The global virulence regulator PhcA negatively controls the Ralstonia solanacearum hrp regulatory cascade by repressing expression of the PrhIR signaling proteins

Positive regulation of the PhcB neighbouring regulator PrhX on expression of the type III secretion system and pathogenesis in Ralstonia solanacearum

Ralstonia solanacearum PhcB and PhcA control a quorum-sensing (QS) system that globally regulates expression of about one third of all genes, including pathogenesis genes. The PhcB-PhcA QS system positively regulates the production of exopolysaccharide (EPS) and negatively regulates hrp gene expression, which is crucial for the type III secretion system (T3SS). Both EPS and the T3SS are essential for pathogenicity. The gene rsc2734 is located upstream of a phcBSR operon and annotated as a response regulator of a two-component system. Here, we demonstrated that RSc2734, hereafter named PrhX, positively regulated hrp gene expression via a PrhA-PrhIR-PrhJ-HrpG signalling cascade. Moreover, PrhX was crucial for R. solanacearum to invade host roots and grow in planta naturally. prhX expression was independent of the PhcB-PhcA QS system. PrhX did not affect the expression of phcB and phcA and the QS-dependent phenotypes, such as EPS production and biofilm formation. Our results provide novel insights into the complex regulatory network of the T3SS and pathogenesis in R. solanacearum.

Novel mandelic acid derivatives suppress virulence of Ralstonia solanacearum via type III secretion system

BACKGROUND

Bacterial wilt induced by Ralstonia solanacearum is regarded as one of the most devastating diseases. However, excessive and repeated use of the same bactericides has resulted in development of bacterial resistance. Targeting bacterial virulence factors, such as type III secretion system (T3SS), without inhibiting bacterial growth is a possible assay to discover new antimicrobial agents.

RESULTS

In this work, identifying new T3SS inhibitors, a series of mandelic acid derivatives with 2-mercapto-1,3,4-thiazole moiety was synthesized. One of them, F-24, inhibited the transcription of hrpY gene significantly. The presence of this compound obviously attenuated hypersensitive response (HR) without inhibiting bacterial growth of R. solanacearum. The transcription levels of those typical T3SS genes were reduced to various degrees. The test of the ability of F-24 in protecting plants demonstrated that F-24 protected tomato plants against bacterial wilt without restricting the multiplication of R. solanacearum. The mechanism of this T3SS inhibition is through the PhcR-PhcA-PrhG-HrpB pathway.

CONCULSION

The screened F-24 could inhibit R. solanacearum T3SS and showed better inhibitory activity than previously reported inhibitors without affecting the growth of the strain, and F-24 is a compound with good potential in the control of R. solanacearum. © 2023 Society of Chemical Industry.

PhcX Is a LqsR-family response regulator that contributes to Ralstonia solanacearum virulence and regulates multiple virulence factors

IMPORTANCE

The bacterial wilt caused by the soil-borne phytopathogen Ralstonia solanacearum is one of the most destructive crop diseases. To achieve a successful infection, R. solanacearum has evolved an intricate regulatory network to orchestrate the expression of an arsenal of virulence factors and fine-tune the allocation of energy. However, despite the wealth of knowledge gained in the past decades, many players and connections are still missing from the network. The importance of our study lies in the identification of PhcX, a novel conserved global regulator with critical roles in modulating the virulence and metabolism of R. solanacearum. PhcX affects many well-characterized regulators and exhibits contrasting modes of regulation from the central regulator PhcA on a variety of virulence-associated traits and genes. Our findings add a valuable piece to the puzzle of how the pathogen regulates its proliferation and infection, which is critical for understanding its pathogenesis and developing disease control strategies.

Functional characterization of two 3-dehydroquinases of AroQ1 and AroQ2 in the shikimate pathway and expression of genes for the type III secretion system in Ralstonia solanacearum

The shikimate pathway is a general route for the biosynthesis of aromatic amino acids (AAAs) in many microorganisms. A 3-dehydroquinase, AroQ, controls the third step of the shikimate pathway that catalyzes the formation of 3-dehydroquinate from 3-dehydroshikimate via a trans-dehydration reaction. Ralstonia solanacearum harbors two 3-dehydroquinases, AroQ1 and AroQ2, sharing 52% similarity in amino acids. Here, we demonstrated that two 3-dehydroquinases, AroQ1 and AroQ2, are essential for the shikimate pathway in R. solanacearum. The growth of R. solanacearum was completely diminished in a nutriment-limited medium with the deletion of both aroQ1 and aroQ2, while substantially impaired in planta. The aroQ1/2 double mutant was able to replicate in planta but grew slowly, which was ~4 orders of magnitude less than the parent strain to proliferate to the maximum cell densities in tomato xylem vessels. Moreover, the aroQ1/2 double mutant failed to cause disease in tomato and tobacco plants, whereas the deletion of either aroQ1 or aroQ2 did not alter the growth of R. solanacearum or pathogenicity on host plants. Supplementary shikimic acid (SA), an important intermediate of the shikimate pathway, substantially restored the diminished or impaired growth of aroQ1/2 double mutant in a limited medium or inside host plants. The necessity of AroQ1 and AroQ2 on the pathogenicity of solanacearum toward host plants was partially due to insufficient SA inside host plants. Moreover, the deletion of both aroQ1 and aroQ2 significantly impaired the expression of genes for the type III secretion system (T3SS) both in vitro and in planta. Its involvement in the T3SS was mediated through the well-characterized PrhA signaling cascade and was independent of growth deficiency under nutrient-limited conditions. Taken together, R. solanacearum 3-dehydroquinases play important roles in bacterial growth, the expression of the T3SS, and pathogenicity in host plants. These results could extend our insights into the understanding of the biological function of AroQ and the sophisticated regulation of the T3SS in R. solanacearum.

Recent advances in esophageal squamous cell precancerous conditions: A review

Esophageal squamous cell carcinoma (ESCC) is a common cancer in many developing countries in Asia and Africa, with a 5-year survival rate of approximately 20%. Most cases are diagnosed at an advanced age when there is no effective treatment strategy. Esophageal precancerous conditions have a much better prognosis, with a 5-year survival rate of over 90% by endoscopic diagnosis and treatment. Nevertheless, limitations, contraindications, and lymph node metastasis incompetency of endoscopy. Thus, the diagnosis and treatment of esophageal precancerous lesions remain a significant challenge. Biomarker investigations provide opportunities for target detection and therapy. Additionally, drug development is ongoing. Changes in lifestyle habits, such as diet balance, smoking and alcohol cessation, are beneficial for the prognosis of esophageal precancerous lesions. Collectively, multiple and sequential diagnoses and treatments are essential for curing esophageal precancerous lesions and reducing the incidence and mortality of ESCC.

