Pseudomonas syringae pv. japonica (Mukoo 1955) Dye et al. 1980 is a junior synonym of Ps. syringae pv. syringae van Hall 1902

An investigation of the biochemical, nutritional and pathogenic reactions of strains of Pseudomonas syringae pv. japonica and Ps. syringae pv. syringae showed them to be indistinguishable. Pseudomonas syringae pv. japonica is a junior synonym of Ps. syringae pv. syringae.     

Pathovar-specific requirement for the Pseudomonas syringae lemA gene in disease lesion formation.

The lemA gene is conserved among strains and pathovars of Pseudomonas syringae. In P. syringae pv. syringae B728a, a causal agent of bacterial brown spot disese of bean, the lemA gene is required for lesion formation on leaves and pods. Using lemA-containing DNA as a probe, we determined that 80 P. syringae pv. syringae strains isolated from bean leaves could be grouped into seven classes based on restriction fragment length polymorphism. Marker exchange mutagenesis showed that the lemA gene was required for lesion formation by representative strains from each restriction fragment length polymorphism class. Hybridization to the lemA locus was detected within six different P. syringae pathovars and within Pseudomonas aeruginosa. Interestingly, a lemA homolog was present and functional within the nonpathogenic strain P. syringae Cit7. We cloned a lemA homolog from a genomic library of P. syringae pv. phaseolicola NPS3121, a causal agent of halo blight of bean, that restored lesion formation to a P. syringae pv. syringae lemA mutant. However, a lemA mutant P. syringae pv. phaseolicola strain retained the ability to produce halo blight disease symptoms on bean plants. Therefore, the lemA gene played an essential role in disease lesion formation by P. syringae pv. syringae isolates, but was not required for pathogenicity of a P. syringae pv. phaseolicola strain.     

Housekeeping gene sequencing and multilocus variable-number tandem-repeat analysis to identify subpopulations within Pseudomonas syringae pv. maculicola and Pseudomonas syringae pv. tomato that correlate with host specificity

Pseudomonas syringae pv. maculicola causes bacterial spot on Brassicaceae worldwide, and for the last 10 years severe outbreaks have been reported in the Loire Valley, France. P. syringae pv. maculicola resembles P. syringae pv. tomato in that it is also pathogenic for tomato and causes the same types of symptoms. We used a collection of 106 strains of P. syringae to characterize the relationships between P. syringae pv. maculicola and related pathovars, paying special attention to P. syringae pv. tomato. Phylogenetic analysis of gyrB and rpoD gene sequences showed that P. syringae pv. maculicola, which causes diseases in Brassicaceae, forms six genetic lineages within genomospecies 3 of P. syringae strains as defined by L. Gardan et al. (Int. J. Syst. Bacteriol. 49[Pt 2]:469-478, 1999), whereas P. syringae pv. tomato forms two distinct genetic lineages. A multilocus variable-number tandem-repeat (VNTR) analysis (MLVA) conducted with eight minisatellite loci confirmed the genetic structure obtained with rpoD and gyrB sequence analyses. These results provide promising tools for fine-scale epidemiological studies on diseases caused by P. syringae pv. maculicola and P. syringae pv. tomato. The two pathovars had distinct host ranges; only P. syringae pv. maculicola strains were pathogenic for Brassicaceae. A subpopulation of P. syringae pv. maculicola strains that are pathogenic for Pto-expressing tomato plants were shown to lack avrPto1 and avrPtoB or to contain a disrupted avrPtoB homolog. Taking phylogenetic and pathological features into account, our data suggest that the DC3000 strain belongs to P. syringae pv. maculicola. This study shows that P. syringae pv. maculicola and P. syringae pv. tomato appear multiclonal, as they did not diverge from a single common ancestral group within the ancestral P. syringae genomospecies 3, and suggests that pathovar specificity within P. syringae may be due to independent genetic events.     

