Protein signaling via type III secretion pathways in phytopathogenic bacteria

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.

An essential role for the VASt domain of the Arabidopsis VAD1 protein in the regulation of defense and cell death in response to pathogens

Several regulators of programmed cell death (PCD) have been identified in plants which encode proteins with putative lipid-binding domains. Among them, VAD1 (Vascular Associated Death) contains a novel protein domain called VASt (VAD1 analog StAR-related lipid transfer) still uncharacterized. The Arabidopsis mutant vad1-1 has been shown to exhibit a lesion mimic phenotype with light-conditional appearance of propagative hypersensitive response-like lesions along the vascular system, associated with defense gene expression and increased resistance to Pseudomonas strains. To test the potential of ectopic expression of VAD1 to influence HR cell death and to elucidate the role of the VASt domain in this function, we performed a structure-function analysis of VAD1 by transient over-expression in Nicotiana benthamiana and by complementation of the mutant vad1-1. We found that (i) overexpression of VAD1 controls negatively the HR cell death and defense expression either transiently in Nicotiana benthamania or in Arabidopsis plants in response to avirulent strains of Pseudomonas syringae, (ii) VAD1 is expressed in multiple subcellular compartments, including the nucleus, and (iii) while the GRAM domain does not modify neither the subcellular localization of VAD1 nor its immunorepressor activity, the domain VASt plays an essential role in both processes. In conclusion, VAD1 acts as a negative regulator of cell death associated with the plant immune response and the VASt domain of this unknown protein plays an essential role in this function, opening the way for the functional analysis of VASt-containing proteins and the characterization of novel mechanisms regulating PCD.

Molecular cloning of ATR5(Emoy2) from Hyaloperonospora arabidopsidis, an avirulence determinant that triggers RPP5-mediated defense in Arabidopsis

RPP5 is the seminal example of a cytoplasmic NB-LRR receptor-like protein that confers downy mildew resistance in Arabidopsis thaliana. In this study, we describe the cloning and molecular characterization of the gene encoding ATR5(Emoy2), an avirulence protein from the downy mildew pathogen Hyaloperonospora arabidopsidis isolate Emoy2. ATR5(Emoy2) triggers defense response in host lines expressing the functional RPP5 allele from Landsberg erecta (Ler-0). ATR5(Emoy2) is embedded in a cluster with two additional ATR5-like (ATR5L) genes, most likely resulting from gene duplications. ATR5L proteins do not trigger RPP5-mediated resistance and the copy number of ATR5L genes varies among H. arabidopsidis isolates. ATR5(Emoy2) and ATR5L proteins contain a signal peptide, canonical EER motif, and an RGD motif. However, they lack the canonical translocation motif RXLR, which characterizes most oomycete effectors identified so far. The signal peptide and the N-terminal regions including the EER motif of ATR5(Emoy2) are not required to trigger an RPP5-dependent immune response. Bioinformatics screen of H. arabidopsidis Emoy2 genome revealed the presence of 173 open reading frames that potentially encode for secreted proteins similar to ATR5(Emoy2), in which they share some motifs such as EER but there is no canonical RXLR motif.

C-terminal region of plant ferredoxin-like protein is required to enhance resistance to bacterial disease in Arabidopsis thaliana

Protein phosphorylation is an important biological process associated with elicitor-induced defense responses in plants. In a previous report, we described how plant ferredoxin-like protein (PFLP) in transgenic plants enhances resistance to bacterial pathogens associated with the hypersensitive response (HR). PFLP possesses a putative casein kinase II phosphorylation (CK2P) site at the C-terminal in which phosphorylation occurs rapidly during defense response. However, the contribution of this site to the enhancement of disease resistance and the intensity of HR has not been clearly demonstrated. In this study, we generated two versions of truncated PFLP, PEC (extant CK2P site) and PDC (deleted CK2P site), and assessed their ability to trigger HR through harpin (HrpZ) derived from Pseudomonas syringae as well as their resistance to Ralstonia solanacearum. In an infiltration assay of HrpZ, PEC intensified harpin-mediated HR; however, PDC negated this effect. Transgenic plants expressing these versions indicate that nonphosphorylated PFLP loses its ability to induce HR or enhance disease resistance against R. solanacearum. Interestingly, the CK2P site of PFLP is required to induce the expression of the NADPH oxidase gene, AtrbohD, which is a reactive oxygen species producing enzyme. This was further confirmed by evaluating the HR on NADPH oxidase in mutants of Arabidopsis. As a result, we have concluded that the CK2P site is required for the phosphorylation of PFLP to enhance disease resistance.

Receptor-mediated signalling in plants: molecular patterns and programmes

A highly evolved surveillance system in plants is able to detect a broad range of signals originating from pathogens, damaged tissues, or altered developmental processes, initiating sophisticated molecular mechanisms that result in defence, wound healing, and development. Microbe-associated molecular pattern molecules (MAMPs), damage-associated molecular pattern molecules (DAMPs), virulence factors, secreted proteins, and processed peptides can be recognized directly or indirectly by this surveillance system. Nucleotide binding-leucine rich repeat proteins (NB-LRR) are intracellular receptors and have been targeted by breeders for decades to elicit resistance to crop pathogens in the field. Receptor-like kinases (RLKs) or receptor like proteins (RLPs) are membrane bound signalling molecules with an extracellular receptor domain. They provide an early warning system for the presence of potential pathogens and activate protective immune signalling in plants. In addition, they act as a signal amplifier in the case of tissue damage, establishing symbiotic relationships and effecting developmental processes. The identification of several important ligands for the RLK-type receptors provided an opportunity to understand how plants differentiate, how they distinguish beneficial and detrimental stimuli, and how they co-ordinate the role of various types of receptors under varying environmental conditions. The diverse roles of extra-and intracellular plant receptors are examined here and the recent findings on how they promote defence and development is reviewed.

