Pseudomonas Syringae Strains Research Articles

Decomposition of dynamic transcriptomic responses during effector-triggered immunity reveals conserved responses in two distinct plant cell populations

Rapid plant immune responses in the appropriate cells are needed for effective defense against pathogens. Although transcriptome analysis is often used to describe overall immune responses, collection of transcriptome data with sufficient resolution in both space and time is challenging. We reanalyzed public Arabidopsis time-course transcriptome data obtained after low-dose inoculation with a Pseudomonas syringae strain expressing the effector AvrRpt2, which induces effector-triggered immunity in Arabidopsis. Double-peak time-course patterns are prevalent among thousands of upregulated genes. We implemented a multi-compartment modeling approach to decompose the double-peak pattern into two single-peak patterns for each gene. The decomposed peaks reveal an “echoing” pattern: the peak times of the first and second peaks correlate well across most upregulated genes. We demonstrated that the two peaks likely represent responses of two distinct cell populations that respond either cell autonomously or indirectly to AvrRpt2. Thus, the peak decomposition has extracted spatial information from the time-course data. The echoing pattern also indicates a conserved transcriptome response with different initiation times between the two cell populations despite different elicitor types. A gene set highly overlapping with the conserved gene set is also upregulated with similar kinetics during pattern-triggered immunity. Activation of a WRKY network via different entry-point WRKYs can explain the similar but not identical transcriptome responses elicited by different elicitor types. We discuss potential benefits of the properties of the WRKY activation network as an immune signaling network in light of pressure from rapidly evolving pathogens.

Investigating the biosynthesis and roles of the auxin phenylacetic acid during Pseudomonas syringae-Arabidopsis thaliana pathogenesis.

Several plant-associated microbes synthesize the auxinic plant growth regulator phenylacetic acid (PAA) in culture; however, the role of PAA in plant-pathogen interactions is not well understood. In this study, we investigated the role of PAA during interactions between the phytopathogenic bacterium Pseudomonas syringae strain PtoDC3000 (PtoDC3000) and the model plant host, Arabidopsis thaliana. Previous work demonstrated that indole-3-acetaldehyde dehydrogenase A (AldA) of PtoDC3000 converts indole-3-acetaldehyde (IAAld) to the auxin indole-3-acetic acid (IAA). Here, we further demonstrated the biochemical versatility of AldA by conducting substrate screening and steady-state kinetic analyses, and showed that AldA can use both IAAld and phenylacetaldehyde as substrates to produce IAA and PAA, respectively. Quantification of auxin in infected plant tissue showed that AldA-dependent synthesis of either IAA or PAA by PtoDC3000 does not contribute significantly to the increase in auxin levels in infected A. thaliana leaves. Using available arogenate dehydratase (adt) mutant lines of A. thaliana compromised for PAA synthesis, we observed that a reduction in PAA-Asp and PAA-Glu is correlated with elevated levels of IAA and increased susceptibility. These results provide evidence that PAA/IAA homeostasis in A. thaliana influences the outcome of plant-microbial interactions.

In Vivo Production of dsRNA Using Bacteriophage ϕ6 in Pseudomonas syringae Cit7 Cells.

RNA interference (RNAi), also known as post-transcriptional gene silencing (PTGS), is one of the emerging genetic engineering techniques to effectively silence or inhibit the expression of target genes. This chapter describes a method for in vivo production of dsRNA in non-pathogenic Pseudomonas syringae strains using phage ϕ6 RNA-dependent RNA polymerase, extraction and purification of dsRNA from bacterial solution, and the use of dsRNA to induce silencing of green fluorescent protein (GFP) in transgenic Nicotiana benthamiana.

