The hrp genes of Pseudomonas cichorii are essential for pathogenicity on eggplant but not on lettuce

Both GacS-regulated lipopeptides and the type three secretion system contribute to Pseudomonas cichorii induced necrosis in lettuce and chicory

Pseudomonas cichorii SF1-54, the causal agent of lettuce midrib rot disease, produces lipopeptides cichofactins and cichopeptins which are important virulence factors. The GacS/GacA two-component system is well known to regulate production of lipopeptides in pseudomonads. Additionally, the functions of the type three secretion system (T3SS) in P. cichorii-plant interactions are not clarified. In this study, we investigated the role of the GacS-regulated lipopeptides and the T3SS in pathogenicity of P. cichorii SF1-54 on two host plants, chicory and lettuce, by constructing mutants in hrpL, which encodes the key sigma factor to control T3SS expression, and gacS. Compared with the wildtype, the hrpL mutant produced lipopeptides at a similar level but the gacS mutant was strongly impaired in lipopeptide production. The mutant deficient in hrpL did not significantly differ from the wildtype in virulence on chicory and lettuce. The gacS mutant exhibited significantly less symptoms on both host plants compared to the wildtype and the hrpL mutant. Intriguingly, the gacS hrpL-double mutant no longer produced lipopeptides, lost virulence and showed impaired colonization on chicory, but was still weakly virulent on lettuce. Thus, contribution of both the GacS-regulated lipopeptides and T3SS to virulence of P. cichorii SF1-54 is host plant dependent.

Identifying more targeted antimicrobials active against select bacterial phytopathogens

AIMS

Phytopathogens are a global threat to the world's food supply. Use of broad-spectrum bactericides and antibiotics to limit or eliminate bacterial infections is becoming less effective as levels of resistance increase, while concurrently becoming less desirable from an ecological perspective due to their collateral damage to beneficial members of plant and soil microbiomes. Bacteria produce numerous antimicrobials in addition to antibiotics, such as bacteriocins with their relatively narrow activity spectra, and inhibitory metabolic by-products, such as organic acids. There is interest in developing these naturally occurring antimicrobials for use as alternatives or supplements to antibiotics.

METHODS AND RESULTS

In this study, we investigate the inhibitory potential of 217 plant associated bacterial isolates from 44 species including plant pathogens, plant growth promoting rhizobacteria, and plant commensals. Over half of the isolates were found to produce antimicrobial substances, of which 68% were active against phytopathogens. Even more intriguing, 98% of phytopathogenic strains were sensitive to the compounds produced specifically by plant growth promoting rhizobacteria.

CONCLUSION

These data argue that plant-associated bacteria produce a broad range of antimicrobial substances, and that the substances produced preferentially target phytopathogenic bacteria.

SIGNIFICANCE AND IMPACT OF STUDY

There is a need for novel antimicrobials for use in agriculture. The methods presented here reveal the potential for simple phenotypic screening methods to provide a broad range of potential drug candidates.

Characterization of Virulence Function of Pseudomonas cichorii Avirulence Protein E1 (AvrE1) during Host Plant Infection

Pseudomonas cichorii secretes effectors that suppress defense mechanisms in host plants. However, the function of these effectors, including avirulence protein E1 (AvrE1), in the pathogenicity of P. cichorii, remains unexplored. In this study, to investigate the function of avrE1 in P. cichorii JBC1 (PcJBC1), we created an avrE1-deficient mutant (JBC1ΔavrE1) using CRISPR/Cas9. The disease severity caused by JBC1ΔavrE1 in tomato plants significantly decreased by reducing water soaking during early infection stage, as evidenced by the electrolyte leakage in infected leaves. The disease symptoms caused by JBC1ΔavrE1 in the cabbage midrib were light-brown spots compared to the dark-colored ones caused by PcJBC1, which indicates the role of AvrE1 in cell lysis. The avrE1-deficient mutant failed to elicit cell death in non-host tobacco plants. Disease severity and cell death caused by JBC1ΔavrE1 in host and non-host plants were restored through heterologous complementation with avrE1 from Pseudomonas syringae pv. tomato DC3000 (PstDC3000). Overall, our results indicate that avrE1 contributes to cell death during early infection, which consequently increases disease development in host plants. The roles of PcJBC1 AvrE1 in host cells remain to be elucidated.

