Xanthomonas arboricola pv. pruni, causal agent of bacterial spot of stone fruits and almond: its genomic and phenotypic characteristics in the X. arboricola species context

Specialized metabolites present in Camellia reticulata nectar inhibit the growth of nectar-inhabiting microorganisms

Plant specialized metabolites are species-specific compounds that help plants adapt and survive in constantly changing ecological environments. Nectar contains various specialized metabolites, essential for maintaining nectar homeostasis. In this study, we employed high-performance liquid chromatography (HPLC) to compare the sugar composition between spoilage nectar and natural nectar, with further analysis of variations in color, odor, pH, and hydrogen peroxide (H₂O₂) content. Microbial strains in Camellia reticulata nectar were isolated and identified using the spread plate method coupled with DNA sequencing. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was implemented to characterize metabolite differences between spoilage and natural nectars. Subsequent in vitro experiments were conducted to validate the effects of screened nectar metabolites on the isolated microbial strains. The results showed that some C. reticulata nectar could spoil and deteriorate, which disrupted nectar homeostasis and significantly reduced the pollination efficiency by pollinators. Spoilage nectar had significant differences in color, odor, sugar composition, pH, and H2O2 content compared to natural nectar. The number of microbial species and quantity in spoilage nectar were much higher. The H2O2 content in natural nectar could reach (55.5 ± 1.80) μM, while it was undetectable in spoilage nectar. A total of 15 distinct microbial strains and 364 differential metabolites were isolated and identified from two types of nectar. In vitro experiments demonstrated that H2O2 could inhibit all the bacteria in C. reticulata nectar except Serratia liquefaciens. 12-Methyltetradecanoic Acid inhibited Bacillus subtilis, Curtobacterium flaccumfaciens, and Rothia terrae, and Myristic Acid only inhibited Rothia terrae. The nectar metabolites screened in this study had no effect on the nectar specialist yeast Metschnikowia reukaufii. In conclusion, the findings of this study revealed that C. reticulata nectar regulates the growth of microorganisms through its metabolites to maintain nectar homeostasis and prevent spoilage. This study improves the understanding of the physiological mechanisms of C. reticulata in maintaining nectar homeostasis and provides theoretical support for controlling nectar diseases and sustaining the reproductive fitness of C. reticulata. Future research could focus on further exploring the complex interactions between different metabolites in C. reticulata nectar and a wider range of microorganisms. Moreover, the development of practical applications based on these findings, such as the development of natural preservatives for nectar-related products or the optimization of pollination efficiency in C. reticulata cultivation, could be an important area for future exploration.

Effects of Temperature on Resistance to Streptomycin in Xanthomonas arboricola pv. pruni

Xanthomonas arboricola pv. pruni (Xap) causes the shot hole disease of stone fruits and almonds. This bacterium is a damaging, widespread pathogen distributed across the major stone fruit producing regions of the world. To control shot hole disease, antibiotics such as streptomycin are mainly used. However, as concerns about antibiotic resistance increase, many restrictions are placed on the use of antibiotics. Additionally, it has been reported that the rise in temperature due to climate change affects disease occurrence and ecology. Therefore, in this study, we determined the minimum inhibitory concentration (MIC) of streptomycin for Xap at an optimal growth temperature of 28°C and investigated the changes in MIC and the occurrence frequency of resistant bacteria at 10°C, 25°C and 30°C. The results of this study showed that the MIC was 30 µg/ml at 28°C. In addition, when the change in streptomycin resistance concentration due to temperature was confirmed, we found that the resistance concentration decreased to 10 µg/ml at 30°C. When the occurrence of resistance according to concentration and temperature conditions was investigated, the occurrence frequency of resistant strains was found to be the highest at 50 µg/ml. In the case of temperature, the occurrence frequency of resistant strains was confirmed to be high at 30°C. These results provide basic data for further reducing the problem of antibiotic resistance by suggesting the possibility of changes in the occurrence of streptomycin-resistant strains depending on the antibiotic treatment environment.

