Immunogold labeling of Hrp pili of Pseudomonas syringae pv. tomato assembled in minimal medium and in planta

FleQ, FleN and c-di-GMP coordinately regulate cellulose production in Pseudomonas syringae pv. tomato DC3000

The second messenger cyclic di-GMP (c-di-GMP) controls the transition between motility and sessility in many bacterial species by a variety of mechanisms, including the production of multiple exopolysaccharides. Pseudomonas syringae pv. tomato (Pto) DC3000 is a plant pathogenic bacteria able to synthesize acetylated cellulose under high c-di-GMP levels thanks to the expression of the wssABCDEFGHI operon. Increased cellulose production enhances air-liquid biofilm formation and generates a wrinkled colony phenotype on solid media. We previously showed that under low levels of c-di-GMP, the regulators FleQ and AmrZ bound to adjacent sequences at the wss promoter inhibiting its expression, but only FleQ responded to the presence of c-di-GMP by activating cellulose production. In the present work, we advance in the knowledge of this complex regulation in Pto DC3000 by shedding light over the role of FleN in this process. The distinctive features of this system are that FleN and FleQ are both required for repression and activation of the wss operon under low and high c-di-GMP levels, respectively. We have also identified three putative FleQ binding sites at the wss promoter and show that FleQ/FleN-ATP binds at those sites under low c-di-GMP levels, inducing a distortion of DNA, impairing RNA polymerase binding, and repressing wss transcription. However, binding of c-di-GMP induces a conformational change in the FleQ/FleN-ATP complex, which relieves the DNA distortion, allows promoter access to the RNA polymerase, and leads to activation of wss transcription. On the other hand, AmrZ is always bound at the wss promoter limiting its expression independently of FleQ, FleN and c-di-GMP levels.

Visualization and characterization of Pseudomonas syringae pv. tomato DC3000 pellicles

Cellulose, whose production is controlled by c-di-GMP, is a commonly found exopolysaccharide in bacterial biofilms. Pseudomonas syringae pv. tomato (Pto) DC3000, a model organism for molecular studies of plant-pathogen interactions, carries the wssABCDEFGHI operon for the synthesis of acetylated cellulose. The high intracellular levels of the second messenger c-di-GMP induced by the overexpression of the heterologous diguanylate cyclase PleD stimulate cellulose production and enhance air-liquid biofilm (pellicle) formation. To characterize the mechanisms involved in Pto DC3000 pellicle formation, we studied this process using mutants lacking flagella, biosurfactant or different extracellular matrix components, and compared the pellicles produced in the absence and in the presence of PleD. We have discovered that neither alginate nor the biosurfactant syringafactin are needed for their formation, whereas cellulose and flagella are important but not essential. We have also observed that the high c-di-GMP levels conferred more cohesion to Pto cells within the pellicle and induced the formation of intracellular inclusion bodies and extracellular fibres and vesicles. Since the pellicles were very labile and this greatly hindered their handling and processing for microscopy, we have also developed new methods to collect and process them for scanning and transmission electron microscopy. These techniques open up new perspectives for the analysis of fragile biofilms in other bacterial strains.

A Career on Both Sides of the Atlantic: Memoirs of a Molecular Plant Pathologist

This article recounts the experiences that shaped my career as a molecular plant pathologist. It focuses primarily on technical and conceptual developments in molecular phytobacteriology, shares some personal highlights and untold stories that impacted my professional development, and describes the early years of agricultural biotechnology. Writing this article required reflection on events occurring over several decades that were punctuated by a mid-career relocation across the Atlantic. I hope it will still be useful, informative, and enjoyable to read. An extended version of the abstract is provided in the Supplemental Materials , available online.

