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

A recently collected Xanthomonas translucens isolate encodes TAL effectors distinct from older, less virulent isolates

Xanthomonas translucens, the causal agent of bacterial leaf streak disease (BLS) in cereals, is a re-emerging pathogen that is becoming increasingly destructive across the world. While BLS has caused yield losses in the past, there is anecdotal evidence that newer isolates may be more virulent. We observed that two X. translucens isolates collected from two sites in Colorado, USA, are more aggressive on current wheat and barley varieties compared to older isolates, and we hypothesize that genetic changes between recent and older isolates contribute to the differences in isolate aggressiveness. To test this, we phenotyped and genetically characterized two X. translucens isolates collected from Colorado in 2018, which we designated CO236 (from barley) and CO237 (from wheat). Using pathovar-specific phenotyping and PCR primers, we determined that CO236 belongs to pathovar translucens (Xtt) and CO237 belongs to pathovar undulosa (Xtu). We sequenced the full genomes of the isolates using Oxford Nanopore long-read sequencing, and compared their whole genomes against published X. translucens genomes. This analysis confirmed our pathovar designations for Xtt CO236 and Xtu CO237, and showed that, at the whole-genome level, there were no obvious genomic structural changes between Xtt CO236 and Xtu CO237 and other respective published pathovar genomes. Focusing on pathovar undulosa (Xtu CO237), we then compared putative type III effectors among all available Xtu isolate genomes and found that they were highly conserved. However, there were striking differences in the presence and sequence of various transcription activator-like effectors between Xtu CO237 and published undulosa genomes, which correlate with isolate virulence. Here, we explore the potential implications of the differences in these virulence factors, and provide possible explanations for the increased virulence of recently emerged isolates.

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.

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.

Comparative Transcriptomic Analysis of Wheat Cultivars in Response to Xanthomonas translucens pv. cerealis and Its T2SS, T3SS, and TALEs Deficient Strains

Xanthomonas translucens pv. cerealis causes bacterial leaf streak disease on small grain cereals. Type II and III secretion systems (T2SS and T3SS) play a pivotal role in the pathogenicity of the bacterium, while no data are available on the transcriptomic profile of wheat cultivars infected with either wild type (WT) or mutants of the pathogen. In this study, WT, TAL-effector mutants, and T2SS/T3SS mutants of X. translucens pv. cerealis strain NXtc01 were evaluated for their effect on the transcriptomic profile of two wheat cultivars, 'Chinese Spring' and 'Yangmai-158', using Illumina RNA-sequencing technology. RNA-Seq data showed that the number of differentially expressed genes (DEGs) was higher in Yangmai-158 than in Chinese Spring, suggesting higher susceptibility of Yangmai-158 to the pathogen. In T2SS, most suppressed DEGs were related to transferase, synthase, oxidase, WRKY, and bHLH transcription factors. The gspD mutants showed significantly decreased disease development in wheat, suggesting an active contribution of T2SS in virulence. Moreover, the gspD mutant restored full virulence and its multiplication in planta by addition of gspD in trans. In the T3SS-deficient strain, downregulated DEGs were associated with cytochrome, peroxidases, kinases, phosphatases, WRKY, and ethylene-responsive transcription factors. In contrast, upregulated DEGs were trypsin inhibitors, cell number regulators, and calcium transporter. Transcriptomic analyses coupled with quantitative real-time-PCR indicated that some genes are upregulated in Δtal1/Δtal2 compared with the tal-free strain, but no direct interaction was observed. These results provide novel insight into wheat transcriptomes in response to X. translucens infection and pave the way for understanding host-pathogen interactions.

Complete Genome Assemblies of All Xanthomonas translucens Pathotype Strains Reveal Three Genetically Distinct Clades

The Xanthomonas translucens species comprises phytopathogenic bacteria that can cause serious damage to cereals and to forage grasses. So far, the genomic resources for X. translucens were limited, which hindered further understanding of the host–pathogen interactions at the molecular level and the development of disease-resistant cultivars. To this end, we complemented the available complete genome sequence of the X. translucens pv. translucens pathotype strain DSM 18974 by sequencing the genomes of all the other 10 X. translucens pathotype strains using PacBio long-read technology and assembled complete genome sequences. Phylogeny based on average nucleotide identity (ANI) revealed three distinct clades within the species, which we propose to classify as clades Xt-I, Xt-II, and Xt-III. In addition to 2,181 core X. translucens genes, a total of 190, 588, and 168 genes were found to be exclusive to each clade, respectively. Moreover, 29 non-transcription activator-like effector (TALE) and 21 TALE type III effector classes were found, and clade- or strain-specific effectors were identified. Further investigation of these genes could help to identify genes that are critically involved in pathogenicity and/or host adaptation, setting the grounds for the development of new resistant cultivars.

