An Innovative Root Inoculation Method to Study Ralstonia solanacearum Pathogenicity in Tomato Seedlings

A study on twitching motility dynamics in Ralstonia solanacearum microcolonies by live imaging

Ralstonia solanacearum is a rod-shaped phytopathogenic bacterium that causes lethal wilt disease in many plants. On solid agar growth medium, in the early hour of the growth of the bacterial colony, the type IV pili-mediated twitching motility, which is important for its virulence and biofilm formation, is prominently observed under the microscope. In this study, we have done a detailed observation of twitching motility in R. solanacearum colony. In the beginning, twitching motility in the microcolonies was observed as a density-dependent phenomenon that influences the shape of the microcolonies. No such phenomenon was observed in Escherichia coli, where twitching motility is absent. In the early phase of colony growth, twitching motility exhibited by the cells at the peripheral region of the colony was more prominent than the cells toward the center of the colony. Using time-lapse photography and merging the obtained photomicrographs into a video, twitching motility was observed as an intermittent phenomenon that progresses in layers in all directions as finger-like projections at the peripheral region of a bacterial colony. Each layer of bacteria twitches on top of the other and produces a multilayered film-like appearance. We found that the duration between the emergence of each layer diminishes progressively as the colony becomes older. This study on twitching motility demonstrates distinctly heterogeneity among the cells within a colony regarding their dynamics and the influence of microcolonies on each other regarding their morphology.

Comprehensive Analysis of Subcellular Localization, Immune Function and Role in Bacterial wilt Disease Resistance of Solanum lycopersicum Linn. ROP Family Small GTPases

ROPs (Rho-like GTPases from plants) belong to the Rho-GTPase subfamily and serve as molecular switches for regulating diverse cellular events, including morphogenesis and stress responses. However, the immune functions of ROPs in Solanum lycopersicum Linn. (tomato) is still largely unclear. The tomato genome contains nine genes encoding ROP-type small GTPase family proteins (namely SlRop1–9) that fall into five distinct groups as revealed by phylogenetic tree. We studied the subcellular localization and immune response induction of nine SlRops by using a transient overexpression system in Nicotiana benthamiana Domin. Except for SlRop1 and SlRop3, which are solely localized at the plasma membrane, most of the remaining ROPs have additional nuclear and/or cytoplasmic distributions. We also revealed that the number of basic residues in the polybasic region of ROPs tends to be correlated with their membrane accumulation. Though nine SlRops are highly conserved at the RHO (Ras Homology) domains, only seven constitutively active forms of SlRops were able to trigger hypersensitive responses. Furthermore, we analyzed the tissue-specific expression patterns of nine ROPs and found that the expression levels of SlRop3, 4 and 6 were generally high in different tissues. The expression levels of SlRop1, 2 and 7 significantly decreased in tomato seedlings after infection with Ralstonia solanacearum (E.F. Smith) Yabuuchi et al. (GMI1000); the others did not respond. Infection assays among nine ROPs showed that SlRop3 and SlRop4 might be positive regulators of tomato bacterial wilt disease resistance, whereas the rest of the ROPs may not contribute to defense. Our study provides systematic evidence of tomato Rho-related small GTPases for localization, immune response, and disease resistance.

Biocontrol Streptomyces Induces Resistance to Bacterial Wilt by Increasing Defense-Related Enzyme Activity in Solanum melongena L

Streptomyces strains were isolated from rhizosphere soil and evaluated for in vitro plant growth and antagonistic potential against Ralstonia solanacearum. Based on their in vitro screening, seven Streptomyces were evaluated for plant growth promotion (PGP) and biocontrol efficacy by in-planta and pot culture study. In the in-planta study, Streptomyces-treated eggplant seeds showed better germination percentage, plant growth, and disease occurrence against R. solanacearum than the control treatment. Hence, all seven Streptomyces cultures were developed as a bioformulation by farmyard manure and used for pot culture study. The highest plant growth, weight, and total chlorophyll content were observed in UP1A-1-treated eggplant followed by UP1A-4, UT4A-49, and UT6A-57. Similarly, the maximum biocontrol efficacy was observed in UP1A-1-treated eggplants against bacterial wilt. The biocontrol potential of Streptomyces is also confirmed through metabolic responses by assessing the activities of the defense-related enzymes peroxidase (POX), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) and as well as the levels of total phenol. Treatment with UP1A-1/ UT4A-49 and challenge with R. solanacearum led to maximum changes in the activities of POX, PPO, and PAL and the levels of total phenol in the eggplants at different time intervals. Alterations in enzymes of UP1A-1 treatment were related to early defense responses in eggplant. Therefore, the treatment with UP1A-1 significantly delayed the establishment of bacterial wilt in eggplant. Altogether, the present study suggested that the treatment of Streptomyces maritimus UP1A-1 fortified farmyard manure has improved the plant growth and stronger disease control against R. solanacearum on eggplant.

