Characterization of the Xanthomonas axonopodis pv. glycines Hrp pathogenicity island

The Harpin-Induced Hypersensitive Reaction in Nicotiana tabacum Requires Wall-Associated Kinase 2

Harpins are proteins secreted by many gram-negative, plant-pathogenic bacteria that stimulate the hypersensitive reaction (HR), a host cell death defence response, when infiltrated into plant leaves as purified proteins. This activity of harpins was first discovered in Nicotiana tabacum (tobacco), which manifests an especially strong and rapid harpin-activated HR that becomes evident within 12-24 h after infiltration. HrpN is the major harpin of the fire blight pathogen Erwinia amylovora. We discovered natural variation in the HrpN-induced HR among tobacco accessions and identified candidate genes using genetic mapping and bulked-segregant analysis with whole genome sequencing. Virus-induced gene silencing of candidate gene Wall-Associated Kinase 2 (WAK2) abrogated the HR in response to HrpN and HpaG, a harpin from the soybean bacterial pustule pathogen Xanthomonas citri pv. glycines. WAK2 silencing also compromised the avirulence activity of harpin HrpZ in the tobacco wildfire pathogen Pseudomonas syringae pv. tabaci. A natural, disruptive mutation in WAK2 correlated with the inability of tobacco accessions to mount the harpin-mediated HR. We conclude that the predicted receptor-like kinase WAK2 is required for the strong HR induced in tobacco leaves by harpin protein infiltration and can potentially mediate resistance to bacterial pathogens based on harpin recognition.

Pathogenesis-Associated Bacterial Amyloids: The Network of Interactions

Amyloids are protein fibrils with a characteristic cross-β structure that is responsible for the unusual resistance of amyloids to various physical and chemical factors, as well as numerous pathogenic and functional consequences of amyloidogenesis. The greatest diversity of functional amyloids was identified in bacteria. The majority of bacterial amyloids are involved in virulence and pathogenesis either via facilitating formation of biofilms and adaptation of bacteria to colonization of a host organism or through direct regulation of toxicity. Recent studies have shown that, beside their commonly known activity, amyloids may be involved in the spatial regulation of proteome by modulating aggregation of other amyloidogenic proteins with multiple functional or pathological effects. Although the studies on the role of microbiome-produced amyloids in the development of amyloidoses in humans and animals have only been started, it is clear that humans as holobionts contain amyloids encoded not only by the host genome, but also by microorganisms that constitute the microbiome. Amyloids acquired from external sources (e.g., food) can interact with holobiont amyloids and modulate the effects of bacterial and host amyloids, thus adding another level of complexity to the holobiont-associated amyloid network. In this review, we described bacterial amyloids directly or indirectly involved in disease pathogenesis in humans and discussed the significance of bacterial amyloids in the three-component network of holobiont-associated amyloids.

An insight into pathogenicity and virulence gene content of Xanthomonas spp. and its biocontrol strategies

The genus Xanthomonas primarily serves as a plant pathogen, targeting a diverse range of economically significant crops on a global scale. Xanthomonas spp. utilizes a collection of toxins, adhesins, and protein effectors as part of their toolkit to thrive in their surroundings, and establish themselves within plant hosts. The bacterial secretion systems (Type 1 to Type 6) assist in delivering the effector proteins to their intended destinations. These secretion systems are specialized multi-protein complexes responsible for transporting proteins into the extracellular milieu or directly into host cells. The potent virulence and systematic infection system result in rapid dissemination of the bacteria, posing significant challenges in management due to complexities and substantial loss incurred. Consequently, there has been a notable increase in the utilization of chemical pesticides, leading to bioaccumulation and raising concerns about adverse health effects. Biological control mechanisms through beneficial microorganism (Bacillus, Pseudomonas, Trichoderma, Burkholderia, AMF, etc.) have proven to be an appropriate alternative in integrative pest management system. This review details the pathogenicity and virulence factors of Xanthomonas, as well as its control strategies. It also encourages the use of biological control agents, which promotes sustainable and environmentally friendly agricultural practices.

Design and Synthesis of Mandelic Acid Derivatives for Suppression of Virulence via T3SS against Citrus Canker

Citrus canker, a highly contagious bacterial disease caused by Xanthomonas citri subsp. citri (Xcc), poses a substantial threat to citrus crops, leading to serious reductions in fruit yield and economic losses. Most commonly used bactericides against Xcc lead to the rapid development of resistant subpopulations. Therefore, it is imperative to create novel drugs, such as type III secretion system (T3SS) inhibitors, that specifically target bacterial virulence factors rather than bacterial viability. In our study, we designed and synthesized a series of mandelic acid derivatives including 2-mercapto-1,3,4-thiazole. Seven substances were found to reduce the level of transcription of hpa1 without affecting bacterial viability. In vivo bioassays indicated that compound F9 significantly inhibited hypersensitive response and pathogenicity. RT-qPCR assays showed that compound F9 visibly suppressed the expression of Xcc T3SS-related genes as well as citrus canker susceptibility gene CsLOB1. Furthermore, the combination with compound F9 and quorum-quenching bacteria HN-8 can also obviously alleviate canker symptoms.

Application and Mechanism of Cryptolepine and Neocryptolepine Derivatives as T3SS Inhibitors for Control of Bacterial Leaf Blight on Rice

Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv oryzae (Xoo) is extremely harmful to rice production. The traditional control approach is to use bactericides that target key bacterial growth factors, but the selection pressure on the pathogen makes resistant strains the dominant bacterial strains, leading to a decline in bactericidal efficacy. Type III secretion system (T3SS) is a conserved and critical virulence factor in most Gram-negative bacteria, and its expression or absence does not affect bacterial growth, rendering it an ideal target for creating drugs against Gram-negative pathogens. In this work, we synthesized a range of derivatives from cryptolepine and neocryptolepine. We found that compound Z-8 could inhibit the expression of Xoo T3SS-related genes without affecting the growth of bacteria. an in vivo bioassay showed that compound Z-8 could effectively reduce the hypersensitive response (HR) induced by Xoo in tobacco and reduce the pathogenicity of Xoo in rice. Furthermore, it exhibited synergy in control of bacterial leaf blight when combined with the quorum quenching bacterial F20.

p-Aminobenzoic acid inhibits the growth of soybean pathogen Xanthomonas axonopodis pv. glycines by altering outer membrane integrity

BACKGROUND

p-Aminobenzoic acid (pABA) is an environmentally friendly bioactive metabolite synthesized by Lysobacter antibioticus. This compound showed an unusual antifungal mode of action based on cytokinesis inhibition. However, the potential antibacterial properties of pABA remain unexplored.

RESULTS

In this study, pABA showed antibacterial activity against Gram-negative bacteria. This metabolite inhibited growth (EC50  = 4.02 mM), and reduced swimming motility, extracellular protease activity, and biofilm formation in the soybean pathogen Xanthomonas axonopodis pv. glycines (Xag). Although pABA was previously reported to inhibit fungal cell division, no apparent effect was observed on Xag cell division genes. Instead, pABA reduced the expression of various membrane integrity-related genes, such as cirA, czcA, czcB, emrE, and tolC. Consistently, scanning electron microscopy observations revealed that pABA caused major alternations in Xag morphology and blocked the formation of bacterial consortiums. In addition, pABA reduced the content and profile of outer membrane proteins and lipopolysaccharides in Xag, which may explain the observed effects. Preventive and curative applications of 10 mM pABA reduced Xag symptoms in soybean plants by 52.1% and 75.2%, respectively.

CONCLUSIONS

The antibacterial properties of pABA were studied for the first time, revealing new insights into its potential application for the management of bacterial pathogens. Although pABA was previously reported to show an antifungal mode of action based on cytokinesis inhibition, this compound inhibited Xag growth by altering the outer membrane's integrity. © 2023 Society of Chemical Industry.

