Genetic organization of the hrp gene cluster and dspAE/BF operon in Erwinia herbicola pv. gypsophilae

The conserved AvrE family of bacterial effectors: functions and targets during pathogenesis

The AvrE family of type III secreted effectors are highly conserved among many agriculturally important phytopathogenic bacteria. Despite their critical roles in the pathogenesis of phytopathogenic bacteria, the molecular functions and virulence mechanisms of these effectors have been largely unknown. However, recent studies have identified host-interacting proteins and demonstrated that AvrE family effectors can form water-permeable channels in the plant plasma membrane (PM) to create a hydrated and nutrient-rich extracellular space (apoplast) required for disease establishment. Here, we summarize these recent discoveries and highlight open questions related to AvrE-targeted host proteins.

Comparative sequence analysis of pPATH pathogenicity plasmids in Pantoea agglomerans gall-forming bacteria

Acquisition of the pathogenicity plasmid pPATH that encodes a type III secretion system (T3SS) and effectors (T3Es) has likely led to the transition of a non-pathogenic bacterium into the tumorigenic pathogen Pantoea agglomerans. P. agglomerans pv. gypsophilae (Pag) forms galls on gypsophila (Gypsophila paniculata) and triggers immunity on sugar beet (Beta vulgaris), while P. agglomerans pv. betae (Pab) causes galls on both gypsophila and sugar beet. Draft sequences of the Pag and Pab genomes were previously generated using the MiSeq Illumina technology and used to determine partial T3E inventories of Pab and Pag. Here, we fully assembled the Pab and Pag genomes following sequencing with PacBio technology and carried out a comparative sequence analysis of the Pab and Pag pathogenicity plasmids pPATHpag and pPATHpab. Assembly of Pab and Pag genomes revealed a ~4 Mbp chromosome with a 55% GC content, and three and four plasmids in Pab and Pag, respectively. pPATHpag and pPATHpab share 97% identity within a 74% coverage, and a similar GC content (51%); they are ~156 kb and ~131 kb in size and consist of 198 and 155 coding sequences (CDSs), respectively. In both plasmids, we confirmed the presence of highly similar gene clusters encoding a T3SS, as well as auxin and cytokinins biosynthetic enzymes. Three putative novel T3Es were identified in Pab and one in Pag. Among T3SS-associated proteins encoded by Pag and Pab, we identified two novel chaperons of the ShcV and CesT families that are present in both pathovars with high similarity. We also identified insertion sequences (ISs) and transposons (Tns) that may have contributed to the evolution of the two pathovars. These include seven shared IS elements, and three ISs and two transposons unique to Pab. Finally, comparative sequence analysis revealed plasmid regions and CDSs that are present only in pPATHpab or in pPATHpag. The high similarity and common features of the pPATH plasmids support the hypothesis that the two strains recently evolved into host-specific pathogens.

Beneficial Effect and Potential Risk of Pantoea on Rice Production

Bacteria from the genus Pantoea have been reported to be widely distributed in rice paddy environments with contradictory roles. Some strains promoted rice growth and protected rice from pathogen infection or abiotic stress, but other strain exhibited virulence to rice, even causing severe rice disease. In order to effectively utilize Pantoea in rice production, this paper analyzed the mechanisms underlying beneficial and harmful effects of Pantoea on rice growth. The beneficial effect of Pantoea on rice plants includes growth promotion, abiotic alleviation and disease inhibition. The growth promotion may be mainly attributed to nitrogen-fixation, phosphate solubilization, plant physiological change, the biosynthesis of siderophores, exopolysaccharides, 1-aminocyclopropane-1-carboxylic acid deaminase and phytohormones, including cytokinin, indole-3-acetic acid (IAA), auxins, abscisic acid and gibberellic acid, while the disease inhibition may be mainly due to the induced resistance, nutrient and spatial competition, as well as the production of a variety of antibiotics. The pathogenic mechanism of Pantoea can be mainly attributed to bacterial motility, production of phytohormones such as IAA, quorum sensing-related signal molecules and a series of cell wall-degrading enzymes, while the pathogenicity-related genes of Pantoea include genes encoding plasmids, such as the pPATH plasmid, the hypersensitive response and pathogenicity system, as well as various types of secretion systems, such as T3SS and T6SS. In addition, the existing scientific problems in this field were discussed and future research prospects were proposed.

Dynamic nutrient acquisition from a hydrated apoplast supports biotrophic proliferation of a bacterial pathogen of maize

Plant pathogens perturb their hosts to create environments suitable for their proliferation, including the suppression of immunity and promotion of water and nutrient availability. Although necrotrophs obtain water and nutrients by disrupting host-cell integrity, it is unknown whether hemibiotrophs, such as the bacterial pathogen Pantoea stewartii subsp. stewartii (Pnss), actively liberate water and nutrients during the early, biotrophic phase of infection. Here, we show that water and metabolite accumulation in the apoplast of Pnss-infected maize leaves precedes the disruption of host-cell integrity. Nutrient acquisition during this biotrophic phase is a dynamic process; the partitioning of metabolites into the apoplast rate limiting for their assimilation by proliferating Pnss cells. The formation of a hydrated and nutritive apoplast is driven by an AvrE-family type III effector, WtsE. Given the broad distribution of AvrE-family effectors, this work highlights the importance of actively acquiring water and nutrients for the proliferation of phytopathogenic bacteria during biotrophy.

The Contributions to Virulence of the Effectors Eop1 and DspE Differ Between Two Clades of Erwinia tracheiphila Strains

Strains of Erwinia tracheiphila, causal agent of bacterial wilt of cucurbits, are divided into distinct clades. Et-melo clade strains wilt Cucumis spp. but not Cucurbita spp., thus exhibiting host specificity, whereas Et-C1 clade strains wilt Cucurbita spp. more rapidly than Cucumis melo, thus exhibiting a host preference. This study investigated the contribution of the effector proteins Eop1 and DspE to E. tracheiphila pathogenicity and host adaptation. Loss of eop1 did not enable Et-melo strains to infect squash (Cucurbita pepo) or an Et-C1 strain to induce a more rapid wilt of muskmelon (Cucumis melo), indicating that Eop1 did not function in host specificity or preference as in the related pathogen E. amylovora. However, overexpression of eop1 from Et-melo strain MDCuke but not from Et-C1 strain BHKY increased the virulence of a BHKY eop1 deletion mutant on muskmelon, demonstrating that the Eop1 variants in the two clades are distinct in their virulence functions. Loss of dspE from Et-melo strains reduced but did not eliminate virulence on hosts muskmelon and cucumber, whereas loss of dspE from an Et-C1 strain eliminated pathogenicity on hosts squash, muskmelon, and cucumber. Thus, the centrality of DspE to virulence differs in the two clades. Et-melo mutants lacking the chaperone DspF exhibited similar virulence to mutants lacking DspE, indicating that DspF is the sole chaperone for DspE in E. tracheiphila, unlike in E. amylovora. Collectively, these results provide the first functional evaluation of effectors in E. tracheiphila and demonstrate clade-specific differences in the roles of Eop1 and DspE.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Synergistic interaction between the type III secretion system of the endophytic bacterium Pantoea agglomerans DAPP-PG 734 and the virulence of the causal agent of olive knot Pseudomonas savastanoi pv. savastanoi DAPP-PG 722

The endophytic bacterium Pantoea agglomerans DAPP-PG 734 was previously isolated from olive knots caused by infection with Pseudomonas savastanoi pv. savastanoi DAPP-PG 722. Whole-genome analysis of this P. agglomerans strain revealed the presence of a Hypersensitive response and pathogenicity (Hrp) type III secretion system (T3SS). To assess the role of the P. agglomerans T3SS in the interaction with P. savastanoi pv. savastanoi, we generated independent knockout mutants in three Hrp genes of the P. agglomerans DAPP-PG 734 T3SS (hrpJ, hrpN, and hrpY). In contrast to the wildtype control, all three mutants failed to cause a hypersensitive response when infiltrated in tobacco leaves, suggesting that P. agglomerans T3SS is functional and injects effector proteins in plant cells. In contrast to P. savastanoi pv. savastanoi DAPP-PG 722, the wildtype strain P. agglomerans DAPP-PG 734 and its Hrp T3SS mutants did not cause olive knot disease in 1-year-old olive plants. Coinoculation of P. savastanoi pv. savastanoi with P. agglomerans wildtype strains did not significantly change the knot size, while the DAPP-PG 734 hrpY mutant induced a significant decrease in knot size, which could be complemented by providing hrpY on a plasmid. By epifluorescence microscopy and confocal laser scanning microscopy, we found that the localization patterns in knots were nonoverlapping for P. savastanoi pv. savastanoi and P. agglomerans when coinoculated. Our results suggest that suppression of olive plant defences mediated by the Hrp T3SS of P. agglomerans DAPP-PG 734 positively impacts the virulence of P. savastanoi pv. savastanoi DAPP-PG 722.