Pseudomonas syringae pv. syringae B728a hydrolyses indole-3-acetonitrile to the plant hormone indole-3-acetic acid

Nitrilase enzymes catalyse the hydrolysis of nitrile compounds to the corresponding carboxylic acid and ammonia, and have been identified in plants, bacteria and fungi. There is mounting evidence to support a role for nitrilases in plant–microbe interactions, but the activity of these enzymes in plant pathogenic bacteria remains unexplored. The genomes of the plant pathogenic bacteria Pseudomonas syringae pv. syringae B728a and Pseudomonas syringae pv. tomato DC3000 contain nitrilase genes with high similarity to characterized bacterial arylacetonitrilases. In this study, we show that the nitrilase of P. syringae pv. syringae B728a is an arylacetonitrilase, which is capable of hydrolysing indole-3-acetonitrile to the plant hormone indole-3-acetic acid, and allows P. syringae pv. syringae B728a to use indole-3-acetonitrile as a nitrogen source. This enzyme may represent an additional mechanism for indole-3-acetic acid biosynthesis by P. syringae pv. syringae B728a, or may be used to degrade and assimilate aldoximes and nitriles produced during plant secondary metabolism. Nitrilase activity was not detected in P. syringae pv. tomato DC3000, despite the presence of a homologous nitrilase gene. This raises the interesting question of why nitrilase activity has been retained in P. syringae pv. syringae B728a and not in P. syringae pv. tomato DC3000.     

Pseudomonas syringae exchangeable effector loci: sequence diversity in representative pathovars and virulence function in P. syringae pv. syringae B728a

Pseudomonas syringae is a plant pathogen whose pathogenicity and host specificity are thought to be determined by Hop/Avr effector proteins injected into plant cells by a type III secretion system. P. syringae pv. syringae B728a, which causes brown spot of bean, is a particularly well-studied strain. The type III secretion system in P. syringae is encoded by hrp (hypersensitive response and pathogenicity) and hrc (hrp conserved) genes, which are clustered in a pathogenicity island with a tripartite structure such that the hrp/hrc genes are flanked by a conserved effector locus and an exchangeable effector locus (EEL). The EELs of P. syringae pv. syringae B728a, P. syringae strain 61, and P. syringae pv. tomato DC3000 differ in size and effector gene composition; the EEL of P. syringae pv. syringae B728a is the largest and most complex. The three putative effector proteins encoded by the P. syringae pv. syringae B728a EEL--HopPsyC, HopPsyE, and HopPsyV--were demonstrated to be secreted in an Hrp-dependent manner in culture. Heterologous expression of hopPsyC, hopPsyE, and hopPsyV in P. syringae pv. tabaci induced the hypersensitive response in tobacco leaves, demonstrating avirulence activity in a nonhost plant. Deletion of the P. syringae pv. syringae B728a EEL strongly reduced virulence in host bean leaves. EELs from nine additional strains representing nine P. syringae pathovars were isolated and sequenced. Homologs of avrPphE (e.g., hopPsyE) and hopPsyA were particularly common. Comparative analyses of these effector genes and hrpK (which flanks the EEL) suggest that the EEL effector genes were acquired by horizontal transfer after the acquisition of the hrp/hrc gene cluster but before the divergence of modern pathovars and that some EELs underwent transpositions yielding effector exchanges or point mutations producing effector pseudogenes after their acquisition.     

Differences between Pseudomonas syringae pv. syringae B728a and Pantoea agglomerans BRT98 in epiphytic and endophytic colonization of leaves

The leaf colonization strategies of two bacterial strains were investigated. The foliar pathogen Pseudomonas syringae pv. syringae strain B728a and the nonpathogen Pantoea agglomerans strain BRT98 were marked with a green fluorescent protein, and surface (epiphytic) and subsurface (endophytic) sites of bean and maize leaves in the laboratory and the field were monitored to see if populations of these strains developed. The populations were monitored using both fluorescence microscopy and counts of culturable cells recovered from nonsterilized and surface-sterilized leaves. The P. agglomerans strain exclusively colonized epiphytic sites on the two plant species. Under favorable conditions, the P. agglomerans strain formed aggregates that often extended over multiple epidermal cells. The P. syringae pv. syringae strain established epiphytic and endophytic populations on asymptomatic leaves of the two plant species in the field, with most of the P. syringae pv. syringae B728a cells remaining in epiphytic sites of the maize leaves and an increasing number occupying endophytic sites of the bean leaves in the 15-day monitoring period. The epiphytic P. syringae pv. syringae B728a populations appeared to originate primarily from multiplication in surface sites rather than from the movement of cells from subsurface to surface sites. The endophytic P. syringae pv. syringae B728a populations appeared to originate primarily from inward movement through the stomata, with higher levels of multiplication occurring in bean than in maize. A rainstorm involving a high raindrop momentum was associated with rapid growth of the P. agglomerans strain on both plant species and with rapid growth of both the epiphytic and endophytic populations of the P. syringae pv. syringae strain on bean but not with growth of the P. syringae pv. syringae strain on maize. These results demonstrate that the two bacterial strains employed distinct colonization strategies and that the epiphytic and endophytic population dynamics of the pathogenic P. syringae pv. syringae strain were dependent on the plant species, whereas those of the nonpathogenic P. agglomerans strain were not.     

