Pathogenic and Comparative Genomic Analysis of Ralstonia pseudosolanacearum Isolated from Casuarina.

Casuarina equisetifolia is crucial in protecting coastal regions of China against typhoon attacks but has faced a substantial challenge due to wilt disease caused by pathogens of the Ralstonia solanacearum species complex (RSSC). Although the initial outbreak of Casuarina wilt in the 1970s was effectively controlled by disease-resistant C. equisetifolia varieties, the disease has recently re-emerged in coastal regions of Guangdong. In this study, we report the isolation, characterization, and comparative genomic analysis of 11 RSSC strains from diseased C. equisetifolia at various locations along the coast of Guangdong. Phylogenomic analysis showed that the strains were closely related and clustered with phylotype I strains previously isolated from peanuts. Single-gene-based analysis further suggested these strains could be derived from strains present in Guangdong since the 1980s, indicating a historical context to their current pathogenicity. Casuarina-isolated strains exhibited notably higher virulence against C. equisetifolia and peanuts than the representative RSSC strains GMI1000 and EP1, suggesting host-specific adaptations that possibly contributed to the recent outbreak. Comparative genomic analysis among RSSC strains revealed a largely conserved genome structure and high levels of conservation in gene clusters encoding extracellular polysaccharide biosynthesis, secretion systems, and quorum sensing regulatory systems. However, we also found a number of unique genes in the Casuarina-isolated strains that were absent in GMI1000 and EP1, and vice versa, pointing to potential genetic factors underpinning their differential virulence. These unique genes offer promising targets for future functional studies. Overall, our findings provide crucial insights into the RSSC pathogens causing Casuarina wilt in Guangdong, guiding future efforts in disease control and prevention.

An immunochromatographic test strip for on-site detection of Ralstonia solanacearum in tobacco leaves and soil.

Tobacco bacterial wilt is one of the most devastating soil-borne diseases in tobacco-producing regions worldwide. It is often responsible for significant economic losses during tobacco production. A rapid, specific, and high-throughput on-site detection method is important for plant disease management. In this study, monoclonal antibody 3H3 and polyclonal antibody 0344 specific for Ralstonia solanacearum were used to prepare a colloidal gold-based immunochromatographic test strip (ITS). Under optimal conditions, the detection limit of the ITS was 105 CFU/mL. The ITS was able to detect different R. solanacearum strains collected from Shandong, Yunnan, Guizhou, and Sichuan provinces in China. Moreover, the ITS was highly specific for R. solanacearum, with no cross-reactivity with Alternaria alternata (Fries) Keissler, Pseudomonas syringae pv. angulata, and P. syringae pv. tabaci. Furthermore, R. solanacearum-spiked tobacco leaves and soil were used to evaluate the matrix interference of the developed ITS, which indicated the test strip was unaffected by leaf size or soil abundance.

Development of Recombinase Polymerase Amplification Combined with Lateral Flow Detection Assay for Rapid and Visual Detection of Ralstonia solanacearum in Tobacco.

Bacterial wilt caused by Ralstonia solanacearum is a serious soilborne disease that results in severe losses to tobacco (Nicotiana tabacum) production in China. In this study, a novel RPA-LFD assay for the rapid visual detection of R. solanacearum was established using recombinase polymerase amplification (RPA) and lateral-flow dipstick (LFD). The RPA-LFD assay was performed at 37°C in 30 min without complex equipment. Targeting the sequence of the RipTALI-9 gene, we designed RPA primers (Rs-rpa-F/R) and an LF probe (Rs-LF-probe) that showed high specificity to R. solanacearum. The sensitivity of RPA-LFD assay to R. solanacearum was the same as that in conventional PCR at 1 pg genomic DNA, 103 CFU/g artificially inoculated tobacco stems, and 104 CFU/g artificially inoculated soil. The RPA-LFD assay could also detect R. solanacearum from plant and soil samples collected from naturally infested tobacco fields. These results suggest that the RPA-LFD assay developed in this study is a rapid, accurate molecular diagnostic tool with high sensitivity for the detection of R. solanacearum.

