Visualization of N-acylhomoserine lactone-mediated cell-cell communication between bacteria colonizing the tomato rhizosphere.

Given that a large proportion of the bacteria colonizing the roots of plants is capable of producing N-acyl-L-homoserine lactone (AHL) molecules, it appears likely that these bacterial pheromones may serve as signals for communication between cells of different species. In this study, we have developed and characterized novel Gfp-based monitor strains that allow in situ visualization of AHL-mediated communication between individual cells in the plant rhizosphere. For this purpose, three Gfp-based AHL sensor plasmids that respond to different spectra of AHL molecules were transferred into AHL-negative derivatives of Pseudomonas putida IsoF and Serratia liquefaciens MG1, two strains that are capable of colonizing tomato roots. These AHL monitor strains were used to visualize communication between defined bacterial populations in the rhizosphere of axenically grown tomato plants. Furthermore, we integrated into the chromosome of AHL-negative P. putida strain F117 an AHL sensor cassette that responds to the presence of long-chain AHLs with the expression of Gfp. This monitor strain was used to demonstrate that the indigenous bacterial community colonizing the roots of tomato plants growing in nonsterile soil produces AHL molecules. The results strongly support the view that AHL signal molecules serve as a universal language for communication between the different bacterial populations of the rhizosphere consortium.     

Diversity of bacterial endophytes in roots of Mexican husk tomato plants (Physalis ixocarpa) and their detection in the rhizosphere

Endophytic bacterial diversity was estimated in Mexican husk tomato plant roots by amplified rDNA restriction analysis and sequence homology comparison of the 16S rDNA genes. Sixteen operational taxonomic units from the 16S rDNA root library were identified based on sequence analysis, including the classes Gammaproteobacteria, Betaproteobacteria, Actinobacteria, and Bacilli. The predominant genera were Stenotrophomonas (21.9%), Microbacterium (17.1%), Burkholderia (14.3%), Bacillus (14.3%), and Pseudomonas (10.5%). In a 16S rDNA gene library of the same plant species' rhizosphere, only common soil bacteria, including Stenotrophomonas, Burkholderia, Bacillus, and Pseudomonas, were detected. We suggest that the endophytic bacterial diversity within the roots of Mexican husk tomato plants is a subset of the rhizosphere bacterial population, dominated by a few genera.     

Protection of tomato seedlings against infection by Pseudomonas syringae pv. tomato by using the plant growth-promoting bacterium Azospirillum brasilense

Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (10(7) versus 10(5) CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (10(7) versus 10(6) CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (10(5) to 10(6) CFU/g [dry weight] of root). However, P. syringae pv. tomato did not survive in the rhizosphere in the presence of A. brasilense. Foliar inoculation of A. brasilense after P. syringae pv. tomato was established on the leaves did not alleviate bacterial speck disease, and A. brasilense did not survive well in the phyllosphere under these conditions, even in a mist chamber. Several applications of a low concentration of buffered malic acid significantly enhanced the leaf population of A. brasilense (>10(8) CFU/g [dry weight] of leaf), decreased the population of P. syringae pv. tomato to almost undetectable levels, almost eliminated disease development, and improved plant growth to the level of uninoculated healthy control plants. Based on our results, we propose that A. brasilense be used in prevention programs to combat the foliar bacterial speck disease caused by P. syringae pv. tomato.     

Micro-organisms in the rhizosphere of plants inoculated withAzotobacter chroococcum

Summary The hypothesis that inoculation withAzotobacter chroococcum affects the growth of plants indirectly through changing the rhizosphere microflora was investigated. Inoculated and uninoculated wheat and tomato plants were grown in the glasshouse in two different soils, and total bacteria, chitinolytic bacteria, actinomycetes, glucosefermenting bacteria, aerobic cellulose-decomposing bacteria, and anaerobes were determined in intervals in the rhizosphere and in the soil. Root-surface fungi were studied using the Harley and Waid's root-washing technique¹⁰. Azotobacter became established in the rhizosphere of wheat and tomato plants and stimulated their growth. All the bacterial groups examined were more abundant in the rhizosphere than in the soil. Inoculation with Azotobacter delayed the colonization of roots by bacteria, actinomycetes, and fungi in the rhizosphere, but had no effect on other organisms. Inoculation did not affect the dominant root-surface fungi, and minor changes were not consistent.Part of a thesis accepted by the University of London for the degree of Ph.D. in Microbiology.     

