture

BACKGROUND

Sclerotinia sclerotiorum (Lib.) de Bary is a major pathogen of witloof chicory. For lack of authorised field treatment, post-harvest sprays with dicarboximide fungicides have been standard practice since the 1970s to prevent root rot and chicory heart decay during the forcing phase. However, the registration of procymidone and vinclozolin has been withdrawn in Europe. The development of organic agriculture and the necessity to reduce fungicide applications in conventional agriculture prompted an assessment of the efficacy of new fungicides and the use of the mycoparasite Coniothyrium minitans (Campbell).

RESULTS

A mixture of the fungicides fludioxonil and cyprodinil (Switch(®) ) applied on chicory roots achieved a very good control of S. sclerotiorum (up to 95%). The use of C. minitans limited root infection, both when applied in the field (50-65% efficacy) and before the forcing period (post-harvest treatment up to 80%).

CONCLUSION

In organic agriculture, two treatments with C. minitans (in field and later at the forcing period) could improve protection against S. sclerotiorum. In conventional agriculture, after the field biological treatment, a post-harvest chemical treatment could be applied. The addition of other prophylactic methods could lead to a high level of performance in practice against decay caused by S. sclerotiorum.

The virulence of a Dickeya dadantii 3937 mutant devoid of osmoregulated periplasmic glucans is restored by inactivation of the RcsCD-RcsB phosphorelay

Dickeya dadantii is a pectinolytic phytopathogen enterobacterium that causes soft rot disease on a wide range of plant species. The virulence of D. dadantii involves several factors, including the osmoregulated periplasmic glucans (OPGs) that are general constituents of the envelope of proteobacteria. In addition to the loss of virulence, opg-negative mutants display a pleiotropic phenotype, including decreased motility and increased exopolysaccharide synthesis. A nitrosoguanidine-induced mutagenesis was performed on the opgG strain, and restoration of motility was used as a screen. The phenotype of the opg mutant echoes that of the Rcs system: high level activation of the RcsCD-RcsB phosphorelay is needed to activate exopolysaccharide synthesis and to repress motility, while low level activation is required for virulence in enterobacteria. Here, we show that mutations in the RcsCDB phosphorelay system restored virulence and motility in a D. dadantii opg-negative strain, indicating a relationship between the Rcs phosphorelay and OPGs.

The Tat pathway of plant pathogen Dickeya dadantii 3937 contributes to virulence and fitness

Protein secretion plays a very important role in the virulence of the bacterium Dickeya dadantii, the causative agent of soft rot disease, in a wide range of plant species. We studied the contribution of the twin-arginine translocation (Tat) protein system to the adaptation of D. dadantii 3937 to different growth conditions and to the interaction with the plant host. First, a list of 44 putative Tat substrates was obtained using bioinformatic programs taking advantage of the availability of the complete sequence of this bacterium. Second, a tatC mutant strain was constructed and analysed. The mutant displayed a pleiotropic phenotype, showing limited growth in an iron-depleted medium, higher sensitivity to copper, reduced motility on soft agar plates and attenuated virulence in witloof chicory leaves. Our results indicate the Tat system as an important determinant of the virulence and fitness of D. dadantii 3937. Potential Tat substrates related to the tatC mutant phenotype are discussed.

