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The overexpression of RXam2, a cassava NLR (nucleotide-binding leucine-rich repeat) gene, by stable transformation and gene expression induction mediated by dTALEs, reduce cassava bacterial blight symptoms. Cassava (Manihot esculenta) is a tropical root crop affected by different pathogens including Xanthomonas phaseoli pv. manihotis (Xpm), the causal agent of cassava bacterial blight (CBB). Previous studies have reported resistance to CBB as a quantitative and polygenic character. This study sought to validate the functional role of a NLR (nucleotide-binding leucine-rich repeat) associated with a QTL to Xpm strain CIO151 called RXam2. Transgenic cassava plants overexpressing RXam2 were generated and analyzed. Plants overexpressing RXam2 showed a reduction in bacterial growth to Xpm strains CIO151, 232 and 226. In addition, designer TALEs (dTALEs) were developed to specifically bind to the RXam2 promoter region. The Xpm strain transformed with dTALEs allowed the induction of the RXam2 gene expression after inoculation in cassava plants and was associated with a diminution in CBB symptoms. These findings suggest that RXam2 contributes to the understanding of the molecular basis of quantitative disease resistance.

Cassava, an important source of carbohydrates in many developing countries, is affected by several diseases including bacterial blight, caused by Xanthomonas phaseoli pv. manihotis (Xpm), responsible for considerable losses in production. A 3-year split-plot design experiment was conducted to evaluate the resistance of 83 Brazilian cassava genotypes to Xpm under field conditions, inoculated or not with bacterial suspension (108 CFU mL−1). The relationship between disease progress and yield traits was also studied. Disease severity was evaluated using an ordinal 0 (no symptoms) to 8 (maximum degree of the disease) scores. At 11 months after planting, the following traits were measured: plant height, above-ground yield (measured as the total weight of leaves and stem), fresh root yield, dry matter content in the roots, and dry root yield. Based on the calculated disease index (DI) and the area under the disease progress curve (AUDPC) values, the accessions were grouped into different classes of resistance/susceptibility. There were 16 accessions classified as moderately resistant (MR), 14 as moderately susceptible (MS), 41 as susceptible (S), and 12 as extremely susceptible (ES). Complete resistance to bacterial blight was not found. Despite the significant reduction in above-ground and root yield average for the inoculated plants, when compared to non-inoculated, there was no correlation between the severity (DI) or disease progress (AUDPC) and the agronomic performance of cassava genotypes. The selection of the best 10 accessions should be a priority, considering that the agronomic performance could increase the above-ground yield, fresh root yield, and dry root yield by 22.17%, 109.61%, and 114.64%, respectively.

The synthesis of Silver Nanoparticles (SNP) from plants is a simple, fast and environmentally safe route. In the present study, the aqueous extract of fresh leaves from Leea coccinea L. was evaluated as a possible source of reducing and stabilizing agents to obtain SNP. The synthesized SNP were characterized by spectroscopic techniques such as UV-Visible spectrophotometry and Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron and Confocal microscopies and the antimicrobial activity was evaluated against to Xanthomonas phaseoli pv. phaseoli using the agar diffusion methods. The results showed that the evaluated extract was promising for the green synthesis of the SNP, which were visually identified by the formation of a dark brown complex and by the presence of a peak of maximum absorption at 470 nm in a UV-VIS spectrum. FTIR spectrum of SNP showed main characteristic signals of aromatic compounds, carboxylic group between others confirmed through phytochemical screening that evidenced the presence of flavonoids, phenols, leucoanthocyanidins, terpenes and steroids groups. Fluorescents SNP with high degree of agglomeration were visualized by microscopies technical used. A promissory antibacterial activity of SNP was demonstrated by a zone of inhibition of the microbial growing. These results suggest the need to go deeper in the physical-chemical characterization and kinetic studies and the biological evaluation to make possible the use of this vegetable source to future develop of antibacterial formulations to the bean seeds protection.

A single loop-mediated isothermal amplification (LAMP) assay was developed for specific detection of both pathogens that cause bacterial blight in common bean, Xanthomonas phaseoli pv. phaseoli (Xpp) and Xanthomonas citri pv. fuscans (Xcf). The objective was to provide a simple, easy-to-use, specific, and sensitive method to investigate the presence of one or both pathogens in plant material and seeds for routine diagnosis. The detection limits for both pathogens were 10 CFU/ml for cell suspensions and 1 fg of DNA, whereas in conventional PCR, the primers detected up to 105 CFU/ml and 1 ng of DNA. Specificity was confirmed by testing DNA from bean leaves, other Xanthomonas species, common fungal and bacterial bean pathogens, and bacteria from the leaf microbiota. The method was tested with bean leaves inoculated with Xpp, and the pathogen could be detected from 4 h up to 15 days postinoculation, even before disease symptoms were visible. When the method was applied to bacterium detection (Xpp or Xcf) in seed lots from infected plants, the bacterium detection rate was 100% (24 of 24). The pathogens were detected in seeds incubated for just 1 h in saline solution (0.85%), reducing the time needed for bacterium detection. The LAMP assay could be useful as a tool in bean bacterial blight management. Rapid and sensitive detection of bacteria in bean seed lots would reduce the risks of planting highly contaminated seeds in environments favorable to blight multiplication.

