Genetic Diversity and Distribution of Korean Isolates of Ralstonia solanacearum

Genetic Diversity of Ralstonia solanacearum Causing Tobacco Bacterial Wilt in Fujian Province and Identification of Biocontrol Streptomyces sp

Tobacco bacterial wilt is a highly destructive soilborne disease caused by the Ralstonia solanacearum species complex, exhibiting a significant risk to global flue-cured tobacco cultivation and resulting in substantial economic loss. In this study, 77 isolates were collected from three prominent flue-cured tobacco cultivation areas in Fujian, China (Nanping, Sanming, and Longyan), in 2021 and 2022. The isolated strains were classified through phylotype-specific multiplex polymerase chain reaction (Pmx-PCR) and physiological tests. The analysis showed that all the strains were associated with phylotype I, race 1, and biovar III. Subsequent phylogenetic analysis using partial egl gene sequences classified the 77 isolates into 5 distinct sequevars: 13, 15, 16, 17, and 34. Notably, a remarkable predominance of sequevar 15 was observed in Fujian Province, while sequevar 16 was first reported on tobacco in China, which was identified in other plants, expanding the understanding of its host range and distribution in the country. In addition, a Streptomyces strain extracted from the rhizosphere soil of tobacco was found to inhibit the growth of multiple sequevars of tobacco R. solanacearum, indicating its broad-spectrum antagonistic properties. Furthermore, pot experiments showed that the strain St35 effectively controlled tobacco bacterial wilt. The isolate St35 was conclusively identified as Streptomyces gancidicus according to the morphological and genetic features. In summary, the present study demonstrated the genetic diversity and distribution of tobacco R. solanacearum strains in the Fujian province of China, as well as the identification of a candidate biological control agent for the management of tobacco bacterial wilt.

Ralstonia solanacearum pandemic lineage strain UW551 overcomes inhibitory xylem chemistry to break tomato bacterial wilt resistance

Plant-pathogenic Ralstonia strains cause bacterial wilt disease by colonizing xylem vessels of many crops, including tomato. Host resistance is the best control for bacterial wilt, but resistance mechanisms of the widely used Hawaii 7996 tomato breeding line (H7996) are unknown. Using growth in ex vivo xylem sap as a proxy for host xylem, we found that Ralstonia strain GMI1000 grows in sap from both healthy plants and Ralstonia-infected susceptible plants. However, sap from Ralstonia-infected H7996 plants inhibited Ralstonia growth, suggesting that in response to Ralstonia infection, resistant plants increase inhibitors in their xylem sap. Consistent with this, reciprocal grafting and defence gene expression experiments indicated that H7996 wilt resistance acts in both above- and belowground plant parts. Concerningly, H7996 resistance is broken by Ralstonia strain UW551 of the pandemic lineage that threatens highland tropical agriculture. Unlike other Ralstonia, UW551 grew well in sap from Ralstonia-infected H7996 plants. Moreover, other Ralstonia strains could grow in sap from H7996 plants previously infected by UW551. Thus, UW551 overcomes H7996 resistance in part by detoxifying inhibitors in xylem sap. Testing a panel of xylem sap compounds identified by metabolomics revealed that no single chemical differentially inhibits Ralstonia strains that cannot infect H7996. However, sap from Ralstonia-infected H7996 contained more phenolic compounds, which are known to be involved in plant antimicrobial defence. Culturing UW551 in this sap reduced total phenolic levels, indicating that the resistance-breaking Ralstonia strain degrades these chemical defences. Together, these results suggest that H7996 tomato wilt resistance depends in part on inducible phenolic compounds in xylem sap.

