Pseudomonas syringae pv. syringae uses proteasome inhibitor syringolin A to colonize from wound infection sites

Decoding the molecular mechanism underlying salicylic acid (SA)-mediated plant immunity: an integrated overview from its biosynthesis to the mode of action

Salicylic acid (SA) is an important phytohormone, well-known for its regulatory role in shaping plant immune responses. In recent years, significant progress has been made in unravelling the molecular mechanisms underlying SA biosynthesis, perception, and downstream signalling cascades. Through the concerted efforts employing genetic, biochemical, and omics approaches, our understanding of SA-mediated defence responses has undergone remarkable expansion. In general, following SA biosynthesis through Avr effectors of the pathogens, newly synthesized SA undergoes various biochemical changes to achieve its active/inactive forms (e.g. methyl salicylate). The activated SA subsequently triggers signalling pathways associated with the perception of pathogen-derived signals, expression of defence genes, and induction of systemic acquired resistance (SAR) to tailor the intricate regulatory networks that coordinate plant immune responses. Nonetheless, the mechanistic understanding of SA-mediated plant immune regulation is currently limited because of its crosstalk with other signalling networks, which makes understanding this hormone signalling more challenging. This comprehensive review aims to provide an integrated overview of SA-mediated plant immunity, deriving current knowledge from diverse research outcomes. Through the integration of case studies, experimental evidence, and emerging trends, this review offers insights into the regulatory mechanisms governing SA-mediated immunity and signalling. Additionally, this review discusses the potential applications of SA-mediated defence strategies in crop improvement, disease management, and sustainable agricultural practices.

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.

Diversity of Strains in the Pseudomonas syringae Complex Causing Bacterial Stem Blight of Alfalfa (Medicago sativa) in the United States

Alfalfa growers in the Intermountain West of the United States have recently seen an increased incidence in bacterial stem blight (BSB), which can result in significant herbage yield losses from the first harvest. BSB has been attributed to Pseudomonas syringae pv. syringae and P. viridiflava; however, little is known about the genetic diversity and pathogenicity of these bacteria or their interaction with alfalfa plants. Here, we present a comprehensive phylogenetic and phenotypic analysis of P. syringae and P. viridiflava strains causing BSB on alfalfa. A multilocus sequence analysis found that they grouped exclusively with P. syringae PG2b and P. viridiflava PG7a. Alfalfa symptoms caused by both bacterial groups were indistinguishable, although there was a large range in mean disease scores for individual strains. Overall, PG2b strains incited significantly greater disease scores than those caused by PG7a strains. Inoculated plants showed browning in the xylem and collapse of epidermal and pith parenchyma cells. Inoculation with a mixture of PG2b and PG7a strains did not result in synergistic activity. The populations of PG2b and PG7a strains were genetically diverse within their clades and did not group by location or haplotype. The PG2b strains had genes for production of the phytotoxin coronatine, which is unusual in PG2b strains. The results indicate that both pathogens are well established on alfalfa across a wide geographic range and that a recent introduction or evolution of more aggressive strains as the basis for emergence of the disease is unlikely.

Characterization of Microbotryum lychnidis-dioicae Secreted Effector Proteins, Their Potential Host Targets, and Localization in a Heterologous Host Plant

Microbotryum lychnidis-dioicae is an obligate fungal species colonizing the plant host, Silene latifolia. The fungus synthesizes and secretes effector proteins into the plant host during infection to manipulate the host for completion of the fungal lifecycle. The goal of this study was to continue functional characterization of such M. lychnidis-dioicae effectors. Here, we identified three putative effectors and their putative host-plant target proteins. MVLG_02245 is highly upregulated in M. lychnidis-dioicae during infection; yeast two-hybrid analysis suggests it targets a tubulin α-1 chain protein ortholog in the host, Silene latifolia. A potential plant protein interacting with MVLG_06175 was identified as CASP-like protein 2C1 (CASPL2C1), which facilitates the polymerization of the Casparian strip at the endodermal cells. Proteins interacting with MVLG_05122 were identified as CSN5a or 5b, involved in protein turnover. Fluorescently labelled MVLG_06175 and MVLG_05122 were expressed in the heterologous plant, Arabidopsis thaliana. MVLG_06175 formed clustered granules at the tips of trichomes on leaves and in root caps, while MVLG_05122 formed a band structure at the base of leaf trichomes. Plants expressing MVLG_05122 alone were more resistant to infection with Fusarium oxysporum. These results indicate that the fungus might affect the formation of the Casparian strip in the roots and the development of trichomes during infection as well as alter plant innate immunity.

Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Methylome Response to Proteasome Inhibition by Pseudomonas syringae Virulence Factor Syringolin A

DNA methylation is an important epigenetic mark required for proper gene expression and silencing of transposable elements. DNA methylation patterns can be modified by environmental factors such as pathogen infection, in which modification of DNA methylation can be associated with plant resistance. To counter the plant defense pathways, pathogens produce effector molecules, several of which act as proteasome inhibitors. Here, we investigated the effect of proteasome inhibition by the bacterial virulence factor syringolin A (SylA) on genome-wide DNA methylation. We show that SylA treatment results in an increase of DNA methylation at centromeric and pericentromeric regions of Arabidopsis chromosomes. We identify several CHH differentially methylated regions (DMRs) that are enriched in the proximity of transcriptional start sites. SylA treatment does not result in significant changes in small RNA composition. However, significant changes in genome transcriptional activity can be observed, including a strong upregulation of resistance genes that are located on chromosomal arms. We hypothesize that DNA methylation changes could be linked to the upregulation of some atypical members of the de novo DNA methylation pathway, namely AGO3, AGO9, and DRM1. Our data suggests that modification of genome-wide DNA methylation resulting from an inhibition of the proteasome by bacterial effectors could be part of an epi-genomic arms race against pathogens. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The Plant Ubiquitin-Proteasome System as a Target for Microbial Manipulation

The plant immune system perceives pathogens to trigger defense responses. In turn, pathogens secrete effector molecules to subvert these defense responses. The initiation and maintenance of defense responses involve not only de novo synthesis of regulatory proteins and enzymes but also their regulated degradation. The latter is achieved through protein degradation pathways such as the ubiquitin-proteasome system (UPS). The UPS regulates all stages of immunity, from the perception of the pathogen to the execution of the response, and, therefore, constitutes an ideal candidate for microbial manipulation of the host. Pathogen effector molecules interfere with the plant UPS through several mechanisms. This includes hijacking general UPS functions or perturbing its ability to degrade specific targets. In this review, we describe how the UPS regulates different immunity-related processes and how pathogens subvert this to promote disease.