Rhizosphere Bacterial Signalling: A Love Parade Beneath Our Feet

Plant roots support the growth and activities of a wide variety of microorganisms that may have a profound effect on the growth and/or health of plants. Among these microorganisms, a high diversity of bacteria have been identified and categorized as deleterious, beneficial, or neutral with respect to the plant. The beneficial bacteria, termed plant growth-promoting rhizobacteria (PGPR), are widely studied by microbiologists and agronomists because of their potential in plant production. Azospirillum, a genus of versatile PGPR, is able to enhance the plant growth and yield of a wide range of economically important crops in different soils and climatic regions. Plant beneficial effects of Azospirillum have mainly been attributed to the production of phytohormones, nitrate reduction, and nitrogen fixation, which have been subject of extensive research throughout the years. These elaborate studies made Azospirillum one of the best-characterized genera of PGPR. However, the genetic and molecular determinants involved in the initial interaction between Azospirillum and plant roots are not yet fully understood. This review will mainly highlight the current knowledge on Azospirillum plant root interactions, in the context of preceding and ongoing research on the association between plants and plant growth-promoting rhizobacteria.

Electrophysiological characterization of the Arabidopsis avrRpt2-specific hypersensitive response in the absence of other bacterial signals

The hypersensitive response (HR) is defined as rapid cell collapse at the infection site and often accompanies plant resistance. The physiological processes leading to HR are not well understood. Here, we report an electrophysiological characterization of bacterial HR caused by a single avirulence gene in the absence of other bacterial signals. We used dexamethasone (dex)-inducible transgenic Arabidopsis (Arabidopsis thaliana) plants containing the avrRpt2 gene from Pseudomonas syringae pv tomato. Membrane depolarization in these plants began 1 to 1.5 h after dex application, hours before electrolyte leakage. Progressive depolarization was a sensitive early indicator of HR that occurred only in Arabidopsis leaf cells expressing both avrRpt2 and a functional RPS2 gene. Hyperpolarization of fully depolarized membranes by fusicoccin, a fungal toxin that activates the H(+)-ATPase, indicates that depolarization did not result from a nonfunctional pump or leaky membranes. Depolarization and electrolyte leakage were inhibited in RPS2 plants by the calcium channel blocker LaCl(3), highly correlating these events and suggesting that Ca(2+) entry into cells is required for both. Also correlated were inhibition of depolarization, electrolyte leakage, and HR following salicylic acid pretreatment. In salicylic acid-pretreated RPS2 seedlings, avrRpt2 transcript was produced after dex treatment. However, AvrRpt2 protein accumulation was greatly reduced, suggesting a possible mechanism for inhibition of HR in plants with induced resistance. This experimental system is a very sensitive assay that lends itself to the dissection of physiological processes leading to HR in plants, and provides a baseline for future research within a genetic framework.

Ectopic expression of Ralstonia solanacearum effector protein PopA early in invasion results in loss of virulence

Ralstonia solanacearum OE1-1 (OE1-1) is pathogenic to tobacco. The type III-secreted effector protein popA of OE1-1 showed 97.6% identity to popA of R. solanacearum GMI1000, which is not pathogenic to tobacco. Reverse transcription-polymerase chain reaction analysis showed that popA in OE1-1 was expressed at 3 h after inoculation (HAI), but not before, in infiltrated-tobacco leaves. Pathogenicity analysis using a popABC operon-deleted mutant of OE1-1 (deltaABC) showed that popABC is not directly involved in the pathogenicity of OE1-1. When Papa, which constitutively expresses popA, was infiltrated into tobacco leaves, popA was expressed by 0.5 HAI. Papa could no longer multiply or spread in tobacco leaves and was no longer virulent. Moreover, the hypersensitive response (HR) and expression of HR-related genes were not induced in Papa-infiltrated leaves. Papa was also avirulent in a tobacco root-dipping inoculation assay. These results suggest that the expression of popA in Papa immediately after invasion triggers the suppression of bacterial proliferation and movement, resulting in loss of virulence. However, Papa retained its virulence when directly inoculated into xylem vessels. This result suggests that tobacco plants can recognize PopA when it is expressed early in disease development, and respond with an effective defense in the intercellular spaces.

Soft rot erwiniae: from genes to genomes

SUMMARY The soft rot erwiniae, Erwinia carotovora ssp. atroseptica (Eca), E. carotovora ssp. carotovora (Ecc) and E. chrysanthemi (Ech) are major bacterial pathogens of potato and other crops world-wide. We currently understand much about how these bacteria attack plants and protect themselves against plant defences. However, the processes underlying the establishment of infection, differences in host range and their ability to survive when not causing disease, largely remain a mystery. This review will focus on our current knowledge of pathogenesis in these organisms and discuss how modern genomic approaches, including complete genome sequencing of Eca and Ech, may open the door to a new understanding of the potential subtlety and complexity of soft rot erwiniae and their interactions with plants.

TAXONOMY

The soft rot erwiniae are members of the Enterobacteriaceae, along with other plant pathogens such as Erwinia amylovora and human pathogens such as Escherichia coli, Salmonella spp. and Yersinia spp. Although the genus name Erwinia is most often used to describe the group, an alternative genus name Pectobacterium was recently proposed for the soft rot species.