Survival of gfp-tagged antagonistic bacteria in the rhizosphere of tomato plants and their effects on the indigenous bacterial community

The survival and colonization patterns of Pseudomonas putida PRD16 and Enterobacter cowanii PRF116 in the rhizosphere of greenhouse-grown tomato plants and the effects of their inoculation on the indigenous bacterial community were followed by selective plating, molecular fingerprinting, and confocal laser scanning microscopy (CLSM) over 3 weeks. Both strains, which showed in vitro antagonistic activity against Ralstonia solanacearum, were previously tagged with gfp. Seed and root inoculation were compared. Although plate counts decreased for both gfp-tagged antagonists, PRD16 showed a better survival in the rhizosphere of tomato roots independent of the inoculation method. Analysis of 16S rRNA gene fragments amplified from total community DNA by denaturing gradient gel electrophoresis and CLSM confirmed the decrease in the relative abundance of the inoculant strains. Pronounced differences in the Pseudomonas community patterns for plants inoculated with PRD16 compared to the control were detected 3 weeks after root inoculation, indicating a longer-lasting effect. Analysis by CLSM showed rather heterogeneous colonization patterns for both inoculant strains. In comparison with seed inoculation, root inoculation led to a much better colonization as evidenced by all three methods. The colonization patterns observed by CLSM provide important information on the sampling strategy required for monitoring inoculant strains in the rhizosphere.     

Bacterial Biosynthesis of 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase and Indole-3-Acetic Acid (IAA) as Endophytic Preferential Selection Traits by Rice Plant Seedlings

In this study, bacteria were isolated from the rhizosphere and inside the roots and nodules of berseem clover plants grown in the field in Iran. Two hundred isolates were obtained from the rhizosphere (120 isolates), interior roots (57 isolates), and nodules (23 isolates) of clover plants grown in rotation with rice plants. Production of chitinase, pectinase, cellulase, siderophore, salicylic acid, hydrogen cyanide, indole acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase, solubilization of phosphate, antifungal activity against various rice plant pathogen fungi, N₂ fixation, and colonization assay on rice seedlings by these strains was evaluated and compared (endophytic isolates vs. rhizosphere bacteria). The results showed both the number and the ability of plant growth-promoting (PGP) traits were different between endophytic and rhizosphere isolates. A higher percentage of endophytic isolates were positive for production of IAA, ACC deaminase, and siderophore than rhizosphere isolates. Therefore, it is suggested that clover plant may shape its own associated microbial community and act as filters for endophyte communities, and rhizosphere isolates with different (PGP) traits. We also studied the PGP effect of the most promising endophytic and rhizosphere isolates on rice seedlings. A significant relationship among IAA and ACC deaminase production, the size of root colonization, and plant growth (root elongation) in comparison with siderophore production and phosphate solubilization for the isolates was observed. The best bacterial isolates (one endophytic isolate and one rhizosphere isolate), based on their ability to promote rice growth and colonize rice roots, were identified. Based on 16S rDNA sequence analysis, the endophytic isolate CEN7 and the rhizosphere isolate CEN8 were closely related to Pseudomonas putida and Pseudomonas fluorescens, respectively. It seems that PGP trait production (such as IAA, ACC deaminase) may be required for endophytic and rhizosphere competence as compared to other PGP traits in rice seedlings under constant flooded conditions. The study also shows that the presence of diverse rhizobacteria with effective growth-promoting traits associated with clover plants may be used for sustainable crop management under field conditions.     

The effect of tryptophan of plant root metabolites on the phyto stimulating activity of rhizobacteria

Aseptic tomato and radish roots were found to exude 2.8-5.3 and 290-390 ng tryptophan per seedling per day. The inoculation of radish plants with rhizosphere pseudomonads increased the root biomass by 1.4 times. The inoculation of tomato plants with the same pseudomonads was ineffective. The beneficial effect of bacterial inoculation on the radish plants can be explained by the fact that the introduced rhizobacteria produce the plant growth-stimulating hormone indole-3-acetic acid. In pot experiments, the addition of this phytohormone to the soil increased the mass of radish roots by 36%. The phytohormonal action of the rhizosphere microflora was found to be efficient provided that the concentration of tryptophan in the rhizosphere is sufficiently high.     