Combination of in vivo proximity labeling and co-immunoprecipitation identifies the host target network of a tumor-inducing effector in the fungal maize pathogen Ustilago maydis

Plant pathogens secrete effectors, which target host proteins to facilitate infection. The Ustilago maydis effector UmSee1 is required for tumor formation in the leaf during infection of maize. UmSee1 interacts with maize SGT1 (suppressor of G2 allele of skp1) and blocks its phosphorylation in vivo. In the absence of UmSee1, U. maydis cannot trigger tumor formation in the bundle sheath. However, it remains unclear which host processes are manipulated by UmSee1 and the UmSee1-SGT1 interaction to cause the observed phenotype. Proximity-dependent protein labeling involving the turbo biotin ligase tag (TurboID) for proximal labeling of proteins is a powerful tool for identifying the protein interactome. We have generated transgenic U. maydis that secretes biotin ligase-fused See1 effector (UmSee1-TurboID-3HA) directly into maize cells. This approach, in combination with conventional co-immunoprecipitation, allowed the identification of additional UmSee1 interactors in maize cells. Collectively, our data identified three ubiquitin-proteasome pathway-related proteins (ZmSIP1, ZmSIP2, and ZmSIP3) that either interact with or are close to UmSee1 during host infection of maize with U. maydis. ZmSIP3 represents a cell cycle regulator whose degradation appears to be promoted in the presence of UmSee1. Our data provide a possible explanation of the requirement for UmSee1 in tumor formation during U. maydis-Zea mays interaction.

Robust transcriptional indicators of immune cell death revealed by spatiotemporal transcriptome analyses

Recognition of a pathogen by the plant immune system often triggers a form of regulated cell death traditionally known as the hypersensitive response (HR). This type of cell death occurs precisely at the site of pathogen recognition, and it is restricted to a few cells. Extensive research has shed light on how plant immune receptors are mechanistically activated. However, two central key questions remain largely unresolved: how does cell death zonation take place, and what are the mechanisms that underpin this phenomenon? Consequently, bona fide transcriptional indicators of HR are lacking, which prevents deeper insight into its mechanisms before cell death becomes macroscopic and precludes early or live observation. In this study, to identify the transcriptional indicators of HR we used the paradigmatic Arabidopsis thaliana-Pseudomonas syringae pathosystem and performed a spatiotemporally resolved gene expression analysis that compared infected cells that will undergo HR upon pathogen recognition with bystander cells that will stay alive and activate immunity. Our data revealed unique and time-dependent differences in the repertoire of differentially expressed genes, expression profiles, and biological processes derived from tissue undergoing HR and that of its surroundings. Furthermore, we generated a pipeline based on concatenated pairwise comparisons between time, zone, and treatment that enabled us to define 13 robust transcriptional HR markers. Among these genes, the promoter of an uncharacterized AAA-ATPase was used to obtain a fluorescent reporter transgenic line that displays a strong spatiotemporally resolved signal specifically in cells that will later undergo pathogen-triggered cell death. This valuable set of genes can be used to define cells that are destined to die upon infection with HR-triggering bacteria, opening new avenues for specific and/or high-throughput techniques to study HR processes at a single-cell level.

AgroLux: bioluminescent Agrobacterium to improve molecular pharming and study plant immunity

Agroinfiltration in Nicotiana benthamiana is widely used to transiently express heterologous proteins in plants. However, the state of Agrobacterium itself is not well studied in agroinfiltrated tissues, despite frequent studies of immunity genes conducted through agroinfiltration. Here, we generated a bioluminescent strain of Agrobacterium tumefaciens GV3101 to monitor the luminescence of Agrobacterium during agroinfiltration. By integrating a single copy of the lux operon into the genome, we generated a stable 'AgroLux' strain, which is bioluminescent without affecting Agrobacterium growth in vitro and in planta. To illustrate its versatility, we used AgroLux to demonstrate that high light intensity post infiltration suppresses both Agrobacterium luminescence and protein expression. We also discovered that AgroLux can detect Avr/Cf-induced immune responses before tissue collapse, establishing a robust and rapid quantitative assay for the hypersensitive response (HR). Thus, AgroLux provides a non-destructive, versatile and easy-to-use imaging tool to monitor both Agrobacterium and plant responses.

Plant susceptible responses: the underestimated side of plant-pathogen interactions

Plant susceptibility to pathogens is usually considered from the perspective of the loss of resistance. However, susceptibility cannot be equated with plant passivity since active host cooperation may be required for the pathogen to propagate and cause disease. This cooperation consists of the induction of reactions called susceptible responses that transform a plant from an autonomous biological unit into a component of a pathosystem. Induced susceptibility is scarcely discussed in the literature (at least compared to induced resistance) although this phenomenon has a fundamental impact on plant-pathogen interactions and disease progression. This review aims to summarize current knowledge on plant susceptible responses and their regulation. We highlight two main categories of susceptible responses according to their consequences and indicate the relevance of susceptible response-related studies to agricultural practice. We hope that this review will generate interest in this underestimated aspect of plant-pathogen interactions.

Comprehensive Review of Endophytic Flora from African Medicinal Plants

Many people in different African countries are suffering from different diseases many of which result in serious life threat and public health problems with high risk of infection and mortality. Due to less accessibility and high cost of modern drugs, people of this continent often depend on traditional medicine using medicinal plants to manage the diseases. Africa has large tropical rain forests, which are very rich in medicinal plants. Many of them have been scientifically proven for their medicinal values. These medicinal plants which constitute a large repertoire of endophytes have not been significantly explored for the isolation of these microorganisms and their bioactive secondary metabolites. This review summarizes the research on endophytes isolated from medicinal plants of Africa, their pharmacological potential and some of their biotechnological aspects. Novel compounds reported from endophytes from Africa with their biological activities have also been reviewed. Information documented in this review might serve as starting point for future researches on endophytes in different African countries.

The plant endosphere world - bacterial life within plants

The plant endosphere is colonized by complex microbial communities and microorganisms, which colonize the plant interior at least part of their lifetime and are termed endophytes. Their functions range from mutualism to pathogenicity. All plant organs and tissues are generally colonized by bacterial endophytes and their diversity and composition depend on the plant, the plant organ and its physiological conditions, the plant growth stage as well as on the environment. Plant-associated microorganisms, and in particular endophytes, have lately received high attention, because of the increasing awareness of the importance of host-associated microbiota for the functioning and performance of their host. Some endophyte functions are known from mostly lab assays, genome prediction and few metagenome analyses; however, we have limited understanding on in planta activities, particularly considering the diversity of micro-environments and the dynamics of conditions. In our review, we present recent findings on endosphere environments, their physiological conditions and endophyte colonization. Furthermore, we discuss microbial functions, the interaction between endophytes and plants as well as methodological limitations of endophyte research. We also provide an outlook on needs of future research to improve our understanding on the role of microbiota colonizing the endosphere on plant traits and ecosystem functioning.