Cassava Bacterial Blight (CBB), caused by Xanthomonas phaseoli pv. manihotis (Xpm), is the most important bacterial disease of cassava (Manihot esculenta). Xpm employs Transcription Activator-Like effectors (TALE) to induce the expression of host susceptibility (S) genes favoring bacterial growth. The identification of resistant and susceptible cassava varieties as well as the identification of the cassava gene MeSweet10a as a target of the effector TAL20, have been conducted on plants grown from mature stem-cuttings. We explored the possibility of using in vitro plants to study cassava-Xpm interactions. Cassava in vitro plants of varieties 60444 and CM6438–14 were susceptible and resistant, respectively, to infection by strain Xpm668. In addition, the expression of MeSweet10a was induced in the susceptible but not in the resistant variety, which was not associated with polymorphisms in the Effector Binding Element (EBE) reported for TAL20. Three other Xpm strains, Xpm531, Xpm681 and Xpm1061, were also able to induce the TAL20 cognate target gene in the susceptible cassava variety 60444. These results demonstrate the usefulness of in vitro plants to study the phenotypic and molecular responses of cassava during Xpm infection.

We report the complete and circularized genome sequences of 17 strains of Xanthomonas citri pv. fuscans and Xanthomonas phaseoli pv. phaseoli, which cause common bacterial blight of bean. These new assemblies combining PacBio and short-read sequencing methods provide high-quality material for studying the evolution of these plant pathogens.

Accurate assessment of plant symptoms plays a key role for measuring the impact of pathogens during plant-pathogen interaction. Common bacterial blight caused by Xanthomonas phaseoli pv. phaseoli and Xanthomonas citri pv. fuscans (Xpp-Xcf) is a major threat to common bean. The pathogenicity of these bacteria is variable among strains, and depends mainly on a type III secretion system and associated type III effectors such as transcription activator-like effectors (TALEs). Because the impact of a single gene is often small and difficult to detect, a discriminating methodology is required to distinguish the slight phenotype changes induced during the progression of the disease. Here, we compared two different inoculation and symptom assessment methods for their ability to distinguish two tal mutants from their corresponding wild-type strains. Interestingly, rub-inoculation of the first leaves combined with symptom assessment by machine learning-based imaging allowed significant distinction between wild-type and mutant strains. By contrast, dip-inoculation of first trifoliate leaves combined with chlorophyll fluorescence imaging did not differentiate the strains. Furthermore, the new method developed here led to the miniaturization of pathogenicity tests and significant time savings.

La bacteriosis vascular de la yuca, causada por la bacteria gram negativa Xanthomonas phaseoli pv. manihotis (Xpm), anteriormente conocida como Xanthomonas axonopodis pv. manihotis, es la principal enfermedad bacteriana que compromete su producción. Con la meta de generar una resistencia durable y de amplio espectro a la bacteriosis es posible explotar los mecanismos naturales presentes en plantas no-hospedero. Arabidopsis es una planta modelo extensamente estudiada, la cual es no-hospedero de Xpm. La meta de este estudio fue determinar si la resistencia no-hospedero de Arabidopsis es consecuencia de la presencia de barreras físicas o si esta depende de determinantes genéticos. En este trabajo se evaluó la capacidad de plantas de Arabidopsis de responder a la inoculación con Xpm. Ninguno de los ocho ecotipos de Arabidopsis evaluados mostraron una respuesta hipersensible a la inoculación con ocho diferentes cepas de Xpm. Aunque no se identificó la presencia de especies reactivas de oxígeno si se encontró un bloqueo en el crecimiento de Xpm en las plantas de Arabidopsis. En conjunto, los resultados aquí presentados sugieren que Arabidopsis no está activando una respuesta contra Xpm y que la resistencia observada puede ser consecuencia de las barreras físicas presentes en Arabidopsis que Xpm no es capaz de superar.

DNA of 8 Xanthomonas phaseoli bacteriophages were studied by restriction analysis using diverse endonucleases. Based on similarity of restricts the phages were divided into 4 groups.

BackgroundThe type VI protein secretion system (T6SS) is important in diverse cellular processes in Gram-negative bacteria, including interactions with other bacteria and with eukaryotic hosts. In this study we analyze the evolution of the T6SS in the genusXanthomonasand evaluate its importance of the T6SS for virulence and in vitro motility inXanthomonas phaseolipv.manihotis(Xpm), the causal agent of bacterial blight in cassava (Manihot esculenta). We delineate the organization of the T6SS gene clusters inXanthomonasand then characterize proteins of this secretion system inXpmstrain CIO151.ResultsWe describe the presence of three different clusters in the genusXanthomonasthat vary in their organization and degree of synteny between species. Using a gene knockout strategy, we also found thatvgrGandhcpare required for maximal aggressiveness ofXpmon cassava plants whileclpVis important for both motility and maximal aggressiveness.ConclusionWe characterized the T6SS in 15 different strains inXanthomonasand our phylogenetic analyses suggest that the T6SS might have been acquired by a very ancient event of horizontal gene transfer and maintained through evolution, hinting at their importance for the adaptation ofXanthomonasto their hosts. Finally, we demonstrated that the T6SS ofXpmis functional, and significantly contributes to virulence and motility. This is the first experimental study that demonstrates the role of the T6SS in theXpm-cassava interaction and the T6SS organization in the genusXanthomonas.