HOST RANGE

Ech mainly affects crops and other plants in tropical and subtropical regions and has a wide host range that includes potato and the important model host African violet (Saintpaulia ionantha). Ecc affects crops and other plants in subtropical and temperate regions and has probably the widest host range, which also includes potato. Eca, on the other hand, has a host range limited almost exclusively to potato in temperate regions only. Disease symptoms: Soft rot erwiniae cause general tissue maceration, termed soft rot disease, through the production of plant cell wall degrading enzymes. Environmental factors such as temperature, low oxygen concentration and free water play an essential role in disease development. On potato, and possibly other plants, disease symptoms may differ, e.g. blackleg disease is associated more with Eca and Ech than with Ecc.

USEFUL WEBSITES

http://www.scri.sari.ac.uk/TiPP/Erwinia.htm, http://www.ahabs.wisc.edu:16080/ approximately pernalab/erwinia/index.htm, http://www.tigr.org/tdb/mdb/mdbinprogress.html, http://www.sanger.ac.uk/Projects/E_carotovora/.

Identification of novel hrp-regulated genes through functional genomic analysis of the Pseudomonas syringae pv. tomato DC3000 genome

Pseudomonas syringae pv. tomato (Pst) strain DC3000 infects the model plants Arabidopsis thaliana and tomato, causing disease symptoms characterized by necrotic lesions surrounded by chlorosis. One mechanism used by Pst DC3000 to infect host plants is the type III protein secretion system, which is thought to deliver multiple effector proteins to the plant cell. The exact number of type III effectors in Pst DC3000 or any other plant pathogenic bacterium is not known. All known type III effector genes of P. syringae are regulated by HrpS, an NtrC family protein, and the HrpL alternative sigma factor, which presumably binds to a conserved cis element (called the "hrp box") in the promoters of type III secretion-associated genes. In this study, we designed a search motif based on the promoter sequences conserved in 12 published hrp operons and putative effector genes in Pst DC3000. Seventy-three predicted genes were retrieved from the January 2001 release of the Pst DC3000 genome sequence, which had 95% genome coverage. The expression of the 73 genes was analysed by microarray and Northern blotting, revealing 24 genes/operons (including eight novel genes), the expression of which was consistently higher in hrp-inducing minimal medium than in nutrient-rich Luria-Bertani broth. Expression of all eight genes was dependent on the hrpS gene. Most were also dependent on the hrpL gene, but at least one was dependent on the hrpS gene, but not on the hrpL gene. An AvrRpt2-based type III translocation assay provides evidence that some of the hrpS-regulated novel genes encode putative effector proteins.

Direct biochemical evidence for type III secretion-dependent translocation of the AvrBs2 effector protein into plant cells

The calmodulin-dependent adenylate cyclase domain (Cya) of the Bordetella pertussis cyclolysin was used as a reporter protein to study the direct translocation of the Xanthomonas effector protein, AvrBs2, into the plant host cell. Adenylate cyclase activity (production of cAMP) depends on the presence of eukaryotic plant calmodulin and is only active after translocation from the prokaryotic cell into the eukaryotic plant cell. Here, we show that infection of pepper plants by Xanthomonas campestris pv. vesicatoria strains expressing the AvrBs2:Cya fusion protein results in detectable increases of cAMP levels in plant cells as early as 3 h after inoculation. Adenylate cyclase activity was shown to be type III secretion-dependent as the Xanthomonas hrp mutations, hrcV or hrpF, failed to produce detectable levels of cAMP in infected pepper plants. Furthermore, the N-terminal secretion and translocation signals of AvrBs2 were shown to be required for activity of the fusion protein in the plant. A single genomic copy of the avrBs2:cya fusion gene expressed under the control of the wild-type avrBs2 promoter was used to compare the effect of a susceptible and resistant plant interaction on the kinetics of effector protein delivery. Implications of these results and additional applications of this reporter construct are discussed.

Differential roles of the Pseudomonas aeruginosa PA14 rpoN gene in pathogenicity in plants, nematodes, insects, and mice

We cloned the rpoN (ntrA, glnF) gene encoding the alternate sigma factor sigma(54) from the opportunistic multihost pathogen Pseudomonas aeruginosa strain PA14. A marker exchange protocol was used to construct the PA14 rpoN insertional mutation rpoN::Gen(r). PA14 rpoN::Gen(r) synthesized reduced levels of pyocyanin and displayed a variety of phenotypes typical of rpoN mutants, including a lack of motility and the failure to grow on nitrate, glutamate, or histidine as the sole nitrogen source. Compared to wild-type PA14, rpoN::Gen(r) was ca. 100-fold less virulent in a mouse thermal injury model and was significantly impaired in its ability to kill the nematode Caenorhabditis elegans. In an Arabidopsis thaliana leaf infectivity assay, although rpoN::Gen(r) exhibited significantly reduced attachment to trichomes, stomata, and the epidermal cell surface, did not attach perpendicularly to or perforate mesophyll cell walls, and proliferated less rapidly in Arabidopsis leaves, it nevertheless elicited similar disease symptoms to wild-type P. aeruginosa PA14 at later stages of infection. rpoN::Gen(r) was not impaired in virulence in a Galleria mellonella (greater wax moth) pathogenicity model. These data indicate that rpoN does not regulate the expression of any genes that encode virulence factors universally required for P. aeruginosa pathogenicity in diverse hosts.

