Campestris B100 Culture Grown in Minimal Medium with a Specific Focus on Nutrient Consumption and Xanthan Biosynthesis

Xanthan, a bacterial polysaccharide, is widespread in industrial applications, particularly as a food additive. However, little is known about the process of xanthan synthesis on the proteome level, even though Xanthomonas campestris is frequently used for xanthan fermentation. A label-free LC-MS/MS method was employed to study the protein changes during xanthan fermentation in minimal medium. According to the reference database, 2416 proteins were identified, representing 54.75 % of the proteome. The study examined changes in protein abundances concerning the growth phase and xanthan productivity. Throughout the experiment, changes in nitrate concentration appeared to affect the abundance of most proteins involved in nitrogen metabolism, except Gdh and GlnA. Proteins involved in sugar nucleotide metabolism stay unchanged across all growth phases. Apart from GumD, GumB, and GumC, the gum proteins showed no significant changes throughout the experiment. GumD, the first enzyme in the assembly of the xanthan-repeating unit, peaked during the early stationary phase but decreased during the late stationary phase. GumB and GumC, which are involved in exporting xanthan, increased significantly during the stationary phase. This study suggests that a potential bottleneck for xanthan productivity does not reside in the abundance of proteins directly involved in the synthesis pathways.

Unveiling the Role of Diffusible Signal Factor-Family Quorum Sensing Signals in Regulating Behavior of Xanthomonas and Lysobacter

Diffusible signal factor (DSF) family signals represent a unique group of quorum sensing (QS) chemicals that modulate a wide range of behaviors for bacteria to adapt to different environments. However, whether DSF-mediated QS signaling acts as a public language to regulate the behavior of biocontrol and pathogenic bacteria remains unknown. In this study, we present groundbreaking evidence demonstrating that RpfFXc1 or RpfFOH11 could be a conserved DSF-family signal synthase in Xanthomonas campestris or Lysobacter enzymogenes. Interestingly, we found that both RpfFOH11 and RpfFXc1 have the ability to synthesize DSF and BDSF signaling molecules. DSF and BDSF positively regulate the biosynthesis of an antifungal factor (heat-stable antifungal factor, HSAF) in L. enzymogenes. Finally, we show that RpfFXc1 and RpfFOH11 have similar functions in regulating HSAF production in L. enzymogenes, as well as the virulence, synthesis of virulence factors, biofilm formation, and extracellular polysaccharide production in X. campestris. These findings reveal a previously uncharacterized mechanism of DSF-mediated regulation in both biocontrol and pathogenic bacteria.

Production and application of xanthan gum-prospects in the dairy and plant-based milk food industry: a review

Xanthan gum (XG) is an important industrial microbial exopolysaccharide. It has found applications in various industries, such as pharmaceuticals, cosmetics, paints and coatings, and wastewater treatment, but especially in the food industry. The thickening and stabilizing properties of XG make it a valuable ingredient in many food products. This review presents a comprehensive overview of the various potential applications of this versatile ingredient in the food industry. Especially in the plant-based food industries due to current interest of consumers in cheaper protein sources and health purposes. However, challenges and opportunities also exist, and this review aims to identify and explore these issues in greater detail. Overall, this article represents a valuable contribution to the scientific understanding of XG and its potential applications in the food industry.

First Report of Bacterial Blight of Pennycress Caused by Xanthomonas campestris in Wisconsin

In May 2023, pennycress (Thlaspi arvense, L.) lines undergoing seed production in the Walnut Street Greenhouse at the University of Wisconsin-Madison displayed symptoms of chlorosis and black necrotic leaf spots (Fig. S1-A). Lesions eventually enlarged to 1-2 cm in diameter, became necrotic, and coalesced to cover a substantial portion of leaves. Symptoms were observed in ~30% of the pennycress lines adversely affecting overall growth and reproduction. Symptomatic leaves were surface sterilized for 30 seconds in 0.75% sodium hypochlorite, rinsed in sterile deionized water, and bacteria were isolated using three-phase streaking of symptomatic tissue onto KB medium (King et al., 1954). Single colonies of three isolates (creamy white to yellow) from this initial isolation were streaked onto KB medium to obtain pure cultures. Individual colonies were transferred for growth overnight in nutrient broth (Difco) and an equal amount of the broth was added to 30% glycerol in deionized (di) water and stored at -80 °C. To validate Koch's Postulates, bacteria were grown from these stocks on Yeast Dextrose Calcium Carbonate medium (Wilson et al., 1967) and were used to inoculate 5-week-old pennycress plants in the greenhouse. The bacteria were grown for 48 hours at 26°C, suspended in 300 ml of 0.05 M PBS buffer (pH=7.2) for inoculum preparation. Plants were inoculated with three bacterial isolates (approx. 108 CFU/ml) by piercing the mid veins or hydathodes with a sterilized toothpick dipped in the suspension. Inoculated plants were then enclosed in clear plastic bags for 24-48 hours and maintained in the greenhouse at a constant temperature of 26°C with a 16-hour photoperiod. After seven days, water-soaked lesions appeared on the inoculated leaves, eventually developing into the characteristic black spots (Fig. S1-B). DNA from the original isolates was extracted, and 16S PCR and sequencing of the positive bands was done. The negative control only produced brown spots at the site of inoculation (Fig. S1-C). The primer sequences were as follows: 27F: AGAGTTTGATCMTGGCTCAG; 1492R: GGTTACCTTGTTACGACTT (Eden et al., 1991; Weisburg et al., 1991). A BLAST analysis showed that the isolates had an E value of 0.0 to the genus Xanthomonas as well as 100% identity. Amplification and sequencing of the bacterium using gyrB amplicons revealed a 99-100% pairwise match with Xc. To enhance taxonomy resolution and confirm the identity of these isolates, the complete genomes of three samples were sequenced using NextSeq2000 Illumina platform (NCBI bioproject ID PRJNA1040293). Average Nucleotide Identity (ANI) analysis was conducted with representative strains from the Xc species (Dubrow et al., 2022), using PanExplorer (Dereeper et al., 2020) featuring integrated FastANI module (Jain et al., 2018). The isolates genomes exhibited over 98% identity and clustered with that of Xc pv. incanae and Xc pv. barbarae (Fig S2). Further work will be required to identify the pathovar of Xc identified in this study through phenotypic host range assay. This marks the first documented case of Xc in pennycress in the Midwestern US. Given the potential use of pennycress as a cover crop in the region, further investigations are warranted to assess its economic impact on production and develop management strategies.

Enzymatic and selective production of alkyl α-d-glucopyranosides by the α-glucosyl transfer enzyme derived from Xanthomonas campestris WU-9701

Several alkyl glucosides exhibit various bioactivities. 1-Octyl β-d-glucopyranoside produced by organic synthesis is used as a nonionic surfactant. However, no convenient method has been developed for the selective production of alkyl α-glucosides (α-AGs), such as 1-octyl α-d-glucopyranoside (α-OG). Therefore, we developed a simple method for selective production of α-AGs using the glucosyl transfer enzyme XgtA, (E.C. 3.2.1.20), derived from Xanthomonas campestris WU-9701. When 0.80 M alkyl alcohol and 2.5 units XgtA were incubated in 2.0 mL of 30 mM HEPES-NaOH buffer (pH 8.0) containing 1.2 M maltose at 45 °C, a specific α-AG corresponding to each alkyl alcohol (C2-C10) was detected. Under the standard conditions, we examined the selective production of α-OG from 1-octanol and maltose using XgtA. The reaction product was isolated and identified as α-OG via 1H nuclear magnetic resonance and nuclear overhauser effect spectroscopy analyses. No other glucosylated products, such as maltotriose, were detected in the reaction mixture. Under the standard conditions at 45 °C for 96 h, 243 mM α-OG (71 g/L) was produced in one batch production. Moreover, the addition of glucose isomerase to the reaction mixture decreased the concentration of glucose released via the reaction and increased the amount of α-OG produced; 359 mM α-OG (105 g/L) was maximally produced at 96 h. In conclusion, this study demonstrates the selective production of α-AGs using a simple enzymatic reaction, and XgtA has the potential to selectively produce various α-AGs.

Hybrid Autofluorescence and Optoacoustic Microscopy for the Label-Free, Early and Rapid Detection of Pathogenic Infections in Vegetative Tissues

Agriculture plays a pivotal role in food security and food security is challenged by pests and pathogens. Due to these challenges, the yields and quality of agricultural production are reduced and, in response, restrictions in the trade of plant products are applied. Governments have collaborated to establish robust phytosanitary measures, promote disease surveillance, and invest in research and development to mitigate the impact on food security. Classic as well as modernized tools for disease diagnosis and pathogen surveillance do exist, but most of these are time-consuming, laborious, or are less sensitive. To that end, we propose the innovative application of a hybrid imaging approach through the combination of confocal fluorescence and optoacoustic imaging microscopy. This has allowed us to non-destructively detect the physiological changes that occur in plant tissues as a result of a pathogen-induced interaction well before visual symptoms occur. When broccoli leaves were artificially infected with Xanthomonas campestris pv. campestris (Xcc), eventually causing an economically important bacterial disease, the induced optical absorption alterations could be detected at very early stages of infection. Therefore, this innovative microscopy approach was positively utilized to detect the disease caused by a plant pathogen, showing that it can also be employed to detect quarantine pathogens such as Xylella fastidiosa.