Pseudomonas capsici sp. nov., a plant-pathogenic bacterium isolated from pepper leaf in Georgia, USA

Three phytopathogenic bacterial strains (Pc19-1^T, Pc19-2 and Pc19-3) were isolated from seedlings displaying water-soaked, dark brown-to-black, necrotic lesions on pepper (Capsicum annuum) leaves in Georgia, USA. Upon isolation on King’s medium B, light cream-coloured colonies were observed and a diffusible fluorescent pigment was visible under ultraviolet light. Analysis of their 16S rRNA gene sequences showed that they belonged to the genus Pseudomonas, with the highest similarity to Pseudomonas cichorii ATCC 10857^T (99.7 %). The fatty acid analysis revealed that the majority of the fatty acids were summed feature 3 (C_{16  :  1} ω7c/C_{16  :  1} ω6c), C_{16  :  0} and summed feature 8 (C_{18  :  1} ω7c/C_{18  :  1} ω6c). Phylogenomic analyses based on whole genome sequences demonstrated that the pepper strains belonged to the Pseudomonas syringae complex with P. cichorii as their closest neighbour, and formed a separate monophyletic clade from other species. Between the pepper strains and P. cichorii, the average nucleotide identity values were 91.3 %. Furthermore, the digital DNA–DNA hybridization values of the pepper strains when compared to their closest relatives, including P. cichorii, were 45.2 % or less. In addition, biochemical and physiological features were examined in this study and the results indicate that the pepper strains represent a novel Pseudomonas species. Therefore, we propose a new species Pseudomonas capsici sp. nov., with Pc19-1^T (=CFBP 8884^T=LMG 32209^T) as the type strain. The DNA G+C content of the strain Pc19-1^T is 58.4 mol%.

Bacterial characterization in ambient submicron particles during severe haze episodes at Ji'nan, China

In January 2014, severe haze episodes which sweep across Chinese cities have attracted public concern and interest at home and abroad. In addition to the physicochemical properties of air pollutants, bacteria are thought to be responsible for the spread of respiratory diseases and various allergies. We attempted the bacterial characterization of submicron particles (PM_0.18-0.32, PM_0.32-0.56, and PM_0.56-1) under severe haze episodes using high-throughput sequencing and real-time quantitative PCR detecting system based on 21 samples collected from January to March 2014 at Ji'nan, China. The high bacterial concentration in PM_0.32-0.56 (7314cells m^-3), PM_0.18-0.32 (7212cells m^-3), and PM_0.56-1 (6982cells m^-3) showed significant negative correlations with SO₂, NO_2, and O₃. Under sufficient sequencing depth, 37 phyla, 71 classes, 137 orders, 236 families, and 378 genera were classified, and the bacterial community structure varied significantly in different size fractions. For example, Holophagaceae (Acidobacteria) in PM_0.32-0.56 showed 6-fold higher abundance than that in PM_0.18-0.32. Moreover, functional categories and bacterial species (Lactococcus piscium, Pseudomonas fragi, Streptococcus agalactiae, and Pseudomonas cichorii) that may potentially be responsible for infections and allergies were also discovered. Source track analysis showed that the ambient bacteria mainly originated from soils, leaf surfaces, and feces. Our results highlighted the importance of airborne microbial communities by understanding the concentration, structure, ecological and health effects, especially those in submicron particles during haze episodes.