The role of NDR1 in avirulence gene-directed signaling and control of programmed cell death in Arabidopsis

Arabidopsis plants containing the ndr1-1 mutation are incapable of mounting a hypersensitive response to bacteria carrying avrRpt2, but show an exaggerated cell death response to bacteria carrying avrB (Century et al., 1995). We show here that ndr1-1 plants are severely impaired in induction of systemic acquired resistance and PR1-driven transcription of a reporter gene in response to Pseudomonas syringae strains carrying avrRpt2 but not in response to P. syringae carrying avrB. The ndr1-1 mutation also impaired salicylic acid (SA) accumulation in response to treatments that produced reactive oxygen species (ROS) and impaired induction of systemic acquired resistance in response to in situ production of ROS. Hydrogen peroxide accumulated in wild-type Arabidopsis leaves beginning 4 to 7 h postinoculation with P. syringae carrying either avrRpt2 or avrB. In ndr1-1 plants, P. syringae carrying avrRpt2 elicited no detectable hydrogen peroxide production. Hydrogen peroxide production in response to bacteria carrying avrB was similar to that of Columbia in kinetics but of lesser intensity at early time points. These data are interpreted to indicate that NDR1 links ROS generation to SA production and that the phenotypic consequences of the ndr1-1 mutation are caused by a reduced ability to accumulate SA upon pathogen infection.

Biological activity of harpin produced by Pantoea stewartii subsp. stewartii

Pantoea stewartii subsp. stewartii causes Stewart's wilt of sweet corn. A hypersensitive response and pathogenicity (Hrp) secretion system is needed to produce water-soaking and wilting symptoms in corn and to cause a hypersensitive response (HR) in tobacco. Sequencing of the hrp cluster revealed a putative harpin gene, hrpN. The product of this gene was overexpressed in Escherichia coli and shown to elicit the HR in tobacco and systemic resistance in radishes. The protein was designated HrpN(Pnss). Like other harpins, it was heat stable and protease sensitive, although it was three- to fourfold less active biologically than Erwinia amylovora harpin. We used antibodies to purified HrpN(Pnss) to verify that hrpN mutants could not produce harpin. This protein was secreted into the culture supernatant and was produced by strains of P. stewartii subsp. indologenes. In order to determine the importance of HrpN(Pnss) in pathogenesis on sweet corn, three hrpN::Tn5 mutants were compared with the wild-type strain with 50% effective dose, disease severity, response time, and growth rate in planta as parameters. In all tests, HrpN(Pnss) was not required for infection, growth, or virulence in corn or endophytic growth in related grasses.

Visualization of secreted Hrp and Avr proteins along the Hrp pilus during type III secretion in Erwinia amylovora and Pseudomonas syringae

Pili are required for protein and/or DNA transfer from bacteria to recipient plant or bacterial cells, based on genetic evidence. However, it has never been shown directly that the effector proteins or DNA are localized along or inside the pili in situ. Failure to visualize an association of effector proteins/DNA with pili is the central issue in the debate regarding the exact function of pili in protein and DNA transfer. In this study, a newly developed in situ immunogold labelling procedure enabled visualization of the specific localization of type III effector proteins of Erwinia amylovora and Pseudomonas syringae pv. tomato along the Hrp pilus, but not along the flagellum or randomly in the intercellular space. In contrast, PelE, a pectate lyase secreted via the type II protein secretion system, was not associated with the Hrp pilus. These results provide direct evidence that type III secretion occurs only at the site of Hrp pilus assembly and that the Hrp pilus guides the transfer of effector proteins outside the bacterial cell, favouring the 'conduit/guiding filament' model.

Highly conserved sequences flank avirulence genes: isolation of novel avirulence genes from Pseudomonas syringae pv. pisi

DNA sequences flanking two avr genes (avrPpiA1 and avrPpiB1) from Pseudomonas syringae pv. pisi show a high degree of similarity. Specific primers designed from the conserved regions were used in PCR amplifications with all P. syringae pv. pisi races. As well as amplifying the expected avrPpiA- and avrPpiB-containing fragments, two additional fragments were amplified: one contained a single open reading frame (ORF1) and was found in races of genomic group II (2, 3A, 4A and 6); the second fragment contained two open reading frames (ORF2 and ORF3), separated by 658 nt, and was detected in all races. All three ORFs had G+C ratios (46.9-48 mol%) that were significantly less than that for P. syringae and each was preceded by a potential hrp box promoter. In P. syringae pv. phaseolicola, ORF1 and ORF2 each elicited a strong non-host hypersensitive reaction on bean leaves; ORF1 was designated avrPpiG, the product of which had strong similarity to AvrRxv, AvrBsT and YopP. ORF2 was identical to a gene, designated avrPpiC, previously isolated from P. syringae pv. pisi race 5. ORF3 was always found in association with avrPpiC and both were detected in a wide range of P. syringae pathovars. In contrast, avrPpiG was only detected in strains of P. syringae pv. pisi genomic group II and P. syringae pv. coronafaciens (ICMP 3113). In P. syringae pv. pisi, avrPpiG was plasmid-borne and avrPpiC and ORF3 were chromosomal. This conservation of flanking sequences has implications for the horizontal transfer of avirulence and virulence genes, suggesting that specific regions of the bacterial genome act as sites for their integration/excision.