Catabolism of raffinose, sucrose, and melibiose in Erwinia chrysanthemi 3937

Erwinia chrysanthemi (Dickeya dadantii) is a plant pathogenic bacterium that has a large capacity to degrade the plant cell wall polysaccharides. The present study reports the metabolic pathways used by E. chrysanthemi to assimilate the oligosaccharides sucrose and raffinose, which are particularly abundant plant sugars. E. chrysanthemi is able to use sucrose, raffinose, or melibiose as a sole carbon source for growth. The two gene clusters scrKYABR and rafRBA are necessary for their catabolism. The phenotypic analysis of scr and raf mutants revealed cross-links between the assimilation pathways of these oligosaccharides. Sucrose catabolism is mediated by the genes scrKYAB. While the raf cluster is sufficient to catabolize melibiose, it is incomplete for raffinose catabolism, which needs two additional steps that are provided by scrY and scrB. The scr and raf clusters are controlled by specific repressors, ScrR and RafR, respectively. Both clusters are controlled by the global activator of carbohydrate catabolism, the cyclic AMP receptor protein (CRP). E. chrysanthemi growth with lactose is possible only for mutants with a derepressed nonspecific lactose transport system, which was identified as RafB. RafR inactivation allows the bacteria to the assimilate the novel substrates lactose, lactulose, stachyose, and melibionic acid. The raf genes also are involved in the assimilation of alpha- and beta-methyl-D-galactosides. Mutations in the raf or scr genes did not significantly affect E. chrysanthemi virulence. This could be explained by the large variety of carbon sources available in the plant tissue macerated by E. chrysanthemi.

Bacterial chemoattraction towards jasmonate plays a role in the entry of Dickeya dadantii through wounded tissues

Jasmonate is a key signalling compound in plant defence that is synthesized in wounded tissues. In this work, we have found that this molecule is also a strong chemoattractant for the phythopathogenic bacteria Dickeya dadantii (ex-Erwinia chysanthemi). Jasmonic acid induced the expression of a subset of bacterial genes possibly involved in virulence/survival in the plant apoplast and bacterial cells pre-treated with jasmonate showed increased virulence in chicory and Saintpaulia leaves. We also showed that tissue wounding induced bacterial spread through the leaf surface. Moreover, the jasmonate-deficient aos1 Arabidopsis thaliana mutant was more resistant to bacterial invasion by D. dadantii than wild-type plants. These results are consistent with the hypothesis that sensing jasmonic acid by this bacterium helps the pathogen to ingress inside plant tissues.

Fenpropimorph slows down the sterol pathway and the development of the arbuscular mycorrhizal fungus Glomus intraradices

The direct impact of fenpropimorph on the sterol biosynthesis pathway of Glomus intraradices when extraradical mycelia alone are in contact with the fungicide was investigated using monoxenic cultures. Bi-compartmental Petri plates allowed culture of mycorrhizal chicory roots in a compartment without fenpropimorph and exposure of extraradical hyphae to the presence of increasing concentrations of fenpropimorph (0, 0.02, 0.2, 2, 20 mg l(-1)). In the fungal compartment, sporulation, hyphal growth, and fungal biomass were already reduced at the lowest fungicide concentration. A decrease in total sterols, in addition to an increase in the amount of squalene and no accumulation of abnormal sterols, suggests that the sterol pathway is severely slowed down or that squalene epoxidase was inhibited by fenpropimorph in G. intraradices. In the root compartment, neither extraradical and intraradical development of the arbuscular mycorrhizal (AM) fungus nor root growth was affected when they were not in direct contact with the fungicide; only hyphal length was significantly affected at 2 mg l(-1) of fenpropimorph. Our results clearly demonstrate a direct impact of fenpropimorph on the AM fungus by a perturbation of its sterol metabolism.

Analysis of the LacI family regulators of Erwinia chrysanthemi 3937, involvement in the bacterial phytopathogenicity

Analysis of the regulators of the LacI family was performed in order to identify those potentially involved in pathogenicity of Erwinia chrysanthemi (Dickeya dadantii). Among the 18 members of the LacI family, the function of 11 members is either known or predicted and only 7 members have, as yet, no proposed function. Inactivation of these seven genes, called lfaR, lfbR, lfcR, lfdR, lfeR, lffR, and lfgR, demonstrated that four of them are important for plant infection. The lfaR and lfcR mutants showed a reduced virulence on chicory, Saintpaulia sp., and Arabidopsis. The lfeR mutant showed a reduced virulence on Arabidopsis. The lfdR mutant was more efficient than the wild-type strain in initiating maceration on Saintpaulia sp. The genetic environment of each regulator was examined to detect adjacent genes potentially involved in a common function. Construction of transcriptional fusions in these neighboring genes demonstrated that five regulators, LfaR, LfcR, LfeR, LffR, and LfgR, act as repressors of adjacent genes. Analysis of these fusions also indicated that the genes controlled by LfaR, LfcR, LfgR, and LffR are expressed during plant infection. Moreover, addition of crude plant extracts to culture medium demonstrated that the expression of the LfaR- and LfgR-controlled genes is specifically induced by plant components.

