Tomato Rhizosphere Research Articles

Functional interplay between antagonistic bacteria and Rhizoctonia solani in the tomato plant rhizosphere.

Microbial interactions with plant roots play an imperial role in tomato plant growth and defense against the Rhizoctonia solani. This study performed a field experiment with two antagonistic bacteria (Pseudomonas and Bacillus) inoculated in healthy and Rhizoctonia solani treated soil in tomato rhizosphere to understand the metabolic pattern and microbial function during plant disease suppression. In the present study, we assessed soil and microbial enzymes, bacterial and fungal cell forming unit (CFU), and carbon utilization profiling through Bio-Eco plates of rhizoplane samples. Antagonist bacteria and pathogen interaction significantly (p < 0.05) influenced the bacterial count, soil enzymes (chitinase and glucanase), and bacterial function (siderophore and chitinase production). These results indicated that these variables had an imperial role in disease suppression during plant development. Furthermore, the metabolic profiling showed that carbon source utilization enhanced under fruit development and ripening stages. These results suggested that carbon sources were essential in plant/pathogen/antagonist interaction. Substrates like β-methyl-D-glucoside, D-mannitol, D-galacturonic acid, N-acetyl-D-glucosamine, and phenylethylamine strongly connect with the suppuration of root rot disease. These carbon sources may help to propagate a healthy microbial community to reduce the pathogen invasion in the plant root system, and these carbon sources can be stimulators of antagonists against pathogens in the future.

Plant Growth Promoting Rhizobacteria for Biocontrol of Tomato Bacterial Wilt Caused by Ralstonia solanacearum

Bacterial wilt induced by Ralstonia solanacearum is one of the most damaging and widespread diseases of tomatoes in the world. Biological control with rhizobacteria is one of the efficient components of integrated pest management methods used to control the disease and enhance production. To this end, plant growth-promoting rhizobacteria (Bacillus isolate BDUA1, and Pseudomonas isolates BBDUA2 and BDUA3) isolated from the tomato rhizosphere were evaluated for their plant growth-promoting traits using standard methods, and selected isolates were also tested for their biocontrol efficacy on tomato bacterial wilt disease under greenhouse conditions. All isolates produced cellulase and lipase, and only BDUA1 and BDUA3 produced protease and amylase. Besides, BDUA1 and BDUA2 showed phosphate solubilization and production of indole-3-acetic acid, HCN, and siderophore, while BDUA3 solubilized phosphate and produced HCN and siderophore. Our results showed that BDUA1 and BDUA2 reduced bacterial wilt incidence on the Maya variety by 51.9% and 48.5%, respectively, and on the Melkesalsa variety by 51.8% and 48.5%, respectively. Treatment of the Melkesalsa variety with BDUA1 displayed the highest height (36.91 cm), followed by treatment with BDUA2 (31.74 cm) on the same variety. BDUA1 induced the highest effect on increasing the dry weight of shoots and roots by 4.16 g and 0.59 g in the Maya variety and in the Melkesalsa variety by 3.63 g and 0.48 g, respectively. Similarly, BDUA2 had the greatest effect on increasing the dry weight of shoots and roots by 3.8 g and 0.54 g of the Maya variety and on the Melkesalsa variety by 3.12 g and 0.41 g, respectively. The overall result showed that BDUA1 and BDUA2 could be used as promising plant growth promotion and biocontrol agents for the management of tomato bacterial wilt disease provided they were validated under field conditions.

Biochar amendment reduces cadmium uptake by stimulating cadmium-resistant PGPR in tomato rhizosphere