The genetic architecture of the adaptive potential of Arabidopsis thaliana in response to Pseudomonas syringae strains isolated from south‐west France

AbstractPhytopathogens are a threat for global food production and security. Emergence or re‐emergence of plant pathogens is highly dependent on the environmental conditions affecting pathogen spread and survival. Under climate change, a geographic expansion of pathogen distribution poleward has been observed, potentially resulting in disease outbreaks on crops and wild plants. Therefore, estimating the adaptive potential of plants to novel epidemics and describing the underlying genetic architecture is a primary need to propose agricultural management strategies reducing pathogen outbreaks and to breed novel plant cultivars adapted to pathogens that might spread under climate change. To address this challenge, we inoculated Pseudomonas syringae strains isolated from Arabidopsis thaliana populations from south‐west of France on the highly genetically polymorphic TOU‐A A. thaliana population from north‐east France. While no adaptive potential was identified in response to most P. syringae strains, the TOU‐A population displayed a variable disease response to the JACO‐CL strain belonging to the P. syringae phylogroup 7 (PG7). This strain carried a reduced type III secretion system (T3SS) characteristic of the PG7 as well as flexible genomic traits and potential novel effectors. Genome‐wide association mapping on 192 TOU‐A accessions revealed a polygenic architecture of disease response to JACO‐CL. The main quantitative trait locus (QTL) region encompasses two R genes and the AT5G18310 gene encoding ubiquitin hydrolase, a target of the AvrRpt2 P. syringae effector. Altogether, our results pave the way for a better understanding of the genetic and molecular basis of the adaptive potential in an ecologically relevant A. thaliana–P. syringae pathosystem.

Characterization of Pseudomonas syringae strains associated with soybean bacterial blight and in vitro inhibitory effect of oregano and thyme essential oils

Pseudomonas savastanoi pv. glycinea is the causative agent of bacterial blight in soybean. Usually, the control of phytopathogenic bacteria is carried out using toxic pesticides for the environment, which generate rapid bacterial resistance. For this reason, it is suggested to do crop rotation and use certified seeds. New natural compounds are being studied as an alternative to avoid these diseases, like essential oils (EO) which have demonstrated antimicrobial activity. This work aimed to study the antimicrobial activity of EO from Origanum vulgare and Thymus vulgaris against Pseudomonas syringae isolated from soybean bacterial blight in the southern area of Córdoba province. In this study, 14 isolates from Coronel Moldes and General Cabrera fields were identified as P. syringae Group I of LOPAT. The strains P. syringae M3-C1 and P. syringae M7-C1 were identified at the species level by housekeeping genes. EO were obtained by hydro-distillation from thyme and oregano plants. The minimum inhibitory concentration (MIC) was carried out by the broth microdilution technique, determining that both EO could inhibit the isolated soybean strains presenting variations in the active concentrations. The bactericidal activity was also assayed (MBC). These EO were also able to inhibit the biofilm formation of the isolates at sub-inhibitory concentrations. These preliminary studies demonstrated the antimicrobial activity of these EO and their potential use as biopesticides; therefore, future studies will be carried out to develop formulations of these compounds.

Characterization of causal agents of bacterial canker on apricot plantations and risk mapping using GIS in Aras Basin (Türkiye)

AbstractBacterial canker of stone fruits caused by Pseudomonas syringae pv. syringae (Pss) and Pseudomonas syringae pv. morsprunorum (Psm race‐1/Psm race‐2) may lead to significant yield and crop losses in apricot (Prunus armeniaca L.) cultivation areas in Türkiye. Strains pathogenic to apricot were isolated from trees with symptoms (mainly necrotized buds and dieback) of bacterial canker in orchards in Aras Basin. Pathogens were characterized using pathogenicity tests, phenotypic assays, end‐point PCR and multilocus sequence analysis (MLSA). Fifteen Pseudomonas syringae strains were isolated from 205 plant samples collected from apricot orchards showing symptoms of bacterial canker. As a consequence of the diagnostic tests, all isolates were identified as P. syringae pv. syringae. In this study, Pss, Psm R1 and Psm R2 strains in stone fruits were separated into different phylogroups (Pg‐2, Pg‐3, sPg‐1b) based on MLSA. Turkish strains obtained from stone fruits, particularly apricot, showed genetic heterogeneity, and clustered in different sub‐phylogroups (sPg‐2b, sPg‐2c, sPg‐2d). All these strains except strain K258 are also clustered in the same sub‐phylogroups (sPg‐2b and sPg‐2d) with other strains from different countries especially Iran, Lebanon, etc. Strain K258 isolated from apricot was clustered in sPg‐2c with Pss strain 642 (USA). The risk of bacterial canker disease in apricot growing areas is considered using GIS in this study. It was determined that a significant part of the Iğdır Plain, the biggest agricultural area in the Aras Basin, is at very high risk.