Differential role of ferritins in iron metabolism and virulence of the plant-pathogenic bacterium Erwinia chrysanthemi 3937

During infection, the phytopathogenic enterobacterium Erwinia chrysanthemi has to cope with iron-limiting conditions and the production of reactive oxygen species by plant cells. Previous studies have shown that a tight control of the bacterial intracellular iron content is necessary for full virulence. The E. chrysanthemi genome possesses two loci that could be devoted to iron storage: the bfr gene, encoding a heme-containing bacterioferritin, and the ftnA gene, coding for a paradigmatic ferritin. To assess the role of these proteins in the physiology of this pathogen, we constructed ferritin-deficient mutants by reverse genetics. Unlike the bfr mutant, the ftnA mutant had increased sensitivity to iron deficiency and to redox stress conditions. Interestingly, the bfr ftnA mutant displayed an intermediate phenotype for sensitivity to these stresses. Whole-cell analysis by Mössbauer spectroscopy showed that the main iron storage protein is FtnA and that there is an increase in the ferrous iron/ferric iron ratio in the ftnA and bfr ftnA mutants. We found that ftnA gene expression is positively controlled by iron and the transcriptional repressor Fur via the small antisense RNA RyhB. bfr gene expression is induced at the stationary phase of growth. The sigmaS transcriptional factor is necessary for this control. Pathogenicity tests showed that FtnA and the Bfr contribute differentially to the virulence of E. chrysanthemi depending on the host, indicating the importance of a perfect control of iron homeostasis in this bacterial species during infection.

Biogenesis of Fe/S proteins and pathogenicity: IscR plays a key role in allowing Erwinia chrysanthemi to adapt to hostile conditions

The Erwinia chrysanthemi genome is predicted to encode three systems, Nif, Isc and Suf, known to assist Fe/S cluster biogenesis and the CsdAE cysteine desulphurase. Single iscU, hscA and fdx mutants were found sensitive to paraquat and exhibited reduced virulence on both chicory leaves and Arabidopsis thaliana. Depletion of the whole Isc system led to a pleiotropic phenotype, including sensitivity to both paraquat and 2,2'-dipyridyl, auxotrophies for branched-chain amino acids, thiamine, nicotinic acid, and drastic alteration in virulence. IscR was able to suppress all of the phenotypes listed above in a sufC-dependent manner while depletion of the Isc system led to IscR-dependent activation of the suf operon. No virulence defects were found associated with csdA or nifS mutations. Surprisingly, we found that the sufC mutant was virulent against A. thaliana, whereas its virulence had been found altered in Saintpaulia. Collectively, these results lead us to propose that E. chrysanthemi possess the Fe/S biogenesis strategy suited to the physico-chemical conditions encountered in its host upon infection. In this view, the IscR regulator, which controls both Isc and Suf, is predicted to play a major role in the ability of E. chrysanthemi to colonize a wide array of different plants.

Endophytic fungi occurring in fennel, lettuce, chicory, and celery--commercial crops in southern Italy