Xanthomonas phaseoli pv. manihotis (Xpm) is the causal agent of cassava bacterial blight, the most important bacterial disease in this crop. There is a paucity of knowledge about the metabolism of Xanthomonas and its relevance in the pathogenic process, with the exception of the elucidation of the xanthan biosynthesis route. Here we report the reconstruction of the genome-scale model of Xpm metabolism and the insights it provides into plant–pathogen interactions. The model, iXpm1556, displayed 1,556 reactions, 1,527 compounds, and 890 genes. Metabolic maps of central amino acid and carbohydrate metabolism, as well as xanthan biosynthesis of Xpm, were reconstructed using Escher (https://escher.github.io/) to guide the curation process and for further analyses. The model was constrained using the RNA-seq data of a mutant of Xpm for quorum sensing (QS), and these data were used to construct context-specific models (CSMs) of the metabolism of the two strains (wild type and QS mutant). The CSMs and flux balance analysis were used to get insights into pathogenicity, xanthan biosynthesis, and QS mechanisms. Between the CSMs, 653 reactions were shared; unique reactions belong to purine, pyrimidine, and amino acid metabolism. Alternative objective functions were used to demonstrate a trade-off between xanthan biosynthesis and growth and the re-allocation of resources in the process of biosynthesis. Important features altered by QS included carbohydrate metabolism, NAD(P)+ balance, and fatty acid elongation. In this work, we modeled the xanthan biosynthesis and the QS process and their impact on the metabolism of the bacterium. This model will be useful for researchers studying host–pathogen interactions and will provide insights into the mechanisms of infection used by this and other Xanthomonas species.

Cassava bacterial blight caused by Xanthomonas phaseoli p.v manihotis (Xpm) is a common disease of cassava (Manihot esculenta). The molecular mechanisms underlying susceptible and resistant response and host defense mechanisms are currently unknown. In this study, we performed large-scale mRNA expression profiling using an RNAseq approach to identify Cassava responses in MBRA685 variety inoculated with Xpm618 (resistant response) and Xpm318 (susceptible response) strains. Bioinformatic analysis revealed significantly differentially expressed genes (DEGs) by Xpm681 in MBRA685 with higher fold change values. Unique prevalent GO categories for Xpm681 response included “protein kinase activity” and “phosphorylation”; “amino acid biosynthetic process” for Xpm318 response and “regulation of transcription” and “transcription factor activity” in both responses. There were particular differences between the cultivars in the intensity of expression of genes with putative roles in transcriptional activity, receptor and signaling processes, secondary metabolism and photosynthesis. The differences between two interactions can be explained quantitatively, the resistant response was weaker than the susceptible response. Quantitative real-time PCR experiments confirmed the reliability of our RNAseq data.

Xanthomonas axonopodis pv. dieffenbachiae, the causal agent of bacterial blight of Araceae (aroids), is a regulated pest in several countries and is included in the EPPO A2 List. Reference strains of Xanthomonas axonopodis pv. dieffenbachiae have recently been reclassified into the species Xanthomonas phaseoli, Xanthomonas citri and Xanthomonas euvesicatoria on the basis of different features, including multilocus sequence analysis, average nucleotide identity and homology in DNA–DNA hybridization analyses. Based on pathogenicity tests, Constantin et al. (2017) proposed naming the pathogens on aroids as X. phaseoli pv. dieffenbachiae, X. phaseoli pv. syngonii and X. citri pv. aracearum. Recommendations are made on how to deal with these changes for the group of pathogenic bacteria for Araceae. The name Xanthomonas axonopodis pv. dieffenbachiae on the EPPO List should be adjusted to the names proposed in the taxonomic study by Constantin et al. (2016). The current EPPO Diagnostic Standard is directed at strains pathogenic on Anthurium. They mainly belong to X. phaseoli pv. dieffenbachiae, but some also to X. citri pv. aracearum that are not detected by the EPPO Diagnostic Standard. Xanthomonas phaseoli pv. syngonii strains are also aggressive, but with a host range restricted to Syngonium. The pathogenicity specific to aroids of the bacterial isolates reclassified as Xanthomonas euvesicatoria was not confirmed and no pathovar epithet has been retained for these strains.

Xanthomonas phaseoli pv. manihotis (Xpm) is the causal agent of Cassava Bacterial Blight (CBB), one of the most important cassava diseases. The aim of this study was to evaluate the susceptibility of sweet cassava genotypes to strains of Xpm from three different geographic regions of Brazil in greenhouse conditions. The inoculation of 14 genotypes of cassava was made by cutting leaflets using scissors previously soaked in a bacterial suspension with 1 × 108 UFC ml-1 and also by inserting a soaked toothpick into the bud of the oldest leaf. The results showed significant differences when the cassava genotypes were individually evaluated in relation to the Xpm strains used; however, the relationship between cassava genotypes and Xpm strains was significant for wilt symptom. The UnB 1111 strain was more aggressive than the UnB 1386 strain based on the average value of the reaction grade, showing the variation that exists between the bacterial isolates from different regions. Considering the reaction of cassava germplasm’s resistance to the three strains used in the study, the BGMC 434 genotype was the only one classified as resistant based on the average reaction grade. The genotypes BGMC 753, BGMC 1289, BGMC 982 and the elite clones BRS 396, BRS 397, BRS 398, 259/08 and BRS 399 were classified as moderately resistant, which indicates the possibility of recommending them for disease favorable regions.

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.

