The Pseudomonas syringae type III-secreted protein HopPtoD2 possesses protein tyrosine phosphatase activity and suppresses programmed cell death in plants

The bacterial plant pathogen Pseudomonas syringae possesses a type III protein secretion system that delivers many virulence proteins into plant cells. A subset of these proteins (called Avr proteins) is recognized by the plant's innate immune system and triggers defences. One defence-associated response is the hypersensitive response (HR), a programmed cell death (PCD) of plant tissue. We have previously identified HopPtoD2 as a type III secreted protein from P. s. pv. tomato DC3000. Sequence analysis revealed that an N-terminal domain shared homology with AvrPphD and a C-terminal domain was similar to protein tyrosine phosphatases (PTPs). We demonstrated that purified HopPtoD2 possessed PTP activity and this activity required a conserved catalytic Cys residue (Cys(378)). Interestingly, HopPtoD2 was capable of suppressing the HR elicited by an avirulent P. syringae strain on Nicotiana benthamiana. HopPtoD2 derivatives that lacked Cys(378) no longer suppressed the HR indicating that HR suppression required PTP activity. A constitutively active MAPK kinase, called NtMEK2DD, is capable of eliciting an HR-like cell death when transiently expressed in tobacco. When NtMEK2DD and HopPtoD2 were co-delivered into plant cells, the HR was suppressed indicating that HopPtoD2 acts downstream of NtMEK2DD. DC3000 hopPtoD2 mutants were slightly reduced in their ability to multiply in planta and displayed an enhanced ability to elicit an HR. The identification of HopPtoD2 as a PTP and a PCD suppressor suggests that the inactivation of MAPK pathways is a virulence strategy utilized by bacterial plant pathogens.     

The bifunctional effector AvrXccC of Xanthomonas campestris pv. campestris requires plasma membrane-anchoring for host recognition

Bacterial pathogens use type III secretion systems (TTSS) to deliver effector proteins into eukaryotic cells for pathogenesis. In bacterial–plant interactions, one effector may function as an avirulence factor to betray the pathogen to the plant surveillance system and induce the hypersensitive response (HR) in the resistant host carrying a corresponding resistance ( R ) gene. However, the same effector can also sustain the growth of the pathogen by acting as a virulence factor to modulate plant physiology in the susceptible host lacking the corresponding R gene. Here, we identified and characterized a bifunctional TTSS effector AvrXccC belonging to the AvrB effector family in Xanthomonas campestris pv. campestris 8004. This effector is required for full bacterial virulence in the susceptible host cabbage ( Brassica oleracea ) and avirulence in the resistant host mustard ( Brassica napiformis L.H. Baily). Expressing avrXccC in mustard-virulent strain Xcc HRI 3849A converts its virulence to avirulence. The effector AvrXccC is anchored to the plant plasma membrane, and the N-terminal myristoylation site (amino acids 2–7: GLcaSK) is essential for its localization. In addition, the avirulence function of AvrXccC for host recognition depends on its plasma membrane localization. Promoter activity assays showed that the expression of avrXccC is hrpG/hrpX -dependent. Moreover, the secretion of AvrXccC displayed hrp -dependency and the core sequence for AvrXccC translocation was defined to the N-terminal 40 amino acids.     

Agrobacterium rhizogenes GALLS protein contains domains for ATP binding, nuclear localization, and type IV secretion

Agrobacterium tumefaciens and Agrobacterium rhizogenes are closely related plant pathogens that cause different diseases, crown gall and hairy root. Both diseases result from transfer, integration, and expression of plasmid-encoded bacterial genes located on the transferred DNA (T-DNA) in the plant genome. Bacterial virulence (Vir) proteins necessary for infection are also translocated into plant cells. Transfer of single-stranded DNA (ssDNA) and Vir proteins requires a type IV secretion system, a protein complex spanning the bacterial envelope. A. tumefaciens translocates the ssDNA-binding protein VirE2 into plant cells, where it binds single-stranded T-DNA and helps target it to the nucleus. Although some strains of A. rhizogenes lack VirE2, they are pathogenic and transfer T-DNA efficiently. Instead, these bacteria express the GALLS protein, which is essential for their virulence. The GALLS protein can complement an A. tumefaciens virE2 mutant for tumor formation, indicating that GALLS can substitute for VirE2. Unlike VirE2, GALLS contains ATP-binding and helicase motifs similar to those in TraA, a strand transferase involved in conjugation. Both GALLS and VirE2 contain nuclear localization sequences and a C-terminal type IV secretion signal. Here we show that mutations in any of these domains abolished the ability of GALLS to substitute for VirE2.     