Social Evolution and Cheating in Plant Pathogens

Plant pathogens are a critical component of the microbiome that exist as populations undergoing ecological and evolutionary processes within their host. Many aspects of virulence rely on social interactions mediated through multiple forms of public goods, including quorum-sensing signals, exoenzymes, and effectors. Virulence and disease progression involve life-history decisions that have social implications with large effects on both host and microbe fitness, such as the timing of key transitions. Considering the molecular basis of sequential stages of plant-pathogen interactions highlights many opportunities for pathogens to cheat, and there is evidence for ample variation in virulence. Case studies reveal systems where cheating has been demonstrated and others where it is likely occurring. Harnessing the social interactions of pathogens, along with leveraging novel sensing and -omics technologies to understand microbial fitness in the field, will enable us to better manage plant microbiomes in the interest of plant health.

Cherry picking by pseudomonads: After a century of research on canker, genomics provides insights into the evolution of pathogenicity towards stone fruits

Bacterial canker disease is a major limiting factor in the growing of cherry and other Prunus species worldwide. At least five distinct clades within the bacterial species complex Pseudomonas syringae are known to be causal agents of the disease. The different pathogens commonly coexist in the field. Reducing canker is a challenging prospect as the efficacy of chemical controls and host resistance may vary against each of the diverse clades involved. Genomic analysis has revealed that the pathogens use a variable repertoire of virulence factors to cause the disease. Significantly, strains of P. syringae pv. syringae possess more genes for toxin biosynthesis and fewer encoding type III effector proteins. There is also a shared pool of key effector genes present on mobile elements such as plasmids and prophages that may have roles in virulence. By contrast, there is evidence that absence or truncation of certain effector genes, such as hopAB, is characteristic of cherry pathogens. Here we highlight how recent research, underpinned by the earlier epidemiological studies, is allowing significant progress in our understanding of the canker pathogens. This fundamental knowledge, combined with emerging insights into host genetics, provides the groundwork for development of precise control measures and informed approaches to breed for disease resistance.

Inhibition of jasmonate-mediated plant defences by the fungal metabolite higginsianin B

Infection of Arabidopsis thaliana by the ascomycete fungus Colletotrichum higginsianum is characterized by an early symptomless biotrophic phase followed by a destructive necrotrophic phase. The fungal genome contains 77 secondary metabolism-related biosynthetic gene clusters, whose expression during the infection process is tightly regulated. Deleting CclA, a chromatin regulator involved in the repression of some biosynthetic gene clusters through H3K4 trimethylation, allowed overproduction of three families of terpenoids and isolation of 12 different molecules. These natural products were tested in combination with methyl jasmonate, an elicitor of jasmonate responses, for their capacity to alter defence gene induction in Arabidopsis. Higginsianin B inhibited methyl jasmonate-triggered expression of the defence reporter VSP1p:GUS, suggesting it may block bioactive jasmonoyl isoleucine (JA-Ile) synthesis or signalling in planta. Using the JA-Ile sensor Jas9-VENUS, we found that higginsianin B, but not three other structurally related molecules, suppressed JA-Ile signalling by preventing the degradation of JAZ proteins, the repressors of jasmonate responses. Higginsianin B likely blocks the 26S proteasome-dependent degradation of JAZ proteins because it inhibited chymotrypsin- and caspase-like protease activities. The inhibition of target degradation by higginsianin B also extended to auxin signalling, as higginsianin B treatment reduced auxin-dependent expression of DR5p:GUS. Overall, our data indicate that specific fungal secondary metabolites can act similarly to protein effectors to subvert plant immune and developmental responses.

Microbial Effector Proteins - A Journey through the Proteolytic Landscape

In the evolutionary arms race between pathogens and plants, pathogens evolved effector molecules that they secrete into the host to subvert plant cellular responses in a process termed the effector-targeted pathway (ETP). During recent years the repertoire of ETPs has increased and mounting evidence indicates that the proteasome and autophagy pathways are central hubs of microbial effectors. Both degradation pathways are implicated in a broad array of cellular responses and thus constitute an attractive target for effector proteins to have a broader impact on the host. In this article we first summarize recent findings on how effectors from various pathogens modulate proteolytic pathways and then provide a network analysis of established effector targets implicated in proteolytic degradation machineries. With this network we emphasize the idea that effectors targeting proteolytic degradation pathways will affect the protein synthesis-transport and degradation triangle. We put in perspective that, in utilizing the effector diversity of microbes, we produce excellent tools to study diverse cellular pathways and their possible interplay with each other.

Phage resistance evolution in vitro is not reflective of in vivo outcome in a plant-bacteria-phage system

The evolution of resistance to parasites is fundamentally important to disease ecology, yet we remain unable to predict when and how resistance will evolve. This is largely due to the context-dependent nature of host-parasite interactions, as the benefit of resistance will depend on the abiotic and biotic environment. Through experimental evolution of the plant pathogenic bacterium Pseudomonas syringae and two lytic bacteriophages across two different environments (high-nutrient media and the tomato leaf apoplast), we demonstrate that de novo evolution of resistance is negligible in planta despite high levels of resistance evolution in vitro. We find no evidence supporting the evolution of phage-selected resistance in planta despite multiple passaging experiments, multiple assays for resistance, and high multiplicities of infection. Additionally, we find that phage-resistant mutants (evolved in vitro) did not realize a fitness benefit over phage-sensitive cells when grown in planta in the presence of phage, despite reduced growth of sensitive cells, evidence of phage replication in planta, and a large fitness benefit in the presence of phage observed in vitro. Thus, this context-dependent benefit of phage resistance led to different evolutionary outcomes across environments. These results underscore the importance of studying the evolution of parasite resistance in ecologically relevant environments.