Draft genome sequence of Xanthomonas arboricola pv. pruni PVCT 262.1 isolated from Prunus dulcis in italy

Here, we report the draft genome sequence of Xanthomonas arboricola pv. pruni strain PVCT 262.1, isolated from almond (Prunus dulcis) leaves affected by bacterial spots in Italy in 2020. Genome size is 5,076,418 bp and G+C content is 65.44%. A total of 4,096 protein-coding genes and 92 RNAs are predicted.

Development of a Long-Amplicon Propidium Monoazide-Quantitative PCR Assay for Detection of Viable Xanthomonas arboricola pv. pruni Cells in Peach Trees

Bacterial spot is one of the most serious diseases of peach caused by the pathogen Xanthomonas arboricola pv. pruni (XAP), leading to early defoliation and unmarketable fruit. The pathogen can overwinter in peach twigs and form spring cankers, which are considered the primary inoculum source for early season leaf and fruitlet infection. The amount of overwintering bacterial inoculum plays a critical role for the bacterial spot development, but no reliable quantification method is available. Thus, we developed a long-amplicon propidium monoazide (PMA)-quantitative PCR (qPCR) assay for specific detection of viable XAP cells. The optimized PMA-qPCR assay used 20 μM of PMAxx for pure bacterial suspensions and 100 μM for peach twig tissues. The Qiagen Plant Pro Kit with an additional lysozyme digestion step was the DNA extraction protocol that yielded the best detection sensitivity with the bacteria-spiked peach twig extracts. The PMA-qPCR assay was tested with different mixtures of viable and heat-killed XAP cells in pure bacterial suspensions and bacteria-spiked peach twig tissues. The results showed that this assay enabled sensitive, specific, and accurate quantification of viable XAP cells as low as 103 CFU/ml with the presence of up to 107 CFU/ml of dead XAP cells, while suppressing the amplification of DNA from dead cells. For mixtures of viable and dead cells, the PMA-qPCR results were linearly correlated with the predicted concentrations of viable XAP (R2 > 0.98). Thus, the PMA-qPCR assay will be a suitable tool for quantifying overwintering XAP population on peach trees.

Antibacterial effect of essential oils and their components against Xanthomonas arboricola pv. pruni revealed by microdilution and direct bioautographic assays

Bacterial spot of stone fruits caused by Xanthomonas arboricola pv. pruni (Xap) is one of the most significant diseases of several Prunus species. Disease outbreaks can result in severe economic losses while the control options are limited. Antibacterial efficacy of essential oils (EOs) of thyme, cinnamon, clove, rosemary, tea tree, eucalyptus, lemon grass, citronella grass, and lemon balm was assessed against two Hungarian Xap isolates. The minimal inhibitory concentration (MIC) was determined by broth microdilution assay and for the identification of active EOs' components a newly introduced high-performance thin-layer chromatography (HPTLC)-Xap (direct bioautography) method combined with solid-phase microextraction-gas chromatography/mass spectrometry (SPME-GC/MS) was applied. All EOs inhibited both bacterium isolates, but cinnamon proved to be the most effective EO with MIC values of 31.25 µg/mL and 62.5 µg/mL, respectively. Compounds in the antibacterial HPTLC zones were identified as thymol in thyme, trans-cinnamaldehyde in cinnamon, eugenol in clove, borneol in rosemary, terpinen-4-ol in tea tree, citral (neral and geranial) in lemon grass and lemon balm, and citronellal and nerol in citronella grass. Regarding active compounds, thymol had the highest efficiency with a MIC value of 50 µg/mL. Antibacterial effects of EOs have already been proven for several Xanthomonas species, but to our knowledge, the studied EOs, except for lemon grass and eucalyptus, were tested for the first time against Xap. Furthermore, in case of Xap, this is the first report demonstrating that direct bioautography is a fast and suitable method for screening anti-Xap components of complex matrices, like EOs.