A yceI Gene Involves in the Adaptation of Ralstonia solanacearum to Methyl Gallate and Other Stresses

Ralstonia solanacearum is a plant-pathogenic bacterium causing plant bacterial wilt, and can be strongly inhibited by methyl gallate (MG). Our previous transcriptome sequencing of MG-treated R. solanacearum showed that the yceI gene AVT05_RS03545 of Rs-T02 was up-regulated significantly under MG stress. In this study, a deletion mutant (named DM3545) and an over-expression strain (named OE3545) for yceI were constructed to confirm this hypothesis. No significant difference was observed among the growth of wild-type strain, DM3545 and OE3545 strains without MG treatment. Mutant DM3545 showed a lower growth ability than that of the wild type and OE3545 strains under MG treatment, non-optimal temperature, or 1% NaCl. The ability of DM3545 for rhizosphere colonization was lower than that of the wild-type and OE3545 strains. The DM3545 strain showed substantially reduced virulence toward tomato plants than its wild-type and OE3545 counterpart. Moreover, DM3545 was more sensitive to MG in plants than the wild-type and OE3545 strains. These results suggest that YceI is involved in the adaptability of R. solanacearum to the presence of MG and the effect of other tested abiotic stresses. This protein is also possibly engaged in the virulence potential of R. solanacearum.

A high-throughput virulence screening method for the Ralstonia solanacearum species complex

Ralstonia solanacearum species complex strains are the causative agents for wilting diseases of many plants, including the economically important brown rot of potato. We developed a high-throughput virulence screen that is implemented in 96-well microtiter plates using seedlings grown in soft water agar to save space, effort, and resources. Nicotiana glutinosa was determined to be the most effective host for this assay, and we confirmed bacterial growth and systemic spread in inoculated seedlings. In our assay, N. glutinosa seeds were sown quickly and easily on top of individual water agar wells of a 96-well plate by pipetting out desired number of seeds in an aqueous suspension. They were inoculated on the same day by first touching a bacterial colony with an autoclaved toothpick and then stabbing the toothpick into the center of the water agar well. Such inoculation method resulted in inocula above a threshold of 2 × 10⁴ CFU per well achieving consistent virulence results and enabling reduction of inoculum preparation efforts to facilitate high-throughput screening. Our assay is suitable for forward genetic screening of a large number of strains, isolates or mutants for disease symptoms under both cool (20 °C) and warm (28 °C) temperature conditions before detailed studies can be narrowed down to a manageable number of desired candidates. Our virulence screen method provides a valuable tool for future work in understanding genetics of virulence of Rssc, especially cool virulence of the highly regulated race 3 biovar 2 group of R. solanacearum, leading toward development of effective control strategies.

Evaluation of seed associated endophytic bacteria from tolerant chilli cv. Firingi Jolokia for their biocontrol potential against bacterial wilt disease

