Identification of regulators of hrp/hop genes of Erwinia carotovora ssp. carotovora and characterization of HrpL(Ecc) (SigmaL(EccM)), an alternative sigma factor

Regulation of corA, the Magnesium, Nickel, Cobalt Transporter, and Its Role in the Virulence of the Soft Rot Pathogen, Pectobacterium versatile Strain Ecc71

Pectobacterium versatile (formally P. carotovorum) causes disease on diverse plant species by synthesizing and secreting copious amount of plant-cell-wall-degrading exoenzymes including pectate lyases, polygalacturonases, cellulases, and proteases. Exoenzyme production and virulence are controlled by many factors of bacterial, host, and environmental origin. The ion channel forming the magnesium, nickel, and cobalt transporter CorA is required for exoenzyme production and full virulence in strain Ecc71. We investigated CorA's role as a virulence factor and its expression in P. versatile. Inhibiting the transport function of CorA by growing a CorA⁺ strain in the presence of specific CorA inhibitor, cobalt (III) hexaammine (Co (III)Hex), has no effect on exoenzyme production. Transcription of pel-1, encoding a pectate lyase isozyme, is decreased in the absence of CorA, suggesting that CorA influences exoenzyme production at the transcriptional level, although apparently not through its transport function. CorA^- and CorA⁺ strains grown in the presence of Co (III)Hex transcriptionally express corA at higher levels than CorA⁺ strains in the absence of an inhibitor, suggesting the transport role of corA contributes to autorepression. The expression of corA is about four-fold lower in HrpL^- strains lacking the hrp-specific extracytoplasmic sigma factor. The corA promoter region contains a sequence with a high similarity to the consensus Hrp box, suggesting that corA is part of Hrp regulon. Our data suggest a complex role, possibly requiring the physical presence of the CorA protein in the virulence of the Pectobacterium versatile strain Ecc71.

The PhoPQ Two-Component System Is the Major Regulator of Cell Surface Properties, Stress Responses and Plant-Derived Substrate Utilisation During Development of Pectobacterium versatile-Host Plant Pathosystems

Pectobacterium versatile (formerly P. carotovorum) is a recently defined species of soft rot enterobacteria capable of infecting many plant hosts and damaging different tissues. Complex transcriptional regulation of virulence properties can be expected for such a versatile pathogen. However, the relevant information is available only for related species and is rather limited. The PhoPQ two-component system, originally described in pectobacteria as PehRS, was previously shown to regulate a single gene, pehA. Using an insertional phoP mutant of Pectobacterium versatile (earlier-P. carotovorum), we demonstrate that PhoP regulates at least 115 genes with a majority of them specific for pectobacteria. The functions performed by PhoP-controlled genes include degradation, transport and metabolism of plant-derived carbon sources (polygalacturonate, arabinose-containing polysaccharides and citrate), modification of bacterial cell envelope and stress resistance. We also demonstrated PhoP involvement in establishing the order of plant cell wall decomposition and utilisation of the corresponding breakdown products. Based on experimental data and in silico analysis, we defined a PhoP binding site motif and provided proof for its universality in enteric bacteria. Scanning P. versatile genome for the locations of this motif suggested a much larger PhoP regulon enriched with the genes important for a plant pathogen, which makes PhoP a global virulence regulator. Potential PhoP targets include many regulatory genes and PhoP control over one of them, expI, was confirmed experimentally, highlighting the link between the PhoPQ two-component and quorum sensing systems. High concentrations of calcium and magnesium ions were found to abolish the PhoPQ-dependent transcription activation but did not relieve repression. Reduced PhoP expression and minimisation of PhoP dependence of regulon members' expression in P. versatile cells isolated from potato tuber tissues suggest that PhoPQ system is a key switch of expression levels of multiple virulence-related genes fine-tuned to control the development of P. versatile-host plant pathosystem.