Antibacterial activity of three essential oils and some monoterpenes against Ralstonia solanacearum phylotype II isolated from potato.

The present study describes the possibility of using some essential oils and monoterpens as bioagents against the growth of Ralstonia solanacearum, a causal bacterium of potato brown rot disease. Eight isolates of the bacterium were recovered from infected potato tubers, showing typical symptoms of the disease, Isolates were identified as R.solanacearum phylotype II, based on biochemical and physiological characteristics, as well as, at the molecular level through PCR analysis. Three essential oils extracted from Corymbia citriodora (leaves), Cupressus sempervirens (aerial parts), and Lantana camara (aerial parts) were evaluated for their antibacterial activity against eight isolates of R. solanacearum phylotype II. Results demonstrated that L. camara essential oil (concentration 5000 µg/mL) had the highest effects against the RsMo2, RsSc1 and Rs48, with inhibition zone (IZ) values of 17.33, 16.33, and 17.50 mm, respectively, also against Rs2 (IZ 14.33 mm), and RsIs2 (IZ 16 mm). C. citriodora oil showed the highest activity against RsBe2 (IZ 14 mm), RsFr4 (IZ 13.66 mm) and RsNe1 (IZ 13.66 mm). Gas Chromatography-Mass Spectrometry (GC-MS-FID) analyzed the chemical composition of these essential oils. It was proved that L. camara leaves contains mainly trans-caryophyllene (16.24%) and α-humulene (9.55%), in C. citriodora oil were α-citronellal (56.55%), α-citronellol (14.89%), and citronellol acetate (13.04%), and in Cup. sempervirens aerial parts were cedrol (22.17%), and Δ3-carene (18.59%). Five monoterpenes were evaluated against the most resistance Ralstonia isolate RsFr5 to the three studied essential oils and found that limonene had the highest effect against it compared with the lowest thymol. The results proved the strong bio effects of the essential oil from L. camara leaves as a natural product contained monoterpenes that can inhibit the growth of tested R. solanacearum phylotype II isolates.

Comparative Evaluation of LAMP, qPCR, Conventional PCR, and ELISA to Detect Ralstonia solanacearum in Kenyan Potato Fields.

Bacterial wilt caused by Ralstonia solanacearum is considered among the most damaging diseases of potato in Sub-Saharan Africa and the most significant biotic constraint of potato production alongside late blight. Unlike late blight, which can be managed by chemical means, R. solanacearum can only be managed through cultural methods and clean seed. Laboratory testing to certify seed before planting is required to confirm the absence of the pathogen in Kenya. A loop-mediated isothermal amplification (LAMP) assay was developed using the UDP-(3-O-acyl)-N-acetylglucosamine deacetylase gene (IpxC) to screen seed potato for R. solanacearum strains. The assay was assessed using DNA extracted from R. solanacearum and other soil and potato pathogens to demonstrate specificity and sensitivity. The LAMP assay was validated using field samples from different potato growing regions of Kenya collected over two growing seasons and compared with established nucleic acid and protein-based assays. The IpxC LAMP assay was found to be specific and sensitive to R. solanacearum, detecting as low as 2.5 pg/µl of R. solanacearum DNA. Of the 47 potentially infected field samples collected, both IpxC LAMP and quantitative polymerase chain reaction (PCR) detected R. solanacearum DNA in 90% of the samples, followed by conventional PCR (86%) and ELISA (75%). This IpxC LAMP assay is a promising diagnostic tool to rapidly screen for R. solanacearum in seed potato with high sensitivity in Kenya. Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Antigenic and pathogenicity activities of Ralstonia solanacearum race 3 biovar 2 molecularly identified and detected by indirect ELISA using polyclonal antibodies generated in rabbits.