AmrZ regulates cellulose production in Pseudomonas syringae pv. tomato DC3000

In Pseudomonas syringae pv. tomato DC3000, the second messenger c-di-GMP has been previously shown to stimulate pellicle formation and cellulose biosynthesis. A screen for genes involved in cellulose production under high c-di-GMP intracellular levels led to the identification of insertions in two genes, wssB and wssE, belonging to the Pto DC3000 cellulose biosynthesis operon wssABCDEFGHI. Interestingly, beside cellulose-deficient mutants, colonies with a rougher appearance than the wild type also arouse among the transposants. Those mutants carry insertions in amrZ, a gene encoding a transcriptional regulator in different Pseudomonas. Here, we provide evidence that AmrZ is involved in the regulation of bacterial cellulose production at transcriptional level by binding to the promoter region of the wssABCDEFGHI operon and repressing cellulose biosynthesis genes. Mutation of amrZ promotes wrinkly colony morphology, increased cellulose production and loss of motility in Pto DC3000. AmrZ regulon includes putative c-di-GMP metabolising proteins, like AdcA and MorA, which may also impact those phenotypes. Furthermore, an amrZ but not a cellulose-deficient mutant turned out to be impaired in pathogenesis, indicating that AmrZ is a key regulator of Pto DC3000 virulence probably by controlling bacterial processes other than cellulose production.

FleQ coordinates flagellum-dependent and -independent motilities in Pseudomonas syringae pv. tomato DC3000

Motility plays an essential role in bacterial fitness and colonization in the plant environment, since it favors nutrient acquisition and avoidance of toxic substances, successful competition with other microorganisms, the ability to locate the preferred hosts, access to optimal sites within them, and dispersal in the environment during the course of transmission. In this work, we have observed that the mutation of the flagellar master regulatory gene, fleQ, alters bacterial surface motility and biosurfactant production, uncovering a new type of motility for Pseudomonas syringae pv. tomato DC3000 on semisolid surfaces. We present evidence that P. syringae pv. tomato DC3000 moves over semisolid surfaces by using at least two different types of motility, namely, swarming, which depends on the presence of flagella and syringafactin, a biosurfactant produced by this strain, and a flagellum-independent surface spreading or sliding, which also requires syringafactin. We also show that FleQ activates flagellum synthesis and negatively regulates syringafactin production in P. syringae pv. tomato DC3000. Finally, it was surprising to observe that mutants lacking flagella or syringafactin were as virulent as the wild type, and only the simultaneous loss of both flagella and syringafactin impairs the ability of P. syringae pv. tomato DC3000 to colonize tomato host plants and cause disease.

Supramolecular Structure and Functional Analysis of the Type III Secretion System in Pseudomonas fluorescens 2P24

The type III secretion system (T3SS) of plant and animal bacterial pathogens directs the secretion and injection of proteins into host cells. Some homologous genes of T3SS were found also in non-pathogenic bacteria, but the organization of its machinery and basic function are still unknown. In this study, we identified a T3SS gene cluster from the plant growth-promoting Pseudomonas fluorescens 2P24 and isolated the corresponding T3SS apparatus. The T3SS gene cluster of strain 2P24 is similar organizationally to that of pathogenic P. syringae, except that it lacks the regulator hrpR and the hrpK1 and hrpH genes, which are involved in translocation of proteins. Electron microscopy revealed that the T3SS supramolecular structure of strain 2P24 was comprised of two distinctive substructures: a long extracellular, filamentous pilus, and a membrane-embedded base. We show that strain 2P24 deploys a harpin homolog protein, RspZ1, to elicit a hypersensitive response when infiltrated into Nicotiana tabacum cv. xanthi leaves with protein that is partially purified, and by complementing the hrpZ1 mutation of pHIR11. The T3SS of strain 2P24 retained ability to secrete effectors, whereas its effector translocation activity appeared to be excessively lost. Mutation of the rscC gene from 2P24 T3SS abolished the secretion of effectors, but the general biocontrol properties were unaffected. Remarkably, strain 2P24 induced functional MAMP-triggered immunity that included a burst of reactive oxygen species, strong suppression of challenge cell death, and disease expansion, while it was not associated with the secretion functional T3SS.

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.

Analysis of the interaction of Clavibacter michiganensis subsp. michiganensis with its host plant tomato by genome-wide expression profiling