Revealing the inventory of type III effectors in Pantoea agglomerans gall-forming pathovars using draft genome sequences and a machine-learning approach

Pantoea agglomerans, a widespread epiphytic bacterium, has evolved into a hypersensitive response and pathogenicity (hrp)-dependent and host-specific gall-forming pathogen by the acquisition of a pathogenicity plasmid containing a type III secretion system (T3SS) and its effectors (T3Es). Pantoea agglomerans pv. betae (Pab) elicits galls on beet (Beta vulgaris) and gypsophila (Gypsophila paniculata), whereas P. agglomerans pv. gypsophilae (Pag) incites galls on gypsophila and a hypersensitive response (HR) on beet. Draft genome sequences were generated and employed in combination with a machine-learning approach and a translocation assay into beet roots to identify the pools of T3Es in the two pathovars. The genomes of the sequenced Pab4188 and Pag824-1 strains have a similar size (∼5 MB) and GC content (∼55%). Mutational analysis revealed that, in Pab4188, eight T3Es (HsvB, HsvG, PseB, DspA/E, HopAY1, HopX2, HopAF1 and HrpK) contribute to pathogenicity on beet and gypsophila. In Pag824-1, nine T3Es (HsvG, HsvB, PthG, DspA/E, HopAY1, HopD1, HopX2, HopAF1 and HrpK) contribute to pathogenicity on gypsophila, whereas the PthG effector triggers HR on beet. HsvB, HsvG, PthG and PseB appear to endow pathovar specificities to Pab and Pag, and no homologous T3Es were identified for these proteins in other phytopathogenic bacteria. Conversely, the remaining T3Es contribute to the virulence of both pathovars, and homologous T3Es were found in other phytopathogenic bacteria. Remarkably, HsvG and HsvB, which act as host-specific transcription factors, displayed the largest contribution to disease development.

Gall-ID: tools for genotyping gall-causing phytopathogenic bacteria

Understanding the population structure and genetic diversity of plant pathogens, as well as the effect of agricultural practices on pathogen evolution, is important for disease management. Developments in molecular methods have contributed to increase the resolution for accurate pathogen identification, but those based on analysis of DNA sequences can be less straightforward to use. To address this, we developed Gall-ID, a web-based platform that uses DNA sequence information from 16S rDNA, multilocus sequence analysis and whole genome sequences to group disease-associated bacteria to their taxonomic units. Gall-ID was developed with a particular focus on gall-forming bacteria belonging to Agrobacterium, Pseudomonas savastanoi, Pantoea agglomerans, and Rhodococcus. Members of these groups of bacteria cause growth deformation of plants, and some are capable of infecting many species of field, orchard, and nursery crops. Gall-ID also enables the use of high-throughput sequencing reads to search for evidence for homologs of characterized virulence genes, and provides downloadable software pipelines for automating multilocus sequence analysis, analyzing genome sequences for average nucleotide identity, and constructing core genome phylogenies. Lastly, additional databases were included in Gall-ID to help determine the identity of other plant pathogenic bacteria that may be in microbial communities associated with galls or causative agents in other diseased tissues of plants. The URL for Gall-ID is http://gall-id.cgrb.oregonstate.edu/.

Direct and Indirect Targeting of PP2A by Conserved Bacterial Type-III Effector Proteins

Bacterial AvrE-family Type-III effector proteins (T3Es) contribute significantly to the virulence of plant-pathogenic species of Pseudomonas, Pantoea, Ralstonia, Erwinia, Dickeya and Pectobacterium, with hosts ranging from monocots to dicots. However, the mode of action of AvrE-family T3Es remains enigmatic, due in large part to their toxicity when expressed in plant or yeast cells. To search for targets of WtsE, an AvrE-family T3E from the maize pathogen Pantoea stewartii subsp. stewartii, we employed a yeast-two-hybrid screen with non-lethal fragments of WtsE and a synthetic genetic array with full-length WtsE. Together these screens indicate that WtsE targets maize protein phosphatase 2A (PP2A) heterotrimeric enzyme complexes via direct interaction with B' regulatory subunits. AvrE1, another AvrE-family T3E from Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000), associates with specific PP2A B' subunit proteins from its susceptible host Arabidopsis that are homologous to the maize B' subunits shown to interact with WtsE. Additionally, AvrE1 was observed to associate with the WtsE-interacting maize proteins, indicating that PP2A B' subunits are likely conserved targets of AvrE-family T3Es. Notably, the ability of AvrE1 to promote bacterial growth and/or suppress callose deposition was compromised in Arabidopsis plants with mutations of PP2A genes. Also, chemical inhibition of PP2A activity blocked the virulence activity of both WtsE and AvrE1 in planta. The function of HopM1, a Pto DC3000 T3E that is functionally redundant to AvrE1, was also impaired in specific PP2A mutant lines, although no direct interaction with B' subunits was observed. These results indicate that sub-component specific PP2A complexes are targeted by bacterial T3Es, including direct targeting by members of the widely conserved AvrE-family.

The AvrE superfamily: ancestral type III effectors involved in suppression of pathogen-associated molecular pattern-triggered immunity

The AvrE superfamily of type III effectors (T3Es) is widespread among type III-dependent phytobacteria and plays a crucial role during bacterial pathogenesis. Members of the AvrE superfamily are vertically inherited core effectors, indicating an ancestral acquisition of these effectors in bacterial plant pathogens. AvrE-T3Es contribute significantly to virulence by suppressing pathogen-associated molecular pattern (PAMP)-triggered immunity. They inhibit salicylic acid-mediated plant defences, interfere with vesicular trafficking and promote bacterial growth in planta. AvrE-T3Es elicit cell death in both host and non-host plants independent of any known plant resistance protein, suggesting an original interaction with the plant immune system. Recent studies in yeast have indicated that they activate protein phosphatase 2A and inhibit serine palmitoyl transferase, the first enzyme of the sphingolipid biosynthesis pathway. In this review, we describe the current picture that has emerged from studies of the different members of this fascinating large family.

Expression of the bacterial type III effector DspA/E in Saccharomyces cerevisiae down-regulates the sphingolipid biosynthetic pathway leading to growth arrest

Erwinia amylovora, the bacterium responsible for fire blight, relies on a type III secretion system and a single injected effector, DspA/E, to induce disease in host plants. DspA/E belongs to the widespread AvrE family of type III effectors that suppress plant defense responses and promote bacterial growth following infection. Ectopic expression of DspA/E in plant or in Saccharomyces cerevisiae is toxic, indicating that DspA/E likely targets a cellular process conserved between yeast and plant. To unravel the mode of action of DspA/E, we screened the Euroscarf S. cerevisiae library for mutants resistant to DspA/E-induced growth arrest. The most resistant mutants (Δsur4, Δfen1, Δipt1, Δskn1, Δcsg1, Δcsg2, Δorm1, and Δorm2) were impaired in the sphingolipid biosynthetic pathway. Exogenously supplied sphingolipid precursors such as the long chain bases (LCBs) phytosphingosine and dihydrosphingosine also suppressed the DspA/E-induced yeast growth defect. Expression of DspA/E in yeast down-regulated LCB biosynthesis and induced a rapid decrease in LCB levels, indicating that serine palmitoyltransferase (SPT), the first and rate-limiting enzyme of the sphingolipid biosynthetic pathway, was repressed. SPT down-regulation was mediated by dephosphorylation and activation of Orm proteins that negatively regulate SPT. A Δcdc55 mutation affecting Cdc55-PP2A protein phosphatase activity prevented Orm dephosphorylation and suppressed DspA/E-induced growth arrest.