In this study, the seed endosphere of a bacterial wilt tolerant chilli cv. Firingi Jolokia was explored in order to find effective agents for bacterial wilt disease biocontrol. A total of 32 endophytic bacteria were isolated from freshly collected seeds and six isolates were selected based on R. solanacearum inhibition assay. These isolates were identified as Bacillus subtilis (KJ-2), Bacillus velezensis (KJ-4), Leuconostoc mesenteroides (KP-1), Lactococcus lactis (LB-3), Bacillus amyloliquefaciens (WK-2), and Bacillus subtilis (WK-3) by 16S rRNA gene sequencing. In the in planta R. solanacearum inhibition assay carried out by seedling root bacterization method, Bacillus subtilis (KJ-2) exhibited highest biocontrol efficacy of 86.6 % on 7^th day post R. solanacearum inoculation and a minimum biocontrol efficacy of 52.9 % was noted for Leuconostoc mesenteroides (KP-1). GC-HRMS analysis detected several known antimicrobial compounds in the extract of the culture supernatant of Bacillus subtilis (KJ-2); which may contribute to inhibition of R. solanacearum. In the growth promotion assay conducted using these isolates, only two of them namely Bacillus subtilis (KJ-2) and Bacillus amyloliquefaciens (WK-2) showed growth promotion in true leafed tomato plants. All the selected seed endophytic isolates were able to control bacterial wilt of tomato at the seedling stage and Bacillus subtilis (KJ-2) was found to be most effective in controlling the disease. The results of the present study highlighted that seed endosphere of bacterial wilt tolerant cultivar is a rich source of R. solanacearum antagonizing bacterial isolates.

An efficient low-cost xylem sap isolation method for bacterial wilt assays in tomato

PREMISE

A portable, simple, yet efficient method was developed for the rapid extraction of xylem sap from the stems and petioles of tomato plants for diagnostic and quantification assays of the xylem-colonizing wilt bacterium Ralstonia solanacearum.

METHODS AND RESULTS

Xylem saps were extracted from tomato stem sections using negative pressure generated from handheld needleless syringes. The samples were collected from plants grown under different soil moisture levels at four days after inoculation with the pathogen. Pipette tips were modified to serve as adapters for the stem sections. The quantification of the bacterial load in the extracted sap was performed by plating sap dilutions in Kelman's triphenyltetrazolium chloride (TTC) medium. Pathogen identity was further confirmed by performing a PCR using R. solanacearum-specific primers.

CONCLUSIONS

Due to its simplicity, portability, and thoroughness of extraction from predetermined tissue sizes, the method can potentially facilitate high-throughput onsite sampling from a large number of samples in a short time, which cannot be achieved with other available techniques.

A Streptomyces sp. NEAU-HV9: Isolation, Identification, and Potential as a Biocontrol Agent against Ralstonia Solanacearum of Tomato Plants

Ralstonia solanacearum is an important soil-borne bacterial plant pathogen. In this study, an actinomycete strain named NEAU-HV9 that showed strong antibacterial activity against Ralstonia solanacearum was isolated from soil using an in vitro screening technique. Based on physiological and morphological characteristics and 98.90% of 16S rRNA gene sequence similarity with Streptomyces panaciradicis 1MR-8^T, the strain was identified as a member of the genus Streptomyces. Tomato seedling and pot culture experiments showed that after pre-inoculation with the strain NEAU-HV9, the disease occurrence of tomato seedlings was effectively prevented for R.solanacearum. Then, a bioactivity-guided approach was employed to isolate and determine the chemical identity of bioactive constituents with antibacterial activity from strain NEAU-HV9. The structure of the antibacterial metabolite was determined as actinomycin D on the basis of extensive spectroscopic analysis. To our knowledge, this is the first report that actinomycin D has strong antibacterial activity against R. solanacearum with a MIC (minimum inhibitory concentration) of 0.6 mg L⁻¹ (0.48 μmol L⁻¹). The in vivo antibacterial activity experiment showed that actinomycin D possessed significant preventive efficacy against R. solanacearum in tomato seedlings. Thus, strain NEAU-HV9 could be used as BCA (biological control agent) against R. solanacearum, and actinomycin D might be a promising candidate for a new antibacterial agent against R. solanacearum.