Lichens are composite organisms formed upon symbiotic association of fungi and algae doing photosynthesis (blue green algae). Lichenized fungi produce unique secondary metabolites which bears a wide spectrum in biological activities viz., anti-HIV, production of antibiotics, anti-protozoan, anticancer etc. This review focuses primarily on the antibacterial activity of lichen’s secondary chemicals related to major antibacterial activity and highlights of each study. The literature published clearly demonstrates that the lichen extracts and their constituent compounds have potential of significant inhibitory activity against various pathogenic bacteria viz., Bacillus cereus, Bacillus subtilis, Chrysobacterium indolthecium, Clavibacter michiganensis subsp. Michiganensis, Enterococcus faecalis, Erwinia carotovora sub sp. carotovora, Escherichia coli, Methicillin-resistant S. aureus, Pseudomonas aeruginosa. Pseudomonas cichorii, Pseudomonas savastoni pv. Fraxinus, Pseudomonas syringae pv. syringae, Pseudomonas vesicularis, Staphylococcus aureus, Staphylococcus aureus, Streptococcus pneumoniae, Xanthomonas axanopodis pv. Malvecearum, Xanthomonas hortanum pv. pelargonii, Xanthomonas phaseoli, Yersiniapseudotuberculosis, even at low concentrations. The literature records however shows no studies reported anything on the specific mechanism of action against pathogenic bacteria. Lichens harbours many biologically active compounds in which less numbers have been experimentally tested. It needs very deep studies for search of active compounds. It needs to do more clinical trials and search of mechanism of action for potent compounds in lichens.

Anthurium bacterial blight caused by Xanthomonas phaseoli pv. dieffenbachiae (formerly Xanthomonas axonopodis pv. dieffenbachiae) is the major phytosanitary threat in many anthurium growing areas worldwide. Reliable and sensitive diagnostic tools are required for surveillance and certification programs. A duplex real–time quantitative PCR assay was developed for the detection and quantification of X. phaseoli pv. dieffenbachiae from anthurium tissue. This PCR assay targeted a X. phaseoli pv. dieffenbachiae–specific gene encoding an ABC transporter and an internal control encoding for chalcone synthase in Anthurium andreanum. A cycle threshold (Ct), using a receiver-operating characteristic approach (ROC), was implemented to ensure that the declaration of a positive sample was reliable. The duplex real–time assay displayed very high performance with regards to analytical specificity (100% inclusivity, 98.9% exclusivity), analytical sensitivity (LOD95% = 894 bacteria/ml corresponding to 18 bacteria per reaction) and repeatability. We demonstrated the pertinence of this real–time quantitative PCR assay for detecting X. phaseoli pv. dieffenbachiae from diseased leaf tissue (collected from outbreaks on anthurium) and from asymptomatic, latently infected anthurium plants. This assay could be useful for surveillance, as well as for indexing propagative plant material for the presence of X. phaseoli pv. dieffenbachiae.

International audience; Diverse molecular markers have been used to analyze the genetic diversity of plant pathogens. Compared with traditional fingerprinting methods, multiple loci variable number of tandem repeat analyses (MLVAs) have gained importance recently due to their reproducibility, high discriminatory power, ease of performance, low cost, and throughput potential. These characteristics are desirable for continuous pathogen monitoring, especially for pathogens with relatively low genetic diversity, and for disease epidemiology studies. Genetic diversity studies of Xanthomonas phaseoli pv. manihotis, which is the causal agent of cassava bacterial blight, have shown variability and changes in the bacterial population over time. Thus, an easy and fast method needs to be developed to type populations of this pathogen in different countries of the world, especially on small scales. In this study, we developed an MLVA scheme to analyze X. phaseoli pv. manihotis variability on a local scale. The MLVA-15 scheme comprises 15 variable number of tandem repeat loci grouped into four multiplex polymerase chain reaction pools. We showed that the MLVA-15 scheme had slightly higher discriminatory ability at the locality level when compared with amplified fragment length polymorphisms. The MLVA-15 scheme allowed for an accurate determination of the number of genotypes in the sample and showed reproducibility and portability. Additionally, this scheme could be used to analyze numerous strains in a reasonable timeframe. The MLVA-15 scheme was highly specific to X. phaseoli but up to eight tandem repeat loci could be amplified from other Xanthomonas spp. Finally, we assessed the utility of the scheme for analyses of X. phaseoli pv. manihotis genetic variability in the Colombian Caribbean region. MLVA-15 distinguished 88.9% of the haplotypes in our sample. Strains originating from the same field and isolated at the same time could be discriminated. In this study, the advantages of the MLVA-15 scheme targeting 6- or 7-bp repeats were demonstrated. Moreover, this scheme was a fast method that was appropriate for routine monitoring of X. phaseoli pv. manihotis populations on a local scale and, thus, was useful for addressing epidemiological questions.

Cassava Bacterial Blight and Cassava Bacterial Necrosis are two bacterial diseases affecting cassava, respectively caused by Xanthomonas phaseoli pv. manihotis (Xpm) and Xanthomonas cassavae (Xc). Since both pathogens may be present on leaves and cause similar symptoms, we developed a new molecular diagnostic tool to detect and distinguish between Xpm and Xc. Based on genome sequences from the target species as well as from non-target species, in silico analysis was performed to select candidate primers through a novel strategy targeting specificity. Experimental validation enabled to establish a duplex-PCR diagnostic tool that will be useful for future surveillance programs and better disease control.