LolA and LolB from the plant-pathogen Xanthomonas campestris forms a stable heterodimeric complex in the absence of lipoprotein

The Gram-negative bacterium Xanthomonas campestris is one of the most problematic phytopathogens, and especially the pathovar campestris (Xcc) that causes a devastating plant disease known as black rot and it is of considerable interest to understand the molecular mechanisms that enable virulence and pathogenicity. Gram-negative bacteria depend on lipoproteins (LPs) that serve many important functions including control of cell shape and integrity, biogenesis of the outer membrane (OM) and establishment of transport pathways across the periplasm. The LPs are localized to the OM where they are attached via a lipid anchor by a process known as the localization of lipoprotein (Lol) pathway. Once a lipid anchor has been synthesized on the nascent LP, the Lol pathway is initiated by a membrane-bound ABC transporter that extracts the lipid anchor of the LP from the IM. The ABC extractor presents the extracted LP to the transport protein LolA, which binds the anchor and thereby shields it from the hydrophilic periplasmic milieu. It is assumed that LolA then carries the LP across the periplasm to the OM. At the periplasmic face of the OM, the LP cargo is delivered to LolB, which completes the Lol pathway by inserting the LP anchor in the inner leaflet of the outer membrane. Earlier studies have shown that loss of Xcc LolA or LolB leads to decreased virulence and pathogenicity during plant infection, which motivates studies to better understand the Lol system in Xcc. In this study, we report the first experimental structure of a complex between LolA and LolB. The crystal structure reveals a stable LolA-LolB complex in the absence of LP. The structural integrity of the LP-free complex is safeguarded by specific protein-protein interactions that do not coincide with interactions predicted to participate in lipid binding. The results allow us to identify structural determinants that enable Xcc LolA to dock with LolB and initiate LP transfer.

Trichoderma hamatum can act as an inter-plant communicator of foliar pathogen infections by colonizing the roots of nearby plants: A new inter-plant "wired communication"

Trichoderma is a genus of filamentous fungi widely studied and used as a biological control agent in agriculture. However, its ability to form fungal networks for inter-plant communication by means of the so-called inter-plant "wired communication" has not yet been addressed. In our study we used the model plant Arabidopsis thaliana, the fungus Trichoderma hamatum (isolated from Brassicaceae plants) and the pathogens Sclerotinia sclerotiorum and Xanthomonas campestris (necrotrophic fungus and hemibiotrophic bacteria, respectively). We performed different combinations of isolated/neighboring plants and root colonization/non-colonization by T. hamatum, as well as foliar infections with the pathogens. In this way, we were able to determine how, in the absence of T. hamatum, there is an inter-plant communication that induces systemic resistance in neighboring plants of plants infected by the pathogens. On the other hand, the plants colonized by T. hamatum roots show a greater systemic resistance against the pathogens. Regarding the role of T. hamatum as an inter-plant communicator, it is the result of an increase in foliar signaling by jasmonic acid (increased expression of LOX1 and VSP2 genes and decreased expression of ICS1 and PR-1 genes), antagonistically increasing root signaling by salicylic acid (increased expression of ICS1 and PR-1 genes and decreased expression of LOX1 and VSP2). This situation prevents root colonization by T. hamatum of the foliarly infected plant and leads to massive colonization of the neighboring plant, where jasmonic acid-mediated systemic defenses are induced.

Chitosan fabricated biogenic silver nanoparticles (Ch@BSNP) protectively modulate the defense mechanism of tomato during bacterial leaf spot (BLS) disease

Herein, the impact of chitosan fabricated biogenic silver nanoparticles (Ch@BSNP) has been evaluated for the protective management of bacterial leaf spot (BLS) disease in tomatoes caused by Xanthomonas campestris (NCIM5028). The Ch@BSNP originated by the Trichoderma viride (MTCC5661) derived extracellular compounds and subsequent chitosan hybridization. Spherical-shaped Ch@BSNP (30-35 nm) treated diseased plants were able to combat the biotic stress, as evidenced by the decreased elevated response of stress markers viz; anthocyanin (34.02%), proline (45.00%), flavonoids (20.26%), lipid peroxidation (10.00%), guaiacol peroxidase (36.58%), ascorbate peroxidase (41.50%), polyphenol oxidase (25.34%) and phenylalanine ammonia-lyase (2.10 fold) as compared to untreated diseased plants. Increased biochemical content specifically sugar (15.43%), phenolics (49.10%), chlorophyll, and carotenoids were measured in Ch@BSNP-treated diseased plants compared to untreated X. campestris-infested plants. The Ch@BSNP considerably reduced stress by increasing net photosynthetic rate and water use efficiency along with decreased transpiration rate and stomatal conductance in comparison to infected plants. Additionally, the expression of defense-regulatory genes viz; growth responsive (AUX, GH3, SAUR), early defense responsive (WRKYTF22, WRKY33, NOS1), defense responsive (PR1, NHO1, NPR1), hypersensitivity responsive (Pti, RbohD, OXI1) and stress hormones responsive (MYC2, JAR1, ERF1) were found to be upregulated in diseased plants while being significantly downregulated in Ch@BSNP-treated diseased plants. Furthermore, fruits obtained from pathogen-compromised plants treated with Ch@BSNP had higher levels of health-promoting compounds including lycopene and beta-carotene than infected plant fruits. This nano-enabled and environmentally safer crop protection strategy may encourage a sustainable agri-system towards the world's growing food demand and promote food security.

Isolation, characterization and genome analysis of an orphan phage FoX4 of the new Foxquatrovirus genus

The growing interest in the therapeutic application of bacteriophages leads to a drastic increase in the number of sequenced genomes. Luckily, recent insights in phage taxonomy facilitate the classification of phages in a comprehensive and data-driven manner as recently proposed by the International Committee on Taxonomy of Viruses. In this research, we present the taxonomical classification of a novel, narrow host range Xanthomonas phage FoX4, isolated from a Brussels sprouts field in Belgium infested with Xanthomonas campestris pv. campestris. The phage has a limited ability to lyse a bacterial culture, yet adsorbs efficiently to its host. Based on its genome sequence and low similarity to previously described phages, the phage comprises the novel phage genus Foxquatrovirus.

Cruciferous Weed Isolates of Xanthomonas campestris Yield Insight into Pathovar Genomic Relationships and Genetic Determinants of Host and Tissue Specificity

Pathovars of Xanthomonas campestris cause distinct diseases on different brassicaceous hosts. The genomic relationships among pathovars as well as the genetic determinants of host range and tissue specificity remain poorly understood despite decades of research. Here, leveraging advances in multiplexed long-read technology, we fully sequenced the genomes of a collection of X. campestris strains isolated from cruciferous crops and weeds in New York and California as well as strains from global collections, to investigate pathovar relationships and candidate genes for host- and tissue-specificity. Pathogenicity assays and genomic comparisons across this collection and publicly available X. campestris genomes revealed a correlation between pathovar and genomic relatedness and provide support for X. campestris pv. barbareae, the validity of which had been questioned. Linking strain host range with type III effector repertoires identified AvrAC (also 'XopAC') as a candidate host-range determinant, preventing infection of Matthiola incana, and this was confirmed experimentally. Furthermore, the presence of a copy of the cellobiosidase gene cbsA with coding sequence for a signal peptide was found to correlate with the ability to infect vascular tissues, in agreement with a previous study of diverse Xanthomonas species; however, heterologous expression in strains lacking the gene gave mixed results, indicating that factors in addition to cbsA influence tissue specificity of X. campestris pathovars. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Selective and high-yield production of ethyl α-d-glucopyranoside by the α-glucosyl transfer enzyme of Xanthomonas campestris WU-9701 and glucose isomerase

Ethyl α-d-glucopyranoside (α-EG) is detected in sake (Japanese rice wine), that has moisturizing and skin conditioning effects. The production of α-EG by fermentation or enzymatic synthesis to date generates unwanted by-products such as maltooligosaccharides and/or organic acids. In this study, we employed a reaction involving selective α-glucosylation of ethanol by the α-glucosyl transfer enzyme (XgtA) of Xanthomonas campestris WU-9701. Under standard conditions, when 0.80 M ethanol and 1.2 M maltose were used as substrates with XgtA (2.5 units) and incubated in 30 mM HEPES-NaOH buffer (pH 8.0) at 45°C, only one form of ethyl glucopyranoside was selectively obtained as a product. The isolated product was identified as ethyl α-d-glucopyranoside by ¹H NMR, ¹H-¹H COSY, and NOESY analyses. In the reaction mixture, other glucosylated products such as maltotriose and ethylmaltoside were not detected. Under optimum conditions, 180 mM (37.5 g/L) α-EG was produced in one batch production for 80 h. Further, the reaction rate of α-EG production decreased with an increase in glucose, especially more than 500 mM. In contrast, the addition of glucose isomerase decreased the concentration of glucose and was useful for maintaining a glucose concentration of less than 500 mM in the reaction mixture. Thus, owing to the enzymatic reaction with XgtA and glucose isomerase, as much as 260 mM (54.1 g/L) α-EG was produced in one batch production for 100 h. Altogether, this study reports the highest concentration of α-EG produced by enzymatic reaction.