Biochar amendment in the soil can exert a positive effect in reducing heavy metal toxicity in plants. However, it remains unclear the extent to which this effect is associated with the modulation of plant growth-promoting rhizobacteria (PGPR). Here, we initially conducted a pot experiment using tomato (Solanum lycopersicum L.) as a model plant grown in soil spiked with cadmium. First, we found biochar amendment to result in reduced cadmium uptake in tomato plants and trackable changes in the tomato rhizosphere microbiome. Then, a rhizosphere transplant experiment validated the importance of this microbiome modulation for cadmium-toxicity amelioration. Sequence-based analyses targeted the isolation of representative isolates of PGPR, including Bacillus and Flavisolibacter spp. that displayed in vitro cadmium tolerance and biosorption capabilities (in addition to abilities to solubilize phosphate and produce indole acetic acid). Last, we performed a soil inoculation experiment and confirmed the effectiveness of these isolates in reducing cadmium toxicity in tomato plants. Besides, we found the inoculation of these taxa as single inoculant and in combination to result in increased activities of specific antioxidant enzymes in tomato tissues. Taken together, this study revealed the ecological and physiological mechanisms by which biochar amendment indirectly alleviate cadmium toxicity in tomato plants, in this case, via the modulation and activity of specific PGPR populations. This study provides new insights into strategies able to promote beneficial PGPR in the rhizosphere with potential application to ameliorate heavy metal toxicity in plants.

Plant-associated Pseudomonas aeruginosa strains harbour multiple virulence traits critical for human infection.

Introduction. Pseudomonas aeruginosa causes fatal infections in immunocompromised individuals and patients with pulmonary disorders.Gap Statement. Agricultural ecosystems are the vast reservoirs of this dreaded pathogen. However, there are limited attempts to analyse the pathogenicity of P. aeruginosa strains associated with edible plants.Aim. This study aims to (i) elucidate the virulence attributes of P. aeruginosa strains isolated from the rhizosphere and endophytic niches of cucumber, tomato, eggplant and chili;and (ii) compare these phenotypes with that of previously characterized clinical isolates.Methodology. Crystal-violet microtitre assay, swarm plate experiment, gravimetric quantification and sheep blood lysis were performed to estimate the biofilm formation, swarming motility, rhamnolipid production and haemolytic activity, respectively, of P. aeruginosa strains. In addition, their pathogenicity was also assessed based on their ability to antagonize plant pathogens (Xanthomonas oryzae, Pythium aphanidermatum, Rhizoctonia solani and Fusarium oxysporum) and kill a select nematode (Caenorhabditis elegans).Results. Nearly 80 % of the plant-associated strains produced rhamnolipid and exhibited at least one type of lytic activity (haemolysis, proteolysis and lipolysis). Almost 50 % of these strains formed significant levels of biofilm and exhibited swarming motility. The agricultural strains showed significantly higher and lower virulence against the bacterial and fungal pathogens, respectively, compared to the clinical strains. In C. elegans, a maximum of 40 and 100% mortality were induced by the agricultural and clinical strains, respectively.Conclusion. This investigation shows that P. aeruginosa in edible plants isolated directly from the farm express virulence and pathogenicity. Furthermore, clinical and agricultural P. aeruginosa strains antagonized the tested fungal phytopathogens, Pythium aphanidermatum, Rhizoctonia solani and Fusarium oxysporum. Thus, we recommend using these fungi as simple eukaryotic model systems to test P. aeruginosa pathogenicity.

Soil and Soilless Tomato Cultivation Promote Different Microbial Communities That Provide New Models for Future Crop Interventions.

The cultivation of soilless tomato in greenhouses has increased considerably, but little is known about the assembly of the root microbiome compared to plants grown in soil. To obtain such information, we constructed an assay in which we traced the bacterial and fungal communities by amplicon-based metagenomics during the cultivation chain from nursery to greenhouse. In the greenhouse, the plants were transplanted either into agricultural soil or into coconut fiber bags (soilless). At the phylum level, bacterial and fungal communities were primarily constituted in all microhabitats by Proteobacteria and Ascomycota, respectively. The results showed that the tomato rhizosphere microbiome was shaped by the substrate or soil in which the plants were grown. The microbiome was different particularly in terms of the bacterial communities. In agriculture, enrichment has been observed in putative biological control bacteria of the genera Pseudomonas and Bacillus and in potential phytopathogenic fungi. Overall, the study describes the different shaping of microbial communities in the two cultivation methods.