New Pseudomonas spp. Are Pathogenic to Citrus

Five putative novel Pseudomonas species shown to be pathogenic to citrus have been characterized in a screening of 126 Pseudomonas strains isolated from diseased citrus leaves and stems in northern Iran. The 126 strains were studied using a polyphasic approach that included phenotypic characterizations and phylogenetic multilocus sequence analysis. The pathogenicity of these strains against 3 cultivars of citrus is demonstrated in greenhouse and field studies. The strains were initially grouped phenotypically and by their partial rpoD gene sequences into 11 coherent groups in the Pseudomonas fluorescens phylogenetic lineage. Fifty-three strains that are representatives of the 11 groups were selected and analyzed by partial sequencing of their 16S rRNA and gyrB genes. The individual and concatenated partial sequences of the three genes were used to construct the corresponding phylogenetic trees. The majority of the strains were identified at the species level: P. lurida (5 strains), P. monteilii (2 strains), P. moraviensis (1 strain), P. orientalis (16 strains), P. simiae (7 strains), P. syringae (46 strains, distributed phylogenetically in at least 5 pathovars), and P. viridiflava (2 strains). This is the first report of pathogenicity on citrus of P. orientalis, P. simiae, P. lurida, P. moraviensis and P. monteilii strains. The remaining 47 strains that could not be identified at the species level are considered representatives of at least 5 putative novel Pseudomonas species that are not yet described.

A DNA-based real-time PCR assay for robust growth quantification of the bacterial pathogen Pseudomonas syringae on Arabidopsis thaliana

BACKGROUND

The interaction of Pseudomonas syringae with Arabidopsis is one of the most commonly used systems to study various bacterial-host interrelationships. Currently, most studies are based on the growth quantification of the pathogen to characterize resistance or virulence targets. However, the standard available method for determining bacterial proliferation in planta is laborious and has several limitations.

RESULTS

Here we present an alternative robust approach, which is based on the quantification of bacterial DNA by real-time PCR. We directly compared this assay with the routinely used plate counting method to access bacterial titers in a number of well described Arabidopsis mutants.

CONCLUSIONS

These studies showed that the DNA-based technique is highly reliable and comparable. Moreover, the technique is easily applicable, robust, and ideal for routine experiments or for larger scale analyses.

Characterization of Cichopeptins, New Phytotoxic Cyclic Lipodepsipeptides Produced by Pseudomonas cichorii SF1-54 and Their Role in Bacterial Midrib Rot Disease of Lettuce

The lettuce midrib rot pathogen Pseudomonas cichorii SF1-54 produces seven bioactive compounds with biosurfactant properties. Two compounds exhibited necrosis-inducing activity on chicory leaves. The structure of the two phytotoxic compounds, named cichopeptin A and B, was tentatively characterized. They are related cyclic lipopeptides composed of an unsaturated C12-fatty acid chain linked to the N-terminus of a 22-amino acid peptide moiety. Cichopeptin B differs from cichopeptin A only in the last C-terminal amino acid residue, which is probably Val instead of Leu/Ile. Based on peptide sequence similarity, cichopeptins are new cyclic lipopeptides related to corpeptin, produced by the tomato pathogen Pseudomonas corrugata. Production of cichopeptin is stimulated by glycine betaine but not by choline, an upstream precursor of glycine betaine. Furthermore, a gene cluster encoding cichopeptin synthethases, cipABCDEF, is responsible for cichopeptin biosynthesis. A cipA-deletion mutant exhibited significantly less virulence and rotten midribs than the parental strain upon spray inoculation on lettuce. However, the parental and mutant strains multiplied in lettuce leaves at a similar rate. These results demonstrate that cichopeptins contribute to virulence of P. cichorii SF1-54 on lettuce.

Cell death-inducing stresses are required for defense activation in DS1-phosphatidic acid phosphatase-silenced Nicotiana benthamiana

We previously identified DS1 plants that showed resistance to compatible Ralstonia solanacearum with accelerated defense responses. Here, we describe activation mechanisms of defense responses in DS1 plants. After inoculation with incompatible R. solanacearum 8107, DS1 plants showed hyperinduction of hypersensitive response (HR) and reactive oxygen species (ROS) generation. Transient expression of PopP1 and AvrA induced hyperinduction of HR and ROS generation. Furthermore, Pseudomonas cichorii (Pc) and a type III secretion system (TTSS)-deficient mutant of P. cichorii showed accelerated induction of HR and ROS generation. Chitin and flg22 did not induce either HR or ROS hyperaccumulation; however, INF1 accelerated HR and ROS in DS1 plants. Activation of these defense responses was closely associated with increased phosphatidic acid (PA) content. Our results show that DS1 plants exhibit PA-mediated sensitization of plant defenses and that cell death-inducing stress is required to achieve full activation of defense responses.