The HrpW protein of Lonsdalea quercina N-5-1 has pectate lyase activity and is required for full bacterial virulence

Lonsdalea quercina N-5-1 is a bacterial pathogen that causes poplar bark cankers. It has been isolated from the branch of Populus × euramericana cv. "74/76" in Henan, China. Previous studies have revealed that the Type III secretion system (T3SS) acts as an essential pathogenic factor in L. quercina N-5-1. HrpW is a putative effector of T3SS in strain N-5-1, which has a typical harpin domain at the amino terminal and a pectate lyase (Pel) domain at its carboxyl terminal. Genetic evidence had shown that, compared to the wild-type and the complementary strain, the hrpW mutation causes a small but significant reduction in virulence when inoculated on the poplar branches. The amino terminal domain of HrpW was found to trigger tobacco hypersensitive response, but the carboxyl terminal domain of HrpW was not. Unlike most HrpW homologs in other bacteria, the carboxyl terminal domain of HrpW of strain N-5-1 exhibited detectable pectate lyase activity. Site-direction mutations (W104A, W171M) further demonstrated that two tryptophan residues were essential to its pectate lyase activity. The results of the present work suggest that HrpW in L. quercina N-5-1 possesses pectate lyase activity and acts as a nonessential but important pathogenic factor in poplar bark canker disease.

How way leads on to way

In this article, I briefly recount the historical events in my native country that led me to become a plant pathologist. I started as a field pathologist specializing in fungal diseases of legumes, moved to biochemical research on virulence factors, and then on to molecular plant-microbe interactions. I describe the impact my graduate studies at the University of California (UC)-Davis had on my career. My life's work and teaching can be said to reflect the development in plant pathology during the past 40 years. I have included a concise review of the development of plant pathology in Israel and the ways it is funded. Dealing with administrative duties while conducting research has contributed to my belief in the importance of multidisciplinary approaches and of preserving the applied approach in the teaching of plant pathology.

Mutational analysis of a predicted double β-propeller domain of the DspA/E effector of Erwinia amylovora

The bacterium Erwinia amylovora causes fire blight, an invasive disease that threatens apple trees, pear trees and other plants of the Rosaceae family. Erwinia amylovora pathogenicity relies on a type III secretion system and on a single effector DspA/E. This effector belongs to the widespread AvrE family of effectors whose biological function is unknown. In this manuscript, we performed a bioinformatic analysis of DspA/E- and AvrE-related effectors. Motif search identified nuclear localization signals, peroxisome targeting signals, endoplasmic reticulum membrane retention signals and leucine zipper motifs, but none of these motifs were present in all the AvrE-related effectors analysed. Protein threading analysis, however, predicted a conserved double β-propeller domain in the N-terminal part of all the analysed effector sequences. We then performed a random pentapeptide mutagenesis of DspA/E, which led to the characterization of 13 new altered proteins with a five amino acids insertion. Eight harboured the insertion inside the predicted β-propeller domain and six of these eight insertions impaired DspA/E stability or function. Conversely, the two remaining insertions generated proteins that were functional and abundantly secreted in the supernatant suggesting that these two insertions stabilized the protein.

Global regulatory networks control the hrp regulon of the gall-forming bacterium Pantoea agglomerans pv. gypsophilae

Gall formation by Pantoea agglomerans pv. gypsophilae is dependent on the hypersensitive response and pathogenicity (hrp) system. Previous studies demonstrated that PagR and PagI, regulators of the quorum-sensing system, induce expression of the hrp regulatory cascade (i.e., hrpXY, hrpS, and hrpL) that activates the HrpL regulon. Here, we isolated the genes of the Gac/Rsm global regulatory pathway (i.e., gacS, gacA, rsmB, and csrD) and of the post-transcriptional regulator rsmA. Our results demonstrate that PagR and PagI also upregulate expression of the Gac/Rsm pathway. PagR acts as a transcriptional activator of each of the hrp regulatory genes and gacA in a N-butanoyl-L-homoserine lactone-dependent manner as shown by gel shift experiments. Mutants of the Gac/Rsm genes or overexpression of rsmA significantly reduced Pantoea agglomerans virulence and colonization of gypsophila. Overexpression of rsmB sRNA abolished gall formation, colonization, and hypersensitive reaction on nonhost plants and prevented transcription of the hrp regulatory cascade, indicating a lack of functional type III secretion system. Expression of rsmB sRNA in the background of the csrD null mutant suggests that CsrD may act as a safeguard for preventing excessive production of rsmB sRNA. Results presented indicate that the hrp regulatory cascade is controlled directly by PagR and indirectly by RsmA, whereas deficiency in RsmA activity is epistatic to PagR induction.

Polar auxin transport is essential for gall formation by Pantoea agglomerans on Gypsophila

The virulence of the bacterium Pantoea agglomerans pv. gypsophilae (Pag) on Gypsophila paniculata depends on a type III secretion system (T3SS) and its effectors. The hypothesis that plant-derived indole-3-acetic acid (IAA) plays a major role in gall formation was examined by disrupting basipetal polar auxin transport with the specific inhibitors 2,3,5-triiodobenzoic acid (TIBA) and N-1-naphthylphthalamic acid (NPA). On inoculation with Pag, galls developed in gypsophila stems above but not below lanolin rings containing TIBA or NPA, whereas, in controls, galls developed above and below the rings. In contrast, TIBA and NPA could not inhibit tumour formation in tomato caused by Agrobacterium tumefaciens. The colonization of gypsophila stems by Pag was reduced below, but not above, the lanolin-TIBA ring. Following Pag inoculation and TIBA treatment, the expression of hrpL (a T3SS regulator) and pagR (a quorum-sensing transcriptional regulator) decreased four-fold and that of pthG (a T3SS effector) two-fold after 24 h. Expression of PIN2 (a putative auxin efflux carrier) increased 35-fold, 24 h after Pag inoculation. However, inoculation with a mutant in the T3SS effector pthG reduced the expression of PIN2 by two-fold compared with wild-type infection. The results suggest that pthG might govern the elevation of PIN2 expression during infection, and that polar auxin transport-derived IAA is essential for gall initiation.

The type III effector HsvG of the gall-forming Pantoea agglomerans mediates expression of the host gene HSVGT

The type III effector HsvG of the gall-forming Pantoea agglomerans pv. gypsophilae is a DNA-binding protein that is imported to the host nucleus and involved in host specificity. The DNA-binding region of HsvG was delineated to 266 amino acids located within a secondary structure region near the N-terminus of the protein but did not display any homology to canonical DNA-binding motifs. A binding site selection procedure was used to isolate a target gene of HsvG, named HSVGT, in Gypsophila paniculata. HSVGT is a predicted acidic protein of the DnaJ family with 244 amino acids. It harbors characteristic conserved motifs of a eukaryotic transcription factor, including a bipartite nuclear localization signal, zinc finger, and leucine zipper DNA-binding motifs. Quantitative real-time polymerase chain reaction analysis demonstrated that HSVGT transcription is specifically induced in planta within 2 h after inoculation with the wild-type P. agglomerans pv. gypsophilae compared with the hsvG mutant. Induction of HSVGT reached a peak of sixfold at 4 h after inoculation and progressively declined thereafter. Gel-shift assay demonstrated that HsvG binds to the HSVGT promoter, indicating that HSVGT is a direct target of HsvG. Our results support the hypothesis that HsvG functions as a transcription factor in gypsophila.