Plasma Membrane-Associated Proteins Identified in Arabidopsis Wild Type, lbr2-2 and bak1-4 Mutants Treated with LPSs from Pseudomonas syringae and Xanthomonas campestris

Plants recognise bacterial microbe-associated molecular patterns (MAMPs) from the environment via plasma membrane (PM)-localised pattern recognition receptor(s) (PRRs). Lipopolysaccharides (LPSs) are known as MAMPs from gram-negative bacteria that are most likely recognised by PRRs and trigger defence responses in plants. The Arabidopsis PRR(s) and/or co-receptor(s) complex for LPS and the associated defence signalling remains elusive. As such, proteomic identification of LPS receptors and/or co-receptor complexes will help to elucidate the molecular mechanisms that underly LPS perception and defence signalling in plants. The Arabidopsis LPS-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI)-related-2 (LBR2) have been shown to recognise LPS and trigger defence responses while brassinosteroid insensitive 1 (BRI1)-associated receptor kinase 1 (BAK1) acts as a co-receptor for several PRRs. In this study, Arabidopsis wild type (WT) and T-DNA knock out mutants (lbr2-2 and bak1-4) were treated with LPS chemotypes from Pseudomonas syringae pv. tomato DC3000 (Pst) and Xanthomonas campestris pv. campestris 8004 (Xcc) over a 24 h period. The PM-associated protein fractions were separated by liquid chromatography and analysed by tandem mass spectrometry (LC-MS/MS) followed by data analysis using Byonic^TM software. Using Gene Ontology (GO) for molecular function and biological processes, significant LPS-responsive proteins were grouped according to defence and stress response, perception and signalling, membrane transport and trafficking, metabolic processes and others. Venn diagrams demarcated the MAMP-responsive proteins that were common and distinct to the WT and mutant lines following treatment with the two LPS chemotypes, suggesting contributions from differential LPS sub-structural moieties and involvement of LBR2 and BAK1 in the LPS-induced MAMP-triggered immunity (MTI). Moreover, the identification of RLKs and RLPs that participate in other bacterial and fungal MAMP signalling proposes the involvement of more than one receptor and/or co-receptor for LPS perception as well as signalling in Arabidopsis defence responses.

Untargeted Metabolomics Profiling of Arabidopsis WT, lbr-2-2 and bak1-4 Mutants Following Treatment with Two LPS Chemotypes

Plants perceive pathogenic threats from the environment that have evaded preformed barriers through pattern recognition receptors (PRRs) that recognise microbe-associated molecular patterns (MAMPs). The perception of and triggered defence to lipopolysaccharides (LPSs) as a MAMP is well-studied in mammals, but little is known in plants, including the PRR(s). Understanding LPS-induced secondary metabolites and perturbed metabolic pathways in Arabidopsis will be key to generating disease-resistant plants and improving global plant crop yield. Recently, Arabidopsis LPS-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI)-related proteins (LBP/BPI related-1) and (LBP/BPI related-2) were shown to perceive LPS from Pseudomonas aeruginosa and trigger defence responses. In turn, brassinosteroid insensitive 1 (BRI1)-associated receptor kinase 1 (BAK1) is a well-established co-receptor for several defence-related PRRs in plants. Due to the lack of knowledge pertaining to LPS perception in plants and given the involvement of the afore-mentioned proteins in MAMPs recognition, in this study, Arabidopsis wild type (WT) and mutant (lbr2-2 and bak1-4) plants were pressure-infiltrated with LPSs purified from Pseudomonas syringae pv. tomato DC3000 (Pst) and Xanthomonas campestris pv. campestris 8004 (Xcc). Metabolites were extracted from the leaves at four time points over a 24 h period and analysed by UHPLC-MS, generating distinct metabolite profiles. Data analysed using unsupervised and supervised multivariate data analysis (MVDA) tools generated results that reflected time- and treatment-related variations after both LPS chemotypes treatments. Forty-five significant metabolites were putatively annotated and belong to the following groups: glucosinolates, hydroxycinnamic acid derivatives, flavonoids, lignans, lipids, oxylipins, arabidopsides and phytohormones, while metabolic pathway analysis (MetPA) showed enrichment of flavone and flavanol biosynthesis, phenylpropanoid biosynthesis, alpha-linolenic acid metabolism and glucosinolate biosynthesis. Distinct metabolite accumulations depended on the LPS chemotype and the genetic background of the lbr2-2 and bak1-4 mutants. This study highlights the role of LPSs in the reprogramming Arabidopsis metabolism into a defensive state, and the possible role of LBR and BAK1 proteins in LPSs perception and thus plant defence against pathogenic bacteria.

campestris

Diffusible signal factors (DSFs) are medium-chain fatty acids that induce bacterial quorum sensing. Among these compounds, BDSF is a structural analog of DSF that is commonly detected in bacterial species, and it is the predominant in planta quorum-sensing signal in Xanthomonas campestris. How BDSF is sensed in Xanthomonas spp. and the functional diversity between BDSF and DSF remain unclear. In this study, we generated genetic and biochemical evidence that BDSF is a low-active regulator of X. campestris pv. campestris quorum sensing, whereas trans-BDSF does not seem to be a signaling compound. BDSF is detected by the sensor histidine kinase RpfC. Although BDSF has relatively low physiological activities, it binds to the RpfC sensor with a high affinity and activates RpfC autophosphorylation to a level that is similar to that induced by DSF in vitro. The inconsistency in the physiological and biochemical activities of BDSF is not due to RpfC-RpfG phosphorylation or RpfG hydrolase. Neither BDSF nor DSF controls the phosphotransferase and phosphatase activities of RpfC or the ability of RpfG hydrolase activity to degrade the bacterial second messenger cyclic di-GMP. We demonstrated that BDSF is prone to degradation by RpfB, a critical fatty acyl coenzyme A ligase involved in the turnover of DSF-family signals. rpfB mutations lead to substantial increases in BDSF-induced quorum sensing. Although DSF and BDSF are similarly detected by RpfC, our data suggest that their differential degradation in cells is the major factor responsible for the diversity in their physiological effects. IMPORTANCE The diffusible signal factor (DSF) family consists of quorum-sensing signals employed by Gram-negative bacteria. These signals are a group of cis-2-unsaturated fatty acids, such as DSF, BDSF, IDSF, CDSF, and SDSF. However, the functional divergence of various DSF signals remains unclear. The present study demonstrates that though BDSF is a low active quorum-sensing signal, it binds histidine kinase RpfC with a higher affinity and activates RpfC autophosphorylation to the similar level as DSF. Rather than regulation of enzymatic activities of RpfC and its cognate response regulator RpfG encoding a c-di-GMP hydrolase, BDSF is prone to degradation in bacterial cells by RpfB, which effectively avoided the inhibition of bacterial growth by accumulating high concentrations of BDSF. Therefore, our study sheds new light on the functional differences of quorum-sensing signals and shows that bacteria balance quorum sensing and growth by fine-tuning concentrations of signaling chemicals.

campestris by Proteomics Analysis

Understanding plant's response mechanisms against pathogenesis is fundamental for the development of resistant crop varieties and more productive agriculture. In this regard, "omic" approaches are heralded as valuable technologies. In this work, combining isobaric tags for relative and absolute quantification (iTRAQ) technology with mass spectrometry, the proteomes from leaves of Brassica oleracea plants infected with Xanthomonas campestris pv. campestris (Xcc), and control plants at two different post-infection times were compared. Stronger proteomic changes were obtained at 12 days post-infection in comparison with 3 days. The responses observed involved different cell processes, from primary metabolism, such as photosynthesis or photorespiration, to other complex processes such as redox homeostasis, hormone signaling, or defense mechanisms. Most of the proteins decreased in the earlier response were involved in energetic metabolism, whereas later response was characterized by a recovery of primary metabolism. Furthermore, our results indicated that proteolysis machinery and reactive oxygen species (ROS) homeostasis could be key processes during this plant-pathogen interaction. Current data provide new insights into molecular mechanisms that may be involved in defense responses of B. oleracea to Xcc.