SseF, a type III effector protein from the mammalian pathogen Salmonella enterica, requires resistance-gene-mediated signalling to activate cell death in the model plant Nicotiana benthamiana

Type III effector proteins (T3Es) of many Gram-negative pathogenic bacteria manipulate highly conserved cellular processes, indicating conservation in virulence mechanisms during the infection of hosts of divergent evolutionary origin. In order to identify conserved effector functions, we used a cross-kingdom approach in which we expressed selected T3Es from the mammalian pathogen Salmonella enterica in leaves of Nicotiana benthamiana and searched for possible virulence or avirulence phenotypes. We show that the T3E SseF of S. enterica triggers hypersensitive response (HR)-like symptoms, a hallmark of effector-triggered immunity in plants, either when transiently expressed in leaves of N. benthamiana by Agrobacterium tumefaciens infiltration or when delivered by Xanthomonas campestris pv vesicatoria (Xcv) through the type III secretion system. The ability of SseF to elicit HR-like symptoms was lost upon silencing of suppressor of G2 allele of skp1 (SGT1), indicating that the S. enterica T3E is probably recognized by an R protein in N. benthamiana. Xcv translocating an AvrRpt2-SseF fusion protein was restricted in multiplication within leaves of N. benthamiana. Bacterial growth was not impaired but symptom development was rather accelerated in a compatible interaction with susceptible pepper (Capsicum annuum) plants. We conclude that the S. enterica T3E SseF is probably recognized by the plant immune system in N. benthamiana, resulting in effector-triggered immunity.     

Unwounded plants elicit Agrobacterium vir gene induction and T-DNA transfer: transformed plant cells produce opines yet are tumour free

Agrobacterium tumefaciens is well known to cause crown gall tumours at plant wound sites and to benefit from this plant association by obtaining nutrients called opines that are produced by these tumours. Tumourigenesis requires expression of the vir regulon in response to chemical signals that are thought to be released from wound sites. Here, we examine chemical interactions between A. tumefaciens and unwounded plants. To determine whether unwounded plants can release significant amounts of vir gene inducers, we constructed an A. tumefaciens strain carrying a PvirB-gfp fusion. This fusion was strongly induced by co-culture with tobacco seedlings that have been germinated without any intentional wounding. The release of phenolic vir gene inducers was confirmed by GC/MS analysis. We also constructed a strain containing the gfp reporter located on an artificial T-DNA and expressed from a plant promoter. A. tumefaciens efficiently transferred this T-DNA into cells of unwounded plants in the absence of exogenous vir gene inducers. Many cells of seedlings colonized by the bacteria also produced octopine, which was detected using a Pocc-gfp reporter strain. This indicates transfer of the native T-DNA. However, these transformed plant cells did not form tumours. These results suggest that successful colonization of plants by A. tumefaciens, including T-DNA transfer and opine production, does not require wounding and does not necessarily cause cell proliferation. Transformation of plant cells without inciting tumours may represent a colonization strategy for this pathogen that has largely been overlooked.     

The Pseudomonas syringae HrpJ protein controls the secretion of type III translocator proteins and has a virulence role inside plant cells

The bacterial plant pathogen Pseudomonas syringae injects effector proteins into plant cells via a type III secretion system (T3SS), which is required for pathogenesis. The protein HrpJ is secreted by P. syringae and is required for a fully functional T3SS. A hrpJ mutant is non-pathogenic and cannot inject effectors into plant cells or secrete the harpin HrpZ1. Here we show that the hrpJ mutant also cannot secrete the harpins HrpW1 and HopAK1 or the translocator HrpK1, suggesting that these proteins are required in the translocation (injection) of effectors into plant cells. Complementation of the hrpJ mutant with secretion incompetent HrpJ derivatives restores the secretion of HrpZ1 and HrpW1 and the ability to elicit a hypersensitive response, a measure of translocation. However, growth in planta and disease symptom production is only partially restored, suggesting that secreted HrpJ may have a direct role in virulence. Transgenic Arabidopsis plants expressing HrpJ-HA complemented the virulence phenotype of the hrpJ mutant expressing a secretion incompetent HrpJ derivative and were reduced in their immune responses. Collectively, these data indicate that HrpJ has a dual role in P. syringae: inside bacterial cells HrpJ controls the secretion of translocator proteins and inside plant cells it suppresses plant immunity.     