Naturally occurring nonpathogenic isolates of the plant pathogen Pseudomonas syringae lack a type III secretion system and effector gene orthologues

Pseudomonas syringae causes plant diseases, and the main virulence mechanism is a type III secretion system (T3SS) that translocates dozens of effector proteins into plant cells. Here we report the existence of a subgroup of P. syringae isolates that do not cause disease on any plant species tested. This group is monophyletic and most likely evolved from a pathogenic P. syringae ancestor through loss of the T3SS. In the nonpathogenic isolate P. syringae 508 the genomic region that in pathogenic P. syringae strains contains the hrp-hrc cluster coding for the T3SS and flanking effector genes is absent. P. syringae 508 was also surveyed for the presence of effector orthologues from the closely related pathogenic strain P. syringae pv. syringae B728a, but none were detected. The absence of the hrp-hrc cluster and effector orthologues was confirmed for other nonpathogenic isolates. Using the AvrRpt2 effector as reporter revealed the inability of P. syringae 508 to translocate effectors into plant cells. Adding a plasmid-encoded T3SS and the P. syringae pv. syringae 61 effector gene hopA1 increased in planta growth almost 10-fold. This suggests that P. syringae 508 supplemented with a T3SS could be used to determine functions of individual effectors in the context of a plant infection, avoiding the confounding effect of other effectors with similar functions present in effector mutants of pathogenic isolates.     

A plant growth-promoting pseudomonad is closely related to the Pseudomonas syringae complex of plant pathogens

Pseudomonas putida GR12-2 is well known as a plant growth-promoting rhizobacterium; however, phylogenetic analysis using the 16S rRNA gene and four housekeeping genes indicated that this strain forms a monophyletic group with the Pseudomonas syringae complex, which is composed of several species of plant pathogens. On the basis of these sequence analyses, we suggest that P. putida GR12-2 be redesignated as P. syringae GR12-2. To compare the ecological roles of P. syringae GR12-2 with its close relatives P. syringae pathovar (pv.) tomato DC3000 and P. syringae pv. syringae B728a, we investigated their ability to cause disease and promote plant growth. When introduced on tobacco or tomato leaves, P. syringae GR12-2 was unable to elicit a hypersensitive response or cause disease, which are characteristic responses of P. syringae DC3000 and B728a, nor were type III secretion system genes required for virulence detected in P. syringae GR12-2 by PCR or DNA hybridization. In contrast to P. syringae GR12-2, neither of the phytopathogens was able to promote root growth when inoculated onto canola seeds. Although commensals and nonpathogens have been reported among the strains of the P. syringae complex, P. syringae GR12-2 is a mutualist and a phytostimulator.     

Detection of tabtoxin-producing strains of Pseudomonas syringae by PCR

AIMS: The present study describes a system based on PCR to distinguish tabtoxin-producing strains of Pseudomonas syringae from other Ps. syringae plant pathogens that produce chlorosis-inducing phytotoxins. METHODS AND RESULTS: Thirty-two strains of Ps. syringae and related species were examined. Two sets of PCR primers were developed to amplify genes (tblA and tabA) required for tabtoxin production. Only a PCR product of 829 bp or 1020 bp was produced in PCR reactions with the tblA or tabA primer sets, respectively, and cells from tabtoxin-producing pathovars of Pseudomonas syringae. All known non-tabtoxin producing bacterial species failed to produce an amplification product with either primer set. CONCLUSIONS: PCR of genes required for tabtoxin production is a simple, rapid and reliable method for identifying tabtoxin-producing strains of Ps. syringae. SIGNIFICANCE AND IMPACT OF THE STUDY: The protocol can effectively distinguish tabtoxin-producing strains of Ps. syringae from other Ps. syringae pathovars and Ps. syringae pv. tabaci strains from other tabtoxin-producing Ps. syringae pathovars.     