Functional characterization of a gamma-glutamyl phosphate reductase ProA in proline biosynthesis and promoting expression of type three secretion system in Ralstonia solanacearum

Ralstonia solanacearum RSc2741 has been predicted as a gamma-glutamyl phosphate reductase ProA catalyzing the second reaction of proline formation from glutamate. Here, we experimentally demonstrated that proA mutants were proline auxotrophs that failed to grow in a minimal medium, and supplementary proline, but not glutamate, fully restored the diminished growth, confirming that ProA is responsible for the biosynthesis of proline from glutamate in R. solanacearum. ProA was previously identified as one of the candidates regulating the expression of genes for type three secretion system (T3SS), one of the essential pathogenicity determinants of R. solanacearum. Supplementary proline significantly enhanced the T3SS expression both in vitro and in planta, indicating that proline is a novel inducer of the T3SS expression. Deletion of proA substantially impaired the T3SS expression both in vitro and in planta even under proline-supplemented conditions, indicating that ProA plays additional roles apart from proline biosynthesis in promoting the expression of the T3SS genes. It was further revealed that the involvement of ProA in the T3SS expression was mediated through the pathway of PrhG-HrpB. Both the proA mutants and the wild-type strain grew in the intercellular spaces of tobacco leaves, while their ability to invade and colonize tobacco xylem vessels was substantially impaired, which was about a 1-day delay for proA mutants to successfully invade xylem vessels and was about one order of magnitude less than the wild-type strain to proliferate to the maximum densities in xylem vessels. It thus resulted in substantially impaired virulence of proA mutants toward host tobacco plants. The impaired abilities of proA mutants to invade and colonize xylem vessels were not due to possible proline insufficiency in the rhizosphere soil or inside the plants. All taken together, these results extend novel insights into the understanding of the biological function of ProA and sophisticated regulation of the T3SS and pathogenicity in R. solanacearum.

Silicon Controls Bacterial Wilt Disease in Tomato Plants and Inhibits the Virulence-Related Gene Expression of Ralstonia solanacearum

Silicon (Si) has a multifunctional role in improving plant growth and enhancing plant disease resistance, but its mechanisms are not fully understood. In this study, we investigated the impacts of silicon application on the control of bacterial wilt and elucidated the molecular mechanisms using transcriptome sequencing. Compared to non-Si treatment, Si application (0.5–2 mM) significantly reduces tomato bacterial wilt index by 46.31–72.23%. However, Si does not influence the growth of R. solanacearum. Si application negatively influences R. solanacearum exopolysaccharide (EPS) synthesis and biofilm formation. Transcriptome analysis showed that Si treatment significantly downregulates the expression of virulence genes’ transcriptional regulator (xpsR), EPS synthesis-related genes (epsD and tek), and type III effectors (HrpB2, SpaO, and EscR) in R. solanacearum. In addition, Si remarkably upregulates the expression of twitch motor-related genes (pilE2, pilE, fimT, and PilX). These findings suggest that silicon-suppressed tomato wilt incidence may be due to the regulation of the virulence-related genes of R. solanacearum by Si. Our research adds new knowledge to the application of Si in the field of disease control.

A CysB regulator positively regulates cysteine synthesis, expression of type III secretion system genes, and pathogenicity in Ralstonia solanacearum

A syringe-like type III secretion system (T3SS) plays essential roles in the pathogenicity of Ralstonia solanacearum, which is a causal agent of bacterial wilt disease on many plant species worldwide. Here, we characterized functional roles of a CysB regulator (RSc2427) in R. solanacearum OE1-1 that was demonstrated to be responsible for cysteine synthesis, expression of the T3SS genes, and pathogenicity of R. solanacearum. The cysB mutants were cysteine auxotrophs that failed to grow in minimal medium but grew slightly in host plants. Supplementary cysteine substantially restored the impaired growth of cysB mutants both in minimal medium and inside host plants. Genes of cysU and cysI regulons have been annotated to function for R. solanacearum cysteine synthesis; CysB positively regulated expression of these genes. Moreover, CysB positively regulated expression of the T3SS genes both in vitro and in planta through the PrhG to HrpB pathway, whilst impaired expression of the T3SS genes in cysB mutants was independent of growth deficiency under nutrient-limited conditions. CysB was also demonstrated to be required for exopolysaccharide production and swimming motility, which contribute jointly to the host colonization and infection process of R. solanacearum. Thus, CysB was identified here as a novel regulator on the T3SS expression in R. solanacearum. These results provide novel insights into understanding of various biological functions of CysB regulators and complex regulatory networks on the T3SS in R. solanacearum.

Involvement of a FAD-Linked Oxidase RSc0454 for Expression of the Type III Secretion System and Pathogenicity in Ralstonia solanacearum

Ralstonia solanacearum RSc0454 is predicted as a FAD-linked oxidase based on protein homologies, while it contains distinct domains of lactate dehydrogenase and succinate dehydrogenase. A previous study demonstrated that RSc0454 exhibits lactate dehydrogenase activity using pyruvate and NADH as substrates, and is essential for pathogenicity of R. solanacearum. Here, we genetically characterized involvement of RSc0454 on bacterial growth and expression of genes for the type III secretion system (T3SS, a pathogenicity determinant) in R. solanacearum. The RSc0454 mutant grew normally in rich medium but grew faintly in host plants, and failed to grow in minimal medium. Supplementary succinate but not lactate could substantially restore some phenotypes of RSc0454 mutants, including faint growth in host plants, diminished growth in the minimal medium, and lost pathogenicity toward host plants. Expression of T3SS genes is directly controlled by a master regulator, HrpB, and hrpB expression is positively regulated by HrpG and PrhG in parallel ways. Deletion of RSc0454 substantially reduced expression levels of hrpB and T3SS both in vitro and in planta. Moreover, RSc0454 is revealed to be required for the T3SS expression via HrpG and PrhG, although through some novel pathway, and impaired expression of these genes was not due to growth deficiency of RSc0454 mutants. RSc0454 is suggested to be important for redox balance inside cells, and supplementary NADH partially restored diminished growth of the RSc0454 mutant in the minimal medium only in the presence of succinate at some moderate concentrations, indicating that the unbalanced redox in the RSc0454 mutant might be responsible for its diminished growth in the minimal medium. Taken together, these results provide novel insights into the understanding of various biological functions of this FAD-linked oxidase RSc0454 and involvement of the redox balance on expression of the T3SS in R. solanacearum.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Dynamic expression of Ralstonia solanacearum virulence factors and metabolism-controlling genes during plant infection

BACKGROUND

Ralstonia solanacearum is the causal agent of bacterial wilt, a devastating plant disease responsible for serious economic losses especially on potato, tomato, and other solanaceous plant species in temperate countries. In R. solanacearum, gene expression analysis has been key to unravel many virulence determinants as well as their regulatory networks. However, most of these assays have been performed using either bacteria grown in minimal medium or in planta, after symptom onset, which occurs at late stages of colonization. Thus, little is known about the genetic program that coordinates virulence gene expression and metabolic adaptation along the different stages of plant infection by R. solanacearum.

RESULTS

We performed an RNA-sequencing analysis of the transcriptome of bacteria recovered from potato apoplast and from the xylem of asymptomatic or wilted potato plants, which correspond to three different conditions (Apoplast, Early and Late xylem). Our results show dynamic expression of metabolism-controlling genes and virulence factors during parasitic growth inside the plant. Flagellar motility genes were especially up-regulated in the apoplast and twitching motility genes showed a more sustained expression in planta regardless of the condition. Xylem-induced genes included virulence genes, such as the type III secretion system (T3SS) and most of its related effectors and nitrogen utilisation genes. The upstream regulators of the T3SS were exclusively up-regulated in the apoplast, preceding the induction of their downstream targets. Finally, a large subset of genes involved in central metabolism was exclusively down-regulated in the xylem at late infection stages.