Genome-wide expression profiles of the phytopathogenic actinomycete Clavibacter michiganensis subsp. michiganensis (Cmm) strain NCPPB382 were analyzed using a 70mer oligonucleotide microarray. Cmm causes bacterial wilt and canker of tomato, a systemic disease leading to substantial economic losses worldwide. Global gene expression was monitored in vitro after long- and short-term incubation with tomato homogenate to simulate conditions in planta and in vivo ten days after inoculation of tomatoes. Surprisingly, both in the presence of tomato homogenate and in planta known virulence genes (celA, chpC, ppaA/C) were down-regulated indicating that the encoded extracellular enzymes are dispensable in late infection stages where plant tissue has already been heavily destroyed. In contrast, some genes of the tomA-region which are involved in sugar metabolism showed an enhanced RNA-level after permanent growth in supplemented medium. Therefore, these genes may be important for utilization of plant derived nutrients. In the plant Cmm exhibited an expression profile completely different from that in vitro. Especially, the strong expression of genes of the wco-cluster (extracellular polysaccharide II), 10 genes encoding surface or pilus assembly proteins, and CMM_2382, coding for a putative perforin suggest a possible role of these genes in the plant-pathogenic interaction.

RIN4 functions with plasma membrane H+-ATPases to regulate stomatal apertures during pathogen attack

Pathogen perception by the plant innate immune system is of central importance to plant survival and productivity. The Arabidopsis protein RIN4 is a negative regulator of plant immunity. In order to identify additional proteins involved in RIN4-mediated immune signal transduction, we purified components of the RIN4 protein complex. We identified six novel proteins that had not previously been implicated in RIN4 signaling, including the plasma membrane (PM) H(+)-ATPases AHA1 and/or AHA2. RIN4 interacts with AHA1 and AHA2 both in vitro and in vivo. RIN4 overexpression and knockout lines exhibit differential PM H(+)-ATPase activity. PM H(+)-ATPase activation induces stomatal opening, enabling bacteria to gain entry into the plant leaf; inactivation induces stomatal closure thus restricting bacterial invasion. The rin4 knockout line exhibited reduced PM H(+)-ATPase activity and, importantly, its stomata could not be re-opened by virulent Pseudomonas syringae. We also demonstrate that RIN4 is expressed in guard cells, highlighting the importance of this cell type in innate immunity. These results indicate that the Arabidopsis protein RIN4 functions with the PM H(+)-ATPase to regulate stomatal apertures, inhibiting the entry of bacterial pathogens into the plant leaf during infection.

Strangers in the matrix: plant cell walls and pathogen susceptibility

Early in infection, pathogens encounter the outer wall of plant cells. Because pathogen hydrolases targeting the plant cell wall are well-known components of virulence, it has been assumed that wall disassembly by the plant itself also contributes to susceptibility, and now this has been established experimentally. Understanding how plant morphological and developmental remodeling and pathogen cell wall targeted virulence influence infections provides new perspectives about plant-pathogen interactions. The plant cell wall can be an effective physical barrier to pathogens, but also it is a matrix where many proteins involved in pathogen perception are delivered. By breaching the wall, a pathogen potentially reveals itself to the plant and activates responses, setting off events that might halt or limit its advance.

Pseudomonas syringae pv. tomato DC3000 uses constitutive and apoplast-induced nutrient assimilation pathways to catabolize nutrients that are abundant in the tomato apoplast

The plant apoplast is the intercellular space that surrounds plant cells, in which metabolic and physiological processes relating to cell wall biosynthesis, nutrient transport, and stress responses occur. The apoplast is also the primary site of infection for hemibiotrophic pathogens such as P. syringae, which obtain nutrients directly from apoplastic fluid. We have used apoplastic fluid extracted from healthy tomato leaves as a growth medium for Pseudomonas spp. in order to investigate the role of apoplastic nutrients in plant colonization by Pseudomonas syringae. We have confirmed that apoplast extracts mimic some of the environmental and nutritional conditions that bacteria encounter during apoplast colonization by demonstrating that expression of the plant-induced type III protein secretion pathway is upregulated during bacterial growth in apoplast extracts. We used a modified phenoarray technique to show that apoplast-adapted P. syringae pv. tomato DC3000 expresses nutrient utilization pathways that allow it to use sugars, organic acids, and amino acids that are highly abundant in the tomato apoplast. Comparative analyses of the nutrient utilization profiles of the genome-sequenced strains P. syringae pv. tomato DC3000, P. syringae pv. syringae B728a, P. syringae pv. phaseolicola 1448A, and the unsequenced strain P. syringae pv. tabaci 11528 with nine other genome-sequenced strains of Pseudomonas provide further evidence that P. syringae strains are adapted to use nutrients that are abundant in the leaf apoplast. Interestingly, P. syringae pv. phaseolicola 1448A lacks many of the nutrient utilization abilities that are present in three other P. syringae strains tested, which can be directly linked to differences in the P. syringae pv. phaseolicola 1448A genome.

