Genome of the actinomycete plant pathogen Clavibacter michiganensis subsp. sepedonicus suggests recent niche adaptation

A Putative Apoplastic Effector of Clavibacter capsici, ChpGCc as Hypersensitive Response and Virulence (Hrv) Protein in Plants

Clavibacter bacteria use secreted apoplastic effectors, such as putative serine proteases, for virulence in host plants and for hypersensitive response (HR) induction in nonhost plants. Previously, we have shown that Clavibacter capsici ChpGCc is important for the necrosis development in pepper (Capsicum annuum) leaves. Here, we determine the function of ChpGCc, along with three paralogous proteins, for HR induction in the apoplastic space of a nonhost plant, Nicotiana tabacum. The full-length and signal peptide-deleted (ΔSP) mature forms of all proteins fused with the tobacco PR1b signal sequence were generated. The full-length and ΔSP forms of ChpGCc and only the ΔSP forms of ChpECc and Pat-1Cc, but none of the ChpCCc, triggered HR. Based on the predicted protein structures, ChpGCc carries amino acids for a catalytic triad and a disulfide bridge in positions like Pat-1Cm. Substituting these amino acids of ChpGCc with alanine abolished or reduced HR-inducing activity. To determine whether these residues are important for necrosis development in pepper, alanine-substituted chpGCc genes were transformed into the C. capsici PF008ΔpCM1 strain, which lacks the intact chpGCc gene. The strain with any variants failed to restore the necrosis-causing ability. These results suggest that ChpGCc has a dual function as a virulence factor in host plants and an HR elicitor in nonhost plants. Based on our findings and previous results, we propose Clavibacter apoplastic effectors, such as ChpGCc, Pat-1Cm, Chp-7Cs, and ChpGCm, as hypersensitive response and virulence (Hrv) proteins that display phenotypic similarities to the hypersensitive response and pathogenicity (Hrp) proteins found in gram-negative bacteria. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Clavibacter nebraskensis causing Goss's wilt of maize: Five decades of detaining the enemy in the New World

Goss's bacterial wilt and leaf blight of maize (Zea mays) caused by the gram-positive coryneform bacterium Clavibacter nebraskensis is an economically important disease in North America. C. nebraskensis is included within the high-risk list of quarantine pathogens by several plant protection organizations (EPPO code: CORBMI), hence it is under strict quarantine control around the world. The causal agent was reported for the first time on maize in Nebraska (USA) in 1969. After an outbreak during the 1970s, prevalence of the disease decreased in the 1980s to the early 2000s, before the disease resurged causing a serious threat to maize production in North America. The re-emergence of Goss's wilt in the corn belt of the United States led to several novel achievements in understanding the pathogen biology and disease control. In this review, we provide an updated overview of the pathogen taxonomy, biology, and epidemiology as well as management strategies of Goss's wilt disease. First, a taxonomic history of the pathogen is provided followed by symptomology and host range, genetic diversity, and pathogenicity mechanisms of the bacterium. Then, utility of high-throughput molecular approaches in the precise detection and identification of the pathogen and the management strategies of the disease are explained. Finally, we highlight the role of integrated pest management strategies to combat the risk of Goss's wilt in the 21st century maize industry.

DISEASE SYMPTOMS

Large (2-15 cm) tan to grey elongated oval lesions with wavy, irregular water-soaked margins on the leaves. The lesions often start at the leaf tip or are associated with wounding caused by hail or wind damage. Small (1 mm in diameter), dark, discontinuous water-soaked spots, known as "freckles", can be observed in the periphery of lesions. When backlit, the freckles appear translucent. Early infection (prior to growth stage V6) may become systemic and cause seedlings to wilt, wither, and die. Coalescence of lesions results in leaf blighting.

HOST RANGE

Maize (Zea mays) is the only economic host of the pathogen. A number of Poaceae species are reported to act as secondary hosts for C. nebraskensis.

TAXONOMIC STATUS OF THE PATHOGEN

Class: Actinobacteria; Order: Micrococcales; Family: Microbacteriaceae; Genus: Clavibacter; Species: Clavibacter nebraskensis.

SYNONYMS

Corynebacterium nebraskense (Schuster, 1970) Vidaver & Mandel 1974; Corynebacterium michiganense pv. nebraskense (Vidaver & Mandel 1974) Dye & Kemp 1977; Corynebacterium michiganense subsp. nebraskense (Vidaver & Mandel 1974) Carlson & Vidaver 1982; Clavibacter michiganense subsp. nebraskense (Vidaver & Mandel 1974) Davis et al. 1984; Clavibacter michiganensis subsp. nebraskensis (Vidaver & Mandel 1974) Davis et al. 1984.

TYPE MATERIALS

ATCC 27794^T ; CFBP 2405^T ; ICMP 3298^T ; LMG 3700^T ; NCPPB 2581^T .

MICROBIOLOGICAL PROPERTIES

Cells are gram-positive, orange-pigmented, pleomorphic club- or rod-shaped, nonspore-forming, nonmotile, and without flagella, approximately 0.5 × 1-2.0 μm.

DISTRIBUTION

The pathogen is restricted to Canada and the United States.

PHYTOSANITARY CATEGORIZATION

EPPO code CORBNE.

Complete Genome Sequence Resource for a Recently Isolated Potato Ring Rot Pathogen, Clavibacter sepedonicus K496

Potato ring rot caused by Clavibacter sepedonicus has been a devastating disease in the U.S. since 1930. In this study, we isolated a recent C. sepedonicus strain, K496, from potato tubers showing discolorations of the vascular cylinder or pith tissues. We de novo assembled the genome sequence of K496 with 1,924,544,313 bp of Nanopore reads (N₅₀ = 13,785 bp) using Flye v2.9 and polished it with 2 × 150 bp paired-end Illumina reads (855,788,703 bp in total). The resulting genome of K496 consists of a single circular chromosome 3,266,016 bp long and a linear plasmid of 135,489 bp. Using the NCBI PGAP v5.3, this genome was predicted to have 3,301 genes, encompassing 3,247 protein-coding genes, 90 pseudogenes, two 5S rRNA-coding, two 16S rRNA-coding, two 23S rRNA-coding sequences, 45 tRNAs, and three noncoding RNAs. The chromosome and plasmid sequences have been deposited at the NCBI GenBank database under the accession numbers CP088266 and CP088267, respectively.

CRISPR-Cas9 driven structural elucidation of the heteroexopolysaccharides from Paenibacillus polymyxa DSM 365

Paenibacillus polymyxa is a Gram-positive soil bacterium known for producing a wide range of exopolysaccharides. However, due to the biopolymer's complexity, structural elucidation has so far been inconclusive. Combinatorial knock-outs of glycosyltransferases were generated in order to separate distinct polysaccharides produced by P. polymyxa. Using a complementary analytical approach consisting of carbohydrate fingerprints, sequence analysis, methylation analysis as well as NMR spectroscopy, the structure of the repeating units of two additional heteroexopolysaccharides termed paenan I and paenan III were elucidated. Results for paenan I identified a trisaccharide backbone consisting of 1➔4-β-d-Glc, 1➔4-β-d-Man and a 1,3,4-branching β-d-Gal residue with a sidechain comprising of a terminal β-d-Gal^3,4-Pyr and 1➔3-β-d-Glc. For paenan III, results indicated a backbone consisting of 1➔3-β-d-Glc, 1,3,4-linked α-d-Man and 1,3,4-linked α-d-GlcA. NMR analysis indicated monomeric β-d-Glc and α-d-Man sidechains for the branching Man and GlcA residues respectively.

Bacterial ring rot of potato caused by Clavibacter sepedonicus: A successful example of defeating the enemy under international regulations

BACKGROUND

Bacterial ring rot of potato (Solanum tuberosum) caused by the gram-positive coryneform bacterium Clavibacter sepedonicus is an important quarantine disease threatening the potato industry around the globe. Since its original description in 1906 in Germany, management of ring rot has been a major problem due to the seedborne nature (via seed tubers not true seeds) of the pathogen allowing the bacterium to be transmitted long distances via infected tubers.

DISEASE SYMPTOMS

On growing potato plants: interveinal chlorosis on leaflets leading to necrotic areas and systemic wilt. On infected tubers: vascular tissues become yellowish brown with a cheesy texture due to bacterial colonization and decay.

HOST RANGE

Potato is the main host of the pathogen, but natural infection also occurs on eggplant, tomato, and sugar beet.