The plant growth-promoting rhizobacterium Bacillus cereus AR156 induces resistance in tomato with induction and priming of defence response

In a previous study, we demonstrated the ability of the rhizobacterium Bacillus cereus AR156 (AR156) to protect tomato against bacterial wilt caused by Ralstonia solanacearum and root-knot disease caused by Meloidogyne incognita. Here, we investigate the ability of AR156 to promote plant growth and its role in the systemic protection of tomatoes cultivated in greenhouses against bacterial speck disease caused by Pseudomonas syringae pv. tomato DC3000 (DC3000). In our experiments, the AR156 population reached 10⁵–10⁶ CFU/g rhizosphere soil, and remained at that level in the rhizosphere of tomato plants for more than 2 months. In terms of its ability to promote plant growth, AR156 increased the average biomass of the tomato by 47.7%. AR156 also elicited induced systemic resistance against DC3000, significantly reduced bacterial speck disease severity 1.6-fold, and inhibited proliferation of the pathogen by approximately 15-fold. This strain triggered the accumulation of defence-related genes (PR1 and PIN2) in tomato leaves and primed the leaves for accelerated defence-related gene expression upon challenge with DC3000. That suggested simultaneous activation of the salicylic acid and the jasmonic acid dependent signalling pathways by AR156 against DC3000. In conclusion, B. cereus AR156 was found to form robust colonies in the roots of tomato and had some beneficial effects, including biological control of bacterial speck disease via ISR and promotion of plant growth.     

Bacterial inoculation of maize affects carbon allocation to roots and carbon turnover in the rhizosphere

To assess the influence of bacteria inoculation on carbon flow through maize plant and rhizosphere,¹⁴C allocation after¹⁴CO₂ application to shoots over a 5-day period was determined. Plants were grown on C- and N-free quartz sand in two-compartment pots, separating root and shoot space. While one treatment remained uninoculated, treatments two and three were inoculated withPantoea agglomerans (D5/23) andPseudomonas fluorescens (Ps I A12), respectively, five days after planting. Bacterial inoculation had profound impacts on carbon distribution within the system. Root/rhizosphere respiration was increased and more carbon was allocated to roots of plants being inoculated. After five days of¹⁴CO₂ application, more ethanol-soluble substances were found in roots of inoculated treatments and lower rhizodeposition indicated intensive C turnover in the rhizosphere. In both inoculated treatments the intensity of photosynthesis measured as net-CO₂-assimilation rates were increased when compared to the uninoculated plants. However, high C turnover in the rhizosphere reduced shoot growth of D5/23 inoculated plants, with no effect on shoot growth of Ps I A12 inoculated plants. A separation of labeled compounds in roots and rhizodeposition revealed that neutral substances (sugars) constituted the largest fraction. The relative fractions of sugars, amino acids and organic acids in roots and rhizodeposition suggest that amino acid exudation was particularly stimulated by bacterial inoculation and that turnover of this substance group is high in the rhizosphere.     

Effect of 2,4-Diacetylphloroglucinol Producing,Overproducing, and Nonproducing Pseudomonas fluorescens F113 in the Rhizosphere of Pea