Genome-wide transcriptional analysis of the Arabidopsis thaliana interaction with the plant pathogen Pseudomonas syringae pv. tomato DC3000 and the human pathogen Escherichia coli O157:H7

Pseudomonas syringae pv. tomato DC3000 (Pst) is a virulent pathogen that causes disease on tomato and Arabidopsis. The type III secretion system (TTSS) plays a key role in pathogenesis by translocating virulence effectors from the bacteria into the plant host cell, while the phytotoxin coronatine (COR) contributes to virulence and disease symptom development. Recent studies suggest that both the TTSS and COR are involved in the suppression of host basal defenses. However, little is known about the interplay between the host gene expression changes associated with basal defenses and the virulence activities of the TTSS and COR during infection. In this study, we used the Affymetrix full genome chip to determine the Arabidopsis transcriptome associated with basal defense to Pst DC3000 hrp mutants and the human pathogenic bacterium Escherichia coli O157:H7. We then used Pst DC3000 virulence mutants to characterize Arabidopsis transcriptional responses to the action of hrp-regulated virulence factors (e.g. TTSS and COR) during bacterial infection. Additionally, we used bacterial fliC mutants to assess the role of the pathogen-associated molecular pattern flagellin in induction of basal defense-associated transcriptional responses. In total, our global gene expression analysis identified 2800 Arabidopsis genes that are reproducibly regulated in response to bacterial pathogen inoculation. Regulation of these genes provides a molecular signature for Arabidopsis basal defense to plant and human pathogenic bacteria, and illustrates both common and distinct global virulence effects of the TTSS, COR, and possibly other hrp-regulated virulence factors during Pst DC3000 infection.

Domain structure of HrpE, the Hrp pilus subunit of Xanthomonas campestris pv. vesicatoria

The plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria possesses a type III secretion (TTS) system necessary for pathogenicity in susceptible hosts and induction of the hypersensitive response in resistant plants. This specialized protein transport system is encoded by a 23-kb hrp (hypersensitive response and pathogenicity) gene cluster. X. campestris pv. vesicatoria produces filamentous structures, Hrp pili, at the cell surface under hrp-inducing conditions. The Hrp pilus acts as a cell surface appendage of the TTS system and serves as a conduit for the transfer of bacterial effector proteins into the plant cell cytosol. The major pilus component, the HrpE pilin, is unique to xanthomonads and is encoded within the hrp gene cluster. In this study, functional domains of HrpE were mapped by linker-scanning mutagenesis and by reporter protein fusions to an N-terminally truncated avirulence protein (AvrBs3Delta2). Thirteen five-amino-acid peptide insertion mutants were obtained and could be grouped into six phenotypic classes. Three permissive mutations were mapped in the N-terminal half of HrpE, which is weakly conserved within the HrpE protein family. Four dominant-negative peptide insertions in the strongly conserved C-terminal region suggest that this domain is critical for oligomerization of the pilus subunits. Reporter protein fusions revealed that the N-terminal 17 amino acid residues act as an efficient TTS signal. From these results, we postulate a three-domain structure of HrpE with an N-terminal secretion signal, a surface-exposed variable region of the N-terminal half, and a C-terminal polymerization domain. Comparisons with a mutant study of HrpA, the Hrp pilin from Pseudomonas syringae pv. tomato DC3000, and hydrophobicity plot analyses of several nonhomologous Hrp pilins suggest a common architecture of Hrp pilins of different plant-pathogenic bacteria.