Discovery of plant phenolic compounds that act as type III secretion system inhibitors or inducers of the fire blight pathogen, Erwinia amylovora

Erwinia amylovora causes a devastating disease called fire blight in rosaceous plants. The type III secretion system (T3SS) is one of the important virulence factors utilized by E. amylovora in order to successfully infect its hosts. By using a green fluorescent protein (GFP) reporter construct combined with a high-throughput flow cytometry assay, a library of phenolic compounds and their derivatives was studied for their ability to alter the expression of the T3SS. Based on the effectiveness of the compounds on the expression of the T3SS pilus, the T3SS inhibitors 4-methoxy-cinnamic acid (TMCA) and benzoic acid (BA) and one T3SS inducer, trans-2-(4-hydroxyphenyl)-ethenylsulfonate (EHPES), were chosen for further study. Both the T3SS inhibitors (TMCA and BA) and the T3SS inducer (EHPES) were found to alter the expression of T3SS through the HrpS-HrpL pathway. Additionally, TMCA altered T3SS expression through the rsmBEa-RsmAEa system. Finally, we found that TMCA and BA weakened the hypersensitive response (HR) in tobacco by suppressing the T3SS of E. amylovora. In our study, we identified phenolic compounds that specifically targeted the T3SS. The T3SS inhibitor may offer an alternative approach to antimicrobial therapy by targeting virulence factors of bacterial pathogens.

The type III secreted effector DspE is required early in solanum tuberosum leaf infection by Pectobacterium carotovorum to cause cell death, and requires Wx(3-6)D/E motifs

Pectobacterium species are enterobacterial plant-pathogens that cause soft rot disease in diverse plant species. Unlike hemi-biotrophic plant pathogenic bacteria, the type III secretion system (T3SS) of Pectobacterium carotovorum subsp. carotovorum (P. carotovorum) appears to secrete only one effector protein, DspE. Previously, we found that the T3SS regulator HrpL and the effector DspE are required for P. carotovorum pathogenesis on leaves. Here, we identified genes up-regulated by HrpL, visualized expression of dspE in leaves, and established that DspE causes host cell death. DspE required its full length and WxxxE-like motifs, which are characteristic of the AvrE-family effectors, for host cell death. We also examined expression in plant leaves and showed that hrpL is required for the expression of dspE and hrpN, and that the loss of a functional T3SS had unexpected effects on expression of other genes during leaf infection. These data support a model where P. carotovorum uses the T3SS early in leaf infection to initiate pathogenesis through elicitation of DspE-mediated host cell death.

Genetic characterization of the HrpL regulon of the fire blight pathogen Erwinia amylovora reveals novel virulence factors

The bacterial pathogen Erwinia amylovora is the causal agent of fire blight, an economically significant disease of apple and pear. Disease initiation by E. amylovora requires the translocation of effector proteins into host cells via the hypersensitive response and pathogenicity (hrp) type III secretion system (T3SS). The alternative sigma factor HrpL positively regulates the transcription of structural and translocated components of the T3SS via hrp promoter elements. To characterize genome-wide HrpL-dependent gene expression in E. amylovora Ea1189, wild-type and Ea1189ΔhrpL strains were cultured in hrp-inducing minimal medium, and total RNA was compared using a custom microarray designed to represent the annotated genes of E. amylovora ATCC 49946. The results revealed 24 genes differentially regulated in Ea1189ΔhrpL relative to Ea1189 with fold-change expression ratios greater than 1.5; of these, 19 genes exhibited decreased transcript abundance and five genes showed increased transcript abundance relative to Ea1189. To expand our understanding of the HrpL regulon and to elucidate direct versus indirect HrpL-mediated effects on gene expression, the genome of E. amylovora ATCC 49946 was examined in silico using a hidden Markov model assembled from known Erwinia spp. hrp promoters. This technique identified 15 putative type III novel hrp promoters, seven of which were validated with quantitative polymerase chain reaction based on expression analyses. It was found that HrpL-regulated genes encode all known components of the hrp T3SS, as well as five putative type III effectors. Eight genes displayed apparent indirect HrpL regulation, suggesting that the HrpL regulon is connected to downstream signalling networks. The construction of deletion mutants of three novel HrpL-regulated genes resulted in the identification of additional virulence factors as well as mutants displaying abnormal motility and biofilm phenotypes.

The role of secretion systems and small molecules in soft-rot Enterobacteriaceae pathogenicity

Soft-rot Enterobacteriaceae (SRE), which belong to the genera Pectobacterium and Dickeya, consist mainly of broad host-range pathogens that cause wilt, rot, and blackleg diseases on a wide range of plants. They are found in plants, insects, soil, and water in agricultural regions worldwide. SRE encode all six known protein secretion systems present in gram-negative bacteria, and these systems are involved in attacking host plants and competing bacteria. They also produce and detect multiple types of small molecules to coordinate pathogenesis, modify the plant environment, attack competing microbes, and perhaps to attract insect vectors. This review integrates new information about the role protein secretion and detection and production of ions and small molecules play in soft-rot pathogenicity.