Immunocytochemical localization of HrpA and HrpZ supports a role for the Hrp pilus in the transfer of effector proteins from Pseudomonas syringae pv. tomato across the host plant cell wall

The Hrp pilus, composed of HrpA subunits, is an essential component of the type III secretion system in Pseudomonas syringae. We used electron microscopy (EM) and immunocytochemistry to examine production of the pilus in vitro from P. syringae pv. tomato strain DC3000 grown under hrp-inducing conditions on EM grids. Pili, when labeled with antibodies to HrpA, developed rapidly in a nonpolar manner shortly after the detection of the hrpA transcript and extended up to 5 microm into surrounding media. Structures at the base of the pilus were clearly differentiated from the basal bodies of flagella. The HrpZ protein, also secreted via the type III system, was found by immunogold labeling to be associated with the pilus in vitro. Accumulation and secretion of HrpA and HrpZ were also examined quantitatively after the inoculation of wild-type DC3000 and hrpA and hrpZ mutants into leaves of Arabidopsis thaliana. The functional pilus crossed the plant cell wall to generate tracks of immunogold labeling for HrpA and HrpZ. Mutants that produced HrpA but did not assemble pili were nonpathogenic, did not secrete HrpA protein, and were compromised for the accumulation of HrpZ. A model is proposed in which the rapidly elongating Hrp pilus acts as a moving conveyor, facilitating transfer of effector proteins from bacteria to the plant cytoplasm across the formidable barrier of the plant cell wall.

Molecular evolution of virulence in natural field strains of Xanthomonas campestris pv. vesicatoria

The avrBs2 avirulence gene of the bacterial plant pathogen Xanthomonas campestris pv. vesicatoria triggers disease resistance in pepper plants containing the Bs2 resistance gene and contributes to bacterial virulence on susceptible host plants. We studied the effects of the pepper Bs2 gene on the evolution of avrBs2 by characterizing the molecular basis for virulence of 20 X. campestris pv. vesicatoria field strains that were isolated from disease spots on previously resistant Bs2 pepper plants. All field strains tested were complemented by a wild-type copy of avrBs2 in their ability to trigger disease resistance on Bs2 plants. DNA sequencing revealed four mutant alleles of avrBs2, two of which consisted of insertions or deletions of 5 nucleotides in a repetitive region of avrBs2. The other two avrBs2 alleles were characterized by point mutations with resulting single amino acid changes (R403P or A410D). We generated isogenic X. campestris pv. vesicatoria strains by chromosomal avrBs2 gene exchange to study the effects of these mutations on the dual functions of avrBs2 in enhancing bacterial virulence and inducing plant resistance by in planta bacterial growth experiments. The deletion of 5 nucleotides led to loss of avrBs2-induced resistance on Bs2 pepper plants and abolition of avrBs2-mediated enhancement of fitness on susceptible plants. Significantly, the point mutations led to minimal reduction in virulence function of avrBs2 on susceptible pepper plants, with either minimal (R403P allele) or an intermediate level of (A410D allele) triggering of resistance on Bs2 plants. Consistent with the divergent selection pressures on avrBs2 exerted by the Bs2 resistance gene, our results show that avrBs2 is evolving to decrease detection by the Bs2 gene while at the same time maintaining its virulence function.

Control of Virulence and Pathogenicity Genes of Ralstonia Solanacearum by an Elaborate Sensory Network

Ralstonia solanacearum causes a lethal bacterial wilt disease of diverse plants. It invades the xylem vessels of roots and disseminates into the stem where it multiplies and wilts by excessive exopolysaccharide production. Many of its key extracytoplasmic virulence and pathogenicity factors are transcriptionally controlled by an extensive network of distinct, interacting signal transduction pathways. The core of this sensory network is the five-gene Phc system that regulates exopolysaccharide, cell-wall-degrading exoenzymes, and other factors in response to a self-produced signal molecule that monitors the pathogen's growth status and environment. Four additional environmentally responsive two-component systems work independently and with the Phc system to fine-tune virulence gene expression. Another critical system is Prh which transduces plant cell-derived signals through a six-gene cascade to activate deployment of the Type III secretion pathway encoded by the hrp pathogenicity genes. Here I summarize knowledge about the regulated targets, signal transduction mechanisms, and crosstalk between Phc, Prh, and other systems. I also provide insight into why R. solanacearum has evolved such a sophisticated sensory apparatus, and how it functions in disease.

Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte

The extremely large number of leaves produced by terrestrial and aquatic plants provide habitats for colonization by a diversity of microorganisms. This review focuses on the bacterial component of leaf microbial communities, with emphasis on Pseudomonas syringae-a species that participates in leaf ecosystems as a pathogen, ice nucleus, and epiphyte. Among the diversity of bacteria that colonize leaves, none has received wider attention than P. syringae, as it gained notoriety for being the first recombinant organism (Ice(-) P. syringae) to be deliberately introduced into the environment. We focus on P. syringae to illustrate the attractiveness and somewhat unique opportunities provided by leaf ecosystems for addressing fundamental questions of microbial population dynamics and mechanisms of plant-bacterium interactions. Leaf ecosystems are dynamic and ephemeral. The physical environment surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation, relative humidity, wind velocity, and leaf wetness. Slightly longer-term changes occur as weather systems pass. Seasonal climatic changes impose still a longer cycle. The physical and physiological characteristics of leaves change as they expand, mature, and senesce and as host phenology changes. Many of these factors influence the development of populations of P. syringae upon populations of leaves. P. syringae was first studied for its ability to cause disease on plants. However, disease causation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generations on the surfaces of leaves as an epiphyte. A number of genes and traits have been identified that contribute to the fitness of P. syringae in the phyllosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system with which to bridge the gap between what is known about the molecular biology of genes linked to pathogenicity and the ecology and epidemiology of associated diseases as they occur in natural settings, the field.