Flagellin induces innate immunity in nonhost interactions that is suppressed by Pseudomonas syringae effectors

Arabidopsis NONHOST1 (NHO1) is required for limiting the in planta growth of nonhost Pseudomonas bacteria but completely ineffective against the virulent bacterium Pseudomonas syringae pv. tomato DC3000. However, the molecular basis underlying this observation remains unknown. Here we show that NHO1 is transcriptionally activated by flagellin. The nonhost bacterium P. syringae pv. tabaci lacking flagellin is unable to induce NHO1, multiplies much better than does the wild-type bacterium, and causes disease symptoms on Arabidopsis. DC3000 also possesses flagellin that is potent in NHO1 induction, but this induction is rapidly suppressed by DC3000 in a type III secretion system-dependent manner. Direct expression of DC3000 effectors in protoplasts indicated that at least nine effectors, HopS1, HopAI1, HopAF1, HopT1-1, HopT1-2, HopAA1-1, HopF2, HopC1, and AvrPto, are capable of suppressing the flagellin-induced NHO1 expression. One of the effectors, HopAI1, is conserved in both animal and plant bacteria. When expressed in transgenic Arabidopsis plants, HopAI1 promotes growth of the nonpathogenic hrpL- mutant bacteria. In addition, the purified phytotoxin coronatine, a known virulence factor of P. syringae, suppresses the flagellin-induced NHO1 transcription. These results demonstrate that flagellin-induced defenses play an important role in nonhost resistance. A remarkable number of DC3000 virulence factors act in the plant cell by suppressing the species level defenses, and that contributes to the specialization of DC3000 on Arabidopsis.

The type III-dependent Hrp pilus is required for productive interaction of Xanthomonas campestris pv. vesicatoria with pepper host plants

The plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria expresses a type III secretion system that is necessary for both pathogenicity in susceptible hosts and the induction of the hypersensitive response in resistant plants. This specialized protein transport system is encoded by a 23-kb hrp (hypersensitive response and pathogenicity) gene cluster. Here we show that X. campestris pv. vesicatoria produces filamentous structures, the Hrp pili, at the cell surface under hrp-inducing conditions. Analysis of purified Hrp pili and immunoelectron microscopy revealed that the major component of the Hrp pilus is the HrpE protein which is encoded in the hrp gene cluster. Sequence homologues of hrpE are only found in other xanthomonads. However, hrpE is syntenic to the hrpY gene from another plant pathogen, Ralstonia solanacearum. Bioinformatic analyses suggest that all major Hrp pilus subunits from gram-negative plant pathogens may share the same structural organization, i.e., a predominant alpha-helical structure. Analysis of nonpolar mutants in hrpE demonstrated that the Hrp pilus is essential for the productive interaction of X. campestris pv. vesicatoria with pepper host plants. Furthermore, a functional Hrp pilus is required for type III-dependent protein secretion. Immunoelectron microscopy revealed that type III-secreted proteins, such as HrpF and AvrBs3, are in close contact with the Hrp pilus during and/or after their secretion. By systematic analysis of nonpolar hrp/hrc (hrp conserved) and hpa (hrp associated) mutants, we found that Hpa proteins as well as the translocon protein HrpF are dispensable for pilus assembly, while all other Hrp and Hrc proteins are required. Hence, there are no other conserved Hrp or Hrc proteins that act downstream of HrpE during type III-dependent protein translocation.

NopB, a soybean cultivar-specificity protein from Sinorhizobium fredii USDA257, is a type III secreted protein

The type III secretion system (TTSS) of plant- and animal-pathogenic bacteria is involved in translocation of virulence factors into the host cell cytosol where they modulate cellular processes. Sinorhizobium fredii USDA257 is a gram-negative soil bacterium that forms nitrogen-fixing nodules on specific soybean cultivars (Glycine max (L.) Merr.). This microsymbiont is known to secrete at least five nodulation outer proteins (Nops) in response to flavonoid induction. Some of these Nops have been shown to be secreted by TTSS in this symbiotic bacterium. We have isolated and purified an 18-kDa extracellular protein from flavonoid-induced cultures of USDA257. The N-terminal amino acid sequence of this purified protein was identical to the published sequence of the soybean cultivar-specificity protein, NopB (formerly NoIB). Inactivation of rhcN, which encodes an ATPase, abolished secretion of NopB. Similarly, a nonpolar nopB deletion mutant was compromised in its ability to secrete several Nops. A construct containing the coding region of nopB under control of a T7 promoter was expressed successfully in Escherichia coli and, subsequently, the recombinant NopB was purified by nickel-affinity column chromatography. Polyclonal antibodies raised against purified recombinant NopB were used in Western blot analysis to demonstrate the association of NopB with pilus-like surface appendages. Deletion analysis indicated that the first 33 N-terminal residues of NopB were necessary and sufficient to mediate the secretion of a green fluorescent protein reporter. Introduction of plasmid-borne extra copies of nopB into USDA257 resulted in lower accumulation of native NopB. We also show that USDA257 and its nonpolar nopB deletion mutant exhibited discernible differences in their ability to nodulate legume hosts.