A Mutation of a Putative NDP-Sugar Epimerase Gene in Ralstonia pseudosolanacearum Attenuates Exopolysaccharide Production and Bacterial Virulence in Tomato Plant

Ralstonia solanacearum species complex (RSSC) is a soil borne plant pathogen causing bacterial wilt on various important crops, including Solanaceae plants. The bacterial pathogens within the RSSC produce exopolysaccharide (EPS), a highly complicated nitrogen-containing heteropolymeric polysaccharide, as a major virulence factor. However, the biosynthetic pathway of the EPS in the RSSC has not been fully characterized. To identify genes in EPS production beyond the EPS biosynthetic gene operon, we selected the EPS-defective mutants of R. pseudosolanacearum strain SL341 from Tn5-inserted mutant pool. Among several EPS-defective mutants, we identified a mutant, SL341P4, with a Tn5-insertion in a gene encoding a putative NDP-sugar epimerase, a putative membrane protein with sugar-modifying moiety, in a reverse orientation to EPS biosynthesis gene cluster. This protein showed similar to other NDP-sugar epimerases involved in EPS biosynthesis in many phytopathogens. Mutation of the NDP-sugar epimerase gene reduced EPS production and biofilm formation in R. pseudosolanacearum. Additionally, the SL341P4 mutant exhibited reduced disease severity and incidence of bacterial wilt in tomato plants compared to the wild-type SL341 without alteration of bacterial multiplication. These results indicate that the NDP-sugar epimerase gene is required for EPS production and bacterial virulence in R. pseudosolanacearum.

QTL Mapping of Resistance to Bacterial Wilt in Pepper Plants (Capsicum annuum) Using Genotyping-by-Sequencing (GBS)

Bacterial wilt (BW) disease, which is caused by Ralstonia solanacearum, is one globally prevalent plant disease leading to significant losses of crop production and yield with the involvement of a diverse variety of monocot and dicot host plants. In particular, the BW of the soil-borne disease seriously influences solanaceous crops, including peppers (sweet and chili peppers), paprika, tomatoes, potatoes, and eggplants. Recent studies have explored genetic regions that are associated with BW resistance for pepper crops. However, owing to the complexity of BW resistance, the identification of the genomic regions controlling BW resistance is poorly understood and still remains to be unraveled in the pepper cultivars. In this study, we performed the quantitative trait loci (QTL) analysis to identify genomic loci and alleles, which play a critical role in the resistance to BW in pepper plants. The disease symptoms and resistance levels for BW were assessed by inoculation with R. solanacearum. Genotyping-by-sequencing (GBS) was utilized in 94 F2 segregating populations originated from a cross between a resistant line, KC352, and a susceptible line, 14F6002-14. A total of 628,437 single-nucleotide polymorphism (SNP) was obtained, and a pepper genetic linkage map was constructed with putative 1550 SNP markers via the filtering criteria. The linkage map exhibited 16 linkage groups (LG) with a total linkage distance of 828.449 cM. Notably, QTL analysis with CIM (composite interval mapping) method uncovered pBWR-1 QTL underlying on chromosome 01 and explained 20.13 to 25.16% by R2 (proportion of explained phenotyphic variance by the QTL) values. These results will be valuable for developing SNP markers associated with BW-resistant QTLs as well as for developing elite BW-resistant cultivars in pepper breeding programs.

Genetic Diversity and Distribution of Korean Isolates of Burkholderia glumae

Burkholderia glumae causes panicle blight of rice (grain rot in Japan and Korea), and the severity of damage is increasing worldwide. During 2017 and 2018, 137 isolates of B. glumae were isolated from symptomatic grain rot of rice cultivated in paddy fields throughout South Korea. Genetic diversity of the isolates was determined using transposase-based PCR (Tnp-PCR) genomic fingerprinting. All 138 isolates, including the B. glumae BGR1 strain, produced toxoflavin in various amounts, and 17 isolates produced an unidentified purple or orange pigment on Luria-Bertani medium and casamino acid-peptone-glucose medium, respectively, at 28°C. Transposase-based PCR genomic fingerprinting was performed using a novel primer designed based on transposase (tnp) gene sequences located at the ends of the toxoflavin efflux transporter operon; this method provided reliable and reproducible results. Through Tnp-PCR genomic fingerprinting, the genetic groups of Korean B. glumae isolates were divided into 11 clusters and three divisions. The Korean B. glumae isolates were mainly grouped in division I (73%). Interestingly, most of the pigment-producing isolates were grouped in divisions II and III; of these, 10 were grouped in cluster VIII, which comprised 67% of this cluster. Results of a phylogenetic analysis based on tofI and hrpB gene sequences were consistent with classification by Tnp-PCR genomic fingerprinting. The BGR1 strain did not belong to any of the clusters, indicating that this strain does not exhibit the typical genetic representation of B. glumae. B. glumae isolates showed diversity in the use of carbon and nitrogen sources, but no correlation with genetic classification by PCR fingerprinting was found. This is the first study to analyze the geographical distribution and genetic diversity of Korean B. glumae isolates.