Novel Ethyl-3-Aryl-2-Nitroacrylate Derivatives as Potential T3SS Inhibitors against Xanthomonas oryzae pv. oryzae on Rice

Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a highly destructive bacterial disease. Traditional prevention methods have utilized antibiotics to target bacterial growth, which has accelerated the emergence of resistant strains. New prevention techniques are developing agents such as type III secretion system (T3SS) inhibitors that target bacterial virulence factors without affecting bacterial growth. To explore novel T3SS inhibitors, a series of ethyl-3-aryl-2-nitroacrylate derivatives were designed and synthesized. Preliminary screening of T3SS inhibitors was based on the inhibition of the hpa1 gene promoter and showed no effect on bacterial growth. Compounds B9 and B10, obtained in the primary screening, significantly inhibited the hypersensitive response (HR) in tobacco and the expression of T3SS genes in the hrp cluster including key regulatory genes. In vivo bioassays showed that T3SS inhibitors obviously inhibited BLB and appeared to be more effective when combined with quorum quenching bacteria F20.

A Phosphate Uptake System Is Required for Xanthomonas citri pv. glycines Virulence in Soybean

The genes encoding the phosphate uptake system in Xanthomonas citri pv. glycines 12-2 were previously found to be upregulated when in soybean leaves. This study thus explored the role of the phosphate uptake system on its virulence to soybean. While phoB and pstSCAB mutants were greatly impaired in both inciting disease symptoms and growth in soybean, the virulence and growth in soybean of a phoU mutant was not reduced when compared with the wild-type strain. The expression of phoB and pstSCAB was highly induced in phosphate-deficient media. In addition, the expression of phoB, assessed with a fusion to a promoterless ice nucleation reporter gene, was greatly increased in soybean leaves, confirming that the soybean apoplast is a phosphorus-limited habitat for X. citri pv. glycines. Global gene expression profiles of phoB and phoU mutants of X. citri pv. glycines conducted under phosphate-limitation conditions in vitro, using RNA-seq, revealed that PhoB positively regulated genes involved in signal transduction, the xcs cluster type II secretion system, cell motility, and chemotaxis, while negatively regulating cell wall and membrane biogenesis, DNA replication and recombination and repair, and several genes with unknown function. PhoU also positively regulated the same genes involved in cell motility and chemotaxis. The severity of bacterial pustule disease was decreased in soybean plants grown under high phosphate fertilization conditions, demonstrating that high phosphate availability in soybean plants can affect infection by X. citri pv. glycines by modulation of the expression of phosphate uptake systems. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

A putative multi-sensor hybrid histidine kinase, BarA Ac , inhibits the expression of the type III secretion system regulator HrpG in Acidovorax citrulli

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli, severely damages watermelon, melon, and other cucurbit crops worldwide. Although many virulence determinants have been identified in A. citrulli, including swimming motility, twitching motility, biofilm formation, and the type III secretion system (T3SS), research on their regulation is lacking. To study virulence regulation mechanisms, we found a putative histidine kinase BarA _Ac that may be related to the T3SS regulator HrpG in A. citrulli. We deleted and characterized barA_Ac (Aave_2063) in A. citrulli Aac5 strain. Compared to the wild-type Aac5, virulence and early proliferation of barA_Ac mutant in host watermelon cotyledons were significantly increased, and induction of hypersensitive response in non-host tobacco was accelerated, while biofilm formation and swimming motility were significantly reduced. In addition, the transcriptomic analysis revealed that the expression of many T3SS-related genes was upregulated in the ΔbarA_Ac deletion mutant when cultured in KB medium. Meanwhile, the ΔbarA_Ac deletion mutant showed increased accumulation of the T3SS regulator HrpG in KB medium, which may account for the increased deployment of T3SS. This suggests that the putative histidine kinase BarA _Ac is able to repress the T3SS expression by inhibiting HrpG in the KB medium, which appears to be important for rational energy allocation. In summary, our research provides further understanding of the regulatory network of A. citrulli virulence.

Exogenous genistein enhances soybean resistance to Xanthomonas axonopodis pv. glycines

BACKGROUND

Xanthomonas axonopodis pv. glycines (Xag) is the causal agent of bacterial pustule disease and results in enormous losses in soybean production. Although isoflavones are known to be involved in soybean resistance against pathogen infection, the effects of exogenous isoflavones on soybean plants remain unexplored.

RESULTS

Irrigation of soybean plants with isoflavone genistein inhibited plant growth for short periods, probably by inhibiting the tyrosine (brassinosteroids) kinase pathway, and increased disease resistance against Xag. The number of lesions was reduced by 59%-63% when applying 50 μg ml-1 genistein. The effects on disease resistance were observed for 15 days after treatment. Genistein also enhanced the disease resistance of soybean against the fungal pathogen Sclerotinia sclerotiorum. Exogenous genistein increased antioxidant capacity, decreased H2 O2 level and promoted the accumulation of phenolics in Xag-infected soybean leaves. Exogenous genistein reduced the amounts of endogenous daidzein, genistein and glycitein and increased the concentration of genistin, which was found to show strong antibacterial activity against the pathogen and to reduce the expression of virulence factor yapH, and flagella formation gene flgK. The expression of several soybean defense genes, such as chalcone isomerase, glutathione S-transferase and 1-aminocyclopropane-1-carboxylate oxidase 1, was upregulated after genistein treatment.

CONCLUSIONS

The effects of exogenous genistein on soybean plants were examined for the first time, revealing new insights into the roles of isoflavones in soybean defense and demonstrating that irrigation with genistein can be a suitable method to induce disease resistance in soybean plants. © 2022 Society of Chemical Industry.

Deciphering the role of plant plasma membrane lipids in response to invasion patterns: how could biology and biophysics help?

Plants have to constantly face pathogen attacks. To cope with diseases, they have to detect the invading pathogen as early as possible via the sensing of conserved motifs called invasion patterns. The first step of perception occurs at the plasma membrane. While many invasion patterns are perceived by specific proteinaceous immune receptors, several studies have highlighted the influence of the lipid composition and dynamics of the plasma membrane in the sensing of invasion patterns. In this review, we summarize current knowledge on how some microbial invasion patterns could interact with the lipids of the plasma membrane, leading to a plant immune response. Depending on the invasion pattern, different mechanisms are involved. This review outlines the potential of combining biological with biophysical approaches to decipher how plasma membrane lipids are involved in the perception of microbial invasion patterns.

Identification of Genes in Xanthomonas euvesicatoria pv. rosa That Are Host Limiting in Tomato

Xanthomonas euvesicatoria pv. rosa strain Xer07 causes a leaf spot on a Rosa sp. and is closely related to X. euvesicatoria pv. euvesicatoria (Xee) and X. perforans (Xp), causal agents of bacterial spot of tomato. However, Xer07 is not pathogenic on tomato and elicits a hypersensitive reaction (HR). We compared the genomes of the three bacterial species to identify the factors that limit Xer07 on tomato. Comparison of pathogenicity associated factors including the type III secretion systems identified two genes, xopA and xer3856, in Xer07 that have lower sequence homology in tomato pathogens. xer3856 is a homolog of genes in X. citri (xac3856) and X. fuscans pv. aurantifolii, both of which have been reported to elicit HRs in tomato. When xer3856 was expressed in X. perforans and infiltrated in tomato leaflets, the transconjugant elicited an HR and significantly reduced bacterial populations compared to the wildtype X. perforans strain. When xer3856 was mutated in Xer07, the mutant strain still triggered an HR in tomato leaflets. The second gene identified codes for type III secreted effector XopA, which contains a harpin domain that is distinct from the xopA homologs in Xee and Xp. The Xer07-xopA, when expressed in X. perforans, did not elicit an HR in tomato leaflets, but significantly reduced bacterial populations. This indicates that xopA and xer3856 genes in combination with an additional factor(s) limit Xer07 in tomato.