A novel nanoparticle loaded with methyl caffeate and caffeic acid phenethyl ester against Ralstonia solanacearum—a plant pathogenic bacteria

Developing a novel agent and understanding the interaction model between multipolymer nanoparticles and bacteria could be worthwhile to induce the protection of crops with the prevalence of frequent hazards because of the use of pesticides and chemical resistance. Unlike metal nanoparticles, multipolymer nanoparticles have bacteriostatic properties against Ralstonia solanacearum that can trigger bacterial wilt by infecting the plant. Therefore, a novel poly(lactic-co-glycolic acid) nanoparticle containing caffeic acid phenethyl ester (CAPE) and methyl caffeate (MC) was prepared with the sustained-release property (for 10 d at pH 6.5); here, 50% of the cumulative release rate was achieved. It was observed that the cytomembrane of R. solanacearum was jeopardized by the nanoparticle by the creation of large holes on the bacterial surface. The nanoparticle has an approximate EC₅₀ value of 0.285 mg mL⁻¹ with active pharmaceutical ingredients (APIs), while the drug dosage could be reduced by 2/3. Furthermore, to reveal the possible mechanism of interaction between the multipolymer nanoparticles and bacteria, a formidable inhibition effect was observed; the pathogenicity-related genes, namely, phcA, phcB, pehC, egl, pilT, and polA, of R. solanacearum were downregulated by 1/2, 1/42, 1/13, 1/6, 1/2, and 1/8, respectively, showing significant effects on the major virulence-related genes. Hence, a novel nanoparticle with excellent antibacterial and sustained-release properties has been prepared, possessing the potential to replace chemical pesticides and serve as a new control strategy for mulberry blight disease.

Developing a novel agent and understanding an interaction model between multipolymer nanoparticles and bacteria could be worthwhile to induce the protection of crops with the prevalence of frequent hazards because of the use of chemical pesticides.

A quick and efficient hydroponic potato infection method for evaluating potato resistance and Ralstonia solanacearum virulence

BACKGROUND

Potato, the third most important crop worldwide, plays a critical role in human food security. Brown rot, one of the most destructive potato diseases caused by Ralstonia solanacearum, results in huge economic losses every year. A quick, stable, low cost and high throughout method is required to meet the demands of identification of germplasm resistance to bacterial wilt in potato breeding programs.

RESULTS

Here we present a novel R. solanacearum hydroponic infection assay on potato plants grown in vitro. Through testing wilt symptom appearance and bacterial colonization in aerial part of plants, we found that the optimum conditions for in vitro potato infection were using an OD₆₀₀ 0.01 bacterial solution suspended with tap water for infection, broken potato roots and an open container. Infection using R. solanacearum strains with differential degree of aggressivity demonstrated that this infection system is equally efficient as soil-drench inoculation for assessment of R. solanacearum virulence on potato. A small-scale assessment of 32 potato germplasms identified three varieties highly resistant to the pathogen, which indicates this infection system is a useful method for high-throughout screening of potato germplasm for resistance. Furthermore, we demonstrate the utility of a strain carrying luminescence to easily quantify bacterial colonization and the detection of latent infections in hydroponic conditions, which can be efficiently used in potato breeding programs.

CONCLUSIONS

We have established a quick and efficient in vitro potato infection system, which may facilitate breeding for new potato cultivars with high resistance to R. solanacearum.

Signal Production and Response Specificity in the phc Quorum Sensing Systems of Ralstonia solanacearum Species Complex

Ralstonia solanacearum strains are devastating plant pathogens with global distribution, a wide host range, and genetic diversity, and they are now also referred to as the R. solanacearum species complex (RSSC). RSSC strains employ the quorum sensing (QS) system composed of the phcBSR operon to regulate their virulence on plants. The RSSC strains previously examined produce either (R)-methyl 3-hydroxymyristate (3-OH MAME) or (R)-methyl 3-hydroxypalmitate (3-OH PAME) as their QS signals. Analogously, the phylogenetic analyses of the signal synthase PhcB and the signal receptor PhcS from 15 RSSC strains revealed that these proteins have two clades dependent on their QS signal types. However, the biochemical mechanism underlying this selectivity of QS signal production remains to be elucidated. We demonstrated that the PhcB methyltransferases synthesize QS signals from the cognate fatty acids (R)-3-hydroxymyristic acid or (R)-3-hydroxypalmitic acid. The RSSC strains used here produced both fatty acids, and thus the selectivity of QS signal production depends on the activity of PhcB enzymes. On the other hand, the enantioselective supply of the precursors functioned in the production of enantiopure QS signals. The opposite QS signals weakly induced the production of virulence factors in the RSSC strains. Furthermore, the complementation of the phcB gene encoding the 3-OH PAME-type synthase to the phcB-deletion mutant of the 3-OH MAME-producing strain did not rescue its virulence on tomato plants. Taken together, we propose that the specific production of 3-OH MAME/3-OH PAME ensures full virulence of the RSSC strains.