Feedback regulation of an Agrobacterium catalase gene katA involved in Agrobacterium–plant interaction

Catalases are known to detoxify H2O2, a major component of oxidative stress imposed on a cell. An Agrobacterium tumefaciens catalase encoded by a chromosomal gene katA has been implicated as an important virulence factor as it is involved in detoxification of H2O2 released during Agrobacterium-plant interaction. In this paper, we report a feedback regulation pathway that controls the expression of katA in A. tumefaciens cells. We observed that katA could be induced by plant tissue sections and by acidic pH on a minimal medium, which resembles the plant environment that the bacteria encounter during the course of infection. This represents a new regulatory factor for catalase induction in bacteria. More importantly, a feedback regulation was observed when the katA-gfp expression was studied in different genetic backgrounds. We found that introduction of a wild-type katA gene encoding a functional catalase into A. tumefaciens cells could repress the katA-gfp expression over 60-fold. The katA gene could be induced by H2O2 and the encoded catalase could detoxify H2O2. In addition, the katA-gfp expression of one bacterial cell could be repressed by other surrounding catalase-proficient bacterial cells. Furthermore, mutation at katA caused a 10-fold increase of the intracellular H2O2 concentration in the bacteria grown on an acidic pH medium. These results suggest that the endogenous H2O2 generated during A. tumefaciens cell growth could serve as the intracellular and intercellular inducer for the katA gene expression and that the acidic pH could pose an oxidative stress on the bacteria. Surprisingly, one mutated KatA protein, exhibiting no significant catalase activity as a result of the alteration of two important residues at the putative active site, could partially repress the katA-gfp expression. The feedback regulation of the katA gene by both catalase activity and KatA protein could presumably maintain an appropriated level of catalase activity and H2O2 inside A. tumefaciens cells.     

Pseudomonas syringae Catalases Are Collectively Required for Plant Pathogenesis

The bacterial pathogen Pseudomonas syringae pv. tomato DC3000 must detoxify plant-produced hydrogen peroxide (H(2)O(2)) in order to survive in its host plant. Candidate enzymes for this detoxification include the monofunctional catalases KatB and KatE and the bifunctional catalase-peroxidase KatG of DC3000. This study shows that KatG is the major housekeeping catalase of DC3000 and provides protection against menadione-generated endogenous H(2)O(2). In contrast, KatB rapidly and substantially accumulates in response to exogenous H(2)O(2). Furthermore, KatB and KatG have nonredundant roles in detoxifying exogenous H(2)O(2) and are required for full virulence of DC3000 in Arabidopsis thaliana. Therefore, the nonredundant ability of KatB and KatG to detoxify plant-produced H(2)O(2) is essential for the bacteria to survive in plants. Indeed, a DC3000 catalase triple mutant is severely compromised in its ability to grow in planta, and its growth can be partially rescued by the expression of katB, katE, or katG. Interestingly, our data demonstrate that although KatB and KatG are the major catalases involved in the virulence of DC3000, KatE can also provide some protection in planta. Thus, our results indicate that these catalases are virulence factors for DC3000 and are collectively required for pathogenesis.     