Expressing the Erwinia amylovora type III effector DspA/E in the yeast Saccharomyces cerevisiae strongly alters cellular trafficking

Erwinia amylovora is responsible for fire blight, a necrotic disease of apples and pears. E. amylovora relies on a type III secretion system (T3SS) to induce disease on host plants. DspA/E belongs to the AvrE family of type III effector. Effectors of the AvrE family are injected via the T3SS in plant cell and are important to promote bacterial growth following infection and to suppress plant defense responses. Their mode of action in the plant cells is unknown. Here we study the physiological effects induced by dspA/E expression in the yeast Saccharomyces cerevisiae. Expression of dspA/E in the yeast inhibits cell growth. This growth inhibition is associated with perturbations of the actin cytoskeleton and endocytosis.

Genomic and phenotypic characterization of a nonpigmented variant of Pantoea vagans biocontrol strain C9-1 lacking the 530-kb megaplasmid pPag3

A 530-kb megaplasmid pPag3 contributing 10.8% of the total genome of Pantoea vagans biocontrol strain C9-1 was sequenced. A rare nonpigmented variant C9-1W was obtained and shown to have lost pPag3, but retained all other plasmids (pPag1, pPag2). Phenotypic characterization of the variant confirmed the function of several annotated genes that may influence ecological fitness and efficacy. Metabolic profiling revealed important plasmid-based carbon utilization phenotypes. Plasmid loss resulted in thiamine auxotrophy, absence of carotenoid pigmentation, desferrioxamine diffusible siderophore biosynthesis, inherent ampicillin resistance and expression of AI-1 quorum-sensing signaling. This confirmed the functional expression of the corresponding genes located on pPag3 in P. vagans.

Regulatory interactions between quorum-sensing, auxin, cytokinin, and the Hrp regulon in relation to gall formation and epiphytic fitness of Pantoea agglomerans pv. gypsophilae

Gall formation by Pantoea agglomerans pv. gypsophilae is controlled by hrp/hrc genes, phytohormones, and the quorum-sensing (QS) regulatory system. The interactions between these three components were investigated. Disruption of the QS genes pagI and pagR and deletion of both substantially reduced the transcription levels of the hrp regulatory genes hrpXY, hrpS, and hrpL, as determined by quantitative reverse-transcriptase polymerase chain reaction. Expression of hrpL in planta was inhibited by addition of 20 microM or higher concentrations of the QS signal C(4)-HSL. The pagR and hrpL mutants caused an equivalent reduction of 1.3 orders in bacterial multiplication on bean leaves, suggesting possible mediation of the QS effect on epiphytic fitness of P. agglomerans pv. gypsophilae by the hrp regulatory system. indole-3-acetic acid (IAA) and cytokinin significantly affected the expression of the QS and hrp regulatory genes. Transcription of pagI, pagR, hrpL, and hrpS in planta was substantially reduced in iaaH mutant (disrupted in IAA biosynthesis via the indole-3-acetamide pathway) and etz mutant (disrupted in cytokinin biosynthesis). In contrast, the ipdC mutant (disrupted in IAA biosynthesis via the indole-3-pyruvate pathway) substantially increased expression of pagI, pagR, hrpL, and hrpS. Results presented suggest the involvement of IAA and cytokinins in regulation of the QS system and hrp regulatory genes.

Recent evolution of bacterial pathogens: the gall-forming Pantoea agglomerans case

Pantoea agglomerans, a widespread epiphyte and commensal bacterium, has evolved into an Hrp-dependent and host-specific tumorigenic pathogen by acquiring a plasmid containing a pathogenicity island (PAI). The PAI was evolved on an iteron plasmid of the IncN family, which is distributed among genetically diverse populations of P. agglomerans. The structure of the PAI supports the premise of a recently evolved pathogen. This review offers insight into a unique model for emergence of new bacterial pathogens. It illustrates how horizontal gene transfer was the major driving force in the creation of the PAI, although a pathoadaptive mechanism might also be involved. It describes the crucial function of plant-produced indole-3-acetic acid (IAA) and cytokinines (CK) in gall initiation as opposed to the significant but secondary role of pathogen-secreted phytohormones. It also unveils the role of type III effectors in determination of host specificity and evolution of the pathogen into pathovars. Finally, it describes how interactions between the quorum sensing system, hrp regulatory genes, and bacterially secreted IAA or CKs affect gall formation and epiphytic fitness.

WtsE, an AvrE-family type III effector protein of Pantoea stewartii subsp. stewartii, causes cell death in non-host plants

Pantoea stewartii subsp. stewartii (Pnss) causes Stewart's bacterial wilt of sweet corn and leaf blight of maize. The pathogenicity of Pnss depends on synthesis of extracellular polysaccharide and an Hrp type III secretion system. WtsE, a type III secreted effector protein, is essential for the virulence of Pnss on corn. It belongs to the AvrE family of effectors, which includes DspA/E from Erwinia amylovora and AvrE1 from Pseudomonas syringae. Previously, WtsE was shown to cause disease-associated cell death in its host plant, sweet corn. Here, we examine the biological activity of WtsE in several non-host plants. WtsE induced cell death in Nicotiana benthamiana, tobacco, beet and Arabidopsis thaliana when it was transiently produced in plant cells following agroinfiltration or translocated into plant cells from Pnss, Escherichia coli or Pseudomonas syringae pv. phaseolicola (Pph). WtsE-induced cell death in N. benthamiana, tobacco and beet resembled a hypersensitive response and in N. benthamiana it was delayed by cycloheximide. Interestingly, WtsE strongly promoted the growth of Pnss in N. benthamiana prior to the onset of cell death. Deletion derivatives of WtsE that failed to induce cell death in N. benthamiana and tobacco also did not complement wtsE mutants of Pnss for virulence in sweet corn, indicating a correlation between the two activities. WtsE also induced cell death in A. thaliana, where it suppressed basal defences induced by Pph. Thus, WtsE has growth-promoting, defence-suppressing and cell death-inducing activities in non-host plants. Expression of WtsE also prevented the growth of yeast, possibly due to an innate toxicity to eukaryotic cells.

Erwinia amylovora type three-secreted proteins trigger cell death and defense responses in Arabidopsis thaliana

Erwinia amylovora is the bacterium responsible for fire blight, a necrotic disease affecting plants of the rosaceous family. E. amylovora pathogenicity requires a functional type three secretion system (T3SS). We show here that E. amylovora triggers a T3SS-dependent cell death on Arabidopsis thaliana. The plants respond by inducing T3SS-dependent defense responses, including salicylic acid (SA)-independent callose deposition, activation of the SA defense pathway, reactive oxygen species (ROS) accumulation, and part of the jasmonic acid/ethylene defense pathway. Several of these reactions are similar to what is observed in host plants. We show that the cell death triggered by E. amylovora on A. thaliana could not be simply explained by the recognition of AvrRpt2 ea by the resistance gene product RPS2. We then analyzed the role of type three-secreted proteins (T3SPs) DspA/E, HrpN, and HrpW in the induction of cell death and defense reactions in A. thaliana following infection with the corresponding E. amylovora mutant strains. HrpN and DspA/E were found to play an important role in the induction of cell death, activation of defense pathways, and ROS accumulation. None of the T3SPs tested played a major role in the induction of SA-independent callose deposition. The relative importance of T3SPs in A. thaliana is correlated with their relative importance in the disease process on host plants, indicating that A. thaliana can be used as a model to study their role.