Efek Perendaman Benih dengan Pupuk Hayati terhadap Pertumbuhan Bibit dan Serangan Penyakit Rebah Semai pada Tomat (Solanum lycopersicum)

The use of biofertilizers is part of an environmentally friendly plant cultivation. In order to analyze the effect of soaking seeds with several types of biofertilizers on seedling growth and damping off disease in tomatoes, a greenhouse experiment has been carried out using biofertilizers and the fungicide Propineb as control treatment. The biofertilizers used were mixed biofertilizer (BION-UP), Consortium isolates of tomato rhizosphere bacteria (CITRB), consortium of Azotobacter chrococcum and Trichoderma harzianum (Azoto-Tricho). The results showed that soaking tomato seeds in bioagens solution increased plant height, number of petioles, fresh weight and dry weight of plants, as well as suppressed damping off disease in tomato nursery. The CITRB biofertilizer has a better effect on plant growth and diseases suppression than another biofertilizers and Propineb fungicide. CITRB enable to increase plant height by 49.97%, number of petioles by 60.00%, plant fresh weight by 107.23%, dry weight of plants by 129.41%, and suppress damping off disease by 56.67%. BION-UP, CITRB and Azoto-Tricho were able to suppress damping off disease by 22.22%, 27.78% and 16.67% respectively. The ability of all biofertilizer to suppress damping off disease incidence were better than Propineb fungicide.

Elicitation of Innate Immunity by a Bacterial Volatile 2-Nonanone at Levels below Detection Limit in Tomato Rhizosphere.

Bacterial volatile compounds (BVCs) exert beneficial effects on plant protection both directly and indirectly. Although BVCs have been detected in vitro, their detection in situ remains challenging. The purpose of this study was to investigate the possibility of BVCs detection under in situ condition and estimate the potentials of in situ BVC to plants at below detection limit. We developed a method for detecting BVCs released by the soil bacteria Bacillus velezensis strain GB03 and Streptomyces griseus strain S4-7 in situ using solid-phase microextraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS). Additionally, we evaluated the BVC detection limit in the rhizosphere and induction of systemic immune response in tomato plants grown in the greenhouse. Two signature BVCs, 2-nonanone and caryolan-1-ol, of GB03 and S4-7 respectively were successfully detected using the soil-vial system. However, these BVCs could not be detected in the rhizosphere pretreated with strains GB03 and S4-7. The detection limit of 2-nonanone in the tomato rhizosphere was 1 µM. Unexpectedly, drench application of 2-nonanone at 10 nM concentration, which is below its detection limit, protected tomato seedlings against Pseudomonas syringae pv. tomato. Our finding highlights that BVCs, including 2-nonanone, released by a soil bacterium are functional even when present at a concentration below the detection limit of SPME-GC-MS.

Biochar stimulates tomato roots to recruit a bacterial assemblage contributing to disease resistance against Fusarium wilt.

Biochar amendment is acknowledged to favor plant resistance against soil-borne diseases. Although plant-beneficial bacteria enrichment in the rhizosphere is often proposed to be associated with this protection, the mechanism behind this stimulating effect remains unelucidated. Here, we tested whether biochar promotes plants to recruit beneficial bacteria to the rhizosphere, and thus develop a disease-suppressive rhizosphere microbiome. In a pot experiment, biochar amendment decreased tomato Fusarium wilt disease severity. Using a transplanting rhizosphere microbiome experiment, we showed that biochar enhanced the suppressiveness of tomato rhizosphere microbiome against Fusarium wilt disease. High-throughput sequencing of 16Sribosomal RNA gene and in vitro cultures further indicated that the recruited suppressive rhizosphere microbiome was associated with the increase of plant-beneficial bacteria, such as Pseudomonas sp. This amendment also enhanced the in vitro chemoattraction and biofilm promotion activity of tomato root exudates. Collectively, our results demonstrate that biochar amendment induces tomato seedlings to efficiently recruit a disease-suppressive rhizosphere microbiome against Fusarium wilt.