Machine learning approaches to predict the Plant-associated phenotype of Xanthomonas strains

BACKGROUND

The genus Xanthomonas has long been considered to consist predominantly of plant pathogens, but over the last decade there has been an increasing number of reports on non-pathogenic and endophytic members. As Xanthomonas species are prevalent pathogens on a wide variety of important crops around the world, there is a need to distinguish between these plant-associated phenotypes. To date a large number of Xanthomonas genomes have been sequenced, which enables the application of machine learning (ML) approaches on the genome content to predict this phenotype. Until now such approaches to the pathogenomics of Xanthomonas strains have been hampered by the fragmentation of information regarding pathogenicity of individual strains over many studies. Unification of this information into a single resource was therefore considered to be an essential step.

RESULTS

Mining of 39 papers considering both plant-associated phenotypes, allowed for a phenotypic classification of 578 Xanthomonas strains. For 65 plant-pathogenic and 53 non-pathogenic strains the corresponding genomes were available and de novo annotated for the presence of Pfam protein domains used as features to train and compare three ML classification algorithms; CART, Lasso and Random Forest.

CONCLUSION

The literature resource in combination with recursive feature extraction used in the ML classification algorithms provided further insights into the virulence enabling factors, but also highlighted domains linked to traits not present in pathogenic strains.

The Induction of the Isoflavone Biosynthesis Pathway Is Associated with Resistance to Common Bacterial Blight in Phaseolus vulgaris L

Xanthomonas axonopodis infects common bean (Phaseolus vulgaris L.) causing the disease common bacterial blight (CBB). The aim of this study was to investigate the molecular and metabolic mechanisms underlying CBB resistance in P. vulgaris. Trifoliate leaves of plants of a CBB-resistant P. vulgaris recombinant inbred line (RIL) and a CBB-susceptible RIL were inoculated with X. axonopodis or water (mock treatment). Leaves sampled at defined intervals over a 48-h post-inoculation (PI) period were monitored for alterations in global transcript profiles. A total of 800 genes were differentially expressed between pathogen and mock treatments across both RILs; approximately half were differentially expressed in the CBB-resistant RIL at 48 h PI. Notably, there was a 4- to 32-fold increased transcript abundance for isoflavone biosynthesis genes, including several isoflavone synthases, isoflavone 2'-hydroxylases and isoflavone reductases. Ultra-high performance liquid chromatography-tandem mass spectrometry assessed leaf metabolite levels as a function of the PI period. The concentrations of the isoflavones daidzein and genistein and related metabolites coumestrol and phaseollinisoflavan were increased in CBB-resistant RIL plant leaves after exposure to the pathogen. Isoflavone pathway transcripts and metabolite profiles were unaffected in the CBB-susceptible RIL. Thus, induction of the isoflavone pathway is associated with CBB-resistance in P. vulgaris.

Antibacterial mechanism of Biochanin A and its efficacy for the control of Xanthomonas axonopodis pv. glycines in soybean

BACKGROUND

Xanthomonas axonopodis pv. glycines (Xag) is a hazardous pathogen able to cause bacterial pustule disease in soybean, reducing crop yield and quality. Although flavonoids rutin and genistein are known to play an important role in soybean defence, soybean is only able to produce Biochanin A in low concentration.

RESULTS

In this work, Biochanin A was found to produce higher antibacterial activity against Xag in comparison with genistein (minimum inhibitory concentration < 100 μg/mL). Biochanin A was able to inhibit DNA synthesis and flagella formation in Xag, and altered the composition of the bacterial membrane. These effects reduced swimming motility, extracellular protease activity and biofilm formation. Further, Biochanin A was tested for the control of Xag in soybean leaves, showing similar, or even higher, inhibitory ability in comparison with some products commonly used for the control of this pathogen.

CONCLUSIONS

The antibacterial properties of Biochanin A against Xag have been studied for the first time, revealing new insights on the potential applications of this isoflavonoid for the management of bacterial pustule disease. © 2020 Society of Chemical Industry.

The stability of the coiled-coil structure near to N-terminus influence the heat resistance of harpin proteins from Xanthomonas

BACKGROUND

Heat resistance is a common characteristic of harpins, a class of proteins found in Gram-negative bacteria, which may be related to the stability of coiled-coil (CC) structure. The CC structure is a ubiquitous protein folding and assembly motif made of α-helices wrapping around each other forming a supercoil. Specifically, whether the stability of the CC structure near to N-terminus of four selected harpin proteins from Xanthomonas (hereafter referred to as Hpa1) would influence their characteristics of heat resistance was investigated. We used bioinformatics approach to predict the structure of Hpa1, used the performance of hypersensitive response (HR)-induction activity of Hpa1 and circular dichroism (CD) spectral analyses to detect the relationship between the stability of the CC structure of Hpa1 and heat resistance.

RESULTS

Each of four-selected Hpa1 has two α-helical regions with one in their N-terminus that could form CC structure, and the other in their C-terminus that could not. And the important amino acid residues involved in the CC motifs are located on helices present on the surface of these proteins, indicating they may engage in the formation of oligo mericaggregates, which may be responsible for HR elicitation by harpins and their high thermal stability. Increased or decreased the probability of forming a CC could either induce a stronger HR response or eliminate the ability to induce HR in tobacco after high temperature treatment. In addition, although the four Hpa1 mutants had little effect on the induction of HR by Hpa1, its thermal stability was significantly decreased. The α-helical content increased with increasing temperature, and the secondary structures of Hpa1 became almost entirely α-helices when the temperature reached 200 °C. Moreover, the stability of the CC structure near to N-terminus was found to be positively correlated with the heat resistance of Hpa1.

CONCLUSIONS

The stability of the CC structure might sever as an inner drive for mediating the heat resistance of harpin proteins. Our results offer a new insight into the interpretation of the mechanism involved in the heat resistance of harpin protein and provide a theoretical basis for further harpin function investigations and structure modifications.

Exogenous Application of Harpin Protein Hpa1 onto Pinellia ternata Induces Systemic Resistance Against Tobacco Mosaic Virus

The harpin protein Hpa1 has various beneficial effects in plants, such as promoting plant growth and inducing pathogen resistance. Our previous study found that Hpa1 could significantly alleviate the mosaic symptoms of tobacco mosaic virus (TMV) in Pinellia ternata, indicating that Hpa1 can effectively stimulate resistance. Here, the potential mechanism of disease resistance and field applicability of Hpa1 against TMV in P. ternata were further investigated. The results showed that 15 µg ml-1 Hpa1 had stronger antiviral activity than the control, and its protective effect was better than its curative effect. Furthermore, Hpa1 could significantly induce an increase in defense-related enzyme activity, including polyphenol oxidase, peroxidase, catalase, and superoxide dismutase, as well as increase the expression of disease resistance-related genes (PR1, PR3, PR5, and PDF1.2). Concurrently, Hpa1 significantly increased the content of some disease resistance-related substances, including hydrogen peroxide, phenolics, and callose, whereas the content of malondialdehyde was reduced. In addition, field application analysis demonstrated that Hpa1 could effectively elicit a defense response against TMV in P. ternata. Our findings propose a mechanism by which Hpa1 can prevent TMV infection in Pinellia by inducing systemic resistance, thereby providing an environmentally friendly approach for the use of Hpa1 in large-scale applications to improve TMV resistance in Pinellia.

Analysis of HrpG regulons and HrpG-interacting proteins by ChIP-seq and affinity proteomics in Xanthomonas campestris

Gamma-proteobacteria Xanthomonas spp. cause at least 350 different plant diseases among important agricultural crops, which result in serious yield losses. Xanthomonas spp. rely mainly on the type III secretion system (T3SS) to infect their hosts and induce a hypersensitive response in nonhosts. HrpG, the master regulator of the T3SS, plays the dominant role in bacterial virulence. In this study, we used chromatin immunoprecipitation followed by sequencing (ChIP-seq) and tandem affinity purification (TAP) to systematically characterize the HrpG regulon and HrpG interacting proteins in vivo. We obtained 186 candidate HrpG downstream genes from the ChIP-seq analysis, which represented the genomic-wide regulon spectrum. A consensus HrpG-binding motif was obtained and three T3SS genes, hpa2, hrcU, and hrpE, were confirmed to be directly transcriptionally activated by HrpG in the inducing medium. A total of 273 putative HrpG interacting proteins were identified from the TAP data and the DNA-binding histone-like HU protein of Xanthomonas campestris pv. campestris (HU_xcc ) was proved to be involved in bacterial virulence by increasing the complexity and intelligence of the bacterial signalling pathways in the T3SS.