Eight molecular-characterized isolates of Ralstonia solanacearum from potato belonging to race 3 biovar 2, their virulence were evaluated on potato cv. Lady Rosette, tomato cv. Strain B, eggplant cv. Balady and pepper cv. Balady and showed high virulence on potato and tomato, and lower virulence on eggplant and pepper. A laboratory study conducted to produce polyclonal antibodies against the potato brown rot bacterium; R. solanacearum cells were generated in female New Zealand white rabbits. A modification were made on the technique of indirect enzyme-linked immunosorbent assay (ELISA) to improve the sensitivity of detection, including antigenic and sensitivity to R. solanacearum race 3 biovar 2 isolates. Determination of the optimum period to collect the antiserum (including, polyclonal antibodies) showed that the best collection dates were at 14, 3 and 7 days, in that order. The efficiency of the antiserum was compared among 42 isolates that cause potato brown rot disease; our polyclonal antiserum (14 days) reacted positively with all tested isolates at a dilution of 1:6.4 × 103. Data indicated the different reactions of eight R. solanacearum isolates at various dilutions (1:1.6 × 103 to 1:5.12 × 106) at 14 days against polyclonal antiserumat a concentration of approximately 1 × 108 CFU/mL and we found the lowest detection level by the indirect ELISA technique was 106 CFU/mL. Finally we recommended the reasonable sensitivity results of the ELISA technique to detect the bacterial pathogen given than the cost of this technique if much lower than that of other expensive molecular techniques.

Matrix approach to the simultaneous detection of multiple potato pathogens by real-time PCR.

AIM: Create a method for highly sensitive, selective, rapid and easy-to-use detection and identification of economically significant potato pathogens, including viruses, bacteria and oomycetes, be it single pathogen, or a range of various pathogens occurring simultaneously. METHODS AND RESULTS: Test-systems for real-time PCR, operating in the unified amplification regime, have been developed for Phytophthora infestans, Pectobacterium atrosepticum, Dickeya dianthicola, Dickeya solani, Ralstonia solanacearum, Pectobacterium carotovorum, Clavibacter michiganensis subsp. sepedonicus, potato viruses Y (ordinary and necrotic forms as well as indiscriminative test system, detecting all forms), A, X, S, M, potato leaf roll virus, potato mop top virus and potato spindle tuber viroid. The test-systems (including polymerase and revertase) were immobilized and lyophilized in miniature microreactors (1·2 µl) on silicon DNA/RNA microarrays (micromatrices) to be used with a mobile AriaDNA® amplifier. CONCLUSIONS: Preloaded 30-reaction micromatrices having shelf life of 3 and 6 months (for RNA- and DNA-based pathogens, respectively) at room temperature with no special conditions were successfully tested on both reference and field samples in comparison with traditional ELISA and microbiological methods, showing perfect performance and sensitivity (1 pg). SIGNIFICANCE AND IMPACT OF THE STUDY: The accurate, rapid and user-friendly diagnostic system in a micromatrix format may significantly contribute to pathogen screening and phytopathological studies.

Phylotype and sequevar of Ralstonia solanacearum which causes bacterial wilt in Curcuma alismatifolia Gagnep.

Bacterial wilt of Curcuma alismatifolia (Patumma) caused by Ralstonia solanacearum is a major disease affecting the quality of rhizome exports. Traditionally, R. solanacearum is classified into five races based on differences in host range and six biovars based on biochemical properties. Recently a classification scheme based on phylotypes and sequevars was presented by the scientific community as a tool for determining phylogenetic relationships within R. solanacearum. This study used traditional and molecular methods to identify R. solanacearum strains from Patumma. All the strains were identified as biovar 4. A phylotype-specific multiplex PCR-based phylotyping of all the isolates detected the phylotype I-specific amplicon of 144 bp and the R. solanacearum-specific 281 bp amplicon. Phylogenetic analyses of endoglucanase (egl) sequences clustered all three strains of Patumma into phylotype I, sequevar 48 with reference strains M2 and M6. The study determined that the R. solanacearum strains from Patumma belong to biovar 4, phylotype I that originated from Asia, and sequevar 48. SIGNIFICANCE AND IMPACT OF THE STUDY: Phylotype and sequevar of Ralstonia solanacearum were associated with geographic region and geographic distribution. This is the first study to identify phylotype and sequevar of R. solanacearum from Patumma in Chiang Mai, Thailand. This will be useful for study of disease epidemiology and could help management for control of bacterial wilt diseases in this host.