Leaf-to-Whole Plant Spread Bioassay for Pepper and Ralstonia solanacearum Interaction Determines Inheritance of Resistance to Bacterial Wilt for Further Breeding

Bacterial wilt (BW) disease from Ralstonia solanacearum is a serious disease and causes severe yield losses in chili peppers worldwide. Resistant cultivar breeding is the most effective in controlling BW. Thus, a simple and reliable evaluation method is required to assess disease severity and to investigate the inheritance of resistance for further breeding programs. Here, we developed a reliable leaf-to-whole plant spread bioassay for evaluating BW disease and then, using this, determined the inheritance of resistance to R. solanacearum in peppers. Capsicum annuum ‘MC4′ displayed a completely resistant response with fewer disease symptoms, a low level of bacterial cell growth, and significant up-regulations of defense genes in infected leaves compared to those in susceptible ‘Subicho’. We also observed the spreading of wilt symptoms from the leaves to the whole susceptible plant, which denotes the normal BW wilt symptoms, similar to the drenching method. Through this, we optimized the evaluation method of the resistance to BW. Additionally, we performed genetic analysis for resistance inheritance. The parents, F₁ and 90 F₂ progenies, were evaluated, and the two major complementary genes involved in the BW resistance trait were confirmed. These could provide an accurate evaluation to improve resistant pepper breeding efficiency against BW.

Development of diagnostic molecular markers for marker-assisted breeding against bacterial wilt in tomato

Bacterial wilt, caused by the Ralstonia pseudosolanacearum species complex, is an important vascular disease that limits tomato production in tropical and subtropical regions. Two major quantitative trait loci (QTL) of bacterial wilt resistance on chromosome 6 (Bwr-6) and 12 (Bwr-12) were previously identified in Solanum lycopersicum 'Hawaii 7996'; however, marker-assisted breeding for bacterial wilt resistance is not well established. To dissect the QTL, six cleaved amplified polymorphic sites (CAPS) and derived CAPS (dCAPS) markers within the Bwr-6 region and one dCAPS marker near Bwr-12 were developed, and resistance levels in 117 tomato cultivars were evaluated. Two markers, RsR6-5 on chromosome 6 and RsR12-1 on chromosome 12, were selected based on the genotypic and phenotypic analysis. The combination of RsR6-5 and RsR12-1 effectively distinguishes resistant and susceptible cultivars. Furthermore, the efficiency of the two markers was validated in the F3 generation derived from the F2 population between E6203 (susceptible) and Hawaii 7998 (resistant). Resistant alleles at both loci led to the resistance to bacterial wilt. These markers will facilitate marker-assisted breeding of tomato resistant to bacterial wilt.

Contribution of the murI Gene Encoding Glutamate Racemase in the Motility and Virulence of Ralstonia solanacearum

Bacterial traits for virulence of Ralstonia solanacearum causing lethal wilt in plants were extensively studied but are not yet fully understood. Other than the known virulence factors of Ralstonia solanacearum, this study aimed to identify the novel gene(s) contributing to bacterial virulence of R. solanacearum. Among the transposon-inserted mutants that were previously generated, we selected mutant SL341F12 strain produced exopolysaccharide equivalent to wild type strain but showed reduced virulence compared to wild type. In this mutant, a transposon was found to disrupt the murI gene encoding glutamate racemase which converts L-glutamate to D-glutamate. SL341F12 lost its motility, and its virulence in the tomato plant was markedly diminished compared to that of the wild type. The altered phenotypes of SL341F12 were restored by introducing a full-length murI gene. The expression of genes required for flagella assembly was significantly reduced in SL341F12 compared to that of the wild type or complemented strain, indicating that the loss of bacterial motility in the mutant was due to reduced flagella assembly. A dramatic reduction of the mutant population compared to its wild type was apparent in planta (i.e., root) than its wild type but not in soil and rhizosphere. This may contribute to the impaired virulence in the mutant strain. Accordingly, we concluded that murI in R. solanacearum may be involved in controlling flagella assembly and consequently, the mutation affects bacterial motility and virulence.

