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Background: Fungal communities around plant roots play crucial roles in maintaining plant health. Nonetheless, the responses of fungal communities to bacterial wilt disease remain poorly understood. Here, the structure and function of fungal communities across four consecutive compartments (bulk soil, rhizosphere, rhizoplane and root endosphere) were investigated under the influence of bacterial wilt disease. Results: The results showed that bacterial wilt disease caused different assembly patterns of fungal communities in the bulk soil, rhizosphere, rhizoplane and endosphere. Under the influence of bacterial wilt disease, a decreased fungal diversity was observed in the rhizoplane and endosphere, and completely different kinds of fungal genera were enriched in the four compartments. The complexity and stability of fungal networks were less affected, but the number of key fungal members in networks were significantly reduced in diseased samples. Functional predictions based on FUNGuild suggested that with the pathogen infection, saprotrophic fungi were increased in the bulk soil, but pathotrophic fungi (potential plant and animal pathogens) were increased in the rhizosphere, rhizoplane and endosphere. Conclusion: This work provides a deep insight into the effects of bacterial wilt disease on fungal communities along the soil-root continuum, and is helpful to identify plant-associated beneficial fungi to resist plant disease. Clinical trial number: Not applicable. [ABSTRACT FROM AUTHOR]

Abstract Tobacco bacterial wilt is a major limiting factor for tobacco production and development, and it is more likely to occur under perennial single cropping of tobacco. In recent years, the rotation of tobacco-radish has gradually become popular. Therefore, we studied the effects of years of tobacco–radish rotation on tobacco bacterial wilt occurrence and rhizosphere microorganisms. The results indicated that both SY and TY could significantly reduce the risk of tobacco bacterial wilt occurrence, and SY had the lowest disease index. The rotation of radish plants significantly increased the soil pH but decreased the contents of alkali-hydrolysed nitrogen and organic matter in the soil. Alkali-hydrolysed nitrogen and pH are the key factors affecting the composition of the bacterial community. Furthermore, radish rotation changed the composition of the soil microbial community, increased the diversity of the bacterial community, and significantly altered the bacterial community structure. At the genus level, the abundance of Sphingomonas species negatively correlated with Ralstonia increased significantly, while the relative abundance of Rhodanobacter species positively correlated with Ralstonia decreased significantly. Disease index, pH and available phosphorus were the main factors affecting the variation in different bacterial genera. The network analysis results showed that Ralstonia was less connected in the network than in the CK group, and the SY treatment group had a more complex bacterial network structure. Overall, 2 years of tobacco and radish rotation improved the bacterial community structure of the rhizosphere soil and alleviated the harm caused by tobacco bacterial wilt, which is highly important for the stability and health of the rhizosphere soil ecosystem.

Tobacco bacterial wilt is a major limiting factor for tobacco production and development, and it is more likely to occur under perennial single cropping of tobacco. In recent years, the rotation of tobacco-radish has gradually become popular. Therefore, we studied the effects of years of tobacco–radish rotation on tobacco bacterial wilt occurrence and rhizosphere microorganisms. The results indicated that both SY and TY could significantly reduce the risk of tobacco bacterial wilt occurrence, and SY had the lowest disease index. The rotation of radish plants significantly increased the soil pH but decreased the contents of alkali-hydrolysed nitrogen and organic matter in the soil. Alkali-hydrolysed nitrogen and pH are the key factors affecting the composition of the bacterial community. Furthermore, radish rotation changed the composition of the soil microbial community, increased the diversity of the bacterial community, and significantly altered the bacterial community structure. At the genus level, the abundance of Sphingomonas species negatively correlated with Ralstonia increased significantly, while the relative abundance of Rhodanobacter species positively correlated with Ralstonia decreased significantly. Disease index, pH and available phosphorus were the main factors affecting the variation in different bacterial genera. The network analysis results showed that Ralstonia was less connected in the network than in the CK group, and the SY treatment group had a more complex bacterial network structure. Overall, 2 years of tobacco and radish rotation improved the bacterial community structure of the rhizosphere soil and alleviated the harm caused by tobacco bacterial wilt, which is highly important for the stability and health of the rhizosphere soil ecosystem. [ABSTRACT FROM AUTHOR]

Curcuma longa L. is a medicinal plant that contains bioactive constituents with various pharmacological properties. However, turmeric is vulnerable to the bacterial wilt disease caused by Ralstonia solanacearum, substantially lowering yields and resulting in death. Thus, this study aims to induce mutations of turmeric cv. 'Trang 2' by applying electron beams (8 MeV) and selecting the resulting populations resistant to bacterial wilt disease. Sixty-day-old plantlets cultured in vitro were exposed to 0, 50, 100 and 150 Gy electron beams with a 240 Gy/min dose rate. The experiment was performed at the Thailand Institute of Nuclear Technology. To explore the potential of electron beam sensitivity in the survival and growth rate, unirradiated plantlets were used to normalize the radiation treatments. Plantlet survival was used to calculate the lethal dose (LD), and the number of new shoots was used to estimate the growth reduction (GR) dose through regression analysis. The survival and growth rates of the plantlets decreased as the radiation dose increased. At doses of 100 and 150 Gy, the turmeric plantlets could not produce new shoots. The median lethal dose (LD50) was 58.6 Gy. A 50 % growth reduction dose (GR50) was observed at 45.3 Gy. Symptom severity ranged from 11.7 to 91.7 %, demonstrating significantly lower levels in the EBRTP-2 and EBRTP-4 M1V5 electron-beam-irradiated populations. Furthermore, the disease incidence was 33.3 % in the EBRTP-6 population. Finally, turmeric populations resistant to bacterial wilt (EBRTP-2, EBRTP-4 and EBRTP-6) were isolated after electron-beam-induced mutation at 50 Gy. [ABSTRACT FROM AUTHOR]