VIRULENCE OF PSEUDOMONAS SYRINGAE STRAINS FROM SORIZ (SORGHUM ORYSOIDUM) AND THEIR SPECIALIZATION

Objective. To study the virulence properties of Pseudomonas syringae strains isolated from soriz and various representatives of sorghum crops in Ukraine and to establish the sensitivity range of plants to these pathogens. Methods. Ten strains freshly isolated from soriz (Sorghum orysoidum) were used to study virulence properties, namely: Pseudomonas syringae 210342, 211141a, 211141, 210341, 21034, 210541b, 210541, and Pseudomonas sp. 210521, 21052, 21054, pathogenic strains of sorghum crops from the collection of the Department of Phytopathogenic Bacteria of D. K. Zabolotny Institute of Microbiology and Virology of the National Academy of Sciences of Ukraine and collection strains of phytopathogens that can be potential causative agents of bacterial diseases of soriz or are causative agents of diseases of agricultural crops that can be used in crop rotation. The virulence properties of strains isolated from sorghum were determined on plants of the genus Sorghum. To establish the range of crops sensitive to these pathogens, a number of plants belonging to other genera and families, both cultivated and weed, were tested. Artificial inoculation was carried out in the greenhouse and in the field as injection of bacterial suspension of 1×109 CFU/mL into the leaf surface and stem at all phases of the development of soriz and sorghum cultures. Accounting for artificial contamination was carried out in 7 to 14 days using a locally modified 6-point scale. Aggressiveness of bacteria was assessed by the size of necrosis that formed at the site of injection of the bacterial suspension. Aggressiveness was calculated as the arithmetic mean of the results. Results. P. syringae strains isolated from soriz were found to be virulent for soriz and sorghum crops, however, heterogeneous in aggressiveness. The most aggressive strains were P. syringae 211141a, 210341, and 21034. Some collection strains, isolated from different cultures, showed higher aggressiveness against sugar and grain sorghum than against soriz and Sudan grass. The studied strains of phytopathogens were found to affect segetal vegetation; the most sensitive to the studied strains were horsetail, field bindweed, goldenrod. They showed high aggressiveness against setaria, ragweed, and common sedge. Strains of P. syringae affecting potato, beet, sunflower and tomato plants were identified. Conclusion. Pseudomonas syringae strains isolated from contaminated sorghum plants are virulent and capable of infecting a wide range of plants, including plants of the sorghum family, other types of agricultural crops, and various types of segetal vegetation. Strains of Pectobacterium carotovorum, Xanthomonas vasicola pv. holcicola, Diskeya chryzanthemy, Pantoea agglomerans can be potential pathogens of sorghum crops. The results indicate that segetal vegetation can serve as a reservoir of bacterial infections and play a role in the transfer of bacterial pathogens.

Antagonistic bacteria against Fusarium spp. isolated from sclerotia of Claviceps gigantea in maize (Zea mays)

