Mutation of Lon protease differentially affects the expression of Pseudomonas syringae type III secretion system genes in rich and minimal media and reduces pathogenicity

c-di-GMP signaling in Pseudomonas syringae complex

The Pseudomonas syringae Complex is one of the model phytopathogenic bacteria for exploring plant-microbe interactions, causing devastating plant diseases and economic losses worldwide. The ubiquitous second messenger bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) plays an important role in the 'lifestyle switch' from single motile cells to biofilm formation and modulates bacterial behavior, thus influencing virulence in Pseudomonas and other bacterial species. However, less is known about the role of c-di-GMP in the P. syringae complex, in which c-di-GMP levels are controlled by diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), such as Chp8, BifA and WspR. Deletion the chemotaxis receptor PscA also influences c-di-GMP levels, suggesting a cross-talk between chemotaxis and c-di-GMP pathways. Another transcription factor, FleQ, plays a dual role (positive or negative) in regulating cellulose synthesis as a c-di-GMP effector, whereas the transcription factor AmrZ regulates local c-di-GMP levels by inhibiting the DGC enzyme AdcA and the PDE enzyme MorA. Our recent research demonstrated that an increase in the c-di-GMP concentration increased biofilm development, siderophore biosynthesis and oxidative stress tolerance, while it decreased the siderophore content, bacterial motility and type III secretion system activity in P. syringae complex. These findings show that c-di-GMP intricately controls virulence in P. syringae complex, indicating that adjusting c-di-GMP levels may be a valuable tactic for defending plants against pathogens. This review highlights recent research on metabolic enzymes, regulatory mechanisms and the phenotypic consequences of c-di-GMP signaling in the P. syringae.

Hierarchical regulation of Burkholderia glumae type III secretion system by GluR response regulator and Lon protease

Expression of type III secretion system (T3SS) genes, which are important for the virulence of phytopathogenic bacteria, is induced in the plant apoplastic environment or artificially amended growth conditions. Wild-type Burkholderia glumae BGR1, which causes rice panicle blight, induced a hypersensitive response (HR) in tobacco plants, whereas the T3SS genes were not significantly expressed in the commonly used hrp induction medium. T3SS gene expression in B. glumae was dependent on HrpB, a well known T3SS gene transcriptional regulator. Here, we report a stepwise mechanism of T3SS gene regulation by the GluR response regulator and Lon protease in addition to HrpB-mediated control of T3SS genes in B. glumae. The gluR mutant showed no HR in tobacco plants and exhibited attenuated virulence in rice plants. GluR directly activated hrpB expression, indicating that hrpB belongs to the GluR regulon. The lon mutation allowed high expression of the T3SS genes in nutrient-rich media. Lon directly activated gluR expression but repressed hrpB expression, indicating that Lon acts as a regulator rather than a protease. However, the lon mutant failed to induce an HR and virulence, suggesting that Lon not only acts as a negative regulator, but also has an essential, yet to be determined role for T3SS. Our results demonstrate the involvement of the two-component system response regulator GluR and Lon in T3SS gene regulation, providing new insight into the complex interplay mechanisms of regulators involved in T3SS gene expression in bacteria-plant interactions.

Regulation of the Pseudomonas syringae Type III Secretion System by Host Environment Signals

Pseudomonas syringae are Gram-negative, plant pathogenic bacteria that use a type III secretion system (T3SS) to disarm host immune responses and promote bacterial growth within plant tissues. Despite the critical role for type III secretion in promoting virulence, T3SS-encoding genes are not constitutively expressed by P. syringae and must instead be induced during infection. While it has been known for many years that culturing P. syringae in synthetic minimal media can induce the T3SS, relatively little is known about host signals that regulate the deployment of the T3SS during infection. The recent identification of specific plant-derived amino acids and organic acids that induce T3SS-inducing genes in P. syringae has provided new insights into host sensing mechanisms. This review summarizes current knowledge of the regulatory machinery governing T3SS deployment in P. syringae, including master regulators HrpRS and HrpL encoded within the T3SS pathogenicity island, and the environmental factors that modulate the abundance and/or activity of these key regulators. We highlight putative receptors and regulatory networks involved in linking the perception of host signals to the regulation of the core HrpRS–HrpL pathway. Positive and negative regulation of T3SS deployment is also discussed within the context of P. syringae infection, where contributions from distinct host signals and regulatory networks likely enable the fine-tuning of T3SS deployment within host tissues. Last, we propose future research directions necessary to construct a comprehensive model that (a) links the perception of host metabolite signals to T3SS deployment and (b) places these host–pathogen signaling events in the overall context of P. syringae infection.