Comparative Genomic Analysis and Rapid Molecular Detection of Xanthomonas euvesicatoria Using Unique ATP-Dependent DNA Helicase recQ, hrpB1, and hrpB2 Genes Isolated from Physalis pubescens in China

Ground cherry (Physalis pubescens) is the most prominent species in the Solanaceae family due to its nutritional content, and prospective health advantages. It is grown all over the world, but notably in northern China. In 2019 firstly bacterial leaf spot (BLS) disease was identified on P. pubescens in China that caused by both BLS pathogens Xanthomonas euvesicatoria pv. euvesicatoria resulted in substantial monetary losses. Here, we compared whole genome sequences of X. euvesicatoria to other Xanthomonas species that caused BLS diseases for high similarities and dissimilarities in genomic sequences through average nucleotide identity (ANI) and BLAST comparison. Molecular techniques and phylogenetic trees were adopted to detect X. euvesicatoria on P. pubescens using recQ, hrpB1, and hrpB2 genes for efficient and precise identification. For rapid molecular detection of X. euvesicatoria, loop-mediated isothermal amplification, polymerase chain reaction (PCR), and real-time PCR techniques were used. Whole genome comparison results showed that the genome of X. euvesicatoria was more closely relative to X. perforans than X. vesicatoria, and X. gardneri with 98%, 84%, and 86% ANI, respectively. All infected leaves of P. pubescens found positive amplification, and negative controls did not show amplification. The findings of evolutionary history revealed that isolated strains XeC10RQ, XeH9RQ, XeA10RQ, and XeB10RQ that originated from China were closely relative and highly homologous to the X. euvesicatoria. This research provides information to researchers on genomic variation in BLS pathogens, and further molecular evolution and identification of X. euvesicatoria using the unique target recQ gene through advance molecular approaches.

Complete Genome Sequence of Xanthomonas arboricola pv. pruni Strain Xcp1 Isolated in 1984 from a Bacterial Spot Spring Canker on Prunus persica var. nucipersica cv. "Redgold"

Xanthomonas arboricola pv. pruni is an important plant pathogen and the causal agent of bacterial spot of stone fruits (Prunus spp). Here, we report a complete genome of X. arboricola pv. pruni strain Xcp1 generated from hybrid PacBio Sequel and Illumina NextSeq2000 sequencing.

High-Throughput Sequencing as a Tool for the Quality Control of Microbial Bioformulations for Agriculture

Microbial bioformulations, due to their positive impact on the growth and development of plants, as well as the absence of harmful effects on the environment and humans, have a vast potential for mass introduction into agriculture. Assessing the quality of bioformulations, especially complex ones, is a difficult task. In this study, we show that high-throughput sequencing can be an effective tool for the quality control and safety of microbial bioformulations. By the method of high-throughput sequencing on the MiSeq platform, we studied 20 samples of commercially available microbial bioformulations. In parallel with this, bioformulations were studied by classical microbiological methods. The analysis showed the presence of extraneous undeclared bacterial genera by the manufacturer. Only 10% of the bioformulations fully corresponded to the commercial composition, and another 10% of the bioformulations did not contain the bacteria declared by the manufacturer in their composition at all. The bacterial composition of 80% of the bioformulations partially corresponded to the composition indicated on the package. The most frequent microbial bioformulations contaminants were Enterococcus, Lactobacillaceae, Klebsiella, Escherichia-Shigella and Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium. Universal methods for the quality control of bioformulations are needed. The advantages of high-throughput sequencing for the evaluation of bioformulations are considered in this work.

Taxonomic Refinement of Xanthomonas arboricola

Xanthomonas arboricola comprises a number of economically important fruit tree pathogens classified within different pathovars. Dozens of nonpathogenic and taxonomically unvalidated strains are also designated as X. arboricola, leading to a complicated taxonomic status in the species. In this study, we have evaluated the whole-genome resources of all available Xanthomonas spp. strains designated as X. arboricola in the public databases to refine the members of the species based on DNA similarity indexes and core genome-based phylogeny. Our results show that, of the nine validly described pathovars within X. arboricola, pathotype strains of seven pathovars are taxonomically genuine, belonging to the core clade of the species regardless of their pathogenicity on the host of isolation (thus the validity of pathovar status). However, strains of X. arboricola pv. guizotiae and X. arboricola pv. populi do not belong to X. arboricola because of the low DNA similarities between the type strain of the species and the pathotype strains of these two pathovars. Thus, we propose to elevate the two pathovars to the rank of a species as X. guizotiae sp. nov. with the type strain CFBP 7408^T and X. populina sp. nov. with the type strain CFBP 3123^T. In addition, other mislabeled strains of X. arboricola were scattered within Xanthomonas spp. that belong to previously described species or represent novel species that await formal description.