Distribution and replication of the pathogenicity plasmid pPATH in diverse populations of the gall-forming bacterium Pantoea agglomerans

Pantoea agglomerans has been transformed from a commensal bacterium into two related gall-forming pathovars by acquisition of pPATH plasmids containing a pathogenicity island (PAI). This PAI harbors an hrp/hrc gene cluster, type III effectors, and phytohormone biosynthetic genes. DNA typing by pulsed-field gel electrophoresis revealed two major groups of P. agglomerans pv. gypsophilae and one group of P. agglomerans pv. betae. The pPATH plasmids of the different groups had nearly identical replicons (98% identity), and the RepA protein showed the highest level of similarity with IncN plasmid proteins. A series of plasmids, designated pRAs, in which the whole replicon region (2,170 bp) or deleted derivatives of it were ligated with nptI were generated for replicon analysis. A basic 929-bp replicon (pRA6) was sufficient for replication in Escherichia coli and in nonpathogenic P. agglomerans. However, the whole replicon region (pRA1) was necessary for expulsion of the pPATH plasmid, which resulted in the loss of pathogenicity. The presence of direct repeats in the replicon region suggests that the pPATH plasmid is an iteron plasmid and that the repeats may regulate its replication. The pPATH plasmids are nonconjugative but exhibit a broad host range, as shown by replication of pRA1 in Erwinia, Pseudomonas, and Xanthomonas. Restriction fragment length polymorphism analyses indicated that the PAIs in the two groups of P. agglomerans pv. gypsophilae are similar but different from those in P. agglomerans pv. betae. The results could indicate that the pPATH plasmids evolved from a common ancestral mobilizable plasmid that was transferred into different strains of P. agglomerans.

Identification of a key functional region in harpins from Xanthomonas that suppresses protein aggregation and mediates harpin expression in E. coli

In the current study, we identified a key functional region in harpins from Xanthomonas that suppressed protein aggregation and mediated its expression in E. coli. Our data suggested that the presence of two common features in harpins [Wei et al. (1992) Science 257:85-88], namely, high glycine content and lack of cysteine residues, were not sufficient for Xanthomonas to elicit hypersensitive response (HR) activity or heat stability. Additionally, bioinformatic analyses revealed that the secondary structure of a conserved N-terminal region consisting of 12 highly hydrophilic amino acids (QGISEKQLDQLL) was alpha-helical. Following site-directed mutagenesis deletion of this region, the three mutated harpin proteins, in cultures induced at 37 degrees C, failed to elicit a HR in tobacco leaves. However, at 24 degrees C, two mutated harpins retained the ability to elicit HR, albeit with lower expression levels than that noted with the wild-type. SDS-PAGE and Western blot data suggested the HpaG mutant protein was found almost entirely in the inclusion body. These data demonstrated that these conserved amino acid residues played a critical role in protein aggregation and inclusion body formation in harpins from Xanthomonas.

Identification of Pseudomonas syringae pv. syringae 61 type III secretion system Hrp proteins that can travel the type III pathway and contribute to the translocation of effector proteins into plant cells

Pseudomonas syringae translocates effector proteins into plant cells via an Hrp1 type III secretion system (T3SS). T3SS components HrpB, HrpD, HrpF, and HrpP were shown to be pathway substrates and to contribute to elicitation of the plant hypersensitive response and to translocation and secretion of the model effector AvrPto1.

Indole-3-acetic acid in microbial and microorganism-plant signaling

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phyto-stimulation. Reviewing the role of bacterial IAA in different microorganism-plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phyto-stimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.

Genomic insights into the contribution of phytopathogenic bacterial plasmids to the evolutionary history of their hosts

Plasmids are common residents of phytopathogenic bacteria and contribute significantly to host evolution in a multi-faceted manner. Plasmids tend to encode determinants of virulence and ecological fitness that can enhance adaptation to a specific niche or can influence niche expansion. Many of these determinants appear to have been acquired from other bacteria via horizontal transfer, illustrating an important function of plasmids in the acquisition of sequences that enable rapid evolution. These genes can ultimately be delivered to the host chromosome through plasmid integration events, thus stabilizing important acquired determinants within the genome. Most plasmids characterized in phytopathogenic bacteria are self-transmissible and possess suites of genes encoding type IV secretion systems. In addition, the phytopathogenic bacterial plasmid "mobilome" includes insertion sequence and other transposable elements that contribute to the movement of sequences within and between genomes. Possession of mosaic and ever-changing plasmids allows phytopathogenic bacteria to maintain a dynamic, flexible genome and possible advantage in host-pathogen and other environmental interactions that belies the concept of plasmids as apparently selfish genetic elements.

Regulation of the type III secretion system in phytopathogenic bacteria

The type III secretion system (TTSS) is a specialized protein secretion machinery used by numerous gram-negative bacterial pathogens of animals and plants to deliver effector proteins directly into the host cells. In plant-pathogenic bacteria, genes encoding the TTSS were discovered as hypersensitive response and pathogenicity (hrp) genes, because mutation of these genes typically disrupts the bacterial ability to cause diseases on host plants and to elicit hypersensitive response on nonhost plants. The hrp genes and the type III effector genes (collectively called TTSS genes hereafter) are repressed in nutrient-rich media but induced when bacteria are infiltrated into plants or incubated in nutrient-deficient inducing media. Multiple regulatory components have been identified in the plant-pathogenic bacteria regulating TTSS genes under various conditions. In Ralstonia solanacearum, several signal transduction components essential for the induction of TTSS genes in plants are dispensable for the induction in inducing medium. In addition to the inducing signals, recent studies indicated the presence of negative signals in the plant regulating the Pseudomonas syringae TTSS genes. Thus, the levels of TTSS gene expression in plants likely are determined by the interactions of multiple signal transduction pathways. Studies of the hrp regulons indicated that TTSS genes are coordinately regulated with a number of non-TTSS genes.

WtsE, an AvrE-family effector protein from Pantoea stewartii subsp. stewartii, causes disease-associated cell death in corn and requires a chaperone protein for stability

The pathogenicity of Pantoea stewartii subsp. stewartii to sweet corn and maize requires a Hrp type III secretion system. In this study, we genetically and functionally characterized a disease-specific (Dsp) effector locus, composed of wtsE and wtsF, that is adjacent to the hrp gene cluster. WtsE, a member of the AvrE family of effector proteins, was essential for pathogenesis on corn and was complemented by DspA/E from Erwinia amylovora. An intact C-terminus of WtsE, which contained a putative endoplasmic reticulum membrane retention signal, was important for function of WtsE. Delivery of WtsE into sweet corn leaves by an Escherichia coli strain carrying the hrp cluster of Erwinia chrysanthemi caused water-soaking and necrosis. WtsE-induced cell death was not inhibited by cycloheximide treatment, unlike the hypersensitive response caused by a known Avr protein, AvrRxol. WtsF, the putative chaperone of WtsE, was not required for secretion of WtsE from P. stewartii, and the virulence of wtsF mutants was reduced only at low inoculum concentrations. However, WtsF was required for full accumulation of WtsE within the bacteria at low temperatures. In contrast, WtsF was needed for efficient delivery of WtsE from E. coli via the Erwinia chrysanthemi Hrp system.

The type III effectors HsvG and HsvB of gall-forming Pantoea agglomerans determine host specificity and function as transcriptional activators

Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) induces galls on both beet and gypsophila. The pathogenicity of both pathovars is dependent on the presence of a plasmid harbouring type III secretion system (TTSS) components and effectors. The HsvG TTSS effectors of Pag (HsvG-Pag) and Pab (HsvG-Pab) determine the host specificity of both pathovars on gypsophila. Here we describe a novel HsvG homologue, HsvB, which determines the host specificity of Pag and Pab on beet. HsvG requires two direct amino acid repeats for pathogenicity on gypsophila, whereas one repeat in HsvB is sufficient for pathogenicity on beet. Exchanging repeats between HsvG-Pag and HsvB-Pab resulted in a switch of host specificities. Transient expression of GFP-HsvG or GFP-HsvB fusions in gypsophila, beet or melon leaves showed that HsvG and HsvB were localized to the nuclei of host and non-host plants. A yeast one-hybrid assay revealed that a single repeat of HsvG or HsvB was sufficient to activate transcription. By employing random binding-site selection and gel-shift assay HsvG was demonstrated to be a double-stranded DNA-binding protein with an ACACC/aAA consensus binding site. These results suggest that HsvG and HsvB are host-specificity determinants and bear the potential to affect the host transcriptional machinery.