The occurrence of endophytic fungi in fennel, lettuce, chicory, and celery crops was investigated in southern Italy. A total of 186 symptomless plants was randomly collected and sampled at the stage of commercial ripeness. Fungal species of Acremonium, Alternaria, Fusarium, and Plectosporium were detected in all four crops; Plectosporium tabacinum was the most common in all crop species and surveyed sites. The effect of eight endophytic isolates (five belonging to Plectosporium tabacinum and three to three species of Acremonium) inoculated on lettuce plants grown in gnotobiosis was assessed by recording plant height, root length and dry weight, collar diameter, root necrosis, and leaf yellowing. P. tabacinum and three species of Acremonium, inoculated on gnotobiotically grown lettuce plants, showed pathogenic activity that varied with the fungal isolate. Lettuce plants inoculated with the isolates Ak of Acremonium kiliense, Ac of Acremonium cucurbitacearum, and P35 of P. tabacinum showed an increased root growth, compared to the non-inoculated control. The high frequency of P. tabacinum isolation recorded in lettuce plants collected in Bari and Metaponto, and in fennel plants from Foggia agricultural districts, suggests a relationship not only between a crop species and P. tabacinum, but also between the occurrence of the endophyte and the crop rotation history of the soil.

Characterization of the Erwinia chrysanthemi Gan locus, involved in galactan catabolism

beta-1,4-Galactan is a major component of the ramified regions of pectin. Analysis of the genome of the plant pathogenic bacteria Erwinia chrysanthemi revealed the presence of a cluster of eight genes encoding proteins potentially involved in galactan utilization. The predicted transport system would comprise a specific porin GanL and an ABC transporter made of four proteins, GanFGK(2). Degradation of galactans would be catalyzed by the periplasmic 1,4-beta-endogalactanase GanA, which released oligogalactans from trimer to hexamer. After their transport through the inner membrane, oligogalactans would be degraded into galactose by the cytoplasmic 1,4-beta-exogalactanase GanB. Mutants affected for the porin or endogalactanase were unable to grow on galactans, but they grew on galactose and on a mixture of galactotriose, galactotetraose, galactopentaose, and galactohexaose. Mutants affected for the periplasmic galactan binding protein, the transporter ATPase, or the exogalactanase were only able to grow on galactose. Thus, the phenotypes of these mutants confirmed the functionality of the gan locus in transport and catabolism of galactans. These mutations did not affect the virulence of E. chrysanthemi on chicory leaves, potato tubers, or Saintpaulia ionantha, suggesting an accessory role of galactan utilization in the bacterial pathogeny.

Efflux pump gene expression in Erwinia chrysanthemi is induced by exposure to phenolic acids

Salicylic acid (SA) is an important signaling molecule in local and systemic plant resistance. Following infection by microbial pathogens and the initial oxidative burst in plants, SA accumulation functions in the amplification of defense gene expression. Production of pathogenesis-related proteins and toxic antimicrobial chemicals serves to protect the plant from infection. Successful microbial pathogens utilize a variety of mechanisms to rid themselves of toxic antimicrobial compounds. Important among these mechanisms are multidrug-resistance pumps that bring about the active efflux of toxic compounds from microbial cells. Here, we show that a combination SA and its precursors, t-cinnamic acid and benzoic acid, can activate expression of specific multidrug efflux pump-encoding genes in the plant pathogen Erwinia chrysanthemi and enhance survival of the bacterium in the presence of model as well as plant-derived antimicrobial chemicals. This ability of plant-pathogenic bacteria to co-opt plant defense-signaling molecules to activate multidrug efflux pumps may have evolved to ensure bacterial survival in susceptible host plants.

Effects of anthracene on development of an arbuscular mycorrhizal fungus and contribution of the symbiotic association to pollutant dissipation

The influence of anthracene, a low molecular weight polycyclic aromatic hydrocarbon (PAH), on chicory root colonization by Glomus intraradices and the effect of the root colonization on PAH degradation were investigated in vitro. The fungus presented a reduced development of extraradical mycelium and a decrease in sporulation, root colonization, and spore germination when exposed to anthracene. Mycorrhization improved the growth of the roots in the medium supplemented containing 140 mg l(-1) anthracene, suggesting a positive contribution of G. intraradices to the PAH tolerance of roots. Anthracene disappearance from the culture medium was quantified; results suggested that nonmycorrhizal chicory roots growing in vitro were able to contribute to anthracene dissipation, and in addition, that mycorrhization significantly enhanced anthracene dissipation. These monoxenic experiments demonstrated a positive contribution of the symbiotic association to anthracene dissipation in the absence of other microorganisms. In addition to anthracene dissipation, intracellular accumulation of anthracene was detected in lipid bodies of plant cells and fungal hyphae, indicating intracellular storage capacity of the pollutant by the roots and the mycorrhizal fungus.