Fungal sclerotia house native bacterial populations of interest for biological control of plant pathogenic fungi. The objectives of this research were: i) to explore the bacterial populations associated with Claviceps gigantea sclerotia in maize from six locations in the State of Mexico, ii) to evaluate the in vitro antagonism of these bacterial populations against three species of Fusarium phytopathogens of maize, iii) to molecularly identify the more efficient antagonists and characterize the in vitro production of metabolites. Bacterial populations in the sclerotia were calculated by direct plate count; in vitro antagonism against Fusarium graminearum, F. sublgutinans, and F. verticillioides was evaluated by confrontation in Waksman agar medium. Antagonistic bacteria were identified by partial sequencing of 16S rRNA gene and evaluated in vitro for the production of indole-3 acetic acid, siderophores, lipolytic and proteolytic activity, and mineral phosphate solubilization. The bacterial density ranged from 2.023 to 2.397 Log10UFC g-1 of sclerotia. Twenty-two strains showed in vitro antagonism against at least one Fusarium species and were identified as members of the genera Bacillus, Delftia, Micromonospora, Pseudomonas, Sphingobacterium, Staphylococcus, and Stenotrophomonas. The 22 antagonists showed in vitro lipolytic, proteolytic, siderophore-producing and phosphate-solubilizing activity; only 12 (55%) produced indole-3-acetic acid. Bacillus subtilis (BA1), Pseudomonas syringae (BA2), and Bacillus amyloliquefaciens (BA18) strains were antagonists against the three Fusarium species and produced all the metabolites evaluated.

Inhibitory Potential of Thymus vulgaris Essential Oil against Growth, Biofilm Formation, Swarming, and Swimming in Pseudomonas syringae Isolates

As a follow-up to previous studies, the effects of Thymus vulgaris essential oil on selected virulence factors (growth, sessile cell survival, swimming, swarming, and exopolysaccharide production) were evaluated in phytopathogenic Pseudomonas syringae strains isolated from soybean fields in Argentina; reference strains Pseudomonas savastanoi pv. glycinea B076 and Pseudomonas aeruginosa PAO1. P. syringae are responsible for bacterial blight, a disease that affects crops worldwide. Plant bacterioses are usually treated with antibiotics and copper compounds, which may contribute to the development of resistance in pathogens and damage the environment. For these reasons, eco-friendly alternatives are necessary. Although aromatic plants are a natural source of antimicrobial substances, the effects of these substances on phytopathogenic bacteria remain largely unexplored. Subinhibitory concentrations of the oil significantly reduced the slope and rate of bacterial growth. In addition, biofilm and exopolysaccharide (EPS) production were inhibited, with swimming and swarming motility patterns being affected at all of the oil concentrations tested. Therefore, TEO could potentially be a highly efficient antipseudomonal agent for treating plant infections caused by P. syringae.

The enzymatic hydrolysate of fucoidan from Sargassum hemiphyllum triggers immunity in plants

Fucoidans are polysaccharides that consist predominantly of sulfated L-fucoses, from which, fucoidan oligosaccharides (FOSs) are prepared through different methods. Fucoidan has versatile physiological activities, like antiviral functions against SARS CoV-2 and bioactivitiy in enhancing immune responses. Although fucoidan or FOS has been widely used in mammals as functional foods and new drugs, its application in plants is still very limited. Moreover, whether fucoidan or its derived hydrolytic products can trigger immune responses in plants remained unknown. In this work, we demonstrate that the fucoidan enzymatic hydrolysate (FEH) prepared from Sargassum hemiphyllum triggers various immune responses, such as ROS production, MAPK activation, gene expression reprogramming, callose deposition, stomatal closure, and plant resistance to the bacterial strain Pseudomonas syringae pv. tomato (Pst) DC3000. Notably, FEH did not induce Arabidopsis root growth inhibition at the concentration used for triggering other immune responses. Our work suggests that EHF can potentially be used as a non-microbial elicitor in agricultural practices to protect plants from pathogen infection.

Sphingolipid Long-Chain Base Signaling in Compatible and Non-Compatible Plant–Pathogen Interactions in Arabidopsis

The chemical diversity of sphingolipids in plants allows the assignment of specific roles to special molecular species. These roles include NaCl receptors for glycosylinositolphosphoceramides or second messengers for long-chain bases (LCBs), free or in their acylated forms. Such signaling function has been associated with plant immunity, with an apparent connection to mitogen-activated protein kinase 6 (MPK6) and reactive oxygen species (ROS). This work used in planta assays with mutants and fumonisin B1 (FB1) to generate varying levels of endogenous sphingolipids. This was complemented with in planta pathogenicity tests using virulent and avirulent Pseudomonas syringae strains. Our results indicate that the surge of specific free LCBs and ceramides induced by FB1 or an avirulent strain trigger a biphasic ROS production. The first transient phase is partially produced by NADPH oxidase, and the second is sustained and is related to programmed cell death. MPK6 acts downstream of LCB buildup and upstream of late ROS and is required to selectively inhibit the growth of the avirulent but not the virulent strain. Altogether, these results provide evidence that a LCB- MPK6- ROS signaling pathway contributes differentially to the two forms of immunity described in plants, upregulating the defense scheme of a non-compatible interaction.