CONCLUSIONS

This is the first report describing R. solanacearum dynamic transcriptional changes within the plant during infection. Our data define four main genetic programmes that define gene pathogen physiology during plant colonisation. The described expression of virulence genes, which might reflect bacterial states in different infection stages, provides key information on the R. solanacearum potato infection process.

A genome-wide scan for genes under balancing selection in the plant pathogen Ralstonia solanacearum

Background

Plant pathogens are under significant selective pressure by the plant host. Consequently, they are expected to have adapted to this condition or contribute to evading plant defenses. In order to acquire long-term fitness, plant bacterial pathogens are usually forced to maintain advantageous genetic diversity in populations. This strategy ensures that different alleles in the pathogen’s gene pool are maintained in a population at frequencies larger than expected under neutral evolution. This selective process, known as balancing selection, is the subject of this work in the context of a common bacterial phytopathogen. We performed a genome-wide scan of Ralstonia solanacearum species complex, an aggressive plant bacterial pathogen that shows broad host range and causes a devastating disease called ‘bacterial wilt’.

Results

Using a sliding window approach, we analyzed 57 genomes from three phylotypes of the R. solanacearum species complex to detect signatures of balancing selection. A total of 161 windows showed extreme values in three summary statistics of population genetics: Tajima’s D, θ_w and Fu & Li’s D*. We discarded any confounding effects due to demographic events by means of coalescent simulations of genetic data. The prospective windows correspond to 78 genes with known function that map in any of the two main replicons (1.7% of total number of genes). The candidate genes under balancing selection are related to primary metabolism and other basal activities (51.3%) or directly associated to virulence (48.7%), the latter being involved in key functions targeted to dismantle plant defenses or to participate in critical stages in the pathogenic process.

Conclusions

We identified various genes under balancing selection that play a significant role in basic metabolism as well as in virulence of the R. solanacearum species complex. These genes are useful to understand and monitor the evolution of bacterial pathogen populations and emerge as potential candidates for future treatments to induce specific plant immune responses.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1456-6) contains supplementary material, which is available to authorized users.

Characterization of medicinal plant-associated biocontrol Bacillus subtilis (SSL2) by liquid chromatography-mass spectrometry and evaluation of compounds by in silico and in vitro methods

This study explores the antimicrobial properties of bioactive secondary metabolites extracted from the medicinal plant (Solanum surattense)-associated Bacillus subtilis strain SSL2. The secondary metabolites were extracted from B. subtilis (SSL2) using ethyl acetate, acetone, butanol, chloroform and methanol solvents. The crude extract was tested against two wilt causing pathogens: Ralstonia solanacearum and Fusarium oxysporum. The results revealed that the ethyl acetate extract has maximum inhibition against both the pathogens tested in this study. Furthermore, liquid chromatography-mass spectrometry (LC-MS) analysis of ethyl acetate extract identified 80 different compounds based on mass-to-charge ratio, database difference, resolution of mass spectrum and so on. Among the 80 compounds, citrulline (m/z = 158.0917), chloramphenicol (m/z = 195.075) and carnitine (m/z 162.11) were further selected based on m/z ratio for in silico and in vitro analyses. The in silico analysis revealed that citrulline, chloramphenicol and carnitine inhibited the virulent genes phcA (R. solanacearum) and ste12 (F. oxysporum). Further, under in vitro condition, citrulline and chloramphenicol were found to inhibit the growth of R. solanacearum and F. oxysporum. On the basis of the biocontrol activity of B. subtilis (SSL2) in in silico and in vitro conditions, the bacteria could be used as a biocontrol agent against both bacterial and fungal wilt-causing pathogens. However, this needs to be tested in pot studies or field conditions before being used as biocontrol agents.Communicated by Ramaswamy H. Sarma.

Functional Characterization of Two Putative DAHP Synthases of AroG1 and AroG2 and Their Links With Type III Secretion System in Ralstonia solanacearum

Type three secretion system (T3SS) is essential for Ralstonia solanacearum to cause disease in host plants and we previously screened AroG1 as a candidate with impact on the T3SS expression. Here, we focused on two putative DAHP synthases of AroG1 and AroG2, which control the first step of the shikimate pathway, a common route for biosynthesis of aromatic amino acids (AAA), to characterize their functional roles and possible links with virulence in R. solanacearum. Deletion of aroG1/2 or aroG1, but not aroG2, significantly impaired the T3SS expression both in vitro and in planta, and the impact of AroG1 on T3SS was mediated with a well-characterized PrhA signaling cascade. Virulence of the aroG1/2 or aroG1 mutants was completely diminished or significantly impaired in tomato and tobacco plants, but not the aroG2 mutants. The aroG1/2 mutants failed to grow in limited medium, but grew slowly in planta. This significantly impaired growth was also observed in the aroG1 mutants both in planta and limited medium, but not in aroG2 mutants. Complementary aroG1 significantly restored the impaired or diminished bacterial growth, T3SS expression and virulence. Supplementary AAA or shikimic acid, an important intermediate of the shikimate pathway, significantly restored diminished growth in limited medium. The promoter activity assay showed that expression of aroG1 and aroG2 was greatly increased to 10-20-folder higher levels with deletion of the other. All these results demonstrated that both AroG1 and AroG2 are involved in the shikimate pathway and cooperatively essential for AAA biosynthesis in R. solanacearum. The AroG1 plays a major role on bacterial growth, T3SS expression and pathogenicity, while the AroG2 is capable to partially carry out the function of AroG1 in the absence of AroG1.

Parallels between experimental and natural evolution of legume symbionts

The emergence of symbiotic interactions has been studied using population genomics in nature and experimental evolution in the laboratory, but the parallels between these processes remain unknown. Here we compare the emergence of rhizobia after the horizontal transfer of a symbiotic plasmid in natural populations of Cupriavidus taiwanensis, over 10 MY ago, with the experimental evolution of symbiotic Ralstonia solanacearum for a few hundred generations. In spite of major differences in terms of time span, environment, genetic background, and phenotypic achievement, both processes resulted in rapid genetic diversification dominated by purifying selection. We observe no adaptation in the plasmid carrying the genes responsible for the ecological transition. Instead, adaptation was associated with positive selection in a set of genes that led to the co-option of the same quorum-sensing system in both processes. Our results provide evidence for similarities in experimental and natural evolutionary transitions and highlight the potential of comparisons between both processes to understand symbiogenesis.

It is unclear if experimental evolution is a good model for natural processes. Here, Clerissi et al. find parallels between the evolution of symbiosis in rhizobia after horizontal transfer of a plasmid over 10 million years ago and experimentally evolved symbionts.