Peptidoglycan hydrolysis mediated by the amidase AmiC and its LytM activator NlpD is critical for cell separation and virulence in the phytopathogen Xanthomonas campestris

The essential stages of bacterial cell separation are described as the synthesis and hydrolysis of septal peptidoglycan (PG). The amidase, AmiC, which cleaves the peptide side‐chains linked to the glycan strands, contributes critically to this process and has been studied extensively in model strains of Escherichia coli. However, insights into the contribution of this protein to other processes in the bacterial cell have been limited. Xanthomonas campestris pv. campestris (Xcc) is a phytopathogen that causes black rot disease in many economically important plants. We investigated how AmiC and LytM family regulators, NlpD and EnvC, contribute to virulence and cell separation in this organism. Biochemical analyses of purified AmiC demonstrated that it could hydrolyse PG and its activity could be potentiated by the presence of the regulator NlpD. We also established that deletion of the genes encoding amiC1 or nlpD led to a reduction in virulence as well as effects on colony‐forming units and cell morphology. Moreover, further genetic and biochemical evidence showed that AmiC1 and NlpD affect the secretion of type III effector XC3176 and hypersensitive response (HR) induction in planta. These findings indicate that, in addition to their well‐studied role(s) in cell separation, AmiC and NlpD make an important contribution to the type III secretion (T3S) and virulence regulation in this important plant pathogen.     

Computational and biochemical analysis of the Xanthomonas effector AvrBs2 and its role in the modulation of Xanthomonas type three effector delivery

Effectors of the bacterial type III secretion system provide invaluable molecular probes to elucidate the molecular mechanisms of plant immunity and pathogen virulence. In this report, we focus on the AvrBs2 effector protein from the bacterial pathogen Xanthomonas euvesicatoria (Xe), the causal agent of bacterial spot disease of tomato and pepper. Employing homology-based structural analysis, we generate a three-dimensional structural model for the AvrBs2 protein and identify catalytic sites in its putative glycerolphosphodiesterase domain (GDE). We demonstrate that the identified catalytic region of AvrBs2 was able to functionally replace the GDE catalytic site of the bacterial glycerophosphodiesterase BhGlpQ cloned from Borrelia hermsii and is required for AvrBs2 virulence. Mutations in the GDE catalytic domain did not disrupt the recognition of AvrBs2 by the cognate plant resistance gene Bs2. In addition, AvrBs2 activation of Bs2 suppressed subsequent delivery of other Xanthomonas type III effectors into the host plant cells. Investigation of the mechanism underlying this modulation of the type III secretion system may offer new strategies to generate broad-spectrum resistance to bacterial pathogens.     

The downy mildew effector proteins ATR1 and ATR13 promote disease susceptibility in Arabidopsis thaliana

The downy mildew (Hyaloperonospora parasitica) effector proteins ATR1 and ATR13 trigger RPP1-Nd/WsB- and RPP13-Nd-dependent resistance, respectively, in Arabidopsis thaliana. To better understand the functions of these effectors during compatible and incompatible interactions of H. parasitica isolates on Arabidopsis accessions, we developed a novel delivery system using Pseudomonas syringae type III secretion via fusions of ATRs to the N terminus of the P. syringae effector protein, AvrRPS4. ATR1 and ATR13 both triggered the hypersensitive response (HR) and resistance to bacterial pathogens in Arabidopsis carrying RPP1-Nd/WsB or RPP13-Nd, respectively, when delivered from P. syringae pv tomato (Pst) DC3000. In addition, multiple alleles of ATR1 and ATR13 confer enhanced virulence to Pst DC3000 on susceptible Arabidopsis accessions. We conclude that ATR1 and ATR13 positively contribute to pathogen virulence inside host cells. Two ATR13 alleles suppressed bacterial PAMP (for Pathogen-Associated Molecular Patterns)-triggered callose deposition in susceptible Arabidopsis when delivered by DC3000 DeltaCEL mutants. Furthermore, expression of another allele of ATR13 in plant cells suppressed PAMP-triggered reactive oxygen species production in addition to callose deposition. Intriguingly, although Wassilewskija (Ws-0) is highly susceptible to H. parasitica isolate Emco5, ATR13Emco5 when delivered by Pst DC3000 triggered localized immunity, including HR, on Ws-0. We suggest that an additional H. parasitica Emco5 effector might suppress ATR13-triggered immunity.     

The Tzs protein from Agrobacterium tumefaciens C58 produces zeatin riboside 5'-phosphate from 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate and AMP

The plant pathogen Agrobacterium tumefaciens produces cytokinins upon induction of the virulence genes by secondary metabolites from wounded plants, and these hormones are believed to stimulate the infection process. To study the biosynthetic pathway, the tzs gene, encoding the Tzs (trans-zeatin-synthesizing) protein from A. tumefaciens, was cloned and the protein was overproduced and purified. Analysis of its reactivity with radioactively labeled substrate demonstrated conversion of 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate, a product of the deoxyxylulose phosphate pathway, with AMP to zeatin riboside 5'-phosphate. This suggests that A. tumefaciens uses an alternative pathway of cytokinin biosynthesis, which had previously been hypothesized to operate in plants.     