Xanthomonas axonopodis pv. dieffenbachiae (Xad) is the causal agent of anthurium bacterial blight and listed as an A2 quarantine organism by EPPO. However, the name Xad covers a variety of strains. Here, 25 Xad strains and 88 phylogenetically related strains, including Xanthomonas type strains and representatives of other pathovars, were examined using a polyphasic taxonomic approach. Multilocus sequence analysis of seven genes showed that strains isolated from Dieffenbachia, Philodendron and Anthurium cluster into three phylogenetic groups (PG I, II and III), while the type strain of X. axonopodis clustered into a fourth group (PG IV). PG I included the type strains of X. citri subsp. citri, X. citri subsp. malvacearum, X. fuscans subsp. fuscans and X. fuscans subsp. aurantifolii. PG II included the type strains of X. euvesicatoria, X. perforans, X. alfalfae subsp. alfalfae and X. alfalfae subsp. citrumelonis. PG III included the type strains of X. phaseoli. Each PG was shown to represent a single species, based on average nucleotide identity values, DNA-DNA hybridization data and phenotypic characteristics. Therefore, strains named as Xad belong to PG I, PG II and PG III, and not to X. axonopodis (PG IV). Taxonomic proposals are made: emendations of the descriptions of X. citri, X. phaseoli and X. axonopodis, to encompass the strains of PG I, PG III and PG IV, respectively; and reclassification of X. perforans and X. alfalfae as X. euvesicatoria and emendation of the description of X. euvesicatoria to encompass all strains of PG II.

Background Common bacterial blight is a devastating bacterial disease of common bean (Phaseolus vulgaris) caused by Xanthomonas citri pv. fuscans and Xanthomonas phaseoli pv. phaseoli. These phylogenetically distant strains are able to cause similar symptoms on common bean, suggesting that they have acquired common genetic determinants of adaptation to common bean. Transcription Activator-Like (TAL) effectors are bacterial type III effectors that are able to induce the expression of host genes to promote infection or resistance. Their capacity to bind to a specific host DNA sequence suggests that they are potential candidates for host adaption. Results To study the diversity of tal genes from Xanthomonas strains responsible for common bacterial blight of bean, whole genome sequences of 17 strains representing the diversity of X. citri pv. fuscans and X. phaseoli pv. phaseoli were obtained by single molecule real time sequencing. Analysis of these genomes revealed the existence of four tal genes named tal23A, tal20F, tal18G and tal18H, respectively. While tal20F and tal18G were chromosomic, tal23A and tal18H were carried on plasmids and shared between phylogenetically distant strains, therefore suggesting recent horizontal transfers of these genes between X. citri pv. fuscans and X. phaseoli pv. phaseoli strains. Strikingly, tal23A was present in all strains studied, suggesting that it played an important role in adaptation to common bean. In silico predictions of TAL effectors targets in the common bean genome suggested that TAL effectors shared by X. citri pv. fuscans and X. phaseoli pv. phaseoli strains target the promoters of genes of similar functions. This could be a trace of convergent evolution among TAL effectors from different phylogenetic groups, and comforts the hypothesis that TAL effectors have been implied in the adaptation to common bean. Conclusions Altogether, our results favour a model where plasmidic TAL effectors are able to contribute to host adaptation by being horizontally transferred between distant lineages. Electronic supplementary material The online version of this article (10.1186/s12864-017-4087-6) contains supplementary material, which is available to authorized users.

Modifications were made in the PCR conditions aiming to overcome the problem of non-amplification of the Xanthomonas phaseoli pv. manihotis (Xpm) fragment, using the primer pair XV / XK described in the literature. The objective of this study was to propose changes in the primers already described (XV / XK_MOD) and validate the use of these new primers in identifying Xpm. The validation procedure was carried out with the primer pair XV and XK_MOD, using different strains of Xpm, other plant pathogenic and endophytic bacteria genera and one isolate of X. phaseoli pv. passiflorae. As a control, additional reactions were conducted in multiplex with the universal primers for the 16S rRNA gene of the bacteria together with XV / XK and XV / XK_MOD. Using the forward primer (XV) described in the literature together with the modified reverse primer (XK_MOD), it was possible to achieve amplification from DNA extracted from in vitro cultures and from infected tissue, but no amplification was noticed for the primer pair described in the literature, confirming the effectiveness of the proposed modification.

We have found a form of resistance lacking host cell death, effective in cucumber against infection of Pseudoperonospora cubensis , in bean against Pseudomonas phaseolicola and Xanthomonas phaseoli , as well as in pepper against the Obuda pepper virus and Xanthomonas vesicatoria . In addition to the own pathogens of the host plants, this defense reaction, accompanied by cell enlargement and stimulated cell division, was also efficient against pathogen species that are in incompatible relationship with the given host plants (the nonhosts). This general defense reaction may manifest itself in different intensities: in cell enlargement of only a few plant cells or enlargement and proliferation of cells to such a degree that results in compaction of even larger tissue parts.