TAXONOMIC STATUS OF THE PATHOGEN

Class: Actinobacteria; Order: Actinomycetales; Family: Microbacteriaceae; Genus: Clavibacter; Species: Clavibacter sepedonicus (Spieckermann and Kotthoff 1914) Li et al. 2018.

SYNONYMS (NONPREFERRED SCIENTIFIC NAMES)

Aplanobacter sepedonicus; Bacterium sepedonicum; Corynebacterium sepedonicum; Corynebacterium michiganense pv. sepedonicum; Clavibacter michiganensis subsp. sepedonicus.

MICROBIOLOGICAL PROPERTIES

Gram-positive, club-shaped cells with creamy to yellowish-cream colonies for which the optimal growth temperature is 20-23°C.

DISTRIBUTION

Asia (China, Japan, Kazakhstan, Nepal, North Korea, Pakistan, South Korea, Uzbekistan, the Asian part of Russia), Europe (Belarus, Bulgaria, Czech Republic, Estonia, Finland, Georgia, Germany, Greece, Hungary, Latvia, Lithuania, Norway, Poland, Romania, European part of Russia, Slovakia, Spain, Sweden, Turkey, Ukraine), and North America (Canada, Mexico, USA).

PHYTOSANITARY CATEGORIZATION

CORBSE: EPPO A2 list no. 51. EU; Annex designation I/A2.

Comparative Genome Analyses of Clavibacter michiganensis Type Strain LMG7333T Reveal Distinct Gene Contents in Plasmids From Other Clavibacter Species

Clavibacter michiganensis, a Gram-positive, plant-pathogenic bacterium belonging to Actinobacteria, is a causal agent of bacterial canker in tomatoes. Although LMG7333^T is the type strain of C. michiganensis, it has not been used in many studies, probably because of a lack of the complete genome sequence being available. Therefore, in this study, the complete genome sequence of this type strain was obtained, and comparative genome analysis was conducted with the genome sequences of two other C. michiganensis strains and type strains of Clavibacter species, of which their complete genome sequences are available. C. michiganensis LMG7333^T carries one chromosome and two plasmids, pCM1 and pCM2, like two other C. michiganensis strains. All three chromosomal DNA sequences were almost identical. However, the DNA sequences of two plasmids of LMG7333^T are similar to those of UF1, but different from those of NCPPB382, indicating that both plasmids carry distinct gene content among C. michiganensis strains. Moreover, 216 protein-coding sequences (CDSs) were only present in the LMG7333^T genome compared with type strains of other Clavibacter species. Among these 216 CDSs, approximately 83% were in the chromosome, whereas others were in both plasmids (more than 6% in pCM1 and 11% in pCM2). However, the ratio of unique CDSs of the total CDSs in both plasmids were approximately 38% in pCM1 and 30% in pCM2, indicating that the high gene content percentage in both plasmids of C. michiganensis are different from those of other Clavibacter species, and plasmid DNAs might be derived from different origins. A virulence assay with C. michiganensis LMG7333^T using three different inoculation methods, root-dipping, leaf-clipping, and stem injection, resulted in typical disease symptoms, including wilting and canker in tomato. Altogether, our results indicate that two plasmids of C. michiganensis carry distinct gene content, and the genome information of the type strain LMG7333^T will help to understand the genetic diversity of the two plasmids of Clavibacter species, including C. michiganensis.

Structural elucidation of the fucose containing polysaccharide of Paenibacillus polymyxa DSM 365

Paenibacillus polymyxa is an avid producer of exopolysaccharides of industrial interest. However, due to the complexity of the polymer composition, structural elucidation of the polysaccharide remained unfeasible for a long time. By using a CRISPR-Cas9 mediated knock-out strategy, all single glycosyltransferases as well as the Wzy polymerases were individually deleted in the corresponding gene cluster for the first time. Thereby, it was observed that the main polymer fraction was completely suppressed (or deleted) and a pure minor fucose containing polysaccharide could be isolated, which was named paenan II. Applying this combinatorial approach, the monosaccharide composition, sequence and linkage pattern of this novel polymer was determined via HPLC-MS, GC-MS and NMR. Furthermore, we demonstrated that the knock-out of the glycosyltransferases PepQ, PepT, PepU and PepV as well as of the Wzy polymerase PepG led to the absence of paenan II, attributing those enzymes to the assembly of the repeating unit.

A Genetic Toolkit for Investigating Clavibacter Species: Markerless Deletion, Permissive Site Identification, and an Integrative Plasmid

The development of knockout mutants and expression variants are critical for understanding genotype-phenotype relationships. However, advances in these techniques in gram-positive actinobacteria have stagnated over the last decade. Actinobacteria in the Clavibacter genus are composed of diverse crop pathogens that cause a variety of wilt and cankering diseases. Here, we present a suite of tools for genetic manipulation in the tomato pathogen Clavibacter michiganensis including a markerless deletion system, an integrative plasmid, and an R package for identification of permissive sites for plasmid integration. The vector pSelAct-KO is a recombination-based, markerless knockout system that uses dual selection to engineer seamless deletions of a region of interest, providing opportunities for repeated higher-order genetic knockouts. The efficacy of pSelAct-KO was demonstrated in C. michiganensis and was confirmed using whole-genome sequencing. We developed permissR, an R package to identify permissive sites for chromosomal integration, which can be used in conjunction with pSelAct-Express, a nonreplicating integrative plasmid that enables recombination into a permissive genomic location. Expression of enhanced green fluorescent protein by pSelAct-Express was verified in two candidate permissive regions predicted by permissR in C. michiganensis. These molecular tools are essential advances for investigating gram-positive actinobacteria, particularly for important pathogens in the Clavibacter genus.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Clavibacter michiganensis Downregulates Photosynthesis and Modifies Monolignols Metabolism Revealing a Crosstalk with Tomato Immune Responses

The gram-positive pathogenic bacterium Clavibacter michiganensis subsp. michiganensis (Cmm) causes bacterial canker disease in tomato, affecting crop yield and fruit quality. To understand how tomato plants respond, the dynamic expression profile of host genes was analyzed upon Cmm infection. Symptoms of bacterial canker became evident from the third day. As the disease progressed, the bacterial population increased in planta, reaching the highest level at six days and remained constant till the twelfth day post inoculation. These two time points were selected for transcriptomics. A progressive down-regulation of key genes encoding for components of the photosynthetic apparatus was observed. Two temporally separated defense responses were observed, which were to an extent interdependent. During the primary response, genes of the phenylpropanoid pathway were diverted towards the synthesis of monolignols away from S-lignin. In dicots, lignin polymers mainly consist of G- and S-units, playing an important role in defense. The twist towards G-lignin enrichment is consistent with previous findings, highlighting a response to generate an early protective barrier and to achieve a tight interplay between lignin recomposition and the primary defense response mechanism. Upon progression of Cmm infection, the temporal deactivation of phenylpropanoids coincided with the upregulation of genes that belong in a secondary response mechanism, supporting an elegant reprogramming of the host transcriptome to establish a robust defense apparatus and suppress pathogen invasion. This high-throughput analysis reveals a dynamic reorganization of plant defense mechanisms upon bacterial infection to implement an array of barriers preventing pathogen invasion and spread.

Bacterial Canker of Tomato: Revisiting a Global and Economically Damaging Seedborne Pathogen

The gram-positive actinobacterium Clavibacter michiganensis is the causal agent of bacterial canker of tomato, an economically impactful disease with a worldwide distribution. This seedborne pathogen systemically colonizes tomato xylem leading to unilateral leaflet wilt, marginal leaf necrosis, stem and petiole cankers, and plant death. Additionally, splash dispersal of the bacterium onto fruit exteriors causes bird's-eye lesions, which are characterized as necrotic centers surrounded by white halos. The pathogen can colonize developing seeds systemically through xylem and through penetration of fruit tissues from the exterior. There are currently no commercially available resistant cultivars, and bactericidal sprays have limited efficacy for managing the disease once the pathogen is in the vascular system. In this review, we summarize research on epidemiology, host colonization, the bacterial genetics underlying virulence, and management of bacterial canker. Finally, we highlight important areas of research into this pathosystem that have the potential to generate new strategies for prevention and mitigation of bacterial canker.