Regulatory network controlling extracellular proteins in Erwinia carotovora subsp. carotovora: FlhDC, the master regulator of flagellar genes, activates rsmB regulatory RNA production by affecting gacA and hexA (lrhA) expression

Erwinia carotovora subsp. carotovora produces an array of extracellular proteins (i.e., exoproteins), including plant cell wall-degrading enzymes and Harpin, an effector responsible for eliciting hypersensitive reaction. Exoprotein genes are coregulated by the quorum-sensing signal, N-acyl homoserine lactone, plant signals, an assortment of transcriptional factors/regulators (GacS/A, ExpR1, ExpR2, KdgR, RpoS, HexA, and RsmC) and posttranscriptional regulators (RsmA, rsmB RNA). rsmB RNA production is positively regulated by GacS/A, a two-component system, and negatively regulated by HexA (PecT in Erwinia chrysanthemi; LrhA [LysR homolog A] in Escherichia coli) and RsmC, a putative transcriptional adaptor. While free RsmA, an RNA-binding protein, promotes decay of mRNAs of exoprotein genes, binding of RsmA with rsmB RNA neutralizes the RsmA effect. In the course of studies of GacA regulation, we discovered that a locus bearing strong homology to the flhDC operon of E. coli also controls extracellular enzyme production. A transposon insertion FlhDC(-) mutant produces very low levels of pectate lyase, polygalacturonase, cellulase, protease, and E. carotovora subsp. carotovora Harpin (Harpin(Ecc)) and is severely attenuated in its plant virulence. The production of these exoproteins is restored in the mutant carrying an FlhDC(+) plasmid. Sequence analysis and transcript assays disclosed that the flhD operon of E. carotovora subsp. carotovora, like those of other enterobacteria, consists of flhD and flhC. Complementation analysis revealed that the regulatory effect requires functions of both flhD and flhC products. The data presented here show that FlhDC positively regulates gacA, rsmC, and fliA and negatively regulates hexA (lrhA). Evidence shows that FlhDC controls extracellular protein production through cumulative effects on hexA and gacA. Reduced levels of GacA and elevated levels of HexA in the FlhDC(-) mutant are responsible for the inhibition of rsmB RNA production, a condition conducive to the accumulation of free RsmA. Indeed, studies with an RsmA(-) FlhDC(-) double mutant and multiple copies of rsmB(+) DNA establish that the negative effect of FlhDC deficiency is exerted via RsmA. The FlhDC-mediated regulation of fliA has no bearing on exoprotein production in E. carotovora subsp. carotovora. Our observations for the first time establish a regulatory connection between FlhDC, HexA, GacA, and rsmB RNA in the context of the exoprotein production and virulence of E. carotovora subsp. carotovora.

Host range and molecular phylogenies of the soft rot enterobacterial genera pectobacterium and dickeya

ABSTRACT Pectobacterium and Dickeya spp. are related broad-host-range entero-bacterial pathogens of angiosperms. A review of the literature shows that these genera each cause disease in species from at least 35% of angiosperm plant orders. The known host ranges of these pathogens partially overlap and, together, these two genera are pathogens of species from 50% of angiosperm plant orders. Notably, there are no reported hosts for either genus in the eudicots clade and no reported Dickeya hosts in the magnoliids or eurosids II clades, although Pectobacterium spp. are pathogens of at least one plant species in the magnoliids and at least one in each of the three eurosids II plant orders. In addition, Dickeya but not Pectobacterium spp. have been reported on a host in the rosids clade and, unlike Pectobacterium spp., have been reported on many Poales species. Natural disease among nonangiosperms has not been reported for either genus. Phylogenetic analyses of sequences concatenated from regions of seven housekeeping genes (acnA, gapA, icdA, mdh, mtlD, pgi, and proA) from representatives of these genera demonstrated that Dickeya spp. and the related tree pathogens, the genus Brenneria, are more diverse than Pectobacterium spp. and that the Pectobacterium strains can be divided into at least five distinct clades, three of which contain strains from multiple host plants.