The alternative sigma factor RpoN is required for hrp activity in Pseudomonas syringae pv. maculicola and acts at the level of hrpL transcription

beta-Glucuronidase (uidA) reporter gene fusions were constructed for the hrpZ, hrpL, and hrpS genes from the phytopathogen Pseudomonas syringae pv. maculicola strain ES4326. These reporters, as well as an avrRpt2-uidA fusion, were used to measure transcriptional activity in ES4326 and a ES4326 rpoN mutant. rpoN was required for the expression of avrRpt2, hrpZ, and hrpL in vitro in minimal media and in vivo when infiltrated into Arabidopsis thaliana leaves. In contrast, the expression of hrpS was essentially the same in wild-type and rpoN mutant strains. Constitutive expression of hrpL in an rpoN mutant restored hrpZ transcription to wild-type levels, restored the hypersensitive response when infiltrated into tobacco (Nicotiana tobacum), and partially restored the elicitation of virulence-related symptoms but not growth when infiltrated into Arabidopsis leaves. These data indicate that rpoN-mediated control of hrp gene expression acts at the level of hrpL and that in planta growth of P. syringae is not required for the elicitation of disease symptoms.

Virulence of the phytopathogen Pseudomonas syringae pv. maculicola is rpoN dependent

We cloned the rpoN (ntrA and glnF) gene encoding sigma(54) from the phytopathogen Pseudomonas syringae pv. maculicola strain ES4326. The P. syringae ES4326 rpoN gene complemented Pseudomonas aeruginosa, Escherichia coli, and Klebsiella aerogenes rpoN mutants for a variety of rpoN mutant phenotypes, including the inability to utilize nitrate as sole nitrogen source. DNA sequence analysis of the P. syringae ES4326 rpoN gene revealed that the deduced amino acid sequence was most similar (86% identity; 95% similarity) to the sigma(54) protein encoded by the Pseudomonas putida rpoN gene. A marker exchange protocol was used to construct an ES4326 rpoN insertional mutation, rpoN::Km(r). In contrast to wild-type ES4326, ES4326 rpoN::Km(r) was nonmotile and could not utilize nitrate, urea, C(4)-dicarboxylic acids, several amino acids, or concentrations of ammonia below 2 mM as nitrogen sources. rpoN was essential for production of the phytotoxin coronatine and for expression of the structural genes encoding coronamic acid. In addition, ES4326 rpoN::Km(r) did not multiply or elicit disease symptoms when infiltrated into Arabidopsis thaliana leaves, did not elicit the accumulation of several Arabidopsis defense-related mRNAs, and did not elicit a hypersensitive response (HR) when infiltrated into tobacco (Nicotiana tabacum) leaves. Furthermore, whereas P. syringae ES4326 carrying the avirulence gene avrRpt2 elicited an HR when infiltrated into Arabidopsis ecotype Columbia leaves, ES4326 rpoN::Km(r) carrying avrRpt2 elicited no response. Constitutive expression of ES4326 hrpL in ES4326 rpoN::Km(r) partially restored defense-related mRNA accumulation, showing a direct role for the hrp cluster in host defense gene induction in a compatible host-pathogen interaction. However, constitutive expression of hrpL in ES4326 rpoN::Km(r) did not restore coronatine production, showing that coronatine biosynthesis requires factors other than hrpL.

Eukaryotic fatty acylation drives plasma membrane targeting and enhances function of several type III effector proteins from Pseudomonas syringae

Bacterial pathogens of plants and animals utilize conserved type III delivery systems to traffic effector proteins into host cells. Plant innate immune systems evolved disease resistance (R) genes to recognize some type III effectors, termed avirulence (Avr) proteins. On disease-susceptible (r) plants, Avr proteins can contribute to pathogen virulence. We demonstrate that several type III effectors from Pseudomonas syringae are targeted to the host plasma membrane and that efficient membrane association enhances function. Efficient localization of three Avr proteins requires consensus myristoylation sites, and Avr proteins can be myristoylated inside the host cell. These prokaryotic type III effectors thus utilize a eukaryote-specific posttranslational modification to access the subcellular compartment where they function.

Two novel proteins, PopB, which has functional nuclear localization signals, and PopC, which has a large leucine-rich repeat domain, are secreted through the hrp-secretion apparatus of Ralstonia solanacearum

The Ralstonia solanacearum hrp gene cluster codes for components of a type III secretion pathway necessary for the secretion of PopA1, a hypersensitive response-like elicitor protein. In the present study, we show that several other Hrp-secreted proteins can be detected by growing wild-type bacteria in minimal medium in the presence of Congo red. Two of these proteins, PopB and PopC, are encoded by genes located downstream of popA and constitute an operon with popA. popABC mutants retain the wild-type ability to cause disease in hosts and to elicit the hypersensitive response on non-hosts. Expression of the popABC operon is controlled by the hrpB regulatory gene and is induced upon co-culture with Arabidopsis cell suspensions. This plant cell-specific induction depends on PrhA, a putative receptor for plant specific signal(s). The transcription of the popABC operon is not modified by the addition of Congo red to the growth medium and the intracellular pools of PopB and PopC are very similar in the absence or presence of Congo red. Preliminary data suggest that Congo red stabilizes secreted proteins in the extracellular medium. PopB is a 173-amino-acid-basic protein that contains a functional bipartite nuclear localization signal. PopC is a 1024-amino-acid protein that carries 22 tandem leucine-rich repeats (LRR). The LRR domain of this protein forms a consensus that perfectly matches the predicted eukaryotic cytoplasmic LRR consensus. We propose that PopB and PopC may be translocated into plant cells via the Hrp pathway.