Ecology, Epidemiology and Disease Management of Ralstonia syzygii in Indonesia

Ralstonia solanacearum species complex phylotype IV strains, which have been primarily isolated from Indonesia, Australia, Japan, Korea, and Malaysia, have undergone recent taxonomic and nomenclatural changes to be placed in the species Ralstonia syzygii. This species contains three subspecies; Ralstonia syzygii subsp. syzygii, a pathogen causing Sumatra disease of clove trees in Indonesia, Ralstonia syzygii subsp. indonesiensis, the causal pathogen of bacterial wilt disease on a wide range of host plants, and Ralstonia syzygii subsp. celebesensis, the causal pathogen of blood disease on Musa spp. In Indonesia, these three subspecies have devastated the cultivation of susceptible host plants which have high economic value. Limited knowledge on the ecology and epidemiology of the diseases has hindered the development of effective control strategies. In this review, we provide insights into the ecology, epidemiology and disease control of these three subspecies of Ralstonia syzygii.

Analysis of Genetic and Pathogenic Diversity of Ralstonia solanacearum Causing Potato Bacterial Wilt in Korea

The Ralstonia solanacearum species complex (RSSC) can be divided into four phylotypes, and includes phenotypically diverse bacterial strains that cause bacterial wilt on various host plants. This study used 93 RSSC isolates responsible for potato bacterial wilt in Korea, and investigated their phylogenetic relatedness based on the analysis of phylotype, biovar, and host range. Of the 93 isolates, twenty-two were identified as biovar 2, eight as biovar 3, and sixty-three as biovar 4. Applied to the phylotype scheme, biovar 3 and 4 isolates belonged to phylotype I, and biovar 2 isolates belonged to phylotype IV. This classification was consistent with phylogenetic trees based on 16S rRNA and egl gene sequences, in which biovar 3 and 4 isolates clustered to phylotype I, and biovar 2 isolates clustered to phylotype IV. Korean biovar 2 isolates were distinct from biovar 3 and 4 isolates pathologically as well as genetically - all biovar 2 isolates were nonpathogenic to peppers. Additionally, in host-determining assays, we found uncommon strains among biovar 2 of phylotype IV, which were the tomato-nonpathogenic strains. Since tomatoes are known to be highly susceptible to RSSC, to the best of our knowledge this is the first report of tomato-nonpathogenic potato strains. These results imply the potential prevalence of greater RSSC diversity in terms of host range than would be predicted based on phylogenetic analysis.

Genotyping by Sequencing Highlights a Polygenic Resistance to Ralstonia pseudosolanacearum in Eggplant (Solanum melongena L.)

Eggplant cultivation is limited by numerous diseases, including the devastating bacterial wilt (BW) caused by the Ralstonia solanacearum species complex (RSSC). Within the RSSC, Ralstonia pseudosolanacearum (including phylotypes I and III) causes severe damage to all solanaceous crops, including eggplant. Therefore, the creation of cultivars resistant to R. pseudosolanacearum strains is a major goal for breeders. An intraspecific eggplant population, segregating for resistance, was created from the cross between the susceptible MM738 and the resistant EG203 lines. The population of 123 doubled haploid lines was challenged with two strains belonging to phylotypes I (PSS4) and III (R3598), which both bypass the published EBWR9 BW-resistance quantitative trait locus (QTL). Ten and three QTLs of resistance to PSS4 and to R3598, respectively, were detected and mapped. All were strongly influenced by environmental conditions. The most stable QTLs were found on chromosomes 3 and 6. Given their estimated physical position, these newly detected QTLs are putatively syntenic with BW-resistance QTLs in tomato. In particular, the QTLs' position on chromosome 6 overlaps with that of the major broad-spectrum tomato resistance QTL Bwr-6. The present study is a first step towards understanding the complex polygenic system, which underlies the high level of BW resistance of the EG203 line.