Analysis of Gum proteins involved in xanthan biosynthesis throughout multiple cell fractions in a "single-tube"

Xanthomonas is a phytopathogenic bacterium and of industrial interest due to its capability to produce xanthan, used as a thickener and emulsifier in the food and non-food industry. Until now, proteome analyses of Xcc lacking a detailed view on the proteins involved in xanthan biosynthesis. The proteins involved in the biosynthesis of this polysaccharide are located near, in or at the cell membrane. This study aims to establish a robust and rapid protocol for a comprehensive proteome analysis of Xcc strains, without the need to isolate different cell fractions. Therefore, a method for the analysis of the whole cell proteome was compared to the isolation of specific fractions regarding the total number of identified proteins, the overlap, and the differences between the approaches. The whole cell proteome analysis with extended peptide separation methods resulted in more than 3254 identified proteins covering 73.1% of the whole proteome. The protocol was used to study xanthan production in a label-free quantification approach. Expression profiles of 8 Gum proteins were compared between the stationary and logarithmic growth phase. Differential expression levels within the operon structure indicate a complex regulatory mechanism for xanthan biosynthesis. Data are available via ProteomeXchange with identifier PXD027261. SIGNIFICANCE: Bacteria are metabolite factories with a wide variety of natural products. Thus, proteome analyses play a crucial role to understand the biological processes within a cell behind the biosynthesis of those metabolites. Proteins involved in the biosynthesis of secreted products are often organised on, in or around the membrane allowing metabolite channelling. Experiments targeting those biosynthesis pathways on protein level often require the analysis of multiple cell fractions like cytosolic, inner, and outer membrane. This is time consuming and demands different protocols. The protocol presented here is a rapid and robust solution to study biosynthetic pathways of biological or biotechnological interest in a single approach on protein level, where gene products are partitioned across multiple cell fractions. The use of a single method also simplifies the comparison of different experiments, for example, production vs. nonproduction conditions.

Microbial community shifts association with physicochemical parameters: Visualizing enset bacterial wilt from different states of enset health

Bacterial wilt of enset caused by Xanthomonas campestris is a devastating disease in Ethiopia, where enset is domesticated and served as a staple food for about 20 million people in the country. While enset is infected by bacteria, it shows different wilting stages. However, the microbial community shifts at the different stages of enset infection and associated physicochemical parameter changes remain poorly understood. This study was aimed to visualize the proportion of enset wilt bacterium from other microbial community and its association with physicochemical parameter at different states of enset health. Soil and enset (zero, first, second and third stages) samples were collected from three districts in Gamo Highlands for physicochemical and biological (culture dependent and16S rRNA gene sequence) analysis. The results of culture dependent analysis which has been complemented by 16S rRNA gene sequence confirmed that increasing trends were observed for Xanthomonadaceae, Pseudomonadaceae, Lactobacillaceae and Flavobacteriaceae, while Bacillaceae and Enterobacteriaceae showed progressive decrease from zero to the third stage. Particularly, the 16S rRNA data showed that Xanthomonadaceae increased significantly from zero to different (2.5 × 10² times at the onset of disease and 1.0-2.0 × 10⁴ times at the second and third) stages of enset infection. Most physicochemical results showed that a decreasing trends from zero to third stage, while few parameters are showing an increasing trend. Moisture content (R² ≥ 0.951, P ≤ 0.049) of the soil and plant samples positively influenced Xanthomonas abundance, while this bacterium showed a strongly negative significant correlation with pH (R² ≥ -0.962, P ≤ 0.038), temperature (R² ≥ -0.958, P ≤ 0.042), OM (R² ≥ -0.952, P ≤ 0.048), and TN (R² ≥ -0.951, P ≤ 0.049). A strongly negative significant correlation (R² ≥ -0.948, P ≤ 0.050) was also observed between Xanthomonas and nutrients (K, Mg, Ca, and Cu). Overall, this study implies that different environmental factors found a key driving force of Xanthomonas proportional increment from low abundance at zero stage to higher abundance at the last stage of enset infection suggesting that considering these factors help to design an effective enset disease management strategy, for which further studies will be needed.

campestris is required for bacterial attachment, stress tolerance, and virulence

BACKGROUND

Xanthomonas campestris pv. campestris (Xcc) is a Gram-negative bacterium that can cause black rot disease in crucifers. The lipoprotein outer membrane localization (Lol) system is involved in the lipoprotein sorting to the outer membrane. Although Xcc has a set of annotated lol genes, there is still little known about the physiological role in this phytopathogen. In this study, we aimed to characterize the role of LolB of Xcc in bacterial attachment, stress tolerance, and virulence.

RESULTS

To characterize the role of LolB, lolB mutant was constructed and phenotypic evaluation was performed. The lolB mutant revealed reductions in bacterial attachment, extracellular enzyme production, and virulence. Mutation of lolB also resulted in reduced tolerance to a myriad of stresses, including heat and a range of membrane-perturbing agents. Trans-complementation of lolB mutant with intact lolB gene reverted these altered phenotypes to the wild-type levels. From subsequent reporter assay and reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) analysis, the expression of genes that encode the major extracellular enzymes and the stress-related proteins was reduced after lolB mutation.

CONCLUSIONS

The results in this work contribute to the functional understanding of lolB in Xanthomonas for the first time, and provide new insights into the function of lolB in bacteria.

Cruciferous Weeds Do Not Act as Major Reservoirs of Inoculum for Black Rot Outbreaks in New York State

Cruciferous weeds have been shown to harbor diverse Xanthomonas campestris pathovars, including the agronomically damaging black rot of cabbage pathogen, X. campestris pv. campestris. However, the importance of weeds as inoculum sources for X. campestris pv. campestris outbreaks in New York remains unknown. To determine if cruciferous weeds act as primary reservoirs for X. campestris pv. campestris, fields that were rotating between cabbage or had severe black rot outbreaks were chosen for evaluation. Over a consecutive 3-year period, 148 cruciferous and noncruciferous weed samples were collected at 34 unique sites located across five New York counties. Of the 148 weed samples analyzed, 48 X. campestris isolates were identified, with a subset characterized using multilocus sequence analysis. All X. campestris isolates originated from weeds belonging to the Brassicaceae family, with predominant weed hosts being shepherd's purse (Capsella bursa-pastoris), wild mustard (Sinapis arvensis), yellow rocket (Barbarea vulgaris), and pennycress (Thlaspi arvense). Identifying pathogenic X. campestris weed isolates was rare, with only eight isolates causing brown necrotic leaf spots or typical V-shaped lesions on cabbage. There was no evidence of cabbage-infecting weed isolates persisting in an infected field by overwintering in weed hosts; however, similar cabbage and weed X. campestris haplotypes were identified in the same field during an active black rot outbreak. X. campestris weed isolates are genetically diverse both within and between fields, but our findings indicate that X. campestris weed isolates do not appear to act as primary sources of inoculum for B. oleracea fields in New York.

Antimicrobial activities of plant essential oil vapours against Acidovorax citrulli and Xanthomonas campestris on Cucurbitaceae, Brassicaceae and Solanaceae seeds

AIM

This study was done to develop a seed decontamination treatment for organic seeds against plant pathogens (Acidovorax citrulli and Xanthomonas campestris) using essential oil (EO) vapours without affecting the seeds' germination rate.

METHODS AND RESULTS

By using a diffusion assay and determining minimum inhibitory and lethal concentrations, we screened two EO vapours which were most inhibitory to A. citrulli (cinnamon bark and garlic EO vapours) and X. campestris (onion and garlic EO vapours). After 48 h of exposure to EO vapours at 25°C and 43% or 85% relative humidity (RH), no significant decrease (p > 0.05) in germination rates was observed compared with those of control seeds. It was observed that EO vapour treatment at 25°C and 43% or 85% RH for 48 h caused significant population reductions (p ≤ 0.05) (ca. 0.3-2.6 log colony forming unit/g) compared to those of untreated seeds.

CONCLUSION

Applications of EO vapours showed significant (p ≤ 0.05) antimicrobial effects against A. citrulli and X. campestris on both laboratory mediums and plant seeds without decreasing the germination rate of seeds.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides useful information for the development of natural seed sterilization treatments using EO vapours.

campestris

Xanthomonas campestris pv. campestris (Xcc) is a Gram-negative bacterium belonging to the Xanthomonodaceae family, causing black rot in crucifers. To control this pathogen, the study investigated the effect of different leaves extracts of Moringa oleifera Lam., a tropical plant, well known for its food properties and with countless applications in many different fields, from nutraceutical (hypoglycemic) to the cosmetic (sunscreen) properties. Nevertheless, several studies pointed to its antibacterial action against both Gram-negative and Gram-positive bacteria. Many bioactive compounds, including flavonoids, phenolic acids, alkaloids, isothiocyanates, tannins and saponins, contained in these extracts, are responsible for its countless activities. The analyses carried out in this study show that the methanolic, hydroalcoholic and hydroalcoholic maltodextrin extracts have both bacteriostatic and bactericidal effects at concentrations of 0.5, 0.5 and 0.1 mg/mL respectively. In particular, the study shows how all extracts can alter membrane permeability, to adversely affect swarming motility, and to alter biofilm formation in Xcc. The in planta experiments showed a reduction of the necrosis area in the infected radishes, although the ability of the extracts to be absorbed by root systems is yet to be understood, in order to reach the target point.