The role of several multidrug resistance systems in Erwinia chrysanthemi pathogenesis

The role of several multidrug resistance (MDR) systems in the pathogenicity of Erwinia chrysanthemi 3937 was analyzed. Using the blast algorithm, we have identified several MDR systems in the E. chrysanthemi genome and selected two acridine resistance (Acr)-like systems, two Emr-like systems, and one member of the major facilitator super-family family to characterize. We generated mutants in genes encoding for these systems and analyzed the virulence of the mutant strains in different hosts and their susceptibility to antibiotics, detergents, dyes, and plant compounds. We have observed that the mutant strains are differentially affected in their virulence in different hosts and that the susceptibility to toxic substances is also differential. Both Acr systems seem to be implicated in the resistance to the plant antimicrobial peptide thionin. Similarly, the emr1AB mutant is unable to grow in the presence of the potato protein tuber extract and shows a decreased virulence in this tissue. These results indicate that the function of these systems in plants could be related to the specificity to extrude a toxic compound that is present in a given host.

PecS and PecT coregulate the synthesis of HrpN and pectate lyases, two virulence determinants in Erwinia chrysanthemi 3937

Erwinia chrysanthemi strain 3937 is a necrotrophic bacterial plant pathogen. Pectinolytic enzymes and, in particular, pectate lyases play a key role in soft rot symptoms; however, the efficient colonization of plants by E. chrysanthemi requires additional factors. These factors include HrpN (harpin), a heat-stable, glycine-rich hydrophilic protein, which is secreted by the type III secretion system. We investigated the expression of hrpN in E. chrysanthemi 3937 in various environmental conditions and different regulatory backgrounds. Using lacZ fusions, hrpN expression was markedly influenced by the carbon source, osmolarity, growth phase, and growth substrate. hrpN was repressed when pectinolysis started and negatively regulated by the repressors of pectate lyase synthesis, PecS and PecT. Primer extension data and in vitro DNA-protein interaction experiments support a model whereby PecS represses hrpN expression by binding to the hrpN regulatory region and inhibiting transcript elongation. The results suggest coordinated regulation of HrpN and pectate lyases by PecS and PecT. A putative model of the synthesis of these two virulence factors in E. chrysanthemi during pathogenesis is presented.

Tol-Pal proteins are critical cell envelope components of Erwinia chrysanthemi affecting cell morphology and virulence

The tol-pal genes are necessary for maintaining the outer-membrane integrity of Gram-negative bacteria. These genes were first described in Escherichia coli, and more recently in several other species. They are involved in the pathogenesis of E. coli, Haemophilus ducreyi, Vibrio cholerae and Salmonella enterica. The role of the tol-pal genes in bacterial pathogenesis was investigated in the phytopathogenic enterobacterium Erwinia chrysanthemi, assuming that this organism might be a good model for such a study. The whole Er. chrysanthemi tol-pal region was characterized. Tol-Pal proteins, except TolA, showed high identity scores with their E. coli homologues. Er. chrysanthemi mutants were constructed by introducing a uidA-kan cassette in the ybgC, tolQ, tolA, tolB, pal and ybgF genes. All the mutants were hypersensitive to bile salts. Mutations in tolQ, tolA, tolB and pal were deleterious for the bacteria, which required high concentrations of sugars or osmoprotectants for their viability. Consistent with this observation, they were greatly impaired in their cell morphology and division, which was evidenced by observations of cell filaments, spherical forms, membrane blebbing and mislocalized bacterial septa. Moreover, tol-pal mutants showed a reduced virulence in a potato tuber model and on chicory leaves. This could be explained by a combination of impaired phenotypes in the tol-pal mutants, such as reduced growth and motility and a decreased production of pectate lyases, the major virulence factor of Er. chrysanthemi.