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

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

Competitive Exclusion of Epiphytic Bacteria by IcePseudomonas syringae Mutants

The growth of ice nucleation-active and near-isogenic ice nucleation-deficient (Ice) Pseudomonas syringae strains coexisting on leaf surfaces was examined to determine whether competition was sufficient to account for antagonism of phylloplane bacteria. The ice nucleation frequency spectra of 46 IceP. syringae mutants, obtained after mutagenesis with ethyl methanesulfonate, differed both quantitatively and qualitatively, but the mutants could be grouped into four distinct phenotypic classes. The numbers of ice nucleation-active bacteria and ice nuclei active at -5 degrees C were reduced on plants colonized with IceP. syringae mutant strains before challenge inoculations with an IceP. syringae wild-type strain. Frost injury to plants pretreated with IceP. syringae strains was also reduced significantly compared with that to control plants and was correlated with the population size of the IceP. syringae strain and with the numbers of ice nuclei active at -5 degrees C. An IceP. syringae strain colonized leaves, flowers, and young fruit of pears in field experiments and significantly reduced the colonization of these tissues by IceP. syringae strains and Erwinia amylovora as compared with untreated trees.     

Functional analysis of the Pseudomonas syringae rulAB determinant in tolerance to ultraviolet B (290-320 nm) radiation and distribution of rulAB among P. syringae pathovars

The effect of the plasmid-encoded rulAB (resistance to ultraviolet radiation) determinant on responses of Pseudomonas syringae to ultraviolet-B (UV-B) radiation and the distribution of rulAB among pathovars of P. syringae were determined. The cloned rulAB determinant and the native rulAB + plasmid pPSR1 both conferred approximately a 10-fold increase in survival on P. syringae pv. syringae FF5 following increasing doses of UV-B radiation. rulAB + P. syringae strains also maintained significantly larger epiphytic populations on leaf surfaces irradiated with UV-B. rulAB -insertional mutants, constructed in two native rulAB + strains, were from 10- to 100-fold more sensitive to UV-B radiation. The UV tolerance phenotype and the rulAB genes were widely distributed among P. syringae pathovars isolated from varied plant hosts throughout the world and within a broad range of genotypic backgrounds of P. syringae pv. syringae. With one exception, the rulAB determinant was harboured on pPT23A-like plasmids; these replicons are indigenous residents of the species P. syringae and also tend to encode determinants of importance in host–pathogen interactions.     

Ethylene Production by Pseudomonas syringae Pathovars In Vitro and In Planta

Significant amounts of ethylene were produced by Pseudomonas syringae pv. glycinea, pv. phaseolicola (which had been isolated from viny weed Pueraria lobata [Willd.] Ohwi [common name, kudzu]), and pv. pisi in synthetic medium. On the other hand, the bean strains of P. syringae pv. phaseolicola and strains of 17 other pathovars did not produce ethylene. P. syringae pv. glycinea and P. syringae pv. phaseolicola produced nearly identical levels of ethylene (about 5 x 10(sup-7) nl h(sup-1) cell(sup-1)), which were about 10 times higher than the ethylene level of P. syringae pv. pisi. Two 22-bp oligonucleotide primers derived from the ethylene-forming enzyme (efe) gene of P. syringae pv. phaseolicola PK2 were investigated for their ability to detect ethylene-producing P. syringae strains by PCR analysis. PCR amplification with this primer set resulted in a specific 0.99-kb fragment in all ethylene-producing strains with the exception of the P. syringae pv. pisi strains. Therefore, P. syringae pv. pisi may use a different biosynthetic pathway for ethylene production or the sequence of the efe gene is less conserved in this bacterium. P. syringae pv. phaseolicola isolated from kudzu and P. syringae pv. glycinea also produced ethylene in planta. It could be shown that the enhanced ethylene production in diseased tissue was due to the production of ethylene by the inoculated bacteria. Ethylene production in vitro and in planta was strictly growth associated.     