The Same against Many: AtCML8, a Ca2+ Sensor Acting as a Positive Regulator of Defense Responses against Several Plant Pathogens

Calcium signals are crucial for the activation and coordination of signaling cascades leading to the establishment of plant defense mechanisms. Here, we studied the contribution of CML8, an Arabidopsis calmodulin-like protein in response to Ralstonia solanacearum and to pathogens with different lifestyles, such as Xanthomonas campestris pv. campestris and Phytophtora capsici. We used pathogenic infection assays, gene expression, RNA-seq approaches, and comparative analysis of public data on CML8 knockdown and overexpressing Arabidopsis lines to demonstrate that CML8 contributes to defense mechanisms against pathogenic bacteria and oomycetes. CML8 gene expression is finely regulated at the root level and manipulated during infection with Ralstonia, and CML8 overexpression confers better plant tolerance. To understand the processes controlled by CML8, genes differentially expressed at the root level in the first hours of infection have been identified. Overexpression of CML8 also confers better tolerance against Xanthomonas and Phytophtora, and most of the genes differentially expressed in response to Ralstonia are differentially expressed in these different pathosystems. Collectively, CML8 acts as a positive regulator against Ralstonia solanaceraum and against other vascular or root pathogens, suggesting that CML8 is a multifunctional protein that regulates common downstream processes involved in the defense response of plants to several pathogens.

Extracellular Amylase Is Required for Full Virulence and Regulated by the Global Posttranscriptional Regulator RsmA in Xanthomonas campestris Pathovar campestris

As with many phytopathogenic bacteria, the virulence of Xanthomonas campestris pv. campestris, the causal agent of black rot disease in cruciferous plants, relies on secretion of a suite of extracellular enzymes that includes cellulase (endoglucanase), pectinase, protease, and amylase. Although the role in virulence of a number of these enzymes has been assessed, the contribution of amylase to X. campestris pv. campestris virulence has yet to be established. In this work, we investigated both the role of extracellular amylase in X. campestris pv. campestris virulence and the control of its expression. Deletion of XC3487 (here renamed amyA_Xcc), a putative amylase-encoding gene from the genome of X. campestris pv. campestris strain 8004, resulted in a complete loss of extracellular amylase activity and significant reduction in virulence. The extracellular amylase activity and virulence of the amyA_Xcc mutant could be restored to the wild-type level by expressing amyA_Xcc in trans. These results demonstrated that amyA_Xcc is responsible for the extracellular amylase activity of X. campestris pv. campestris and indicated that extracellular amylase plays an important role in X. campestris pv. campestris virulence. We also found that the expression of amyA_Xcc is strongly induced by starch and requires activation by the global posttranscriptional regulator RsmA. RsmA binds specifically to the 5'-untranslated region of amyA_Xcc transcripts, suggesting that RsmA regulates amyA_Xcc directly at the posttranscriptional level. Unexpectedly, in addition to posttranscriptional regulation, the use of a transcriptional reporter demonstrated that RsmA also regulates amyA_Xcc expression at the transcriptional level, possibly by an indirect mechanism.

Safety and efficacy of an additive consisting of xanthan gum produced by Xanthomonas campestris strains ■■■■■, ■■■■■ for all animal species (Biopolymer International)

Following a request from the European Commission, the Panel on Additives and Products or Substances used in Animal Feed (FEEDAP) was asked to deliver a scientific opinion on xanthan gum as a feed additive for all animal species. Xanthan gum is manufactured using different production strains belonging to the X. campestris species. The identity of the strains producing xanthan gum was not unambiguously established, data on antimicrobial susceptibility were incomplete, and it was not possible to exclude the presence in the additive of viable cells/DNA of the production strains. Consequently, no conclusions could be drawn on the safety of the X. campestris strains ■■■■■. Considering the above and in the absence of adequate information on the additive under assessment, the FEEDAP Panel cannot conclude on the safety of xanthan gum produced by the X. campestris strains ■■■■■ for the target species, the consumer, the user and the environment. Xanthan gum is considered as an efficacious stabiliser and thickener in feedingstuffs for all animal species at the proposed conditions of use.

Root zone warming represses foliar diseases in tomato by inducing systemic immunity

Plants employ systemic-induced resistance as part of their defence arsenal against pathogens. In recent years, the application of mild heating has been found to induce resistance against several pathogens. In the present study, we investigated the effect of root zone warming (RZW) in promoting tomato's resistance against the necrotrophic fungus Botrytis cinerea (Bc), the hemibiotrophic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) and the biotrophic fungus Oidium neolycopersici (On). We demonstrate that RZW enhances tomato's resistance to Bc, On and Xcv through a process that is dependent on salicylic acid and ethylene. RZW induced tomato immunity, resulting in increased defence gene expression, reactive oxygen species (ROS) and ethylene output when plants were challenged, even in the absence of pathogens. Overall, the results provide novel insights into the underlying mechanisms of warming-induced immune responses against phytopathogens with different lifestyles in tomato.

Efficient production of l-menthyl α-glucopyranoside from l-menthol via whole-cell biotransformation using recombinant Escherichia coli

l-Menthyl α-D-glucopyranoside (α-MenG) is a glycoside derivative of l-menthol with improved water-solubility and new flavor property as a food additive. α-MenG can be synthesized through biotransformation, but its scale-up production was rarely reported. In this study, the properties of an α-glucosidase from Xanthomonas campestris pv. campestris 8004 (Agl-2) in catalyzing the glucosylation of menthol was investigated. Agl-2 can almost completely glycosylate l-menthol (> 99%) when using 1.2 M maltose as glycosyl donor. Accumulated glucose resulted from maltose hydrolysis and transglycosylation caused the inhibition of the glucosylation rate (40% reduction of the glucosylation rate in the presence of 1.2 M glucose) which can be avoided through whole-cell catalysis with recombinant E. coli. Interestingly, in spite of the poor solubility of menthol, the productivity of α-MenG reached 24.7 g/(L·h) in a 2 L catalyzing system, indicating industrialization of the reported approach.

campestris

Epigenetics is the study of heritable alterations in phenotypes that are not caused by changes in DNA sequence. In the present study, we characterized the genetic and phenotypic alterations of the bacterial plant pathogen Xanthomonas campestris pv. campestris (Xcc) under different treatments with several epigenetic modulating chemicals. The use of DNA demethylating chemicals unambiguously caused a durable decrease in Xcc bacterial virulence, even after its reisolation from infected plants. The first-time use of chemicals to modify the activity of sirtuins also showed some noticeable results in terms of increasing bacterial virulence, but this effect was not typically stable. Changes in treated strains were also confirmed by using methylation sensitive amplification (MSAP), but with respect to registered SNPs induction, it was necessary to consider their contribution to the observed polymorphism. The molecular basis of the altered virulence was deciphered by using dualRNA-seq analysis of treated Xcc strains infecting Brassica rapa plants. The results of the present study should promote more intensive research in the generally understudied field of bacterial epigenetics, where artificially induced modification by epigenetic modulating chemicals can significantly increase the diversity of bacterial properties and potentially contribute to the further development of the fields, such as bacterial ecology and adaptation.

Semi-continuous production of xanthan in biofilm reactor using Xanthomonas campestris

Semi-continuous production of xanthan gum using self-immobilized Xanthomonas campestris cells in biofilm reactors was studied. Fermentation was carried out using two different designs of biofilm reactor equipped with a) stainless-steel support (SSS) and b) polyethylene support (PES). Fermentation was performed in three cycles with refreshing the media at the beginning of each: cycle 1, 0-27 h; cycle 2, 27-54 h; and cycle 3, 54-78.5 h. Results showed that the glucose consumption and the pH reduction in the PES biofilm reactor was faster compared to the SSS biofilm reactor. Scanning electron microscopy showed that the SSS was capable to immobilize more cells during the growth of X. campestris. The maximum concentration of xanthan gum in the SSS biofilm reactor obtained after 27 h (3.47 ± 0.71 g/L), while the maximum concentration of xanthan in the PES biofilm reactor obtained after 78.5 h (3.21 ± 0.68 g/L). Thermal stability analysis of xanthan using differential scanning calorimetry showed the presence of two fractures attributed to dehydration and degradation of polymer. The thermogram represented both endothermal and exothermal behaviour of xanthan polymer. Furthermore, the functional groups and molecular structure of the xanthan produced in this study was evaluated using Fourier transform infrared spectrometry and also proton nuclear magnetic resonance. in addition, the surface tension of (0.2 %, w/v) xanthan gum solution was in a range of 52.16-56.5 mN/m. Rheological analysis of xanthan showed that the G' values were higher than the G″ in all frequencies demonstrating a relatively high elasticity of the produced xanthan gum.