Microbial Diversity of Psychrotolerant Bacteria Isolated from Wild Flora of Andes Mountains and Patagonia of Chile towards the Selection of Plant Growth-Promoting Bacterial Consortia to Alleviate Cold Stress in Plants

Cold stress decreases the growth and productivity of agricultural crops. Psychrotolerant plant growth-promoting bacteria (PGPB) may protect and promote plant growth at low temperatures. The aims of this study were to isolate and characterize psychrotolerant PGPB from wild flora of Andes Mountains and Patagonia of Chile and to formulate PGPB consortia. Psychrotolerant strains were isolated from 11 wild plants (rhizosphere and phyllosphere) during winter of 2015. For the first time, bacteria associated with Calycera, Orites, and Chusquea plant genera were reported. More than 50% of the 130 isolates showed ≥33% bacterial cell survival at temperatures below zero. Seventy strains of Pseudomonas, Curtobacterium, Janthinobacterium, Stenotrophomonas, Serratia, Brevundimonas, Xanthomonas, Frondihabitans, Arthrobacter, Pseudarthrobacter, Paenarthrobacter, Brachybacterium, Clavibacter, Sporosarcina, Bacillus, Solibacillus, Flavobacterium, and Pedobacter genera were identified by 16S rRNA gene sequence analyses. Ten strains were selected based on psychrotolerance, auxin production, phosphate solubilization, presence of nifH (nitrogenase reductase) and acdS (1-aminocyclopropane-1-carboxylate (ACC) deaminase) genes, and anti-phytopathogenic activities. Two of the three bacterial consortia formulated promoted tomato plant growth under normal and cold stress conditions. The bacterial consortium composed of Pseudomonas sp. TmR5a & Curtobacterium sp. BmP22c that possesses ACC deaminase and ice recrystallization inhibition activities is a promising candidate for future cold stress studies.

Complete Genome Sequencing Provides Novel Insight Into the Virulence Repertories and Phylogenetic Position of Dry Beans Pathogen Curtobacterium flaccumfaciens pv. flaccumfaciens

Bacterial wilt of dry beans (family Fabaceae) caused by the actinobacterial agent Curtobacterium flaccumfaciens pv. flaccumfaciens is one of the most important diseases threatening edible legume production around the globe. Despite the economic losses due to the bacterial wilt disease, the pathogen has not so far been investigated for its genomic features, pathogenicity determinants, and virulence strategies. Here we present the first complete genome sequence of a highly virulent bacteriocin-producing C. flaccumfaciens pv. flaccumfaciens strain P990. The bacterium has a circular chromosome consisting of 3,736 kbp with the G+C% content of 71.0%. Furthermore, a 147-kbp circular plasmid (pCff1) with 66.1% G+C% content as well as two circular plasmid-like DNAs with sizes of 25 and 22 kbp were detected within the genomic contents of C. flaccumfaciens pv. flaccumfaciens. Phylogenetic analyses revealed that only a few number of Curtobacterium sp. strains deposited in the public databases could be classified within the species C. flaccumfaciens. Comparative genomics of C. flaccumfaciens pv. flaccumfaciens using the genome sequences of actinobacterial plant pathogens revealed the presence of a set of unique low G+C% content genomic islands in the C. flaccumfaciens pv. flaccumfaciens genome. Homologs of pathogenicity-determinant loci capable of producing 1,4-beta-xylanase (xysA), pectate lyase (pelA1 and pelA2), serine protease (chpC, chpG, and pat-1), and sortase (srtA) were detected in C. flaccumfaciens pv. flaccumfaciens genome. The genomic data presented here extend our understanding of the C. flaccumfaciens pv. flaccumfaciens genomic features and pave the ways of research on functional and interaction genetics to combat the risk of bacterial wilt disease in the 21st century's dry bean industry.

Complete Genome Sequence Resource of a Strain of Curtobacterium flaccumfaciens pv. flaccumfaciens, the Causal Agent of Bacterial Wilt of Common Bean, from Turkey

Curtobacterium flaccumfaciens pv. flaccumfaciens is the causal agent of bacterial wilt of common bean (Phaseolus vulgaris), a disease that can reduce yields of this economically important crop worldwide. Current genomics resources for this bacterial pathogen are limited. Therefore, long-read sequencing was used to determine the complete genome sequence of a pathogenic C. flaccumfaciens pv. flaccumfaciens strain isolated from common bean leaves showing irregular necrotic lesions with yellow borders collected in a commercial field in Turkey in 2015.

Comparative Genomics and Phylogenetic Analyses Suggest Several Novel Species within the Genus Clavibacter, Including Nonpathogenic Tomato-Associated Strains

Members of the genus Clavibacter are economically important bacterial plant pathogens infecting a set of diverse agricultural crops (e.g., alfalfa, corn, potato, tomato, and wheat). Tomato-associated Clavibacter sp. strains account for a great portion of the genetic diversity of the genus, and C. michiganensis sensu stricto (formerly C. michiganensis subsp. michiganensis), causing bacterial canker disease, is considered one of the most destructive seed-borne agents for the crop worldwide. However, current taxonomic descriptions of the genus do not reflect the existing diversity of the strains, resulting in unsatisfactory results in quarantine surveys for the pathogens. In this study, we used all the available genome sequences of Clavibacter sp. strains, including the type strains of newly described subspecies, to provide precise insight into the diversity of tomato-associated members of the genus and further clarify the taxonomic status of the strains using genotypic and phenotypic features. The results of phylogenetic analyses revealed the existence of nine hypothetical new species among the investigated strains. None of the three new subspecies (i.e., C. michiganensis subsp. californiensis, C. michiganensis subsp. chilensis, and C. michiganensis subsp. phaseoli) is included within the tomato-pathogenic C. michiganensis sensu stricto lineage. Although comparative genomics revealed the lack of chp and tomA pathogenicity determinant gene clusters in the nonpathogenic strains, a number of pathogenicity-related genes were noted to be present in all the strains regardless of their pathogenicity characteristics. Altogether, our results indicate a need for a formal taxonomic reconsideration of tomato-associated Clavibacter sp. strains to facilitate differentiation of the lineages in quarantine inspections.IMPORTANCE Clavibacter spp. are economically important bacterial plant pathogens infecting a set of diverse agricultural crops, such as alfalfa, corn, pepper, potato, tomato, and wheat. A number of plant-pathogenic members of the genus (e.g., C. michiganensis sensu stricto and C. sepedonicus, infecting tomato and potato plants, respectively) are included in the A2 (high-risk) list of quarantine pathogens by the European and Mediterranean Plant Protection Organization (EPPO). Although tomato-associated members of Clavibacter spp. account for a significant portion of the genetic diversity in the genus, only the strains belonging to C. michiganensis sensu stricto (formerly C. michiganensis subsp. michiganensis) cause bacterial canker disease of tomato and are subjected to the quarantine inspections. Hence, discrimination between the pathogenic and nonpathogenic Clavibacter sp. strains associated with tomato seeds and transplants plays a pivotal role in the accurate detection and cost-efficient management of the disease. On the other hand, detailed information on the genetic contents of different lineages of the genus would lead to the development of genome-informed specific detection techniques. In this study, we have provided an overview of the phylogenetic and genomic differences between the pathogenic and nonpathogenic tomato-associated Clavibacter sp. strains. We also noted that the taxonomic status of newly introduced subspecies of C. michiganensis (i.e., C. michiganensis subsp. californiensis, C. michiganensis subsp. chilensis, and C. michiganensis subsp. phaseoli) should be reconsidered.

The Evolution, Ecology, and Mechanisms of Infection by Gram-Positive, Plant-Associated Bacteria

Gram-positive bacteria are prominent members of plant-associated microbial communities. Although many are hypothesized to be beneficial, some are causative agents of economically important diseases of crop plants. Because the features of Gram-positive bacteria are fundamentally different relative to those of Gram-negative bacteria, the evolution and ecology as well as the mechanisms used to colonize and infect plants also differ. Here, we discuss recent advances in our understanding of Gram-positive, plant-associated bacteria and provide a framework for future research directions on these important plant symbionts.