Phylogenetic Assignment of Ralstonia pseudosolanacearum (Ralstonia solanacearum Phylotype I) Isolated from Rosa spp.

During the last two years, greenhouse cultivation of rose (Rosa spp.) in the Netherlands has been challenged by an uncommon bacterial disease. Affected plants suffered from chlorosis, stunting, wilting, and necrosis. The bacterial isolates obtained from the different Rosa spp. cultivars were all identified as phylotype I, sequevar 33 from the 'Ralstonia solanacearum species complex' (RSSC), actually reclassified as Ralstonia pseudosolanacearum. The work in this paper considers the genetic diversity and the phylogenetic position of 129 R. pseudosolanacearum isolates from the outbreak. This was assessed by AFLP based on four different primer combinations and MLP using partial sequences of the egl, mutS, and fliC genes. The AFLP revealed identical profiles for all the isolates, irrespective of their association with Rosa sp. propagating material, Rosa spp. plants for cut flowers, or water used in the different greenhouse cultivations. These AFLP profiles were unique and diverged from profiles of all other reference isolates in the RSSC included. Furthermore, MLP on egl, fliC, and mutS gene sequences clearly demonstrated that all R. pseudosolanacearum isolates clustered in phylotype I, as a distinct monophyletic group. Interestingly, this monophyletic group also included phylotype I strain Rs-09-161 from eggplant (Solanum melongena), isolated in 2009 in India. AFLP and MLP were both efficient in revealing the genetic divergence from the RSSC isolates included. The phylogenetic tree constructed from the AFLP profiles was, in general, in agreement with the one obtained from MLP. Both phylogenetic trees displayed a similar clustering, supported by high posterior probabilities. Both methodologies clearly demonstrated that the R. pseudosolanacearum isolates from Rosa spp. grouped in a monophyletic group inside phylotype I, with a particular correspondence to a strain present in India, as revealed in MLP.

New Multilocus Variable-Number Tandem-Repeat Analysis (MLVA) Scheme for Fine-Scale Monitoring and Microevolution-Related Study of Ralstonia pseudosolanacearum Phylotype I Populations.

Bacterial wilt caused by the Ralstonia solanacearum species complex (RSSC) is considered one of the most harmful plant diseases in the world. Special attention should be paid to R. pseudosolanacearum phylotype I due to its large host range, its worldwide distribution, and its high evolutionary potential. So far, the molecular epidemiology and population genetics of this bacterium are poorly understood. Until now, the genetic structure of the RSSC has been analyzed on the worldwide and regional scales. Emerging questions regarding evolutionary forces in RSSC adaptation to hosts now require genetic markers that are able to monitor RSSC field populations. In this study, we aimed to evaluate the multilocus variable-number tandem-repeat analysis (MLVA) approach for its ability to discriminate genetically close phylotype I strains and for population genetics studies. We developed a new MLVA scheme (MLVA-7) allowing us to genotype 580 R. pseudosolanacearum phylotype I strains extracted from susceptible and resistant hosts and from different habitats (stem, soil, and rhizosphere). Based on specificity, polymorphism, and the amplification success rate, we selected seven fast-evolving variable-number tandem-repeat (VNTR) markers. The newly developed MLVA-7 scheme showed higher discriminatory power than the previously published MLVA-13 scheme when applied to collections sampled from the same location on different dates and to collections from different locations on very small scales. Our study provides a valuable tool for fine-scale monitoring and microevolution-related study of R. pseudosolanacearum phylotype I populations.IMPORTANCE Understanding the evolutionary dynamics of adaptation of plant pathogens to new hosts or ecological niches has become a key point for the development of innovative disease management strategies, including durable resistance. Whereas the molecular mechanisms underlying virulence or pathogenicity changes have been studied thoroughly, the population genetics of plant pathogen adaptation remains an open, unexplored field, especially for plant-pathogenic bacteria. MLVA has become increasingly popular for epidemiosurveillance and molecular epidemiology studies of plant pathogens. However, this method has been used mostly for genotyping and identification on a regional or global scale. In this study, we developed a new MLVA scheme, targeting phylotype I of the soilborne Ralstonia solanacearum species complex (RSSC), specifically to address the bacterial population genetics on the field scale. Such a MLVA scheme, based on fast-evolving loci, may be a tool of choice for field experimental evolution and spatial genetics studies.