Biosurfactant lipopeptides and polyketide biosynthetic gene in rhizobacterium Achromobacter kerstersii to induce systemic resistance in tomatoes

Discovery of novel fungicides for disease management improve on our panoply of known pharmaceuticals. The prime mechanisms of action of agricultural fungicides are by pathogen antagonism or destruction and induced plant systemic resistance. In this study, we describe lipopeptides from tomato rhizobacterium, Achromobacter kerstersii JKP9 exhibiting effective antagonism to Fusarium wilt. Induced systemic resistance potentials were screened after inoculation of bioactive rhizosphere bacterial consortium (Mycoclear) into tomato rhizosphere. Mild biosurfactant properties were observed in drop collapse, and oil displacement assays. The isolated lipopeptides were chemically characterized using FT-IR, HR-NMR. Fatty acid tails were profiled by GC-MS/MS and lipopeptide types were predicted in LC-MS/MS. Polyketide synthase II gene (PKS2) for antibiotic biosynthesis was detected in Achromobacter kerstersii JKP9. Fungal membrane damages and cell wall disruptions due to the interactions of lipopeptides were studied using live/dead fluorescent probes in a dosage-dependent manner. The characterized lipopeptides exhibited broad spectrum of antifungal action against virulent Fusarium spp. and Sarocladium sp. Hence, our findings support cultivable rhizobacteria and their lipopeptides, as promising rhizotechnological tools, rather than synthetics to warrant wilt-free landscape for agriculture.

N-Acetylglucosamine Promotes Tomato Plant Growth by Shaping the Community Structure and Metabolism of the Rhizosphere Microbiome.

ABSTRACTCommunication between plants and microorganisms is vital because it influences their growth, development, defense, propagation, and metabolism in achieving maximal fitness. N-acetylglucosamine (N-GlcNAc), the building block of bacterial and fungal cell walls, was first reported to promote tomato plant growth via stimulation of microorganisms typically known to dominate the tomato root rhizosphere, such as members of Proteobacteria and Actinobacteria. Using KEGG pathway analysis of the rhizosphere microbial operational taxonomic units, the streptomycin biosynthesis pathway was enriched in the presence of N-GlcNAc. The biosynthesis of 3-hydroxy-2-butanone (acetoin) and 2,3-butanediol, two foremost types of plant growth promotion-related volatile organic compounds, were activated in both Bacillus subtilis and Streptomyces thermocarboxydus strains when they were cocultured with N-GlcNAc. In addition, the application of N-GlcNAc increased indole-3-acetic acid production in a dose-dependent manner in strains of Bacillus cereus, Proteus mirabilis, Pseudomonas putida, and S. thermocarboxydus that were isolated from an N-GlcNAc-treated tomato rhizosphere. Overall, this study found that N-GlcNAc could function as microbial signaling molecules to shape the community structure and metabolism of the rhizosphere microbiome, thereby regulating plant growth and development and preventing plant disease through complementary plant–microbe interactions.IMPORTANCE While the benefits of using plant growth-promoting rhizobacteria (PGPRs) to enhance crop production have been recognized and studied extensively under laboratory conditions, the success of their application in the field varies immensely. More fundamentally explicit processes of positive, plant–PGPRs interactions are needed. The utilization of organic amendments, such as chitin and its derivatives, is one of the most economical and practical options for improving soil and substrate quality as well as plant growth and resilience. In this study, we observed that the chitin monomer N-GlcNAc, a key microbial signaling molecule produced through interactions between chitin, soil microbes, and the plants, positively shaped the community structure and metabolism of the rhizosphere microbiome of tomatoes. Our findings also provide a new direction for enhancing the benefits and stability of PGPRs in the field.

Molecular characterization of endophytic and ectophytic plant growth promoting bacteria isolated from tomato plants (Solanum lycopersicum L.) grown in different soil types