Molecular Epidemiology of Bacterial Wilt in the Madagascar Highlands Caused by Andean (Phylotype IIB-1) and African (Phylotype III) Brown Rot Strains of the Ralstonia solanacearum Species Complex

The Ralstonia solanacearum species complex (RSSC) is a highly diverse cluster of bacterial strains found worldwide, many of which are destructive and cause bacterial wilt (BW) in a wide range of host plants. In 2009, potato production in Madagascar was dramatically affected by several BW epidemics. Controlling this disease is critical for Malagasy potato producers. The first important step toward control is the characterization of strains and their putative origins. The genetic diversity and population structure of the RSSC were investigated in the major potato production areas of the Highlands. A large collection of strains (n = 1224) was assigned to RSSC phylotypes based on multiplex polymerase chain reaction (PCR). Phylotypes I and III have been present in Madagascar for a long time but rarely associated with major potato BW outbreaks. The marked increase of BW prevalence was found associated with phylotype IIB sequevar 1 (IIB-1) strains (n = 879). This is the first report of phylotype IIB-1 strains in Madagascar. In addition to reference strains, epidemic IIB-1 strains (n = 255) were genotyped using the existing MultiLocus Variable-Number Tandem Repeat Analysis (MLVA) scheme RS2-MLVA9, producing 31 haplotypes separated into two related clonal complexes (CCs). One major CC included most of the worldwide haplotypes distributed across wide areas. A regional-scale investigation suggested that phylotype IIB-1 strains were introduced and massively spread via latently infected potato seed tubers. Additionally, the genetic structure of phylotype IIB-1 likely resulted from a bottleneck/founder effect. The population structure of phylotype III, described here for the first time in Madagascar, exhibited a different pattern. Phylotype III strains (n = 217) were genotyped using the highly discriminatory MLVA scheme RS3-MLVA16. High genetic diversity was uncovered, with 117 haplotypes grouped into 11 CCs. Malagasy phylotype III strains were highly differentiated from continental African strains, suggesting no recent migration from the continent. Overall, population structure of phylotype III involves individual small CCs that correlate to restricted geographic areas in Madagascar. The evidence suggests, if at all, that African phylotype III strains are not efficiently transmitted through latently infected potato seed tubers.

Bacteriophage-Based Bacterial Wilt Biocontrol for an Environmentally Sustainable Agriculture

Bacterial wilt diseases caused by Ralstonia solanacearum, R. pseudosolanacearum, and R. syzygii subsp. indonesiensis (former R. solanacearum species complex) are among the most important plant diseases worldwide, severely affecting a high number of crops and ornamentals. Difficulties of bacterial wilt control by non-biological methods are related to effectiveness, bacterial resistance and environmental impact. Alternatively, a great many biocontrol strategies have been carried out, with the advantage of being environmentally friendly. Advances in bacterial wilt biocontrol include an increasing interest in bacteriophage-based treatments as a promising re-emerging strategy. Bacteriophages against the bacterial wilt pathogens have been described with either lytic or lysogenic effect but, they were proved to be active against strains belonging to R. pseudosolanacearum and/or R. syzygii subsp. indonesiensis, not to the present R. solanacearum species, and only two of them demonstrated successful biocontrol potential in planta. Despite the publication of three patents on the topic, until now no bacteriophage-based product is commercially available. Therefore, there is still much to be done to incorporate valid bacteriophages in an integrated management program to effectively fight bacterial wilt in the field.