An effector gene hopA1 influences on virulence, host specificity, and lifestyles of Pseudomonas cichorii JBC1

Pseudomonas cichorii is a devastating pathogen which infects a wide range of ornamental as well as agricultural crops worldwide. Characterization of virulence genes helps to understand pathogens' infection processes, which may lead to development of resistant crops. For functional validation of novel genes, we re-constructed pUCP18 vector with λ phage red operon and sacB gene (pUCP18_RedS), which simplified conventional marker exchange system. The effector gene hopA1 of P. cichorii JBC1 was marker exchanged with PCR product of kanamycin gene flanked by hopA1 flanking region using pUCP18_RedS. The virulence and internal growth of hopA1 defective mutant (ΔhopA1) in tomato seedlings was significantly reduced compared to wild type (WT) and hopA1 complemented strain (ΔhopA1::phopA1). The analysis on role of hopA1 in host range revealed that P. cichorii was hopA1-dependent to infect cabbage, tomato, soybean, hot pepper, and cucumber, but not melon and eggplant. Despite the similarity in growth pattern, the biofilm formation and swarming motility of ΔhopA1 were significantly reduced compared to WT and ΔhopA1::phopA1. The results of this study indicate that hopA1 plays a significant role not only in virulence and host specificity, but also motility and biofilm formation of P. cichorii which may influence the infection processes.

Commonalities and differences of T3SSs in rhizobia and plant pathogenic bacteria

Plant pathogenic bacteria and rhizobia infect higher plants albeit the interactions with their hosts are principally distinct and lead to completely different phenotypic outcomes, either pathogenic or mutualistic, respectively. Bacterial protein delivery to plant host plays an essential role in determining the phenotypic outcome of plant-bacteria interactions. The involvement of type III secretion systems (T3SSs) in mediating animal- and plant-pathogen interactions was discovered in the mid-80's and is now recognized as a multiprotein nanomachine dedicated to trans-kingdom movement of effector proteins. The discovery of T3SS in bacteria with symbiotic lifestyles broadened its role beyond virulence. In most T3SS-positive bacterial pathogens, virulence is largely dependent on functional T3SSs, while in rhizobia the system is dispensable for nodulation and can affect positively or negatively the mutualistic associations with their hosts. This review focuses on recent comparative genome analyses in plant pathogens and rhizobia that uncovered similarities and variations among T3SSs in their genetic organization, regulatory networks and type III secreted proteins and discusses the evolutionary adaptations of T3SSs and type III secreted proteins that might account for the distinguishable phenotypes and host range characteristics of plant pathogens and symbionts.

New linear lipopeptides produced by Pseudomonas cichorii SF1-54 are involved in virulence, swarming motility, and biofilm formation

Pseudomonas cichorii is the causal agent of lettuce midrib rot, characterized by a dark-brown to green-black discoloration of the midrib. Formation of necrotic lesions by several plant-pathogenic pseudomonads is associated with production of phytotoxic lipopeptides, which contribute to virulence. Therefore, the ability of P. cichorii SF1-54 to produce lipopeptides was investigated. A cell-free culture filtrate of SF1-54 showed surfactant, antimicrobial, and phytotoxic activities which are typical for lipopeptides. High-performance liquid chromatography analysis of P. cichorii SF1-54 culture filtrate revealed the presence of seven compounds with lipopeptide characteristics. Two related lipopeptides, named cichofactin A and B, were studied in more detail: they are linear lipopeptides with a decanoic and dodecanoic lipid chain, respectively, connected to the N-terminus of an eight-amino-acid peptide moiety. Both cichofactins are new members of the syringafactin lipopeptide family. Furthermore, two nonribosomal peptide synthethase-encoding genes, cifA and cifB, were identified as responsible for cichofactin biosynthesis. A cifAB deletion mutant no longer produced cichofactins and was impaired in swarming motility but showed enhanced biofilm formation. Upon spray inoculation on lettuce, the cichofactin-deficient mutant caused significantly less rotten midribs than the wild type, indicating that cichofactins are involved in pathogenicity of P. cichorii SF1-54. Further analysis revealed that P. cichorii isolates vary greatly in swarming motility and cichofactin production.