A LysR-Type Transcriptional Regulator LcrX Is Involved in Virulence, Biofilm Formation, Swimming Motility, Siderophore Secretion, and Growth in Sugar Sources in Xanthomonas axonopodis Pv. glycines

Xanthomonas axonopodis pv. glycines (Xag) is a Gram-negative bacterium that causes bacterial pustule disease in soybean. To acclimate to new environments, the expression of genes in bacteria is controlled directly or indirectly by diverse transcriptional factors. Among them, LysR type transcriptional regulators are well-characterized and abundant in bacteria. In a previous study, comparative proteomic analysis revealed that LysR type carbohydrate-related transcriptional regulator in Xag (LcrX) was more abundant in XVM2, which is a minimal medium, compared with a rich medium. However, the functions of LcrX in Xag have not been characterized. In this study, we generated an LcrX-overexpressing strain, Xag(LcrX), and the knockout mutant strain, XagΔlcrX(EV), to elucidate the functions of LcrX. Bacterial multiplication of Xag(LcrX) in soybean was significantly impaired, indicating that LcrX is related to virulence. Comparative proteomic analysis revealed that LcrX is mainly involved in carbohydrate metabolism/transport and inorganic ion transport/metabolism. Based on the results of proteomics analysis, diverse phenotypic assays were carried out. A gel electrophoresis mobility shift assay demonstrated that LcrX specifically bound to the putative promoter regions of genes encoding putative fructose 1,6-bisphosphatase and protease. Through a 96-well plate assay under various conditions, we confirmed that the growth of Xag(LcrX) was dramatically affected in the presence of various carbon sources, while the growth of XagΔlcrX(EV) was only slightly changed. Biofilm formation activity was reduced in Xag(LcrX) but enhanced in XagΔlcrX(EV). The production of siderophores was also decreased in Xag(LcrX) but not altered in XagΔlcrX(EV). In contrast, LcrX was not associated with exopolysaccharide production, protease activity, or bacterial motility. These findings provide new insights into the functions of a carbohydrate-related transcriptional regulator in Xag.

HpaXpm, a novel harpin of Xanthomonas phaseoli pv. manihotis, acts as an elicitor with high thermal stability, reduces disease, and promotes plant growth

BACKGROUND

Harpins are proteins secreted by the type III secretion system of Gram-negative bacteria during pathogen-plant interactions that can act as elicitors, stimulating defense and plant growth in many types of non-host plants. Harpin-treated plants have higher resistance, quality and yields and, therefore, harpin proteins may potentially have many valuable agricultural applications. Harpins are characterized by high thermal stability at 100 °C. However, it is unknown whether harpins are still active at temperatures above 100 °C or whether different temperatures affect the activity of the harpin protein in different ways. The mechanism responsible for the heat stability of harpins is also unknown.

RESULTS

We identified a novel harpin, HpaXpm, from the cassava blight bacteria Xanthomonas phaseoli pv. manihotis HNHK. The predicted secondary structure and 3-D structure indicated that the HpaXpm protein has two β-strand domains and two major α-helical domains located at the N- and C-terminal regions, respectively. A phylogenetic tree generated using the maximum likelihood method grouped HpaXpm in clade I of the Hpa1 group along with harpins produced by other Xanthomonas spp. (i.e., HpaG-Xag, HpaG-Xcm, Hpa1-Xac, and Hpa1Xm). Phenotypic assays showed that HpaXpm induced the hypersensitive response (HR), defense responses, and growth promotion in non-host plants more effectively than Hp1Xoo (X. oryzae pv. oryzae). Quantitative real-time PCR analysis indicated that HpaXpm proteins subjected to heat treatments at 100 °C, 150 °C, or 200 °C were still able to stimulate the expression of function-related genes (i.e., the HR marker genes Hin1 and Hsr203J, the defense-related gene NPR1, and the plant growth enhancement-related gene NtEXP6); however, the ability of heat-treated HpaXpm to induce HR was different at different temperatures.

CONCLUSIONS

These findings add a new member to the harpin family. HpaXpm is heat-stable up to 200 °C and is able to stimulate powerful beneficial biological functions that could potentially be more valuable for agricultural applications than those stimulated by Hpa1Xoo. We hypothesize that the extreme heat resistance of HpaXpm is because the structure of harpin is very stable and, therefore, the HpaXpm structure is less affected by temperature.

Tal1NXtc01 in Xanthomonas translucens pv. cerealis Contributes to Virulence in Bacterial Leaf Streak of Wheat

Xanthomonas translucens pv. cerealis (Xtc) causes bacterial leaf streak (BLS) of important cereal crops, including wheat (Triticum aestivum) and barley (Hordeum vulgare). Transcription activator-like effectors (TALEs) play vital roles in many plant diseases caused by Xanthomonas spp., however, TALEs have not been previously characterized in Xtc. In this study, the whole genome of NXtc01, a virulent strain of Xtc from Xinjiang, China, was sequenced and compared with genomes of other Xanthomonas spp. Xtc NXtc01 consists of a single 4,622,298 bp chromosome that encodes 4,004 genes. Alignment of the NXtc01 sequence with the draft genome of Xtc strain CFBP 2541 (United States) revealed a single giant inversion and differences in the location of two tal genes, which were designated tal1 and tal2. In NXtc01, both tal genes are located on the chromosome, whereas tal2 is plasmid-encoded in CFBP 2541. The repeat variable diresidues (RVDs) at the 12th and 13th sites within Tal2 repeat units were identical in both strains, whereas Tal1 showed differences in the third RVD. Xtc NXtc01 and CFBP 2541 encoded 35 and 33 non-TALE type III effectors (T3Es), respectively. tal1, tal2, and tal-free deletion mutants of Xtc NXtc01 were constructed and evaluated for virulence. The tal1 and tal-free deletion mutants were impaired with respect to symptom development and growth in wheat, suggesting that tal1 is a virulence factor in NXtc01. This was confirmed in gain-of-function experiments that showed the introduction of tal1, but not tal2, restored virulence to the tal-free mutant. Furthermore, we generated a hrcC deletion mutant of NXtc01; the hrcC mutant was non-pathogenic on wheat and unable to elicit a hypersensitive response in the non-host Nicotiana benthamiana. Our data provide a platform for exploring the roles of both TALEs and non-TALEs in promoting BLS on wheat.

The OmpA Gene of Xanthomonas axonopodis pv. glycines is Involved in Pathogenesis of Pustule Disease on Soybean

The goal of this study was to elucidate the role of the outer membrane protein A (ompA) gene of Xanthomonas axonopodis pv. glycines in bacterial pustule pathogenesis of soybean. An ompA mutant of X. axonopodis pv. glycines KU-P-SW005 was shown to significantly decrease cellulase, pectate lyase, and polysaccharide production. The production of these proteins in the ompA mutant was approximately five times lower than that of the wildtype. The ompA mutant also exhibited modified biofilm development. More importantly, the mutant reduced disease severity to the soybean. Ten days after inoculation, the virulence rating of the susceptible soybean cv. SJ4 inoculated with the ompA mutant was 11.23%, compared with 87.98% for the complemented ompA mutant. Production of cellulase, pectate lyase, polysaccharide was restored, biofilm, and pustule numbers were restored in the complemented ompA mutant that did not differ from the wild type. Taken together, these data suggest that OmpA-mediated invasion plays an important role in protein secretion during pathogenesis to soybean.