Plant bacterial wilt is caused by Ralstonia solanacearum, a soilborne pathogen that infects plant conduits, leading to wilt disease. It is extremely difficult to cure plants infected with Ralstonia solanacearum; however, bactericide-loaded beads with hot-/wet-responsive properties may be able to release a biocide in line with the increase in the hot-/wet-associated activity of Ralstonia solanacearum, effectively killing the pathogenic cells and providing high levels of plant protection. A biopesticide, Tetramycin, was embedded in corn kernel powder (CKP)-based cores. An oil-phase mixture was sprayed onto the core surface to form a hot-/wet-responsive intermediate shell (IMS). Subsequently, a layer of ethyl cellulose (EC) and hydroxypropyl methyl cellulose (HPMC) was coated onto the IMS to create a single wet-responsive outer shell (OTS). The ratios of the components in the cores, including the corn kernel powder (CKP), xanthan gum (XG), and Tetramycin, were optimized, as well as those of the IMS, including pentaerythrityl tetrastearate (PETS), pentaerythrityl tetraoleate (PETO), polyethylene glycol stearate (PEG400MS), and polyethylene glycol monooleate (PEG400MO), and those of the outer shell (OTS), including ethyl cellulose (EC) and hydroxypropyl methyl cellulose (HPMC). A texture performance analysis, differential scanning calorimetry (DSC) analysis, thermogravimetric analysis (TGA), temperature and humidity response performance tests, scanning electron microscope (SEM) observations, and a field effectiveness test were conducted to characterize the Tetramycin-loaded beads. The results indicated that the optimal formula for the bead cores comprised a mass ratio of CKP/Tetramycin solution/XG = 13.5:23:2. The preferred mass ratio for IMS was PETS/PETO/PEG400MO = 10:30:10, and the formula for the applicable OTS consisted of a mass ratio of EC/HPMC = 5:1. In soil with a temperature of 30–35 °C and humidity of 30%, the release period of the Tetramycin-loaded beads, with a cumulative release rate of over 95%, could last up to 35 days. Furthermore, the Tetramycin-loaded beads exhibited a gradual and multi-cyclic release process under alternating hot/wet and dry/cold environments. The relative preventive efficacy of 54.74% on tobacco was revealed at a field-testing scale. A significant reduction in the abundance of Ralstonia solanacearum was also observed under treatment with the Tetramycin-loaded beads. The early fungal community structure exhibited higher consistency compared to the control. However, in the later stage, the diversity differences between the soil layers were restored. In conclusion, Tetramycin-loaded beads that could effectively respond to temperature and humidity fluctuations were developed, resulting in enhanced disease prevention efficacy and offering broad prospects for the prevention and control of Ralstonia solanacearum in agricultural settings. [ABSTRACT FROM AUTHOR]

The effects of companion planting on soil antibiotic resistance genes (ARGs) and associated microbial composition have remained largely unclear until now. In this study, we assessed the changes in the soil microbiome and ARGs frequencies for tomato growing soils that were companion planted with basil (TB) or cilantro (TC) using a metagenome approach. The abundance of the phytopathogen Ralstonia solanacearum was significantly lower in the TC or TB treatments compared to the tomato monoculture soils (TT). A significant enrichment of Pseudomonas and Aquabacterium and a depletion of Nocardioides and Streptomyces were observed in the TC treatment. Interestingly, both TC and TB companion planting reduced the absolute abundance and the number of subtypes of ARGs. The TC soil showed the lowest numbers of unique ARG subtypes, especially the ARGs resistant to vancomycin and rifamycin, as well as those associated with multidrug resistance. Furthermore, network analysis further revealed that Nocardioides and Streptomyces were potential hosts of ARGs, whereas Flavobacterium negatively correlated with mdtG, suggesting a suppressive effect in reducing ARGs. Together, our results suggest that the companion planting of tomatoes with basil or cilantro can reduce the risk of ARG accumulation, making it a feasible farming management tool to promote soil and plant health in sustainable agriculture. [ABSTRACT FROM AUTHOR]

ABSTRACT Bacteriophages, viruses that specifically target plant pathogenic bacteria, have emerged as a promising alternative to traditional agrochemicals. However, it remains unclear how phages should be applied to achieve efficient pathogen biocontrol and to what extent their efficacy is shaped by indirect interactions with the resident microbiota. Here, we tested if the phage biocontrol efficacy of Ralstonia solanacearum phytopathogenic bacterium can be improved by increasing the phage cocktail application frequency and if the phage efficacy is affected by pathogen-suppressing bacteria already present in the rhizosphere. We find that increasing phage application frequency improves R. solanacearum density control, leading to a clear reduction in bacterial wilt disease in both greenhouse and field experiments with tomato. The high phage application frequency also increased the diversity of resident rhizosphere microbiota and enriched several bacterial taxa that were associated with the reduction in pathogen densities. Interestingly, these taxa often belonged to Actinobacteria known for antibiotics production and soil suppressiveness. To test if they could have had secondary effects on R. solanacearum biocontrol, we isolated Actinobacteria from Nocardia and Streptomyces genera and tested their suppressiveness to the pathogen in vitro and in planta. We found that these taxa could clearly inhibit R. solanacearum growth and constrain bacterial wilt disease, especially when combined with the phage cocktail. Together, our findings unravel an undiscovered benefit of phage therapy, where phages trigger a second line of defense by the pathogen-suppressing bacteria that already exist in resident microbial communities.IMPORTANCERalstonia solanacearum is a highly destructive plant-pathogenic bacterium with the ability to cause bacterial wilt in several crucial crop plants. Given the limitations of conventional chemical control methods, the use of bacterial viruses (phages) has been explored as an alternative biological control strategy. In this study, we show that increasing the phage application frequency can improve the density control of R. solanacearum, leading to a significant reduction in bacterial wilt disease. Furthermore, we found that repeated phage application increased the diversity of rhizosphere microbiota and specifically enriched Actinobacterial taxa that showed synergistic pathogen suppression when combined with phages due to resource and interference competition. Together, our study unravels an undiscovered benefit of phages, where phages trigger a second line of defense by the pathogen-suppressing bacteria present in resident microbial communities. Phage therapies could, hence, potentially be tailored according to host microbiota composition to unlock the pre-existing benefits provided by resident microbiota.

