Xanthomonas translucens pv. undulosa Identified on Common Weedy Grasses in Naturally Infected Wheat Fields in Minnesota

Detection and Differentiation of Xanthomonas translucens Pathovars translucens and undulosa from Wheat and Barley by Duplex Quantitative PCR

Two probe-based quantitative PCR (qPCR) systems, namely P-Xtt and P-Xtu, were developed to diagnose cereal bacterial leaf streak pathogens Xanthomonas translucens pv. translucens and pv. undulosa, respectively. P-Xtt is specific to pv. translucens, and P-Xtu is specific to pv. undulosa, pv. cerealis, pv. secalis, and pv. pistaciae. P-Xtt and P-Xtu worked on all accessible strains of pv. translucens and pv. undulosa, respectively. Both systems could detect 100 copies of the target gBlock DNA. The two systems could be used in both singleplex qPCR and duplex qPCR with similar efficiencies. On genomic DNA from strains of various X. translucens pathovars, both singleplex and duplex qPCR could specifically detect and differentiate pv. translucens and pv. undulosa. The duplex qPCR could detect pv. translucens and pv. undulosa from genomic DNA of 1,000 bacterial cells. On infected barley and wheat grain samples and on one infected wheat leaf sample, the duplex qPCR showed similar efficiency compared to a previously published qPCR system but with the additional capability of pathovar differentiation. The duplex qPCR system developed in this study will be useful in studies on bacterial leaf streak and detection/differentiation of the pathogens.

Comparative Genomics of Xanthomonas translucens pv. undulosa Strains Isolated from Weedy Grasses and Cultivated Wild Rice

Bacterial leaf streak (BLS) of wheat (Triticum aestivum), caused by Xanthomonas translucens pv. undulosa, is a disease of major concern in the Northern Great Plains. The host range for X. translucens pv. undulosa is relatively broad, including several small grains and perennial grasses. In Minnesota, X. translucens pv. undulosa was isolated from weedy grasses in and around wheat fields that exhibited BLS symptoms and from cultivated wild rice (Zizania palustris) with symptomatic leaf tissue. Currently, no genomic resources are available for X. translucens pv. undulosa strains isolated from non-wheat hosts. In this study, we sequenced and assembled the complete genomes of five strains isolated from weedy grass hosts, foxtail barley (Hordeum jubatum), green foxtail (Setaria viridis), and wild oat (Avena fatua), and from cultivated wild rice and wheat. These five genomes were compared with the publicly available genomes of seven X. translucens pv. undulosa strains originating from wheat and one genome of an X. translucens pv. secalis strain originating from rye (Secale cereale). Global alignments of the genomes revealed little variation in genomic structures. Average nucleotide identity-based phylogeny and life identification numbers revealed that the strains share ≥99.25% identity. We noted differences in the presence of Type III secreted effectors, including transcription activator-like effectors. Despite differences between strains, we did not identify unique features distinguishing strains isolated from wheat and non-wheat hosts. This study contributes to the availability of genomic data for X. translucens pv. undulosa from non-wheat hosts, thus increasing our understanding of the diversity within the pathogen population.

Epidemiology, Host Resistance, and Genomics of the Small Grain Cereals Pathogen Xanthomonas translucens: New Advances and Future Prospects

Bacterial leaf streak (BLS) primarily affects barley and wheat and is mainly caused by the pathogens Xanthomonas translucens pv. translucens and X. translucens pv. undulosa, respectively. BLS is distributed globally and poses a risk to food security and the supply of malting barley. X. translucens pv. cerealis can infect both wheat and barley but is rarely isolated from these hosts in natural infections. These pathogens have undergone a confusing taxonomic history, and the biology has been poorly understood, making it difficult to develop effective control measures. Recent advancements in the ability and accessibility to sequence bacterial genomes have shed light on phylogenetic relationships between strains and identified genes that may play a role in virulence, such as those that encode Type III effectors. In addition, sources of resistance to BLS have been identified in barley and wheat lines, and ongoing efforts are being made to map these genes and evaluate germplasm. Although there are still gaps in BLS research, progress has been made in recent years to further understand epidemiology, diagnostics, pathogen virulence, and host resistance.

Detection and Diagnosis of Bacterial Leaf Streak on Small Grain Cereals: From Laboratory to Field

Bacterial leaf streak of small-grain cereals is an economically important disease of wheat and barley crops. The disease occurs in many countries across the globe, with particular importance in regions characterized by high precipitation or areas in which sprinkler irrigation is used. Three genetically distinct lineages of the Gram-negative bacterium Xanthomonas translucens (X. translucens pv. undulosa, X. translucens pv. translucens, and X. translucens pv. cerealis) are responsible for most of the bacterial leaf streak infections on wheat and barley crops. Considering the seedborne nature of the pathogens, they are included in the A2 (high-risk) list of quarantine organisms for some European countries; hence, they are under strict quarantine control and zero tolerance. Due to the taxonomic complexities within X. translucens, the exact geographic distribution of each pathovar has not yet been determined. In this mini review, we provide an updated overview of the detection and diagnosis of the bacterial leaf streak pathogens. First, a short history of the leaf streak pathogens is provided, followed by the symptomology and host range of the causal agents. Then, the utility of conventional methods and high-throughput molecular approaches in the precise detection and identification of the pathogens is explained. Finally, we highlight the role of quarantine inspections and early detection of the pathogen in combating the risk of bacterial leaf streak in the 21st century's small-grains cereals' industry.