BackgroundSuccessful rhizosphere colonization by plant growth promoting rhizobacteria (PGPR) is of crucial importance to perform the desired plant growth promoting activities. Since rhizocompetence is a dynamic process influenced by surrounding environmental conditions. In the present study, we hypothesized that bacterial isolates obtained from different tomato plant microhabitats (balk soil, rhizosphere, endorhiza, phyllosphere, and endoshoot) grown in different soils (sand, clay, and peat moss) will show different rhizocompetence abilities. ResultsTo evaluate this hypothesis, bacterial isolates were obtained from different plant microhabitats and screened for their phosphate solubilizing and nitrogen fixing activates. BOX-PCR fingerprint profiles showed high genotypic diversity among the tested isolates and that same genotypes were shared between different soils and/or plant microhabitats. 16S rRNA gene sequences of 25 PGP isolates, representing different plant spheres and soil types, were affiliated to eight genera: Enterobacter, Paraburkholderia, Klebsiella, Bacillus, Paenibacillus, Stenotrophomonas, Pseudomonas, and Kosakonia. The rhizocompetence of each isolate was evaluated in the rhizosphere of tomato plants grown on a mixture of the three soils. Different genotypes of the same bacterial species displayed different rhizocompetence potentials. However, isolates obtained from the above-ground parts of the plant showed high rhizocompetence. In addition, biological control-related genes, ituD and srfC, were detected in the obtained spore forming bacterial isolates. ConclusionThis study evaluates, for the first time, the relationship between plant microhabitat and the rhizocompetence ability in tomato rhizosphere. The results indicated that soil type and plant sphere can influence both the genotypic diversity and rhizocompetence ability of the same bacterial species. Bacterial isolates obtained in this study are promising to be used as an environmentally friendly substitution of chemical fertilizers.

Enhanced anti-herbivore defense of tomato plants against Spodoptera litura by their rhizosphere bacteria

BackgroundThe use of beneficial microorganisms as an alternative for pest control has gained increasing attention. The objective of this study was to screen beneficial rhizosphere bacteria with the ability to enhance tomato anti-herbivore resistance.ResultsRhizosphere bacteria in tomato field from Fuqing, one of the four locations where rhizosphere bacteria were collected in Fujian, China, enhanced tomato resistance against the tobacco cutworm Spodoptera litura, an important polyphagous pest. Inoculation with the isolate T6–4 obtained from the rhizosphere of tomato field in Fuqing reduced leaf damage and weight gain of S. litura larvae fed on the leaves of inoculated tomato plants by 27% in relative to control. Analysis of 16S rRNA gene sequence identities indicated that the isolate T6–4 was closely related to Stenotrophomonas rhizophila supported with 99.37% sequence similarity. In the presence of S. litura infestation, inoculation with the bacterium led to increases by a 66.9% increase in protease inhibitor activity, 53% in peroxidase activity and 80% in polyphenol oxidase activity in the leaves of inoculated plants as compared to the un-inoculated control. Moreover, the expression levels of defense-related genes encoding allene oxide cyclase (AOC), allene oxide synthase (AOS), lipoxygenase D (LOXD) and proteinase inhibitor (PI-II) in tomato leaves were induced 2.2-, 1.7-, 1.4- and 2.7-fold, respectively by T6–4 inoculation.ConclusionThese results showed that the tomato rhizosphere soils harbor beneficial bacteria that can systemically induce jasmonate-dependent anti-herbivore resistance in tomato plants.

Diverse interactions of five core type III effectors from Ralstonia solanacearum with plants

Ralstonia solanacearum is a widespread plant bacterial pathogen that can launch a range of type III effectors (T3Es) to cause disease. In this study, we isolate a pathogenic R. solanacearum strain named P380 from tomato rhizosphere. Five out of 12 core T3Es of strain P380 are introduced into Pseudomonas syringae DC3000D36E separately to determine their functions in interacting with plants. DC3000D36E that harbors each effector suppresses FliC-triggered Pti5 and ACRE31 expression, ROS burst, and callose deposition. RipAE, RipU, and RipW elicit cell death as well as upregulate the MAPK cascades in Nicotiana benthamiana. The derivatives RipC1ΔDXDX(T/V) and RipWΔDKXXQ but not RipAEK310R fail to suppress ROS burst. Moreover, RipAEK310R and RipWΔDKXXQ retain the cell death elicitation ability. RipAE and RipW are associated with salicylic acid and jasmonic acid pathways, respectively. RipAE and RipAQ significantly promote the propagation of DC3000D36E in plants. The five core T3Es localize in diverse subcellular organelles of nucleus, plasma membrane, endoplasmic reticulum, and Golgi network. The suppressor of G2 allele of Skp1 is required for RipAE but not RipU-triggered cell death in N. benthamiana. These results indicate that the core T3Es in R. solanacearum play diverse roles in plant-pathogen interactions.