Inference of Convergent Gene Acquisition Among Pseudomonas syringae Strains Isolated From Watermelon, Cantaloupe, and Squash

Pseudomonas syringae sensu stricto (phylogroup 2; referred to as P. syringae) consists of an environmentally ubiquitous bacterial population associated with diseases of numerous plant species. Recent studies using multilocus sequence analysis have indicated the clonal expansion of several P. syringae lineages, located in phylogroups 2a and 2b, in association with outbreaks of bacterial spot disease of watermelon, cantaloupe, and squash in the United States. To investigate the evolutionary processes that led to the emergence of these epidemic lineages, we sequenced the genomes of six P. syringae strains that were isolated from cucurbits grown in the United States, Europe, and China over a period of more than a decade, as well as eight strains that were isolated from watermelon and squash grown in six different Florida counties during the 2013 and 2014 seasons. These data were subjected to comparative analyses along with 42 previously sequenced genomes of P. syringae stains collected from diverse plant species and environments available from GenBank. Maximum likelihood reconstruction of the P. syringae core genome revealed the presence of a hybrid phylogenetic group, comprised of cucurbit strains collected in Florida, Italy, Serbia, and France, which emerged through genome-wide homologous recombination between phylogroups 2a and 2b. Functional analysis of the recombinant core genome showed that pathways involved in the ATP-dependent transport and metabolism of amino acids, bacterial motility, and secretion systems were enriched for recombination. A survey of described virulence factors indicated the convergent acquisition of several accessory type 3 secreted effectors (T3SEs) among phylogenetically distinct lineages through integrative and conjugative element and plasmid loci. Finally, pathogenicity assays on watermelon and squash showed qualitative differences in virulence between strains of the same clonal lineage, which correlated with T3SEs acquired through various mechanisms of horizontal gene transfer (HGT). This study provides novel insights into the interplay of homologous recombination and HGT toward pathogen emergence and highlights the dynamic nature of P. syringae sensu lato genomes.

Built to bind: biosynthetic strategies for the formation of small-molecule protease inhibitors

The discovery and characterization of natural product protease inhibitors has inspired the development of numerous pharmaceutical agents.

Chemical strategies to unravel bacterial-eukaryotic signaling

The common language of bacteria and higher life forms is a lexicon of small molecules that the research community is only beginning to decipher. While many new signaling molecules have been discovered in recent years, the identification of their targets is mostly lagging. This review will focus on the latest chemical-probe based research aimed at understanding how bacteria interact chemically with mammals and plants. In general, chemical biology strategies remain under-utilized in this complex field of research, with a few key exceptions, and we hope that this review encourages others to implement these techniques in their research. Specifically, we highlight the chemical biology techniques used in recent studies, especially activity-based protein profiling, that have been applied to unravel the chemical mechanisms of interkingdom interactions.

Ecology and Genomic Insights into Plant-Pathogenic and Plant-Nonpathogenic Endophytes

Plants are colonized on their surfaces and in the rhizosphere and phyllosphere by a multitude of different microorganisms and are inhabited internally by endophytes. Most endophytes act as commensals without any known effect on their plant host, but multiple bacteria and fungi establish a mutualistic relationship with plants, and some act as pathogens. The outcome of these plant-microbe interactions depends on biotic and abiotic environmental factors and on the genotype of the host and the interacting microorganism. In addition, endophytic microbiota and the manifold interactions between members, including pathogens, have a profound influence on the function of the system plant and the development of pathobiomes. In this review, we elaborate on the differences and similarities between nonpathogenic and pathogenic endophytes in terms of host plant response, colonization strategy, and genome content. We furthermore discuss environmental effects and biotic interactions within plant microbiota that influence pathogenesis and the pathobiome.

Stomatal Defense a Decade Later

A decade has passed since the discovery of stomatal defense, and the field has expanded considerably with significant understanding of the basic mechanisms underlying the process.

Tomato root microbiota and Phytophthora parasitica-associated disease

BACKGROUND

Interactions between pathogenic oomycetes and microbiota residing on the surface of the host plant root are unknown, despite being critical to inoculum constitution. The nature of these interactions was explored for the polyphagous and telluric species Phytophthora parasitica.

RESULTS

Composition of the rhizospheric microbiota of Solanum lycopersicum was characterized using deep re-sequencing of 16S rRNA gene to analyze tomato roots either free of or partly covered with P. parasitica biofilm. Colonization of the host root surface by the oomycete was associated with a shift in microbial community involving a Bacteroidetes/Proteobacteria transition and Flavobacteriaceae as the most abundant family. Identification of members of the P. parasitica-associated microbiota interfering with biology and oomycete infection was carried out by screening for bacteria able to (i) grow on a P. parasitica extract-based medium (ii), exhibit in vitro probiotic or antibiotic activity towards the oomycete (iii), have an impact on the oomycete infection cycle in a tripartite interaction S. lycopersicum-P. parasitica-bacteria. One Pseudomonas phylotype was found to exacerbate disease symptoms in tomato plants. The lack of significant gene expression response of P. parasitica effectors to Pseudomonas suggested that the increase in plant susceptibility was not associated with an increase in virulence. Our results reveal that Pseudomonas spp. establishes commensal interactions with the oomycete. Bacteria preferentially colonize the surface of the biofilm rather than the roots, so that they can infect plant cells without any apparent infection of P. parasitica.

CONCLUSIONS

The presence of the pathogenic oomycete P. parasitica in the tomato rhizosphere leads to a shift in the rhizospheric microbiota composition. It contributes to the habitat extension of Pseudomonas species mediated through a physical association between the oomycete and the bacteria.

Differences in internalization and growth of Escherichia coli O157:H7 within the apoplast of edible plants, spinach and lettuce, compared with the model species Nicotiana benthamiana

Internalization of food-borne bacteria into edible parts of fresh produce plants represents a serious health risk. Therefore, internalization of verocytotoxigenic E. coli O157:H7 isolate Sakai was assessed in two species associated with outbreaks, spinach (Spinacia oleracea) and lettuce (Lactuca sativa) and compared to the model species Nicotiana benthamiana. Internalization occurred in the leaves and roots of spinach and lettuce throughout a 10 day time-course. The plant species, tissue type and inoculum dose all impacted the outcome. A combination of low inoculum dose (~102 CFU) together with light microscopy imaging highlighted marked differences in the fate of endophytic E. coli O157:H7 Sakai. In the fresh produce species, bacterial growth was restricted but viable cells persisted over 20 days, whereas there was > 400-fold (~2.5 Log10 ) increase in growth in N. benthamiana. Colony formation occurred adjacent to epidermal cells and mesophyll cells or close to vascular bundles of N. benthamiana and contained components of a biofilm matrix, including curli expression and elicitation, extracellular DNA and a limited presence of cellulose. Together the data show that internalization is a relevant issue in crop production and that crop species and tissue need to be considered as food safety risk parameters.