The bacterial type III secretion system-associated pilin HrpA has an unusually long mRNA half-life

Secondary structures affect mRNA stability and may play a role in protein secretion. We have studied the mRNA of hrpA, which codes for the major structural unit of the type III secretion system-associated pilus of Pseudomonas syringae pv. tomato, Erwinia carotovora and Pseudomonas syringae pv. phaseolicola. We show that hrpA mRNA has an unusually long half-life, approximately 33-47 min. We mapped regions in the transcript that affected hrpA mRNA accumulation. Apparently, sequences at both 5' and 3' ends affect accumulation. Altering the hypothetical, stable GC rich loop structure in the 3' end of the transcript decreased transcript levels.

Deciphering host resistance and pathogen virulence: the Arabidopsis/Pseudomonas interaction as a model

SUMMARY The last decade has witnessed steady progress in deciphering the molecular basis of plant disease resistance and pathogen virulence. Although contributions have been made using many different plant and pathogen species, studies of the interactions between Arabidopsis thaliana and Pseudomonas syringae have yielded a particularly significant body of information. The present review focuses on recent findings regarding R gene products and the guard hypothesis, RAR1/SGT1 and other examples where protein processing activity is implicated in disease resistance or susceptibility, the use of microarray expression profiling to generate information and experimental leads, and important molecular- and genome-level discoveries regarding P. syringae effectors that mediate bacterial virulence. The development of the Arabidopsis-Pseudomonas model system is also reviewed briefly, and we close with a discussion of characteristics to consider when selecting other pathosystems as experimentally tractable models for future research.

Getting across--bacterial type III effector proteins on their way to the plant cell

Pathogenicity of most Gram-negative bacterial plant pathogens depends on hrp (hypersensitive response and pathogenicity) genes, which control the ability to cause disease and to elicit specific defense responses in resistant plants. hrp genes encode a specialized type III secretion (TTS) system that mediates the vectorial delivery of bacterial effector proteins across both bacterial membranes as well as across the eukaryotic plasma membrane into the host cell cytosol. One well-studied effector protein is AvrBs3 from Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot in pepper and tomato. AvrBs3 induces hypertrophy symptoms in susceptible plants and triggers a resistance gene-specific cell death reaction in resistant plants. Intriguingly, AvrBs3 has characteristic features of eukaryotic transcription factors, suggesting that it modulates the host's transcriptome. Here, we discuss the TTS system of X.campestris pv. vesicatoria in the light of current knowledge on type III-dependent protein secretion in plant pathogenic bacteria.

hrp genes of Erwinia chrysanthemi 3937 are important virulence factors

We developed improved virulence assays for Erwinia chrysanthemi 3937 on African violet varieties and devised a new method for the construction of precise bacterial gene knockouts. These methods were tested by constructing mutations in genes suspected to be involved with plant interactions. The virulence of the hrpG and hrcC mutant strains (both gene products presumed to be involved in protein secretion) was greatly reduced on leaves of semitolerant African violet varieties. An hrpN mutant strain produced delayed symptoms on African violet leaves and an hrpN delta pel (delta pel = five major pectate lyase genes deleted) double mutant was nonpathogenic. The hrcC and hrpG mutants did not produce a rapid hypersensitive response (HR) in tobacco, unlike the wild-type bacterium, and the hrpN mutant gave a reduced HR. The results, therefore, establish the importance of hrp genes in the virulence of E. chrysanthemi and their ability to elicit HR on nonhosts. The data also suggest that other effector proteins secreted by the Hrp system are required for full virulence and HR elicitation.