Genetic Determinants for Pyomelanin Production and Its Protective Effect against Oxidative Stress in Ralstonia solanacearum

Ralstonia solanacearum is a soil-borne plant pathogen that infects more than 200 plant species. Its broad host range and long-term survival under different environmental stress conditions suggest that it uses a variety of mechanisms to protect itself against various types of biotic and abiotic stress. R. solanacearum produces a melanin-like brown pigment in the stationary phase when grown in minimal medium containing tyrosine. To gain deeper insight into the genetic determinants involved in melanin production, transposon-inserted mutants of R. solanacearum strain SL341 were screened for strains with defective melanin-producing capability. In addition to one mutant already known to be involved in pyomelanin production (viz., strain SL341D, with disruption of the hydroxphenylpyruvate dioxygenase gene), we identified three other mutants with disruption in the regulatory genes rpoS, hrpG, and oxyR, respectively. Wild-type SL341 produced pyomelanin in minimal medium containing tyrosine whereas the mutant strains did not. Likewise, homogentisate, a major precursor of pyomelanin, was detected in the culture filtrate of the wild-type strain but not in those of the mutant strains. A gene encoding hydroxyphenylpyruvate dioxygenase exhibited a significant high expression in wild type SL341 compared to other mutant strains, suggesting that pyomelanin production is regulated by three different regulatory proteins. However, analysis of the gene encoding homogentisate dioxygenase revealed no significant difference in its relative expression over time in the wild-type SL341 and mutant strains, except for SL341D, at 72 h incubation. The pigmented SL341 strain also exhibited a high tolerance to hydrogen peroxide stress compared with the non-pigmented SL341D strain. Our study suggests that pyomelanin production is controlled by several regulatory factors in R. solanacearum to confer protection under oxidative stress.

Genetic Diversity and Pathogenicity of Ralstonia solanacearum Causing Tobacco Bacterial Wilt in China

Bacterial wilt caused by Ralstonia solanacearum is the most serious soilborne disease of tobacco (Nicotiana tabacum) in China. In this study, 89 strains were collected in 2012 to 2014 from across the four major tobacco-growing areas in China. The strains were identified as phylotype I by multiplex polymerase chain reaction and further divided into seven sequevars based on polymorphisms in the endoglucanase (egl) gene. Among the seven sequevars, four (15, 17, 34, and 44) have been previously described as pathogens of tobacco and two (13 and 14), which are reported here on tobacco, were previously found only on other plants. In addition, a new sequevar named 54 was identified. Strains from tobacco from different regions showed different levels of genetic diversity based on partial egl gene sequences. The farther north the distribution, the lower the gene diversity found. Pathogenicity of 27 representative strains was assessed by inoculation onto three tobacco cultivars of varying susceptibility. Through cluster analysis of area under the disease progress curve values, the 27 strains were classified into different pathotypes based on virulence; however, no obvious associations were found between sequevar and pathotype. These results will assist in determining geographical distribution of strains, and provide the foundation for breeding and integrated management programs in China.

Induction of the viable but nonculturable state of Ralstonia solanacearum by low temperature in the soil microcosm and its resuscitation by catalase

Ralstonia solanacearum is the causal agent of bacterial wilt on a wide variety of plants, and enters a viable but nonculturable (VBNC) state under stress conditions in soil and water. Here, we adopted an artificial soil microcosm (ASM) to investigate the VBNC state of R. solanacearum induced by low temperature. The culturability of R. solanacearum strains SL341 and GMI1000 rapidly decreased at 4°C in modified ASM (mASM), while it was stably maintained at 25°C in mASM. We hypothesized that bacterial cells at 4°C in mASM are viable but nonculturable. Total protein profiles of SL341 cells at 4°C in mASM did not differ from those of SL341 culturable cells at 25°C in mASM. Moreover, the VBNC cells maintained in the mASM retained respiration activity. Catalase treatment effectively restored the culturability of nonculturable cells in mASM, while temperature increase or other treatments used for resuscitation of other bacteria were not effective. The resuscitated R. solanacearum from VBNC state displayed normal level of bacterial virulence on tomato plants compared with its original culturable bacteria. Expression of omp, oxyR, rpoS, dps, and the 16S rRNA gene quantified by RT-qPCR did not differ significantly between the culturable and VBNC states of R. solanacearum. Our results suggested that the VBNC bacterial cells in mASM induced by low temperature exist in a physiologically unique state.