Crystal structure of XCC3289 from Xanthomonas campestris: homology with the N-terminal substrate-binding domain of Lon peptidase

LonA peptidase is a major component of the protein quality-control mechanism in both prokaryotes and the organelles of eukaryotes. Proteins homologous to the N-terminal domain of LonA peptidase, but lacking its other domains, are conserved in several phyla of prokaryotes, including the Xanthomonadales order. However, the function of these homologous proteins (LonNTD-like proteins) is not known. Here, the crystal structure of the LonNTD-like protein from Xanthomonas campestris (XCC3289; UniProt Q8P5P7) is reported at 2.8 Å resolution. The structure was solved by molecular replacement and contains one polypeptide in the asymmetric unit. The structure was refined to an Rfree of 29%. The structure of XCC3289 consists of two domains joined by a long loop. The N-terminal domain (residues 1-112) consists of an α-helix surrounded by β-sheets, whereas the C-terminal domain (residues 123-193) is an α-helical bundle. The fold and spatial orientation of the two domains closely resembles those of the N-terminal domains of the LonA peptidases from Escherichia coli and Mycobacterium avium. The structure is also similar to that of cereblon, a substrate-recognizing component of the E3 ubiquitin ligase complex. The N-terminal domains of both LonA and cereblon are known to be involved in specific protein-protein interactions. This structural analysis suggests that XCC3289 and other LonNTD-like proteins might also be capable of such protein-protein interactions.

I-SceI Endonuclease-Mediated Plant Genome Editing by Protein Transport through a Bacterial Type III Secretion System

Xanthomonas campestris is one of bacteria carrying a type III secretion system which transports their effector proteins into host plant cells to disturb host defense system for their infection. To establish a genome editing system without introducing any foreign gene, we attempted to introduce genome editing enzymes through the type III secretion system. In a test of protein transfer, X. campestris pv. campestris (Xcc) transported a considerable amount of a reporter protein sGFP-CyaA into tobacco plant cells under the control of the type III secretion system while maintaining cell viability. For proof of concept for genome editing, we used a reporter tobacco plant containing a luciferase (LUC) gene interrupted by a meganuclease I-SceI recognition sequence; this plant exhibits chemiluminescence of LUC only when a frameshift mutation is introduced at the I-SceI recognition site. Luciferase signal was observed in tobacco leaves infected by Xcc carrying an I-SceI gene which secretes I-SceI protein through the type III system, but not leaves infected by Xcc carrying a vector control. Genome-edited tobacco plant could be regenerated from a piece of infected leaf piece by repeated selection of LUC positive calli. Sequence analysis revealed that the regenerated tobacco plant possessed a base deletion in the I-SceI recognition sequence that activated the LUC gene, indicating genome editing by I-SceI protein transferred through the type III secretion system of Xcc.

The phytopathogen Xanthomonas campestris utilizes the divergently transcribed pobA/pobR locus for 4-hydroxybenzoic acid recognition and degradation to promote virulence

Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot in crucifers. Our previous findings revealed that Xcc can degrade 4-hydroxybenzoic acid (4-HBA) via the β-ketoadipate pathway. This present study expands on this knowledge in several ways. First, we show that infective Xcc cells induce in situ biosynthesis of 4-HBA in host plants, and Xcc can efficiently degrade 4-HBA via the pobA/pobR locus, which encodes a 4-hydroxybenzoate hydroxylase and an AraC-family transcription factor respectively. Next, the transcription of pobA is specifically induced by 4-HBA and is positively regulated by PobR, which is constitutively expressed in Xcc. 4-HBA directly binds to PobR dimers, resulting in activation of pobA expression. Point mutation and subsequent isothermal titration calorimetry and size exclusion chromatography analysis identified nine key conserved residues required for 4-HBA binding and/or dimerization of PobR. Furthermore, overlapping promoters harboring fully overlapping -35 elements were identified between the divergently transcribed pobA and pobR. The 4-HBA/PobR dimer complex specifically binds to a 25-bp site, which encompasses the -35 elements shared by the overlapping promoters. Finally, GUS histochemical staining and subsequent quantitative assay showed that both pobA and pobR genes are transcribed during Xcc infection of Chinese radish, and the strain ΔpobR exhibited compromised virulence in Chinese radish. These findings suggest that the ability of Xcc to survive the 4-HBA stress might be important for its successful colonization of host plants.

Crystal structure of α-glucosyl transfer enzyme XgtA from Xanthomonas campestris WU-9701

The α-glucosyl transfer enzyme XgtA is a novel type α-Glucosidase (EC 3.2.1.20) produced by Xanthomonas campestris WU-9701. One of the unique properties of XgtA is that it shows extremely high α-glucosylation activity toward alcoholic and phenolic -OH groups in compounds using maltose as an α-glucosyl donor and allows for the synthesis of various useful α-glucosides with high yields. XgtA shows no hydrolytic activity toward sucrose and no α-glucosylation activity toward saccharides to produce oligosaccharides. In this report, the crystal structure of XgtA was solved at 1.72 Å resolution. The crystal belonged to space group P22₁2₁, with unit-cell parameters a = 73.07, b = 83.48, and c = 180.79 Å. The β→α loop 4 of XgtA, which is proximal to the catalytic center, formed a unique structure that is not observed in XgtA homologs. Furthermore, XgtA was found to contain unique amino acid residues around its catalytic center. The unique structure of XgtA provides an insight into the mechanism for the regulation of substrate specificity in this enzyme.

Characterization, Phylogeny, and Genome Analyses of Nonpathogenic Xanthomonas campestris Strains Isolated from Brassica Seeds

Xanthomonads were detected by using the Xan-D(CCF) medium from the brassica seeds, and their pathogenicity was determined by plant inoculation tests. It was found that some seed lots were infested with Xanthomonas campestris pv. campestris, some with X. campestris pv. raphani, and some with nonpathogenic xanthomonads. The nonpathogenic xanthomonad strains were identified as X. campestris, and the multilocus sequence analysis showed that the nonpathogenic X. campestris strains were grouped together with pathogenic X. campestris, but not with nonpathogenic strains of X. arboricola. In addition, all isolated X. campestris pv. campestris and X. campestris pv. raphani strains were positive in the hrpF-PCR, but the nonpathogenic strains were negative. It was further found that nonpathogenic X. campestris strain nE1 does not contain the entire pathogenicity island (hrp gene cluster; type III secretion system) and all type III effector protein genes based on the whole genome sequence analyses. The nonpathogenic X. campestris strain nE1 could acquire the entire pathogenicity island from the endemic X. campestris pv. campestris and X. campestris pv. raphani strains by conjugation, but type III effector genes were not cotransferred. The studies showed that the nonpathogenic X. campestris strains indeed exist on the brassica seeds, but it could be differentiated by the PCR assays on the hrp and type III effector genes. Nevertheless, the nonpathogenic X. campestris strains cannot be ignored because they may be potential gene resources to increase genetic diversity in the endemic pathogenic X. campestris pv. campestris and X. campestris pv. raphani strains.

Chemical Structure, Biological Roles, Biosynthesis and Regulation of the Yellow Xanthomonadin Pigments in the Phytopathogenic Genus Xanthomonas

Xanthomonadins are membrane-bound yellow pigments that are typically produced by phytopathogenic bacterial Xanthomonas spp., Xylella fastidiosa, and Pseudoxanthomonas spp. They are also produced by a diversity of environmental bacterial species. Considerable research has revealed that they are a unique group of halogenated, aryl-polyene, water-insoluble pigments. Xanthomonadins have been shown to play important roles in epiphytic survival and host-pathogen interactions in the phytopathogen Xanthomonas campestris pv. campestris, which is the causal agent of black rot in crucifers. Here, we review recent advances in the understanding of xanthomonadin chemical structures, physiological roles, biosynthetic pathways, regulatory mechanisms, and crosstalk with other signaling pathways. The aim of the present review is to provide clues for further in-depth research on xanthomonadins from Xanthomonas and other related bacterial species.