Role of the PhoP-PhoQ system in the virulence of Erwinia chrysanthemi strain 3937: involvement in sensitivity to plant antimicrobial peptides, survival at acid Hh, and regulation of pectolytic enzymes

Erwinia chrysanthemi is a phytopathogenic bacterium that causes soft-rot diseases in a broad number of crops. The PhoP-PhoQ system is a key factor in pathogenicity of several bacteria and is involved in the bacterial resistance to different factors, including acid stress. Since E. chrysanthemi is confronted by acid pH during pathogenesis, we have studied the role of this system in the virulence of this bacterium. In this work, we have isolated and characterized the phoP and phoQ mutants of E. chrysanthemi strain 3937. It was found that: (i) they were not altered in their growth at acid pH; (ii) the phoQ mutant showed diminished ability to survive at acid pH; (iii) susceptibility to the antimicrobial peptide thionin was increased; (iv) the virulence of the phoQ mutant was diminished at low and high magnesium concentrations, whereas the virulence of the phoP was diminished only at low magnesium concentrations; (v) in planta Pel activity of both mutant strains was drastically reduced; and (vi) both mutants lagged behind the wild type in their capacity to change the apoplastic pH. These results suggest that the PhoP-PhoQ system plays a role in the virulence of this bacterium in plant tissues, although it does not contribute to bacterial growth at acid pH.

Pseudomonads associated with midrib rot and soft rot of butterhead lettuce and endive

During the past ten years, bacterial soft rot and midrib rot of glasshouse-grown butterhead lettuce (Lactuca sativa L. var. capitata) and field-grown endive (Cichorium endivia L.) has become increasingly common in the region of Flanders, Belgium. Severe losses and reduced market quality caused by bacterial rot represent an important economical threat for the production sector. Symptoms of midrib rot are a brownish rot along the midrib of one or more inner leaves, often accompanied by soft rot of the leaf blade. Twenty-five symptomatic lettuce and endive samples were collected from commercial growers at different locations in Flanders. Isolations of dominant bacterial colony types on dilution plates from macerated diseased tissue extracts yielded 282 isolates. All isolates were characterized by colony morphology and fluorescence on pseudomonas agar F medium, oxidase reaction, and soft rot ability on detached chicory leaves. Whole-cell fatty acid methyl esters profile analyses identified the majority of isolates (85%) as belonging to the Gammaproteobacteria, which included members of the family Enterobacteriaceae (14%) and of the genera Pseudomonas (73%), Stenotrophomonas (9%), and Acinetobacter (3%). Predominant bacteria were a diverse group of fluorescent Pseudomonas species. They were further differentiated based on the non-host hypersensitive reaction on tobacco and the ability to rot potato slices into 4 phenotypic groups: HR-/P- (57 isolates), HR-/P+ (54 isolates), HR+/P (16 isolates) and HR+/P+ (35 isolates). Artificial inoculation of suspensions of HR-, pectolytic fluorescent pseudomonads in the leaf midrib of lettuce plants produced various symptoms of soft rot, but they did not readily cause symptoms upon spray inoculation. Fluorescent pseudomonads with phenotype HR+ were consistently isolated from typical dark midrib rot symptoms, and selected isolates reproduced the typical midrib rot symptoms when spray-inoculated onto healthy lettuce plants.