Pest categorisation of Clavibacter sepedonicus

Following a request from the European Commission, the EFSA Panel on Plant Health performed a pest categorisation of Clavibacter sepedonicus, a well-defined and distinguishable bacterial plant pathogen of the family Microbacteriaceae. C. sepedonicus causes bacterial ring rot of potato and is reported from North America, Asia and Europe. The bacterium is mostly tuber transmitted, but it can also enter host plants through wounds or via contaminated equipment. C. sepedonicus is regulated in Council Directive 2000/29/EC (Annex IAII, as Clavibacter michiganensis subsp. sepedonicus) as a harmful organism whose introduction into the EU is banned. In addition, Council Directive 1993/85/EEC concerns the measures to be taken within EU Member States (MS) against C. sepedonicus to (a) detect it and determine its distribution, (b) prevent its occurrence and spread, and (c) control it with the aim of eradication. The pest is present in several EU MS, but in all cases with a restricted distribution and under official control. C. sepedonicus could enter the EU and spread primarily via host plants for planting (i.e. potato tubers). The pest could establish in the EU, as the main host (potato) is commonly grown and climatic conditions are favourable. Direct potato losses following infection by C. sepedonicus can be substantial and are due to the destruction of the vascular tissue, wilting of the plant and rotting of the tubers. Infected hosts can remain asymptomatic. The main knowledge gaps are the geographic distribution of the pest and the host range. The criteria assessed by the Panel for consideration of C. sepedonicus as a potential quarantine pest are met, while, for regulated non-quarantine pests, the criterion on the widespread presence in the EU is not met.

A genomics perspective on natural product biosynthesis in plant pathogenic bacteria

This review summarizes findings from genomics-inspired natural product research in plant pathogenic bacteria and discusses emerging trends in this field.

Differentiation of Clavibacter michiganensis subsp. sepedonicus using PCR melting profile and variable number of tandem repeat methods

The potato phytopathogen Clavibacter michiganensis subsp. sepedonicus (Cms) is a causative agent of bacterial ring rot, which is a serious threat to crops. In EU member countries, Cms is subject to quarantine and has to be combated. The knowledge about the transmission of C. michiganensis strains is limited due to a lack of methods which could be used for epidemiological analysis. In this study, PCR melting profile (PCR MP) and variable number tandem repeat methods were used in Cms epidemiological analysis for the first time. PCR MP was based on the melting temperature analysis of BamHI restriction fragments of chromosomal DNA. Respectively, for the variable number tandem repeat (VNTR) method, six loci were identified and used in the differentiation of Cms isolates. PCR MP was used for 93 Cms isolated in Poland. Both PCR MP and VNTR methods were used for the differentiation of 47 Cms strains in this collection. Both these methods were found to be useful for the epidemiological analysis of Cms. SIGNIFICANCE AND IMPACT OF THE STUDY: The potato phytopathogen, Clavibacter michiganensis subsp. sepedonicus (Cms), is a serious threat to crops and lead to significant economic losses. The only way to control and eliminate the disease caused by this pathogen is the use of certified seed potato and strict quarantine of infected fields. Here, for the first time, two molecular typing methods (PCR melting profile (PCR MP) and variable number tandem repeat (VNTR)) were evaluated in respect of their potential in differentiation of Cms isolates. As a result, we obtained characteristic profiles of DNA fragments (PCR MP) and numeric patterns (VNTR), which enable the intraspecies genotyping of Cms strains confirming the effectiveness of PCR MP and VNTR methods in differentiation of Cms strains.

Detection of Clavibacter michiganensis subsp. michiganensis Assisted by Micro-Raman Spectroscopy under Laboratory Conditions

Clavibacter michiganensis subsp. michiganesis (Cmm) is a quarantine-worthy pest in México. The implementation and validation of new technologies is necessary to reduce the time for bacterial detection in laboratory conditions and Raman spectroscopy is an ambitious technology that has all of the features needed to characterize and identify bacteria. Under controlled conditions a contagion process was induced with Cmm, the disease epidemiology was monitored. Micro-Raman spectroscopy (532 nm λ laser) technique was evaluated its performance at assisting on Cmm detection through its characteristic Raman spectrum fingerprint. Our experiment was conducted with tomato plants in a completely randomized block experimental design (13 plants × 4 rows). The Cmm infection was confirmed by 16S rDNA and plants showed symptoms from 48 to 72 h after inoculation, the evolution of the incidence and severity on plant population varied over time and it kept an aggregated spatial pattern. The contagion process reached 79% just 24 days after the epidemic was induced. Micro-Raman spectroscopy proved its speed, efficiency and usefulness as a non-destructive method for the preliminary detection of Cmm. Carotenoid specific bands with wavelengths at 1146 and 1510 cm-1 were the distinguishable markers. Chemometric analyses showed the best performance by the implementation of PCA-LDA supervised classification algorithms applied over Raman spectrum data with 100% of performance in metrics of classifiers (sensitivity, specificity, accuracy, negative and positive predictive value) that allowed us to differentiate Cmm from other endophytic bacteria (Bacillus and Pantoea). The unsupervised KMeans algorithm showed good performance (100, 96, 98, 91 y 100%, respectively).

Analysis of Extreme Phenotype Bulk Copy Number Variation (XP-CNV) Identified the Association of rp1 with Resistance to Goss's Wilt of Maize

Goss's wilt (GW) of maize is caused by the Gram-positive bacterium Clavibacter michiganensis subsp. nebraskensis (Cmn) and has spread in recent years throughout the Great Plains, posing a threat to production. The genetic basis of plant resistance is unknown. Here, a simple method for quantifying disease symptoms was developed and used to select cohorts of highly resistant and highly susceptible lines known as extreme phenotypes (XP). Copy number variation (CNV) analyses using whole genome sequences of bulked XP revealed 141 genes containing CNV between the two XP groups. The CNV genes include the previously identified common rust resistant locus rp1. Multiple Rp1 accessions with distinct rp1 haplotypes in an otherwise susceptible accession exhibited hypersensitive responses upon inoculation. GW provides an excellent system for the genetic dissection of diseases caused by closely related subspecies of C. michiganesis. Further work will facilitate breeding strategies to control GW and provide needed insight into the resistance mechanism of important related diseases such as bacterial canker of tomato and bacterial ring rot of potato.

Multiple plasmid-borne virulence genes of Clavibacter michiganensis ssp. capsici critical for disease development in pepper

Clavibacter michiganensis ssp. capsici is a Gram-positive plant-pathogenic bacterium causing bacterial canker disease in pepper. Virulence genes and mechanisms of C. michiganensis ssp. capsici in pepper have not yet been studied. To identify virulence genes of C. michiganensis ssp. capsici, comparative genome analyses with C. michiganensis ssp. capsici and its related C. michiganensis subspecies, and functional analysis of its putative virulence genes during infection were performed. The C. michiganensis ssp. capsici type strain PF008 carries one chromosome (3.056 Mb) and two plasmids (39 kb pCM1_Cmc and 145 kb pCM2_Cmc ). The genome analyses showed that this bacterium lacks a chromosomal pathogenicity island and celA gene that are important for disease development by C. michiganensis ssp. michiganensis in tomato, but carries most putative virulence genes in both plasmids. Virulence of pCM1_Cmc -cured C. michiganensis ssp. capsici was greatly reduced compared with the wild-type strain in pepper. The complementation analysis with pCM1_Cmc -located putative virulence genes showed that at least five genes, chpE, chpG, ppaA1, ppaB1 and pelA1, encoding serine proteases or pectate lyase contribute to disease development in pepper. In conclusion, C. michiganensis ssp. capsici has a unique genome structure, and its multiple plasmid-borne genes play critical roles in virulence in pepper, either separately or together.

Comparative Genomic Analyses of Clavibacter michiganensis subsp. insidiosus and Pathogenicity on Medicago truncatula

Clavibacter michiganensis is the most economically important gram-positive bacterial plant pathogen, with subspecies that cause serious diseases of maize, wheat, tomato, potato, and alfalfa. Much less is known about pathogenesis involving gram-positive plant pathogens than is known for gram-negative bacteria. Comparative genome analyses of C. michiganensis subspecies affecting tomato, potato, and maize have provided insights on pathogenicity. In this study, we identified strains of C. michiganensis subsp. insidiosus with contrasting pathogenicity on three accessions of the model legume Medicago truncatula. We generated complete genome sequences for two strains and compared these to a previously sequenced strain and genome sequences of four other subspecies. The three C. michiganensis subsp. insidiosus strains varied in gene content due to genome rearrangements, most likely facilitated by insertion elements, and plasmid number, which varied from one to three depending on strain. The core C. michiganensis genome consisted of 1,917 genes, with 379 genes unique to C. michiganensis subsp. insidiosus. An operon for synthesis of the extracellular blue pigment indigoidine, enzymes for pectin degradation, and an operon for inositol metabolism are among the unique features. Secreted serine proteases belonging to both the pat-1 and ppa families were present but highly diverged from those in other subspecies.