A high-throughput forward genetic screen identifies genes required for virulence of Pseudomonas syringae pv. maculicola ES4326 on Arabidopsis

Successful pathogenesis requires a number of coordinated processes whose genetic bases remain to be fully characterized. We utilized a high-throughput, liquid media-based assay to screen transposon disruptants of the phytopathogen Pseudomonas syringae pv. maculicola ES4326 to identify genes required for virulence on Arabidopsis. Many genes identified through this screen were involved in processes such as type III secretion, periplasmic glucan biosynthesis, flagellar motility, and amino acid biosynthesis. A small set of genes did not fall into any of these functional groups, and their disruption resulted in context-specific effects on in planta bacterial growth.

Implication of an Aldehyde Dehydrogenase Gene and a Phosphinothricin N-Acetyltransferase Gene in the Diversity of Pseudomonas cichorii Virulence

Pseudomonas cichorii harbors the hrp genes. hrp-mutants lose their virulence on eggplant but not on lettuce. A phosphinothricin N-acetyltransferase gene (pat) is located between hrpL and an aldehyde dehydrogenase gene (aldH) in the genome of P. cichorii. Comparison of nucleotide sequences and composition of the genes among pseudomonads suggests a common ancestor of hrp and pat between P. cichorii strains and P. viridiflava strains harboring the single hrp pathogenicity island. In contrast, phylogenetic diversification of aldH corresponded to species diversification amongst pseudomonads. In this study, the involvement of aldH and pat in P. cichorii virulence was analyzed. An aldH-deleted mutant (ΔaldH) and a pat-deleted mutant (Δpat) lost their virulence on eggplant but not on lettuce. P. cichorii expressed both genes in eggplant leaves, independent of HrpL, the transcriptional activator for the hrp. Inoculation into Asteraceae species susceptible to P. cichorii showed that the involvement of hrp, pat and aldH in P. cichorii virulence is independent of each other and has no relationship with the phylogeny of Asteraceae species based on the nucleotide sequences of ndhF and rbcL. It is thus thought that not only the hrp genes but also pat and aldH are implicated in the diversity of P. cichorii virulence on susceptible host plant species.

Cell-associated hemolysis activity in the clinical strain of Pseudomonas fluorescens MFN1032

BACKGROUND

MFN1032 is a clinical Pseudomonas fluorescens strain able to grow at 37 degrees C. MFN1032 cells induce necrosis and apoptosis in rat glial cells at this temperature. This strain displays secretion-mediated hemolytic activity involving phospholipase C and cyclolipopeptides. Under laboratory conditions, this activity is not expressed at 37 degrees C. This activity is tightly regulated and is subject to phase variation.

RESULTS

We found that MFN1032 displays a cell-associated hemolytic activity distinct from the secreted hemolytic activity. Cell-associated hemolysis was expressed at 37 degrees C and was only detected in vitro in mid log growth phase in the presence of erythrocytes. We studied the regulation of this activity in the wild-type strain and in a mutant defective in the Gac two-component pathway. GacS/GacA is a negative regulator of this activity. In contrast to the Pseudomonas fluorescens strains PfO-1 and Pf5, whose genomes have been sequenced, the MFN1032 strain has the type III secretion-like genes hrcRST belonging to the hrpU operon. We showed that disruption of this operon abolished cell-associated hemolytic activity. This activity was not detected in P.fluorescens strains carrying similar hrc genes, as for the P. fluorescens psychrotrophic strain MF37.

CONCLUSIONS

To our knowledge this the first demonstration of cell-associated hemolytic activity of a clinical strain of Pseudomonas fluorescens. Moreover, this activity seems to be related to a functional hrpU operon and is independent of biosurfactant production. Precise link between a functional hrpU operon and cell-associated hemolytic activity remains to be elucidated.