Altered Gene Expression and Intracellular Changes of the Viable But Nonculturable State in Ralstonia solanacearum by Copper Treatment

Environmental stresses induce several plant pathogenic bacteria into a viable but nonculturable (VBNC) state, but the basis for VBNC is largely uncharacterized. We investigated the physiology and morphology ofthe copper-induced VBNC state in the plant pathogen Ralstonia solanacearum in liquid microcosm. Supplementation of 200 μM copper sulfate to the liquid microcosm completely suppressed bacterial colony formation on culture media; however, LIVE/DEAD BacLight bacterial viability staining showed that the bacterial cells maintained viability, and that the viable cells contain higher level of DNA. Based on electron microscopic observations, the bacterial cells in the VBNC state were unchanged in size, but heavily aggregated and surrounded by an unknown extracellular material. Cellular ribosome contents, however, were less, resulting in a reduction of the total RNA in VBNC cells. Proteome comparison and reverse transcription PCR analysis showed that the Dps protein production was up-regulated at the transcriptional level and that 2 catalases/peroxidases were present at lower level in VBNC cells. Cell aggregation and elevated levels of Dps protein are typical oxidative stress responses. H2O2 levels also increased in VBNC cells, which could result if catalase/peroxidase levels are reduced. Some of phenotypic changes in VBNC cells of R. solanacearum could be an oxidative stress response due to H2O2 accumulation. This report is the first of the distinct phenotypic changes in cells of R. solanacearum in the VBNC state.

Diversity among Ralstonia solanacearum strains isolated from the southeastern United States

This is the first comprehensive study of a collection of Ralstonia solanacearum strains from the southeastern United States to be characterized based on biovar, pathogenicity, hypersensitive reaction on tobacco, and phylogenetic analyses of the egl sequence. Rigorous phylogenetic analysis of the commonly used egl gene produced robust phylogenies that differed significantly from a neighbor-joining tree differed from and previously published phylogenies for R. solanacearum strains. These robust trees placed phylotype IV within the phylotype I clade, which may suggest that phylogenies based solely on egl may be misleading. As a result of phylogenetic analyses in this study, we determined that U.S. strains from Georgia, North Carolina, South Carolina, and older Florida strains isolated from solanaceous crops all belong to phylotype II sequevar 7. However, many strains recently isolated in Florida from tomato and other crops were more diverse than the southeastern United States population. These unique Florida strains grouped with strains mostly originating from the Caribbean and Central America. One of the exotic strains, which in a previous study was determined to be established in northern Florida, was characterized more extensively. Upon using Musa-specific multiplex polymerase chain reaction, this strain produced a unique banding pattern, which has not previously been reported. Inoculation of this strain into Musa spp. did not result in wilt symptoms; however, the plants were stunted and root masses were significantly reduced. Furthermore, following root inoculation, the bacterium, unlike a typical Florida race 1 biovar 1 strain, was recovered from the roots and stems, indicating systemic movement. This is the first report of an R. solanacearum strain isolated in the United States that is deleterious to the growth of Musa plants.

Contrasting recombination patterns and demographic histories of the plant pathogen Ralstonia solanacearum inferred from MLSA

We used multilocus sequence analysis (MLSA) on a worldwide collection of the plant pathogenic Ralstonia solanacearum (Betaproteobacteria) to retrace its complex evolutionary history. Using genetic imprints left during R. solanacearum evolution, we were able to delineate distinct evolutionary complex displaying contrasting dynamics. Among the phylotypes already described (I, IIA, IIB, III, IV), eight groups of strains with distinct evolutionary patterns, named clades, were identified. From our recombination analysis, we identified 21 recombination events that occurred within and across these lineages. Although appearing the most divergent and ancestral phylotype, phylotype IV was inferred as a gene donor for the majority of the recombination events that we detected. Whereas this phylotype apparently fuelled the species diversity, ongoing diversification was mainly detected within phylotype I, IIA and III. These three groups presented a recent expanding population structure, a high level of homologous recombination and evidences of long-distance migrations. Factors such as adaptation to a specific host or intense trading of infected crops may have promoted this diversification. Whether R. solanacearum lineages will eventually evolve in distinct species remains an open question. The intensification of cropping and increase of geographical dispersion may favour situations of phylotype sympatry and promote higher exchange of key factors for host adaptation from their common genetic pool.