Re-classification of Clavibacter michiganensis subspecies on the basis of whole-genome and multi-locus sequence analyses

Although the genus Clavibacter was originally proposed to accommodate all phytopathogenic coryneform bacteria containing B2γ diaminobutyrate in the peptidoglycan, reclassification of all but one species into other genera has resulted in the current monospecific status of the genus. The single species in the genus, Clavibacter michiganensis, has multiple subspecies, which are all highly host-specific plant pathogens. Whole genome analysis based on average nucleotide identity and digital DNA-DNA hybridization as well as multi-locus sequence analysis (MLSA) of seven housekeeping genes support raising each of the C. michiganensis subspecies to species status. On the basis of whole genome and MLSA data, we propose the establishment of two new species and three new combinations: Clavibacter capsici sp. nov., comb. nov. and Clavibacter tessellarius sp. nov., comb. nov., and Clavibacter insidiosus comb. nov., Clavibacter nebraskensis comb. nov. and Clavibacter sepedonicus comb. nov.

Genomic Analysis of Clavibacter michiganensis Reveals Insight Into Virulence Strategies and Genetic Diversity of a Gram-Positive Bacterial Pathogen

Clavibacter michiganensis subsp. michiganensis is a gram-positive bacterial pathogen that proliferates in the xylem vessels of tomato, causing bacterial canker disease. In this study, we sequenced and assembled genomes of 11 C. michiganensis subsp. michiganensis strains isolated from infected tomato fields in California as well as five Clavibacter strains that colonize tomato endophytically but are not pathogenic in this host. The analysis of the C. michiganensis subsp. michiganensis genomes supported the monophyletic nature of this pathogen but revealed genetic diversity among strains, consistent with multiple introduction events. Two tomato endophytes that clustered phylogenetically with C. michiganensis strains capable of infecting wheat and pepper and were also able to cause disease in these plants. Plasmid profiles of the California strains were variable and supported the essential role of the pCM1-like plasmid and the CelA cellulase in virulence, whereas the absence of the pCM2-like plasmid in some pathogenic C. michiganensis subsp. michiganensis strains revealed it is not essential. A large number of secreted C. michiganensis subsp. michiganensis proteins were carbohydrate-active enzymes (CAZymes). Glycome profiling revealed that C. michiganensis subsp. michiganensis but not endophytic Clavibacter strains is able to extensively alter tomato cell-wall composition. Two secreted CAZymes found in all C. michiganensis subsp. michiganensis strains, CelA and PelA1, enhanced pathogenicity on tomato. Collectively, these results provide a deeper understanding of C. michiganensis subsp. michiganensis diversity and virulence strategies.

Comparative genomics of Clavibacter michiganensis subspecies, pathogens of important agricultural crops

Subspecies of Clavibacter michiganensis are important phytobacterial pathogens causing devastating diseases in several agricultural crops. The genome organizations of these pathogens are poorly understood. Here, the complete genomes of 5 subspecies (C. michiganensis subsp. michiganensis, Cmi; C. michiganensis subsp. sepedonicus, Cms; C. michiganensis subsp. nebraskensis, Cmn; C. michiganensis subsp. insidiosus, Cmi and C. michiganensis subsp. capsici, Cmc) were analyzed. This study assessed the taxonomic position of the subspecies based on 16S rRNA and genome-based DNA homology and concludes that there is ample evidence to elevate some of the subspecies to species-level. Comparative genomics analysis indicated distinct genomic features evident on the DNA structural atlases and annotation features. Based on orthologous gene analysis, about 2300 CDSs are shared across all the subspecies; and Cms showed the highest number of subspecies-specific CDS, most of which are mobile elements suggesting that Cms could be more prone to translocation of foreign genes. Cms and Cmi had the highest number of pseudogenes, an indication of potential degenerating genomes. The stress response factors that may be involved in cold/heat shock, detoxification, oxidative stress, osmoregulation, and carbon utilization are outlined. For example, the wco-cluster encoding for extracellular polysaccharide II is highly conserved while the sucrose-6-phosphate hydrolase that catalyzes the hydrolysis of sucrose-6-phosphate yielding glucose-6-phosphate and fructose is highly divergent. A unique second form of the enzyme is only present in Cmn NCPPB 2581. Also, twenty-eight plasmid-borne CDSs in the other subspecies were found to have homologues in the chromosomal genome of Cmn which is known not to carry plasmids. These CDSs include pathogenesis-related factors such as Endocellulases E1 and Beta-glucosidase. The results presented here provide an insight of the functional organization of the genomes of five core C. michiganensis subspecies, enabling a better understanding of these phytobacteria.

Genome Analysis and Development of a Multiplex TaqMan Real-Time PCR for Specific Identification and Detection of Clavibacter michiganensis subsp. nebraskensis

The reemergence of the Goss's bacterial wilt and blight disease in corn in the United States and Canada has prompted investigative research to better understand the genome organization. In this study, we generated a draft genome sequence of Clavibacter michiganensis subsp. nebraskensis strain DOAB 395 and performed genome and proteome analysis of C. michiganensis subsp. nebraskensis strains isolated in 2014 (DOAB 397 and DOAB 395) compared with the type strain, NCPPB 2581 (isolated over 40 years ago). The proteomes of strains DOAB 395 and DOAB 397 exhibited a 99.2% homology but had 92.1 and 91.8% homology, respectively, with strain NCPPB 2581. The majority (99.9%) of the protein sequences had a 99.6 to 100% homology between C. michiganensis subsp. nebraskensis strains DOAB 395 and DOAB 397, with only four protein sequences (0.1%) exhibiting a similarity <70%. In contrast, 3.0% of the protein sequences of strain DOAB 395 or DOAB 397 showed low homologies (<70%) with the type strain NCPPB 2581. The genome data were exploited for the development of a multiplex TaqMan real-time polymerase chain reaction (PCR) tool for rapid detection of C. michiganensis subsp. nebraskensis. The specificity of the assay was validated using 122 strains of Clavibacter and non-Clavibacter spp. A blind test and naturally infected leaf samples were used to confirm specificity. The sensitivity (0.1 to 1.0 pg) compared favorably with previously reported real-time PCR assays. This tool should fill the current gap for a reliable diagnostic technique.

(Meta)genomic insights into the pathogenome of Cellulosimicrobium cellulans

Despite having serious clinical manifestations, Cellulosimicrobium cellulans remain under-reported with only three genome sequences available at the time of writing. Genome sequences of C. cellulans LMG16121, C. cellulans J36 and Cellulosimicrobium sp. strain MM were used to determine distribution of pathogenicity islands (PAIs) across C. cellulans, which revealed 49 potential marker genes with known association to human infections, e.g. Fic and VbhA toxin-antitoxin system. Oligonucleotide composition-based analysis of orthologous proteins (n = 791) across three genomes revealed significant negative correlation (P < 0.05) between frequency of optimal codons (F_opt) and gene G+C content, highlighting the G+C-biased gene conversion (gBGC) effect across Cellulosimicrobium strains. Bayesian molecular-clock analysis performed on three virulent PAI proteins (Fic; D-alanyl-D-alanine-carboxypeptidase; transposase) dated the divergence event at 300 million years ago from the most common recent ancestor. Synteny-based annotation of hypothetical proteins highlighted gene transfers from non-pathogenic bacteria as a key factor in the evolution of PAIs. Additonally, deciphering the metagenomic islands using strain MM’s genome with environmental data from the site of isolation (hot-spring biofilm) revealed (an)aerobic respiration as population segregation factor across the in situ cohorts. Using reference genomes and metagenomic data, our results highlight the emergence and evolution of PAIs in the genus Cellulosimicrobium.