The Hrp pilus of Pseudomonas syringae elongates from its tip and acts as a conduit for translocation of the effector protein HrpZ

The type III secretion system (TTSS) is an essential requirement for the virulence of many Gram-negative bacteria infecting plants, animals and man. Pathogens use the TTSS to deliver effector proteins from the bacterial cytoplasm to the eukaryotic host cell, where the effectors subvert host defences. Plant pathogens have to translocate their effector proteins through the plant cell wall barrier. The best candidates for directing effector protein traffic are bacterial appendages attached to the membrane-bound components of the TTSS. We have investigated the protein secretion route in relation to the TTSS appendage, termed the Hrp pilus, of the plant pathogen Pseudomonas syringae pv. tomato. By pulse expression of proteins combined with immunoelectron microscopy, we show that the Hrp pilus elongates by the addition of HrpA pilin subunits at the distal end, and that the effector protein HrpZ is secreted only from the pilus tip. Our results indicate that both HrpA and HrpZ travel through the Hrp pilus, which functions as a conduit for the long-distance translocation of effector proteins.

Identification of Pseudomonas syringae pv. tomato genes induced during infection of Arabidopsis thaliana

Phytopathogenic bacteria possess a large number of genes that allow them to grow and cause disease on plants. Many of these genes should be induced when the bacteria come in contact with plant tissue. We used a modified in vivo expression technology (IVET) approach to identify genes from the plant pathogen Pseudomonas syringae pv. tomato that are induced upon infection of Arabidopsis thaliana and isolated over 500 in planta-expressed (ipx) promoter fusions. Sequence analysis of 79 fusions revealed several known and potential virulence genes, including hrp/hrc, avr and coronatine biosynthetic genes. In addition, we identified metabolic genes presumably important for adaptation to growth in plant tissue, as well as several genes with unknown function that may encode novel virulence factors. Many ipx fusions, including several corresponding to novel genes, are dependent on HrpL, an alternative RNA polymerase sigma factor that regulates the expression of virulence genes. Expression analysis indicated that several ipx fusions are strongly induced upon inoculation into plant tissue. Disruption of one ipx gene, conserved effector locus (CEL) orf1, encoding a putative lytic murein transglycosylase, resulted in decreased virulence of P. syringae. Our results demonstrate that this screen can be used successfully to isolate genes that are induced in planta, including many novel genes potentially involved in pathogenesis.

The role of bacterial pili in protein and DNA translocation

Gram-negative bacteria have surface appendages that assemble via different secretion machineries. Recently, new experimental approaches have contributed to a better understanding of the molecular mechanisms of flagellar and pilus assembly, and protein secretion. These findings can be applied to plant pathogenic bacteria, which probably transfer effector proteins directly into their eukaryotic host cells. Here, it is suggested that assembly of Hrp pili occurs in the periplasm and that unfolded effector proteins attach to pilins within the pili, thus effecting protein translocation. A two-domain structure for the HrpA pilin from Pseudomonas syringae is also predicted.

Genetic organization of the Pantoea stewartii subsp. stewartii hrp gene cluster and sequence analysis of the hrpA, hrpC, hrpN, and wtsE operons

The hrp/wts gene cluster of Pantoea stewartii subsp. stewartii is required for pathogenicity on sweet corn and the ability to elicit a hypersensitive response (HR) in tobacco. Site-directed transposon mutagenesis and nucleotide sequencing were used to identify hrp/wts genes within the left 20 kb of this cluster. Seventeen open reading frames (ORFs) comprise seven genetic complementation groups. These ORFs share homology with hrp and dsp genes from Erwinia amylovora, Erwinia chrysanthemi, and Pseudomonas syringae pathovars and have been designated, in map order, wtsF, wtsE, hrpN, hrpV, hrpT, hrcC, hrpG, hrpF, hrpE, hrpD, hrcJ, hrpB, hrpA, hrpS, hrpY, hrpX, and hrpL. Putative hrp consensus promoter sequences were identified upstream of hrpA, hrpF, hrpN, and wtsE. Expression of the hrpA, hrpC, and wtsE operons was regulated by HrpS. Transposon mutations in all of the hrp operons abolished pathogenicity and HR elicitation, except for the hrpN and hrpV mutants, which were still pathogenic. hrpS, hrpXY, and hrpL regulatory mutations abolished HrpN synthesis, whereas secretory mutations in the hrpC, hrpA, and hrpJ operons permitted intracellular HrpN synthesis. wtsEF mutants were not pathogenic but still produced HrpN and elicited the HR. wtsE encodes a 201-kDa protein that is similar to DspE in E. amylovora and AvrE in P. syringae pv. tomato, suggesting that this protein is a major virulence factor involved in the elicitation of water-soaked lesions.