The hrpB and hrpG regulatory genes of Ralstonia solanacearum are required for different stages of the tomato root infection process

hrp genes, encoding type III secretion machinery, have been shown to be key determinants for pathogenicity in the vascular phytopathogenic bacterium Ralstonia solanacearum GMI1000. Here, we show phenotypes of R. solanacearum mutant strains disrupted in the prhJ, hrpG, or hrpB regulatory genes with respect to root infection and vascular colonization in tomato plants. Tests of bacterial colonization and enumeration in tomato plants, together with microscopic observations of tomato root sections, revealed that these strains display different phenotypes in planta. The phenotype of a prhJ mutant resembles that of the wild-type strain. An hrpB mutant shows reduced infection, colonization, and multiplication ability in planta, and induces a defense reaction similar to a vascular hypersensitive response at one protoxylem pole of invaded plants. In contrast, the hrpG mutant exhibited a wild-type level of infection at secondary root axils, but the ability of the infecting bacteria to penetrate into the vascular cylinder was significantly impaired. This indicates that bacterial multiplication at root infection sites and transit through the endodermis constitute critical stages in the infection process, in which hrpB and hrpG genes are involved. Moreover, our results suggest that the hrpG gene might control, in addition to hrp genes, other functions required for vascular colonization.

Effector proteins of phytopathogenic bacteria: bifunctional signals in virulence and host recognition

Phytopathogenic bacteria deliver effectors of disease into plant hosts via a Type III secretion system. These Type III effectors have genetically determined roles in virulence. They also are among the components recognized by the putative receptors of the plant innate immune system. Recent breakthroughs include localization of some of these Type III effectors to specific host cell compartments, and the first dissection of pathogenicity islands that carry them.

Role of the Hrp type III protein secretion system in growth of Pseudomonas syringae pv. syringae B728a on host plants in the field

hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv. syringae (Pss) and other phytopathogenic bacterial species. A subset of these genes encodes a type III secretion system through which virulence factors are thought to be delivered to plant cells. In this study, we sought to better understand the role that hrp genes play in interactions of Pss with its host as they occur naturally under field conditions. Population sizes of hrp mutants with defects in genes that encode components of the Hrp secretion system (DeltahrcC::nptII and hrpJ:: OmegaSpc) and a protein secreted via the system (DeltahrpZ::nptII) were similar to B728a on germinating seeds. However, phyllosphere (i.e., leaf) population sizes of the hrcC and hrpJ secretion mutants, but not the hrpZ mutant, were significantly reduced relative to B728a. Thus, the Hrp type III secretion system, but not HrpZ, plays an important role in enabling Pss to flourish in the phyllosphere, but not the spermosphere. The hrcC and hrpJ mutants caused brown spot lesions on primary leaves at a low frequency when they were inoculated onto seeds at the time of planting. Pathogenic reactions also were found when the hrp secretion mutants were co-infiltrated into bean leaves with a non-lesion-forming gacS mutant of B728a. In both cases, the occurrence of disease was associated with elevated population sizes of the hrp secretion mutants. The role of the Hrp type III secretion system in pathogenicity appears to be largely mediated by its requirement for growth of Pss in the phyllosphere. Without growth, disease does not occur.

Early events in host-pathogen interactions

Research focused on early events in host-pathogen interactions has provided new insights into fundamental aspects of microbial pathogenicity and plant responses. Considerable progress has been made in understanding regulation of the delivery of pathogenicity determinants from bacteria into plant cells, signal cascades involved in fungal pathogenicity, the co-ordinating role of the plant cytoskeleton in plant defence and calcium flux as a primary signalling function during the hypersensitive reaction.

Characterization of the Pseudomonas syringae pv. tomato AvrRpt2 protein: demonstration of secretion and processing during bacterial pathogenesis

Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000) expressing avrRpt2 is specifically recognized by plant cells expressing RPS2 activity, resulting in localized cell death and plant resistance. Furthermore, transient expression of this bacterial avrRpt2 gene in plant cells results in RPS2-dependent cell death. This indicates that the AvrRpt2 protein is recognized inside RPS2 plant cells and is sufficient for the activation of disease resistance-mediated cell death in planta. We explored the possibility that Pst DC3000 delivers AvrRpt2 protein to plant cells via the hrp (type III) secretion pathway. We now provide direct evidence that mature AvrRpt2 protein is secreted from Pst DC3000 and that secretion is hrp dependent. We also show that AvrRpt2 is N-terminally processed when Arabidopsis thaliana plants are infected with Pst DC3000 expressing avrRpt2. Similar N-terminal processing of AvrRpt2 occurred when avrRpt2 was stably expressed in A. thaliana. No cleavage of AvrRpt2 was detected in bacteria expressing avrRpt2 in culture or in the plant extracellular fluids. The N-terminus of AvrRpt2 was not required for RPS2 recognition in planta. However, this region of AvrRpt2 was essential for Pst DC3000-mediated elicitation of RPS2-dependent cell death in A. thaliana leaves.