MEDIATOR18 and MEDIATOR20 confer susceptibility to Fusarium oxysporum in Arabidopsis thaliana

The conserved protein complex known as Mediator conveys transcriptional signals by acting as an intermediary between transcription factors and RNA polymerase II. As a result, Mediator subunits play multiple roles in regulating developmental as well as abiotic and biotic stress pathways. In this report we identify the head domain subunits MEDIATOR18 and MEDIATOR20 as important susceptibility factors for Fusarium oxysporum infection in Arabidopsis thaliana. Mutants of MED18 and MED20 display down-regulation of genes associated with jasmonate signaling and biosynthesis while up-regulation of salicylic acid associated pathogenesis related genes and reactive oxygen producing and scavenging genes. We propose that MED18 and MED20 form a sub-domain within Mediator that controls the balance of salicylic acid and jasmonate associated defense pathways.

Subunit-selective proteasome activity profiling uncovers uncoupled proteasome subunit activities during bacterial infections

The proteasome is a nuclear-cytoplasmic proteolytic complex involved in nearly all regulatory pathways in plant cells. The three different catalytic activities of the proteasome can have different functions, but tools to monitor and control these subunits selectively are not yet available in plant science. Here, we introduce subunit-selective inhibitors and dual-color fluorescent activity-based probes for studying two of the three active catalytic subunits of the plant proteasome. We validate these tools in two model plants and use this to study the proteasome during plant-microbe interactions. Our data reveal that Nicotiana benthamiana incorporates two different paralogs of each catalytic subunit into active proteasomes. Interestingly, both β1 and β5 activities are significantly increased upon infection with pathogenic Pseudomonas syringae pv. tomato DC3000 lacking hopQ1-1 [PtoDC3000(ΔhQ)] whilst the activity profile of the β1 subunit changes. Infection with wild-type PtoDC3000 causes proteasome activities that range from strongly induced β1 and β5 activities to strongly suppressed β5 activities, revealing that β1 and β5 activities can be uncoupled during bacterial infection. These selective probes and inhibitors are now available to the plant science community, and can be widely and easily applied to study the activity and role of the different catalytic subunits of the proteasome in different plant species.

Proteasome Activity Profiling Uncovers Alteration of Catalytic β2 and β5 Subunits of the Stress-Induced Proteasome during Salinity Stress in Tomato Roots

The stress proteasome in the animal kingdom facilitates faster conversion of oxidized proteins during stress conditions by incorporating different catalytic β subunits. Plants deal with similar kind of stresses and also carry multiple paralogous genes encoding for each of the three catalytic β subunits. Here, we investigated the existence of stress proteasomes upon abiotic stress (salt stress) in tomato roots. In contrast to Arabidopsis thaliana, tomato has a simplified proteasome gene set with single genes encoding each β subunit except for two genes encoding β2. Using proteasome activity profiling on tomato roots during salt stress, we discovered a transient modification of the catalytic subunits of the proteasome coinciding with a loss of cell viability. This stress-induced active proteasome disappears at later time points and coincides with the need to degrade oxidized proteins during salt stress. Subunit-selective proteasome probes and MS analysis of fluorescent 2D gels demonstrated that the detected stress-induced proteasome is not caused by an altered composition of subunits in active proteasomes, but involves an increased molecular weight of both labeled β2 and β5 subunits, and an additional acidic pI shift for labeled β5, whilst labeled β1 remains mostly unchanged. Treatment with phosphatase or glycosidases did not affect the migration pattern. This stress-induced proteasome may play an important role in PCD during abiotic stress.

Regulation of Stomatal Defense by Air Relative Humidity

It has long been observed that environmental conditions play crucial roles in modulating immunity and disease in plants and animals. For instance, many bacterial plant disease outbreaks occur after periods of high humidity and rain. A critical step in bacterial infection is entry into the plant interior through wounds and natural openings, such as stomata, which are adjustable microscopic pores in the epidermal tissue. Several studies have shown that stomatal closure is an integral part of the plant immune response to reduce pathogen invasion. In this study, we found that high humidity can effectively compromise Pseudomonas syringae-triggered stomatal closure in both Phaseolus vulgaris and Arabidopsis (Arabidopsis thaliana), which is accompanied by early up-regulation of the jasmonic acid (JA) pathway and simultaneous down-regulation of salicylic acid (SA) pathway in guard cells. Furthermore, SA-dependent response, but not JA-dependent response, is faster in guard cells than in whole leaves, suggesting that the SA signaling in guard cells may be independent from other cell types. Thus, we conclude that high humidity, a well-known disease-promoting environmental condition, acts in part by suppressing stomatal defense and is linked to hormone signaling in guard cells.

The Proteasome Acts as a Hub for Plant Immunity and Is Targeted by Pseudomonas Type III Effectors

Recent evidence suggests that the ubiquitin-proteasome system is involved in several aspects of plant immunity and that a range of plant pathogens subvert the ubiquitin-proteasome system to enhance their virulence. Here, we show that proteasome activity is strongly induced during basal defense in Arabidopsis (Arabidopsis thaliana). Mutant lines of the proteasome subunits RPT2a and RPN12a support increased bacterial growth of virulent Pseudomonas syringae pv tomato DC3000 (Pst) and Pseudomonas syringae pv maculicola ES4326. Both proteasome subunits are required for pathogen-associated molecular pattern-triggered immunity responses. Analysis of bacterial growth after a secondary infection of systemic leaves revealed that the establishment of systemic acquired resistance (SAR) is impaired in proteasome mutants, suggesting that the proteasome also plays an important role in defense priming and SAR In addition, we show that Pst inhibits proteasome activity in a type III secretion-dependent manner. A screen for type III effector proteins from Pst for their ability to interfere with proteasome activity revealed HopM1, HopAO1, HopA1, and HopG1 as putative proteasome inhibitors. Biochemical characterization of HopM1 by mass spectrometry indicates that HopM1 interacts with several E3 ubiquitin ligases and proteasome subunits. This supports the hypothesis that HopM1 associates with the proteasome, leading to its inhibition. Thus, the proteasome is an essential component of pathogen-associated molecular pattern-triggered immunity and SAR, which is targeted by multiple bacterial effectors.