Copper Tolerance in Xanthomonas arboricola pv. pruni in South Carolina Peach Orchards

Bacterial spot of peach, caused by Xanthomonas arboricola pv. pruni, causes yield loss every year in southeastern U.S. peach orchards. Management is mainly driven by season-long applications of copper-based products, site location, and choice of cultivar. Although tolerance to copper has not been reported in X. arboricola pv. pruni in the United States, adaptation of populations from frequent use is a concern. We collected X. arboricola pv. pruni from shoot cankers, leaves, and fruit of cultivar O'Henry over 2 years from three conventional farms and one organic farm in South Carolina, one orchard per farm. The four farms had been using copper extensively for years to control bacterial spot. X. arboricola pv. pruni was isolated from four canker types (bud canker, tip canker, nonconcentric canker, and concentric canker) in early spring (bud break), as well as from leaf and fruit tissues later in the season at the phenological stages of pit hardening and final swell. X. arboricola pv. pruni was most frequently isolated from cankers of the organic farm (24% of the cankers) and most isolates (45%) came from bud cankers. X. arboricola pv. pruni isolates were assessed for sensitivity to copper using minimal glucose yeast agar and nutrient agar amended with 38 μg/ml or 51 μg/ml of Cu²⁺. Two phenotypes of copper tolerance in X. arboricola pv. pruni were discovered: low copper tolerance (LCT; growth up to 38 μg/ml Cu²⁺) and high copper tolerance (HCT; growth up to 51 μg/ml Cu²⁺). A total of 26 (23 LCT and 3 HCT) out of 165 isolates in 2018 and 32 (20 LCT and 12 HCT) out of 133 isolates in 2019 were tolerant to copper. Peach leaves on potted trees were sprayed with copper rates typically applied at the stages of delayed dormancy (high rate; 2,397 μg/ml Cu²⁺), shuck split (medium rate; 599 μg/ml Cu²⁺), and during summer cover sprays (low rate; 120 μg/ml Cu²⁺), and subsequently inoculated with sensitive, LCT, and HCT strains. Results indicated that the low and medium rates of copper reduced bacterial spot incidence caused by the sensitive strain but not by the LCT and HCT strains. This study confirms existence of X. arboricola pv. pruni tolerance to copper in commercial peach orchards in the southeastern United States, and suggests its contribution to bacterial spot development under current management practices.

Genetic and Phenotypic Characterization of Xanthomonas Species Pathogenic in Wheat in Uruguay

Bacterial diseases affecting wheat production in Uruguay are an issue of growing concern yet remain largely uninvestigated in the region. Surveys of 61 wheat fields carried out from 2017 to 2019 yielded a regional collection of 63 strains identified by 16S rRNA gene analysis as Xanthomonas spp. A real-time PCR protocol with species-specific primers previously reported allowed the identification of 44 strains as X. translucens, the causal agent of bacterial leaf streak (BLS) in wheat and other cereal crops. Multilocus sequence analysis of four housekeeping genes (dnaK, fyuA, gyrB, and rpoD) revealed that these strains were most closely related to X. translucens pv. undulosa, the pathovar that is most commonly associated with BLS of wheat. Multilocus sequence typing was applied to examine the genetic diversity of X. translucens strains. Strains were assigned to four different sequence types, three of which were previously reported globally. Additionally, 17 Xanthomonas strains not belonging to X. translucens were obtained from diseased wheat leaves. Phylogenetic analysis showed that these strains are closely related to X. prunicola and clustered together with previously uncharacterized Xanthomonas strains isolated from wheat in Minnesota. In planta pathogenicity assays carried out on a BLS-susceptible wheat cultivar showed that X. translucens pv. undulosa strains caused brown necrosis symptoms typical of BLS, whereas non-translucens Xanthomonas sp. strains elicited an atypical symptom of dry necrosis. These findings suggest that local wheat fields are affected by X. translucens pv. undulosa and by a new wheat pathogen within the Xanthomonas genus.