Characterization and Comparison of Clavibacter michiganensis subsp. nebraskensis Strains Recovered from Epiphytic and Symptomatic Infections of Maize in Iowa

Clavibacter michiganensis subsp. nebraskensis (Cmn), the causal organism of Goss’s wilt and leaf blight of maize, can be detected in the phyllosphere of its host prior to disease development. We compared the morphology and pathogenicity of 37 putative isolates of Cmn recovered from asymptomatic and symptomatic maize leaves. Thirty-three of the isolates produced mucoid orange colonies, irrespective of the source of isolation and all but four of these isolates were pathogenic on maize. The remaining 4 isolates recovered from asymptomatic leaves had large fluidal yellow colonies, and were non-pathogenic on maize. Isolates varied in their aggressiveness on a susceptible hybrid of maize but no significant differences in aggressiveness were detected between epiphytic isolates and those recovered from diseased maize tissues. The genomics of Cmn is poorly understood; therefore as a first step to determining what genes may play a role in virulence, we compared 33 putative virulence gene sequences from 6 pathogenic and a non-pathogenic isolate recovered from the phyllosphere. Sequence polymorphisms were detected in 5 genes, cellulase A, two endoglucanases, xylanase B and a pectate lyase but there was no relationship with pathogenicity. Further research is needed to determine what genes play a role in virulence of Cmn. Our data show however, that the virulence factors in Cmn likely differ from those reported for the closely related subspecies michiganensis and sepedonicus.

A Proteomic Study of Clavibacter Michiganensis Subsp. Michiganensis Culture Supernatants

Clavibacter michiganensis, subsp. michiganensis is a Gram-positive plant pathogen infecting tomato (Solanum lycopersicum). Despite a considerable economic importance due to significant losses of infected plants and fruits, knowledge about virulence factors of C. michiganensis subsp. michiganensis and host-pathogen interactions on a molecular level are rather limited. In the study presented here, the proteome of culture supernatants from C. michiganensis subsp. michiganensis NCPPB382 was analyzed. In total, 1872 proteins were identified in M9 and 1766 proteins in xylem mimicking medium. Filtration of supernatants before protein precipitation reduced these to 1276 proteins in M9 and 976 proteins in the xylem mimicking medium culture filtrate. The results obtained indicate that C. michiganensis subsp. michiganensis reacts to a sucrose- and glucose-depleted medium similar to the xylem sap by utilizing amino acids and host cell polymers as well as their degradation products, mainly peptides, amino acids and various C5 and C6 sugars. Interestingly, the bacterium expresses the previously described virulence factors Pat-1 and CelA not exclusively after host cell contact in planta but already in M9 minimal and xylem mimicking medium.

Putative Serine Protease Effectors of Clavibacter michiganensis Induce a Hypersensitive Response in the Apoplast of Nicotiana Species

Clavibacter michiganensis subspp. michiganensis and sepedonicus cause diseases on solanaceous crops. The genomes of both subspecies encode members of the pat-1 family of putative serine proteases known to function in virulence on host plants and induction of hypersensitive responses (HR) on nonhosts. One gene of this family in C. michiganensis subsp. sepedonicus, chp-7, is required for triggering HR in Nicotiana tabacum. Here, further investigation revealed that mutation of the putative catalytic serine residue at position 232 to threonine abolished the HR induction activity of Chp-7, suggesting that enzymatic activity is required. Purified Chp-7 triggered an HR in N. tabacum leaves in the absence of the pathogen, indicating Chp-7 itself is the HR elicitor from C. michiganensis subsp. sepedonicus. Ectopic expression of chp-7 constructs in N. tabacum leaves revealed that Chp-7 targeted to the apoplast triggered an HR while cytoplasmic Chp-7 did not, indicating that Chp-7 induces the HR in the apoplast of N. tabacum leaves. Chp-7 also induced HR in N. sylvestris, a progenitor of N. tabacum, but not in other Nicotiana species tested. ChpG, a related protein from C. michiganensis subsp. michiganensis, also triggered HR in N. tabacum and N. sylvestris. Unlike Chp-7, ChpG triggered HR in N. clevelandii and N. glutinosa.

Validation and Application of a Real-time PCR Protocol for the Specific Detection and Quantification of Clavibacter michiganensis subsp. sepedonicus in Potato

Clavibacter michiganensis subsp. sepedonicus (Cms) multiplies very rapidly, passing through the vascular strands and into the stems and petioles of a diseased potato. Therefore, the rapid and specific detection of this pathogen is highly important for the effective control of the pathogen. Although several PCR assays have been developed for detection, they cannot afford specific detection of Cms. Therefore, in this study, a computational genome analysis was performed to compare the sequenced genomes of the C. michiganensis subspecies and to identify an appropriate gene for the development of a subspecies-specific PCR primer set (Cms89F/R). The specificity of the primer set based on the putative phage-related protein was evaluated using genomic DNA from seven isolates of Cms and 27 other reference strains. The Cms89F/R primer set was more specific and sensitive than the existing assays in detecting Cms in in vitro using Cms cells and its genomic DNA. This assay was also able to detect at least 1.47×10(2) copies/μl of cloned-amplified target DNA, 5 fg of DNA using genomic DNA or 10(-6) dilution point of 0.12 at OD600 units of cells per reaction using a calibrated cell suspension.

Complete Genome Sequence of Clavibacter michiganensis subsp. insidiosus R1-1 Using PacBio Single-Molecule Real-Time Technology

We report here the complete genome sequence of Clavibacter michiganensis subsp. insidiosus R1-1, isolated in Minnesota, USA. The R1-1 genome, generated by a de novo assembly of PacBio sequencing data, is the first complete genome sequence available for this subspecies.

Characterizing the Genetic Diversity of the Clavibacter michiganensis subsp. michiganensis Population in New York

New York Clavibacter michiganensis subsp. michiganensis isolates, collected from disparate bacterial canker of tomato outbreaks over the past 11 years, were characterized with a multilocus sequence analysis (MLSA) scheme that differentiated the 51 isolates into 21 haplotypes with a discriminatory power of 0.944. The MLSA scheme consisted of five housekeeping genes (kdpA, sdhA, dnaA, ligA, and gyrB) and three putative pathogenicity genes (celA, tomA, and nagA). Repetitive polymerase chain reaction (PCR), with the BOX-A1R primer, confirmed the high diversity of C. michiganensis subsp. michiganensis isolates in New York by demonstrating that all six PCR patterns (A, B, 13C, 65C, 81C, and D) were present, with PCR patterns C and A being the most common. The MLSA scheme provided higher resolving power than the current repetitive-PCR approach. The plasmid profiles of New York isolates were diverse and differed from reference strain NCPPB382. PCR analysis indicated that the presence of putative pathogenicity genes varied between isolates and highlighted the ephemeral nature of pathogenicity genes in field populations of C. michiganensis subsp. michiganensis. Analysis of molecular variance between Serbian and New York C. michiganensis subsp. michiganensis isolates demonstrated that the two populations were not significantly different, with 98% genetic variation within each population and only 2% genetic variation between populations.

Comparative genome analysis of pathogenic and non-pathogenic Clavibacter strains reveals adaptations to their lifestyle

BACKGROUND

The genus Clavibacter harbors economically important plant pathogens infecting agricultural crops such as potato and tomato. Although the vast majority of Clavibacter strains are pathogenic, there is an increasing number of non-pathogenic isolates reported. Non-pathogenic Clavibacter strains isolated from tomato seeds are particularly problematic because they affect the current detection and identification tests for Clavibacter michiganensis subsp. michiganensis (Cmm), which is regulated with a zero tolerance in tomato seed. Their misidentification as pathogenic Cmm hampers a clear judgment on the seed quality and health.

RESULTS

To get more insight in the genetic features linked to the lifestyle of these bacteria, a whole-genome sequence of the tomato seed-borne non-pathogenic Clavibacter LMG 26808 was determined. To gain a better understanding of the molecular determinants of pathogenicity, the genome sequence of LMG 26808 was compared with that of the pathogenic Cmm strain (NCPPB 382). The comparative analysis revealed that LMG 26808 does not contain plasmids pCM1 and pCM2 and also lacks the majority of important virulence factors described so far for pathogenic Cmm. This explains its apparent non-pathogenic nature in tomato plants. Moreover, the genome analysis of LMG 26808 detected sequences from a plasmid originating from a member of Enterobacteriaceae/Klebsiella relative. Genes received that way and coding for antibiotic resistance may provide a competitive advantage for survival of LMG 26808 in its ecological niche. Genetically, LMG 26808 was the most similar to the pathogenic Cmm NCPPB 382 but contained more mobile genetic elements. The genome of this non-pathogenic Clavibacter strain contained also a high number of transporters and regulatory genes.