Integrated regulatory network in Pseudomonas syringae reveals dynamics of virulence

Pseudomonas syringae, a Gram-negative plant pathogen, expresses multitudinous transcriptional regulators to control the type III secretion system (T3SS) and response to diverse environmental challenges. Although the mechanisms of virulence-associated regulators of P. syringae have been studied for decades, the overall crosstalk underlying these regulators is still elusive. Here, we identify five T3SS regulators (EnvZ-OmpR, CbrAB2, PhoPQ, PilRS, and MgrA), and find that the two-component systems EnvZ-OmpR and CbrAB2 negatively regulate the T3SS. To elucidate crosstalk between 16 virulence-associated regulators in P. syringae, we map an online intricate network called "PSRnet" (Pseudomonas syringae regulatory network) by combining the differentially expressed genes (DEGs) of these 16 regulators by RNA sequencing (RNA-seq) and their binding loci by chromatin immunoprecipitation sequencing (ChIP-seq). Consequently, we identify 238 and 153 functional genes involved in the T3SS and other virulence-related pathways in KB and MM media, respectively. Our results provide insights into the mechanism of plant infections caused by P. syringae.

Signal transduction schemes in Pseudomonas syringae

To cope with their continually fluctuating surroundings, pathovars of the unicellular phytopathogen Pseudomonas syringae have developed rapid and sophisticated signalling networks to sense extracellular stimuli, which allow them to adjust their cellular composition to survive and cause diseases in host plants. Comparative genomic analyses of P. syringae strains have identified various genes that encode several classes of signalling proteins, although how this bacterium directly perceives these environmental cues remains elusive. Recent work has revealed new mechanisms of a cluster of bacterial signal transduction systems that mainly include two-component systems (such as RhpRS, GacAS, CvsRS and AauRS), extracytoplasmic function sigma factors (such as HrpL and AlgU), nucleotide-based secondary messengers, methyl-accepting chemotaxis sensor proteins and several other intracellular surveillance systems. In this review, we compile a list of the signal transduction mechanisms that P. syringae uses to monitor and respond in a timely manner to intracellular and external conditions. Further understanding of these surveillance processes will provide new perspectives from which to combat P. syringae infections.

Cleavage of PrePL by Lon promotes growth and pathogenesis in Magnaporthe oryzae

ATP-dependent Lon proteases function in bacterial pathogenesis by regulating the expression of the Type III secretion system; however, little is known about how Lon proteases regulate fungal pathogenesis. We previously investigated Lon-binding proteins involved in fungal pathogenesis that interact with PrePL, the smallest Magnaporthe oryzae Lon-binding protein. Here, we show that Lon cleaves PrePL and produces Pc, an extracellular 11-kDa isoform with catalase and peroxidase activity. The ΔPrePL loss-of-function strain showed stronger sporulation and accelerated disease development, suggesting a temporally specific negative regulatory mechanism controlled by PrePL in disease progression. Neither the truncated Pc, nor the full-length PrePL missing the Lon cleavage site complemented the ΔPrePL phenotype, suggesting that full-length PrePL and Pc both function in fungal development. PrePL targeted to the mitochondria undergoes hydrolysis by Lon to produce Pc, which accumulates in the fungal apoplast. Importantly, recombinant Pc induced plant defence responses and cell death after being infiltrated into selected plant leaves, indicating that it functions as an avirulence factor. This work thus reveals a novel pathogenic factor in the fungal Lon-mediated pathway. Additionally, our results provide new insight into the functions of a full-length protein and its cleaved isoform in fungal pathogenesis.

c-di-GMP inhibits LonA-dependent proteolysis of TfoY in Vibrio cholerae

The LonA (or Lon) protease is a central post-translational regulator in diverse bacterial species. In Vibrio cholerae, LonA regulates a broad range of behaviors including cell division, biofilm formation, flagellar motility, c-di-GMP levels, the type VI secretion system (T6SS), virulence gene expression, and host colonization. Despite LonA’s role in cellular processes critical for V. cholerae’s aquatic and infectious life cycles, relatively few LonA substrates have been identified. LonA protease substrates were therefore identified through comparison of the proteomes of wild-type and ΔlonA strains following translational inhibition. The most significantly enriched LonA-dependent protein was TfoY, a known regulator of motility and the T6SS in V. cholerae. Experiments showed that TfoY was required for LonA-mediated repression of motility and T6SS-dependent killing. In addition, TfoY was stabilized under high c-di-GMP conditions and biochemical analysis determined direct binding of c-di-GMP to LonA results in inhibition of its protease activity. The work presented here adds to the list of LonA substrates, identifies LonA as a c-di-GMP receptor, demonstrates that c-di-GMP regulates LonA activity and TfoY protein stability, and helps elucidate the mechanisms by which LonA controls important V. cholerae behaviors.