The Ralstonia solanacearum species complex is a group of globally important plant pathogens. Bacteria in this very large and genetically diverse group all colonize the xylem elements of angiosperm plants and cause high-impact wilting diseases of many crops. Because they threaten economic and food security, several R. solanacearum species complex subgroups are strictly regulated as quarantine pests. Biologically meaningful and consistent nomenclature is essential for organisms that have major economic and regulatory importance, such as plant-pathogenic Ralstonia. There are currently three species of Ralstonia wilt pathogens: R. pseudosolanacearum (corresponding to two phylogenetic groups that are described in the literature as phylotypes I and III), R. solanacearum (phylotypes IIA, IIB, and IIC), and R. syzygii (phylotype IV, containing three subspecies: subsp. syzygii, subsp. celebensis, and subsp. indonesiensis). A recent paper proposed reclassifying phylotype I as a new species named “Ralstonia nicotianae.” The purpose of this commentary is to register our objection to the taxon “Ralstonia nicotianae.” [Figure: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

The major disease known as bacterial wilt, which affects tomato plants in Indonesia, is brought on by Ralstonia syzygii subsp. indonesiensis. In Simpang Empat District, Karo Regency, Indonesia, R. syzygii subsp. indonesiensis causes bacterial withery in tomato plants. The purpose of this study is to map its spread. This study is an experimental one that used the survey method to determine the prevalence of disease, the incidence of disease, and the pathogenic isolation of tomato plants at nine different locations throughout Simpang Empat District. The pathogen was isolated from the sample and identified in the lab as R. syzygii subsp. indonesiensis, according to the results. The outcome of the presence of bacterial wilt brought on by R. syzygii subsp. indonesiensis bacteria with various illness occurrences. At a proportion of disease incidence of 57.07%, Ndokum Siroga Location 1 in the community experienced the most disease.

Summary: A prophage tail‐like protein (Bg_9562) of Burkholderia gladioli strain NGJ1 possesses broad‐spectrum antifungal activity, and it is required for the bacterial ability to forage over fungi. Here, we analyzed whether heterologous overexpression of Bg_9562 or exogenous treatment with purified protein can impart disease tolerance in tomato. The physiological relevance of Bg_9562 during endophytic growth of NGJ1 was also investigated.Bg_9562 overexpressing lines demonstrate fungal and bacterial disease tolerance. They exhibit enhanced expression of defense genes and activation of mitogen‐activated protein kinases. Treatment with Bg_9562 protein induces defense responses and imparts immunity in wild‐type tomato.The defense‐inducing ability lies within 18–51 aa region of Bg_9562 and is due to sequence homology with the bacterial flagellin epitope. Interaction studies suggest that Bg_9562 is perceived by FLAGELLIN‐SENSING 2 homologs in tomato. The silencing of SlSERK3s (BAK1 homologs) prevents Bg_9562‐triggered immunity. Moreover, type III secretion system‐dependent translocation of Bg_9562 into host apoplast is important for elicitation of immune responses during colonization of NGJ1.Our study emphasizes that Bg_9562 is important for the endophytic growth of B. gladioli, while the plant perceives it as an indirect indicator of the presence of bacteria to mount immune responses. The findings have practical implications for controlling plant diseases. [ABSTRACT FROM AUTHOR]

Bacterial wilt disease caused by Ralstonia solanacearum is a widespread, severe plant disease. Tomato (Solanum lycopersicum), one of the most important vegetable crops worldwide, is particularly susceptible to this disease. Biological control offers numerous advantages, making it a highly favorable approach for managing bacterial wilt. In this study, the results demonstrate that treatment with the biological control strain Bacillus subtilis R31 significantly reduced the incidence of tomato bacterial wilt. In addition, R31 directly inhibits the growth of R. solanacearum, and lipopeptides play an important role in this effect. The results also show that R31 can stably colonize the rhizosphere soil and root tissues of tomato plants for a long time, reduce the R. solanacearum population in the rhizosphere soil, and alter the microbial community that interacts with R. solanacearum. This study provides an important theoretical basis for elucidating the mechanism of B. subtilis as a biological control agent against bacterial wilt and lays the foundation for the optimization and promotion of other agents such as R31. [ABSTRACT FROM AUTHOR]

Potato (Solanum tuberosum L.) is considered one of the four major and important food crops around the world. This study planned to control bacterial wilt disease of potato using some bio-agents which isolated and identified from potato plant soil. Twenty isolates were selected and identified as the following three isolates belonging to Streptomyces spp. (Streptomyces antibioticus (SA1), S. albus (SA2), and S. mutabilis (SM1); five isolates belong to pseudomonas species (Pseudomonas fluorescence (PsF1), P. aeruginosa (PsA1), P. putida (PsP1), P. alcaligenes (PsA2) and P. pseudoalcaligones (PsP2)) and twelve isolates were belonging to Bacillus spp. (Bacillus subtilis (BS1-8), B. cereus(BC1), B. badius (BB1-2), B. pumilus (BP1). In vitro, these isolates were examined against the growth R. solanacearum bacterium, where some isolates (BS3, PsF1, BS8, BS6, SM1, BS5, and BS4) were the most effective compared with other isolates. Bacillus subtilis (BS8); Pseudomonas fluorescence (PsF1) and Streptomyces mutabilis (SM1) isolates were selected as bio-agents to control potato bacterial wilt disease under in vivo condition, where these isolates led to reduced disease severity and to increase potato yield compared with the control. The application of bio-agents as drench treatment was more effective than tuber treatment, and isolates of S. mutabilis (SM1) and B. subtilis (BS8) were more effective than Pseudomonas fluorescence (PsF1) isolate. [ABSTRACT FROM AUTHOR]

The group of strains constituting the Ralstonia solanacearum species complex (RSSC) is a prominent model for the study of plant-pathogenic bacteria because of its impact on agriculture, owing to its wide host range, worldwide distribution, and long persistence in the environment. RSSC strains have led to numerous studies aimed at deciphering the molecular bases of virulence, and many biological functions and mechanisms have been described to contribute to host infection and pathogenesis. In this review, we put into perspective recent advances in our understanding of virulence in RSSC strains, both in terms of the inventory of functions that participate in this process and their evolutionary dynamics. We also present the different strategies that have been developed to combat these pathogenic strains through biological control, antimicrobial agents, plant genetics, or microbiota engineering. [ABSTRACT FROM AUTHOR]