A putative LysR-type transcriptional regulator PrhO positively regulates the type III secretion system and contributes to the virulence of Ralstonia solanacearum

LysR-type transcriptional regulators (LTTRs) are ubiquitous and abundant amongst bacteria and control a variety of cellular processes. Here, we investigated the effect of Rsc1880 (a putative LTTR, hereafter designated as PrhO) on the pathogenicity of Ralstonia solanacearum. Deletion of prhO substantially reduced the expression of the type III secretion system (T3SS) both in vitro and in planta, and resulted in significantly impaired virulence in tomato and tobacco plants. Complementary prhO completely restored the reduced virulence and T3SS expression to that of the wild-type. Moreover, PrhO-dependent T3SS and virulence were conserved amongst R. solanacearum species. However, deletion of prhO did not alter biofilm formation, swimming mobility and in planta growth. The expression of some type III effectors was significantly reduced in prhO mutants, but the hypersensitive response was not affected in tobacco leaves. Consistent with the key regulatory role of HrpB on T3SS, PrhO positively regulated the T3SS through HrpB. Furthermore, PrhO regulated hrpB expression via two close paralogues, HrpG and PrhG, which are two-component response regulators and positively regulate hrpB expression in a parallel manner. However, deletion of prhO did not alter the expression of phcA, prhJ and prhN, which are also involved in hrpB regulation. In addition, PrhO was expressed in a cell density-dependent manner, but negatively repressed by itself. No regulation was observed for HrpB, PhcA and PrhN on prhO expression. Taken together, we genetically demonstrated that PrhO is a novel virulence regulator of R. solanacearum, which positively regulates T3SS expression through HrpG, PrhG and HrpB and contributes to virulence.

Involvement of NpdA, a Putative 2-Nitropropane Dioxygenase, in the T3SS Expression and Full Virulence in Ralstonia solanacearum OE1-1

Previously, we isolated several genes that potentially affected the expression of type III secretion system (T3SS) in Ralstonia solanacearum OE1-1. Here, we focused on the rsp0316, which encodes a putative 2-nitropropane dioxygenase (hereafter designated NpdA). The deletion of npdA substantially reduced the T3SS expression and virulence in OE1-1, and the complementation with functional NpdA could completely restore its reduced T3SS expression and virulence to that of wild type. The NpdA was highly conserved among diverse R. solanacearum species and the NpdA-dependent expression of T3SS was not specific to OE1-1 strain, but not the virulence. The NpdA was important for the T3SS expression in planta, while it was not required for the bacterial growth in planta. Moreover, the NpdA was not required for the elicitation of hypersensitive response (HR) of R. solanacearum strains in tobacco leaves. The T3SS in R. solanacearum is directly controlled by the AraC-type transcriptional regulator HrpB and regulated by a complex regulation network. The NpdA affected the T3SS expression mediated with HrpB but through some novel pathway. All these results from genetic studies demonstrate that NpdA is a novel factor for the T3SS expression in diverse R. solanacearum species in medium, but specifically for the T3SS expression in strain OE1-1 in planta. And the NpdA-dependent expression of T3SS in planta plays an important role in pathogenicity of R. solanacearum OE1-1 in host plants.

Characterization of an Alteromonas long-type ulvan lyase involved in the degradation of ulvan extracted from Ulva ohnoi

Ulvan is a sulfated polysaccharide found in the cell wall of the green algae Ulva. We first isolated several ulvan-utilizing Alteromonas sp. from the feces of small marine animals. The strain with the highest ulvan-degrading activity, KUL17, was analyzed further. We identified a 55-kDa ulvan-degrading protein secreted by this strain and cloned the gene encoding for it. The deduced amino acid sequence indicated that the enzyme belongs to polysaccharide lyase family 24 and thus the protein was named ulvan lyase. The predicted molecular mass of this enzyme is 110 kDa, which is different from that of the identified protein. By deletion analysis, the catalytic domain was proven to be located on the N-terminal half of the protein. KUL17 contains two ulvan lyases, one long and one short, but the secreted and cleaved long ulvan lyase was demonstrated to be the major enzyme for ulvan degradation.

Regulation Involved in Colonization of Intercellular Spaces of Host Plants in Ralstonia solanacearum

A soil-borne bacterium Ralstonia solanacearum invading plant roots first colonizes the intercellular spaces of the root, and eventually enters xylem vessels, where it replicates at high levels leading to wilting symptoms. After invasion into intercellular spaces, R. solanacearum strain OE1-1 attaches to host cells and expression of the hrp genes encoding components of the type III secretion system (T3SS). OE1-1 then constructs T3SS and secrets effectors into host cells, inducing expression of the host gene encoding phosphatidic acid phosphatase. This leads to suppressing plant innate immunity. Then, OE1-1 grows on host cells, inducing quorum sensing (QS). The QS contributes to regulation of OE1-1 colonization of intercellular spaces including mushroom-type biofilm formation on host cells, leading to its virulence. R. solanacearum strains AW1 and K60 produce methyl 3-hydroxypalmitate (3-OH PAME) as a QS signal. The methyltransferase PhcB synthesizes 3-OH PAME. When 3-OH PAME reaches a threshold level, it increases the ability of the histidine kinase PhcS to phosphorylate the response regulator PhcR. This results in elevated levels of functional PhcA, the global virulence regulator. On the other hand, strains OE1-1 and GMI1000 produce methyl 3-hydroxymyristate (3-OH MAME) as a QS signal. Among R. solanacearum strains, the deduced PhcB and PhcS amino acid sequences are related to the production of QS signals. R. solanacearum produces aryl-furanone secondary metabolites, ralfuranones, which are extracellularly secreted and required for its virulence, dependent on the QS. Interestingly, ralfuranones affect the QS feedback loop. Taken together, integrated signaling via ralfuranones influences the QS, contributing to pathogen virulence.

Ferulic Acid, But Not All Hydroxycinnamic Acids, Is a Novel T3SS Inducer of Ralstonia solanacearum and Promotes Its Infection Process in Host Plants under Hydroponic Condition

Hydroxycinnamic acids (HCAs) are typical monocyclic phenylpropanoids, including cinnamic acid (Cin), coumaric acid (Cou), caffeic acid (Caf), ferulic acid (FA) and their isomers, and involved in the interactions between pathogens and host plants. Here, we focused on the impact of HCAs on expression of type III secretion system (T3SS) in Ralstonia solanacearum. FA significantly induced the expression of the T3SS and some type III effectors (T3Es) genes in hrp-inducing medium, while did not the other HCAs. However, exogenously supplemented FA did not affect the T3SS expression in planta and the elicitation of the hypersensitive response (HR) in tobacco leaves. Consistent with its central roles in pathogenicity, the FA-induced expression of the T3SS led to significant promotion on infection process of R. solanacearum in tomato plants under hydroponics cultivation. Moreover, the FA-induced expression of the T3SS was specifically mediated by the well-characterized signaling cascade PrhA-prhI/R-PrhJ-HrpG-HrpB, independent of the other known regulatory pathways. In summary, our results demonstrated that FA, a novel inducer of the T3SS in R. solanacearum, was able to promote its infection process in host plants under hydroponics condition.