Oxytetracycline and Streptomycin Resistance Genes in Xanthomonas arboricola pv. pruni, the Causal Agent of Bacterial Spot in Peach

Xanthomonas arboricola pv. pruni (Xap) causes bacterial spot, a major worldwide disease of Prunus species. Very few chemical management options are available for this disease and frequent applications of oxytetracycline (OTC) in the United States peach orchards have raised concerns about resistance development. During 2017-2020, 430 Xap strains were collected from ten peach orchards in South Carolina. Seven OTC-resistant (OTC R ) Xap strains were found in 2017 and 2020 from four orchards about 20-270 km apart. Interestingly, the seven strains were also resistant to streptomycin (STR). Six strains grew on media amended with ≤100 μg/mL OTC, while one strain, R1, grew on ≤250 μg/mL OTC. Genome sequence analysis of four representative OTC R strains revealed a 14-20 kb plasmid carrying tetC, tetR, and strAB in each strain. These three genes were transferable to Xanthomonas perforans via conjugation, and they were PCR confirmed in all seven OTC R Xap strains. When tetC and tetR were cloned and expressed together in a sensitive strain, the transconjugants showed resistance to ≤100 μg/mL OTC. When tetC was cloned and expressed alone in a sensitive strain, the transconjugants showed resistance to ≤250 μg/mL OTC. TetC and tetR expression was inducible by OTC in all six wild-type strains resistant to ≤100 μg/mL OTC. However, in the R1 strain resistant to ≤250 μg/mL OTC, tetR was not expressed, possibly due to the presence of Tn3 in the tetR gene, and in this case tetC was constitutively expressed. These data suggest that tetC confers OTC resistance in Xap strains, and tetR regulates the level of OTC resistance conferred by tetC. To our knowledge, this is the first report of OTC resistance in plant pathogenic xanthomonads.

Xanthomonas arboricola pv. juglandis and pv. corylina: Brothers or distant relatives? Genetic clues, epidemiology, and insights for disease management

BACKGROUND

The species Xanthomonas arboricola comprises up to nine pathovars, two of which affect nut crops: pv. juglandis, the causal agent of walnut bacterial blight, brown apical necrosis, and the vertical oozing canker of Persian (English) walnut; and pv. corylina, the causal agent of the bacterial blight of hazelnut. Both pathovars share a complex population structure, represented by different clusters and several clades. Here we describe our current understanding of symptomatology, population dynamics, epidemiology, and disease control.

TAXONOMIC STATUS

Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Lysobacterales (earlier synonym of Xanthomonadales); Family Lysobacteraceae (earlier synonym of Xanthomonadaceae); Genus Xanthomonas; Species X. arboricola; Pathovars: pv. juglandis and pv. corylina.

HOST RANGE AND SYMPTOMS

The host range of each pathovar is not limited to a single species, but each infects mainly one plant species: Juglans regia (X. arboricola pv. juglandis) and Corylus avellana (X. arboricola. pv. corylina). Walnut bacterial blight is characterized by lesions on leaves and fruits, and cankers on twigs, branches, and trunks; brown apical necrosis symptoms consist of apical necrosis originating at the stigmatic end of the fruit. A peculiar symptom, the vertical oozing canker developing along the trunk, is elicited by a particular genetic lineage of the bacterium. Symptoms of hazelnut bacterial blight are visible on leaves and fruits as necrotic lesions, and on woody parts as cankers. A remarkable difference is that affected walnuts drop abundantly, whereas hazelnuts with symptoms do not.

DISTRIBUTION

Bacterial blight of walnut has a worldwide distribution, wherever Persian (English) walnut is cultivated; the bacterial blight of hazelnut has a more limited distribution, although disease outbreaks are currently more frequently reported. X. arboricola pv. juglandis is regulated almost nowhere, whereas X. arboricola pv. corylina is regulated in most European and Mediterranean Plant Protection Organization (EPPO) countries.