Pseudomonas fluorescens F113lacZY and modified strains carrying different function modifications were assessed for their impact in the rhizosphere of pea. Strain F113lacZY naturally produces the anti-fungal metabolite 2,4-diacetylphloroglucinol (Phl) useful in plant disease control. The first modified strain of F113 was repressed in production of Phl, creating the Phl negative strain F113G22. The second was a plasmid based overproducer of Phl (F113Rif (pCUGP)). Both the F113lacZY and the F113Rif (pCUGP) strains increased the rhizoplane fungal populations, whereas the same strains reduced the rhizosphere soil fungal populations with respect to the control. Similar results were found with the rhizoplane and rhizosphere soil bacterial populations. The F113G22 treatment resulted in a significantly greater indigenous fluorescent Pseudomonas population than the F113lacZY and F113Rif (pCUGP) treatments and a greater total Pseudomonas population than the control, F113lacZY, and F113Rif (pCUGP) treatments. Overproduction of Phl did not affect the establishment of the introduced Pseudomonas population. None of the inocula displaced the indigenous populations, but the F113G22 inocula had an additive effect on the total Pseudomonas population. P (phosphatase), S (sulphatase), and N (urease) cycle enzyme activities were increased while C (glucosidase, NAGase) cycle activities were decreased by the F113lacZY and F113Rif (pCUGP) treatments, suggesting C leakage from the roots. Overall, most effects of inoculation compared to the wild type were found with the non-Phl-producing strain. Overproduction of Phl had little environmental effect in relation to wild-type inocula.     

The Effect of Tryptophan Present in Plant Root Exudates on the Phytostimulating Activity of Rhizobacteria

Aseptic tomato and radish roots were found to exude, respectively, 2.8–5.3 and 290–390 ng tryptophan per seedling per day. The inoculation of radish plants with rhizosphere pseudomonads increased the root biomass by 1.4 times. The inoculation of tomato plants with the same pseudomonads was ineffective. The beneficial effect of bacterial inoculation on the radish plants can be explained by the fact that the introduced rhizobacteria produce the plant growth–stimulating hormone indole-3-acetic acid. In pot experiments, the addition of this phytohormone to the soil increased the mass of radish roots by 36%. The phytohormonal action of the rhizosphere microflora was found to be efficient provided that the concentration of tryptophan in the rhizosphere is sufficiently high.     

Protection of Tomato Seedlings against Infection by Pseudomonas syringae pv. Tomato by Using the Plant Growth-Promoting Bacterium Azospirillum brasilense

Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (10⁷ versus 10⁵ CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (10⁷ versus 10⁶ CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (10⁵ to 10⁶ CFU/g [dry weight] of root). However, P. syringae pv. tomato did not survive in the rhizosphere in the presence of A. brasilense. Foliar inoculation of A. brasilense after P. syringae pv. tomato was established on the leaves did not alleviate bacterial speck disease, and A. brasilense did not survive well in the phyllosphere under these conditions, even in a mist chamber. Several applications of a low concentration of buffered malic acid significantly enhanced the leaf population of A. brasilense (>10⁸ CFU/g [dry weight] of leaf), decreased the population of P. syringae pv. tomato to almost undetectable levels, almost eliminated disease development, and improved plant growth to the level of uninoculated healthy control plants. Based on our results, we propose that A. brasilense be used in prevention programs to combat the foliar bacterial speck disease caused by P. syringae pv. tomato.     

Phenylacetic acid-producing Rhizoctonia solani represses the biosynthesis of nematicidal compounds in vitro and influences biocontrol of Meloidogyne incognita in tomato by Pseudomonas fluorescens strain CHA0 and its GM derivatives