This study provides insights into the mechanisms and consequences of LonA-mediated regulated proteolysis in Vibrio cholerae, the causal organism of the acute diarrheal disease cholera that is endemic in more than 47 countries across the globe. Lon is broadly conserved in bacterial systems; uncovering the molecular connection between c-di-GMP signaling and LonA-mediated proteolysis of V. cholerae will provide conceptual frameworks for the development of intervention strategies to combat virulence by bacterial pathogens.

Lon Protease Is Important for Growth Under Stressful Conditions and Pathogenicity of the Phytopathogen, Bacterium Dickeya solani

The Lon protein is a protease implicated in the virulence of many pathogenic bacteria, including some plant pathogens. However, little is known about the role of Lon in bacteria from genus Dickeya. This group of bacteria includes important potato pathogens, with the most aggressive species, D. solani. To determine the importance of Lon for pathogenicity and response to stress conditions of bacteria, we constructed a D. solani Δlon strain. The mutant bacteria showed increased sensitivity to certain stress conditions, in particular osmotic and high-temperature stresses. Furthermore, qPCR analysis showed an increased expression of the lon gene in D. solani under these conditions. The deletion of the lon gene resulted in decreased motility, lower activity of secreted pectinolytic enzymes and finally delayed onset of blackleg symptoms in the potato plants. In the Δlon cells, the altered levels of several proteins, including virulence factors and proteins associated with virulence, were detected by means of Sequential Window Acquisition of All Theoretical Mass Spectra (SWATH-MS) analysis. These included components of the type III secretion system and proteins involved in bacterial motility. Our results indicate that Lon protease is important for D. solani to withstand stressful conditions and effectively invade the potato plant.

The C3HC type zinc-finger protein (ZFC3) interacting with Lon/MAP1 is important for mitochondrial gene regulation, infection hypha development and longevity of Magnaporthe oryzae

Background

The rice blast is a typical fungal disease caused by Magnaporthe oryzae, and the mitochondrial ATP-dependent Lon protease (MAP1) has been proven to be involved in blast development. We previously screened a C3HC type Zinc-finger domain protein (ZFC3), which is interacted with MAP1. The purpose of this research was to study the biological function of ZFC3 protein in M. oryzae.

Results

We first confirmed that the ZFC3-RFP fusion protein is localized within the mitochondria. The deleted mutant strains of ZFC3 (∆ZFC3) showed the enhanced expression level of mtATP6, particularly mtATP8, and almost unchanged nATP9. ΔZFC3 produces more conidia and more tolerance to multiple stressors. The knock-out strain shows more melanin accumulation suggests the susceptibility to aging. ΔZFC3 displays faster early-stage hypha infiltration involved in MAP1-mediated pathogenicity in host rice.

Conclusion

These results support the view that ZFC3 is a key regulator involved in gene regulation, stress response, cell wall integrity, longevity, conidiation, infection hypha development and MAP1-mediated pathogenicity in M. oryzae.

Two components of the rhpPC operon coordinately regulate the type III secretion system and bacterial fitness in Pseudomonas savastanoi pv. phaseolicola

Many plant bacterial pathogens including Pseudomonas species, utilize the type III secretion system (T3SS) to deliver effector proteins into plant cells. Genes encoding the T3SS and its effectors are repressed in nutrient-rich media but are rapidly induced after the bacteria enter a plant or are transferred into nutrient-deficient media. To understand how the T3SS genes are regulated, we screened for P. savastanoi pv. phaseolicola (Psph) mutants displaying diminished induction of avrPto-luc, a reporter for the T3SS genes, in Arabidopsis. A mutant carrying transposon insertion into a gene coding for a small functional unknown protein, designated as rhpC, was identified that poorly induced avrPto-luc in plants and in minimal medium (MM). Interestingly, rhpC is located immediately downstream of a putative metalloprotease gene named rhpP, and the two genes are organized in an operon rhpPC; but rhpP and rhpC displayed different RNA expression patterns in nutrient-rich King’s B medium (KB) and MM. Deletion of the whole rhpPC locus did not significantly affect the avrPto-luc induction, implying coordinated actions of rhpP and rhpC in regulating the T3SS genes. Further analysis showed that RhpC was a cytoplasmic protein that interacted with RhpP and targeted RhpP to the periplasm. In the absence of RhpC, RhpP was localized in the cytoplasm and caused a reduction of HrpL, a key regulator of the T3SS genes, and also reduced the fitness of Psph. Expression of RhpP alone in E. coli inhibited the bacterial growth. The detrimental effect of RhpP on the fitness of Psph and E. coli required metalloprotease active sites, and was repressed when RhpC was co-expressed with RhpP. The coordination between rhpP and rhpC in tuning the T3SS gene expression and cell fitness reveals a novel regulatory mechanism for bacterial pathogenesis. The function of RhpP in the periplasm remains to be studied.