EPIDEMIOLOGY AND CONTROL

For both pathogens infected nursery material is the main pathway for their introduction and spread into newly cultivated areas; additionally, infected nursery material is the source of primary inoculum. X. arboricola pv. juglandis is also disseminated through pollen. Disease control is achieved through the phytosanitary certification of nursery material (hazelnut), although approved certification schemes are not currently available. Once the disease is present in walnut/hazelnut groves, copper compounds are widely used, mostly in association with dithiocarbamates; where allowed, antibiotics (preferably kasugamycin) are sprayed. The emergence of strains highly resistant to copper currently represents the major threat for effective management of the bacterial blight of walnut. USEFUL WEBSITES: https://gd.eppo.int/taxon/XANTJU, https://gd.eppo.int/taxon/XANTCY, https://www.euroxanth.eu, http://www.xanthomonas.org.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Identification and Characterization of Xanthomonas arboricola pv. juglandis Causing Bacterial Blight of Walnuts in Korea

The present study describes the bacterial blight of walnut, caused by Xanthomonas arboricola pv. juglandis (Xaj) in the northern Gyeongbuk province, Korea. Disease symptoms that appear very similar to anthracnose symptoms were observed in walnut trees in June 2016. Pathogens were isolated from disease infected leaves, fruits, shoots, bud, flower bud of walnut, and cultured onto yeast dextrose carbonate agar plates. Isolated bacteria with bacterial blight symptoms were characterized for their nutrient utilization profiles using Biolog GN2 and Vitek 2. In addition, isolates were subjected to physiological, biochemical, and morphological characterizations. Furthermore, isolates were identified using 16S rDNA sequence analysis, and multi-locus sequence analysis using atpD, dnaK, efp, and rpoD. To confirm pathogenicity, leaves, fruits, and stems of 3-year-old walnut plants were inoculated with bacterial pathogen suspensions as a foliar spray. One week after inoculation, the gray spots on leaves and yellow halos around the spots were developed. Fruits and stems showed browning symptoms. The pathogen Xaj was re-isolated from all symptomatic tissues to fulfill Koch's postulates, while symptoms were not appeared on control plants. On the other hand, the symptoms were very similar to the symptoms of anthracnose caused by Colletotrichum gloeosporioides. When walnut plants were inoculated with combined pathogens of Xaj and C. gloeosporioides, disease symptoms were greater in comparison with when inoculated alone. Xaj population size was more in the month of April than March due to their dormancy in March, and sensitive to antibiotics such as oxytetracycline and streptomycin, while resistant to copper sulfate.

Comparative Genomics of Xanthomonas euroxanthea and Xanthomonas arboricola pv. juglandis Strains Isolated from a Single Walnut Host Tree

The recent report of distinct Xanthomonas lineages of Xanthomonas arboricola pv. juglandis and Xanthomonas euroxanthea within the same walnut tree revealed that this consortium of walnut-associated Xanthomonas includes both pathogenic and nonpathogenic strains. As the implications of this co-colonization are still poorly understood, in order to unveil niche-specific adaptations, the genomes of three X. euroxanthea strains (CPBF 367, CPBF 424^T, and CPBF 426) and of an X. arboricola pv. juglandis strain (CPBF 427) isolated from a single walnut tree in Loures (Portugal) were sequenced with two different technologies, Illumina and Nanopore, to provide consistent single scaffold chromosomal sequences. General genomic features showed that CPBF 427 has a genome similar to other X. arboricola pv. juglandis strains, regarding its size, number, and content of CDSs, while X. euroxanthea strains show a reduction regarding these features comparatively to X. arboricola pv. juglandis strains. Whole genome comparisons revealed remarkable genomic differences between X. arboricola pv. juglandis and X. euroxanthea strains, which translates into different pathogenicity and virulence features, namely regarding type 3 secretion system and its effectors and other secretory systems, chemotaxis-related proteins, and extracellular enzymes. Altogether, the distinct genomic repertoire of X. euroxanthea may be particularly useful to address pathogenicity emergence and evolution in walnut-associated Xanthomonas.

Xanthomonas euroxanthea sp. nov., a new xanthomonad species including pathogenic and non-pathogenic strains of walnut