The vascular plant-pathogenic bacterium Ralstonia solanacearum produces biofilms required for its virulence on the surfaces of tomato cells adjacent to intercellular spaces

The mechanism of colonization of intercellular spaces by the soil-borne and vascular plant-pathogenic bacterium Ralstonia solanacearum strain OE1-1 after invasion into host plants remains unclear. To analyse the behaviour of OE1-1 cells in intercellular spaces, tomato leaves with the lower epidermis layers excised after infiltration with OE1-1 were observed under a scanning electron microscope. OE1-1 cells formed microcolonies on the surfaces of tomato cells adjacent to intercellular spaces, and then aggregated surrounded by an extracellular matrix, forming mature biofilm structures. Furthermore, OE1-1 cells produced mushroom-type biofilms when incubated in fluids of apoplasts including intercellular spaces, but not xylem fluids from tomato plants. This is the first report of biofilm formation by R. solanacearum on host plant cells after invasion into intercellular spaces and mushroom-type biofilms produced by R. solanacearum in vitro. Sugar application led to enhanced biofilm formation by OE1-1. Mutation of lecM encoding a lectin, RS-IIL, which reportedly exhibits affinity for these sugars, led to a significant decrease in biofilm formation. Colonization in intercellular spaces was significantly decreased in the lecM mutant, leading to a loss of virulence on tomato plants. Complementation of the lecM mutant with native lecM resulted in the recovery of mushroom-type biofilms and virulence on tomato plants. Together, our findings indicate that OE1-1 produces mature biofilms on the surfaces of tomato cells after invasion into intercellular spaces. RS-IIL may contribute to biofilm formation by OE1-1, which is required for OE1-1 virulence.

Evaluation of antibacterial activity of Stenotrophomonas maltophilia against Ralstonia solanacearum under different application conditions

AIM

The aim of this study was the monitoring of different mechanisms involved in the antibacterial activity of the biocontrol agent, Stenotrophomonas maltophilia (PD4560), against Ralstonia solanacearum in vitro and in vivo. Optimization of conditions that favour these mechanisms was the second target of this study.

METHODS AND RESULTS

Proteolytic activity of Sten. maltophilia (PD 4560), was tested on skimmed milk medium. The biocontrol agent was able to produce an alkaline serine protease enzyme with a molecular weight of 40 KDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses. Spraying of salicylic acid (SA) led to an increase in the efficacy of Sten. maltophilia in controlling the Ralstonia potato wilt while spraying of ammonium sulphate (AmS) did not affect the biocontrol efficacy. The efficacy was correlated with the expression of protease enzyme genes; Prt genes (mainly PrtP and Prt4) and PR genes (mainly PR-1 and PRQ) as evaluated using real-time polymerase chain reaction analysis.

CONCLUSIONS

The biocontrol activity of Sten. maltophilia can be attributed to the direct mechanism alkaline serine proteolytic enzyme production and through induction of host systemic acquired resistance as indirect mechanism. Tuber bulking was the most suitable physiological growth stage to apply either SA or the biocontrol agent.

SIGNIFICANCE AND IMPACT OF THE STUDY

Both SA and peat-moss as an organic carrier enhanced the antibacterial efficiency of the biocontrol agent. Application of Sten. maltophilia is more suitable under alkaline soil conditions.

Ralstonia solanacearum RSp0194 Encodes a Novel 3-Keto-Acyl Carrier Protein Synthase III

Fatty acid synthesis (FAS), a primary metabolic pathway, is essential for survival of bacteria. Ralstonia solanacearum, a β-proteobacteria member, causes a bacterial wilt affecting more than 200 plant species, including many economically important plants. However, thus far, the fatty acid biosynthesis pathway of R. solanacearum has not been well studied. In this study, we characterized two forms of 3-keto-ACP synthase III, RsFabH and RsFabW, in R. solanacearum. RsFabH, the homologue of Escherichia coli FabH, encoded by the chromosomal RSc1050 gene, catalyzes the condensation of acetyl-CoA with malonyl-ACP in the initiation steps of fatty acid biosynthesis in vitro. The RsfabH mutant lost de novo fatty acid synthetic ability, and grows in medium containing free fatty acids. RsFabW, a homologue of Pseudomonas aeruginosa PA3286, encoded by a megaplasmid gene, RSp0194, condenses acyl-CoA (C₂-CoA to C₁₀-CoA) with malonyl-ACP to produce 3-keto-acyl-ACP in vitro. Although the RsfabW mutant was viable, RsfabW was responsible for RsfabH mutant growth on medium containing free fatty acids. Our results also showed that RsFabW could condense acyl-ACP (C₄-ACP to C₈-ACP) with malonyl-ACP, to produce 3-keto-acyl-ACP in vitro, which implies that RsFabW plays a special role in fatty acid synthesis of R. solanacearum. All of these data confirm that R. solanacearum not only utilizes acetyl-CoA, but also, utilizes medium-chain acyl-CoAs or acyl-ACPs as primers to initiate fatty acid synthesis.

PrhN, a putative marR family transcriptional regulator, is involved in positive regulation of type III secretion system and full virulence of Ralstonia solanacearum

The MarR-family of transcriptional regulators are involved in various cellular processes, including resistance to multiple antibiotics and other toxic chemicals, adaptation to different environments and pathogenesis in many plant and animal pathogens. Here, we reported a new MarR regulator PrhN, which was involved in the pathogenesis of Ralstonia solanacearum. prhN mutant exhibited significantly reduced virulence and stem colonization compared to that of wild type in tomato plants. prhN mutant caused identical hypersensitive response (HR) on resistant plants to the wild type. Deletion of prhN gene substantially reduced the expression of type III secretion system (T3SS) in vitro and in planta (mainly in tomato plants), which is essential for pathogenicity of R. solanacearum, and the complemented PrhN could restore its virulence and T3SS expression to that of wild type. T3SS is directly controlled by AraC-type transcriptional regulator HrpB, and the transcription of hrpB is activated by HrpG and PrhG. HrpG and PrhG are homologs but are regulated by the PhcA positively and negatively, respectively. Deletion of prhN gene also abolished the expression of hrpB and prhG, but didn't change the expression of hrpG and phcA. Together, these results indicated that PrhN positively regulates T3SS expression through PrhG and HrpB. PrhN and PhcA should regulate prhG expression in a parallel way. This is the first report on the pathogenesis of MarR regulator in R. solanacearum, and this new finding will improve our understanding on the various biological functions of MarR regulator and the complex regulatory network on hrp regulon in R. solanacearum.

rpoN1, but not rpoN2, is required for twitching motility, natural competence, growth on nitrate, and virulence of Ralstonia solanacearum

The plant pathogen Ralstonia solanacearum has two genes encoding for the sigma factor σ(54): rpoN1, located in the chromosome and rpoN2, located in a distinct "megaplasmid" replicon. In this study, individual mutants as well as a double mutant of rpoN were created in R. solanacearum strain GMI1000 in order to determine the extent of functional overlap between these two genes. By virulence assay we observed that rpoN1 is required for virulence whereas rpoN2 is not. In addition rpoN1 controls other important functions such twitching motility, natural transformation and growth on nitrate, unlike rpoN2. The rpoN1 and rpoN2 genes have different expression pattern, the expression of rpoN1 being constitutive whereas rpoN2 expression is induced in minimal medium and in the presence of plant cells. Moreover, the expression of rpoN2 is dependent upon rpoN1. Our work therefore reveals that the two rpoN genes are not functionally redundant in R. solanacearum. A list of potential σ(54) targets was identified in the R. solanacearum genome and suggests that multiple traits are under the control of these regulators. Based on these findings, we provide a model describing the functional connection between RpoN1 and the PehR pathogenicity regulator and their dual role in the control of several R. solanacearum virulence determinants.