The induction of the type III secretion system (T3SS) is of great importance to the pathogenesis of bacterial pathogens in host plants. Pseudomonas savastanoi pv. phaseolicola (Psph) causes halo blight disease on beans. We discovered that the bicistronic genes in the rhpPC locus of Psph act coordinately to regulate the T3SS gene expression, bacterial fitness, and pathogenicity. rhpP encodes a metalloprotease that can degrade the key T3SS regulator protein HrpL and reduce bacterial fitness. rhpC encodes a chaperone that inhibits the RhpP activity and mediates translocation of RhpP to the periplasm. The level of rhpP RNA is high in KB but decreases in MM, but the rhpC RNA is low in KB but increases in MM. The elevated rhpC/rhpP transcript ratio in MM plus the inhibition of RhpC on RhpP activity in cytoplasm provide double insurance that warrants high induction of the T3SS genes in MM and bacterial fitness. The coordination between rhpP and rhpC reveals a new mechanism regulating bacterial pathogenicity, and may provide an important target for controlling bacterial pathogens.

The Role of Proteases in the Virulence of Plant Pathogenic Bacteria

A pathogenic lifestyle is inextricably linked with the constant necessity of facing various challenges exerted by the external environment (both within and outside the host). To successfully colonize the host and establish infection, pathogens have evolved sophisticated systems to combat the host defense mechanisms and also to be able to withstand adverse environmental conditions. Proteases, as crucial components of these systems, are involved in a variety of processes associated with infection. In phytopathogenic bacteria, they play important regulatory roles and modulate the expression and functioning of various virulence factors. Secretory proteases directly help avoid recognition by the plant immune systems, and contribute to the deactivation of the defense response pathways. Finally, proteases are important components of protein quality control systems, and thus enable maintaining homeostasis in stressed bacterial cells. In this review, we discuss the known protease functions and protease-regulated signaling processes associated with virulence of plant pathogenic bacteria.

Profiling the extended phenotype of plant pathogens: Challenges in Bacterial Molecular Plant Pathology

One of the most fundamental questions in plant pathology is what determines whether a pathogen grows within a plant? This question is frequently studied in terms of the role of elicitors and pathogenicity factors in the triggering or overcoming of host defences. However, this focus fails to address the basic question of how the environment in host tissues acts to support or restrict pathogen growth. Efforts to understand this aspect of host-pathogen interactions are commonly confounded by several issues, including the complexity of the plant environment, the artificial nature of many experimental infection systems and the fact that the physiological properties of a pathogen growing in association with a plant can be very different from the properties of the pathogen in culture. It is also important to recognize that the phenotype and evolution of pathogen and host are inextricably linked through their interactions, such that the environment experienced by a pathogen within a host, and its phenotype within the host, is a product of both its interaction with its host and its evolutionary history, including its co-evolution with host plants. As the phenotypic properties of a pathogen within a host cannot be defined in isolation from the host, it may be appropriate to think of pathogens as having an 'extended phenotype' that is the product of their genotype, host interactions and population structure within the host environment. This article reflects on the challenge of defining and studying this extended phenotype, in relation to the questions posed below, and considers how knowledge of the phenotype of pathogens in the host environment could be used to improve disease control. What determines whether a pathogen grows within a plant? What aspects of pathogen biology should be considered in describing the extended phenotype of a pathogen within a host? How can we study the extended phenotype in ways that provide insights into the phenotypic properties of pathogens during natural infections?

Lon Protease Is Involved in RhpRS-Mediated Regulation of Type III Secretion in Pseudomonas syringae

Pseudomonas syringae depends on the type III secretion system (T3SS) to directly translocate effectors into host cells. Previously, we reported a nonpathogenic rhpS mutant, suggesting that the two-component transduction system rhpRS is an important regulator of T3SS in P. syringae. rhpRS regulates itself and a variety of downstream genes under an inverted repeat element promoter in a phosphorylation-dependent manner. Here, we identify lon as a suppressor of the rhpS mutant through transposon screening. A lon/rhpS double mutant restored the phenotypes of the rhpS mutant. The expression level of lon was higher in rhpS and other T3SS-deficient mutants than the wild-type strain, suggesting a negative feedback mechanism between lon and T3SS genes. lon was also induced by a novel T3SS inhibitor, acetate, which substantially compromises the activation of T3SS genes in minimal medium and bacterial growth in host plants.

The LonA Protease Regulates Biofilm Formation, Motility, Virulence, and the Type VI Secretion System in Vibrio cholerae

The presence of the Lon protease in all three domains of life hints at its biological importance. The prokaryotic Lon protease is responsible not only for degrading abnormal proteins but also for carrying out the proteolytic regulation of specific protein targets. Posttranslational regulation by Lon is known to affect a variety of physiological traits in many bacteria, including biofilm formation, motility, and virulence. Here, we identify the regulatory roles of LonA in the human pathogen Vibrio cholerae. We determined that the absence of LonA adversely affects biofilm formation, increases swimming motility, and influences intracellular levels of cyclic diguanylate. Whole-genome expression analysis revealed that the message abundance of genes involved in biofilm formation was decreased but that the message abundances of those involved in virulence and the type VI secretion system were increased in a lonA mutant compared to the wild type. We further demonstrated that a lonA mutant displays an increase in type VI secretion system activity and is markedly defective in colonization of the infant mouse. These findings suggest that LonA plays a critical role in the environmental survival and virulence of V. cholerae.