Main conclusion: Plant and the soil-associated microbiome is important for imparting bacterial wilt disease tolerance in plants. Plants are versatile organisms that are endowed with the capacity to withstand various biotic and abiotic stresses despite having no locomotory abilities. Being the agent for bacterial wilt (BW) disease, Ralstonia solanacearum (RS) colonizes the xylem vessels and limits the water supply to various plant parts, thereby causing wilting. The havoc caused by RS leads to heavy losses in crop productivity around the world, for which a sustainable mitigation strategy is urgently needed. As several factors can influence plant–microbe interactions, comprehensive understanding of plant and soil-associated microbiome under the influence of RS and various environmental/edaphic conditions is important to control this pathogen. This review mainly focuses on microbiome dynamics associated with BW disease and also provide update on microbial/non-microbial approaches employed to control BW disease in crop plants. [ABSTRACT FROM AUTHOR]

Abstract Background Bacterial viruses, phages, play a key role in nutrient turnover and lysis of bacteria in terrestrial ecosystems. While phages are abundant in soils, their effects on plant pathogens and rhizosphere bacterial communities are poorly understood. Here, we used metagenomics and direct experiments to causally test if differences in rhizosphere phage communities could explain variation in soil suppressiveness and bacterial wilt plant disease outcomes by plant-pathogenic Ralstonia solanacearum bacterium. Specifically, we tested two hypotheses: (1) that healthy plants are associated with stronger top-down pathogen control by R. solanacearum-specific phages (i.e. ‘primary phages’) and (2) that ‘secondary phages’ that target pathogen-inhibiting bacteria play a stronger role in diseased plant rhizosphere microbiomes by indirectly ‘helping’ the pathogen. Results Using a repeated sampling of tomato rhizosphere soil in the field, we show that healthy plants are associated with distinct phage communities that contain relatively higher abundances of R. solanacearum-specific phages that exert strong top-down pathogen density control. Moreover, ‘secondary phages’ that targeted pathogen-inhibiting bacteria were more abundant in the diseased plant microbiomes. The roles of R. solanacearum-specific and ‘secondary phages’ were directly validated in separate greenhouse experiments where we causally show that phages can reduce soil suppressiveness, both directly and indirectly, via top-down control of pathogen densities and by alleviating interference competition between pathogen-inhibiting bacteria and the pathogen. Conclusions Together, our findings demonstrate that soil suppressiveness, which is most often attributed to bacteria, could be driven by rhizosphere phage communities that regulate R. solanacearum densities and strength of interference competition with pathogen-suppressing bacteria. Rhizosphere phage communities are hence likely to be important in determining bacterial wilt disease outcomes and soil suppressiveness in agricultural fields. Video Abstract

Ralstonia solanacearum is one of the major plant pathogens causing bacterial wilt disease in a variety of plant species. In Vietnam, according to our knowledge, we first discovered R. pseudosolanacearum, which is one of four phylotypes of R. solanacearum, as a causal agent wilting in cucumber (Cucumis sativus). Due to the latent infection of R. pseudosolanacearum and its heterogenous species complex, controlling the disease becomes difficult.Therefore, the study of R. pseudosolanacearum has great significance to generate effective disease management and treatment. Here, we assembled the isolate R. pseudosolanacearum strain T2C-Rasto, which possessed 183 contigs with 67.03% GC content of 5,628,295 bp in. This assembly included 4,893 protein sequences, 52 tRNA genes, and 3 rRNA genes. In addition, the virulence genes involved in the colonization of the bacterium and wilting to the host were defined in twitching motility (pilT, pilJ, pilH and pilG), chemotaxis (cheA and cheW), type VI secretion system (ompA, hcp, paar, tssB, tssC, tssF, tssG, tssK, tssH, tssJ, tssL and tssM), type III secretion system (hrpB and hrpF).

SUMMARY: Phytopathogens pose a severe threat to agriculture and strengthening the plant defense response is an important strategy for disease control. Here, we report that AtRAV1, an AP2 and B3 domain‐containing transcription factor, is required for basal plant defense in Arabidopsis thaliana. The atrav1 mutant lines demonstrate hyper‐susceptibility against fungal pathogens (Rhizoctonia solani and Botrytis cinerea), whereas AtRAV1 overexpressing lines exhibit disease resistance against them. Enhanced expression of various defense genes and activation of mitogen‐activated protein kinases (AtMPK3 and AtMPK6) are observed in the R. solani infected overexpressing lines, but not in the atrav1 mutant plants. An in vitro phosphorylation assay suggests AtRAV1 to be a novel phosphorylation target of AtMPK3. Bimolecular fluorescence complementation and yeast two‐hybrid assays support physical interactions between AtRAV1 and AtMPK3. Overexpression of the native as well as phospho‐mimic but not the phospho‐defective variant of AtRAV1 imparts disease resistance in the atrav1 mutant A. thaliana lines. On the other hand, overexpression of AtRAV1 fails to impart disease resistance in the atmpk3 mutant. These analyses emphasize that AtMPK3‐mediated phosphorylation of AtRAV1 is important for the elaboration of the defense response in A. thaliana. Considering that RAV1 homologs are conserved in diverse plant species, we propose that they can be gainfully deployed to impart disease resistance in agriculturally important crop plants. Indeed, overexpression of SlRAV1 (a member of the RAV1 family) imparts disease tolerance against not only fungal (R. solani and B. cinerea), but also against bacterial (Ralstonia solanacearum) pathogens in tomato, whereas silencing of the gene enhances disease susceptibility. [ABSTRACT FROM AUTHOR]