Specific Reconstruction on pRC Plasmid to Facilitate Its Universal Chromosomal Integration in Different Ralstonia solanacearum Species Complex Strains

The pRC system is an efficient tool for genetic studies in Ralstonia solanacearum, ensuring direct insertion of foreign gene elements into Ralstonia chromosome downstream of glms. This system is designed for double recombination across glms and the downstream region, which considerably simplifies genetic studies of complementation, overexpression, pathogenicity, and in-vivo promoter activity assays with monocopy in R. solanacearum, one of the most destructive plant-pathogenic bacteria worldwide. R. solanacearum is extremely heterogeneous and is currently referred to as a Ralstonia solanacearum species complex (RSSC). The glms gene is greatly conserved, but its downstream regions are mostly different in the RSSC, which limits the application of the current pRC plasmid in the RSSC. We compared all existing 132 genome sequences in a precise genomic glms downstream region and confirmed that the pRC system is appropriate for application of chromosomal integration in all RSSC strains but needs respective reconstruction on current pRC plasmids, since glms downstream regions are greatly variable in the RSSC. RSSC strains can be grouped according to identical glms downstream regions. This grouping provides valuable information for gene insertion in this chromosomal region, as it facilitates universal application of the pRC system in RSSC strains.

Activation of Ralfuranone/Ralstonin Production by Plant Sugars Functions in the Virulence of Ralstonia solanacearum

Plant pathogenic bacteria possess sophisticated mechanisms to detect the presence of host plants by sensing host-derived compounds. Ralstonia solanacearum, the causative agent of bacterial wilt on solanaceous plants, employs quorum sensing to control the production of the secondary metabolite ralfuranones/ralstonins, which have been suggested to be involved in virulence. Here, we report that d-galactose and d-glucose, plant sugars, activate the production of ralfuranones/ralstonins in R. solanacearum. As a result, two new derivatives, ralfuranone M (1) and ralstonin C (2), were found in the culture extracts, and their structures were elucidated by spectroscopic and chemical methods. Ralstonin C (2) is a cyclic lipopeptide containing a unique fatty acid, (2S,3S,Z)-3-amino-2-hydroxyicos-13-enoic acid, whereas ralfuranone M (1) has a common aryl-furanone structure with other ralfuranones. d-Galactose and d-glucose activated the expression of the biosynthetic ralfuranone/ralstonin genes and in part became the biosynthetic source of ralfuranones/ralstonins. Ralfuranones and ralstonins were detected from the xylem fluid of the infected tomato plants, and their production-deficient mutants exhibited reduced virulence on tomato and tobacco plants. Taken together, these results suggest that activation of ralfuranone/ralstonin production by host sugars functions in R. solanacearum virulence.

An Innovative Approach to Study Ralstonia solanacearum Pathogenicity in 6 to 7 Days Old Tomato Seedlings by Root Dip Inoculation

Ralstonia solanacearum (F1C1) is a Gram-negative plant pathogenic bacterium that causes lethal wilt disease in a wide range of plant species. This pathogen is very well known for its unpredictable behavior during infection and wilting its host. Because of its mysterious infection behavior, virulence and pathogenicity standardization are still a big challenge in the case of R. solanacearum. Here, we report an innovative pathogenicity assay of R. solanacearum (F1C1) in the early stage of tomato seedlings by root dip inoculation. In this assay, we employed 6-7days old tomato seedlings for infection grown under nutrients free and gnotobiotic condition. After that, pathogenicity assay was performed by maintaining the inoculated seedlings in 1.5 or 2 ml sterile microfuge tubes. During infection, wilting symptom starts appearing from ~48 h post inoculation and the pathogenicity assay gets completed within seven days of post inoculation. This method is rapid, consistent as well as less resource dependent in terms of labor, space and cost to screen large numbers of plants. Hence, this newly developed assay is an easy and useful approach to study pathogen virulence functions and its interaction with the host plant during wilting and disease progression at the seedling stage.