CONCLUSIONS

The genome sequence of the non-pathogenic Clavibacter strain LMG 26808 and the comparative analyses with other pathogenic Clavibacter strains provided a better understanding of the genetic bases of virulence and adaptation mechanisms present in the genus Clavibacter.

Complete Genome of Clavibacter michiganensis subsp. sepedonicusis Siphophage CN1A

Clavibacter michiganensis subsp. sepedonicusis is a Gram-positive actinomycete that is the causative agent of the potato disease ring rot. Here, we announce the complete genome sequence of the Clavibacter michiganensis subsp. sepedonicusis siphophage CN1A. CN1A is only the second fully sequenced Clavibacter michiganensis subsp. sepedonicusis phage reported to date. Core and unique features of its genome are described.

Genome-wide transcriptome analysis of Clavibacter michiganensis subsp. michiganensis grown in xylem mimicking medium

The interaction between Clavibacter michiganensis subsp. michiganensis with its host, the tomato plant (Solanum lycopersicum), is poorly understood and only few virulence factors are known. While studying of the bacteria in planta is time-consuming and difficult, the analysis in vitro would facilitate research. Therefore, a xylem mimicking medium (XMM) for C. michiganensis subsp. michiganensis was established in this study based on an apoplast medium for Xanthomonas campestris pv. vesicatoria. In contrast to the apoplast medium, XMM contains no sugars, but amino acids which serve as nitrogen and carbon source. As a result, growth in XMM induced transcriptional changes of genes encoding putative sugar, amino acid and iron uptake systems. In summary, mRNA levels of about 8% of all C. michiganensis subsp. michiganensis genes were changed when XMM-grown bacteria were compared to M9 minimal medium-grown cells. Almost no transcriptional changes of genes encoding hydrolytic enzymes were detected, leading to the idea that XMM reflects the situation in the beginning of infection and therefore allows the characterization of virulence factors in this early stage of infection. The addition of the plant wound substance acetosyringone to the XMM medium led to a change in transcript amount, including genes coding for proteins involved in protein transport, iron uptake and regulation processes.

Engineering unnatural variants of plantazolicin through codon reprogramming

Plantazolicin (PZN) is a polyheterocyclic natural product derived from a ribosomal peptide that harbors remarkable antibiotic selectivity for the causative agent of anthrax, Bacillus anthracis. To simultaneously establish the structure-activity relationship of PZN and the substrate tolerance of the biosynthetic pathway, an Escherichia coli expression strain was engineered to heterologously produce PZN analogues. Variant PZN precursor genes were produced by site-directed mutagenesis and later screened by mass spectrometry to assess post-translational modification and export by E. coli. From a screen of 72 precursor peptides, 29 PZN variants were detected. This analogue collection provided insight into the selectivity of the post-translational modifying enzymes and established the boundaries of the natural biosynthetic pathway. Unlike other studied thiazole/oxazole-modified microcins, the biosynthetic machinery appeared to be finely tuned toward the production of PZN, such that the cognate enzymes did not process even other naturally occurring sequences from similar biosynthetic clusters. The modifying enzymes were exquisitely selective, installing heterocycles only at predefined positions within the precursor peptides while leaving neighboring residues unmodified. Nearly all substitutions at positions normally harboring heterocycles prevented maturation of a PZN variant, though some exceptions were successfully produced lacking a heterocycle at the penultimate residue. No variants containing additional heterocycles were detected, although several peptide sequences yielded multiple PZN variants as a result of varying oxidation states of select residues. Eleven PZN variants were produced in sufficient quantity to facilitate purification and assessment of their antibacterial activity, providing insight into the structure-activity relationship of PZN.

The role of prophage in plant-pathogenic bacteria

A diverse set of phage lineages is associated with the bacterial plant-pathogen genomes sequenced to date. Analysis of 37 genomes revealed 5,169 potential genes (approximately 4.3 Mbp) of phage origin, and at least 50% had no function assigned or are nonessential to phage biology. Some phytopathogens have transcriptionally active prophage genes under conditions that mimic plant infection, suggesting an association between plant disease and prophage transcriptional modulation. The role of prophages within genomes for cell biology varies. For pathogens such as Pectobacterium, Pseudomonas, Ralstonia, and Streptomyces, involvement of prophage in disease symptoms has been demonstrated. In Xylella and Xanthomonas, prophage activity is associated with genome rearrangements and strain differentiation. For other pathogens, prophage roles are yet to be established. This review integrates available information in a unique interface ( http://propnav.esalq.usp.br ) that may be assessed to improve research in prophage biology and its association with genome evolution and pathogenicity.

Genome-enabled perspectives on the composition, evolution, and expression of virulence determinants in bacterial plant pathogens

Genome sequence analyses of bacterial plant pathogens are revealing important insights into the molecular determinants of pathogenicity and, through transcript characterization, responses to environmental conditions, evidence for small RNAs, and validation of uncharacterized genes. Genome comparison sheds further light on the processes impacting pathogen evolution and differences in gene repertoire among isolates contributing to niche specialization. Information derived from pathogen genome analysis is providing tools for use in diagnosis and interference with host-pathogen interactions for the purpose of disease control. However, the existing information infrastructure fails to adequately integrate the increasing numbers of sequence data sets, bioinformatic analyses, and experimental characterization, as required for effective systems-level analysis. Enhanced standardization of data formats at the point of publication is proposed as a possible solution.

pFiD188, the linear virulence plasmid of Rhodococcus fascians D188

Rhodococcus fascians is currently the only phytopathogen of which the virulence genes occur on a linear plasmid. To get insight into the origin of this replicon and into the virulence strategy of this broad-spectrum phytopathogen, the sequence of the linear plasmid of strain D188, pFiD188, was determined. Analysis of the 198,917 bp revealed four syntenic regions with linear plasmids of R. erythropolis, R. jostii, and R. opacus, suggesting a common origin of these replicons. Mutational analysis of pFi_086 and pFi_102, similar to cutinases and type IV peptidases, respectively, showed that conserved region R2 was involved in plasmid dispersal and pointed toward a novel function for actinobacterial cutinases in conjugation. Additionally, pFiD188 had three regions that were unique for R. fascians. Functional analysis of the stk and nrp loci of regions U2 and U3, respectively, indicated that their role in symptom development was limited compared with that of the previously identified fas, att, and hyp virulence loci situated in region U1. Thus, pFiD188 is a typical rhodococcal linear plasmid with a composite structure that encodes core functions involved in plasmid maintenance and accessory functions, some possibly acquired through horizontal gene transfer, implicated in virulence and the interaction with the host.

Phylogenetic framework and molecular signatures for the main clades of the phylum Actinobacteria

The phylum Actinobacteria harbors many important human pathogens and also provides one of the richest sources of natural products, including numerous antibiotics and other compounds of biotechnological interest. Thus, a reliable phylogeny of this large phylum and the means to accurately identify its different constituent groups are of much interest. Detailed phylogenetic and comparative analyses of >150 actinobacterial genomes reported here form the basis for achieving these objectives. In phylogenetic trees based upon 35 conserved proteins, most of the main groups of Actinobacteria as well as a number of their superageneric clades are resolved. We also describe large numbers of molecular markers consisting of conserved signature indels in protein sequences and whole proteins that are specific for either all Actinobacteria or their different clades (viz., orders, families, genera, and subgenera) at various taxonomic levels. These signatures independently support the existence of different phylogenetic clades, and based upon them, it is now possible to delimit the phylum Actinobacteria (excluding Coriobacteriia) and most of its major groups in clear molecular terms. The species distribution patterns of these markers also provide important information regarding the interrelationships among different main orders of Actinobacteria. The identified molecular markers, in addition to enabling the development of a stable and reliable phylogenetic framework for this phylum, also provide novel and powerful means for the identification of different groups of Actinobacteria in diverse environments. Genetic and biochemical studies on these Actinobacteria-specific markers should lead to the discovery of novel biochemical and/or other properties that are unique to different groups of Actinobacteria.