The C-terminal extension of PrhG impairs its activation of hrp expression and virulence in Ralstonia solanacearum

Ralstonia solanacearum is the second most destructive bacterial plant pathogens worldwide and HrpG is the master regulator of its pathogenicity. PrhG is a close paralogue of HrpG and both belong to OmpR/PhoB family of two-component response regulators. Despite a high similarity (72% global identity and 96% similarity in helix-loop-helix domain), they display distinct roles in pathogenicity. HrpG is necessary for the bacterial growth in planta and pathogenicity, while PrhG is dispensable for bacterial growth in planta and contributes little to pathogenicity. The main difference between HrpG and PrhG is the 50-amino-acid-long C-terminal extension in PrhG (amino-acid residues 230-283), which is absent in HrpG. When this extension is deleted, truncated PrhGs (under the control of its native promoter) allowed complete recovery of bacterial growth in planta and wild-type virulence of hrpG mutant. This novel finding demonstrates that the extension region in PrhG is responsible for the functional difference between HrpG and PrhG, which may block the binding of PrhG to target promoters and result in impaired activation of hrp expression by PrhG and reduced virulence of R. solanacearum.

Ralstonia solanacearum requires PopS, an ancient AvrE-family effector, for virulence and To overcome salicylic acid-mediated defenses during tomato pathogenesis

During bacterial wilt of tomato, the plant pathogen Ralstonia solanacearum upregulates expression of popS, which encodes a type III-secreted effector in the AvrE family. PopS is a core effector present in all sequenced strains in the R. solanacearum species complex. The phylogeny of popS mirrors that of the species complex as a whole, suggesting that this is an ancient, vertically inherited effector needed for association with plants. A popS mutant of R. solanacearum UW551 had reduced virulence on agriculturally important Solanum spp., including potato and tomato plants. However, the popS mutant had wild-type virulence on a weed host, Solanum dulcamara, suggesting that some species can avoid the effects of PopS. The popS mutant was also significantly delayed in colonization of tomato stems compared to the wild type. Some AvrE-type effectors from gammaproteobacteria suppress salicylic acid (SA)-mediated plant defenses, suggesting that PopS, a betaproteobacterial ortholog, has a similar function. Indeed, the popS mutant induced significantly higher expression of tomato SA-triggered pathogenesis-related (PR) genes than the wild type. Further, pretreatment of roots with SA exacerbated the popS mutant virulence defect. Finally, the popS mutant had no colonization defect on SA-deficient NahG transgenic tomato plants. Together, these results indicate that this conserved effector suppresses SA-mediated defenses in tomato roots and stems, which are R. solanacearum’s natural infection sites. Interestingly, PopS did not trigger necrosis when heterologously expressed in Nicotiana leaf tissue, unlike the AvrE homolog DspE_Pcc from the necrotroph Pectobacterium carotovorum subsp. carotovorum. This is consistent with the differing pathogenesis modes of necrosis-causing gammaproteobacteria and biotrophic R. solanacearum.

The type III-secreted AvrE effector family is widely distributed in high-impact plant-pathogenic bacteria and is known to suppress plant defenses for virulence. We characterized the biology of PopS, the only AvrE homolog made by the bacterial wilt pathogen Ralstonia solanacearum. To our knowledge, this is the first study of R. solanacearum effector function in roots and stems, the natural infection sites of this pathogen. Unlike the functionally redundant R. solanacearum effectors studied to date, PopS is required for full virulence and wild-type colonization of two natural crop hosts. R. solanacearum is a biotrophic pathogen that causes a nonnecrotic wilt. Consistent with this, PopS suppressed plant defenses but did not elicit cell death, unlike AvrE homologs from necrosis-causing plant pathogens. We propose that AvrE family effectors have functionally diverged to adapt to the necrotic or nonnecrotic lifestyle of their respective pathogens.

Novel plant inputs influencing Ralstonia solanacearum during infection

Ralstonia solanacearum is a soil and water-borne pathogen that can infect a wide range of plants and cause the devastating bacterial wilt disease. To successfully colonize a host, R. solanacearum requires the type III secretion system (T3SS), which delivers bacterial effector proteins inside the plant cells. HrpG is a central transcriptional regulator that drives the expression of the T3SS and other virulence determinants. hrpG transcription is highly induced upon plant cell contact and its product is also post-transcriptionally activated by metabolic signals present when bacteria are grown in minimal medium (MM). Here, we describe a transcriptional induction of hrpG at early stages of bacterial co-culture with plant cells that caused overexpression of the downstream T3SS effector genes. This induction was maintained in a strain devoid of prhA, the outer membrane receptor that senses bacterial contact with plant cells, demonstrating that this is a response to an unknown signal. Induction was unaffected after disruption of the known R. solanacearum pathogenicity regulators, indicating that it is controlled by a non-described system. Moreover, plant contact-independent signals are also important in planta, as shown by the hrpG induction triggered by apoplastic and xylem extracts. We also found that none of the amino acids or sugars present in the apoplast and xylem saps studied correlated with hrpG induction. This suggests that a small molecule or an environmental condition is responsible for the T3SS gene expression inside the plants. Our results also highlight the abundance and diversity of possible carbon, nitrogen and energy sources likely used by R. solanacearum during growth in planta.

Ralstonia solanacearum, a widespread bacterial plant pathogen in the post-genomic era

Ralstonia solanacearum is a soil-borne bacterium causing the widespread disease known as bacterial wilt. Ralstonia solanacearum is also the causal agent of Moko disease of banana and brown rot of potato. Since the last R. solanacearum pathogen profile was published 10 years ago, studies concerning this plant pathogen have taken a genomic and post-genomic direction. This was pioneered by the first sequenced and annotated genome for a major plant bacterial pathogen and followed by many more genomes in subsequent years. All molecular features studied now have a genomic flavour. In the future, this will help in connecting the classical field of pathology and diversity studies with the gene content of specific strains. In this review, we summarize the recent research on this bacterial pathogen, including strain classification, host range, pathogenicity determinants, regulation of virulence genes, type III effector repertoire, effector-triggered immunity, plant signalling in response to R. solanacearum, as well as a review of different new pathosystems.

TAXONOMY

Bacteria; Proteobacteria; β subdivision; Ralstonia group; genus Ralstonia.

DISEASE SYMPTOMS

Ralstonia solanacearum is the agent of bacterial wilt of plants, characterized by a sudden wilt of the whole plant. Typically, stem cross-sections will ooze a slimy bacterial exudate. In the case of Moko disease of banana and brown rot of potato, there is also visible bacterial colonization of banana fruit and potato tuber.

DISEASE CONTROL

As a soil-borne pathogen, infected fields can rarely be reused, even after rotation with nonhost plants. The disease is controlled by the use of resistant and tolerant plant cultivars. The prevention of spread of the disease has been achieved, in some instances, by the application of strict prophylactic sanitation practices.