Plant Health Status Affects the Functional Diversity of the Rhizosphere Microbiome Associated With Solanum lycopersicum

The microorganisms inhabiting soil perform unique functions in the growth and development of plants. However, little is known about how plant health status affects their potential functions. We examined the functional diversity of the microbiome inhabiting the rhizosphere of powdery mildew diseased and healthy tomato plants alongside the bulk soils in South Africa's Northwest Province employing a shotgun metagenomics approach. We envisaged that the functional categories would be abundant in the healthy rhizosphere (HR) of the tomato plant. We collected soil from the rhizosphere of healthy, powdery mildew diseased tomato plants (DR), and bulk soil (BR). After that, their DNA was extracted. The extracted DNA was subjected to shotgun metagenomic sequencing. Our result using the SEED subsystem revealed that a total of fifteen (15) functional categories dominated the healthy rhizosphere, seven (7) functional categories dominated the diseased rhizosphere. At the same time, six (6) functions dominated the bulk soil. Alpha (α) diversity assessment did not reveal a significant difference (p &amp;gt; 0.05) in all the soil samples, but a considerable difference was observed for beta (β) diversity (P = 0.01). The functional categories obtained in this research were highly abundant in HR. Therefore, this study shows that the functions groups of the rhizosphere microbiomes were more abundant in HR samples as compared to others. The high prevalence of functions groups associated with rhizobiomes in the tomato rhizosphere indicates the need for more research to establish the functional genes associated with these rhizosphere microbiomes.

Effects of Biochar on the Growth and Development of Tomato Seedlings and on the Response of Tomato Plants to the Infection of Systemic Viral Agents.

Biochar is a rich carbon product obtained by pyrolysis of biomass under a limited supply of oxygen. It is composed mainly of aromatic molecules, but its agronomic value is hard to evaluate and difficult to predict due to its great variable characteristics depending on the type of starting biomass and the conditions of pyrolysis. Anyway, it could be used as soil amendment because it increases the soil fertility of acidic soils, increases the agricultural productivity, and seems to provide protection against some foliar and soilborne diseases. In this study, the effects of biochar, obtained from olive pruning, have been evaluated on tomato seedlings growth and on their response to systemic agents' infection alone or added with beneficial microorganisms (Bacillus spp. and Trichoderma spp.). First, experimental data showed that biochar seems to promote the development of the tomato seedlings, especially at concentrations ranging from 1 to 20% (w/w with peat) without showing any antimicrobial effects on the beneficial soil bacteria at the tomato rhizosphere level and even improving their growth. Thus, those concentrations were used in growing tomato plants experimentally infected with tomato spotted wilt virus (TSWV) and potato spindle tuber viroid (PSTVd). The biochar effect was estimated by evaluating three parameters, namely, symptom expression, number of infected plants, and pathogen quantification, using RT-qPCR technique and −ΔΔCt analysis. Biochar at 10–15% and when added with Trichoderma spp. showed that it reduces the replication of PSTVd and the expression of symptoms even if it was not able to block the start of infection. The results obtained on TSWV-infected plants suggested that biochar could contribute to reducing both infection rate and virus replication. For systemic viral agents, such as PSTVd and TSWV, there are no curative control methods, and therefore, the use of prevention means, as can be assumed the use biochar, for example, in the nursery specialized in horticultural crops, can be of great help. These results can be an encouraging starting point to introduce complex biochar formulates among the sustainable managing strategies of plant systemic diseases.

Mixtures of suppressive bacteria enhance biological control of tomato bacterial wilt