High temperatures affect the hypersensitive reaction, disease resistance and gene expression induced by a novel harpin HpaG-Xcm

Harpin proteins are produced by plant-pathogenic Gram-negative bacteria and regulate bacterial pathogenicity by inducing plant growth and defence responses in non-hosts. HpaG-Xcm, a novel harpin protein, was identified from Xanthomonas citri pv. mangiferaeindicae, which causes bacterial black spot of mango. Here, we describe the predicted structure and functions of HpaG-Xcm and investigate the mechanism of heat resistance. The HpaG-Xcm amino acid sequence contains seven motifs and two α-helices, in the N- and C-terminals, respectively. The N-terminal α-helical region contains two heptads, which form the coiled-coil (CC) structure. The CC region, which is on the surface of HpaG-Xcm, forms oligomeric aggregates by forming hydrophobic interactions between hydrophobic amino acids. Like other harpins, HpaG-Xcm was heat stable, promoted root growth and induced a hypersensitive response (HR) and systemic acquired resistance in non-host plants. Subjecting HpaG-Xcm to high temperatures altered the gene expression induced by HpaG-Xcm in tobacco leaves, probably due to changes in the spatial structure of HpaG-Xcm. Phenotypic tests revealed that the high-temperature treatments reduced the HR and disease resistance induced by HpaG-Xcm but had little effect on growth promotion. These findings indicate that the stability of interactions between CC and plants may be associated with thermal stability of HpaG-Xcm.

Beyond the expected: the structural and functional diversity of bacterial amyloids

Intense research has confirmed the formerly theoretical distribution of amyloids in nature, and studies on different systems have illustrated the role of these proteins in microbial adaptation and in interactions with the environment. Two lines of research are expanding our knowledge on functional amyloids: (i) structural studies providing insights into the molecular machineries responsible for the transition from monomer to fibers and (ii) studies showing the way in which these proteins might participate in the microbial fitness in natural settings. Much is known about how amyloids play a role in the social behavior of bacteria, or biofilm formation, and in the adhesion of bacteria to surfaces; however, we are still in the initial stages of understanding a complementary involvement of amyloids in bacteria-host interactions. This review will cover the following two topics: first, the key aspects of the microbial platforms dedicated to the assembly of the fibers, and second, the mechanisms by which bacteria utilize the morphological and biochemical variability of amyloids to modulate the immunological response of the host, plants and humans, contributing to (i) infection, in the case of pathogenic bacteria or (ii) promotion of the health of the host, in the case of beneficial bacteria.

Functional Amyloid and Other Protein Fibers in the Biofilm Matrix

Biofilms are ubiquitous in the natural and man-made environment. They are defined as microbes that are encapsulated in an extracellular, self-produced, biofilm matrix. Growing evidence from the genetic and biochemical analysis of single species biofilms has linked the presence of fibrous proteins to a functional biofilm matrix. Some of these fibers have been described as functional amyloid or amyloid-like fibers. Here we provide an overview of the biophysical and biological data for a wide range of protein fibers found in the biofilm matrix of Gram-positive and Gram-negative bacteria.

Involvement of hrpX and hrpG in the Virulence of Acidovorax citrulli Strain Aac5, Causal Agent of Bacterial Fruit Blotch in Cucurbits

Acidovorax citrulli causes bacterial fruit blotch, a disease that poses a global threat to watermelon and melon production. Despite its economic importance, relatively little is known about the molecular mechanisms of pathogenicity and virulence of A. citrulli. Like other plant-pathogenic bacteria, A. citrulli relies on a type III secretion system (T3SS) for pathogenicity. On the basis of sequence and operon arrangement analyses, A. citrulli was found to have a class II hrp gene cluster similar to those of Xanthomonas and Ralstonia spp. In the class II hrp cluster, hrpG and hrpX play key roles in the regulation of T3SS effectors. However, little is known about the regulation of the T3SS in A. citrulli. This study aimed to investigate the roles of hrpG and hrpX in A. citrulli pathogenicity. We found that hrpG or hrpX deletion mutants of the A. citrulli group II strain Aac5 had reduced pathogenicity on watermelon seedlings, failed to induce a hypersensitive response in tobacco, and elicited higher levels of reactive oxygen species in Nicotiana benthamiana than the wild-type strain. Additionally, we demonstrated that HrpG activates HrpX in A. citrulli. Moreover, transcription and translation of the type 3-secreted effector (T3E) gene Aac5_2166 were suppressed in hrpG and hrpX mutants. Notably, hrpG and hrpX appeared to modulate biofilm formation. These results suggest that hrpG and hrpX are essential for pathogenicity, regulation of T3Es, and biofilm formation in A. citrulli.

Comparative Proteomic Analysis of Three Xanthomonas spp. Cultured in Minimal and Rich Media

Bacteria change their gene expression when exposed to different nutrient conditions. The levels of proteins do not always correlate with those of RNAs, hence proteomic analysis is required for understanding how bacteria adapt to different conditions. Herein, differentially abundant proteins from Xanthomonas oryzae pv. oryzae (Xoo), X. campestris pv. vesicatoria (Xcv), and X. axonopodis pv. glycines (Xag), which were cultured in rich media and in minimal media, were determined using label-free shotgun proteomic analysis and clusters of orthologous groups classification. The detected proteins from all three species ranged from 1190 to 1187. Among them, 702, 584, and 529 proteins from Xoo, Xcv, and Xag, respectively, were more (> twofold) abundant depending on the media, indicating that about 11.4-13.8% of proteins from the three species were differentially expressed. The levels of abundant proteins in minimal media were significantly higher than those in rich media for all three species, demonstrating how Xanthomonas species actively change their protein expression in different nutrient conditions. These results will lead to new insights in elucidation of cellular mechanisms involved in virulence and adaption of bacteria to harsh environments for further studies. The MS proteomics data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD006310.

HPAF de <i>Xanthomonas axonopodis</i> PV. <i>manihotis</i> regula negativamente genes relacionados con metabolismo y defensa en hojas de yuca

Las bacterias fitopatógenas de los géneros Erwinia, Pantoea, Pseudomonas, Ralstonia y Xanthomonas causan una gran cantidad de enfermedades en diversos cultivos. La base molecular que explica parcialmente la patogenicidad de estas bacterias radica en la translocación de proteínas efectoras hacia el interior de las células hospederas a través del sistema de secreción tipo tres (SST3). Xanthomonas axonopodis pv. manihotis (Xam) es un bacilo gram negativo y es el agente causal de la bacteriosis vascular de la yuca (Manihot esculenta Crantz). Los estudios derivados de la secuenciación del genoma de diversas cepas de Xam han permitido la identificación de hrp-associated F (HpaF) como un efector principal presente en todas las cepas secuenciadas de Latinoamérica, África y Asia. En este trabajo se evaluó la importancia de HpaF en la virulencia de Xam empleando dos estrategias. Primeramente, se pudo determinar a nivel histológico cambios morfológicos en las células de yuca causadas por HpaF. En segundo lugar, se empleó una estrategia de transcriptómica comparativa empleando un microarreglo de ADNc de yuca y ARN obtenido de plantas de yuca inoculadas con cepas de Xam mutadas en hpaF (ΔhpaF) o complementadas (ΔhpaF +hpaF). Los datos obtenidos sugieren que HpaF es un factor de virulencia de Xam ya que regula negativamente genes involucrados en el metabolismo y defensa de la planta.

HpaB-Dependent Secretion of Type III Effectors in the Plant Pathogens Ralstonia solanacearum and Xanthomonas campestris pv. vesicatoria

Plant pathogenic bacteria exerts their pathogenicity through the injection of large repertoires of type III effectors (T3Es) into plant cells, a mechanism controlled in part by type III chaperones (T3Cs). In Ralstonia solanacearum, the causal agent of bacterial wilt, little is known about the control of type III secretion at the post-translational level. Here, we provide evidence that the HpaB and HpaD proteins do act as bona fide R. solanacearum class IB chaperones that associate with several T3Es. Both proteins can dimerize but do not interact with each other. After screening 38 T3Es for direct interactions, we highlighted specific and common interacting partners, thus revealing the first picture of the R. solanacearum T3C-T3E network. We demonstrated that the function of HpaB is conserved in two phytopathogenic bacteria, R. solanacearum and Xanthomonas campestris pv. vesicatoria (Xcv). HpaB from Xcv is able to functionally complement a R. solanacearum hpaB mutant for hypersensitive response elicitation on tobacco plants. Likewise, Xcv is able to translocate a heterologous T3E from R. solanacearum in an HpaB-dependent manner. This study underlines the central role of the HpaB class IB chaperone family and its potential contribution to the bacterial plasticity to acquire and deliver new virulence factors.