AIMS: The aim of the present investigation was to determine the influence of Rhizoctonia solani and its pathogenicity factor on the production of nematicidal agent(s) by Pseudomonas fluorescens strain CHA0 and its GM derivatives in vitro and nematode biocontrol potential by bacterial inoculants in tomato. METHODS AND RESULTS: One (Rs7) of the nine R. solani isolates from infected tomato roots inhibited seedling emergence and caused root rot in tomato. Thin layer chromatography revealed that culture filtrates of two isolates (Rs3 and Rs7) produced brown spots at Rf-values closely similar to synthetic phenylacetic acid (PAA), a phytotoxic factor. Filtrates from isolate Rs7, amended with the growth medium of P. fluorescens, markedly repressed nematicidal activity and PhlA'-'LacZ reporter gene expression of the bacteria in vitro. On the contrary, isolate Rs4 enhanced nematicidal potential of a 2,4-diacetylphloroglucinol overproducing mutant, CHA0/pME3424, of P. fluorescens strain CHA0 in vitro. Therefore, R. solani isolates Rs4 and Rs7 were tested more rigorously for their potential to influence biocontrol effectiveness of the bacterial agents. Methanol extract of the culture filtrates of PAA-producing isolate Rs7 resulting from medium amended with phenylalanine enhanced fungal repression of the production of nematicidal agents by bacteria, while amendments with zinc or molybdenum eliminated such fungal repression, thereby restoring bacterial potential to cause nematode mortality in vitro. A pot experiment was carried out, 3-week-old tomato seedlings were infested with R. solani isolates Rs4 or Rs7 and/or inoculated with Meloidogyne incognita, the root-knot nematode. The infested soil was treated with aqueous cell suspensions (10(8) CFU) of P. fluorescens strain CHA0 or its GM derivatives or left untreated (as a control). Observations taken 45 days after nematode inoculation revealed that, irrespective of the bacterial treatments, galling intensity per gram of fresh tomato roots was markedly higher in soil amended with isolate Rs4 than in Rs7-amended soils. Soil amendments with R. solani and the bacterial antagonists resulted in substantial reductions of the number of galls per gram of root. These results are contradictory to those obtained under in vitro conditions where culture filtrates of PAA-positive Rs7 repressed the production of nematicidal compounds. Plants grown in Rs7-amended soils, with or without bacterial inoculants, had lesser shoot and root weights than plants grown in nonamended or Rs4-amended soils. Moreover, amendments with Rs7 substantially retarded root growth and produced necrotic lesions that reduced the number of entry sites for invasion and subsequent infection by nematodes. Populations of P. fluorescens in the tomato rhizosphere were markedly higher in Rs7-amended soils. CONCLUSIONS: PAA-producing virulent R. solani drastically affects the potential of P. fluorescens to cause death of M. incognita juveniles in vitro and influences bacterial effectiveness to suppress nematodes in tomato roots. SIGNIFICANCE AND IMPACT OF THE STUDY: As most agricultural soils are infested with root-infecting fungi, including R. solani, it is likely that some PAA-producing isolates of the fungus may also be isolated from such soils. The inhibitory effect of PAA-producing R. solani on the biosynthesis of nematicidal agent(s) critical in biocontrol may reduce or even eliminate the effectiveness of fluorescent pseudomonads against root-knot nematodes, both in nursery beds and in field conditions. Introduction of bacterial inoculants, for the control of any plant pathogen, should be avoided in soils infested with PAA-producing R. solani. Alternatively, the agents could be applied together with an appropriate quantity of fungicide or chemicals such as zinc to create an environment more favourable for bacterial biocontrol action.     

Influence of Pythium oligandrum on the bacterial communities that colonize the nutrient solutions and the rhizosphere of tomato plants

The influence exerted by the biocontrol oomycete Pythium oligandrum on the bacterial populations proliferating in the rhizosphere of tomato plants grown in a hydroponic system and in the circulating solutions is studied in the present experiment. Quantitative PCR and single-strand conformation polymorphism were used to investigate the genetic structure and dynamics of the bacterial communities colonizing the root systems and the various circulating solutions. Quantitative PCR assays showed that bacteria heavily colonized the rhizosphere of tomato plants with, however, no significant density changes throughout the cultural season (April-September). Single strand conformation polymorphism fingerprints revealed the occurrence of transient perturbations in the rhizospheric indigenous bacterial communities following P.?oligandrum introduction in the root system of plants. This effect was, however, transient and did not persist until the end of the cropping season. Interestingly, the genetic structure of the bacterial microflora colonizing either the roots or the nutrient solutions evolved throughout the cropping season. This temporal evolution occurred whatever the presence and persistence of P.?oligandrum in the rhizosphere. Evidence is also provided that bacterial microflora that colonize the root system are different from the ones colonizing the circulating solutions. The relationships between these 2 microflora (at the root and solution levels) are discussed.     