La caraota (Phaseolus vulgaris L.) representa uno de los alimentos básicos en la dieta del venezolano. La productividad se ve seriamente afectada por la bacteriosis común causada por Xanthomonas phaseoli. La incorporación de genes de resistencia, a través de programas de cruzamiento se presenta como una alternativa de control. Con el fin de evaluar la resistencia a la bacteriosis común, se estudiaron familias F2:4 de caraota provenientes del cruce XAN-154 y MEM-0301013. Las plantas fueron inoculadas con heridas, utilizando un cojín de alfileres sobre las protófilas, con una cepa proveniente de Tucutunemo. Las variables evaluadas, tamaño de las manchas (mm) y porcentaje del área foliar afectada (% AFA), permitieron estudiar el avance de la enfermedad. Para el momento de máximo desarrollo de la enfermedad, 25 días después de inoculadas (DDI) las plantas, se determinó la distribución de la frecuencia en los distintos grados de la escala de reacción (1 al 9). Se realizó la prueba de bondad de ajuste (χ²), arrojando que la segregación observada correspondió a un gen mayor dominante en F2. Se encontraron dos familias F2:4 (16 y 36) completamente resistentes, el padre XAN-154 y el genotipo XAN-149 presentaron resistencia alta. La mayoría de las familias F2:4 mostraron resistencia alta a moderada. La resistencia aportada por el progenitor XAN-154, pudiera utilizarse como fuente de resistencia monogénica en la conformación de otras poblaciones para mejoramiento genético. Se evidenció la presencia de poligenes a la resistencia, posiblemente aportados por ambos progenitores; estos genes menores pudieran aprovecharse en futuros programas de mejoramiento. The black bean (Phaseolus vulgaris L.) is one of the staples in the venezuelan diet. Productivity is seriously affected by the common bacterial blight caused by Xanthomonas phaseoli. Incorporating resistance genes through breeding programs is presented as an alternative control. In order to evaluate the resistance to common bacterial blight, black beans families F2:4 were studied, from crossing XAN-154 and MEM-0301013. The plants were inoculated with Tucutunemo strain, using the multiple needle method on protophylls. The variables evaluated, spot size (mm) and percentage of leaf area affected (% FAA) allowed us to study the disease advance. For the moment of maximum development of disease, 25 days after plants inoculation (DDI), the frequency distribution of the different degrees of reaction (scale from 1 to 9) was determined. The goodness of fit test (χ2) was performed, yielding that segregation observed corresponded to a single dominant gene in F2. Two families F2:4 (16 and 36) were found completely resistant, the father XAN-154 and genotype XAN-149 showed high resistance. Most families F2:4 showed high to moderate resistance. The resistance provided by the parent XAN-154, could be used as a source of monogenic resistance for the conformation of other populations in genetic breeding. There was evidence of the presence of polygenes for resistance possibly provided by both parents; these genes of smaller effect, could be used in future breeding programs.

Anguillospora longissima, isolated from root as endophytic freshwater hyphomycetes, was evaluated for its bioactivity (antibacterial potential) against five bacterial strains, namely, Gram-positive (Bacillus subtilisMTCC 121) and Gram-negative (Agrobacterium tumefaciensMTCC 609,Escherichia coliMTCC 40,Erwinia chrysanthemum,andXanthomonas pseudomonas). Antimicrobial activity was assessed by measuring the zone of inhibition with preliminary and secondary antimicrobial assays. The applied fungus was found significant for all tested bacterial strains as showen by their zone of inhibition. In preliminary antimicrobial assay, maximum zone of inhibition was recorded against Gram-negative human pathogenic bacterial strainEscherichia coli(23 mm) followed byErwinia chrysanthemi(22 mm),Agrobacterium tumefaciens(21 mm), andXanthomonas phaseoli(21 mm), while minimum zone of inhibition was observed againstBacillus subtilis(20 mm). In secondary antimicrobial assay, the maximum zone of inhibition was recorded againstErwinia chrysanthemi(11 mm) followed byAgrobacterium tumefaciens(10 mm),Xanthomonas phaseoli(10 mm), andBacillus subtilis(9 mm) and minimum inhibition was found againstEscherichia coli(8 mm).

Surface and internal populations of Xanthomonas campestris pv. phaseoli, causal agent of common bacterial blight of bean, on and in flower buds, blossoms and pods of seven bean (Phaseolus vulgaris) genotypes were studied. Bean plants were grown in the field and artificially inoculated at the seedling stage (18 days old). The pathogen was recovered in high numbers from flower buds, blossoms, pods and seed of both resistant and susceptible bean genotypes. Significant differences (P = 0.05) in population levels of X. c. pv. phaseoli between stages of reproductive tissue development were observed. Infected seed from resistant bean genotypes had no visible symptoms. Such seed may play an important role in the epidemiology of common bacterial blight because they are difficult to detect and may occur at low frequency in seed lots, as was the case in the current study.

The antibacterial activity of methanol, ethanol, chloroform, and hexane extracts of the leaves of Himalayan gymnospermous plantGinkgo bilobaL. was assessed against five animal and plant pathogenic strains (Agrobacterium tumefaciens, Bacillus subtilis, Escherichia coli, Erwinia chrysanthemi, and Xanthomonas phaseoli) employing disc-diffusion and broth-dilution assays. The methanol extract showed the highest activity (zone of inhibition of 15–21 mm) followed by ethanol (14–19 mm), chloroform (15–20 mm), and hexane (14–19 mm) extracts at 250 μg/mL. A minimum inhibitory concentration (MIC) of 7.8 μg/mL was found for the methanol extract against most of the pathogens tested.

Various organic and aqueous extracts of aerial part of Boenninghausenia albiflora (Rutaceae) obtained by infusion and maceration were screened for their antimicrobial activity against eight animal and plant pathogenic bacteria (Bacillus subtilis, Staphylococcus aureus, Erwinia chrysanthemi, Escherichia coli, Proteus vulgaris, Xanthomonas phaseoli, Agrobacterium tumefaciens and Xanthomonas campestris) using disc diffusion method. Out of total 32 tests performed, 27 tests showed positive antibacterial activity at 1000 mg/ml concentration, whereas 15 instances exhibited zone of inhibition ³ 10 mm at same concentration. The activity shown by some of the extracts was found higher than ampicillin (10 mcg) and erythromycin (15 mcg), standard antibiotic used.