Complete Genome Sequence of the Ice-Nucleation-Active Pseudomonas syringae Strain MUP20, Isolated from Frost-Damaged Wheat (Triticum aestivum cv. Scepter) in Western Australia.

Pseudomonas syringae MUP20 was isolated from Western Australian frost-damaged wheat. The MUP20 complete genome contained a 6,045,198-bp single circular chromosome with a GC content of 59.03%. IMG/M genome annotation identified 5,245 protein-coding genes, 1 of which encoded an ice nucleation protein containing 20 occurrences of a highly repetitive PF00818 domain.

Cooperative virulence via the collective action of secreted pathogen effectors.

Although virulence is typically attributed to single pathogenic strains, here we investigated whether effectors secreted by a population of non-virulent strains could function as public goods to enable the emergence of collective virulence. We disaggregated the 36 type III effectors of the phytopathogenic bacterium Pseudomonas syringae strain PtoDC3000 into a 'metaclone' of 36 coisogenic strains, each carrying a single effector in an effectorless background. Each coisogenic strain was individually unfit, but the metaclone was collectively as virulent as the wild-type strain on Arabidopsis thaliana, suggesting that effectors can drive the emergence of cooperation-based virulence through their public action. We show that independently evolved effector suits can equally drive this cooperative behaviour by transferring the effector alleles native to the strain PmaES4326 into the conspecific but divergent strain PtoDC3000. Finally, we transferred the disaggregated PtoDC3000 effector arsenal into Pseudomonas fluorescens and show that their cooperative action was sufficient to convert this rhizosphere-inhabiting beneficial bacterium into a phyllosphere pathogen. These results emphasize the importance of microbial community interactions and expand the ecological scale at which disease may be attributed.

Pangenomic analysis reveals plant NAD+ manipulation as an important virulence activity of bacterial pathogen effectors

Nicotinamide adenine dinucleotide (NAD+) has emerged as a key component in prokaryotic and eukaryotic immune systems. The recent discovery that Toll/interleukin-1 receptor (TIR) proteins function as NAD+ hydrolases (NADase) links NAD+-derived small molecules with immune signaling. We investigated pathogen manipulation of host NAD+ metabolism as a virulence strategy. Using the pangenome of the model bacterial pathogen Pseudomonas syringae, we conducted a structure-based similarity search from 35,000 orthogroups for type III effectors (T3Es) with potential NADase activity. Thirteen T3Es, including five newly identified candidates, were identified that possess domain(s) characteristic of seven NAD+-hydrolyzing enzyme families. Most Pseudomonas syringae strains that depend on the type III secretion system to cause disease, encode at least one NAD+-manipulating T3E, and many have several. We experimentally confirmed the type III-dependent secretion of a novel T3E, named HopBY, which shows structural similarity to both TIR and adenosine diphosphate ribose (ADPR) cyclase. Homologs of HopBY were predicted to be type VI effectors in diverse bacterial species, indicating potential recruitment of this activity by microbial proteins secreted during various interspecies interactions. HopBY efficiently hydrolyzes NAD+ and specifically produces 2'cADPR, which can also be produced by TIR immune receptors of plants and by other bacteria. Intriguingly, this effector promoted bacterial virulence, indicating that 2'cADPR may not be the signaling molecule that directly initiates immunity. This study highlights a host-pathogen battleground centered around NAD+ metabolism and provides insight into the NAD+-derived molecules involved in plant immunity.

Overexpression of an Antioxidant Enzyme APX1 in cpr5 Mutant Restores its Pleiotropic Growth Phenotype.