The effects of host age and spatial location on bacterial community composition in the English Oak tree (Quercus robur)

Drivers of bacterial community assemblages associated with plants are diverse and include biotic factors, such as competitors and host traits, and abiotic factors, including environmental conditions and dispersal mechanisms. We examine the roles of spatial distribution and host size, as an approximation for age, in shaping the microbiome associated with Quercus robur woody tissue using culture-independent 16S rRNA gene amplicon sequencing. In addition to providing a baseline survey of the Q. robur microbiome, we screened for the pathogen of acute oak decline. Our results suggest that age is a predictor of bacterial community composition, demonstrating a surprising negative correlation between tree age and alpha diversity. We find no signature of dispersal limitation within the Wytham Woods plot sampled. Together, these results provide evidence for niche-based hypotheses of community assembly and the importance of tree age in bacterial community structure, as well as highlighting that caution must be applied when diagnosing dysbiosis in a long-lived plant host.

Screen of Non-annotated Small Secreted Proteins of Pseudomonas syringae Reveals a Virulence Factor That Inhibits Tomato Immune Proteases

Pseudomonas syringae pv. tomato DC3000 (PtoDC3000) is an extracellular model plant pathogen, yet its potential to produce secreted effectors that manipulate the apoplast has been under investigated. Here we identified 131 candidate small, secreted, non-annotated proteins from the PtoDC3000 genome, most of which are common to Pseudomonas species and potentially expressed during apoplastic colonization. We produced 43 of these proteins through a custom-made gateway-compatible expression system for extracellular bacterial proteins, and screened them for their ability to inhibit the secreted immune protease C14 of tomato using competitive activity-based protein profiling. This screen revealed C14-inhibiting protein-1 (Cip1), which contains motifs of the chagasin-like protease inhibitors. Cip1 mutants are less virulent on tomato, demonstrating the importance of this effector in apoplastic immunity. Cip1 also inhibits immune protease Pip1, which is known to suppress PtoDC3000 infection, but has a lower affinity for its close homolog Rcr3, explaining why this protein is not recognized in tomato plants carrying the Cf-2 resistance gene, which uses Rcr3 as a co-receptor to detect pathogen-derived protease inhibitors. Thus, this approach uncovered a protease inhibitor of P. syringae, indicating that also P. syringae secretes effectors that selectively target apoplastic host proteases of tomato, similar to tomato pathogenic fungi, oomycetes and nematodes.

Phytohormone pathways as targets of pathogens to facilitate infection

Plants are constantly threatened by potential pathogens. In order to optimize the output of defense against pathogens with distinct lifestyles, plants depend on hormonal networks to fine-tune specific responses and regulate growth-defense tradeoffs. To counteract, pathogens have evolved various strategies to disturb hormonal homeostasis and facilitate infection. Many pathogens synthesize plant hormones; more importantly, toxins and effectors are produced to manipulate hormonal crosstalk. Accumulating evidence has shown that pathogens exert extensive effects on plant hormone pathways not only to defeat immunity, but also modify habitat structure, optimize nutrient acquisition, and facilitate pathogen dissemination. In this review, we summarize mechanisms by which a wide array of pathogens gain benefits from manipulating plant hormone pathways.

Comparative genomics of Pseudomonas syringae pv. syringae strains B301D and HS191 and insights into intrapathovar traits associated with plant pathogenesis

Pseudomonas syringae pv. syringae is a common plant-associated bacterium that causes diseases of both monocot and dicot plants worldwide. To help delineate traits critical to adaptation and survival in the plant environment, we generated complete genome sequences of P. syringae pv. syringae strains B301D and HS191, which represent dicot and monocot strains with distinct host specificities. Intrapathovar comparisons of the B301D (6.09 Mb) and HS191 (5.95 Mb plus a 52 kb pCG131 plasmid) genomes to the previously sequenced B728a genome demonstrated that the shared genes encompass about 83% of each genome, and include genes for siderophore biosynthesis, osmotolerance, and extracellular polysaccharide production. Between 7% and 12% of the genes are unique among the genomes, and most of the unique gene regions carry transposons, phage elements, or IS elements associated with horizontal gene transfer. Differences are observed in the type III effector composition for the three strains that likely influences host range. The HS191 genome had the largest number at 25 of effector genes, and seven effector genes are specific to this monocot strain. Toxin production is another major trait associated with virulence of P. syringae pv. syringae, and HS191 is distinguished by genes for production of syringopeptin SP25 and mangotoxin.

The Xanthomonas campestris type III effector XopJ proteolytically degrades proteasome subunit RPT6

Many animal and plant pathogenic bacteria inject type III effector (T3E) proteins into their eukaryotic host cells to suppress immunity. The Yersinia outer protein J (YopJ) family of T3Es is a widely distributed family of effector proteins found in both animal and plant pathogens, and its members are highly diversified in virulence functions. Some members have been shown to possess acetyltransferase activity; however, whether this is a general feature of YopJ family T3Es is currently unknown. The T3E Xanthomonas outer protein J (XopJ), a YopJ family effector from the plant pathogen Xanthomonas campestris pv vesicatoria, interacts with the proteasomal subunit Regulatory Particle AAA-ATPase6 (RPT6) in planta to suppress proteasome activity, resulting in the inhibition of salicylic acid-related immune responses. Here, we show that XopJ has protease activity to specifically degrade RPT6, leading to reduced proteasome activity in the cytoplasm as well as in the nucleus. Proteolytic degradation of RPT6 was dependent on the localization of XopJ to the plasma membrane as well as on its catalytic triad. Mutation of the Walker B motif of RPT6 prevented XopJ-mediated degradation of the protein but not XopJ interaction. This indicates that the interaction of RPT6 with XopJ is dependent on the ATP-binding activity of RPT6, but proteolytic cleavage additionally requires its ATPase activity. Inhibition of the proteasome impairs the proteasomal turnover of Nonexpressor of Pathogenesis-Related1 (NPR1), the master regulator of salicylic acid responses, leading to the accumulation of ubiquitinated NPR1, which likely interferes with the full induction of NPR1 target genes. Our results show that YopJ family T3Es are not only highly diversified in virulence function but also appear to possess different biochemical activities.