Plant-associated microorganisms are believed to be part of the so-called extended plant phenotypes, affecting plant growth and health. Understanding how plant-associated microorganisms respond to pathogen invasion is crucial to controlling plant diseases through microbiome manipulation. In this study, healthy and diseased (bacterial wilt disease, BWD) tomato (Solanum lycopersicum L.) plants were harvested, and variations in the rhizosphere and root endosphere microbial communities were subsequently investigated using amplicon and shotgun metagenome sequencing. BWD led to a significant increase in rhizosphere bacterial diversity in the rhizosphere but reduced bacterial diversity in the root endosphere. The ecological null model indicated that BWD enhanced the bacterial deterministic processes in both the rhizosphere and root endosphere. Network analysis showed that microbial co-occurrence complexity was increased in BWD-infected plants. Moreover, higher universal ecological dynamics of microbial communities were observed in the diseased rhizosphere. Metagenomic analysis revealed the enrichment of more functional gene pathways in the infected rhizosphere. More importantly, when tomato plants were infected with BWD, some plant-harmful pathways such as quorum sensing were significantly enriched, while some plant-beneficial pathways such as streptomycin biosynthesis were depleted. These findings broaden the understanding of plant–microbiome interactions and provide new clues to the underlying mechanism behind the interaction between the plant microbiome and BWD. [ABSTRACT FROM AUTHOR]

Abstract Natural soil has the ability to suppress the soil-borne pathogen to a certain extent, and the assemblage of soil microbiome plays a crucial role in maintaining such ability. Long-term monoculture accelerates the forms of soil microbiome and leads to either disease conducive or suppressive soils. Here, we explored the impact of soil conditions on bacterial wilt disease (healthy or diseased) under long-term tobacco monoculture on the assemblage of bacterial and fungal communities in bulk and rhizosphere soils during the growth periods. With Illumina sequencing, we compared the bacterial and fungal composition of soil samples from tobacco bacterial wilt diseased fields and healthy fields in three growth periods. We found that Proteobacteria and Ascomycota were the most abundant phylum for bacteria and fungi, respectively. Factors of soil conditions and tobacco growth periods can significantly influence the microbial composition in bulk soil samples, while the factor of soil conditions mainly determined the microbial composition in rhizosphere soil samples. Next, rhizosphere samples were further analyzed with LEfSe to determine the discriminative taxa affected by the factor of soil conditions. For bacteria, the genus Ralstonia was found in the diseased soils, whereas the genus Flavobacterium was the only shared taxon in healthy soils; for fungi, the genus Chaetomium was the most significant taxon in healthy soils. Besides, network analysis confirmed that the topologies of networks of healthy soils were higher than that of diseased soils. Together, our results suggest that microbial assemblage in the rhizosphere will be largely affected by soil conditions especially after long-term monoculture.

Background: Bacterial viruses, phages, play a key role in nutrient turnover and lysis of bacteria in terrestrial ecosystems. While phages are abundant in soils, their effects on plant pathogens and rhizosphere bacterial communities are poorly understood. Here, we used metagenomics and direct experiments to causally test if differences in rhizosphere phage communities could explain variation in soil suppressiveness and bacterial wilt plant disease outcomes by plant-pathogenic Ralstonia solanacearum bacterium. Specifically, we tested two hypotheses: (1) that healthy plants are associated with stronger top-down pathogen control by R. solanacearum-specific phages (i.e. 'primary phages') and (2) that 'secondary phages' that target pathogen-inhibiting bacteria play a stronger role in diseased plant rhizosphere microbiomes by indirectly 'helping' the pathogen. Results: Using a repeated sampling of tomato rhizosphere soil in the field, we show that healthy plants are associated with distinct phage communities that contain relatively higher abundances of R. solanacearum-specific phages that exert strong top-down pathogen density control. Moreover, 'secondary phages' that targeted pathogen-inhibiting bacteria were more abundant in the diseased plant microbiomes. The roles of R. solanacearum-specific and 'secondary phages' were directly validated in separate greenhouse experiments where we causally show that phages can reduce soil suppressiveness, both directly and indirectly, via top-down control of pathogen densities and by alleviating interference competition between pathogen-inhibiting bacteria and the pathogen. Conclusions: Together, our findings demonstrate that soil suppressiveness, which is most often attributed to bacteria, could be driven by rhizosphere phage communities that regulate R. solanacearum densities and strength of interference competition with pathogen-suppressing bacteria. Rhizosphere phage communities are hence likely to be important in determining bacterial wilt disease outcomes and soil suppressiveness in agricultural fields. 9CDhWVdEuw3PsngifdxC8S Video Abstract [ABSTRACT FROM AUTHOR]

Tobacco bacterial wilt caused by Ralstonia solanacearum invades tobacco plants during the whole growth period affecting yield and quality. However, the transcriptome profiling of tobacco plant in response to bacterial wilt has not been well studied. In this study, we identified the transcriptional profiles of bacterial wilt-resistant (ac Yanyan97) and -susceptible (ac Honghuadajinyuan) tobacco cultivars infected with R. solanacearum at six time points by RNA sequencing. Gene expression analysis showed that the resistant cultivar manifested a faster change in the expression of defense-related genes than the susceptible cultivar during R. solanacearum infection, by which more differentially expressed genes (DEGs) were up-regulated rather than down-regulated at all time points. Functional analysis indicated that DEGs were involved in plant hormones, glutathione and secondary metabolic pathways associated with tobacco resistance to bacterial wilt induced by R. solanacearum. Through subsequent Short Time-series Expression Miner (STEM) and weighted correlation network (WGCNA) analyses, the phenylpropanoid metabolic pathway was identified as a key pathway for tobacco defense against R. solanacearum infestation. In summary, our results provide transcriptomic profiles of tobacco response to R. solanacearum infestation. [ABSTRACT FROM AUTHOR]

The present study evaluated the priming efficacy of chitosan and chitosan-derived nanoparticles (CNPs) against bacterial wilt of tomato. In the current study, seed-treated CNPs plus pathogen-inoculated tomato seedlings recorded significant protection of 62 % against pathogen-induced wilt disease and subsequently better growth. The induced resistance was witnessed by a prominent increase in lignin, callose and H 2 O 2 deposition, followed by superoxide radical accumulation in leaves. Additionally, chitosan and CNPs-treated tomato plants recorded a remarkable increase in the upregulation of phenylalanine ammonia-lyase (PAL), peroxidase (POX), polyphenol oxidase (PPO), catalase (CAT) and β-1, 3 glucanase (GLU) in comparison with untreated plants. The chitosan and CNPs-induced antioxidant enzymes were positively correlated with the stimulation of corresponding gene expression in CNPs treated plants related to pathogen-inoculated ones. The results of this study describe that how the application of chitosan and CNPs elicit defense responses at the cellular, biochemical and gene expression in tomato plants against bacterial wilt disease, thereby improve growth and yield. [ABSTRACT FROM AUTHOR]