Structure determination and interception of biosynthetic intermediates for the plantazolicin class of highly discriminating antibiotics

The soil-dwelling, plant growth-promoting bacterium Bacillus amyloliquefaciens FZB42 is a prolific producer of complex natural products. Recently, a new FZB42 metabolite, plantazolicin (PZN), has been described as a member of the growing thiazole/oxazole-modified microcin (TOMM) family. TOMMs are biosynthesized from inactive, ribosomal peptides and undergo a series of cyclodehydrations, dehydrogenations, and other modifications to become bioactive natural products. Using high-resolution mass spectrometry, chemoselective modification, genetic interruptions, and other spectroscopic tools, we have determined the molecular structure of PZN. In addition to two conjugated polyazole moieties, the amino-terminus of PZN has been modified to N(α),N(α)-dimethylarginine. PZN exhibited a highly selective antibiotic activity toward Bacillus anthracis, but no other tested human pathogen. By altering oxygenation levels during fermentation, PZN analogues were produced that bear variability in their heterocycle content, which yielded insight into the order of biosynthetic events. Lastly, genome-mining has revealed the existence of four additional PZN-like biosynthetic gene clusters. Given their structural uniqueness and intriguing antimicrobial specificity, the PZN class of antibiotics may hold pharmacological value.

Cloning and characterization of a novel cellobiase gene, cba3, encoding the first known β-glucosidase of glycoside hydrolase family 1 of Cellulomonas biazotea

A novel cellobiase gene, designated cba3, was cloned from Cellulomonas biazotea. Although cellobiase genes of C. biazotea were previously cloned, published and/or patented, they encoded β-glucosidases all belonging to glycoside hydrolase family 3 (GH3); the new Cba3 cellobiase was identified to be a glycoside hydrolase family 1 (GH1) member, which represents the first discovered GH1 β-glucosidase of C. biazotea. Escherichia coli transformants expressing recombinant Cba3 were shown to grow readily in minimal media using cellobiose as the sole carbon source, supporting the conclusion that Cba3 is a genuine cellobiase. The full-length cba3 gene was revealed by sequencing to be 1344 bp long. Cba3 deletants lacking either the N-terminal 10 amino acids or the C-terminal 10 residues were found to be biologically inactive, supporting the importance of both ends in catalysis. Like other GH1 β-glucosidases, Cba3 was shown to contain the highly conserved NEP and ENG motifs, which are crucial for enzymatic activity. Despite lacking a classical N-terminal signal peptide, Cba3 was demonstrated to be a secretory protein. The findings that Cba3 is a cellobiase, and that it was expressed well as an extracellular protein in E. coli, support the potential of Cba3 for use with other cellulases in the hydrolysis of cellulosic biomass.

Cow teat skin, a potential source of diverse microbial populations for cheese production

The diversity of the microbial community on cow teat skin was evaluated using a culture-dependent method based on the use of different dairy-specific media, followed by the identification of isolates by 16S rRNA gene sequencing. This was combined with a direct molecular approach by cloning and 16S rRNA gene sequencing. This study highlighted the large diversity of the bacterial community that may be found on teat skin, where 79.8% of clones corresponded to various unidentified species as well as 66 identified species, mainly belonging to those commonly found in raw milk (Enterococcus, Pediococcus, Enterobacter, Pantoea, Aerococcus, and Staphylococcus). Several of them, such as nonstarter lactic acid bacteria (NSLAB), Staphylococcus, and Actinobacteria, may contribute to the development of the sensory characteristics of cheese during ripening. Therefore, teat skin could be an interesting source or vector of biodiversity for milk. Variations of microbial counts and diversity between the farms studied have been observed. Moreover, Staphylococcus auricularis, Staphylococcus devriesei, Staphylococcus arlettae, Streptococcus bovis, Streptococcus equinus, Clavibacter michiganensis, Coprococcus catus, or Arthrobacter gandavensis commensal bacteria of teat skin and teat canal, as well as human skin, are not common in milk, suggesting that there is a breakdown of microbial flow from animal to milk. It would then be interesting to thoroughly study this microbial flow from teat to milk.

The Jackprot Simulation Couples Mutation Rate with Natural Selection to Illustrate How Protein Evolution Is Not Random

Protein evolution is not a random process. Views which attribute randomness to molecular change, deleterious nature to single-gene mutations, insufficient geological time, or population size for molecular improvements to occur, or invoke "design creationism" to account for complexity in molecular structures and biological processes, are unfounded. Scientific evidence suggests that natural selection tinkers with molecular improvements by retaining adaptive peptide sequence. We used slot-machine probabilities and ion channels to show biological directionality on molecular change. Because ion channels reside in the lipid bilayer of cell membranes, their residue location must be in balance with the membrane's hydrophobic/philic nature; a selective "pore" for ion passage is located within the hydrophobic region. We contrasted the random generation of DNA sequence for KcsA, a bacterial two-transmembrane-domain (2TM) potassium channel, from Streptomyces lividans, with an under-selection scenario, the "jackprot," which predicted much faster evolution than by chance. We wrote a computer program in JAVA APPLET version 1.0 and designed an online interface, The Jackprot Simulation http://faculty.rwu.edu/cbai/JackprotSimulation.htm, to model a numerical interaction between mutation rate and natural selection during a scenario of polypeptide evolution. Winning the "jackprot," or highest-fitness complete-peptide sequence, required cumulative smaller "wins" (rewarded by selection) at the first, second, and third positions in each of the 161 KcsA codons ("jackdons" that led to "jackacids" that led to the "jackprot"). The "jackprot" is a didactic tool to demonstrate how mutation rate coupled with natural selection suffices to explain the evolution of specialized proteins, such as the complex six-transmembrane (6TM) domain potassium, sodium, or calcium channels. Ancestral DNA sequences coding for 2TM-like proteins underwent nucleotide "edition" and gene duplications to generate the 6TMs. Ion channels are essential to the physiology of neurons, ganglia, and brains, and were crucial to the evolutionary advent of consciousness. The Jackprot Simulation illustrates in a computer model that evolution is not and cannot be a random process as conceived by design creationists.

The plant pathogen Pseudomonas syringae pv. tomato is genetically monomorphic and under strong selection to evade tomato immunity

Recently, genome sequencing of many isolates of genetically monomorphic bacterial human pathogens has given new insights into pathogen microevolution and phylogeography. Here, we report a genome-based micro-evolutionary study of a bacterial plant pathogen, Pseudomonas syringae pv. tomato. Only 267 mutations were identified between five sequenced isolates in 3,543,009 nt of analyzed genome sequence, which suggests a recent evolutionary origin of this pathogen. Further analysis with genome-derived markers of 89 world-wide isolates showed that several genotypes exist in North America and in Europe indicating frequent pathogen movement between these world regions. Genome-derived markers and molecular analyses of key pathogen loci important for virulence and motility both suggest ongoing adaptation to the tomato host. A mutational hotspot was found in the type III-secreted effector gene hopM1. These mutations abolish the cell death triggering activity of the full-length protein indicating strong selection for loss of function of this effector, which was previously considered a virulence factor. Two non-synonymous mutations in the flagellin-encoding gene fliC allowed identifying a new microbe associated molecular pattern (MAMP) in a region distinct from the known MAMP flg22. Interestingly, the ancestral allele of this MAMP induces a stronger tomato immune response than the derived alleles. The ancestral allele has largely disappeared from today's Pto populations suggesting that flagellin-triggered immunity limits pathogen fitness even in highly virulent pathogens. An additional non-synonymous mutation was identified in flg22 in South American isolates. Therefore, MAMPs are more variable than expected differing even between otherwise almost identical isolates of the same pathogen strain.

Genetic diversity and population structure of Clavibacter michiganensis subsp. nebraskensis

Clavibacter michiganensis subsp. nebraskensis (CMN) is a gram-positive bacterium and an incitant of Goss's bacterial wilt and leaf blight or "leaf freckles" in corn. A population structure of a wide temporal and geographic collection of CMN strains (n = 131), originating between 1969 and 2009, was determined using amplified fragment length polymorphism (AFLP) analysis and repetitive DNA sequence-based BOX-PCR. Analysis of the composite data set of AFLP and BOX-PCR fingerprints revealed two groups with a 60% cutoff similarity: a major group A (n = 118 strains) and a minor group B (n = 13 strains). The clustering in both groups was not correlated with strain pathogenicity. Group A contained two clusters, A1 (n = 78) and A2 (n = 40), with a linkage of 75%. Group A strains did not show any correlation with historical, geographical, morphological, or physiological properties of the strains. Group B was very heterogeneous and eight out of nine clusters were represented by a single strain. The mean similarity between clusters in group B varied from 13% to 63%. All strains in group B were isolated after 1999. The percentage of group B strains among all strains isolated after 1999 (n = 69) was 18.8%. Implications of the findings are discussed.