IMPORTANCE

Bacteria utilize intracellular proteases to degrade damaged proteins and adapt to changing environments. The Lon protease has been shown to be important for environmental adaptation and plays a crucial role in regulating the motility, biofilm formation, and virulence of numerous plant and animal pathogens. We find that LonA of the human pathogen V. cholerae is in line with this trend, as the deletion of LonA leads to hypermotility and defects in both biofilm formation and colonization of the infant mouse. In addition, we show that LonA regulates levels of cyclic diguanylate and the type VI secretion system. Our observations add to the known regulatory repertoire of the Lon protease and the current understanding of V. cholerae physiology.

Proteins interacting with mitochondrial ATP-dependent Lon protease (MAP1) in Magnaporthe oryzae are involved in rice blast disease

The ATP-dependent Lon protease is involved in many physiological processes. In bacteria, Lon regulates pathogenesis and, in yeast, Lon protects mitochondia from oxidative damage. However, little is known about Lon in fungal phytopathogens. MAP1, a homologue of Lon in Magnaporthe oryzae, was recently identified to be important for stress resistance and pathogenesis. Here, we focus on a novel pathogenic pathway mediated by MAP1. Based on an interaction system between rice and a tandem affinity purification (TAP)-tagged MAP1 complementation strain, we identified 23 novel fungal proteins from infected leaves using a TAP approach with mass spectrometry, and confirmed that 14 of these proteins physically interact with MAP1 in vivo. Among these 14 proteins, 11 candidates, presumably localized to the mitochondria, were biochemically determined to be substrates of MAP1 hydrolysis. Deletion mutants were created and functionally analysed to further confirm the involvement of these proteins in pathogenesis. The results indicated that all mutants showed reduced conidiation and sensitivity to hydrogen peroxide. Appressorial formations were not affected, although conidia from certain mutants were morphologically altered. In addition, virulence was reduced in four mutants, enhanced (with lesions forming earlier) in two mutants and remained unchanged in one mutant. Together with the known virulence-related proteins alternative oxidase and enoyl-CoA hydratase, we propose that most of the Lon-interacting proteins are involved in the pathogenic regulation pathway mediated by MAP1 in M. oryzae. Perturbation of this pathway may represent an effective approach for the inhibition of rice blast disease.

The Stringent Response Mediated by (p)ppGpp Is Required for Virulence of Pseudomonas syringae pv. tomato and Its Survival on Tomato

The hypersensitive response and pathogenicity (hrp) type III secretion system (T3SS) is a key pathogenicity factor in Pseudomonas syringae pv. tomato DC3000 (DC3000). In this study, the role of the second messenger (p)ppGpp on virulence and survival of DC3000 was investigated. Results have demonstrated that (p)ppGpp-deficient mutant (ppGpp(0)) of DC3000 exhibited lower levels of expression of the T3SS and genes of other virulence traits, such as coronatine toxin. The ppGpp(0) mutant of DC3000 was greatly impaired in causing disease and in growth in planta. Furthermore, (p)ppGpp was required for swarming motility, pyoverdine production, the oxidative stress response, as well as γ-amino butyric acid utilization. Screening of amino acids, major signals in activation of ppGpp biosynthesis, revealed that promoter activities of the avrPto gene could be either activated or suppressed by various amino acids in a ppGpp-dependent or -independent manner. Moreover, the ppGpp(0) mutant exhibited increased cell size and decreased survival on plant surfaces. Altogether, these findings indicate that ppGpp acts as an internal signal that regulates the T3SS as well as other virulence factors in pseudomonads and suggest that bacterial pathogens utilize intracellular messengers to sense environmental and nutritional signals for rapid, precise, and reversible control of their pathogenesis and survival.

Pseudomonas aeruginosa Lon and ClpXP proteases: roles in linking carbon catabolite repression system with quorum-sensing system

Quorum sensing (QS) plays critical roles in virulence gene expression and the pathogenesis of Pseudomonas aeruginosa, an important human pathogen. However, the regulatory effects, especially that occur directly upstream of the QS system, remain largely unknown. Here, we review recent advances in the understanding of the key component of carbon catabolite repression (CCR) system and protein quality control (PQC) system in regulating the QS system in P. aeruginosa. We propose that PQC proteases Lon and ClpXP may have an important role in linking CCR with QS, and thus contribute to the integration of nutritional cues into the regulatory network governing the virulence factors expression in P. aeruginosa.