Isolation and Characterization of Biosurfactant-Producing Bacteria From Oil Well Batteries With Antimicrobial Activities Against Food-Borne and Plant Pathogens

Microbial biosurfactants, produced by fungi, yeast, and bacteria, are surface-active compounds with emulsifying properties that have a number of known activities, including the solubilization of microbial biofilms. In an on-going survey to uncover new or enhanced antimicrobial metabolite-producing microbes from harsh environments, such as oil-rich niches, 123 bacterial strains were isolated from three oil batteries in the region of Chauvin, Alberta, and characterized by 16S rRNA gene sequencing. Based on their nucleotide sequences, the strains are associated with 3 phyla (Actinobacteria, Proteobacteria and Firmicutes), as well as 17 other discrete genera that shared high homology with known sequences, with the majority of these strains identified to the species level. The most prevalent strains associated with the three oil wells belonged to the Bacillus genus. Thirty-four of the 123 strains were identified as biosurfactant-producers, among which Bacillus methylotrophicus strain OB9 exhibited the highest biosurfactant activity based on multiple screening methods and a comparative analysis with the commercially available biosurfactant, Tween 20. B. methylotrophicus OB9 was selected for further antimicrobial analysis and addition of live cultures of B. methylotrophicus OB9 (or partially purified biosurfactant fractions thereof) were highly effective on biofilm disruption in agar diffusion assays against several Gram-negative food-borne bacteria and plant pathogens. Upon co-culturing with B. methylotrophicus OB9, the number of either Salmonella enterica subsp. enterica Newport SL1 or Xanthomonas campestris B07.007 cells significantly decreased after 6 h and were not retrieved from co-cultures following 12 h exposure. These results also translated to studies on plants, where bacterized tomato seedlings with OB9 significantly protected the tomato leaves from Salmonella enterica Newport SL1 contamination, as evidenced by a 40% reduction of log10 CFU of Salmonella/mg leaf tissue compared to non-bacterized tomato leaves. When B. methylotrophicus 0B9 was used for bacterized lettuce, the growth of X. campestris B07.007, the causal agent of bacterial leaf spot of lettuce, was completely inhibited. While limited, these studies are noteworthy as they demonstrate the inhibition spectrum of B. methylotrophicus 0B9 against both human and plant pathogens; thereby making this bacterium attractive for agricultural and food safety applications in a climate where microbial-biofilm persistence is an increasing problem.

campestris

A characteristic feature of phytopathogenic Xanthomonas bacteria is the production of yellow membrane-bound pigments called xanthomonadins. Previous studies showed that 3-hydroxybenzoic acid (3-HBA) was a xanthomonadin biosynthetic intermediate and also, that it had a signaling role. The question of whether the structural isomers 4-HBA and 2-HBA (salicylic acid) have any role in xanthomonadin biosynthesis remained unclear. In this study, we have selectively eliminated 3-HBA, 4-HBA, or the production of both by expression of the mhb, pobA, and pchAB gene clusters in the Xanthomonas campestris pv. campestris strain XC1. The resulting strains were different in pigmentation, virulence factor production, and virulence. These results suggest that both 3-HBA and 4-HBA are involved in xanthomonadin biosynthesis. When both 3-HBA and 4-HBA are present, X. campestris pv. campestris prefers 3-HBA for Xanthomonadin-A biosynthesis; the 3-HBA-derived Xanthomonadin-A was predominant over the 4-HBA-derived xanthomonadin in the wild-type strain XC1. If 3-HBA is not present, then 4-HBA is used for biosynthesis of a structurally uncharacterized Xanthomonadin-B. Salicylic acid had no effect on xanthomonadin biosynthesis. Interference with 3-HBA and 4-HBA biosynthesis also affected X. campestris pv. campestris virulence factor production and reduced virulence in cabbage and Chinese radish. These findings add to our understanding of xanthomonadin biosynthetic mechanisms and further help to elucidate the biological roles of xanthomonadins in X. campestris pv. campestris adaptation and virulence in host plants.

Molecular characterization of bacterial leaf streak resistance in hard winter wheat

Bacterial leaf streak (BLS) caused by Xanthomonas campestris pv. translucens is one of the major bacterial diseases threatening wheat production in the United States Northern Great Plains (NGP) region. It is a sporadic but widespread wheat disease that can cause significant loss in grain yield and quality. Identification and characterization of genomic regions in wheat that confer resistance to BLS will help track resistance genes/QTLs in future wheat breeding. In this study, we evaluated a hard winter wheat association mapping panel (HWWAMP) containing 299 hard winter wheat lines from the US hard winter wheat growing region for their reactions to BLS. We observed a range of BLS responses among the lines, importantly, we identified ten genotypes that showed a resistant reaction both in greenhouse and field evaluation. -Genome-wide association analysis with 15,990 SNPs was conducted using an exponentially compressed mixed linear model. Five genomic regions (p < 0.001) that regulate the resistance to BLS were identified on chromosomes 1AL, 1BS, 3AL, 4AL, and 7AS. The QTLs Q.bls.sdsu-1AL, Q.bls.sdsu-1BS, Q.bls.sdsu-3AL, Q.bls.sdsu-4AL, and Q.bls.sdsu-7AS explain a total of 42% of the variation. In silico analysis of sequences in the candidate regions on chromosomes 1AL, 1BS, 3AL, 4AL, and 7AS identified 10, 25, 22, eight, and nine genes, respectively with known plant defense-related functions. Comparative analysis with rice showed two syntenic regions in rice that harbor genes for bacterial leaf streak resistance. The ten BLS resistant genotypes and SNP markers linked to the QTLs identified in our study could facilitate breeding for BLS resistance in winter wheat.

Biosynthesis, structure and antioxidant activities of xanthan gum from Xanthomonas campestris with additional furfural

Lignocellulosic-like materials are potentially low-cost fermentation substrates, but their pretreatment brings about by-products. This work investigated the effects of furfural on xanthan gum (XG) production, and product quality was evaluated by structure, viscosity and antioxidant capacities. Xanthomonas campestris maintained steady polysaccharide yield (above 13 g·L^-1) with enhanced cell growth at low furfural concentrations (below 3.2 g·L^-1). The products were verified as XG by FT-IR, XRD, NMR and monosaccharide analysis. Moreover, they were found to have reduced acetyl, rising pyruvate and up-to-down glucuronic acid groups as increasing furfural concentration. Furthermore, XG product with 1 g·L^-1 furfural addition showed the best hydroxyl scavenging effects, though reducing powers presented no variation. It was demonstrated that furfural, the common hydrolysis by-product, was not necessarily an inhibitor for fermentation, and an appropriate amount of furfural was beneficial to XG production with steady yield and good quality.

vitians

Bacterial leaf spot (BLS) caused by Xanthomonas campestris pv. vitians (Xcv) places a major constraint on lettuce production worldwide. The most sustainable strategy known to date for controlling BLS is the use of resistant cultivars. The nutrient elemental signature (ionome) of ten lettuce cultivars with three levels of resistance was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) to determine which nutrient balances are linked to resistance to BLS, and to assess the effect of Xcv infection on the ionome. The elemental concentrations were preprocessed with isometric log-ratios to define nutrient balances. Using this approach, 4 out of 11 univariate nutrient balances were found to significantly influence the resistance of lettuce cultivars to BLS (P < 0.05). These significant balances were the overall nutritional status balancing all measured nutrients with their complementary in the dry mass, as well as balances [Mn | Zn,Cu], [Zn | Cu], and [S,N | P]. Moreover, the infection of lettuce cultivars mostly affected the lettuce ionome on the [N,S | P] balance, where infection tended to lean the balance toward the N,S part relatively to P. This study shows that nutrient uptake in lettuce can be affected by BLS infection and that nutrient status influences resistance to BLS infection.

Valorisation of chicken feathers for xanthan gum production using Xanthomonas campestris MO-03

Xanthan gum is an important commercial polysaccharide produced by Xanthomonas species. In this study, xanthan production was investigated using a local isolate of Xanthomonas campestris MO-03 in medium containing various concentrations of chicken feather peptone (CFP) as an enhancer substrate. CFP was produced with a chemical process and its chemical composition was determined. The addition of CFP (1–8 g/l) increased the conversion of sugar to xanthan gum in comparison with the control medium, which did not contain additional supplements. The highest xanthan production (24.45 g/l) was found at the 6 g/l CFP containing control medium in 54 h. This value was 1.73 fold higher than that of control medium (14.12 g/l). Moreover, addition of CFP improved the composition of xanthan gum; the pyruvate content of xanthan was 3.86% (w/w), higher than that of the control (2.2%, w/w). The xanthan gum yield was also influenced by the type of organic nitrogen sources. As a conclusion, CFP was found to be a suitable substrate for xanthan gum production.

Two ancestral genes shaped the Xanthomonas campestris TAL effector gene repertoire

Xanthomonas transcription activator-like effectors (TALEs) are injected inside plant cells to promote host susceptibility by enhancing transcription of host susceptibility genes. TALE-encoding (tal) genes were thought to be absent from Brassicaceae-infecting Xanthomonas campestris (Xc) genomes based on four reference genomic sequences. We discovered tal genes in 26 of 49 Xc strains isolated worldwide and used a combination of single molecule real time (SMRT) and tal amplicon sequencing to yield a near-complete description of the TALEs found in Xc (Xc TALome). The 53 sequenced tal genes encode 21 distinct DNA binding domains that sort into seven major DNA binding specificities. In silico analysis of the Brassica rapa promoterome identified a repertoire of predicted TALE targets, five of which were experimentally validated using quantitative reverse transcription polymerase chain reaction. The Xc TALome shows multiple signs of DNA rearrangements that probably drove its evolution from two ancestral tal genes. We discovered that Tal12a and Tal15a of Xcc strain Xca5 contribute together in the development of disease symptoms on susceptible B. oleracea var. botrytis cv Clovis. This large and polymorphic repertoire of TALEs opens novel perspectives for elucidating TALE-mediated susceptibility of Brassicaceae to black rot disease and for understanding the molecular processes underlying TALE evolution.