Breeding crops with enhanced immunity is an effective strategy to reduce yield loss caused by pathogens. The constitutive expresser of pathogenesis-related genes (cpr5) mutant shows enhanced pathogen resistance but retarded growth; thus, it restricts the application of cpr5 in breeding crops with disease resistance. Reactive oxygen species (ROS) play important roles in plant growth and defense. In this study, we determined that the cpr5 mutant exhibited excessive ROS accumulation. However, the mutation of respiratory burst oxidase homolog D (RBOHD), a reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase responsible for the production of ROS signaling in plant immunity, did not suppress excessive ROS levels in cpr5. Furthermore, the cpr5 mutant showed low levels of ascorbate peroxidase 1 (APX1), an important cytosolic ROS-scavenging enzyme. APX1 overexpression in the cpr5 background removed excessive ROS and restored the pleiotropic growth phenotype. Notably, APX1 overexpression did not reduce the resistance of cpr5 mutant to virulent strain Pseudomonas syringae pv. tomato (Pst) DC3000 and avirulent strain Pst DC3000 (avrRpt2). These results suggest that the removal of excessive ROS by APX1 overexpression restored the cpr5 growth phenotype while conserving pathogen resistance. Hence, our study provides a theoretical and empirical basis for utilizing CPR5 in the breeding of crops with disease resistance by effective oxidative stress management via APX1 expression.

The Arabidopsis thaliana–Streptomyces Interaction Is Controlled by the Metabolic Status of the Holobiont

How specific interactions between plant and pathogenic, commensal, or mutualistic microorganisms are mediated and how bacteria are selected by a plant are important questions to address. Here, an Arabidopsis thaliana mutant called chs5 partially deficient in the biogenesis of isoprenoid precursors was shown to extend its metabolic remodeling to phenylpropanoids and lipids in addition to carotenoids, chlorophylls, and terpenoids. Such a metabolic profile was concomitant to increased colonization of the phyllosphere by the pathogenic strain Pseudomonas syringae pv. tomato DC3000. A thorough microbiome analysis by 16S sequencing revealed that Streptomyces had a reduced colonization potential in chs5. This study revealed that the bacteria-Arabidopsis interaction implies molecular processes impaired in the chs5 mutant. Interestingly, our results revealed that the metabolic status of A. thaliana was crucial for the specific recruitment of Streptomyces into the microbiota. More generally, this study highlights specific as well as complex molecular interactions that shape the plant microbiota.

Biocontrol Potential of Endophytic Plant-Growth-Promoting Bacteria against Phytopathogenic Viruses: Molecular Interaction with the Host Plant and Comparison with Chitosan.

Endophytic plant-growth-promoting bacteria (ePGPB) are interesting tools for pest management strategies. However, the molecular interactions underlying specific biocontrol effects, particularly against phytopathogenic viruses, remain unexplored. Herein, we investigated the antiviral effects and triggers of induced systemic resistance mediated by four ePGPB (Paraburkholderia fungorum strain R8, Paenibacillus pasadenensis strain R16, Pantoea agglomerans strain 255-7, and Pseudomonas syringae strain 260-02) against four viruses (Cymbidium Ring Spot Virus—CymRSV; Cucumber Mosaic Virus—CMV; Potato Virus X—PVX; and Potato Virus Y—PVY) on Nicotiana benthamiana plants under controlled conditions and compared them with a chitosan-based resistance inducer product. Our studies indicated that ePGPB- and chitosan-treated plants presented well-defined biocontrol efficacy against CymRSV and CMV, unlike PVX and PVY. They exhibited significant reductions in symptom severity while promoting plant height compared to nontreated, virus-infected controls. However, these phenotypic traits showed no association with relative virus quantification. Moreover, the tested defense-related genes (Enhanced Disease Susceptibility-1 (EDS1), Non-expressor of Pathogenesis-related genes-1 (NPR1), and Pathogenesis-related protein-2B (PR2B)) implied the involvement of a salicylic-acid-related defense pathway triggered by EDS1 gene upregulation.