Molecular mechanisms of two-component system RhpRS regulating type III secretion system in Pseudomonas syringae

Pseudomonas syringae uses the two-component system RhpRS to regulate the expression of type III secretion system (T3SS) genes and bacterial virulence. However, the molecular mechanisms and the regulons of RhpRS have yet to be fully elucidated. Here, we show that RhpS functions as a kinase and a phosphatase on RhpR and as an autokinase upon itself. RhpR is phosphorylated by the small phosphodonor acetyl phosphate. A specific RhpR-binding site containing the inverted repeat (IR) motif GTATC-N6-GATAC, was mapped to its own promoter by a DNase I footprint analysis. Electrophoretic mobility shift assay indicated that P-RhpR has a higher binding affinity to the IR motif than RhpR. To identify additional RhpR targets in P. syringae, we performed chromatin immunoprecipitation followed by high-throughput DNA sequencing (ChIP-seq) and detected 167 enriched loci including the hrpR promoter, suggesting the direct regulation of T3SS cascade genes by RhpR. A genome-wide microarray analysis showed that, in addition to the T3SS cascade genes, RhpR differentially regulates a large set of genes with various functions in response to different growth conditions. Together, these results suggested that RhpRS is a global regulator that allows P. syringae to sense and respond to environmental changes by coordinating T3SS expression and many other biological processes.

Identification of Burkholderia cenocepacia strain H111 virulence factors using nonmammalian infection hosts

Burkholderia cenocepacia H111, a strain isolated from a cystic fibrosis patient, has been shown to effectively kill the nematode Caenorhabditis elegans. We used the C. elegans model of infection to screen a mini-Tn5 mutant library of B. cenocepacia H111 for attenuated virulence. Of the approximately 5,500 B. cenocepacia H111 random mini-Tn5 insertion mutants that were screened, 22 showed attenuated virulence in C. elegans. Except for the quorum-sensing regulator cepR, none of the mutated genes coded for the biosynthesis of classical virulence factors such as extracellular proteases or siderophores. Instead, the mutants contained insertions in metabolic and regulatory genes. Mutants attenuated in virulence in the C. elegans infection model were also tested in the Drosophila melanogaster pricking model, and those also attenuated in this model were further tested in Galleria mellonella. Six of the 22 mutants were attenuated in D. melanogaster, and five of these were less pathogenic in the G. mellonella model. We show that genes encoding enzymes of the purine, pyrimidine, and shikimate biosynthesis pathways are critical for virulence in multiple host models of infection.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Lon protease of Azorhizobium caulinodans ORS571 is required for suppression of reb gene expression

Bacterial Lon proteases play important roles in a variety of biological processes in addition to housekeeping functions. In this study, we focused on the Lon protease of Azorhizobium caulinodans, which can fix nitrogen both during free-living growth and in stem nodules of the legume Sesbania rostrata. The nitrogen fixation activity of an A. caulinodans lon mutant in the free-living state was not significantly different from that of the wild-type strain. However, the stem nodules formed by the lon mutant showed little or no nitrogen fixation activity. By microscopic analyses, two kinds of host cells were observed in the stem nodules formed by the lon mutant. One type has shrunken host cells containing a high density of bacteria, and the other type has oval or elongated host cells containing a low density or no bacteria. This phenotype is similar to a praR mutant highly expressing the reb genes. Quantitative reverse transcription-PCR analyses revealed that reb genes were also highly expressed in the lon mutant. Furthermore, a lon reb double mutant formed stem nodules showing higher nitrogen fixation activity than the lon mutant, and shrunken host cells were not observed in these stem nodules. These results suggest that Lon protease is required to suppress the expression of the reb genes and that high expression of reb genes in part causes aberrance in the A. caulinodans-S. rostrata symbiosis. In addition to the suppression of reb genes, it was found that Lon protease was involved in the regulation of exopolysaccharide production and autoagglutination of bacterial cells.

Type Three Secretion System in Pseudomonas savastanoi Pathovars: Does Timing Matter?

Pseudomonas savastanoi pv. savastanoi is the causal agent of Olive knot disease, relying on the Type Three Secretion System (TTSS) for its pathogenicity. In this regard, nothing was known about the two other pathovars belonging to this species, pv. nerii and pv. fraxini, characterized by a different host range. Here we report on the organization of the entire TTSS cluster on the three pathovars, and a phylogenetic analysis including the TTSS of those bacteria belonging to the P. syringae complex sequenced so far, highlighting the evolution of each operon (hrpC, hrpJ, hrpRS, hrpU and hrpZ). Moreover, by Real-Time PCR we analyzed the in vitro expression of four main TTSS genes, revealing different activation patterns in the three pathovars, hypothetically related to their diverse virulence behaviors.