Production and comparison of peptide siderophores from strains of distantly related pathovars of Pseudomonas syringae and Pseudomonas viridiflava LMG 2352

The production of peptide siderophores and the variation in siderophore production among strains of Pseudomonas syringae and Pseudomonas viridiflava were investigated. An antibiose test was used to select a free amino acid-containing agar medium favorable for production of fluorescent siderophores by two P. syringae strains. A culture technique in which both liquid and solid asparagine-containing culture media were used proved to be reproducible and highly effective for inducing production of siderophores in a liquid medium by the fluorescent Pseudomonas strains investigated. Using asparagine as a carbon source appeared to favor siderophore production, and relatively high levels of siderophores were produced when certain amino acids were used as the sole carbon and energy sources. Purified chelated siderophores of strains of P. syringae pv. syringae, P. syringae pv. aptata, P. syringae pv. morsprunorum, P. syringae pv. tomato, and P. viridiflava had the same amino acid composition and spectral characteristics and were indiscriminately used by these strains. In addition, nonfluorescent strains of P. syringae pv. aptata and P. syringae pv. morsprunorum were able to use the siderophores in biological tests. Our results confirmed the proximity of P. syringae and P. viridiflava; siderotyping between pathovars of P. syringae was not possible. We found that the spectral characteristics of the chelated peptide siderophores were different from the spectral characteristics of typical pyoverdins. Our results are discussed in relation to the ecology of the organisms and the conditions encountered on plant surfaces.     

Biological and molecular detection of toxic lipodepsipeptide-producing pseudomonas syringae strains and PCR identification in plants

Toxin-based identification procedures are useful for differentiating Pseudomonas syringae pathovars. A biological test on peptone-glucose-NaCl agar in which the yeast Rhodotorula pilimanae was used proved to be more reliable for detecting lipodepsipeptide-producing strains of P. syringae than the more usual test on potato dextrose agar in which Geotrichum candidum is used. A PCR test performed with primers designed to amplify a 1, 040-bp fragment in the coding sequence of the syrD gene, which was assumed to be involved in syringomycin and syringopeptin secretion, efficiently detected the gene in pathovars that produce the lipodepsipeptides. Comparable results were obtained in both tests performed with strains of the syringomycin-producing organisms P. syringae pv. syringae, P. syringae pv. atrofaciens, and P. syringae pv. aptata, but the PCR test failed with a syringotoxin-producing Pseudomonas fuscovaginae strain. The specificity of the test was verified by obtaining negative PCR test results for related pathovars or species that do not produce the toxic lipodepsipeptides. P. syringae pv. syringae was detected repeatedly in liquid medium inoculated with diseased vegetative tissue and assayed by the PCR test. Our procedure was also adapted to detect P. syringae pv. morsprunorum with a cfl gene-based PCR test.     

62-kb Plasmids Harboring <Emphasis Type="Italic">rulAB</Emphasis> Homologues Confer UV-tolerance and Epiphytic Fitness to <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">syringae</Emphasis> Mango Isolates

The presence of genetic determinants homologous to rulAB genes for ultraviolet (UV) radiation resistance was determined in a collection of Pseudomonas syringae pv. syringae strains isolated from mango. The potential role of these plasmids in UV tolerance and ecological fitness in the mango phyllosphere was also evaluated. Nearly all of the 62-kb plasmids present in the P. syringae pv. syringae strains hybridized with a rulAB probe, but these 62-kb plasmids showed differences in restriction patterns. In vitro assays of tolerance to UV radiation of P. syringae pv. syringae strains showed a higher survival of the strains harboring the 62-kb plasmids compared to strains lacking plasmids when exposed to UVC or UVA+B fractions. Similar results were observed when transconjugants harboring the 62-kb plasmid were tested. Survival assays were carried out under field conditions, and a higher survival of P. syringae pv. syringae strains harboring 62-kb plasmids under direct solar radiation on the adaxial surface of leaves was also observed. When the assays were carried out in shady areas or on the abaxial surface of leaves, survival time was comparable for all the assayed strains, whether or not they contained a 62-kb plasmid hybridizing to rulAB. Our results indicate that P. syringae pv. syringae strains harboring 62-kb plasmids show an increase in ecological fitness when colonizing the mango phyllosphere.     