Genomics-Enabled Novel Insight Into the Pathovar-Specific Population Structure of the Bacterial Leaf Streak Pathogen Xanthomonas translucens in Small Grain Cereals

The Gram-negative bacterium Xanthomonas translucens infects a wide range of gramineous plants with a notable impact on small grain cereals. However, genomics-informed intra-species population structure and virulence repertories of the pathogen have rarely been investigated. In this study, the complete genome sequences of seven X. translucens strains representing an entire set of genetic diversity of two pathovars X. translucens pv. undulosa and X. translucens pv. translucens is provided and compared with those of seven publicly available complete genomes of the pathogen. Organization of the 25 type III secretion system genes in all the 14 X. translucens strains was exactly the same, while TAL effector genes localized singly or in clusters across four loci in X. translucens pv. translucens and five to six loci in X. translucens pv. undulosa. Beside two previously unreported endogenous plasmids in X. translucens pv. undulosa, and variations in repeat variable diresidue (RVD) of the 14 strains, tal1a of X. translucens pv. translucens strain XtKm8 encode the new RVDs HE and YI which have not previously been reported in xanthomonads. Further, a number of truncated tal genes were predicted among the 14 genomes lacking conserved BamHI site at N-terminus and SphI site at C-terminus. Our data have doubled the number of complete genomes of X. translucens clarifying the population structure and genomics of the pathogen to pave the way in the small grain cereals industry for disease resistance breeding in the 21st century's agriculture.

Identification of a virulence tal gene in the cotton pathogen, Xanthomonas citri pv. malvacearum strain Xss-V2-18

Background

Bacterial blight of cotton (BBC), which is caused by the bacterium Xanthomonas citri pv. malvacearum (Xcm), is a destructive disease in cotton. Transcription activator-like effectors (TALEs), encoded by tal-genes, play critical roles in the pathogenesis of xanthomonads. Characterized strains of cotton pathogenic Xcm harbor 8–12 different tal genes and only one of them is functionally decoded. Further identification of novel tal genes in Xcm strains with virulence contributions are prerequisite to decipher the Xcm-cotton interactions.

Results

In this study, we identified six tal genes in Xss-V₂–18, a highly-virulent strain of Xcm from China, and assessed their role in BBC. RFLP-based Southern hybridization assays indicated that Xss-V₂–18 harbors the six tal genes on a plasmid. The plasmid-encoded tal genes were isolated by cloning BamHI fragments and screening clones by colony hybridization. The tal genes were sequenced by inserting a Tn5 transposon in the DNA encoding the central repeat region (CRR) of each tal gene. Xcm TALome evolutionary relationship based on TALEs CRR revealed relatedness of Xss-V₂–18 to MSCT1 and MS14003 from the United States. However, Tal2 of Xss-V₂–18 differs at two repeat variable diresidues (RVDs) from Tal6 and Tal26 in MSCT1 and MS14003, respectively, inferred functional dissimilarity. The suicide vector pKMS1 was then used to construct tal deletion mutants in Xcm Xss-V₂–18. The mutants were evaluated for pathogenicity in cotton based on symptomology and growth in planta. Four mutants showed attenuated virulence and all contained mutations in tal2. One tal2 mutant designated M2 was further investigated in complementation assays. When tal2 was introduced into Xcm M2 and expressed in trans, the mutant was complemented for both symptoms and growth in planta, thus indicating that tal2 functions as a virulence factor in Xcm Xss-V₂–18.

Conclusions

Overall, the results demonstrated that Tal2 is a major pathogenicity factor in Xcm strain Xss-V₂–18 that contributes significantly in BBC. This study provides a foundation for future efforts aimed at identifying susceptibility genes in cotton that are targeted by Tal2.

Transcription Activator-Like Effectors Diversity in Iranian Strains of Xanthomonas translucens

Bacterial leaf streak caused by different pathovars of Xanthomonas translucens is the most important seedborne bacterial disease of small grain cereals. However, variations in the virulence-associated genomic areas of the pathogen remain uninvestigated. In this study, the diversity of transcription activator-like effectors (TALE) was investigated using the Southern blotting of BamHI-digested genomic DNAs in the Iranian strains of X. translucens. All 65 X. translucens strains were assigned into 13 genotypes, where 57 X. translucens pv. undulosa strains were placed in genotypes 1 to 8, and seven X. translucens pv. translucens strains were placed in genotypes 9 to 12. Interestingly, we did not find any TALE genes in the strain XtKm7 (genotype 13), which showed to be pathogenic only on barley. Virulence and aggressiveness of these strains in greenhouse conditions were in agreement with the TALE-based clustering of the strains in the pathovar level, though variations were observed in the aggressiveness of X. translucens pv. undulosa strains. In general, strains containing higher numbers of putative TALE genes were more virulent on wheat and barley than strains containing fewer. This is the first TALE-based genetic diversity analysis on X. translucens strains and provides novel insights into the virulence repertories and genomic characteristics of the pathogen. Further investigations using TALE mutagenesis and complementation analysis are warranted to precisely elucidate the role of each detected X. translucens TALE in bacterial virulence and aggressiveness either on wheat or barley.