Virulence systems of Pseudomonas syringae pv. tomato promote bacterial speck disease in tomato by targeting the jasmonate signaling pathway

Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000) causes bacterial speck disease on tomato. The pathogenicity of Pst DC3000 depends on both the type III secretion system that delivers virulence effector proteins into host cells and the phytotoxin coronatine (COR), which is thought to mimic the action of the plant hormone jasmonic acid (JA). We found that a JA-insensitive mutant (jai1) of tomato was unresponsive to COR and highly resistant to Pst DC3000, whereas host genotypes that are defective in JA biosynthesis were as susceptible to Pst DC3000 as wild-type (WT) plants. Treatment of WT plants with exogenous methyl-JA (MeJA) complemented the virulence defect of a bacterial mutant deficient in COR production, but not a mutant defective in the type III secretion system. Analysis of host gene expression using cDNA microarrays revealed that COR works through Jai1 to induce the massive expression of JA and wound response genes that have been implicated in defense against herbivores. Concomitant with the induction of JA and wound response genes, the type III secretion system and COR repressed the expression of pathogenesis-related (PR) genes in Pst DC3000-infected WT plants. Resistance of jai1 plants to Pst DC3000 was correlated with a high level of PR gene expression and reduced expression of JA/wound response genes. These results indicate that COR promotes bacterial virulence by activating the host's JA signaling pathway, and further suggest that the type III secretion system might also modify host defense by targeting the JA signaling pathway in susceptible tomato plants.     

Stimulation of Agrobacterium tumefaciens virulence with indole-3-acetic acid

The phytopathogen Agrobacterium tumefaciens incites the production of crown-gall on a wide range of dicotyledonous plants. Gall formation is dependent upon indole-3-acetic acid (IAA) and cytokinin production by the transformed plant cells. Upon incubation of Agrobacterium tumefaciens C58 with the plant hormone indole-3-acetic acid (IAA), bacterial virulence on cucumber plants was stimulated up to tenfold. Stimulation was maximized after exposure of bacteria to 50 or 100 μg ml^-1 IAA for 3 h. This was shown to be at the early log phase of bacterial growth.
The authors suggest that the excretion of IAA by the transformed plant cells stimulates bacterial virulence mechanism(s) encoded by the Ti plasmid, the chromosome, or both.     

A local accumulation of the Ralstonia solanacearum PopA protein in transgenic tobacco renders a compatible plant–pathogen interaction incompatible

Plants activate disease resistance responses when they recognize pathogen-derived molecules (elicitors). Frequently, recognition results in a hypersensitive response (HR), which is characterized by local host cell death at the infection site. Here we describe a genetic engineering approach to generate an HR in plants, whether or not an invading micro-organism produces a recognized elicitor. To that aim we created transgenic tobacco plants in which the pathogen-inducible promoter of the hsr203J gene from tobacco controls the expression of the popA elicitor gene from Ralstonia solanacearum. Because PopA itself also induces the hsr203J promoter, transgenic plants rapidly accumulate the bacterial elicitor in the pathogen infection sites. The elicitor becomes converted in plant tissues into its fully active derivatives PopA1-PopA3, showing that the previously observed processing events are not dependent on the bacterial type III secretion system. The outcome of induced PopA accumulation is a localized HR and a high degree of resistance of the transgenic plants to an oomycete pathogen. The system is functional in hybrids between different tobacco varieties, and we show that the engineered resistance, but not the associated cell death, is dependent on the salicylic acid signalling cascade. Although the approach is powerful in generating oomycete resistance, the induced HR might affect plant health. Its application thus requires a careful selection of individual transgenic lines and trials with various pathogens.     