USEFUL WEBSITES

Stock centre: International Centre for Microbial Resources-French Collection for Plant-associated Bacteria CIRM-CFBP, IRHS UMR 1345 INRA-ACO-UA, 42 rue Georges Morel, 49070 Beaucouzé Cedex, France, http://www.angers-nantes.inra.fr/cfbp/. Ralstonia Genome browser: https://iant.toulouse.inra.fr/R.solanacearum. GMI1000 insertion mutant library: https://iant.toulouse.inra.fr/R.solanacearumGMI1000/GenomicResources. MaGe Genome Browser: https://www.genoscope.cns.fr/agc/microscope/mage/viewer.php?

Functional analysis of Ralstonia solanacearum PrhG regulating the hrp regulon in host plants

Genes in the hrp regulon encode component proteins of the type III secretion system and are essential for the pathogenicity of Ralstonia solanacearum. The hrp regulon is controlled by HrpB. We isolated several genes regulating hrpB expression from the Japanese strain OE1-1 using minitransposon mutagenesis. Among them, we mainly focused on two genes, hrpG and prhG, which are the positive regulators of hrpB. Although the global virulence regulator PhcA negatively regulated hrpG expression via prhIR, it positively regulated prhG expression. We further investigated the contrasting regulation of hrpG and prhG by PhcA and speculated that R. solanacearum may switch from HrpG to PrhG for hrpB activation in a cell density-dependent manner. Although the prhG mutant proliferated similarly to the wild-type in leaf intercellular spaces and in xylem vessels of the host plants, it was less virulent than the wild-type. The expression of the popA operon, which belongs to the hrp regulon, was significantly reduced in the prhG mutant by more than half in the leaf intercellular spaces and more than two-thirds in the xylem vessels when compared with the wild-type.

Ralstonia solanacearum RSc0411 (lptC) is a determinant for full virulence and has a strain-specific novel function in the T3SS activity

Previously, we have identified an avirulent Ralstonia solanacearum mutant carrying a transposon insertion in RSc0411, a gene homologous to the Escherichia coli LPS-transporting protein LptC. However, how the disruption of RSc0411 affects the bacterium-plant interactions and leads to decreased pathogenicity was not known. Here we show that the disruption of RSc0411 leads to pleiotropic defects, including reducing bacterial motility, biofilm formation, root attachment, rough-form LPS production and virulence in tomato and increasing membrane permeability. Disruption of the orthologous RSc0411 present in other R. solanacearum strains proves that most of these functions are conserved in the species. In contrast, trans-complementation analyses show that only RSc0411 orthologues from closely related bacteria can rescue the defects of the disruption mutant. These results enable us to propose a function for RSc0411, and for the clustered genes, in LPS biogenesis, and for the first time, to our knowledge, also a role of a gene from the DUF1239 gene family in bacterial pathogenicity. In addition and notably, the RSc0411 mutant displays a strain-specific phenotype for hypersensitive response (HR), in which the RSc0411 disruption impairs the HR caused by strain Pss190 but not that by strain Pss1308. Consistent with this strain-specific defect, the mutation clearly affects expression of the type III secretion system (T3SS) in Pss190 but not in other strains, suggesting that the HR-deficient phenotype of the RSc0411 mutant in Pss190 is due to impairment of the T3SS and thus RSc0411 has a strain-specific role in the T3SS activity of R. solanacearum.

The in planta transcriptome of Ralstonia solanacearum: conserved physiological and virulence strategies during bacterial wilt of tomato

Plant xylem fluid is considered a nutrient-poor environment, but the bacterial wilt pathogen Ralstonia solanacearum is well adapted to it, growing to 10⁸ to 10⁹ CFU/g tomato stem. To better understand how R. solanacearum succeeds in this habitat, we analyzed the transcriptomes of two phylogenetically distinct R. solanacearum strains that both wilt tomato, strains UW551 (phylotype II) and GMI1000 (phylotype I). We profiled bacterial gene expression at ~6 × 10⁸ CFU/ml in culture or in plant xylem during early tomato bacterial wilt pathogenesis. Despite phylogenetic differences, these two strains expressed their 3,477 common orthologous genes in generally similar patterns, with about 12% of their transcriptomes significantly altered in planta versus in rich medium. Several primary metabolic pathways were highly expressed during pathogenesis. These pathways included sucrose uptake and catabolism, and components of these pathways were encoded by genes in the scrABY cluster. A UW551 scrA mutant was significantly reduced in virulence on resistant and susceptible tomato as well as on potato and the epidemiologically important weed host Solanum dulcamara. Functional scrA contributed to pathogen competitive fitness during colonization of tomato xylem, which contained ~300 µM sucrose. scrA expression was induced by sucrose, but to a much greater degree by growth in planta. Unexpectedly, 45% of the genes directly regulated by HrpB, the transcriptional activator of the type 3 secretion system (T3SS), were upregulated in planta at high cell densities. This result modifies a regulatory model based on bacterial behavior in culture, where this key virulence factor is repressed at high cell densities. The active transcription of these genes in wilting plants suggests that T3SS has a biological role throughout the disease cycle.

Ralstonia solanacearum is a widespread plant pathogen that causes bacterial wilt disease. It inflicts serious crop losses on tropical farmers, with major economic and human consequences. It is also a model for the many destructive microbes that colonize the water-conducting plant xylem tissue, which is low in nutrients and oxygen. We extracted bacteria from infected tomato plants and globally identified the biological functions that R. solanacearum expresses during plant pathogenesis. This revealed the unexpected presence of sucrose in tomato xylem fluid and the pathogen’s dependence on host sucrose for virulence on tomato, potato, and the common weed bittersweet nightshade. Further, R. solanacearum was highly responsive to the plant environment, expressing several metabolic and virulence functions quite differently in the plant than in pure culture. These results reinforce the utility of studying pathogens in interaction with hosts and suggest that selecting for reduced sucrose levels could generate wilt-resistant crops.

A luminescent reporter evidences active expression of Ralstonia solanacearum type III secretion system genes throughout plant infection

Although much is known about the signals that trigger transcription of virulence genes in plant pathogens, their prevalence and timing during infection are still unknown. In this work, we address these questions by analysing expression of the main pathogenicity determinants in the bacterial pathogen Ralstonia solanacearum. We set up a quantitative, non-invasive luminescent reporter to monitor in planta transcription from single promoters in the bacterial chromosome. We show that the new reporter provides a real-time measure of promoter output in vivo - either after re-isolation of pathogens from infected plants or directly in situ - and confirm that the promoter controlling exopolysaccharide (EPS) synthesis is active in bacteria growing in the xylem. We also provide evidence that hrpB, the master regulator of type III secretion system (T3SS) genes, is transcribed in symptomatic plants. Quantitative RT-PCR assays demonstrate that hrpB and type III effector transcripts are abundant at late stages of plant infection, suggesting that their function is required throughout disease. Our results challenge the widespread view in R. solanacearum pathogenicity that the T3SS, and thus injection of effector proteins, is only active to manipulate plant defences at the first stages of infection, and that its expression is turned down when bacteria reach high cell densities and EPS synthesis starts.