General phytoplasma detection by a q-PCR method using mycoplasma primers.

Phytoplasmas and mycoplasmas are bacteria belonging to the class Mollicutes. In this study, a fine tuning of quantitative polymerase chain reaction (qPCR) with a universal mycoplasma primer pair (GPO3F/MGSO) targeting the 16S rRNA gene was carried out on phytoplasmas. The dissociation curves of DNAs from Catharanthus roseus phytoplasma-infected micropropagated shoots and from phytoplasma field-infected plant samples showed a single peak at 82.5 °C (±0.5) specifically detecting phytoplasmas belonging to several ribosomal groups. Assay specificity was determined with DNA of selected bacteria: 'Candidatus Liberibacter solanacearum', Xylella fastidiosa, Ralstonia solanacearum and Clavibacter michiganensis. No amplification curves were observed with any of these tested bacteria except 'Ca. L. solanacearum' that was amplified with a melting temperature at 85 °C. Absolute quantification of phytoplasma titer was calculated using standard curves prepared from serial dilutions of plasmids containing the cloned fragment GPO3F/MGSO from European stone fruit yellows phytoplasma. Phytoplasma copy number ranged from 106 to 103 according with the sample. The sensitivity evaluated comparing plasmid serial dilutions resulted 10-6 for conventional PCR and 10-7 for qPCR. The latter method resulted therefore able to detect very low concentrations of phytoplasma in plant material.

Ralstonia solanacearum Differentially Colonizes Roots of Resistant and Susceptible Tomato Plants.

Ralstonia solanacearum is the causal agent of bacterial wilt and infects over 200 plant species in 50 families. The soilborne bacterium is lethal to many solanaceous species, including tomato. Although resistant plants can carry high pathogen loads (between 105 and 108 CFU/g fresh weight), the disease is best controlled by the use of resistant cultivars, particularly resistant rootstocks. How these plants have latent infections yet maintain resistance is not clear. R. solanacearum first infects the plant through the root system and, thus, early root colonization events may be key to understanding resistance. We hypothesized that the distribution and timing of bacterial invasion differed in roots of resistant and susceptible tomato cultivars. Here, we use a combination of scanning electron microscopy and light microscopy to investigate R. solanacearum colonization in roots of soil-grown resistant and susceptible tomato cultivars at multiple time points after inoculation. Our results show that colonization of the root vascular cylinder is delayed in resistant 'Hawaii7996' and that, once bacteria enter the root vascular tissues, colonization in the vasculature is spatially restricted. Our data suggest that resistance is due, in part, to the ability of the resistant cultivar to restrict bacterial root colonization in space and time.

Rapid differentiation of Ralstonia solanacearum avirulent and virulent strains by cell fractioning of an isolate using high performance liquid chromatography.