Ralstonia solanacearum (Smith) is a soil-borne pathogen that causes bacterial wilt disease and is a complex species in races, biovars, phylotypes, and strains of various pathogenicities. As a result, the pathogen is difficult to control. An alternative control is by applying actinomycetes and bacteriophages on tomato grafted with resistant rootstock. This study aims to find the best combination in controlling bacterial wilt disease among grafted tomato plants and/or actinomycetes and/or bacteriophage treatments to increase yields. The graftings were between Amelia (East West, Indonesia) or H7996 (AVRDC) as rootstocks, and Servo (East West, Indonesia) as the susceptible scion. The grafting used the tube method, and the experimental design was a randomized completely block design with the grafted plants treated by actinomycetes and/or bacteriophages with three replications. The results indicated that actinomycetes application on plants grafted with Amelia rootstock or H7996 and the application of bacteriophages on susceptible varieties (Servo) could be the best alternative treatment in controlling the bacterial wilt disease. The susceptible varieties with the actinomycetes or bacteriophages application could even increase fruit yields higher than the grafted tomatoes treated with actinomycetes or bacteriophages. In fact, the susceptible tomato varieties with bacteriophage treatment showed high fruit yield. [ABSTRACT FROM AUTHOR]

Bacterial wilt caused by the soil-borne pathogen Ralstonia solanacearum is one of the most devastating diseases in solanaceous plants. No agrochemicals are available to manage bacterial wilt effectively. A Bacillus amyloliquefaciens strain designated PMB01 was recovered from the cabbage rhizosphere and was found to be capable of inhibiting the growth of R. solanacearum. The PMB01 strain was highly resistant to extreme pH, heat, high salt salinity, and various fungicides. In contrast, PMB01 was sensitive to copper-based compounds, streptomycin, and tetracycline. The efficacy of the PMB01 strain in suppressing R. solanacearum and bacterial wilt in tomatoes was significantly improved when the culture medium was supplemented with 1% (w/v) soybean meal. PMB01 was in a 500-liter tank for the pilot production, and the resultant broth could effectively reduce the severity of tomato bacterial wilt in greenhouse trials. The PMB01 fermentation broth was mixed with 10% corn starch and 30% maltodextrin to make a wettable powder (WP). PMB01 could survive in the wettable powder for more than two years without losing its antagonistic activity. In ten field trials, tomato plants treated with 50, 100, or 200-fold dilutions of PMB01 WP reduced bacterial wilt severity by more than 67% compared to the mock (water control) treatment. This work revealed that the effectiveness of the rhizobacterium PMB01 to antagonize R. solanacearum was greatly improved when the culture medium was supplemented with 1% (w/v) soybean meal, indicating that PMB01 is an ideal bio-agent candidate. A durable format suitable for storage was also developed. Similar concepts may be applied to other bio-agent candidates to improve their effectiveness in disease management. [ABSTRACT FROM AUTHOR]

Root-associated compartments, including the rhizosphere, rhizoplane, and endosphere, live with diverse microbial communities which profoundly affect plant growth and health. However, a systematic understanding of the microbiome assembly across the rhizosphere, rhizoplane, and endosphere under pathogen invasion remains elusive. Using 16S high-throughput sequencing, we studied how bacterial wilt disease affected the variation and assembly of the three continuous root-associated microbiomes of tobacco. The results indicated that microorganisms were gradually filtered from the rhizosphere to the endosphere. With the pathogen invasion, the rhizosphere, rhizoplane and endosphere microbiomes selected and recruited different beneficial bacterial taxa. Some recruited bacteria were also identified as keystone members in networks (i.e., Bosea in the endosphere). The microbiomes of endosphere and rhizoplane were more sensitive to plant disease than the rhizosphere microbiome. Still, response strategies of the rhizoplane and endosphere to disease were obviously different. Microbial networks of the rhizoplane became complex in diseased samples and genes involved in sporulation formation and cell cycle were enriched. However, microbial networks of the diseased endosphere were disrupted, and functional genes related to nitrogen utilization and chemotaxis were significantly increased, indicating the importance of nitrogen resources supply of plants for the endosphere microbiome under pathogen invasion. Our results provide novel insights for understanding the different responses of the root-associated microbiomes to plant disease. [ABSTRACT FROM AUTHOR]

Root-associated compartments, including the rhizosphere, rhizoplane, and endosphere, live with diverse microbial communities which profoundly affect plant growth and health. However, a systematic understanding of the microbiome assembly across the rhizosphere, rhizoplane, and endosphere under pathogen invasion remains elusive. Using 16S high-throughput sequencing, we studied how bacterial wilt disease affected the variation and assembly of the three continuous root-associated microbiomes of tobacco. The results indicated that microorganisms were gradually filtered from the rhizosphere to the endosphere. With the pathogen invasion, the rhizosphere, rhizoplane and endosphere microbiomes selected and recruited different beneficial bacterial taxa. Some recruited bacteria were also identified as keystone members in networks (i.e., Bosea in the endosphere). The microbiomes of endosphere and rhizoplane were more sensitive to plant disease than the rhizosphere microbiome. Still, response strategies of the rhizoplane and endosphere to disease were obviously different. Microbial networks of the rhizoplane became complex in diseased samples and genes involved in sporulation formation and cell cycle were enriched. However, microbial networks of the diseased endosphere were disrupted, and functional genes related to nitrogen utilization and chemotaxis were significantly increased, indicating the importance of nitrogen resources supply of plants for the endosphere microbiome under pathogen invasion. Our results provide novel insights for understanding the different responses of the root-associated microbiomes to plant disease.