The proposals to reinstate Xanthomonas citri (ex Hasse 1915) Gabriel et al. 1989 and X. phaseoli (Smith 1897) Gabriel et al. 1989 are examined in terms of conventional criteria for describing new species. We suggest that the descriptions presented are insufficient in terms of modern practice for the purposes of formal classification in the genus Xanthomonas. To create guidelines for future reinstatements, the Judicial Commission of the International Committee on Systematic Bacteriology is requested to arrange for the preparation and promulgation of minimal standards for Xanthomonas. The pathovars proposed, X. campestris pv. aurantifolii Gabriel et al. 1989 and X. campestris pv. citrumelo Gabriel et al. 1989, are considered to be defective in terms of the International Society for Plant Pathology's standards for naming pathovars.

Systematic sampling of waterways and irrigation runoff from agricultural lands in the North Platte Project of Nebraska in July and August of 1972–1974 demonstrated that phytopathogenic organisms were disseminated. The organisms monitored included the bean common blight bacterium Xanthomonas phaseoli, the bean white mold fungus Whetzelinia sclerotiorum and various nematodes. Although many types of nematodes often were recovered from irrigation water, Heterodera sp. cysts which cause significant disease problems in the valley were found infrequently. Patterns of movement of the bacterial and fungal organisms were correlated with previous or current season infection of bean plants. The short-term survival of X. phaseoli in sterile deionized water may explain the detection of this organism only in runoff or ditches receiving runoff from common blight infected bean fields. Sclerotial bodies of W. sclerotiorum remained viable for at least 10–21 days in flowing water and were found throughout the irrigation waterways. Irrigation of beans with contaminated water can result in both common blight and white mold diseases. Dissemination of phytopathogenic organisms in irrigation reuse systems as well as agricultural land runoff should be considered in irrigation planning and system design.

Four interspecific backcross populations derived from hybridizations between Phaseolus vulgaris × (P. vulgaris × P. coccineus) were produced to transfer common blight (Xanthomonas campestris pv phaseoli) resistance from a blight–resistant accession of P. coccineus to P. vulgaris white bean cultivars and to determine the association between blight resistance and some agronomic characteristics. Frequency distribution of leaf blight resistance ratings in the BCF2 and BCF3 generations and of percent pod blight infection in the BCF3 generation appeared to be continuous, ranging between the respective parental resistance reactions. This indicated that the transfer of common blight resistance from P. coccineus to P. vulgaris was feasible. Scores for leaf and pod blight were not associated indicating differential blight resistance reactions of the two plant organs. Leaf blight was negatively correlated with days to blossom but was not correlated with growth habit and pod set except for one cross in which leaf blight and pod set were associated. Percent pod blight was negatively correlated with days to blossom in two crosses. Pod blight was not associated with growth habit and pod set except for one cross in which it was associated with pod set. Days to blossom and pod set were associated. Implications of these findings in the breeding of blight resistant white bean cultivars are discussed.Key words: Interspecific crosses, Phaseolus coccineus, Phaseolus vulgaris, Xanthomonas campestris pv phaseoli, common blight, correlation

In this review, genetic variation has been reported in bacterial pathogens of Phaseolus vulgaris L., Xanthomonas phaseoli, Pseudomonas phaseolicola, corynebacterium flaccumfaciens, and Pseudomonas syringae based on degrees of virulence and cultural characteristics. P. phaseolicola, which was reported as consisting of 2 races, was found to comprise numerous strains. A correlation between appearance on special culture medium and virulence was reported for strains of X. phaseoli. Occurrence of the bacterial pathogens complicates control through breeding and maintaining resistant cultivars.

The inheritance of the reaction to Xanthomonas phaseoli (E. F. Smith) Dowson Nebraska isolate Xp-816, cause of common blight disease of beans Phaseolus vulgaris L. was studied in crosses between the late flowering, indeterminate, blight tolerant dry bean PI 207262 (Colombia) and susceptible cvs. GN 1140, an early flowering and indeterminate dry bean; Dark Red Kidney, a late flowering and determinate dry bean; and Gallatin 50, an early and determinate green bean. The tolerant disease reaction was dominant in the F1. A continuous distribution of disease reaction ratings, skewed in the direction of dominance, occurred in the F2 derived from the first 2 crosses while a slight bimodal distribution was observed in the F2 of the last cross. A low narrow sense heritability estimate of 14% was calculated by the regression of F3 progeny means on individual F2 plants, in the cross GN 1140×PI 207262. The occurrence of a small number of nonsegregating families in a low number of F3 families indicates that a small number of major genes were involved in controlling the disease reaction. Linkage did not appear to be involved between genes controlling early flowering (early maturity) and common blight tolerance. Coupling linkage occurred between genes controlling determinate plant habit and early flowering. A crossover value of 8.4% was estimated. Recombinants for early maturity, determinate habit, and blight tolerance were obtained. Transgressive segregation for early flowering and common blight susceptibility occurred in progeny derived from the cross of the two late-flowering blight tolerant lines, PI 207262 and GN Nebraska 1, sel. 27, indicating that the parents possessed different genes controlling these traits.