Ralstonia solanacearum is one of the most destructive plant bacterial pathogens worldwide. The population dynamics and genetic stability are important issues, especially when an avirulent strain is used for biocontrol. In this study, we developed a rapid method to differentiate the virulent and avirulent strains of R. solanacearum and to predict the biocontrol efficiency of an avirulent strain using high performance liquid chromatography (HPLC). Three chromatographic peaks P1, P2 and P3 were observed on the HPLC spectra among 68 avirulent and 28 virulent R. solanacearum strains. Based on the HPLC peaks, 96 strains total were assigned to three categories. For avirulent strains, the intense peak is P1, while for virulent strains, P3 is the majority. Based on the HLPC spectra of R. solanacearum strains, a chromatography titer index (CTI) was established as CTIi = Si/(S1+S2+S3) × 100% (i represents an individual HPLC peak; S1, S2 and S3 represent peak areas of P1, P2 and P3, respectively). The avirulent strains had high values of CTI1 ranging from 63.6 to 100.0%, while the virulent strains displayed high values of CTI3 ranging from 90.2 to 100.0%. Biological inoculation studies of 68 avirulent strains revealed that the biocontrol efficacy was the best when CTI1 = 100%. The purity and genetic stability of R. solanacearum strains were confirmed in the P1 fraction of avirulent strain FJAT-1957 and P3 fraction of virulent strain FJAT-1925 after 30 generations of consecutive subculture. These results confirmed that fractioning by HPLC and their deduced CTI can be used for rapid and efficient evaluation and prediction of an isolate of R. solanacearum. To the best of our knowledge, this is the first report that HPLC fractioning can be used for rapid differentiation of virulent and avirulent strains of R. solanacearum.

Simultaneous Detection of Brown Rot- and Soft Rot-Causing Bacterial Pathogens from Potato Tubers Through Multiplex PCR.

Ralstonia solanacearum (Smith) Yabuuchi et al. and Erwinia carotovora subsp. carotovora (Jones) Bergey et al. (Pectobacterium carotovorum subsp. carotovorum) are the two major bacterial pathogens of potato causing brown rot (wilt) and soft rot diseases, respectively, in the field and during storage. Reliable and early detection of these pathogens are keys to avoid occurrence of these diseases in potato crops and reduce yield loss. In the present study, multiplex polymerase chain reaction (PCR) protocol was developed for simultaneous detection of R. solanacearum and E. carotovora subsp. carotovora from potato tubers. A set of oligos targeting the pectatelyase (pel) gene of E. carotovora subsp. carotovora and the universal primers based on 16S r RNA gene of R. solanacearum were used. The standardized multiplex PCR protocol could detect R. solanacearum and E. carotovora subsp. carotovora up to 0.01 and 1.0 ng of genomic DNA, respectively. The protocol was further validated on 96 stored potato tuber samples, collected from different potato-growing states of India, viz. Uttarakhand, Odisha, Meghalaya and Delhi. 53.1 % tuber samples were positive for R. solanacearum, and 15.1 % of samples were positive for E. carotovora subsp. carotovora, and both the pathogens were positive in 26.0 % samples when BIO-PCR was used. This method offers sensitive, specific, reliable and fast detection of two major bacterial pathogens from potato tubers simultaneously, particularly pathogen-free seed certification in large scale.

Development and Comparison of TaqMan-Based Real-Time PCR Assays for Detection and Differentiation of Ralstonia solanacearum strains.

Bacterial wilt caused by Ralstonia solanacearum is destructive to many plant species worldwide. The race 3 biovar 2 (r3b2) strains of R. solanacearum infect potatoes in temperate climates and are listed as select agents by the U.S. government. TaqMan-based real-time quantitative PCR (qPCR) is commonly used in federal and state diagnostic laboratories over conventional PCR due to its speed and sensitivity. We developed the Rs16S primers and probe set and compared it with a widely used set (RS) for detecting R. solanacearum species complex strains. We also developed the RsSA3 primers and probe set and compared it with the previously published B2 and RsSA2 sets for specific detection of r3b2 strains. Both comparisons were done under standardized qPCR master mix and cycling conditions. The Rs16S and RS assays detected all 90 R. solanacearum species complex strains and none of the five outgroups, but the former was more sensitive than the latter. For r3b2 strain detection, the RsSA2 and RsSA3 sets specifically detected the 34 r3b2 strains and none of the 56 R. solanacearum non-r3b2 strains or out-group strains. The B2 set, however, detected five non-r3b2 R. solanacearum strains and was less sensitive than the other two sets under the same testing conditions. We conclude that the Rs16S, RsSA2, and RsSA3 sets are best suited under the standardized conditions for the detection of R. solanacearum species complex and r3b2 strains by TaqMan-based qPCR assays.