Xanthan gum production from acid hydrolyzed broomcorn stem as a sole carbon source by Xanthomonas campestris

Xanthan gum is an exo-polysaccharide industrially produced by fermentation using simple sugars. In this study, broomcorn stem was introduced as a low-cost- and widely available carbon source for xanthan gum fermentation. Broomcorn stem was hydrolyzed using sulphuric acid to liberate reducing sugar which was then used as a carbon source for biosynthesis of xanthan gum by Xanthomonas campesteris. Effects of hydrolysis time (15, 30, 45 and 60 min), sulphuric acid concentration (2, 4, 6 and 8% v/v) and solid loading (3, 4, 5 and 6% w/v) on the yield of reducing sugar and consequent xanthan production were investigated. Maximum reducing sugar yield (55.2%) and xanthan concentration (8.9 g/L) were obtained from hydrolysis of 4% (w/v) broomcorn stem with 6% (v/v) sulphuric acid for 45 min. The fermentation product was identified and confirmed as xanthan gum using Fourier transform infrared spectroscopy analysis. Thermogrvimetric analysis showed that thermal stability of synthesized xanthan gum was similar to those reported in previous studies. The molecular weight of the produced xanthan (2.23 × 10⁶ g/mol) was determined from the intrinsic viscosity. The pyruvate and acetyl contents in xanthan gum were 4.21 and 5.04%, respectively. The chemical composition results indicated that this biopolymer contained glucose, mannose and glucoronic acid with molecular ratio of 1.8:1.5:1.0. The kinetics of batch fermentation was also investigated. The kinetic parameters of the model were determined by fermentation results and evaluated. The results of this study are noteworthy for the sustainable xanthan gum production from low-value agricultural waste.

Modes of Action of a Bi-domain Plant Defensin MtDef5 Against a Bacterial Pathogen Xanthomonas campestris

Defensins are small cysteine-rich endogenous host defense peptides expressed in all higher plants. They are thought to be important players in the defense arsenal of plants against fungal and oomycete pathogens. However, little is known regarding the antibacterial activity of these peptides. The genome of the model legume Medicago truncatula contains 63 genes each encoding a defensin with a tetradisulfide array. A unique bi-domain defensin, designated MtDef5, was recently characterized for its potent broad-spectrum antifungal activity. This 107-amino acid defensin contains two domains, 50 amino acids each, linked by a short peptide APKKVEP. Here, we characterize antibacterial activity of this defensin and its two domains, MtDef5A and MtDef5B, against two economically important plant bacterial pathogens, Gram-negative Xanthomonas campestris and Gram-positive Clavibacter michiganensis. MtDef5 inhibits the growth of X. campestris, but not C. michiganensis, at micromolar concentrations. MtDef5B, but not MtDef5A, exhibits more potent antibacterial activity than its parent MtDef5. MtDef5 and each of its two domains induce distinct morphological changes and cell death in X. campestris. They permeabilize the bacterial plasma membrane and translocate across membranes to the cytoplasm. They bind to negatively charged DNA indicating these peptides may kill bacterial cells by inhibiting DNA synthesis and/or transcription. The cationic amino acids present in the two γ-core motifs of MtDef5 that were previously shown to be important for its antifungal activity are also important for its antibacterial activity. MtDef5 and its more potent single domain MtDef5B have the potential to be deployed as antibacterial agents for control of a Xanthomonas wilt disease in transgenic crops.

Probing the Role of Cyclic di-GMP Signaling Systems in Disease Using Chinese Radish

The determination of the genome sequences of pathogenic bacteria has facilitated functional analyses that aim to understand the molecular basis of virulence. In particular, genome sequence information of the pathogen Xanthomonas campestris pathovar campestris has allowed researchers to identify and functionally analyze the role of intracellular signaling involving cyclic di-GMP in black rot disease of crucifers. Here, we describe leaf clipping and spraying methods for testing the virulence of wild type and derived mutants of X. campestris in Chinese radish. These methods address different facets of the disease cycle, which requires the ability to survive epiphytically before entry into the plant and growth and systemic spread within the xylem.

Effects of Nonionic Surfactants on Xanthan Gum Production: a Survey on Cellular Interactions

Background

Xanthomonas campestris is a biopolymer producing gram negative bacterium. Production of xanthan biopolymer can be affected by different extrinsic factors as well as surfactants. Hitherto, effects of nonionic surfactants on xanthan production have been studied in a limited number of articles.

Objective

In the present study, nonionic surfactants were used to pursue their effects on improvement of xanthan production. Moreover, a number of cellular consequences upon the treatment were investigated with impacts on gum production.

Materials and Methods

Effects of different nonionic surfactants (Tween 20, Tween 80 and Triton X100) on xanthan production and Xanthomonas campestris cells were assessed by ultramicroscopy (SEM), changes in culture turbidity, leakage of sugars and ATP, and quality of xanthan (i.e. pyruvate content and determination of polymer molecular weight).

Results

The nonionic surfactant Tween 20 increased ATP (3.2 folds) and sugar leakage (3.1 folds). Furthermore, they caused cell shape alteration. Tween 80 improved both xanthan production (11 g.L^-1) and viscosity of the product (1368 cP), while the total biomass remained unchanged (2.2 g.L^-1). Molecular weight of xanthan was enhanced (from 23 to 59 million Da). Toxic effect of 5% (v/v) Triton X 100 decreased the turbidity of culture to 120 NTU and total biomass was diminished to 1 g.L^-1. Tween 20 caused the loss of ATP and sugar leakage and led to lower xanthan production. It had no effect on biomass content.

Conclusions

In general, amounts of surfactants that bacterial cells can tolerate seem to be helpful in substrate and metabolite transportation, and enzyme activities involved in xanthan biosynthesis and release. Surfactants induce harsh damages to cell barriers, preventing the growth and adversely affecting quantity and quality of xanthan gum.

Construction and application of a Xanthomonas campestris CGMCC15155 strain that produces white xanthan gum

In the industrial production of xanthan gum using Xanthomonas campestris CGMCC15155, large amounts of ethanol are required to extract xanthan gum from the fermentation broth and remove xanthomonadin impurities. To reduce the amount of ethanol and the overall production cost of xanthan gum, a xanthomonadin‐deficient strain of CGMCC15155 was constructed by inserting the Vitreoscilla globin (vgb) gene, under the control of the LacZ promoter, into the region of the pigA gene, which is involved in xanthomonadin synthesis. The insertion of vgb inactivated pigA, resulting in the production of white xanthan gum. The lack of xanthomonadins resulted in a decreased yield of xanthan gum. However, the expression product of vgb gene, VHb, could increase the metabolism of X. campestris, which allowed the production of xanthan gum to reach wild‐type levels in the engineered strain. The yield, molecular weight, and rheological properties of the xanthan gum synthesized by the engineered and wild‐type bacteria were essentially the same. When the same volume of ethanol was used, the whiteness values of the xanthan gum extracted from engineered and wild‐type bacteria were 65.20 and 38.17, respectively. To extract xanthan gum with the same whiteness, three and seven times the fermentation volume of ethanol was required for the engineered and wild‐type strains, respectively. Thus, the engineered train reduced the requirement for ethanol in xanthan gum production by 133.3%. The results demonstrated that the engineered bacteria used less ethanol, thus reducing the downstream processing cost in xanthan gum production.

Methods to Study Quorum Sensing-Dependent Virulence and Movement of Phytopathogens In Planta

Cell-to-cell communication mediated by the diffusible signal factor (DSF) is a common form of gene regulation and plays an important role in virulence of many plant pathogenic bacteria including Xanthomonas spp. Here we describe several approaches to study the involvement of DSF-dependent QS system of the plant pathogenic bacteria Xanthomonas campestris pv. pelargonii (Xhp) as an example of the Xanthomonas spp. The methods described include detection and measurement of DSF, movement in planta, colonization, and aggregate formation.

mangiferaeindicae IBSBF 2103

The conditions of storage, cultivation and maintenance of microbial cultures should preserve the microbiological homogeneity, phenotypic and genotypic characteristics to ensure better reproducibility of metabolic production. To evaluate the influence of the storage condition on the composition of cell fatty acids, genetic profile and biochemical characteristics of Xanthomonas campestris pv. mangiferaeindicae IBSBF 2103, as well as, to identify its relationship with the yielding and viscosity of the xanthan gum produced, this study monitored the strain preserved in two simple and widely used conditions, ultra-freezer (-80 °C) and refrigeration (3-8 °C) during 5 months. Were identified and quantified 13 fatty acids. The cells preserved at -80 °C showed more stable concentration of all fatty acids, producing more xanthan gum and with higher viscosity. The chromosomal analysis obtained with the enzyme XbaI revealed 17 distinct fragments with maximum size of 485 kilobases, without variations among the subcultures maintained in both storage conditions. The X. campestris pv. mangiferaeindicae subcultures preserved at -80 °C showed less pronounced phenotypic variations, which had positive influence in the qualitative and quantitative characteristics of the xanthan gum produced.