The RhaS activator controls the Erwinia chrysanthemi 3937 genes rhiN, rhiT and rhiE involved in rhamnogalacturonan catabolism

Erwinia chrysanthemi causes soft-rot diseases of various plants by enzymatic degradation of the pectin in plant cell walls. The linear regions of pectin are composed of an acidic sugar, D-galacturonic acid. The ramified regions of pectin also include neutral sugars, and are rich in L-rhamnose residues. E. chrysanthemi is able to degrade these polysaccharides, polygalacturonate and rhamnogalacturonate. In E. chrysanthemi, the production of pectinases acting on linear regions is induced in the presence of polygalacturonate by a mechanism involving the repressor KdgR. The induction of the two adjacent E. chrysanthemi genes, designated rhiT and rhiN, is maximal after the simultaneous addition of both polygalacturonate and L-rhamnose. The rhiT product is homologous to the oligogalacturonide transporter TogT of E. chrysanthemi. The rhiN product is homologous to various proteins of unknown function, including a protein encoded by the plant-inducible locus picA of Agrobacterium tumefaciens. Both rhiT and rhiN are highly induced during plant infection. Various data suggest that RhiT and RhiN are involved in rhamnogalacturonate catabolism. RhiN is able to degrade the oligomers liberated by the rhamnogalacturonate lyase RhiE. The induction of the rhiTN operon in the presence of polygalacturonate results from control by the repressor KdgR. The additional induction of these genes by rhamnose is directly mediated by RhaS, a protein homologous to the activator of rhamnose catabolism in Escherichia coli. The virulence of an E. chrysanthemi rhaS mutant towards different host plants was clearly reduced. In this phytopathogenic bacterial species, RhaS positively regulates the transcription of the rhaBAD operon, involved in rhamnose catabolism, of the rhiE gene and of the rhiTN operon. The regulator RhaS plays a larger role in E. chrysanthemi than in other enterobacteria. Indeed, the RhaS control is not restricted to the catabolism of rhamnose but is extended to the degradation of plant polysaccharides that contain this sugar.

Erwinia chrysanthemi tolC is involved in resistance to antimicrobial plant chemicals and is essential for phytopathogenesis

TolC is the outer-membrane component of several multidrug resistance (MDR) efflux pumps and plays an important role in the survival and virulence of many gram-negative bacterial animal pathogens. We have identified and characterized the outer-membrane protein-encoding gene tolC in the bacterial plant pathogen Erwinia chrysanthemi EC16. The gene was found to encode a 51-kDa protein with 70% identity to its Escherichia coli homologue. The E. chrysanthemi gene was able to functionally complement the E. coli tolC gene with respect to its role in MDR efflux pumps. A tolC mutant of E. chrysanthemi was found to be extremely sensitive to antimicrobial agents, including several plant-derived chemicals. This mutant was unable to grow in planta and its ability to cause plant tissue maceration was severely compromised. The tolC mutant was shown to be defective in the efflux of berberine, a model antimicrobial plant chemical. These results suggest that by conferring resistance to the antimicrobial compounds produced by plants, the E. chrysanthemi tolC plays an important role in the survival and colonization of the pathogen in plant tissue.

The ybiT gene of Erwinia chrysanthemi codes for a putative ABC transporter and is involved in competitiveness against endophytic bacteria during infection

We investigated the role in bacterial infection of a putative ABC transporter, designated ybiT, of Erwinia chrysanthemi AC4150. The deduced sequence of this gene showed amino acid sequence similarity with other putative ABC transporters of gram-negative bacteria, such as Escherichia coli and Pseudomonas aeruginosa, as well as structural similarity with proteins of Streptomyces spp. involved in resistance to macrolide antibiotics. The gene contiguous to ybiT, designated as pab (putative antibiotic biosynthesis) showed sequence similarity with Pseudomonas and Streptomyces genes involved in the biosynthesis of antibiotics. A ybiT mutant (BT117) was constructed by marker exchange. It retained full virulence in potato tubers and chicory leaves, but it showed reduced ability to compete in planta against the wild-type strain or against selected saprophytic bacteria. These results indicate that the ybiT gene plays a role in the in planta fitness of the bacteria.