Phytohormone production and colonization of canola (Brassica napus L.) roots by Pseudomonas fluorescens 6-8 under gnotobiotic conditions

Pseudomonas fluorescens 6-8, a rhizosphere isolate previously shown to enhance root elongation of canola ( Brassica napus L.), was characterized for its ability to produce indole-3-acetic acid and cytokinins in pure culture and in the rhizosphere of canola under gnotobiotic conditions in comparison with the cytokinin-producing strain P. fluorescens G20-18 and its mutant CNT2. Strain 6-8 produced isopentenyl adenosine, zeatin riboside, and dihydroxyzeatin riboside at levels similar to those of G20-18, but only very low concentrations of indole-3-acetic acid. In a gnotobiotic assay canola inoculated with 6-8 and G20-18 had higher concentrations of isopentenyl adenosine and zeatin riboside in the rhizosphere and greater root length than the noninoculated control. The ability of strain 6-8 to colonize canola roots was assessed following transformation with the green fluorescent protein and inoculation onto canola seed in a gnotobiotic assay. Higher populations of strain 6-8 were observed on the proximal region of the root closest to the seed than on the mid and distal portions 9?days after seed inoculation. The ability of P. fluorescens 6-8 to produce cytokinins, colonize the roots of canola seedlings, and enhance root elongation may contribute to its ability to survive in the rhizosphere and may benefit seedling growth.     

Potential for integrated biological and chemical control of damping-off disease caused by Pythium ultimum in tomato

Pythium ultimum (Trow) is one of the main causes of damping-off disease in many parts of the world. Control of the disease depends mainly on application of chemical fungicides. However, soil treatments with fungicides are not always feasible due to economical and ecological reasons. Soil-borne, non-pathogenic bacteria of the genus Pseudomonas fluorescens with the ability to antagonise fungal phytopathogens, represent a realistic alternative to chemical fungicides and show great promise with respect to protect plant roots from fungal-induced diseases. In an attempt to find an integrated control system of damping-off disease in tomato, fungicides including azoxystrobin, metalaxyl-M and pyraclostrobin were applied alone and in combination with P. fluorescens isolate CW2. The fungicides were tested in in vitro for their antagonistic potential against P. ultimum and for compatibility with CW2. It was found that the fungicides were fungitoxic to P. ultimum, but did not inhibit the growth of the P. fluorescens. The efficacy of the fungicides alone and in combination with CW2 was also tested in greenhouse experiments against damping-off disease on tomato. Two concentrations (5 and 10?μg?ml^?1) were applied. Damping-off incidence of tomato seedlings in Humosoil^?:sand mixture infested with P. ultimum was reduced following seed treatment with the fungicides. However, the degree of control obtained varied significantly depending on the fungicide used. Combined seed treatment with P. fluorescens and the fungicides resulted in a significant improvement in disease control and improved plant growth as indicated by shoot and root dry weights. Metalaxyl-M treatment applied alone or in combination with P. fluorescens, significantly protected tomato seedlings against damping-off. Strobilurin fungicides stimulated plant growth compared to metalaxyl-M. Combined treatment of tomato seeds with strobilurin fungicides and CW2 showed a moderate to good disease control and an increase in shoot and root dry weights.     

Tomato roots secrete tomatine to modulate the bacterial assemblage of the rhizosphere

Saponins are the group of plant specialized metabolites which are widely distributed in angiosperm plants and have various biological activities. The present study focused on α-tomatine, a major saponin present in tissues of tomato (Solanum lycopersicum) plants. α-Tomatine is responsible for defense against plant pathogens and herbivores, but its biological function in the rhizosphere remains unknown. Secretion of tomatine was higher at the early growth than the green-fruit stage in hydroponically grown plants, and the concentration of tomatine in the rhizosphere of field-grown plants was higher than that of the bulk soil at all growth stages. The effects of tomatine and its aglycone tomatidine on the bacterial communities in the soil were evaluated in vitro, revealing that both compounds influenced the microbiome in a concentration-dependent manner. Numerous bacterial families were influenced in tomatine/tomatidine-treated soil as well as in the tomato rhizosphere. Sphingomonadaceae species, which are commonly observed and enriched in tomato rhizospheres in the fields, were also enriched in tomatine- and tomatidine-treated soils. Moreover, a jasmonate-responsive ETHYLENE RESPONSE FACTOR 4 mutant associated with low tomatine production caused the root-associated bacterial communities to change with a reduced abundance of Sphingomonadaceae. Taken together, our results highlight the role of tomatine in shaping the bacterial communities of the rhizosphere and suggest additional functions of tomatine in belowground biological communication.