The Predicted Lytic Transglycosylase HpaH from Xanthomonas campestris pv. vesicatoria Associates with the Type III Secretion System and Promotes Effector Protein Translocation

The pathogenicity of the Gram-negative plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria depends on a type III secretion (T3S) system, which spans both bacterial membranes and translocates effector proteins into plant cells. The assembly of the T3S system presumably involves the predicted lytic transglycosylase (LT) HpaH, which is encoded adjacent to the T3S gene cluster. Bacterial LTs degrade peptidoglycan and often promote the formation of membrane-spanning macromolecular protein complexes. In the present study, we show that HpaH localizes to the bacterial periplasm and binds to peptidoglycan as well as to components of the T3S system, including the predicted periplasmic inner rod proteins HrpB1 and HrpB2 as well as the pilus protein HrpE. In vivo translocation assays revealed that HpaH promotes the translocation of various effector proteins and of early substrates of the T3S system, suggesting a general contribution of HpaH to type III-dependent protein export. Mutant studies and the analysis of reporter fusions showed that the N-terminal region of HpaH contributes to protein function and is proteolytically cleaved. The N-terminally truncated HpaH cleavage product is secreted into the extracellular milieu by a yet-unknown transport pathway, which is independent of the T3S system.

The signal peptide-like segment of hpaXm is required for its association to the cell wall in transgenic tobacco plants

Harpins, encoded by hrp (hypersensitive response and pathogenicity) genes of Gram-negative plant pathogens, are elicitors of hypersensitive response (HR). HpaXm is a novel harpin-like protein described from cotton leaf blight bacteria, Xanthomonas citri subsp. malvacearum-a synonym of X. campestris pv. malvacearum (Smith 1901-1978). A putative signal peptide (1-MNSLNTQIGANSSFL-15) of hpaXm was predicted in the nitroxyl-terminal (N-terminal)by SignalP (SignalP 3.0 server). Here, we explored the function of the N-terminal leader peptide like segment of hpaXm using transgenic tobacco (Nicotiana tabacum cv. Xanthi nc.). Transgenic tobacco lines expressing the full-length hpaXm and the signal peptide-like segment-deleted mutant hpaXmΔLP were developed using transformation mediated by Agrobacterium tumefaciens. The target genes were confirmed integrated into the tobacco genomes and expressed normally. Using immune colloidal-gold detection technique, hpaXm protein was found to be transferred to the cytoplasm, the cell membrane, and organelles such as chloroplasts, mitochondria, and nucleus, as well as the cell wall. However, the deletion mutant hpaXmΔLP expressed in transgenic tobacco was found unable to cross the membrane to reach the cell wall. Additionally, soluble proteins extracted from plants transformed with hpaXm and hpaXmΔLP were bio-active. Defensive micro-HR induced by the transgene expression of hpaXm and hpaXmΔLP were observed on transgenic tobacco leaves. Disease resistance bioassays to tobacco mosaic virus (TMV) showed that tobacco plants transformed with hpaXm and with hpaXmΔLP exhibited enhanced resistance to TMV. In summary, the N-terminal signal peptide-like segment (1-45 bp) in hpaXm sequence is not necessary for transgene expression, bioactivity of hpaXm and resistance to TMV in transgenic tobacco, but is required for the protein to be translocated to the cell wall.

Global Pattern of Gene Expression of Xanthomonas axonopodis pv. glycines Within Soybean Leaves

To better understand the behavior of Xanthomonas axonopodis pv. glycines, the causal agent of bacterial pustule of soybean within its host, its global transcriptome within soybean leaves was compared with that in a minimal medium in vitro, using deep sequencing of mRNA. Of 5,062 genes predicted from a draft genome of X. axonopodis pv. glycines, 534 were up-regulated in the plant, while 289 were down-regulated. Genes encoding YapH, a cell-surface adhesin, as well as several others encoding cell-surface proteins, were down-regulated in soybean. Many genes encoding the type III secretion system and effector proteins, cell wall-degrading enzymes and phosphate transporter proteins were strongly expressed at early stages of infection. Several genes encoding RND multidrug efflux pumps were induced in planta and by isoflavonoids in vitro and were required for full virulence of X. axonopodis pv. glycines, as well as resistance to soybean phytoalexins. Genes encoding consumption of malonate, a compound abundant in soybean, were induced in planta and by malonate in vitro. Disruption of the malonate decarboxylase operon blocked growth in minimal media with malonate as the sole carbon source but did not significantly alter growth in soybean, apparently because genes for sucrose and fructose uptake were also induced in planta. Many genes involved in phosphate metabolism and uptake were induced in planta. While disruption of genes encoding high-affinity phosphate transport did not alter growth in media varying in phosphate concentration, the mutants were severely attenuated for growth in soybean. This global transcriptional profiling has provided insight into both the intercellular environment of this soybean pathogen and traits used by X. axonopodis pv. glycines to promote disease.

Transcriptome-Based Identification of Differently Expressed Genes from Xanthomonas oryzae pv. oryzae Strains Exhibiting Different Virulence in Rice Varieties

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight (BB) in rice (Oryza sativa L.). In this study, we investigated the genome-wide transcription patterns of two Xoo strains (KACC10331 and HB1009), which showed different virulence patterns against eight rice cultivars, including IRBB21 (carrying Xa21). In total, 743 genes showed a significant change (p-value < 0.001 in t-tests) in their mRNA expression levels in the HB1009 (K3a race) strain compared with the Xoo KACC10331 strain (K1 race). Among them, four remarkably enriched GO terms, DNA binding, transposition, cellular nitrogen compound metabolic process, and cellular macromolecule metabolic process, were identified in the upregulated genes. In addition, the expression of 44 genes was considerably higher (log2 fold changes > 2) in the HB1009 (K3a race) strain than in the Xoo KACC10331 (K1 race) strain. Furthermore, 13 and 12 genes involved in hypersensitive response and pathogenicity (hrp) and two-component regulatory systems (TCSs), respectively, were upregulated in the HB1009 (K3a race) strain compared with the Xoo KACC10331 (K1 race) strain, which we determined using either quantitative real-time PCR analysis or next-generation RNA sequencing. These results will be helpful to improve our understanding of Xoo and to gain a better insight into the Xoo–rice interactions.

Comparative Genomics of Pathogenic and Nonpathogenic Strains of Xanthomonas arboricola Unveil Molecular and Evolutionary Events Linked to Pathoadaptation

The bacterial species Xanthomonas arboricola contains plant pathogenic and nonpathogenic strains. It includes the pathogen X. arboricola pv. juglandis, causing the bacterial blight of Juglans regia. The emergence of a new bacterial disease of J. regia in France called vertical oozing canker (VOC) was previously described and the causal agent was identified as a distinct genetic lineage within the pathovar juglandis. Symptoms on walnut leaves and fruits are similar to those of a bacterial blight but VOC includes also cankers on trunk and branches. In this work, we used comparative genomics and physiological tests to detect differences between four X. arboricola strains isolated from walnut tree: strain CFBP 2528 causing walnut blight (WB), strain CFBP 7179 causing VOC and two nonpathogenic strains, CFBP 7634 and CFBP 7651, isolated from healthy walnut buds. Whole genome sequence comparisons revealed that pathogenic strains possess a larger and wider range of mobile genetic elements than nonpathogenic strains. One pathogenic strain, CFBP 7179, possessed a specific integrative and conjugative element (ICE) of 95 kb encoding genes involved in copper resistance, transport and regulation. The type three effector repertoire was larger in pathogenic strains than in nonpathogenic strains. Moreover, CFBP 7634 strain lacked the type three secretion system encoding genes. The flagellar system appeared incomplete and nonfunctional in the pathogenic strain CFBP 2528. Differential sets of chemoreceptor and different repertoires of genes coding adhesins were identified between pathogenic and nonpathogenic strains. Besides these differences, some strain-specific differences were also observed. Altogether, this study provides valuable insights to highlight the mechanisms involved in ecology, environment perception, plant adhesion and interaction, leading to the emergence of new strains in a dynamic environment.