Genome-Wide Association Mapping of Bacterial Leaf Streak Resistance in Two Elite Barley Breeding Panels

Bacterial leaf streak (BLS), caused chiefly by the pathogen Xanthomonas translucens pv. translucens, is becoming an increasingly important foliar disease of barley in the Upper Midwest. The deployment of resistant cultivars is the most economical and practical method of control. To identify sources of BLS resistance, we evaluated two panels of breeding lines from the University of Minnesota (UMN) and Anheuser-Busch InBev (ABI) barley improvement programs for reaction to strain CIX95 in the field at St. Paul and Crookston, MN, in 2020 and 2021. The percentage of resistant lines in the UMN and ABI panels with mid-season maturity was 1.8% (6 of 333 lines) and 5.2% (13 of 251 lines), respectively. Both panels were genotyped with the barley 50K iSelect SNP array, and then a genome-wide association study was performed. A single, highly significant association was identified for BLS resistance on chromosome 6H in the UMN panel. This association was also identified in the ABI panel. Seven other significant associations were detected in the ABI panel: two each on chromosomes 1H, 2H, and 3H and one on chromosome 5H. Of the eight associations identified in the panels, five were novel. The discovery of resistance in elite breeding lines will hasten the time needed to develop and release a BLS-resistant cultivar.

DNA Markers, Pathogenicity Test, and Multilocus Sequence Analysis to Differentiate and Characterize Cereal-Specific Xanthomonas translucens Strains

Xanthomonas translucens contains a group of bacterial pathogens that are closely related and have been divided into several pathovars based on their host range. X. translucens pv. undulosa (Xtu) and X. translucens pv. translucens (Xtt) are two important pathovars that cause bacterial leaf streak disease on wheat and barley, respectively. In this study, DNA markers were developed to differentiate Xtu and Xtt and were then used to characterize a collection of X. translucens strains with diverse origins, followed by confirmation and characterization with pathogenicity tests and multilocus sequence analysis/typing (MLSA/MLST). We first developed cleaved amplified polymorphic sequence markers based on the single-nucleotide polymorphisms within a cereal pathovar-specific DNA sequence. In addition, two Xtt-specific markers, designated Xtt-XopM and Xtt-SP1, were developed from comparative genomics among the sequenced Xtt/Xtu genomes. Using the developed markers, a collection of X. translucens strains were successfully identified as Xtu or Xtt. Pathogenicity tests on wheat and barley plants and MLSA of four housekeeping genes validated the pathovar assignation of those strains. Furthermore, MLSA revealed distinct subclades within both Xtu and Xtt groups. Seven and three sequence types were identified from MLST for Xtu and Xtt strains, respectively. The establishment of efficient Xtt/Xtu differentiation methods and characterization of those strains will be useful in studying disease epidemiology and host-pathogen interactions and breeding programs when screening for sources of resistance for these two important bacterial pathogens.

Development of Genome-Driven, Lifestyle-Informed Markers for Identification of the Cereal-Infecting Pathogens Xanthomonas translucens Pathovars undulosa and translucens

Bacterial leaf streak, bacterial blight, and black chaff caused by Xanthomonas translucens pathovars are major diseases affecting small grains. Xanthomonas translucens pv. translucens and X. translucens pv. undulosa are seedborne pathogens that cause similar symptoms on barley, but only X. translucens pv. undulosa causes bacterial leaf streak of wheat. Recent outbreaks of X. translucens have been a concern for wheat and barley growers in the Northern Great Plains; however, there are limited diagnostic tools for pathovar differentiation. We developed a multiplex PCR based on whole-genome differences to distinguish X. translucens pv. translucens and X. translucens pv. undulosa. We validated the primers across different Xanthomonas and non-Xanthomonas strains. To our knowledge, this is the first multiplex PCR to distinguish X. translucens pv. translucens and X. translucens pv. undulosa. These molecular tools will support disease management strategies enabling detection and pathovar incidence analysis of X. translucens. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Bacterial Leaf Streak Diseases of Plants: Symptom Convergence in Monocot Plants by Distant Pathogenic Xanthomonas Species

Bacterial leaf streak (BLS) is a disease of monocot plants caused by Xanthomonas translucens on small grains, X. vasicola on maize and sorghum, and X. oryzae on rice. These three pathogens cause remarkably similar symptomology in their host plants. Despite causing similar symptoms, BLS pathogens are dispersed throughout the larger Xanthomonas phylogeny. Each aforementioned species includes strain groups that do not cause BLS and instead cause vascular disease. In this commentary, we hypothesize that strains of X. translucens, X. vasicola, and X. oryzae convergently evolved to cause BLS due to shared evolutionary pressures. We examined the diversity of secreted effectors, which may be important virulence factors for BLS pathogens and their evolution. We discuss evidence that differences in gene regulation and abilities to manipulate plant hormones may also separate BLS pathogens from other Xanthomonas species or pathovars. BLS is becoming an increasing issue across the three pathosystems. Overall, we hope that a better understanding of conserved mechanisms used by BLS pathogens will enable researchers to translate findings across production systems and guide approaches to control this (re)emerging threat.