Detection of Quorum Sensing Molecules and Biofilm Formation in Ralstonia solanacearum.

Many bacteria use small diffusible signaling molecules to communicate each other termed as quorum sensing (QS). Most Gram-negative bacteria use acyl homoserine lactone (AHL) as QS signal molecules. Using these signaling molecules, bacteria are able to express specific genes in response to population density. This work aimed to detect the production of QS signal molecules and biofilm formation in Ralstonia solanacearum isolated from various diseased tomato plants with symptoms of bacterial wilt. A total of 30 R. solanacearum strains were investigated for the production of QS signal molecules using Chromobacterium violaceum CV026 and Agrobacterium tumefaciens NT1 (pZLR4) biosensor systems. All 30 bacterial isolates from various bacterial wilt-affected tomato plants produced AHL molecules that induced the biosensor. The microtiter plate assay demonstrated that of the 30 bacterial isolates, 60 % formed biofilm, among which four isolates exhibited a higher degree of biofilm formation. The biofilm-inducing factor was purified from these four culture supernatants. The structure of the responsible molecule was solved using nuclear magnetic resonance and mass spectroscopy and was determined to be 2-hydroxy-4-((methylamino)(phenyl)methyl) cyclopentanone (HMCP), which was confirmed by chemical synthesis and NMR. The Confocal laser scanning microscopic analysis showed well-developed biofilm architecture of bacteria when treated with HMCP. The knowledge we obtained from this study will be useful for further researcher on the role of HMCP molecule in biofilm formation.

Tropical strains of Ralstonia solanacearum Outcompete race 3 biovar 2 strains at lowland tropical temperatures.

Bacterial wilt, caused by members of the heterogenous Ralstonia solanacearum species complex, is an economically important vascular disease affecting many crops. Human activity has widely disseminated R. solanacearum strains, increasing their global agricultural impact. However, tropical highland race 3 biovar 2 (R3bv2) strains do not cause disease in tropical lowlands, even though they are virulent at warm temperatures. We tested the hypothesis that differences in temperature adaptation and competitive fitness explain the uneven geographic distribution of R. solanacearum strains. Using three phylogenetically and ecologically distinct strains, we measured competitive fitness at two temperatures following paired-strain inoculations of their shared host, tomato. Lowland tropical strain GMI1000 was only weakly virulent on tomato under temperate conditions (24°C for day and 19°C for night [24/19°C]), but highland tropical R3bv2 strain UW551 and U.S. warm temperate strain K60 were highly virulent at both 24/19°C and 28°C. Strain K60 was significantly more competitive than both GMI1000 and UW551 in tomato rhizospheres and stems at 28°C, and GMI1000 also outcompeted UW551 at 28°C. The results were reversed at cooler temperatures, at which highland strain UW551 generally outcompeted GMI1000 and K60 in planta. The superior competitive index of UW551 at 24/19°C suggests that adaptation to cool temperatures could explain why only R3bv2 strains threaten highland agriculture. Strains K60 and GMI1000 each produced different bacteriocins that inhibited growth of UW551 in culture. Such interstrain inhibition could explain why R3bv2 strains do not cause disease in tropical lowlands.

Antibacterial activities against rice bacterial leaf blight and tomato bacterial wilt of 2-mercapto-5-substituted-1,3,4-oxadiazole/thiadiazole derivatives.