Pseudomonas syringae HrpJ is a type III secreted protein that is required for plant pathogenesis, injection of effectors, and secretion of the HrpZ1 Harpin

The bacterial plant pathogen Pseudomonas syringae requires a type III protein secretion system (TTSS) to cause disease. The P. syringae TTSS is encoded by the hrp-hrc gene cluster. One of the genes within this cluster, hrpJ, encodes a protein with weak similarity to YopN, a type III secreted protein from the animal pathogenic Yersinia species. Here, we show that HrpJ is secreted in culture and translocated into plant cells by the P. syringae pv. tomato DC3000 TTSS. A DC3000 hrpJ mutant, UNL140, was greatly reduced in its ability to cause disease symptoms and multiply in Arabidopsis thaliana. UNL140 exhibited a reduced ability to elicit a hypersensitive response (HR) in nonhost tobacco plants. UNL140 was unable to elicit an AvrRpt2- or AvrB1-dependent HR in A. thaliana but maintained its ability to secrete AvrB1 in culture via the TTSS. Additionally, UNL140 was defective in its ability to translocate the effectors AvrPto1, HopB1, and AvrPtoB. Type III secretion assays showed that UNL140 secreted HrpA1 and AvrPto1 but was unable to secrete HrpZ1, a protein that is normally secreted in culture in relatively large amounts, into culture supernatants. Taken together, our data indicate that HrpJ is a type III secreted protein that is important for pathogenicity and the translocation of effectors into plant cells. Based on the failure of UNL140 to secrete HrpZ1, HrpJ may play a role in controlling type III secretion, and in its absence, specific accessory proteins, like HrpZ1, may not be extracellularly localized, resulting in disabled translocation of effectors into plant cells.     

Ca2+-dependent lipid binding and membrane integration of PopA, a harpin-like elicitor of the hypersensitive response in tobacco

PopA is released by type III secretion from the bacterial plant pathogen Ralstonia solanacearum and triggers the hypersensitive response (HR) in tobacco. The function of PopA remains obscure, mainly because mutants lacking this protein are not altered in their ability to interact with plants. In an attempt to identify the site of PopA activity in plant cells, we generated transgenic tobacco plants expressing the popA gene under the control of an inducible promoter. Immunocytologic analysis revealed that the HR phenotype of these plants correlated with the presence of PopA at the plant plasma membrane. Membrane localization was observed irrespective of whether the protein was designed to accumulate in the cytoplasm or to be secreted by the plant cell, suggesting a general lipid-binding ability. We found that the protein had a high affinity for sterols and sphingolipids in vitro and that it required Ca2+ for both lipid binding and oligomerization. In addition, the protein was integrated into liposomes and membranes from Xenopus laevis oocytes where it formed ion-conducting pores. These characteristics suggest that PopA is part of a system that aims to attach the host cell plasma membrane and to allow molecules cross this barrier.     

Identification of a twin-arginine translocation system in Pseudomonas syringae pv. tomato DC3000 and its contribution to pathogenicity and fitness

The bacterial plant pathogen Pseudomonas syringae pv. tomato DC3000 (DC3000) causes disease in Arabidopsis thaliana and tomato plants, and it elicits the hypersensitive response in nonhost plants such as Nicotiana tabacum and Nicotiana benthamiana. While these events chiefly depend upon the type III protein secretion system and the effector proteins that this system translocates into plant cells, additional factors have been shown to contribute to DC3000 virulence and still many others are likely to exist. Therefore, we explored the contribution of the twin-arginine translocation (Tat) system to the physiology of DC3000. We found that a tatC mutant strain of DC3000 displayed a number of phenotypes, including loss of motility on soft agar plates, deficiency in siderophore synthesis and iron acquisition, sensitivity to copper, loss of extracellular phospholipase activity, and attenuated virulence in host plant leaves. In the latter case, we provide evidence that decreased virulence of tatC mutants likely arises from a synergistic combination of (i) compromised fitness of bacteria in planta; (ii) decreased efficiency of type III translocation; and (iii) cytoplasmically retained virulence factors. Finally, we demonstrate a novel broad-host-range genetic reporter based on the green fluorescent protein for the identification of Tat-targeted secreted virulence factors that should be generally applicable to any gram-negative bacterium. Collectively, our evidence supports the notion that virulence of DC3000 is a multifactorial process and that the Tat system is an important virulence determinant of this phytopathogenic bacterium.