α-Tomatine is the major toxic saponin secreted from tomato roots at high levels during early growth stages and plays an important role in the formation of bacterial communities in the rhizosphere.     

Evaluation of a Native and Introduced Isolate of Pochonia chlamydosporia against Meloidogyne javanica

Growth chamber and plastic tunnel experiments were conducted to compare the ability of a native and introduced isolate of Pochonia chlamydosporia to colonize the rhizosphere of selected plant species and survive in soil. Effects of the isolates on population density of Meloidogyne javanica and yield of tomato after single or multiple fungal applications were also determined. In growth chamber experiments, both isolates showed a similar ability to colonise the rhizosphere of selected vegetables, except for the introduced isolate, which produced more colony forming units cm^-2 of root surface on tomato and cabbage than the native one. In the tunnel house, both isolates parasitized eggs of M. javanica, and the native but not the introduced isolate increased parasitism after multiple applications. The native isolate was recovered more frequently from soil, and was a better colonizer of tomato roots than the introduced one irrespective of the number of fungal applications. Multiple fungal applications of either isolate reduced the nematode gall rating, and the native isolate also reduced the final egg population in roots. Neither isolates reduced final nematode densities in soil or affected tomato yield when compared to untreated plots.     

Fluorescent pseudomonads in the rhizosphere of plants and their relation to root exudates

Fluorescent pseudomonads were present in chernozem soil not influenced by plant roots (10(3)-10(4) per g dry soil) in the rhizosphere soil of various plants (10(4)-10(5) per g soil) and on roots (10(3) to 10(7) per g fresh roots), depending on the species and age of the plant. Relative species representation of fluorescent pseudomonads changed on the roots and in the plant rhizosphere as compared with free soil. Pseudomonas fluorescens, representing 60-93% of the population of fluorescent pseudomonads predominated on the roots of all plants investigated. Somewhat different results were obtained in rhizosphere soil. Relatively higher numbers of P. fluorescens were detected in the rhizosphere soil of cucumber and maize, numbers in the rhizosphere soil of wheat were practically the same as in free soil and higher numbers of P. putida were found in the rhizosphere soil of barley. Almost all components contained in the root exudates of the plants studied, including beta-pyrazolylalanine from the root exudates of cucumbers were utilized as carbon and energy sources. Root exudates of wheat and maize were utilized by the strain P. putida K2 with an efficiency of 73-91%, depending on species and age of the plant.     

Effect of Bacillus mesonae H20-5 Treatment on Rhizospheric Bacterial Community of Tomato Plants under Salinity Stress

Plant growth-promoting bacteria improve plant growth under abiotic stress conditions. However, their effects on microbial succession in the rhizosphere are poorly understood. In this study, the inoculants of Bacillus mesonae strain H20-5 were administered to tomato plants grown in soils with different salinity levels (EC of 2, 4, and 6 dS/m). The bacterial communities in the bulk and rhizosphere soils were examined 14 days after H20-5 treatment using Illumina MiSeq sequencing of the bacterial 16S rRNA gene. Although the abundance of H20-5 rapidly decreased in the bulk and rhizosphere soils, a shift in the bacterial community was observed following H20-5 treatment. The variation in bacterial communities due to H20-5 treatment was higher in the rhizosphere than in the bulk soils. Additionally, the bacterial species richness and diversity were greater in the H20-5 treated rhizosphere than in the control. The composition and structure of the bacterial communities varied with soil salinity levels, and those in the H20-5 treated rhizosphere soil were clustered. The members of Actinobacteria genera, including Kineosporia, Virgisporangium, Actinoplanes, Gaiella, Blastococcus, and Solirubrobacter, were enriched in the H20-5 treated rhizosphere soils. The microbial co-occurrence network of the bacterial community in the H20-5 treated rhizosphere soils had more modules and keystone taxa compared to the control. These findings revealed that the strain H20-5 induced systemic tolerance in tomato plants and influenced the diversity, composition, structure, and network of bacterial communities. The bacterial community in the H20-5 treated rhizosphere soils also appeared to be relatively stable to soil salinity changes.