Tissue-specific FLAGELLIN-SENSING 2 (FLS2) expression in roots restores immune responses in Arabidopsis fls2 mutants

The flagellin receptor of Arabidopsis, At-FLAGELLIN SENSING 2 (FLS2), has become a model for mechanistic and functional studies on plant immune receptors. Responses to flagellin or its active epitope flagellin 22 (flg22) have been extensively studied in Arabidopsis leaves. However, the perception of microbe-associated molecular patterns (MAMPs) and the immune responses in roots are poorly understood. Here, we show that isolated root tissue is able to induce pattern-triggered immunity (PTI) responses upon flg22 perception, in contrast to elf18 (the active epitope of elongation factor thermo unstable (EF-Tu)). Making use of fls2 mutant plants and tissue-specific promoters, we generated transgenic Arabidopsis lines expressing FLS2 only in certain root tissues. This allowed us to study the spatial requirements for flg22 responses in the root. Remarkably, the intensity of the immune responses did not always correlate with the expression level of the FLS2 receptor, but depended on the expressing tissue, supporting the idea that MAMP perception and sensitivity in different tissues contribute to a proper balance of defense responses according to the expected exposure to elicitors. In summary, we conclude that each investigated root tissue is able to perceive flg22 if FLS2 is present and that tissue identity is a major element of MAMP perception in roots.

Microbial effectors target multiple steps in the salicylic acid production and signaling pathway

Microbes attempting to colonize plants are recognized through the plant immune surveillance system. This leads to a complex array of global as well as specific defense responses, which are often associated with plant cell death and subsequent arrest of the invader. The responses also entail complex changes in phytohormone signaling pathways. Among these, salicylic acid (SA) signaling is an important pathway because of its ability to trigger plant cell death. As biotrophic and hemibiotrophic pathogens need to invade living plant tissue to cause disease, they have evolved efficient strategies to downregulate SA signaling by virulence effectors, which can be proteins or secondary metabolites. Here we review the strategies prokaryotic pathogens have developed to target SA biosynthesis and signaling, and contrast this with recent insights into how plant pathogenic eukaryotic fungi and oomycetes accomplish the same goal.

Interactions of Xanthomonas type-III effector proteins with the plant ubiquitin and ubiquitin-like pathways

In eukaryotes, regulated protein turnover is required during many cellular processes, including defense against pathogens. Ubiquitination and degradation of ubiquitinated proteins via the ubiquitin-proteasome system (UPS) is the main pathway for the turnover of intracellular proteins in eukaryotes. The extensive utilization of the UPS in host cells makes it an ideal pivot for the manipulation of cellular processes by pathogens. Like many other Gram-negative bacteria, Xanthomonas species secrete a suite of type-III effector proteins (T3Es) into their host cells to promote virulence. Some of these T3Es exploit the plant UPS to interfere with immunity. This review summarizes T3E examples from the genus Xanthomonas with a proven or suggested interaction with the host UPS or UPS-like systems and also discusses the apparent paradox that arises from the presence of T3Es that inhibit the UPS in general while others rely on its activity for their function.

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

Background

Pseudomonas syringae is pathogenic to a large number of plant species. For host colonization and disease progression, strains of this bacterium utilize an array of type III-secreted effectors and other virulence factors, including small secreted molecules such as syringolin A, a peptide derivative that inhibits the eukaryotic proteasome. In strains colonizing dicotyledonous plants, the compound was demonstrated to suppress the salicylic-acid-dependent defense pathway. Here, we analyze virulence factors of three strains colonizing wheat (Triticum aestivum): P. syringae pathovar syringae (Psy) strains B64 and SM, as well as P. syringae BRIP34876. These strains have a relatively small repertoire of only seven to eleven type III secreted effectors (T3Es) and differ in their capacity to produce syringolin A. The aim of this study was to analyze the contribution of various known virulence factors in the context of a small T3E repertoire.

Results

We demonstrate that syringolin A production enhances disease symptom development upon direct infiltration of strains into wheat leaves. However, it is not universally required for colonization, as Psy SM, which lacks syringolin biosynthesis genes, reaches cell densities comparable to syringolin A producer P. syringae BRIP34876. Next, we show that despite the small set of T3E-encoding genes, the type III secretion system remains the key pathogenicity determinant in these strains, and that phenotypic effects of deleting T3E-coding genes become apparent only when multiple effectors are removed.

Conclusions

Whereas production of syringolin A is not required for successful colonization of wheat leaves by P. syringae strains, its production results in increased lesion formation. Despite the small number of known T3Es encoded by the analyzed strains, the type III secretion system is essential for endophytic growth of these strains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0304-5) contains supplementary material, which is available to authorized users.

Expression patterns of flagellin sensing 2 map to bacterial entry sites in plant shoots and roots

Pathogens can colonize all plant organs and tissues. To prevent this, each cell must be capable of autonomously triggering defence. Therefore, it is generally assumed that primary sensors of the immune system are constitutively present. One major primary sensor against bacterial infection is the flagellin sensing 2 (FLS2) pattern recognition receptor (PRR). To gain insights into its expression pattern, the FLS2 promoter activity in β-glucuronidase (GUS) reporter lines was monitored. The data show that pFLS2::GUS activity is highest in cells and tissues vulnerable to bacterial entry and colonization, such as stomata, hydathodes, and lateral roots. GUS activity is also high in the vasculature and, by monitoring Ca(2+) responses in the vasculature, it was found that this tissue contributes to flg22-induced Ca(2+) burst. The FLS2 promoter is also regulated in a tissue- and cell type-specific manner and is responsive to hormones, damage, and biotic stresses. This results in stimulus-dependent expansion of the FLS2 expression domain. In summary, a tissue- and cell type-specific map of FLS2 expression has been created correlating with prominent entry sites and target tissues of plant bacterial pathogens.

Dynamic hydrolase activities precede hypersensitive tissue collapse in tomato seedlings

Hydrolases such as subtilases, vacuolar processing enzymes (VPEs) and the proteasome play important roles during plant programmed cell death (PCD). We investigated hydrolase activities during PCD using activity-based protein profiling (ABPP), which displays the active proteome using probes that react covalently with the active site of proteins. We employed tomato (Solanum lycopersicum) seedlings undergoing synchronized hypersensitive cell death by co-expressing the avirulence protein Avr4 from Cladosporium fulvum and the tomato resistance protein Cf-4. Cell death is blocked in seedlings grown at high temperature and humidity, and is synchronously induced by decreasing temperature and humidity. ABPP revealed that VPEs and the proteasome are not differentially active, but that activities of papain-like cysteine proteases and serine hydrolases, including Hsr203 and P69B, increase before hypersensitive tissue collapse, whereas the activity of a carboxypeptidase-like enzyme is reduced. Similar dynamics were observed for these enzymes in the apoplast of tomato challenged with C. fulvum. Unexpectedly, these challenged plants also displayed novel isoforms of secreted putative VPEs. In the absence of tissue collapse at high humidity, the hydrolase activity profile is already altered completely, demonstrating that changes in hydrolase activities precede hypersensitive tissue collapse.