Bacterial wilt disease poses a significant threat to the production and quality of tobacco. Disease-resistant varieties of tobacco are the primary means of preventing bacterial wilt, but the soil microbial mechanisms that alleviate this disease require further in-depth research. In our study, we analyzed the physicochemical and microbial properties of rhizosphere soil in four different disease-resistant varieties of tobacco. The treatments included: T1 (Chang Bo Huang; 100 % infection rate); T2 (Yun Yan 87; 80 % infection rate); T3 (K326; 50 % infection rate) and T4 (YAN YAN 97; 0 % infection rate). Our results showed that the T4 treatment improved the fresh biomass of tobacco by 35.42 % compared to the T1 treatment. In addition, a significant increase in soil pH and soil organic matter (SOM) from 4.98 and 1.24 % under T1 treatment to 5.86 and 1.38 % under T4 treatment, respectively. As the disease resistance of the tobacco variety improved was found, there was a noticeable upward trend in the number of operational taxonomic units (OTUs) in the rhizosphere soil, showing higher sensitivity compared to fungi. Additionally, rhizosphere soil from susceptible varieties exhibited higher levels of metabolites, resulting in the accumulation of acidic substances (6-aminopenicillanic acid, garcinia acid, and 4-decan-4-ylbenzenesulfonic acid). Moreover, these soil metabolites might have attracted certain pathogenic fungi (Monoblepharomycota), contributing to the spread of soil diseases. This study suggests that the rhizosphere soil of disease-resistant tobacco variety provides a better microecological environment which could inhibit accumulation of allelopathic substances and reduce the spread of bacterial wilt disease. [ABSTRACT FROM AUTHOR]

Natural soil has the ability to suppress the soil-borne pathogen to a certain extent, and the assemblage of soil microbiome plays a crucial role in maintaining such ability. Long-term monoculture accelerates the forms of soil microbiome and leads to either disease conducive or suppressive soils. Here, we explored the impact of soil conditions on bacterial wilt disease (healthy or diseased) under long-term tobacco monoculture on the assemblage of bacterial and fungal communities in bulk and rhizosphere soils during the growth periods. With Illumina sequencing, we compared the bacterial and fungal composition of soil samples from tobacco bacterial wilt diseased fields and healthy fields in three growth periods. We found that Proteobacteria and Ascomycota were the most abundant phylum for bacteria and fungi, respectively. Factors of soil conditions and tobacco growth periods can significantly influence the microbial composition in bulk soil samples, while the factor of soil conditions mainly determined the microbial composition in rhizosphere soil samples. Next, rhizosphere samples were further analyzed with LEfSe to determine the discriminative taxa affected by the factor of soil conditions. For bacteria, the genus Ralstonia was found in the diseased soils, whereas the genus Flavobacterium was the only shared taxon in healthy soils; for fungi, the genus Chaetomium was the most significant taxon in healthy soils. Besides, network analysis confirmed that the topologies of networks of healthy soils were higher than that of diseased soils. Together, our results suggest that microbial assemblage in the rhizosphere will be largely affected by soil conditions especially after long-term monoculture. [ABSTRACT FROM AUTHOR]

As a kind of soil-borne epidemic disease, bacterial wilt (BW) is one of the most serious diseases in tomatoes in southern China, which may significantly reduce food quality and the total amount of yield. Hyperspectral remote sensing can detect crop diseases in the early stages and offers potential for BW detection in tomatoes. Tomatoes in southern China are commonly cultivated in greenhouses or bird nets, limiting the application of remote sensing based on natural sunlight. To resolve these issues, we collected the spectrum of tomatoes firstly using the HS-VN1000B Portable Intelligent Spectrometer, which is equipped with a simulated solar light source. We then proposed a tomato BW detection model based on some optimal spectral features. Specifically, these optimal features, including vegetation indexes and principal components (PCs), were extracted by the sequential forward selection (SFS), the simulated annealing (SA), and the genetic algorithm (GA) and were finally fed into the support vector machine (SVM) classifier to detect diseased tomatoes. The results showed that the infected and healthy tomatoes exhibit different spectral characteristics for both leave and stem spectra, especially for near-infrared bands. In addition, the BW detecting model built by the combination of GA and SVM (GA-SVM) achieved the best performance with overall accuracies (OA) of 90.7% for leaves and 92.6% for stems. Compared with the results based on leaves, spectral features of stems provided better accuracy, indicating that the symptom of early infection of BW is more significant in tomato stems than in leaves. Further, the reliability of the GA-SVM tomato stem model was verified in our 2022 experiment with an OA of 88.6% and an F1 score of 0.80. Our study provides an effective means to detect BW disease of tomatoes in the early stages, which could help farmers manage their tomato production and effectively prevent pesticide abuse. [ABSTRACT FROM AUTHOR]

Bacterial wilt (BW) disease caused by Ralstonia solanacearum is a major constraint in eggplant (Solanum melongena L.) cultivation, resulting in economic loss in tropics, sub-tropics and temperate regions of the world. Identification of genetically diverse parents for development of BW resistant varieties having preferred large fruit with more flesh and less seed content, in disease hot spot areas, is lacking. Twenty-five eggplant genotypes were screened for tolerance to BW disease and other economic traits to identify cultivated genotypes that can be utilized in breeding. Based on screening under artificial and field condition 'Utkal Anusree' appeared resistant, while 'Utkal Madhuri' was moderately resistant against bacterial isolate (Biovar IIIA). Principal components of variable traits, fruit weight, disease incidence of BW, number of fruit per plant and plant height had eigen values >1 and accounting for 100% of total variation. Genotypes were grouped into 6 clusters which do not represent places of origin indicating genotypes in a cluster were geographically diverse; genotypes obtained from the same region were genetically different. Based on multivariate analysis and average values of economic traits, genotypes 'Bidhan Suphala,' 'Bidhan Supreme,' 'Garia,' 'Punjab Sadabahar,' 'Utkal Anusree' and 'Utkal Madhuri' could be potential donors for BW disease resistant breeding. Breeders might be able to introgress bacterial wilt tolerance into commercial genotypes to retrieve potential recombinants with preferred economic traits. [ABSTRACT FROM AUTHOR]

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. olanacearum. 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.