Comparative anatomy of gall development on Gypsophila paniculata induced by bacteria with different mechanisms of pathogenicity

Galls induced on Gypsophila paniculata by Pantoea agglomerans pv. gypsophilae (Pag) and Agrobacterium tumefaciens (At), bacteria with different mechanisms of pathogenicity, were compared morphologically and anatomically. The pathogenicity of Pag is dependent on the presence of an indigenous plasmid that harbors hrp gene cluster, genes encoding Hop virulence proteins and biosynthetic genes for auxin (IAA) and cytokinins (CKs), whereas that of At involves host transformation. The Pag-induced gall was rough, brittle and exhibited limited growth, in contrast to the smooth, firm appearance and continuous growth of the At-induced gall. Anatomical analysis revealed the presence of cells with enlarged nuclei and multiple nucleoli, giant cells and suberin deposition in Pag that were absent from At-induced galls. Although circular vessels were observed in both gall types, they were more numerous and the vascular system was more organized in At. An aerenchymal tissue was observed in the upper part of the galls. Ethylene emission from Pag galls, recorded 6 days after inoculation, was eight times as great as that from non-infected controls. In contrast, a significant decrease in ethylene production was observed in Gypsophila cuttings infected with Pag mutants deficient in IAA and CK production. The results presented are best accounted for by the two pathogens having distinct pathogenicity mechanisms that lead to their differential recognition by the host as non-self (Pag) and self (At).

The hrpN gene of Erwinia amylovora stimulates tobacco growth and enhances resistance to Botrytis cinerea

Erwinia amylovora is a member of the harpin proteins that induces pathogen resistance and hypersensitive cell death in plants. To obtain tobacco plants displaying a hypersensitive response, the hrpN gene from Erwinia amylovora was cloned into vector pMJC-GB under the control of the rice cytochrome promoter and transfected into tobacco. Southern hybridization with a hrpN probe revealed that the gene was present in one copy in the transgenic plants. In addition, hrpN transcripts could be detected in transgenic plants but not in wild-type tobacco. The wild type gave 75 products in RAPD analysis with 12 primers while the transgenic plants gave 73, suggesting that hrpN gene had been integrated into the transgenic plant genomic DNA. The distribution of cell cycle phases in the wild type and transgenic plants was G0-G1: 71.25%, G2-M: 20.41%, S: 8.33%, while in transgenic plant was G0-G1: 54.95%, G2-M: 43.82%, S: 10.23%. The sizes of stomata and guard cells on transgenic leaves were similar to those of the wild type, but the epidermal cells were clearly smaller. The transgenic plants showed accelerated growth and development as well as enhanced resistance to Botrytis cinerea.

Type III effector proteins from the plant pathogen Xanthomonas and their role in the interaction with the host plant

Pathogenicity of Xanthomonas campestris pathovar (pv.) vesicatoria and most other Gram-negative bacterial plant pathogens largely depends on a type III secretion (TTS) system which is encoded by hypersensitive response and pathogenicity (hrp) genes. These genes are induced in the plant and are essential for the bacterium to be virulent in susceptible hosts and for the induction of the hypersensitive response (HR) in resistant host and non-host plants. The TTS machinery secretes proteins into the extracellular milieu and effector proteins into the plant cell cytosol. In the plant, the effectors presumably interfere with cellular processes to the benefit of the pathogen or have an avirulence activity that betrays the bacterium to the plant surveillance system. Type III effectors were identified by their avirulence activity, co-regulation with the TTS system and homology to known effectors. A number of effector proteins are members of families, e.g., the AvrBs3 family in Xanthomonas. AvrBs3 localizes to the nucleus of the plant cell where it modulates plant gene expression. Another family that is also present in Xanthomonas is the YopJ/AvrRxv family. The latter proteins appear to act as SUMO cysteine proteases in the host. Here, we will present an overview about the regulation of the TTS system and its substrates and discuss the function of the AvrRxv and AvrBs3 family members in more detail.

DspA/E, a type III effector essential for Erwinia amylovora pathogenicity and growth in planta, induces cell death in host apple and nonhost tobacco plants

Erwinia amylovora is responsible for fire blight, a necrotic disease of apples and pears. E. amylovora relies on a type III secretion system (TTSS) to induce disease on hosts and hypersensitive response (HR) on nonhost plants. The DspA/E protein is essential for E. amylovora pathogenicity and is secreted via the TTSS in vitro. DspA/E belongs to a type III effector family that is conserved in several phytopathogenic bacteria. In E. amylovora, DspA/E has been implicated in the generation of an oxidative stress during disease and the suppression of callose deposition. We investigated the fate of DspA/E in planta. DspA/E delivered artificially to apple or tobacco cells by agroinfection induced necrotic symptoms, indicating that DspA/E was probably injected via the TTSS. We confirmed that DspA/E acts as a major cell-death inducer during disease and HR, because the dspA/E mutant is severely impaired in its ability to induce electrolyte leakage in apple and tobacco leaves. Expression of the defense marker gene PR1 was delayed when dspA/E was transiently expressed in tobacco, suggesting that DspA/E-mediated necrosis may be associated with an alteration of defense responses.

Alternative sigma factors and their roles in bacterial virulence

Sigma factors provide promoter recognition specificity to RNA polymerase holoenzyme, contribute to DNA strand separation, and then dissociate from the core enzyme following transcription initiation. As the regulon of a single sigma factor can be composed of hundreds of genes, sigma factors can provide effective mechanisms for simultaneously regulating expression of large numbers of prokaryotic genes. One newly emerging field is identification of the specific roles of alternative sigma factors in regulating expression of virulence genes and virulence-associated genes in bacterial pathogens. Virulence genes encode proteins whose functions are essential for the bacterium to effectively establish an infection in a host organism. In contrast, virulence-associated genes can contribute to bacterial survival in the environment and therefore may enhance the capacity of the bacterium to spread to new individuals or to survive passage through a host organism. As alternative sigma factors have been shown to regulate expression of both virulence and virulence-associated genes, these proteins can contribute both directly and indirectly to bacterial virulence. Sigma factors are classified into two structurally unrelated families, the sigma70 and the sigma54 families. The sigma70 family includes primary sigma factors (e.g., Bacillus subtilis sigma(A)) as well as related alternative sigma factors; sigma54 forms a distinct subfamily of sigma factors referred to as sigma(N) in almost all species for which these proteins have been characterized to date. We present several examples of alternative sigma factors that have been shown to contribute to virulence in at least one organism. For each sigma factor, when applicable, examples are drawn from multiple species.

Identification of Erwinia amylovora genes induced during infection of immature pear tissue

The enterobacterium Erwinia amylovora is a devastating plant pathogen causing necrotrophic fire blight disease of apple, pear, and other rosaceous plants. In this study, we used a modified in vivo expression technology system to identify E. amylovora genes that are activated during infection of immature pear tissue, a process that requires the major pathogenicity factors of this organism. We identified 394 unique pear fruit-induced (pfi) genes on the basis of sequence similarity to known genes and separated them into nine putative function groups including host-microbe interactions (3.8%), stress response (5.3%), regulation (11.9%), cell surface (8.9%), transport (13.5%), mobile elements (1.0%), metabolism (20.3%), nutrient acquisition and synthesis (15.5%), and unknown or hypothetical proteins (19.8%). Known virulence genes, including hrp/hrc components of the type III secretion system, the major effector gene dspE, type II secretion, levansucrase (lsc), and regulators of levansucrase and amylovoran biosynthesis, were upregulated during pear tissue infection. Known virulence factors previously identified in E. (Pectobacterium) carotovora and Pseudomonas syringae were identified for the first time in E. amylovora and included HecA hemagglutinin family adhesion, Peh polygalacturonase, new effector HopPtoC(EA), and membrane-bound lytic murein transglycosylase MltE(EA). An insertional mutation within hopPtoC(EA) did not result in reduced virulence; however, an mltE(EA) knockout mutant was reduced in virulence and growth in immature pears. This study suggests that E. amylovora utilizes a variety of strategies during plant infection and to overcome the stressful and poor nutritional environment of its plant hosts.