Molecular and Genomic Characterization of the Pseudomonas syringae Phylogroup 4: An Emerging Pathogen of Arabidopsis thaliana and Nicotiana benthamiana

Environmental fluctuations such as increased temperature, water availability, and air CO2 concentration triggered by climate change influence plant disease dynamics by affecting hosts, pathogens, and their interactions. Here, we describe a newly discovered Pseudomonas syringae strain found in a natural population of Arabidopsis thaliana collected from the southwest of France. This strain, called Psy RAYR-BL, is highly virulent on natural Arabidopsis accessions, Arabidopsis model accession Columbia 0, and tobacco plants. Despite the severe disease phenotype caused by the Psy RAYR-BL strain, we identified a reduced repertoire of putative Type III virulence effectors by genomic sequencing compared to P. syringae pv tomato (Pst) DC3000. Furthermore, hopBJ1Psy is found exclusively on the Psy RAYR-BL genome but not in the Pst DC3000 genome. The plant expression of HopBJ1Psy induces ROS accumulation and cell death. In addition, HopBJ1Psy participates as a virulence factor in this plant-pathogen interaction, likely explaining the severity of the disease symptoms. This research describes the characterization of a newly discovered plant pathogen strain and possible virulence mechanisms underlying the infection process shaped by natural and changing environmental conditions.

Characterization of Pseudomonas syringae Isolated from Systemic Infection of Zucchini in Australia.

Zucchini plants with symptoms including twisted petioles, necrotic leaves, crown rot, and internal fruit rot were found in Bundaberg, Australia, at a commercial field for the first time during late autumn 2016, resulting in direct yield losses of 70 to 80%. Three Pseudomonas syringae strains isolated from symptomatic leaf (KL004-k1), crown (77-4C), and fruit (KFR003-1) were characterized and their pathogenicity evaluated on pumpkin, rockmelon, squash, and zucchini. Biochemical assays showed typical results for P. syringae. The three isolates differed, however, in that two produced fluorescent pigment (KFR003-1 and 77-4C) whereas the third, KL004-k1, was nonfluorescent. Multilocus sequence analysis classified the isolates to phylogroup 2b. The single-nucleotide polymorphism analysis of core genome from the Australian and closely related international isolates of P. syringae showed two separate clusters. The Australian isolates were clustered based on fluorescent phenotype. Pathogenicity tests demonstrated that all three isolates moved systemically within the inoculated plants and induced necrotic leaf symptoms in zucchini plants. Their identities were confirmed with specific PCR assays for P. syringae and phylogroup 2. Pathogenicity experiments also showed that the Eva variety of zucchini was more susceptible than the Rosa variety for all three isolates. Isolate KL004-k1 was more virulent than 77-4C on pumpkin, rockmelon, squash, and zucchini. This study expands the knowledge of P. syringae isolates that infect cucurbits and provides useful information for growers about the relative susceptibility of a range of cucurbit species.

Diversity, pathogenicity and biocontrol efficacy of Pseudomonas syringae isolated from plants in northern Jordan

Diversity, pathogenicity, and biocontrol efficacy of Pseudomonas syringae strains isolated from a wide range of habitats in Jordan were investigated. Thirty-five infected plant samples (vegetable leaves and woody plant twig) were randomly collected. King's B agar medium was implemented and 124 colonies of P. syringae candidates were selected. Phenotypic, biochemical and pathogenicity tests were further carried out to confirm the identity of P. syringae isolates. The frequency of fluorescent isolates varied between 11- 50 %. Even though all isolates were shown to induce chlorosis of tobacco leaves, only two were able to macerate potato slices causing their rotting. This may be due to variation in secretion of cellulase and pectinase enzymes involve in pathogenicity as clearly discussed in this study. Interestingly, 55% of isolates were found to clearly inhibit growth of E. coli. Exceptional inhibition was noticed in P. syringae isolated from tomato and a possible effect of syringomycin was suggested. Therefore, P. syringae obtained in this study was recommended to have a pivotal role in the biological control system. Results obtained from the experiment of ice nucleation activity revealed that this phenotypic feature dominates woody plants.