Ralstonia solanacearum is a bacterial phytopathogen of worldwide attention as it causes wilt disease in tomato leading to huge economic loss. This poses a need for identifying host non-specific drug targets to design effective antibacterial agents. Hence, in this study novel therapeutic targets were prioritized by implementing integrative in silico analysis spanning genome to metabolic pathway levels. Initial comparison of chromosome encoded proteins of pathogen against host proteome revealed non-host homologous protein sequences in the pathogen. Further, these proteins were probed for essentiality analysis using database of essential genes as reference, which prioritized the host non-homologous proteins that are essential for survival. Among the 3380 chromosome encoded proteins of the pathogen, 115 were identified as essential and host non-homologous. Subsequent metabolic pathway analysis revealed 24 proteins to be involved in pathogen-specific pathways. On further expression profiling based on codon usage, 17 of these 24 proteins were found to be highly expressed, as per the Codon Adaptation Index (CAI) score. In addition, subcellular localization, virulence factor analysis, drugbank screening and Protein–Protein Interactions (PPIs) were also implemented in order to stringently prioritize the targets based on potentiality and druggability. On cumulative analysis of all these results, 3 unique proteins (Q8XU97, Q8Y1J8 and Q8XVG8) of this pathogen were recognized as highly potential and novel antimicrobial targets. As these proteins resulted from a multilevel stringent prioritization process and were also found to be involved in key survival pathways, these shall form as druggable targets for the design and development of safe bactericidal agents to combat this economically important pathogen. [ABSTRACT FROM AUTHOR]

Bacterial wilt caused by the soil-borne pathogen Ralstonia solanacearum is one of the most devastating diseases in solanaceous plants. No agrochemicals are available to manage bacterial wilt effectively. A Bacillus amyloliquefaciens strain designated PMB01 was recovered from the cabbage rhizosphere and was found to be capable of inhibiting the growth of R. solanacearum. The PMB01 strain was highly resistant to extreme pH, heat, high salt salinity, and various fungicides. In contrast, PMB01 was sensitive to copper-based compounds, streptomycin, and tetracycline. The efficacy of the PMB01 strain in suppressing R. solanacearum and bacterial wilt in tomatoes was significantly improved when the culture medium was supplemented with 1% (w/v) soybean meal. PMB01 was in a 500-liter tank for the pilot production, and the resultant broth could effectively reduce the severity of tomato bacterial wilt in greenhouse trials. The PMB01 fermentation broth was mixed with 10% corn starch and 30% maltodextrin to make a wettable powder (WP). PMB01 could survive in the wettable powder for more than two years without losing its antagonistic activity. In ten field trials, tomato plants treated with 50, 100, or 200-fold dilutions of PMB01 WP reduced bacterial wilt severity by more than 67% compared to the mock (water control) treatment. This work revealed that the effectiveness of the rhizobacterium PMB01 to antagonize R. solanacearum was greatly improved when the culture medium was supplemented with 1% (w/v) soybean meal, indicating that PMB01 is an ideal bio-agent candidate. A durable format suitable for storage was also developed. Similar concepts may be applied to other bio-agent candidates to improve their effectiveness in disease management.

Recent studies have observed differing microbiomes between disease-suppressive and disease-conducive soils. However, it remains unclear whether the microbial keystone taxa in suppressive soil are critical for the suppression of diseases. Bacterial wilt is a common soil-borne disease caused by Ralstonia solanacearum that affects tobacco plants. In this study, two contrasting tobacco fields with bacterial wilt disease incidences of 0% (disease suppressive) and 100% (disease conducive) were observed. Through amplicon sequencing, as expected, a high abundance of Ralstonia was found in the disease-conducive soil, while large amounts of potential beneficial bacteria were found in the disease-suppressive soil. In the fungal community, an abundance of the Fusarium genus, which contains species that cause Fusarium wilt, showed a positive correlation (p < 0.001) with the abundance of Ralstonia. Network analysis revealed that the healthy plants had more complex bacterial networks than the diseased plants. A total of 9 and 13 bacterial keystone taxa were identified from the disease-suppressive soil and healthy root, respectively. Accumulated abundance of these bacterial keystones showed a negative correlation (p < 0.001) with the abundance of Ralstonia. To complement network analysis, culturable strains were isolated, and three species belonging to Pseudomonas showed high 16S rRNA gene similarity (98.4–100%) with keystone taxa. These strains displayed strong inhibition on pathogens and reduced the incidence of bacterial wilt disease in greenhouse condition. This study highlighted the importance of keystone species in the protection of crops against pathogen infection and proposed an approach to obtain beneficial bacteria through identifying keystone species, avoiding large-scale bacterial isolation and cultivation.

Ralstonia solanacearum is a devastating soilborne pathogen which poses significant yield and economic losses to tobacco production globally. The impact of R. solanacearum on rhizosphere bacteriome and soil physicochemical characteristics in resistant and susceptible tobacco cultivars is poorly understood. This study aims to determine the effect of R. solanacearum on soil physicochemical parameters and rhizosphere bacteriome of resistant (K326) and susceptible (Hongda) tobacco cultivars at various growth stages. Results demonstrated that the contents of available potassium and phosphorus, as well as soil pH were significantly increased in K326 soils (CK and T2) compared with Hongda (T1) after 21, 42, and 63 days post-inoculation (dpi) of R. solanacearum except for available nitrogen which showed an opposite trend. The qPCR results showed a significant decrease in R. solanacearum population in rhizosphere of K326 (T2) compared to the Hongda (T1) at 21 and 63 dpi than that after 42 dpi. The rhizosphere bacteriome analysis through 16S rRNA amplicon sequencing revealed that rhizosphere bacterial community composition was significantly different between two tobacco cultivars (Hongda and K326) and this effect was more prominent after 63 dpi (93 days after post-transplantation), suggesting that each cultivar recruits a unique set of bacterial communities. There was no obvious difference observed in the rhizosphere bacteriome of CK (K326) and T2 (K326), which might be attributed to the same genetic makeup and inherent resistance of K326 to bacterial wilt infection. Analysis of co-occurrence networks revealed that the microbial network in T1 (Hongda) was more complex than those in T2 (K326) and CK (K326), while the networks in CK and T2 were almost identical. The present research highlights the time-course relationship between environmental factors and rhizosphere bacteriome of tobacco cultivars showing different levels of resistance against R. solanacearum. Conclusively, studying the plant-soil-microbe interaction system in susceptible and resistant tobacco cultivars may enable us to develop effective integrated disease control plans for the healthy production of tobacco crops. [ABSTRACT FROM AUTHOR]