Analysis of promoters recognized by HrpL, an alternative sigma-factor protein from Pantoea agglomerans pv. gypsophilae

HrpL, an alternative sigma factor, activates the transcription of the Hrp regulon by its binding to a common "hrp box" promoter. Based on computational techniques, the hrp box previously was defined as a consensus bipartite cis element, 5'-GGAACC-N(15-16)-CCACNNA-3'. The present report combines a quantitative in vivo assay for measuring Hrp promoter activity with site-specific mutagenesis to analyze the effect of consensus and nonconsensus nucleotides on promoter activity. The analysis was carried out with Hop effectors of the tumorigenic bacterium Pantoea agglomerans pv. gypsophilae, in which HrpL is indispensable for gall formation. Mutational analysis indicates that the hrp box consensus can be divided into crucial and noncrucial nucleotides. The first 5 nucleotides (nt) of the--35 consensus motif (GGAAC) and the 3 nt of the--10 motif (ACNNA) are crucial, whereas other consensus and adjacent nonconsensus nucleotides exert a significant effect on the promoter's strength. With spacing of 13 or 17 nt between the two motifs, significant activity was still retained. Gel shift assays indicated that deletion of GG from the--35 consensus motif eliminated HrpL binding, whereas mutations in the--10 consensus motif or modification of the spacing, which eliminates promoter activity, did not elicit any effect. The degeneracy in Hrp promoters of four hrp and type III effector genes of P agglomerans pv. gypsophilae indicated significant differences in promoter activity, whereas increasing the promoter strength of the Hop effector, HsvG, resulted in overexpression of gall formation.

The Hrp pathogenicity island of Erwinia amylovora and identification of three novel genes required for systemic infectiondouble dagger

SUMMARY Sequence analysis of the region bordering the hrp/dsp gene cluster of Erwinia amylovora strain Ea321, which causes fire blight, revealed characteristics of pathogenicity islands (PAIs). Included are genes for a phage integrase, a tRNA(Phe), several orthologues of genes of YAPI, a PAI of Yersinia pseudotuberculosis, and several putative virulence genes with HrpL-dependent promoter motifs. The island is designated the Hrp PAI of E. amylovora. It is comprised of a chromosomal region of c. 62 kb with 60 open reading frames (ORFs). Comparison of the Hrp PAI of E. amylovora with those of four closely related bacteria showed that orfB, a homologue of avrBsT of Xanthomonas campestris pv. vesicatoria, and orfA, its putative chaperone gene, are present only in the Hrp PAI of E. amylovora. As regions flanking the hrp/dsp gene cluster are quite diverse, addition and deletion may have occurred during divergent evolution of the five bacteria. Among ORFs of the PAI of Ea321, three new HrpL-dependent genes were identified. Because they are required for full virulence in apple, they were designated hsvC, hsvB and hsvA (hrp-associated systemic virulence). They encode a homologue of an amidinotransferase for phaseolotoxin biosynthesis and homologues of a nikkomycin-biosynthetic protein of Pseudomonas syringae.

Use of a pooled transposon mutation grid to demonstrate roles in disease development for Erwinia carotovora subsp. atroseptica putative type III secreted effector (DspE/A) and helper (HrpN) proteins

Soft rot Erwinia spp., like other closely related plant pathogens, possess a type III secretion system (TTSS) (encoded by the hrp gene cluster) implicated in disease development. We report the sequence of the entire hrp gene cluster and adjacent dsp genes in Erwinia carotovora subsp. atroseptica SCRI1039. The cluster is similar in content and structural organization to that in E. amylovora. However, eight putative genes of unknown function located within the E. carotovora subsp. atroseptica cluster do not have homologues in the E. amylovora cluster. An arrayed set of Tn5 insertional mutants (mutation grid) was constructed and pooled to allow rapid isolation of mutants for any given gene by polymerase chain reaction screening. This novel approach was used to obtain mutations in two structural genes (hrcC and hrcV), the effector gene dspE/A, and the helper gene hrpN. An improved pathogenicity assay revealed that these mutations led to significantly reduced virulence, showing that both the putative E. carotovora subsp. atroseptica TTSS-delivered effector and helper proteins are required for potato infection.

A family of conserved bacterial effectors inhibits salicylic acid-mediated basal immunity and promotes disease necrosis in plants

Salicylic acid (SA)-mediated host immunity plays a central role in combating microbial pathogens in plants. Inactivation of SA-mediated immunity, therefore, would be a critical step in the evolution of a successful plant pathogen. It is known that mutations in conserved effector loci (CEL) in the plant pathogens Pseudomonas syringae (the Delta CEL mutation), Erwinia amylovora (the dspA/E mutation), and Pantoea stewartii subsp. stewartii (the wtsE mutation) exert particularly strong negative effects on bacterial virulence in their host plants by unknown mechanisms. We found that the loss of virulence in Delta CEL and dspA/E mutants was linked to their inability to suppress cell wall-based defenses and to cause normal disease necrosis in Arabidopsis and apple host plants. The Delta CEL mutant activated SA-dependent callose deposition in wild-type Arabidopsis but failed to elicit high levels of callose-associated defense in Arabidopsis plants blocked in SA accumulation or synthesis. This mutant also multiplied more aggressively in SA-deficient plants than in wild-type plants. The hopPtoM and avrE genes in the CEL of P. syringae were found to encode suppressors of this SA-dependent basal defense. The widespread conservation of the HopPtoM and AvrE families of effectors in various bacteria suggests that suppression of SA-dependent basal immunity and promotion of host cell death are important virulence strategies for bacterial infection of plants.

The Erwinia chrysanthemi EC16 hrp/hrc gene cluster encodes an active Hrp type III secretion system that is flanked by virulence genes functionally unrelated to the Hrp system

Erwinia chrysanthemi is a host-promiscuous plant pathogen that possesses a type III secretion system (TTSS) similar to that of the host-specific pathogens E. amylovora and Pseudomonas syringae. The regions flanking the TTSS-encoding hrp/hrc gene clusters in the latter pathogens encode various TTSS-secreted proteins. DNA sequencing of the complete E. chrysanthemi hrp/hrc gene cluster and approximately 12 kb of the flanking regions (beyond the previously characterized hecA adhesin gene in the left flank) revealed that the E. chrysanthemi TTSS genes were syntenic and similar (>50% amino-acid identity) with their E. amylovora orthologs. However, the hrp/hrc cluster was interrupted by a cluster of four genes, only one of which, a homolog of lytic transglycosylases, is implicated in TTSS functions. Furthermore, the regions flanking the hrp/hrc cluster lacked genes that were likely to encode TTSS substrates. Instead, some of the genes in these regions predict ABC transporters and methyl-accepting chemotaxis proteins that could have alternative roles in virulence. Mutations affecting all of the genes in the regions flanking or interrupting the hrp/hrc cluster were constructed in E. chrysanthemi CUCPB5047, a mutant whose reduced pectolytic capacity can enhance the phenotype of minor virulence factors. Mutants were screened in witloof chicory leaves and then in potato tubers and Nicotiana clevelandii seedlings. Mu dII1734 insertion in one gene, designated virA, resulted in strongly reduced virulence in all three tests. virA is immediately downstream of hecA, has an unusually low G+C content of 38%, and predicts an unknown protein of 111 amino acids. The E. chrysanthemi TTSS was shown to be active by its ability to translocate AvrPto-Cya (a P. syringae TTSS effector fused to an adenylate cyclase reporter that is active in the presence of eukaryote calmodulin) into N. benthamiana leaf cells. However, VirA(1-61)-Cya was not translocated into plant cells, and virA expression was not affected by mutations in E. chrysanthemi Hrp regulator genes hrpL and hrpS. Thus, the 44-kb region of the E. chrysanthemi EC16 genome that is centered on the hrplhrc cluster encodes a potpourri of virulence factors, but none of these appear to be a TTSS effector.