Tomato bacterial wilt is one of the most destructive soil-borne plant diseases in the world, causing considerable economic losses. Biological control is an effective and environmentally friendly strategy to reduce disease incidence and yield loss. In this study, six Streptomyces-like strains, NEAU-Z2, NEAU-D18, NEAU-88, NEAU-174, NEAU-C12 and NEAU-66 exhibiting high antagonistic activities against Ralstonia solanacearum were isolated from tomato rhizosphere soil and used for further study. Of which strains NEAU-D18 and NEAU-88 were compatible with each other and the mixed strain (NEAU-D18 + NEAU-88) showing the strongest antibacterial activity against R. solanacearum were prepared. Hydroponic experiment at seedling stage showed that strains NEAU-D18 and NEAU-88 had the best control efficacy at the spore concentration of 108 cfu ml−1, which were 66.6% and 80%, respectively. Therefore, strains NEAU-D18 and NEAU-88 were inoculated into tomato seedling roots with different proportions (i.e., 1:1, 1:2, and 2:1) at the spore concentration of 108 cfu ml−1, and the ratio of 1:1 showed the best control efficacy (100%) on tomato bacterial wilt. Pot experiment also indicated that the mixed strain (NEAU-D18 + NEAU-88) had better biocontrol efficacy than that of single strains. Moreover, the mixed strain and R. solanacearum could utilize 31 of 51 tested compounds of tomato root exudates, indicating the mixed strain was more competitive than the single strains (27, 29) in theory. Furthermore, in vitro nutritional competition experiment showed that the incidence of tomato bacterial wilt with the addition of compounds that could not be utilized by R. solanacearum and mixed strain was equal to that of the control group (CK4-1), but the biocontrol efficacy was great different after adding six compounds that could be utilized by R. solanacearum and mixed strain, which confirmed that the nutritional competition occurred between the mixed strain and R. solanacearum. In addition, solid-state fermentation was used to prepare bioorganic fertilizer of mixed strain. The application of bioorganic fertilizer could effectively suppress the occurrence of tomato bacterial wilt, and the incidence was only 21.67 at 14 days post-challenge inoculation, while 85% of tomato plants developed bacterial wilt with organic fertilizer treatment. In conclusion, this study demonstrated that the mixed strain (NEAU-D18 + NEAU-88) showed better control efficacy than that of single strains and was a potential biocontrol agent for controlling tomato bacterial wilt.

Deterministic Process Dominated Belowground Community Assembly When Suffering Tomato Bacterial Wilt Disease

Soil microbial communities are closely associated with ecosystem functions. However, unravelling the complex nature of the microbial world and successfully utilizing all positive interactions for multipurpose environmental benefits is still a major challenge. Here, we describe the soil bacterial communities in different niches of healthy and diseased tomatoes under natural conditions. A higher abundance of the pathogen Ralstonia solanacearum and lower bacterial diversity were observed in the disease samples. The healthy tomato rhizosphere harbored more plant-beneficial microbes, including Bacillus and Streptomyces. Also, the co-occurrence network in the healthy rhizosphere samples was more complicated, so as to better adapt to the soil-borne pathogen invasion. Both the beta nearest-taxon-index (βNTI) and normalized stochasticity ratio (NST) analyses demonstrated that healthy rhizosphere communities were less phylogenetically clustered and mainly dominated by dispersal limitation, while homogeneous selection was the major assembly process driving the rhizosphere community of diseased samples. The results obtained with community assembly methods and co-occurrence network analysis revealed that healthy rhizosphere bacterial communities possessed potentially broader environmental stress (soil-borne pathogen stress) adaptability compared with diseased rhizosphere bacterial communities. In conclusion, this study contributed to widening our understanding of the potential mechanisms of soil bacterial community composition and assembly responding to soil-borne pathogen invasion.

Evaluation of Rhizosphere Bacterial Antagonists against Ralstonia solanacearum Causing Tomato (Lycopersicon esculentum) Wilt in Central Ethiopia

