Virulence determinants of Pseudomonas syringae strains isolated from grasses in the context of a small type III effector repertoire

Genetic dissection of the tissue-specific roles of type III effectors and phytotoxins in the pathogenicity of Pseudomonas syringae pv. syringae to cherry

When compared with other phylogroups (PGs) of the Pseudomonas syringae species complex, P. syringae pv. syringae (Pss) strains within PG2 have a reduced repertoire of type III effectors (T3Es) but produce several phytotoxins. Effectors within the cherry pathogen Pss 9644 were grouped based on their frequency in strains from Prunus as the conserved effector locus (CEL) common to most P. syringae pathogens; a core of effectors common to PG2; a set of PRUNUS effectors common to cherry pathogens; and a FLEXIBLE set of T3Es. Pss 9644 also contains gene clusters for biosynthesis of toxins syringomycin, syringopeptin and syringolin A. After confirmation of virulence gene expression, mutants with a sequential series of T3E and toxin deletions were pathogenicity tested on wood, leaves and fruits of sweet cherry (Prunus avium) and leaves of ornamental cherry (Prunus incisa). The toxins had a key role in disease development in fruits but were less important in leaves and wood. An effectorless mutant retained some pathogenicity to fruit but not wood or leaves. Striking redundancy was observed amongst effector groups. The CEL effectors have important roles during the early stages of leaf infection and possibly acted synergistically with toxins in all tissues. Deletion of separate groups of T3Es had more effect in P. incisa than in P. avium. Mixed inocula were used to complement the toxin mutations in trans and indicated that strain mixtures may be important in the field. Our results highlight the niche-specific role of toxins in P. avium tissues and the complexity of effector redundancy in the pathogen Pss 9644.

Blue-light perception by epiphytic Pseudomonas syringae drives chemoreceptor expression, enabling efficient plant infection

Adaptation and efficient colonization of the phyllosphere are essential processes for the switch to an epiphytic stage in foliar bacterial pathogens. Here, we explore the interplay among light perception and global transcriptomic alterations in epiphytic populations of the hemibiotrophic pathogen Pseudomonas syringae pv. tomato DC3000 (PsPto) following contact with tomato leaves. We found that blue-light perception by PsPto on leaf surfaces is required for optimal colonization. Blue light triggers the activation of metabolic activity and increases the transcript levels of five chemoreceptors through the function of light oxygen voltage and BphP1 photoreceptors. The inactivation of PSPTO_1008 and PSPTO_2526 chemoreceptors causes a reduction in virulence. Our results indicate that during PsPto interaction with tomato plants, light perception, chemotaxis, and virulence are highly interwoven processes.

Role of Jasmonic Acid Pathway in Tomato Plant-Pseudomonas syringae Interaction

The jasmonic acid pathway has been considered as the backbone of the response against necrotrophic pathogens. However, a hemi-biotrophic pathogen, such as Pseudomonas syringae, has taken advantage of the crosstalk between the different plant hormones in order to manipulate the responses for its own interest. Despite that, the way in which Pseudomonas syringae releases coronatine to activate jasmonic acid-derived responses and block the activation of salicylic acid-mediated responses is widely known. However, the implication of the jasmonic intermediates in the plant-Pseudomonas interaction is not studied yet. In this work, we analyzed the response of both, plant and bacteria using SiOPR3 tomato plants. Interestingly, SiOPR3 plants are more resistant to infection with Pseudomonas. The gene expression of bacteria showed that, in SiOPR3 plants, the activation of pathogenicity is repressed in comparison to wild type plants, suggesting that the jasmonic acid pathway might play a role in the pathogenicity of the bacteria. Moreover, treatments with JA restore the susceptibility as well as activate the expression of bacterial pathogenicity genes. The observed results suggest that a complete jasmonic acid pathway is necessary for the susceptibility of tomato plants to Pseudomonas syringae.