HopZ4 from Pseudomonas syringae, a member of the HopZ type III effector family from the YopJ superfamily, inhibits the proteasome in plants

The YopJ family of type III effector proteins (T3E) is one of the largest and most widely distributed families of effector proteins, whose members are highly diversified in virulence functions. In the present study, HopZ4, a member of the YopJ family of T3E from the cucumber pathogen Pseudomonas syringae pv. lachrymans is described. HopZ4 shares high sequence similarity with the Xanthomonas T3E XopJ, and a functional analysis suggests a conserved virulence function between these two T3E. As has previously been shown for XopJ, HopZ4 interacts with the proteasomal subunit RPT6 in yeast and in planta to inhibit proteasome activity during infection. The inhibitory effect on the proteasome is dependent on localization of HopZ4 to the plasma membrane as well as on an intact catalytic triad of the effector protein. Furthermore, HopZ4 is able to complement loss of XopJ in Xanthomonas spp., as it prevents precocious host cell death during a compatible Xanthomonas-pepper interaction. The data presented here suggest that different bacterial species employ inhibition of the proteasome as a virulence strategy by making use of conserved T3E from the YopJ family of bacterial effector proteins.

Intervention of Phytohormone Pathways by Pathogen Effectors

The constant struggle between plants and microbes has driven the evolution of multiple defense strategies in the host as well as offense strategies in the pathogen. To defend themselves from pathogen attack, plants often rely on elaborate signaling networks regulated by phytohormones. In turn, pathogens have adopted innovative strategies to manipulate phytohormone-regulated defenses. Tactics frequently employed by plant pathogens involve hijacking, evading, or disrupting hormone signaling pathways and/or crosstalk. As reviewed here, this is achieved mechanistically via pathogen-derived molecules known as effectors, which target phytohormone receptors, transcriptional activators and repressors, and other components of phytohormone signaling in the host plant. Herbivores and sap-sucking insects employ obligate pathogens such as viruses, phytoplasma, or symbiotic bacteria to intervene with phytohormone-regulated defenses. Overall, an improved understanding of phytohormone intervention strategies employed by pests and pathogens during their interactions with plants will ultimately lead to the development of new crop protection strategies.

Real time live imaging of phytopathogenic bacteria Xanthomonas campestris pv. campestris MAFF106712 in 'plant sweet home'

Xanthomonas is one of the most widespread phytobacteria, causing diseases on a variety of agricultural plants. To develop novel control techniques, knowledge of bacterial behavior inside plant cells is essential. Xanthomonas campestris pv. campestris, a vascular pathogen, is the causal agent of black rot on leaves of Brassicaceae, including Arabidopsis thaliana. Among the X. campestris pv. campestris stocks in the MAFF collection, we selected XccMAFF106712 as a model compatible pathogen for the A. thaliana reference ecotype Columbia (Col-0). Using modified green fluorescent protein (AcGFP) as a reporter, we observed real time XccMAFF106712 colonization in planta with confocal microscopy. AcGFP-expressing bacteria colonized the inside of epidermal cells and the apoplast, as well as the xylem vessels of the vasculature. In the case of the type III mutant, bacteria colonization was never detected in the xylem vessel or apoplast, though they freely enter the xylem vessel through the wound. After 9 days post inoculation with XccMAFF106712, the xylem vessel became filled with bacterial aggregates. This suggests that Xcc colonization can be divided into main four steps, (1) movement in the xylem vessel, (2) movement to the next cell, (3) adhesion to the host plant cells, and (4) formation of bacterial aggregates. The type III mutant abolished at least steps (1) and (2). Better understanding of Xcc colonization is essential for development of novel control techniques for black rot.

Production of proteasome inhibitor syringolin A by the endophyte Rhizobium sp. strain AP16

Syringolin A, the product of a mixed nonribosomal peptide synthetase/polyketide synthase encoded by the syl gene cluster, is a virulence factor secreted by certain Pseudomonas syringae strains. Together with the glidobactins produced by a number of beta- and gammaproteobacterial human and animal pathogens, it belongs to the syrbactins, a structurally novel class of proteasome inhibitors. In plants, proteasome inhibition by syringolin A-producing P. syringae strains leads to the suppression of host defense pathways requiring proteasome activity, such as the ones mediated by salicylic acid and jasmonic acid. Here we report the discovery of a syl-like gene cluster with some unusual features in the alphaproteobacterial endophyte Rhizobium sp. strain AP16 that encodes a putative syringolin A-like synthetase whose components share 55% to 65% sequence identity (72% to 79% similarity) at the amino acid level. As revealed by average nucleotide identity (ANI) calculations, this strain likely belongs to the same species as biocontrol strain R. rhizogenes K84 (formely known as Agrobacterium radiobacter K84), which, however, carries a nonfunctional deletion remnant of the syl-like gene cluster. Here we present a functional analysis of the syl-like gene cluster of Rhizobium sp. strain AP16 and demonstrate that this endophyte synthesizes syringolin A and some related minor variants, suggesting that proteasome inhibition by syrbactin production can be important not only for pathogens but also for endophytic bacteria in the interaction with their hosts.

Genomics-Based Exploration of Virulence Determinants and Host-Specific Adaptations of Pseudomonas syringae Strains Isolated from Grasses

The Pseudomonas syringae species complex has recently been named the number one plant pathogen, due to its economic and environmental impacts, as well as for its role in scientific research. The bacterium has been repeatedly reported to cause outbreaks on bean, cucumber, stone fruit, kiwi and olive tree, as well as on other crop and non-crop plants. It also serves as a model organism for research on the Type III secretion system (T3SS) and plant-pathogen interactions. While most of the current work on this pathogen is either carried out on one of three model strains found on dicot plants with completely sequenced genomes or on isolates obtained from recent outbreaks, not much is known about strains isolated from grasses (Poaceae). Here, we use comparative genomics in order to identify putative virulence-associated genes and other Poaceae-specific adaptations in several newly available genome sequences of strains isolated from grass species. All strains possess only a small number of known Type III effectors, therefore pointing to the importance of non-Type III secreted virulence factors. The implications of this finding are discussed.

The role of bacterial phytotoxins in inhibiting the eukaryotic proteasome

The ubiquitin-26S proteasome degradation system (UPS) plays a pivotal role in almost all aspects of plant life, including defending against pathogens. Although the proteasome is important for plant immunity, it has been found to be also exploited by pathogens using effectors to increase their virulence. Recent work on the XopJ effector and syringolin A/syrbactins has highlighted host proteasome inhibition as a virulence strategy of pathogens. This review will focus on these recent developments.