Regulation of the co-evolved HrpR and HrpS AAA+ proteins required for Pseudomonas syringae pathogenicity

The bacterial AAA+ enhancer-binding proteins (EBPs) HrpR and HrpS (HrpRS) of Pseudomonas syringae (Ps) activate σ⁵⁴-dependent transcription at the hrpL promoter; triggering type-three secretion system-mediated pathogenicity. In contrast with singly acting EBPs, the evolution of the strictly co-operative HrpRS pair raises questions of potential benefits and mechanistic differences this transcription control system offers. Here, we show distinct properties of HrpR and HrpS variants, indicating functional specialization of these non-redundant, tandemly arranged paralogues. Activities of HrpR, HrpS and their control proteins HrpV and HrpG from Ps pv. tomato DC3000 in vitro establish that HrpRS forms a transcriptionally active hetero-hexamer, that there is a direct negative regulatory role for HrpV through specific binding to HrpS and that HrpG suppresses HrpV. The distinct HrpR and HrpS functionalities suggest how partial paralogue degeneration has potentially led to a novel control mechanism for EBPs and indicate subunit-specific roles for EBPs in σ⁵⁴-RNA polymerase activation.

HrpR and HrpS enhancer-binding proteins of Pseudomonas syringae activate σ⁵⁴-dependent transcription of the HrpL promoter and are required for type-three secretion pathogenicity. Here, the authors demonstrate that, despite being co-regulated, HrpR and HrpS each have distinct functions for activating σ⁵⁴.

The novel two-component regulatory system BfiSR regulates biofilm development by controlling the small RNA rsmZ through CafA

The formation of biofilms by the opportunistic pathogen Pseudomonas aeruginosa is a developmental process governed by a novel signal transduction system composed of three two-component regulatory systems (TCSs), BfiSR, BfmSR, and MifSR. Here, we show that BfiSR-dependent arrest of biofilm formation coincided with reduced expression of genes involved in virulence, posttranslational/transcriptional modification, and Rhl quorum sensing but increased expression of rhlAB and the small regulatory RNAs rsmYZ. Overexpression of rsmZ, but not rsmY, coincided with impaired biofilm development similar to inactivation of bfiS and retS. We furthermore show that BfiR binds to the 5' untranslated region of cafA encoding RNase G. Lack of cafA expression coincided with impaired biofilm development and increased rsmYZ levels during biofilm growth compared to the wild type. Overexpression of cafA restored ΔbfiS biofilm formation to wild-type levels and reduced rsmZ abundance. Moreover, inactivation of bfiS resulted in reduced virulence, as revealed by two plant models of infection. This work describes the regulation of a committed biofilm developmental step following attachment by the novel TCS BfiSR through the suppression of sRNA rsmZ via the direct regulation of RNase G in a biofilm-specific manner, thus underscoring the importance of posttranscriptional mechanisms in controlling biofilm development and virulence.

In planta gene expression analysis of Xanthomonas oryzae pathovar oryzae, African strain MAI1

BACKGROUND

Bacterial leaf blight causes significant yield losses in rice crops throughout Asia and Africa. Although both the Asian and African strains of the pathogen, Xanthomonas oryzae pv. oryzae (Xoo), induce similar symptoms, they are nevertheless genetically different, with the African strains being more closely related to the Asian X. oryzae pv. oryzicola (Xoc).

RESULTS

Changes in gene expression of the African Xoo strain MAI1 in the susceptible rice cultivar Nipponbare were profiled, using an SSH Xoo DNA microarray. Microarray hybridization was performed comparing bacteria recovered from plant tissues at 1, 3, and 6 days after inoculation (dai) with bacteria grown in vitro. A total of 710 bacterial genes were found to be differentially expressed, with 407 up-regulated and 303 down-regulated. Expression profiling indicated that less than 20% of the 710 bacterial transcripts were induced in the first 24 h after inoculation, whereas 63% were differentially expressed at 6 dai. The 710 differentially expressed genes were one-end sequenced. 535 sequences were obtained from which 147 non-redundant sequences were identified. Differentially expressed genes were related to metabolism, secretion and transport, pathogen adherence to plant tissues, plant cell-wall degradation, IS elements, and virulence. In addition, various other genes encoding proteins with unknown function or showing no similarity to other proteins were also induced. The Xoo MAI1 non-redundant set of sequences was compared against several X. oryzae genomes, revealing a specific group of genes that was present only in MAI1. Numerous IS elements were also found to be differentially expressed. Quantitative real-time PCR confirmed 86% of the identified profile on a set of 14 genes selected according to the microarray analysis.

CONCLUSIONS

This is the first report to compare the expression of Xoo genes in planta across different time points during infection. This work shows that as-yet-unidentified and potentially new virulence factors are appearing in an emerging African pathogen. It also confirms that African Xoo strains do differ from their Asian counterparts, even at the transcriptional level.