A chromosomal insertion toolbox for promoter probing, mutant complementation, and pathogenicity studies in Ralstonia solanacearum

We describe here the construction of a delivery system for stable and directed insertion of gene constructs in a permissive chromosomal site of the bacterial wilt pathogen Ralstonia solanacearum. The system consists of a collection of suicide vectors-the Ralstonia chromosome (pRC) series-that carry an integration element flanked by transcription terminators and two sequences of homology to the chromosome of strain GMI1000, where the integration element is inserted through a double recombination event. Unique restriction enzyme sites and a GATEWAY cassette enable cloning of any promoter::gene combination in the integration element. Variants endowed with different selectable antibiotic resistance genes and promoter::gene combinations are described. We show that the system can be readily used in GMI1000 and adapted to other R. solanacearum strains using an accessory plasmid. We prove that the pRC system can be employed to complement a deletion mutation with a single copy of the native gene, and to measure transcription of selected promoters in monocopy both in vitro and in planta. Finally, the system has been used to purify and study secretion type III effectors. These novel genetic tools will be particularly useful for the construction of recombinant bacteria that maintain inserted genes or reporter fusions in competitive situations (i.e., during plant infection).

The filamentous phage ϕRSS1 enhances virulence of phytopathogenic Ralstonia solanacearum on tomato

Ralstonia solanacearum is the causative agent of bacterial wilt in many important crops. ϕRSS1 is a filamentous phage that infects R. solanacearum strains. Upon infection, it alters the physiological state and the behavior of host cells. Here, we show that R. solanacearum infected by ϕRSS1 becomes more virulent on host plants. Some virulence and pathogenicity factors, such as extracellular polysaccharide (EPS) synthesis and twitching motility, increased in the bacterial host cells infected with ϕRSS1, resulting in early wilting. Tomato plants inoculated with ϕRSS1-infected bacteria wilted 2 to 3 days earlier than those inoculated with wild-type bacteria. Infection with ϕRSS1 induced early expression of phcA, the global virulence regulator. phcA expression was detected in ϕRSS1-infected cells at cell density as low as 10(4) CFU/ml. Filamentous phages are assembled on the host cell surface and many phage particles accumulate on the cell surface. These surface-associated phage particles (phage proteins) may change the cell surface nature (hydrophobicity) to give high local cell densities. ϕRSS1 infection also enhanced PilA and type IV pilin production, resulting in increased twitching motility.

Fructose-bisphophate aldolase exhibits functional roles between carbon metabolism and the hrp system in rice pathogen Xanthomonas oryzae pv. oryzicola

Fructose-bisphophate aldolase (FbaB), is an enzyme in glycolysis and gluconeogenesis in living organisms. The mutagenesis in a unique fbaB gene of Xanthomonas oryzae pv. oryzicola, the causal agent of rice bacterial leaf streak, led the pathogen not only unable to use pyruvate and malate for growth and delayed its growth when fructose was used as the sole carbon source, but also reduced extracellular polysaccharide (EPS) production and impaired bacterial virulence and growth in rice. Intriguingly, the fbaB promoter contains an imperfect PIP-box (plant-inducible promoter) (TTCGT-N₉-TTCGT). The expression of fbaB was negatively regulated by a key hrp regulatory HrpG and HrpX cascade. Base substitution in the PIP-box altered the regulation of fbaB with the cascade. Furthermore, the expression of fbaB in X. oryzae pv. oryzicola RS105 strain was inducible in planta rather than in a nutrient-rich medium. Except other hrp-hrc-hpa genes, the expression of hrpG and hrpX was repressed and the transcripts of hrcC, hrpE and hpa3 were enhanced when fbaB was deleted. The mutation in hrcC, hrpE or hpa3 reduced the ability of the pathogen to acquire pyruvate and malate. In addition, bacterial virulence and growth in planta and EPS production in RΔfbaB mutant were completely restored to the wild-type level by the presence of fbaB in trans. This is the first report to demonstrate that carbohydrates, assimilated by X. oryzae pv. oryzicola, play critical roles in coordinating hrp gene expression through a yet unknown regulator.

Pathogenomics of the Ralstonia solanacearum species complex

Ralstonia solanacearum is a major phytopathogen that attacks many crops and other plants over a broad geographical range. The extensive genetic diversity of strains responsible for the various bacterial wilt diseases has in recent years led to the concept of an R. solanacearum species complex. Genome sequencing of more than 10 strains representative of the main phylogenetic groups has broadened our knowledge of the evolution and speciation of this pathogen and led to the identification of novel virulence-associated functions. Comparative genomic analyses are now opening the way for refined functional studies. The many molecular determinants involved in pathogenicity and host-range specificity are described, and we also summarize current understanding of their roles in pathogenesis and how their expression is tightly controlled by an intricate virulence regulatory network.

prhKLM genes of Ralstonia solanacearum encode novel activators of hrp regulon and are required for pathogenesis in tomato

The genes in the hrp regulon encode the proteins composing type III secretion system in Ralstonia solanacearum. The hrp regulon is positively controlled by HrpB, and hrpB expression is activated by both HrpG and PrhG. We have identified three genes, prhK, prhL, and prhM, which positively control the hrp regulon in strain OE1-1. These genes are likely to form an operon, and this operon is well conserved in the genera Ralstonia and Burkholderia. This indicates that the operon is not specific to the plant pathogens. Mutations in each of these three genes abolished hrpB and prhG expression. prhK, prhL, and prhM mutant strains lost pathogenicity toward tomato completely, and they were less virulent toward tobacco. PrhK and PrhL share sequence similarity with allophanate hydrolase and PrhM with LamB. This suggests that the three gene products are not transcriptional regulators in the strict sense, but regulate hrp regulon indirectly. This novel class of virulence-related genes will mark the beginning of new findings regarding the overall infection mode of R. solanacearum.

PrhG, a transcriptional regulator responding to growth conditions, is involved in the control of the type III secretion system regulon in Ralstonia solanacearum

The ability of Ralstonia solanacearum to cause disease in plants depends on its type III secretion system (T3SS). The expression of the T3SS and its effector substrates is coordinately controlled by a regulatory cascade, at the bottom of which is HrpB. Transcription of the hrpB gene is activated by a plant-responsive regulator named HrpG, which is a master regulator of a wide array of pathogenicity functions in R. solanacearum. We have identified in the genome of strain GMI1000 a close paralog of hrpG (83% overall similarity at the protein level) that we have named prhG. Despite this high similarity, the expression pattern of prhG is remarkably different from that of hrpG: prhG expression is activated after growth of bacteria in minimal medium but not in the presence of host cells, while hrpG expression is specifically induced in response to plant cell signals. We provide genetic evidence that prhG is a transcriptional regulator that, like hrpG, controls the expression of hrpB and the hrpB-regulated genes under minimal medium conditions. However, the regulatory functions of prhG and hrpG are distinct: prhG has no influence on hrpB expression when the bacteria are in the presence of plant cells, and transcriptomic profiling analysis of a prhG mutant revealed that the PrhG and HrpG regulons have only one pathogenicity target in common, hrpB. Functional complementation experiments indicated that PrhG and HrpG are individually sufficient to activate hrpB expression in minimal medium. Rather surprisingly, a prhG disruption mutant had little impact on pathogenicity, which may indicate that prhG has a minor role in the activation of T3SS genes when R. solanacearum grows parasitically inside the plant. The cross talk between pathogenicity regulatory proteins and environmental signals described here denotes that an intricate network is at the basis of the bacterial disease program.