Improved antibiotic resistance gene cassette for marker exchange mutagenesis in Ralstonia solanacearum and Burkholderia species

Marker exchange mutagenesis is a fundamental approach to understanding gene function at a molecular level in bacteria. New plasmids carrying a kanamycin resistance gene or a trimethoprim resistance gene were constructed to provide antibiotic resistance cassettes for marker exchange mutagenesis in Ralstonia solanacearum and many antibiotic-resistant Burkholderia spp. Insertion sequences present in the flanking sequences of the antibiotic resistance cassette were removed to prevent aberrant gene replacement and polar mutation during mutagenesis in wild-type bacteria. Plasmids provided in this study would be convenient for use in gene cassettes for gene replacement in other Gram-negative bacteria.

An HrpB-dependent but type III-independent extracellular aspartic protease is a virulence factor of Ralstonia solanacearum

The host specificity of Ralstonia solanacearum, the causal organism of bacterial wilt on many solanaceous crops, is poorly understood. To identify a gene conferring host specificity of the bacterium, SL341 (virulent to hot pepper but avirulent to potato) and SL2029 (virulent to potato but avirulent to hot pepper) were chosen as representative strains. We identified a gene, rsa1, from SL2029 that confers avirulence to SL341 in hot pepper. The rsa1 gene encoding an 11.8-kDa protein possessed the perfect consensus hrp(II) box motif upstream of the gene. Although the expression of rsa1 was activated by HrpB, a transcriptional activator for hrp gene expression, Rsa1 protein was secreted in an Hrp type III secretion-independent manner. Rsa1 exhibited weak homology with an aspartic protease, cathepsin D, and possessed protease activity. Two specific aspartic protease inhibitors, pepstatin A and diazoacetyl-d,l-norleucine methyl ester, inhibited the protease activity of Rsa1. Substitution of two aspartic acid residues with alanine at positions 54 and 59 abolished protease activity. The SL2029 rsa1 mutant was much less virulent than the wild-type strain, but did not induce disease symptoms in hot pepper. These data indicate that Rsa1 is an extracellular aspartic protease and plays an important role for the virulence of SL2029 in potato.

Characterization of filamentous bacteriophage PE226 infecting Ralstonia solanacearum strains

AIMS

The aim of this study was to isolate and characterize new bacteriophages that infect a wide range of plant pathogenic Ralstonia solanacearum strains.

METHODS AND RESULTS

Fifteen bacteriophages were isolated from pepper, tomato and tobacco plant rhizospheres infected with R. solanacearum. A host specificity analysis of the isolated phages using nine strains of R. solanacearum indicated great phage diversity in a single soil. Two phages, PE226 and TM227, showed clear plaques on all nine bacterial hosts tested and were virtually identical in morphology and genome. PE226, an Inovirus, is a long, flexible, filamentous phage carrying a circular (+) sense single-strand DNA genome of 5475 nucleotides. DNA sequences of PE226 exhibited nine open reading frames (ORF) that were not highly similar to those of other phages infecting R. solanacearum. The genome organization of PE226 was partially similar to that of p12J of Ralstonia pickettii. One ORF of PE226 showed identity to the zot gene encoding zonula occludens toxin of Vibrio cholera. Orf7 of PE226 was also present in the genome of R. solanacearum strain SL341. However, SL341, a highly virulent strain in tomato, was still sensitive to phage PE226.

CONCLUSIONS

A new, flexible, filamentous phage PE226 infected wide range of R. solanacearum strains and carried unique circular single-strand DNA genome with an ORF encoding Zot-like protein.

SIGNIFICANCE AND IMPACT OF THE STUDY

PE226 may be a new type of temperate phage, based on its lytic nature on a wide range of hosts and the presence of a zot homologue in a host bacterial genome.