Comparative Secretome Analysis of Ralstonia solanacearum Type 3 Secretion-Associated Mutants Reveals a Fine Control of Effector Delivery, Essential for Bacterial Pathogenicity

Ralstonia solanacearum, the causal agent of bacterial wilt, exerts its pathogenicity through more than a hundred secreted proteins, many of them depending directly on the functionality of a type 3 secretion system. To date, only few type 3 effectors have been identified as required for bacterial pathogenicity, notably because of redundancy among the large R. solanacearum effector repertoire. In order to identify groups of effectors collectively promoting disease on susceptible hosts, we investigated the role of putative post-translational regulators in the control of type 3 secretion. A shotgun secretome analysis with label-free quantification using tandem mass spectrometry was performed on the R. solanacearum GMI1000 strain. There were 228 proteins identified, among which a large proportion of type 3 effectors, called Rip (Ralstonia injected proteins). Thanks to this proteomic approach, RipBJ was identified as a new effector specifically secreted through type 3 secretion system and translocated into plant cells. A focused Rip secretome analysis using hpa (hypersensitive response and pathogenicity associated) mutants revealed a fine secretion regulation and specific subsets of Rips with different secretion patterns. We showed that a set of Rips (RipF1, RipW, RipX, RipAB, and RipAM) are secreted in an Hpa-independent manner. We hypothesize that these Rips could be preferentially involved in the first stages of type 3 secretion. In addition, the secretion of about thirty other Rips is controlled by HpaB and HpaG. HpaB, a candidate chaperone was shown to positively control secretion of numerous Rips, whereas HpaG was shown to act as a negative regulator of secretion. To evaluate the impact of altered type 3 effectors secretion on plant pathogenesis, the hpa mutants were assayed on several host plants. HpaB was required for bacterial pathogenicity on multiple hosts whereas HpaG was found to be specifically required for full R. solanacearum pathogenicity on the legume plant Medicago truncatula.

Phylogenetic and Variable-Number Tandem-Repeat Analyses Identify Nonpathogenic Xanthomonas arboricola Lineages Lacking the Canonical Type III Secretion System

Xanthomonas arboricola is conventionally known as a taxon of plant-pathogenic bacteria that includes seven pathovars. This study showed that X. arboricola also encompasses nonpathogenic bacteria that cause no apparent disease symptoms on their hosts. The aim of this study was to assess the X. arboricola population structure associated with walnut, including nonpathogenic strains, in order to gain a better understanding of the role of nonpathogenic xanthomonads in walnut microbiota. A multilocus sequence analysis (MLSA) was performed on a collection of 100 X. arboricola strains, including 27 nonpathogenic strains isolated from walnut. Nonpathogenic strains grouped outside clusters defined by pathovars and formed separate genetic lineages. A multilocus variable-number tandem-repeat analysis (MLVA) conducted on a collection of X. arboricola strains isolated from walnut showed that nonpathogenic strains clustered separately from clonal complexes containing Xanthomonas arboricola pv. juglandis strains. Some nonpathogenic strains of X. arboricola did not contain the canonical type III secretion system (T3SS) and harbored only one to three type III effector (T3E) genes. In the nonpathogenic strains CFBP 7640 and CFBP 7653, neither T3SS genes nor any of the analyzed T3E genes were detected. This finding raises a question about the origin of nonpathogenic strains and the evolution of plant pathogenicity in X. arboricola. T3E genes that were not detected in any nonpathogenic isolates studied represent excellent candidates to be those responsible for pathogenicity in X. arboricola.

The Amino Acid Arginine 210 of the Response Regulator HrpG of Xanthomonas citri subsp. citri Is Required for HrpG Function in Virulence

Xanthomonas citri subsp. citri colonizes its hosts through the trafficking of effector proteins to the plant cell by the type III protein secretion system. In X. citri subsp. citri, as in other plant pathogens, the hrp cluster encodes the type III protein secretion system and is regulated by the transcription factors HrpG and HrpX. HrpG belongs to the OmpR family's response regulator of EnvZ/OmpR two-component signal transduction system. Here, we show that the arginine 210 residue is crucial for the transcriptional activity of HrpG revealed by the absence of disease in host plants and hypersensitive response in non-host plants when a strain carrying this point mutation is used in plant infiltration assays. Also, this strain showed decreased expression levels of hrp genes in bacteria grown in culture or when they were recovered from citrus leaves. Moreover, we show for the first time that HrpG binds to both hrpX and its own promoter, and the change of the arginine 210 by a cysteine does not prevent the binding to both promoters. Nevertheless, in vitro hrpX transcription was observed only with HrpG whereas no transcription was detected with the R210C mutant. HrpG was able to interact with itself as well as with the mutant R210C suggesting that it functions as a dimer. The mutant protein R210C showed altered protease sensitivity, suggesting that Arg210 is essential for protein active conformation and thus for transcriptional activity. Our results indicate that arginine 210 in HrpG, as it may occur with this conserved residue in other members of this family of response regulators, is not required for DNA binding whereas is essential for hrp genes transcription and therefore for pathogenicity and HR induction.

Comparative Genomics of Pathogenic and Nonpathogenic Strains of Xanthomonas arboricola Unveil Molecular and Evolutionary Events Linked to Pathoadaptation

The bacterial species Xanthomonas arboricola contains plant pathogenic and nonpathogenic strains. It includes the pathogen X. arboricola pv. juglandis, causing the bacterial blight of Juglans regia. The emergence of a new bacterial disease of J. regia in France called vertical oozing canker (VOC) was previously described and the causal agent was identified as a distinct genetic lineage within the pathovar juglandis. Symptoms on walnut leaves and fruits are similar to those of a bacterial blight but VOC includes also cankers on trunk and branches. In this work, we used comparative genomics and physiological tests to detect differences between four X. arboricola strains isolated from walnut tree: strain CFBP 2528 causing walnut blight (WB), strain CFBP 7179 causing VOC and two nonpathogenic strains, CFBP 7634 and CFBP 7651, isolated from healthy walnut buds. Whole genome sequence comparisons revealed that pathogenic strains possess a larger and wider range of mobile genetic elements than nonpathogenic strains. One pathogenic strain, CFBP 7179, possessed a specific integrative and conjugative element (ICE) of 95 kb encoding genes involved in copper resistance, transport and regulation. The type three effector repertoire was larger in pathogenic strains than in nonpathogenic strains. Moreover, CFBP 7634 strain lacked the type three secretion system encoding genes. The flagellar system appeared incomplete and nonfunctional in the pathogenic strain CFBP 2528. Differential sets of chemoreceptor and different repertoires of genes coding adhesins were identified between pathogenic and nonpathogenic strains. Besides these differences, some strain-specific differences were also observed. Altogether, this study provides valuable insights to highlight the mechanisms involved in ecology, environment perception, plant adhesion and interaction, leading to the emergence of new strains in a dynamic environment.

Oligomerization, conformational stability and thermal unfolding of Harpin, HrpZPss and its hypersensitive response-inducing c-terminal fragment, C-214-HrpZPss

HrpZ-a harpin from Pseudomonas syringae-is a highly thermostable protein that exhibits multifunctional abilities e.g., it elicits hypersensitive response (HR), enhances plant growth, acts as a virulence factor, and forms pores in plant plasma membranes as well as artificial membranes. However, the molecular mechanism of its biological activity and high thermal stability remained poorly understood. HR inducing abilities of non-overlapping short deletion mutants of harpins put further constraints on the ability to establish structure-activity relationships. We characterized HrpZPss from Pseudomonas syringae pv. syringae and its HR inducing C-terminal fragment with 214 amino acids (C-214-HrpZPss) using calorimetric, spectroscopic and microscopic approaches. Both C-214-HrpZPss and HrpZPss were found to form oligomers. We propose that leucine-zipper-like motifs may take part in the formation of oligomeric aggregates, and oligomerization could be related to HR elicitation. CD, DSC and fluorescence studies showed that the thermal unfolding of these proteins is complex and involves multiple steps. The comparable conformational stability at 25°C (∼10.0 kcal/mol) of HrpZPss and C-214-HrpZPss further suggest that their structures are flexible, and the flexibility allows them to adopt proper conformation for multifunctional abilities.