A breeding and genetic program to develop Phaseolus vulgaris bean varieties tolerant to the bacterial pathogens Pseudomonas phaseolicola, Xanthomonas phaseoli and Corynebacterium flaccumfaciens was conducted from 1962 to the present tme. The research progress is reported. Great Northern (GN) Nebraska #1, sel. 27 and PI 150414 dry bean lines were highly tolerant to races 1 and 2 of the halo blight bacterium (P. phaseolicola). The tolerant inoculated leaf, pod, and non-systemic chlorosis reactions to this bacterium were each controlled by different major genes. Coupling linkage was detected between genes controlling the leaf and systemic chlorosis reactions. GN Nebraska #1, sel. 27 and PI 207262 were tolerant to isolates (USA) of X. phaseoli, causal pathogen of common blight disease. Reaction to this bacterium was inherited quantitatively. Narrow sense heritability estimates of the disease reaction were low. Genes controlling late maturity and tolerant reaction were found to be linked in crosses with GN Nebraska #1, sel. 27 but linkage was not apparent in one cross with PI 207262. A different reaction of pod and foliage was observed in some bean lines. Susceptibility increased with the onset of plant maturity. PI 165078 was tolerant to C. flaccumfaciens and the disease reaction was simply inherited. The dry bean varieties ‘GN Tara’ and ‘GN Jules’, tolerant X. phaseoli, and ‘GN Emerson’ tolerant to C. flaccumfaciens and X. phaseoli were released in recent years. Breeding approaches to develop bean varieties tolerant to these bacterial pathogens is discussed.

Yield of white beans increased markedly (250, 1,090 and 1,545 kg/ha) when grown in field plots at soil temperatures of 11.6 (T1), 19.7 (T2), and 29.7 C (T3) (20-cm depth 0830 h). This represents a change in yield of 71.1 kg/ha per 1 C change in the seasonal temperature (T2) of the soil from 23 June to 21 August. The number of degree-day units for the soil (> 5 C) corresponding to these temperatures (20-cm depth) were 268, 602, and 1,004. A decrease in the Soil Moisture Index (SMI) from 0.69 to 0.10 and an increase from 0.69 to 1.00 lowered yield by 35% and 6%, respectively (T2), for the seasonal temperature. Loss in yield from plants infected with bacterial blight (Xanthomonas phaseoli E.F. Sm. Dows.) by spraying the organism onto leaves of 21-day-old seedlings was negligible. However, loss in yield from plants infected by injecting the organism into stems was 9.0, 43 and 52% for plants grown at cool (T1), seasonal (T2), and warm (T3) soil temperatures, respectively. Percent of pods infected by spraying was increased, whereas percent of pods infected by injection was decreased at the higher soil temperatures. In general under cool seasonal temperatures, protein content of the infected and noninfected beans decreased with higher moisture that coincided with only a small change in yield. The protein content, however, remained fairly constant under large yield increases resulting from higher soil tempratures. The data are interpreted in relation to spread of the disease in fields from seedborne infection and subsequent spread of the disease to adjacent healthy plants.

One strain each of Xanthomonas campestris and 'Xanthomonas phaseoli', three strains of 'Xanthomonas oryzae', and five strains each of Escherichia coli and Klebsiella pneumoniae were found to produce a mixture of (1----2)-beta-D-gluco-oligosaccharides and/or low-Mr (1----2)-beta-D-glucans in their culture media. The saccharides from the strains of Xanthomonas were all composed of unbranched, linear (1----2)-beta-D-glucosaccharides with degrees of polymerization (DPs) of 8 to about 20, and a cyclic (1----2)-beta-D-glucan (DP16) containing one (1----6)-linkage and one alpha-linkage. The saccharides produced by the five strains of E. coli and four strains of K. pneumoniae were glucans with branches at O-6, and DPs of 10 to 15, whereas one strain of K. pneumoniae produced unbranched linear (1----2)-beta-D-glucosaccharides with DPs of 6 to about 20.

The brown melanoid pigment of Xanthomonas phaseoli var. fuscans is a reliable cultural character to distinguish this pathogen, the cause of fuscous blight of beans (Phaseolus vulgaris), from X. phaseoli, the causal organism of common bean blight. The pigment appears to be exogenous because it developed mainly in nutrient agar and only slightly in water agar held side by side in the same petri plate inoculated by a single streak of X. phaseoli var. fuscans. Both pathogens produced similar tyrosinase-type diffusible enzymes in nutrient agar plates as detected by DL-dopa (3,4-dihydroxy-DL-phenylalanine) reaction. Presently unknown specific factors, besides the detectable enzymes, may be involved in pigment production.High glucose concentration in culture media caused inhibition of pigment development. One of the reasons for this inhibition seemed to be the repressive effect of glucose on the synthesis or secretion of tyrosinase-type enzymes as well as on the autooxidation of substrates, like dopa. Therefore, the amount of glucose in culture media should be carefully regulated when the characteristic brown pigment of X. phaseoli var. fuscans is used to distinguish this pathogen from X. phaseoli.

Three-week-old kidney bean Phaseolus vulgaris L. plants were exposed to either constant or continuously varying concentrations of SO 2 at times before or after the application of Xanthomonas phaseoli (E. F. Smith) Dows. to leaf surfaces. The growth rate of the bacterium during the 10-day period after application to the leaf surface was not affected by exposures of up to 520 μg SO 2 m −3 (approximately 0·20 ppm at STP) for 6 h on each of 10 days or four 3-h exposures to peak concentrations of 2600 μg SO 2 m −3 (approximately 1·0 ppm at STP). Twenty days after application of the bacterium there was some indication that reduced populations were present on leaves of plants exposed to SO 2 . It does not appear, however, that SO 2 in these concentrations or durations significantly affects the populations of this bacterium during the epidemiologically important resident phase.