Tomato (Lycopersicon esculentum) is one of the most commonly grown vegetables in Ethiopia. However, diseases such as bacterial wilt caused by Ralstonia solanacearum have been limiting the production. The rhizosphere is an important source of antagonistic bacteria against soilborne pathogens. This study aimed to investigate the antagonistic potential of rhizosphere bacteria against R. solanacearum in vitro. The pathogen was isolated from wilted tomato plants and tested for hypersensitivity reactions to ascertain the virulent R. solanacearum. Antagonistic rhizobacteria were also isolated from the rhizosphere of healthy tomatoes. Isolates were identified based on cultural characteristics and biochemical tests. The antagonistic effect of rhizobacteria against R. solanacearum was tested in vitro. In addition, the growth of rhizobacterial isolates was determined at different levels of temperature, pH, and NaCl. Of the 36 randomly collected colonies, 7 isolates were identified as Ralstonia spp., all of which were grouped under R. solanacearum biovar III. Similarly, 57 rhizobacteria were isolated, and only 14 had shown antagonistic effects against R. solanacearum. The antagonistic rhizobacteria were identified as Pseudomonas or Bacillus species. Significantly higher (p ≤ 0.05) antagonistic activity (14.66 mm inhibition zone) was recorded by Pseudomonas isolate (P6) than recorded by the rest of the isolates and the positive control. Nine rhizobacterial isolates (out of 14) demonstrated higher or equal inhibition zones recorded by the positive controls. All isolates grew at temperatures ranging 15–45°C, pH 5–9, and 2–5% NaCl. The Bacillus spp. grew at all conditions except at pH 3, showing that they can tolerate wide range of growth conditions. The results of this study showed the presence of potential antagonistic bacteria against R. solanacearum in the study area, which can be used for the control of bacterial wilt of tomato as an alternative management option. Further study is required to determine the efficacy at greenhouse and field conditions.

Bacillus licheniformis JF-22 to Control Meloidogyne incognita and Its Effect on Tomato Rhizosphere Microbial Community

Meloidogyne incognita is one of the most destructive soil pests, causing serious economic losses in tomato production. Here, in vitro experiments demonstrated that the Bacillus licheniformis strain JF-22 has the potential to prevent M. incognita infection. A pot experiment confirmed that B. licheniformis strain JF-22 isolated from the tomato rhizosphere soil and planted in the tomato root-knot nematode disease area effectively prevented and controlled M. incognita, reducing its negative effect on tomato growth. Additionally, the composition of volatile substances secreted by B. licheniformis strain JF-22 was analyzed using solid-phase microextraction and gas chromatography–mass spectrometry. We detected acetoin, 2,3-Butanediol, [R-(R*,R*) ]-, and hexamethyl cyclotrisiloxane as the main components among these volatiles. Using MiSeq sequencing technology and bioinformatics, we analyzed the influence of B. licheniformis strain JF-22 on the microbial community of the tomato rhizosphere. B. licheniformis strain JF-22 changed the composition of the microbial community; particularly, it significantly reduced the diversity of the fungal community. Furthermore, using the FUNGuild and PICRUSt databases, we predicted the effect of JF-22 on microbial community function. In conclusion, B. licheniformis strain JF-22 may be considered as a potential biocontrol agent against M. incognita.

Plant Growth Promoting Properties of a Novel Species of Rhizobium sp. Strain 11515TR Isolated from Tomato Rhizosphere in Los Baños, Laguna, Philippines

The Rhizobium sp. strain 11515TR was established to be a novel species based on whole- genome sequencing and thus merits the evaluation of its beneficial properties to ascertain its potential as a microbial inoculant. This study was aimed at determining its growth-promoting properties and antagonistic activity, nodulation capability, and in planta growth-promoting ability. Genome and predicted proteome analysis revealed that it possesses genes for cellulases, siderophores, nodulation, NifU family protein, and a nitrogen fixation protein (ImpE). The turbidimetric assay showed that it could produce indole acetic acid. In plate assays, it was found negative for phosphate and potassium solubilization, as well as for extracellular cellulase and pectinase. It exhibited in vitro antagonistic effect against Fusarium oxysporum but not against Pseudomonas syringae, and Xanthomonas oryzae. It nodulated Centrosema pubescens, Phaseolus vulgaris, P. lunatus, and Vigna unguiculata. It did not nodulate Glycine max, Lablab purpureus, Macroptilium atropurpureum, Pisum sativum, Psophocarpus tetragonolobus, Vigna radiata, and Vigna unguiculata ssp. sesquipedalis. Under screenhouse conditions, the number and length of beans produced by Rhizobium sp. strain 11515TR-inoculated cowpea were comparable with the plants amended with commercially available Rhizobium inoculant and with chemical fertilizer. Results of this study showed that the novel Rhizobium sp. strain 11515TR has plant growth-promoting and biocontrol potentials that may contribute to its development as a possible microbial inoculant. Further work needs to be conducted to augment and confirm these results.