Use of a pooled transposon mutation grid to demonstrate roles in disease development for Erwinia carotovora subsp. atroseptica putative type III secreted effector (DspE/A) and helper (HrpN) proteins

Soft rot Erwinia spp., like other closely related plant pathogens, possess a type III secretion system (TTSS) (encoded by the hrp gene cluster) implicated in disease development. We report the sequence of the entire hrp gene cluster and adjacent dsp genes in Erwinia carotovora subsp. atroseptica SCRI1039. The cluster is similar in content and structural organization to that in E. amylovora. However, eight putative genes of unknown function located within the E. carotovora subsp. atroseptica cluster do not have homologues in the E. amylovora cluster. An arrayed set of Tn5 insertional mutants (mutation grid) was constructed and pooled to allow rapid isolation of mutants for any given gene by polymerase chain reaction screening. This novel approach was used to obtain mutations in two structural genes (hrcC and hrcV), the effector gene dspE/A, and the helper gene hrpN. An improved pathogenicity assay revealed that these mutations led to significantly reduced virulence, showing that both the putative E. carotovora subsp. atroseptica TTSS-delivered effector and helper proteins are required for potato infection.

Characterization of the hrp pathogenicity cluster of Erwinia carotovora subsp. carotovora: high basal level expression in a mutant is associated with reduced virulence

Extracellularly targeted proteins are crucial for virulence of gram-negative phytopathogenic bacteria. Erwinia carotovora subsp. carotovora employs the so-called type II (GSP) pathway to secrete a number of pectinases and cellulases, which cause the typical tissue maceration symptoms of soft-rot disease. The type III (hrp) pathway is the major virulence determinant in the genera Pseudomonas, Ralstonia and Xanthomonas, and in non-macerating species of Erwinia. The hrp cluster was recently partially characterized from E. carotovora sp. carotovora, and shown to affect virulence during early stages of infection. Here we have isolated and characterized 15 hrp genes comprising the remaining part of the cluster. The genes hrpL, hrpXY and hrpS were deduced to be transcribed as separate units, whereas the 11 remaining genes from hrpJ to hrcU form a single large operon. The hrpX gene, which codes for the sensory kinase of the two-component regulatory locus hrpXY was insertionally inactivated by placing a transposon (entranceposon) in the gene. The resulting mutant bacterium expresses the hrp genes at high basal level even in a non-inducing medium. This relative overexpression was shown to be due to the hrpX::entranceposon insertion causing enhanced transcription of the downstream hrpY gene. The hrpX(-)-hrpYC mutant bacterium exhibited a slower growth rate and the appearance of disease symptoms in infected Arabidopsis plants was delayed, as compared to the wild-type strain. The need for hrp gene expression for virulence has been documented in both non-macerating plant pathogens and in soft-rotting Erwinia sp. but this is the first demonstration that high basal-level expression of hrp -regulated genes may actually have a negative impact on disease progress in a susceptible host plant.

Regulation of Erwinia carotovora hrpL(Ecc) (sigma-L(Ecc)), which encodes an extracytoplasmic function subfamily of sigma factor required for expression of the HRP regulon

In Erwinia carotovora subsp. carotovora (Ecc) strain 71 (Ecc71), HrpL(Ecc), an alternate sigma factor of the extracytoplasmic function subfamily, plays a central role in the expression of the hrp (hypersensitive reaction and pathogenicity) regulon. We document here that sigma-54 (RpoN) is required for full expression of hrpL(Ecc) and that HrpS, in conjunction with sigma-54, activates hrpL(Ecc) transcription. We also made the novel observation that integration host factor is required for the activation of the hrpL(Ecc) promoter. Our findings reveal that the RsmA/rsmB RNA-mediated post-transcriptional system, known to control extracellular enzyme and harpin production, affects hrpL(Ecc) expression as well. For example, hrpL(Ecc) RNA levels are barely detected in an RsmB- strain. Conversely, hrpL(Ecc) mRNA levels are much higher in RsmA- bacteria than in the RsmA+ parent. This effect is due to RsmA-promoted decay of hrpL(Ecc) RNA. Moreover, the following regulators known to control the production of either RsmA, rsmB RNA, or both also affect hrpL(Ecc) expression: GacA (response regulator of a two-component system), KdgR (an IcII type repressor), HexA (a LysR type repressor), RsmC (a putative transcriptional adapter). Based upon the data now available for Ecc and extrapolating from the evidence in other systems, we propose a tentative model that depicts the Hrp regulatory system of Ecc and explains the basis for coregulation of extracellular enzyme production and expression of the Hrp regulon.