We describe a novel species isolated from walnut (Juglans regia) which comprises non-pathogenic and pathogenic strains on walnut. The isolates, obtained from a single ornamental walnut tree showing disease symptoms, grew on yeast extract-dextrose-carbonate agar as mucoid yellow colonies characteristic of Xanthomonas species. Pathogenicity assays showed that while strain CPBF 424T causes disease in walnut, strain CPBF 367 was non-pathogenic on walnut leaves. Biolog GEN III metabolic profiles disclosed some differences between strains CPBF 367 and CPBF 424T and other xanthomonads. Multilocus sequence analysis with seven housekeeping genes (fyuA, gyrB, rpoD, atpD, dnaK, efp, glnA) grouped these strains in a distinct cluster from Xanthomonas arboricola pv. juglandis and closer to Xanthomonas prunicola and Xanthomonas arboricola pv. populi. Average nucleotide identity (ANI) analysis results displayed similarity values below 93 % to X. arboricola strains. Meanwhile ANI and digital DNA-DNA hybridization similarity values were below 89 and 50 % to non-arboricola Xanthomonas strains, respectively, revealing that they do not belong to any previously described Xanthomonas species. Furthermore, the two strains show over 98 % similarity to each other. Genomic analysis shows that strain CPBF 424T harbours a complete type III secretion system and several type III effector proteins, in contrast with strain CPBF 367, shown to be non-pathogenic in plant bioassays. Taking these data altogether, we propose that strains CPBF 367 and CPBF 424T belong to a new species herein named Xanthomonas euroxanthea sp. nov., with CPBF 424T (=LMG 31037T=CCOS 1891T=NCPPB 4675T) as the type strain.

Wheat Straw Return Influences Nitrogen-Cycling and Pathogen Associated Soil Microbiota in a Wheat-Soybean Rotation System

Returning straw to soil is an effective way to sustain or improve soil quality and crop yields. However, a robust understanding of the impact of straw return on the composition of the soil microbial communities under field conditions has remained elusive. In this study, we characterized the effects of wheat straw return on soil bacterial and fungal communities in a wheat–soybean rotation system over a 3-year period, using Illumina-based 16S rRNA, and internal transcribed region (ITS) amplicon sequencing. Wheat straw return significantly affected the α-diversity of the soil bacterial, but not fungal, community. It enhanced the relative abundance of the bacterial phylum Proteobacteria and the fungal phylum Zygomycota, but reduced that of the bacterial phylum Acidobacteria, and the fungal phylum Ascomycota. Notably, it enriched the relative abundance of nitrogen-cycling bacterial genera such as Bradyrhizobium and Rhizobium. Preliminary analysis of soil chemical properties indicated that straw return soils had significantly higher total nitrogen (TN) contents than no straw return soils. In addition, the relative abundance of fungal genera containing pathogens was significantly lower in straw return soils relative to control soils, such as Fusarium, Alternaria, and Myrothecium. These results suggested a selection effect from the 3-year continuous straw return treatment and the soil bacterial and fungal communities were moderately changed.

Xanthomonas citri subsp. citri and Xanthomonas arboricola pv. pruni: Comparative analysis of two pathogens producing similar symptoms in different host plants

Comparative studies in Xanthomonas have provided a vast amount of data that enabled to deepen in the knowledge of those factors associated with virulence and Xanthomonas plant interaction. The species of this genus present a wide range of host plants and a large number of studies have been focused to elucidate which mechanism are involved in this characteristic. In this study, comparative genomic and phenotypic analysis were performed between X. citri subsp. citri (Xcc), one of the most studied pathogens within Xanthomonas, and X. arboricola pv. pruni (Xap), a pathogen which has aroused great interest in recent time. The work was aimed to find those elements that contribute to their host divergence despite the convergence in the symptoms that each species cause on Citrus spp. and Prunus spp., respectively. This study reveals a set of genes that could be putatively associated with the adaptation of these pathogens to their hosts, being the most remarkable those involved in environmental sensing systems such as the case of the TonB-dependent transporters, the sensors of the two-component system and the methyl accepting chemotaxis proteins. Other important variants were found in processes related to the decomposition of the cell wall as could be appreciated by their dissimilar set of cell-wall degrading enzymes. Type three effectors, as one of the most important factors in delineating the host specificity in Xanthomonas, also showed a different array when comparing both species, being some of them unique to each pathogen. On the other hand, only small variations could be connected to other features such as the motility appendages and surface adhesion proteins, but these differences were accompanied by a dissimilar capacity to attach on host and non-host leaf surface. The molecular factors found in this work provide the basis to perform a more in-depth functional analyses that unveil those actual factors associated with pathogenesis and host specificity in Xcc and Xap.