Unraveling the photoactive annihilation mechanism of nanostructures as effective green tools for inhibiting the proliferation of the phytopathogenic bacterium Pseudomonas syringae

The infectious proliferation of phytopathogenic microorganisms depends on a complex sequence of biological events involving host defense, environmental conditions, and chemical and physical interactions between the surface of a plant and microorganisms, which in numerous cases display resistance to conventional microbicides. Among these microorganisms, Pseudomonas syringae (P. syringae) is a Gram-negative bacterium that attacks wounded parts of plants before invading healthy tissues. In order to control P. syringae, considering it to be a phytopathogenic model, an effective method featuring silver nanoparticles (AgNPs) functionalized on titanate nanotubes (Nts) used as photoactive antibacterial agents was investigated to understand the effective photoactive annihilation mechanism. The high dispersion of AgNPs over the Nts boosted charge carrier separation by generating reactive oxygen species (ROS) under visible-light, which stressed the bacteria and enhanced the biocidal effect by quickly preventing the rod-shaped P. syringae bacteria from proliferating. Biological transmission and scanning electron microscopy revealed damaged P. syringae cells that underwent the formation of outer membrane vesicles, caused by photo-assisted annihilation, which is considered to be an indication of a critical defense mechanism. The unusual synergistic properties of the Nts, and their low cost and practical synthesis, made these nanocomposites promising green tools that can positively and swiftly photokill P. syringae within 30 min.

Molecular Evolution of Pseudomonas syringae Type III Secreted Effector Proteins

Diverse Gram-negative pathogens like Pseudomonas syringae employ type III secreted effector (T3SE) proteins as primary virulence factors that combat host immunity and promote disease. T3SEs can also be recognized by plant hosts and activate an effector triggered immune (ETI) response that shifts the interaction back toward plant immunity. Consequently, T3SEs are pivotal in determining the virulence potential of individual P. syringae strains, and ultimately help to restrict P. syringae pathogens to a subset of potential hosts that are unable to recognize their repertoires of T3SEs. While a number of effector families are known to be present in the P. syringae species complex, one of the most persistent challenges has been documenting the complex variation in T3SE contents across a diverse collection of strains. Using the entire pan-genome of 494 P. syringae strains isolated from more than 100 hosts, we conducted a global analysis of all known and putative T3SEs. We identified a total of 14,613 putative T3SEs, 4,636 of which were unique at the amino acid level, and show that T3SE repertoires of different P. syringae strains vary dramatically, even among strains isolated from the same hosts. We also find substantial diversification within many T3SE families, and in many cases find strong signatures of positive selection. Furthermore, we identify multiple gene gain and loss events for several families, demonstrating an important role of horizontal gene transfer (HGT) in the evolution of P. syringae T3SEs. These analyses provide insight into the evolutionary history of P. syringae T3SEs as they co-evolve with the host immune system, and dramatically expand the database of P. syringae T3SEs alleles.

TIR-only protein RBA1 recognizes a pathogen effector to regulate cell death in Arabidopsis

Detection of pathogens by plants is mediated by intracellular nucleotide-binding site leucine-rich repeat (NLR) receptor proteins. NLR proteins are defined by their stereotypical multidomain structure: an N-terminal Toll-interleukin receptor (TIR) or coiled-coil (CC) domain, a central nucleotide-binding (NB) domain, and a C-terminal leucine-rich repeat (LRR). The plant innate immune system contains a limited NLR repertoire that functions to recognize all potential pathogens. We isolated Response to the bacterial type III effector protein HopBA1 (RBA1), a gene that encodes a TIR-only protein lacking all other canonical NLR domains. RBA1 is sufficient to trigger cell death in response to HopBA1. We generated a crystal structure for HopBA1 and found that it has similarity to a class of proteins that includes esterases, the heme-binding protein ChaN, and an uncharacterized domain of Pasteurella multocida toxin. Self-association, coimmunoprecipitation with HopBA1, and function of RBA1 require two previously identified TIR-TIR dimerization interfaces. Although previously described as distinct in other TIR proteins, in RBA1 neither of these interfaces is sufficient when the other is disrupted. These data suggest that oligomerization of RBA1 is required for function. Our identification of RBA1 demonstrates that "truncated" NLRs can function as pathogen sensors, expanding our understanding of both receptor architecture and the mechanism of activation in the plant immune system.