Increase of Bacterial Leaf Streak in Hard Red Spring Wheat in North Dakota and Yield Loss Considerations

Bacterial leaf streak (BLS), caused by Xanthomonas translucens pv. undulosa, has increased in both prevalence and severity in the major hard red spring wheat (HRSW)-producing state North Dakota. The disease is readily observed after flag leaf emergence and can quickly lead to defoliation and severe yield loss. The objectives of this research were to document the prevalence and incidence of BLS in North Dakota and provide estimations of yield and economic losses. Trained field scouts determined the incidence and prevalence of BLS in ND on HRSW plants between Feekes growth stage (FGS) 8 and FGS 11.2 from 2015 to 2021, and data were used to determine BLS-affected hectares. Yield data in combination with BLS ratings were obtained from HRSW performance trials to estimate the impact of BLS on yield. The combination of variety identity, hectarage data, BLS-affected hectarage estimates, and yield loss estimates was used to estimate economic losses from BLS in 2019 and 2020. Our data suggest that BLS-affected hectares ranged from 747 to 141,680 between 2015 and 2021. Yield loss was observed at multiple HRSW performance trial locations, with estimated yield losses as high as 60% on susceptible varieties. The amount of BLS-affected hectares was the highest in 2019 and 2020, and direct economic losses for North Dakota HRSW producers were estimated to be as high as $4.7 and $8.0 million, respectively. These data highlight the importance of BLS in HRSW and the need to procure resources for breeding efforts and grower education on management of BLS.

Sources of Bacterial Leaf Streak Resistance Identified in a Diverse Collection of Barley Germplasm

Bacterial leaf streak (BLS) is a sporadic yet damaging disease of cereals that is growing in importance across the Upper Midwest production region. In barley (Hordeum vulgare ssp. vulgare), this disease is caused primarily by the bacterium Xanthomonas translucens pv. translucens. Accessions resistant to BLS have been reported in past studies, but few have been rigorously validated in the field. To identify accessions carrying diverse resistance alleles to BLS, a largescale germplasm screening study was undertaken against strain CIX95 of X. translucens pv. translucens in St. Paul and Crookston, Minnesota, in 2020 and 2021. The germplasm screened was diverse and included adapted breeding lines from two improvement programs, two landrace panels (one global and one from Ethiopia/Eritrea), introgression lines from wild barley (H. vulgare ssp. spontaneum) in the genetic background of barley cultivar 'Rasmusson', and an assemblage of accessions previously reported to carry BLS resistance. Of the 2,094 accessions evaluated in this study, 32 (1.5%) exhibited a consistently high level of resistance across locations and years and had heading dates similar to standard cultivars grown in the region. Accessions resistant to BLS were identified from all germplasm panels tested, providing genetically diverse sources for barley improvement programs focused on breeding for resistance to this important bacterial disease.

Detection of Xanthomonas translucens pv. undulosa, pv. translucens, and pv. secalis by Quantitative PCR

A probe-based quantitative PCR (qPCR) protocol was developed for detection and evaluation of the wheat bacterial leaf streak pathogen Xanthomonas translucens pathovar (pv.) undulosa. The protocol can also detect X. translucens pv. translucens and X. translucens pv. secalis but can't differentiate the three pathovars. When tested on nontarget DNA (i.e., from plant; bacteria other than X. translucens pv. undulosa, X. translucens pv. translucens, and X. translucens pv. secalis; and culture of microorganisms from wheat grains), the qPCR showed a high specificity. On purified X. translucens pv. undulosa DNA, the qPCR was more sensitive than a loop-mediated isothermal amplification assay. When DNA samples from a set of serial dilutions of X. translucens pv. undulosa cells were tested, the qPCR method could repeatedly generate quantification cycle (Cq) values from the dilutions containing ≥1,000 cells. Since 2 µl of the total 50 µl of DNA was used in one reaction, one qPCR reaction could detect the presence of the bacteria in samples containing as few as 40 bacterial cells. The qPCR could detect the bacteria from both infected grain and leaf tissues. For seed testing, a protocol for template preparation was standardized, which allowed one qPCR reaction to test DNA from the surface of one wheat grain. Thus, the qPCR system could detect X. translucens pv. undulosa, X. translucens pv. translucens, and/or X. translucens pv. secalis in samples where the bacteria had an average concentration of ≥40 cells per grain.