Ecological Similarity and Coexistence of Epiphytic Ice-Nucleating (Ice) Pseudomonas syringae Strains and a Non-Ice-Nucleating (Ice) Biological Control Agent

De Wit replacement series were used to study competitive interactions between epiphytic IcePseudomonas syringae strains and the biological frost control agents IceP. syringae TLP2del1 and Pseudomonas fluorescens A506. Mixtures containing two strains in different proportions but at a constant total population size were inoculated onto potato leaves. The population sizes of each strain and the total population size were determined when the community had reached equilibrium. A near-isogenic P. syringae strain pair exhibited an interaction similar to that expected for strains competing equally for limiting environmental resources. Replacement series with nonisogenic Ice and IceP. syringae strain pairs suggested that these strains competed for limiting resources according to their relative competitive abilities. There was no evidence of any niche differentiation between the IceP. syringae strains and the IceP. syringae strain. The growth responses of epiphytes following addition of nutrients to the phyllosphere indicated that the epiphytic P. syringae populations were nutrient limited and that, under growth chamber conditions, the populations were more limited by the availability of carbon than by the availability of nitrogen. Determination of in vitro carbon source utilization profiles provided further evidence for the lack of niche differentiation between the Ice and the IceP. syringae strains. Niche overlap indices calculated for the IceP. syringae strains with respect to IceP. syringae TLP2del1 were uniformly high, indicating ecological similarity, and were consistent with the observed low level of coexistence. The biological frost control agent P. fluorescens A506 replaced P. syringae. This was correlated with a high degree of niche overlap between these species.     

Molecular cloning of a Pseudomonas syringae pv. syringae gene cluster that enables Pseudomonas fluorescens to elicit the hypersensitive response in tobacco plants.

A cosmid clone isolated from a genomic library of Pseudomonas syringae pv. syringae 61 restored to all Tn5 mutants of this strain studied the ability to elicit the hypersensitive response (HR) in tobacco. Cosmid pHIR11 also enabled Escherichia coli TB1 to elicit an HR-like reaction when high levels of inoculum (10(9) cells per ml) were infiltrated into tobacco leaves. The cosmid, which contains a 31-kilobase DNA insert, was mobilized by triparental matings into Pseudomonas fluorescens 55 (a nonpathogen that normally causes no plant reactions), P. syringae pv. syringae 226 (a tomato pathogen that causes the HR in tobacco), and P. syringae pv. tabaci (a tobacco pathogen that causes the HR in tomato). The plant reaction phenotypes of all of the transconjugants were altered. P. fluorescens(pHIR11) caused the HR in tobacco and tomato leaves and stimulated an apparent proton influx in suspension-cultured tobacco cells that was indistinguishable from the proton influx caused by incompatible pathogenic pseudomonads. P. syringae pv. tabaci(pHIR11) and P. syringae pv. syringae 226(pHIR11) elicited the HR rather than disease symptoms on their respective hosts and were no longer pathogenic. pHIR11 was mutagenized with TnphoA (Tn5 IS50L::phoA). One randomly chosen mutant, pHIR11-18, no longer conferred the HR phenotype to P. fluorescens. The mutation was marker-exchanged into the genomes of P. syringae pv. syringae strains 61 and 226. The TnphoA insertions in the two pseudomonads abolished their ability to elicit any plant reactions in all plants tested. The results indicate that a relatively small portion of the P. syringae genome is sufficient for the elicitation of plant reactions.     

Genetic diversity and relationships of two pathovars of Pseudomonas syringae

To determine genetic relationships within and between two pathovars of Pseudomonas syringae, strains typical of P. syringae pv. tomato (P. s. tomato) and selected strains of P. syringae pv. syringae (P. s. syringae) were characterized by three methods. DNA-DNA hybridization experiments showed that strains of P. s. tomato and P. s. syringae were, respectively, 86-100% and 37-47% homologous to DNA from a P. s. tomato reference strain when tested under stringent conditions. An analysis of electrophoretic variation in enzymes encoded by 26 loci placed 17 P. s. tomato strains studied in a group of four electrophoretic types, and these strains had a mean genetic diversity per locus of 0.076. Six P. s. syringae strains formed a second group of six electrophoretic types, which had a higher mean genetic diversity per locus of 0.479. The mean genetic distance separating P. s. tomato from P. s. syringae (D = 0.94) was unexpectedly large for strains of a single species. An analysis of restriction fragment length polymorphisms (RFLPs) with three cloned hybridization probes demonstrated that each of the P. s. tomato and P. s. syringae strains was unique. A method was developed to quantify the RFLP difference between pairs of strains, and cluster analysis revealed relationships among P. s. tomato, but not among P. s. syringae, that were similar to those based on enzyme polymorphisms. Implications of these findings for bacterial systematics and epidemiology are discussed.