Knot Formation Caused by Pseudomonas syringae subsp. savastanoi on Olive Plants Is hrp-Dependent

ABSTRACT The virulence of Pseudomonas syringae subsp. savastanoi, which causes hyperplastic symptoms (knots) on olive plants, is associated with secreted phytohormones. We identified a Tn5-induced mutant of P. syringae subsp. savastanoi that did not cause disease symptoms on olive plants although it was still able to produce phytohormones. In addition, the mutant failed to elicit a hypersensitive response in a nonhost plant. Molecular characterization of the mutant revealed that a single Tn5 insertion occurred within an open reading frame encoding a protein 92% identical to the HrcC protein of P. syringae pv. syringae. Moreover, sequence analysis revealed that the gene encoding the HrcC protein in P. syringae subsp. savastanoi was part of an operon that included five genes arranged as in other phytopathogenic bacteria. These results imply that hrp/hrc genes are functional in P. syringae subsp. savastanoi and that they play a key role in the pathogenicity of this plant pathogen.

pthG from Pantoea agglomerans pv. gypsophilae encodes an avirulence effector that determines incompatibility in multiple beet species

SUMMARY Pantoea agglomerans pv. gypsophilae (Pag) causes root and crown gall disease on gypsophila, whereas P. agglomerans pv. betae (Pab) induces the disease on beet as well as gypsophila. Both pathovars harbour a pathogenicity plasmid (pPATH(Pag) or pPATH(Pab)) that determines disease development. We have previously isolated and partially characterized a pleiotropic gene from the pPATH(Pag), designated as pthG, that encodes a virulence factor in gypsophila and an elicitor of a hypersensitive-like response in beet roots. The present study was undertaken to characterize pthG further as an avr gene. The infiltration of beet leaves with strains expressing PthG (i.e. Pag or Pab containing pthG in trans) caused an hypersensitive reaction (HR) response within 48 h, whereas strains lacking intact pthG (i.e. Pab or Pag mutated in pthG) resulted in gall formation after 5 days. A hypersensitive reaction was elicited by PthG on multiple beet species, whereas a marker exchange mutant of Pag in pthG extended its host range on these beet species. A marker exchange mutant of Pag in hrpJ, encoding a component of the Type III secretion system, prevented HR elicitation. Mutations in each of the hrp regulatory genes (hrpY, hrpS and hrpL) substantially reduced the transcriptional activity of pthG in gypsophila cuttings. PthG could only be detected inside Pag cells during over-expression of hrpS or hrpL. Particle bombardment of GFP-PthG fusion caused cell death in beet, but not in non-host (melon) leaves. Present and previous results have established pthG as a broad-host-range avr gene that functions in multiple host plant species and the first functional avr gene in Pantoea spp.

GacA, the response regulator of a two-component system, acts as a master regulator in Pseudomonas syringae pv. tomato DC3000 by controlling regulatory RNA, transcriptional activators, and alternate sigma factors

Concerted investigations of factors affecting host-pathogen interactions are now possible with the model plant Arabidopsis thaliana and its model pathogen Pseudomonas syringae pv. tomato DC3000, as their whole genome sequences have become available. As a prelude to analysis of the regulatory genes and their targets, we have focused on GacA, the response regulator of a two-component system. The DC3000 gene was cloned by testing for the reversal of phenotypes of an Erwinia GacA- mutant. A GacA- mutant of DC3000 constructed by marker exchange produces much-reduced levels of transcripts of three alternate sigma factors: HrpL, required for the production of effector proteins and their translocation via the type III secretion system; RpoS, required for stress responses and secondary metabolite production; and RpoN, required for an assortment of metabolic processes and expression of hrpL. GacA deficiency also reduces the expression of hrpR and hrpS, which specify enhancer-binding proteins of the NtrC family required for hrpL transcription; ahlI and ahlR, the genes for quorum sensing signal; salA, a regulatory gene known to control virulence; CorS, a sensor kinase; CorR, the cognate response regulator that controls coronatine biosynthetic genes; and rsmB and rsmZ, which specify untranslatable regulatory RNA species. gacA expression itself is regulated by environmental conditions in DC3000, since transcript levels are affected by growth phase and media composition. The observations that high levels of gacA RNA occur in the hrp-inducing medium and GacA deficiency reduces the levels of rpoS expression implicate an important role of GacA in stress responses of DC3000. Consistent with the effects on hrpL expression, the GacA- mutant produces lower levels of transcripts of avr, hrp, and hop genes controlled by HrpL. In addition, GacA deficiency results in reduced levels of transcripts of several HrpL-independent genes. As would be expected, these effects on gene expression cause drastic changes in bacterial behavior: virulence towards A. thaliana and tomato; multiplication in planta; efficiency of the induction of the hypersensitive reaction (HR); production of pigment and N-acyl-homoserine lactone (AHL), the presumed quorum-sensing signal; and swarming motility. Our findings establish that GacA, located at the top in a regulatory cascade in DC3000, functions as a central regulator by controlling an assortment of transcriptional and posttranscriptional factors.

Pathogenicity and other genomic islands in plant pathogenic bacteria

SUMMARY Pathogenicity islands (PAIs) were first described in uropathogenic E. coli. They are now defined as regions of DNA that contain virulence genes and are present in the genome of pathogenic strains, but absent from or only rarely present in non-pathogenic variants of the same or related strains. Other features include a variable G+C content, distinct boundaries from the rest of the genome and the presence of genes related to mobile elements such as insertion sequences, integrases and transposases. Although PAIs have now been described in a wide range of both plant and animal pathogens it has become evident that the general features of PAIs are displayed by a number of regions of DNA with functions other than pathogenicity, such as symbiosis and antibiotic resistance, and the general term genomic islands has been adopted. This review will describe a range of genomic islands in plant pathogenic bacteria including those that carry effector genes, phytotoxins and the type III protein secretion cluster. The review will also consider some medically important bacteria in order to discuss the range, acquisition and stabilization of genomic islands.

Pantoea agglomerans pvs. gypsophilae and betae, recently evolved pathogens?

SUMMARY Pantoea agglomerans pvs. gypsophilae and betae

TAXONOMY

Bacteria; Proteobacteria; gamma subdivision; order Enterobacteriales; family Enterobacteriaceae; species Pantoea agglomerans. Microbiological properties: Gram-negative, non-capsulated, non-spore-forming, predominately motile rode. Disease symptoms: Gall formation at wound sites, mainly in the crown region of the stem. The host range of P. agglomerans pv. gypsophilae is restricted to Gypsophila paniculata, whereas P. agglomerans pv. betae is pathogenic on Beta vulgaris and gypsophila. Disease control: Pathogenic-free transplants and sanitation. No resistant cultivars are available. Major virulence determinants: Pathogenicity plasmid (pPATH), hrp cluster, type III virulence effectors, phytohormones.

The regulatory cascade that activates the Hrp regulon in Erwinia herbicola pv. gypsophilae

The pathogenicity of Erwinia herbicola pv. gypsophilae (Ehg) is dependent on a plasmid (pPATH(Ehg)) that harbors the hrp gene cluster and additional virulence genes. The hrp regulatory cascade of Ehg comprises an hrpXY operon encoding a two-component system; hrpS encoding a transcriptional factor of the NtrC family and hrpL encoding an alternative sigma factor. Results obtained suggest the following signal transduction model for activating the Hrp regulon: phosphorylated HrpY activates hrpS, HrpS activates hrpL, and HrpL activates genes containing "hrp box" promoter. This model was supported by studies on the effects of mutations in the regulatory genes on pathogenicity and complementation analysis. Nonpolar mutations in hrpX did not affect virulence or transcription of downstream genes. Site-directed mutagenesis of the conserved aspartate 57 in HrpY suggested that its phosphorylation is crucial for activating the hrp regulatory cascade. Studies on the effects of mutations in the hrp regulatory genes on transcriptional activity of downstream genes or of their isolated promoters in planta showed dependency of hrpS expression on active HrpY, of hrpL expression on active HrpS, and of hrpN or hrpJ expression on active HrpL. These results were also partially supported by overexpression of regulatory genes under in vitro conditions. The hrpXY is constitutively expressed with high basal levels under repressive conditions, in contrast to hrpS and hrpL, which exhibit low basal expression levels and are environmentally regulated.