In this study, a series of 2-mercapto-5-substituted-1,3,4-oxadiazole/thiadiazole derivatives were synthesized and evaluated for their antibacterial activities against rice bacterial leaf blight and tomato bacterial wilt caused by Xanthomonas oryzae pv. oryzae (Xoo) and Ralstonia solanacearum (R. solanacearum) via the turbidimeter test in vitro. Antibacterial bioassays indicated that most compounds demonstrated appreciable antibacterial bioactivities against Xoo and R. solanacearum. Among the title compounds, compound 4i demonstrated the best inhibitory effect against Xoo and R. solanacearum with half-maximal effective concentration (EC50) values of 14.69 and 15.14µg/mL, respectively, which were even better than those of commercial agents Bismerthiazol and Thiodiazole Copper. In vivo antibacterial activities tests under greenhouse conditions revealed that the control efficiency of compound 4i against rice bacterial leaf blight and tobacco bacterial wilt were better than those of Bismerthiazol and Thiodiazole Copper. Meanwhile, field trials also indicated that compound 4i demonstrated appreciable control efficiency against rice bacterial leaf blight and tomato bacterial wilt.

A novel, sensitive method to evaluate potato germplasm for bacterial wilt resistance using a luminescent Ralstonia solanacearum reporter strain.

Several breeding programs are under way to introduce resistance to bacterial wilt caused by Ralstonia solanacearum in solanaceous crops. The lack of screening methods allowing easy measurement of pathogen colonization and the inability to detect latent (i.e., symptomless) infections are major limitations when evaluating resistance to this disease in plant germplasm. We describe a new method to study the interaction between R. solanacearum and potato germplasm that overcomes these restrictions. The R. solanacearum UY031 was genetically modified to constitutively generate light from a synthetic luxCDABE operon stably inserted in its chromosome. Colonization of this reporter strain on different potato accessions was followed using life imaging. Bacterial detection in planta by this nondisruptive system correlated with the development of wilting symptoms. In addition, we demonstrated that quantitative detection of the recombinant strain using a luminometer can identify latent infections on symptomless potato plants. We have developed a novel, unsophisticated, and accurate method for high-throughput evaluation of pathogen colonization in plant populations. We applied this method to compare the behavior of potato accessions with contrasting resistance to R. solanacearum. This new system will be especially useful to detect latency in symptomless parental lines before their inclusion in long-term breeding programs for disease resistance.

Dynamics in the resistant and susceptible peanut (Arachis hypogaea L.) root transcriptome on infection with the Ralstonia solanacearum.

BACKGROUND: Bacterial wilt caused by Ralstonia solanacearum is a serious soil-borne disease of peanut (Arachis hypogaea L). The molecular basis of peanut response to R. solanacearum remains unknown. To understand the resistance mechanism behind peanut resistance to R. solanacearum, we used RNA-Seq to perform global transcriptome profiling on the roots of peanut resistant (R) and susceptible (S) genotypes under R. solanacearum infection. RESULTS: A total of 4.95 x 108 raw sequence reads were generated and subsequently assembled into 271, 790 unigenes with an average length of 890 bp and a N50 of 1, 665 bp. 179, 641 unigenes could be annotated by public protein databases. The pairwise transcriptome comparsions of time course (6, 12, 24, 48 and 72 h post inoculation) were conducted 1) between inoculated and control samples of each genotype, 2) between inoculated samples of R and S genotypes. The linear dynamics of transcriptome profile was observed between adjacent samples for each genotype, two genotypes shared similar transcriptome pattern at early time points with most significant up regulation at 12 hour, and samples from R genotype at 24 h and S genotype at 48 h showed similar transcriptome pattern, significant differences of transcriptional profile were observed in pairwise comparisons between R and S genotypes. KEGG analysis showed that the primary metabolisms were inhibited in both genotypes and stronger inhibition in R genotype post inoculation. The defense related genes (R gene, LRR-RLK, cell wall genes, etc.) generally showed a genotype-specific down regulation and different expression between both genotypes. CONCLUSION: This transcriptome profiling provided the largest data set that explores the dynamic in crosstalk between peanut and R. solanacearum. The results suggested that the down-regulation of primary metabolism is contributed to the resistance difference between R and S genotypes. The genotype-specific expression pattern of defense related DEGs also contributed to the resistance difference between R and S genotype. This study will strongly contribute to better understand the molecular interaction between plant and R. solanacearum.