Negative regulation of the Hrp type III secretion system in Pseudomonas syringae pv. phaseolicola

Many plant-pathogenic bacteria require type III secretion systems (T3SS) to cause disease in compatible hosts and to induce the hypersensitive response in resistant plants. T3SS gene expression is induced within the plant and responds to host and environmental factors. In Pseudomonas syringae, expression is downregulated by the Lon protease in rich medium and by HrpV under inducing conditions. HrpV acts as an anti-activator by binding HrpS. HrpG, which can also bind HrpV, has been reported to act as an anti-anti-activator. Previous studies have used mostly in vitro inducing conditions, different pathovars, and methodology. We have used single and double lon and hrpV mutants of P. syringae pv. phaseolicola 1448a, as well as strains ectopically expressing the regulators, to examine their role in coordinating expression of the T3SS. We applied real-time polymerase chain reaction to analyze gene expression both in vitro and in planta, and assessed bacterial fitness using competitive indices. Our results indicate that i) Lon downregulates expression of the hrp/hrc genes in all conditions, probably by constitutively degrading naturally unstable HrpR; ii) HrpV and HrpT downregulate expression of the hrp/hrc genes in all conditions; and iii) HrpG has an additional, HrpV-independent role, regulating expression of the hrpC operon.

Pseudomonas syringae strains naturally lacking the classical P. syringae hrp/hrc Locus are common leaf colonizers equipped with an atypical type III secretion system

Pseudomonas syringae is best known as a plant pathogen that causes disease by translocating immune-suppressing effector proteins into plant cells through a type III secretion system (T3SS). However, P. syringae strains belonging to a newly described phylogenetic subgroup (group 2c) are missing the canonical P. syringae hrp/hrc cluster coding for a T3SS, flanking effector loci, and any close orthologue of known P. syringae effectors. Nonetheless, P. syringae group 2c strains are common leaf colonizers and grow on some tested plant species to population densities higher than those obtained by other P. syringae strains on nonhost species. Moreover, group 2c strains have genes necessary for the production of phytotoxins, have an ice nucleation gene, and, most interestingly, contain a novel hrp/hrc cluster, which is only distantly related to the canonical P. syringae hrp/hrc cluster. This hrp/hrc cluster appears to encode a functional T3SS although the genes hrpK and hrpS, present in the classical P. syringae hrp/hrc cluster, are missing. The genome sequence of a representative group 2c strain also revealed distant orthologues of the P. syringae effector genes avrE1 and hopM1 and the P. aeruginosa effector genes exoU and exoY. A putative life cycle for group 2c P. syringae is discussed.

Regulation and quality control by Lon-dependent proteolysis

After their first discovery in Escherichia coli, Lon homologues were found to be widely distributed among prokaryotes to eukaryotes. The ATP-dependent Lon protease belongs to the AAA(+) (ATPases associated with a variety of cellular activities) superfamily, and is involved in both general quality control by degrading abnormal proteins and in the specific control of several regulatory proteins. As such, this enzyme has a pivotal role in quality control and cellular physiology. This review focuses on mechanisms of degradation both from the protease and substrate points of view, and discusses the role of Lon in global regulation, stress response and virulence.

Pseudomonas syringae pv. phaseolicola Mutants Compromised for type III secretion system gene induction

Pseudomonas syringae bacteria utilize the type III secretion system (T3SS) to deliver effector proteins into host cells. The T3SS and T3 effector genes (together called the T3 genes hereafter) are repressed in nutrient-rich medium but rapidly induced after the bacteria are transferred into minimal medium or infiltrated into plants. The induction of the T3 genes is mediated by HrpL, an alternative sigma factor that recognizes the conserved hrp box motif in the T3 gene promoters. The induction of hrpL is mediated by HrpR and HrpS, two homologous proteins that bind the hrpL promoter. To identify additional genes involved in regulation of the T3 genes, we screened for the P. syringae pv. phaseolicola NPS3121 transposon-tagged mutants with reduced induction of avrPto-luc and hrpL-luc, reporter genes for promoters of effector gene avrPto and hrpL, respectively. Determination of the transposon-insertion sites revealed genes with putative functions in signal transduction and transcriptional regulation, protein synthesis, and basic metabolism. A transcriptional regulator (AefR(NPS3121)) was identified in our screen that is homologous to AefR of P. syringae pv. syringae strain B728a, a regulator of the quorum-sensing signal and epiphytic traits, but was not known to regulate the T3 genes. AefR(NPS3121) in P. syringae pv. phaseolicola NPS3121 and AefR in P. syringae pv. syringae B728a behave similarly in regulating the quorum-sensing signal in liquid medium but differ in regulating the epiphytic traits, including swarming motility, leaf entry, and epiphytic survival.