Regulation of expression of avirulence gene avrRxv and identification of a family of host interaction factors by sequence analysis of avrBsT

Resistance in tomato line Hawaii 7998 as well as in several nonhost plants to Xanthomonas campestris pv. vesicatoria tomato strain (XcvT) is mediated in part by the avirulence gene avrRxv. Analysis of growth of wild-type and avrRxv deletion strains indicates that avrRxv plays a crucial role in the ability of XcvT 92-14 to induce resistance on Hawaii 7998. We used avrRxv reporter gene fusions and Northern (RNA) blot analysis to test several growth environments for inductive potential. We found that avrRxv is constitutively expressed at high levels and that growth in planta, in tobacco conditioned medium, and in hrp-inductive medium XVM2 did not affect the high levels of expression. In addition, hrp structural and regulatory mutant backgrounds had no effect. We mutated the bipartite plant inducible promoter (PIP)-box sequence and found that avrRxv activity appears to be independent of an intact PIP-box element. We present the sequence of the avrRxv homologue called avrBsT and align the six AvrRxv host interaction factor family members including mammalian pathogen virulence factors YopJ and YopP from Yersinia spp. and AvrA from Salmonella typhimurium, and open reading frame Y4LO with unknown function from the symbiont Rhizobium sp.

prhJ and hrpG, two new components of the plant signal-dependent regulatory cascade controlled by PrhA in Ralstonia solanacearum

hrp gene expression in the phytopathogenic bacterium Ralstonia solanacearum GMI1000 is induced through the HrpB regulator in minimal medium and upon co-culture with plant cell suspensions. The putative outer membrane protein PrhA is specifically involved in hrp gene activation in the presence of plant cells and has been proposed to be a receptor of a plant-dependent signal transduction pathway. Here, we report on the identification of two regulatory genes, hrpG and prhJ, located at the right-hand end of the hrp gene cluster, that are required for full pathogenicity. HrpG belongs to the OmpR subclass of two-component response regulators and is homologous to HrpG, the activator of hrp genes in Xanthomonas campestris pv. vesicatoria. PrhJ is a novel hrp regulatory protein, sharing homology with the LuxR/UhpA family of transcriptional activators. As for HrpG of X. c. pv. vesicatoria, HrpG is required for hrp gene expression in minimal medium, but, in addition, we show that it also controls hrpB gene activation upon co-culture with Arabidopsis thaliana and tomato cell suspensions. In contrast, PrhJ is specifically involved in hrp gene expression in the presence of plant cells. hrpG and prhJ gene transcription is plant cell inducible through the PrhA-dependent pathway. From these results, we propose a regulatory cascade in which plant cell signal(s) sensed by PrhA are transduced to the prhJ gene, whose predicted product controls hrpG gene expression. HrpG then activates the hrpB regulatory gene, and, subsequently, the remaining hrp transcriptional units in all known inducing conditions.

Genetically engineered broad-spectrum disease resistance in tomato

Resistance in tomato to the bacterial pathogen Pseudomonas syringae pathovar tomato requires Pto and Prf. Mutations that eliminate Prf show a loss of both Pto resistance and sensitivity to the organophosphate insecticide fenthion, suggesting that Prf controls both phenotypes. Herein, we report that the overexpression of Prf leads to enhanced resistance to a number of normally virulent bacterial and viral pathogens and leads to increased sensitivity to fenthion. These plants express levels of salicylic acid comparable to plants induced for systemic acquired resistance (SAR) and constitutively express pathogenesis related genes. These results suggest that the overexpression of Prf activates the Pto and Fen pathways in a pathogen-independent manner and leads to the activation of SAR. Transgene-induced SAR has implications for the generation of broad spectrum disease resistance in agricultural crop plants.

Recognition specificity for the bacterial avirulence protein AvrPto is determined by Thr-204 in the activation loop of the tomato Pto kinase

The Pto kinase confers resistance in tomato to P. syringae pv. tomato strains expressing the AvrPto protein. Physical interaction of the Pto kinase and AvrPto protein in the plant cell initiates host defense responses. The recognition event between these two proteins is very specific; AvrPto does not interact with other closely related kinases, including the Fen kinase, which shares 80% amino acid identity with Pto. By using Pto-Fen chimeric proteins and site-directed mutagenesis, we found that Thr-204 is required for Pto interaction with AvrPto in a yeast two-hybrid system and for recognition specificity in a tobacco leaf transient assay. Substitution of Thr-204 into the Fen kinase allowed that kinase to interact with AvrPto and to confer an AvrPto-specific defense response in tobacco leaves. Thus, simple mutations appear capable of giving rise to new resistance gene specificities.

Determinants of pathogenicity and avirulence in plant pathogenic bacteria

Many plant pathogenic bacteria possess a conserved protein secretion system that is thought to transfer Avr (avirulence) proteins, with potential activities in both parasitism and defense elicitation, into plant cells. avr genes may be acquired horizontally by these bacteria, and avr gene compositions are highly variable. In the past year, heterologous expression experiments have revealed that the products of avr genes can be interchanged among different genera of bacteria with retention of secretion, pathogenicity, and avirulence activities, suggesting mechanisms for rapid coevolution of these parasites with changing plant hosts.

Current concepts of active defense in plants

A growing body of evidence indicates that elicitation of primary active defense responses results from a recognition event frequently involving protein-protein interactions. Most pathogen avirulence determinants eliciting resistance gene-dependent responses have been shown to be proteins with no apparent enzymic activity. Disruption of the tertiary and quaternary structure of these proteins abolishes their elicitor activity. Critical to their elicitor activity is their display by the pathogen. Resistance genes are proposed to function as receptors for the eliciting proteins. The most consistent feature of resistance gene products is the presence of potential protein binding domains in the form of leucine-rich repeat regions, and there is direct evidence for the physical interaction of elicitor proteins and receptor proteins in several cases. Thus in many but not all cases the primary recognition event eliciting an active defense response during incompatible interactions appears to be a protein-protein interaction occurring between a specific pathogen protein and a strategically placed receptor protein in the host cell. The interaction of elicitor protein with the receptor protein activates a signal transduction pathway leading to programmed cell death and an oxidative burst.