Osmoregulated periplasmic glucan synthesis is required for Erwinia chrysanthemi pathogenicity

Erwinia chrysanthemi is a phytopathogenic enterobacterium causing soft rot disease in a wide range of plants. Osmoregulated periplasmic glucans (OPGs) are intrinsic components of the gram-negative bacterial envelope. We cloned the opgGH operon of E. chrysanthemi, encoding proteins involved in the glucose backbone synthesis of OPGs, by complementation of the homologous locus mdoGH of Escherichia coli. OpgG and OpgH show a high level of similarity with MdoG and MdoH, respectively, and mutations in the opgG or opgH gene abolish OPG synthesis. The opg mutants exhibit a pleiotropic phenotype, including overproduction of exopolysaccharides, reduced motility, bile salt hypersensitivity, reduced protease, cellulase, and pectate lyase production, and complete loss of virulence. Coinoculation experiments support the conclusion that OPGs present in the periplasmic space of the bacteria are necessary for growth in the plant host.

Relative effects on virulence of mutations in the sap, pel, and hrp loci of Erwinia chrysanthemi

We constructed strains of Erwinia chrysanthemi EC16 with multiple mutations involving three virulence systems in this bacterium, namely pel (coding for the major pectate lyases pelABCE), hrp (hypersensitive response and pathogenicity), and sap (sensitivity to antimicrobial peptides). The relative effects on virulence of those mutations have been analyzed on potato tubers and chicory leaves. In potato tubers, the sap mutation (BT105) had a greater effect in the reduction of the virulence than the pel (CUCPB5006) and hrp (CUCPB5039) mutations. This reduction was similar to that observed in the pel-hrp double mutant (CUCPB5037). The analysis of the strains affected in Pel-Sap (BT106), Hrp-Sap (BT107), and Pel-Hrp-Sap (BT108) suggested that the effects of these mutations are additive. In chicory leaves, the mutation in the sap locus appeared to have a greater effect than in potato tubers. The competitive indices of strains BT105, UM1005 (Pel-), CUCPB5039, and CUCPB5037 have been estimated in vivo and in vitro. These results indicate that the mutation in the hrp locus can be complemented in vivo by coinfection, whereas the mutations in pel and sap cannot.

Pectate lyase PelI of Erwinia chrysanthemi 3937 belongs to a new family

Erwinia chrysanthemi 3937 secretes five major isoenzymes of pectate lyases encoded by the pel4, pelB, pelC, pelD, and pelE genes and a set of secondary pectate lyases, two of which, pelL and pelZ, have been already identified. We cloned the pelI gene, encoding a ninth pectate lyase of E. chrysanthemi 3937. The pelI reading frame is 1,035 bases long, corresponding to a protein of 344 amino acids including a typical amino-terminal signal sequence of 19 amino acids. The purified mature PelI protein has an isoelectric point of about 9 and an apparent molecular mass of 34 kDa. PelI has a preference for partially methyl esterified pectin and presents an endo-cleaving activity with an alkaline pH optimum and an absolute requirement for Ca2+ ions. PelI is an extracellular protein secreted by the Out secretory pathway of E. chrysanthemi. The PelI protein is very active in the maceration of plant tissues. A pelI mutant displayed reduced pathogenicity on chicory leaves, but its virulence did not appear to be affected on potato tubers or Saintpaulia ionantha plants. The pelI gene constitutes an independent transcriptional unit. As shown for the other pel genes, the transcription of pelI is dependent on various environmental conditions. It is induced by pectic catabolic products and affected by growth phase, oxygen limitation, temperature, nitrogen starvation, and catabolite repression. Regulation of pelI expression appeared to be dependent on the three repressors of pectinase synthesis, KdgR, PecS, and PecT, and on the global activator of sugar catabolism, cyclic AMP receptor protein. A functional KdgR binding site was identified close to the putative pelI promoter. Analysis of the amino acid sequence of PelI revealed high homology with a pectate lyase from Erwinia carotovora subsp. carotovora (65% identity) and low homology with pectate lyases of the phytopathogenic fungus Nectria haematococca (Fusarium solani). This finding indicates that PelI belongs to pectate lyase class III. Using immunoblotting experiments, we detected PelI homologs in various strains of E. chrysanthemi and E. carotovora subsp. carotovora but not in E. carotovora subsp. atroseptica.