Fold modulating function: bacterial toxins to functional amyloids

Many bacteria produce cytolytic toxins that target host cells or other competing microbes. It is well known that environmental factors control toxin expression, however, recent work suggests that some bacteria manipulate the fold of these protein toxins to control their function. The β-sheet rich amyloid fold is a highly stable ordered aggregate that many toxins form in response to specific environmental conditions. When in the amyloid state, toxins become inert, losing the cytolytic activity they display in the soluble form. Emerging evidence suggest that some amyloids function as toxin storage systems until they are again needed, while other bacteria utilize amyloids as a structural matrix component of biofilms. This amyloid matrix component facilitates resistance to biofilm disruptive challenges. The bacterial amyloids discussed in this review reveal an elegant system where changes in protein fold and solubility dictate the function of proteins in response to the environment.

Draft genome sequence of Xanthomonas axonopodis pv. glycines 8ra possessing transcription activator-like effectors used for genetic engineering

Xanthomonas axonopodis pv. glycines 8ra is a causal agent of bacterial pustule disease in soybean. This bacterium possesses transcription activator-like (TAL) effectors which are useful for genetic/protein engineering applications in higher organisms including plants and humans. Here, we report that the draft genome sequence consists of 5,337,885-bp double-stranded DNA encoding 4674 open reading frames (ORFs) in 13 different contigs. This genome sequence would be useful in applications of TAL effectors in genetic engineering and in elucidating virulence factors against plants.

Transgenic Expression of the Functional Fragment Hpa110-42 of the Harpin Protein Hpa1 Imparts Enhanced Resistance to Powdery Mildew in Wheat

Powdery mildew, one of devastating diseases of wheat worldwide, is caused by Erysiphe graminis f. sp. tritici, a fungal species with constant population changes, which often poses challenges in disease management with host resistance. Transgenic approaches that utilize broad-spectrum resistance may limit changes of pathogen populations and contribute to effective control of the disease. The harpin protein Hpa1, produced by the rice bacterial blight pathogen, can induce resistance to bacterial blight and blast in rice. The fragment comprising residues 10 through 42 of Hpa1, Hpa1_10-42, is reportedly three- to eightfold more effective than the full-length protein. This study evaluated the transgenic expression of the Hpa1_10-42 gene for resistance to powdery mildew in wheat caused by E. graminis f. sp. tritici. Nine Hpa1_10-42 transgenic wheat lines were generated. The genomic integration of Hpa1_10-42 was confirmed, and expression of the transgene was detected at different levels in the individual transgenic lines. Following inoculation with the E. graminis f. sp. tritici isolate Egt15 in the greenhouse, five transgenic lines had significantly higher levels of resistance to powdery mildew compared with nontransformed plants. Thus, transgenic expression of Hpa1_10-42 conferred resistance to one isolate of E. graminis f. sp. tritici in wheat in the greenhouse.

The HrpW protein of Lonsdalea quercina N-5-1 has pectate lyase activity and is required for full bacterial virulence

Lonsdalea quercina N-5-1 is a bacterial pathogen that causes poplar bark cankers. It has been isolated from the branch of Populus × euramericana cv. "74/76" in Henan, China. Previous studies have revealed that the Type III secretion system (T3SS) acts as an essential pathogenic factor in L. quercina N-5-1. HrpW is a putative effector of T3SS in strain N-5-1, which has a typical harpin domain at the amino terminal and a pectate lyase (Pel) domain at its carboxyl terminal. Genetic evidence had shown that, compared to the wild-type and the complementary strain, the hrpW mutation causes a small but significant reduction in virulence when inoculated on the poplar branches. The amino terminal domain of HrpW was found to trigger tobacco hypersensitive response, but the carboxyl terminal domain of HrpW was not. Unlike most HrpW homologs in other bacteria, the carboxyl terminal domain of HrpW of strain N-5-1 exhibited detectable pectate lyase activity. Site-direction mutations (W104A, W171M) further demonstrated that two tryptophan residues were essential to its pectate lyase activity. The results of the present work suggest that HrpW in L. quercina N-5-1 possesses pectate lyase activity and acts as a nonessential but important pathogenic factor in poplar bark canker disease.

Peptides as triggers of plant defence

Plants are confronted with several biotic stresses such as microbial pathogens and other herbivores. To defend against such attackers, plants possess an array of pattern recognition receptors (PRRs) that sense the danger and consequently initiate a defence programme that prevents further damage and spreading of the pest. Characteristic pathogenic structures, so-called microbe-associated molecular patterns (MAMPs), serve as signals that allow the plant to sense invaders. Additionally, pathogens wound or damage the plant and the resulting release of damage-associated molecular patterns (DAMPs) serves as a warning signal. This review focuses on peptides that serve as triggers or amplifiers of plant defence and thus follow the definition of a MAMP or a DAMP.

Comparative proteomic analysis reveals that T3SS, Tfp, and xanthan gum are key factors in initial stages of Citrus sinensis infection by Xanthomonas citri subsp. citri

The bacteria Xanthomonas citri subsp. citri (Xac) is the causal agent of citrus canker. The disease symptoms are characterized by localized host cell hyperplasia followed by tissue necrosis at the infected area. An arsenal of bacterial pathogenicity- and virulence-related proteins is expressed to ensure a successful infection process. At the post-genomic stage of Xac, we used a proteomic approach to analyze the proteins that are displayed differentially over time when the pathogen attacks the host plant. Protein extracts were prepared from infectious Xac grown in inducing medium (XAM1) for 24 h or from host citrus plants for 3 or 5 days after infection, detached times to evaluate the adaptation and virulence of the pathogen. The protein extracts were proteolyzed, and the peptides derived from tryptic digestion were investigated using liquid chromatography and tandem mass spectrometry. Changes in the protein expression profile were compared with the Xac genome and the proteome recently described under non-infectious conditions. An analysis of the proteome of Xac under infectious conditions revealed proteins directly involved in virulence such as the type III secretion system (T3SS) and effector proteins (T3SS-e), the type IV pilus (Tfp), and xanthan gum biosynthesis. Moreover, four new mutants related to proteins detected in the proteome and with different functions exhibited reduced virulence relative to the wild-type proteins. The results of the proteome analysis of infectious Xac define the processes of adaptation to the host and demonstrate the induction of the virulence factors of Xac involved in plant-pathogen interactions.

Harpins, multifunctional proteins secreted by gram-negative plant-pathogenic bacteria

Harpins are glycine-rich and heat-stable proteins that are secreted through type III secretion system in gram-negative plant-pathogenic bacteria. Many studies show that these proteins are mostly targeted to the extracellular space of plant tissues, unlike bacterial effector proteins that act inside the plant cells. Over the two decades since the first harpin of pathogen origin, HrpN of Erwinia amylovora, was reported in 1992 as a cell-free elicitor of hypersensitive response (HR), diverse functional aspects of harpins have been determined. Some harpins were shown to have virulence activity, probably because of their involvement in the translocation of effector proteins into plant cytoplasm. Based on this function, harpins are now considered to be translocators. Their abilities of pore formation in the artificial membrane, binding to lipid components, and oligomerization are consistent with this idea. When harpins are applied to plants directly or expressed in plant cells, these proteins trigger diverse beneficial responses such as induction of defense responses against diverse pathogens and insects and enhancement of plant growth. Therefore, in this review, we will summarize the functions of harpins as virulence factors (or translocators) of bacterial pathogens, elicitors of HR and immune responses, and plant growth enhancers.