Importance of Biofilm Formation in Plant Growth Promoting Rhizobacterial Action

Whole genome sequencing and characterization of Pantoea agglomerans DBM 3797, endophyte, isolated from fresh hop (Humulus lupulus L.)

Background

This paper brings new information about the genome and phenotypic characteristics of Pantoea agglomerans strain DBM 3797, isolated from fresh Czech hop (Humulus lupulus) in the Saaz hop-growing region. Although P. agglomerans strains are frequently isolated from different materials, there are not usually thoroughly characterized even if they have versatile metabolism and those isolated from plants may have a considerable potential for application in agriculture as a support culture for plant growth.

Methods

P. agglomerans DBM 3797 was cultured under aerobic and anaerobic conditions, its metabolites were analyzed by HPLC and it was tested for plant growth promotion abilities, such as phosphate solubilization, siderophore and indol-3-acetic acid productions. In addition, genomic DNA was extracted, sequenced and de novo assembly was performed. Further, genome annotation, pan-genome analysis and selected genome analyses, such as CRISPR arrays detection, antibiotic resistance and secondary metabolite genes identification were carried out.

Results and discussion

The typical appearance characteristics of the strain include the formation of symplasmata in submerged liquid culture and the formation of pale yellow colonies on agar. The genetic information of the strain (in total 4.8 Mb) is divided between a chromosome and two plasmids. The strain lacks any CRISPR-Cas system but is equipped with four restriction-modification systems. The phenotypic analysis focused on growth under both aerobic and anaerobic conditions, as well as traits associated with plant growth promotion. At both levels (genomic and phenotypic), the production of siderophores, indoleacetic acid-derived growth promoters, gluconic acid, and enzyme activities related to the degradation of complex organic compounds were found. Extracellular gluconic acid production under aerobic conditions (up to 8 g/l) is probably the result of glucose oxidation by the membrane-bound pyrroloquinoline quinone-dependent enzyme glucose dehydrogenase. The strain has a number of properties potentially beneficial to the hop plant and its closest relatives include the strains also isolated from the aerial parts of plants, yet its safety profile needs to be addressed in follow-up research.

Biocontrol potential of Bacillus subtilis CTXW 7-6-2 against kiwifruit soft rot pathogens revealed by whole-genome sequencing and biochemical characterisation

Soft rot causes significant economic losses in the kiwifruit industry. This study isolated strain CTXW 7-6-2 from healthy kiwifruit tissue; this was a gram-positive bacterium that produced the red pigment pulcherrimin. The phylogenetic tree based on 16S ribosomal RNA, gyrA, rpoB, and purH gene sequences identified CTXW 7-6-2 as a strain of Bacillus subtilis. CTXW 7-6-2 inhibited hyphal growth of pathogenic fungi that cause kiwifruit soft rot, namely, Botryosphaeria dothidea, Phomopsis sp., and Alternaria alternata, by 81.76, 69.80, and 32.03%, respectively. CTXW 7-6-2 caused the hyphal surface to become swollen and deformed. Volatile compounds (VOC) produced by the strain inhibited the growth of A. alternata and Phomopsis sp. by 65.74 and 54.78%, respectively. Whole-genome sequencing revealed that CTXW 7-6-2 possessed a single circular chromosome of 4,221,676 bp that contained 4,428 protein-coding genes, with a guanine and cytosine (GC) content of 43.41%. Gene functions were annotated using the National Center for Biotechnology Information (NCBI) non-redundant protein, Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes, Clusters of Orthologous Groups of proteins, Gene Ontology, Pathogen-Host Interactions, Carbohydrate-Active enZYmes, and Rapid Annotations using Subsystem Technology databases, revealing non-ribosomal pathways associated with antifungal mechanisms, biofilm formation, chemotactic motility, VOC 3-hydroxy-2-butanone, cell wall-associated enzymes, and synthesis of various secondary metabolites. antiSMASH analysis predicted that CTXW 7-6-2 can produce the active substances bacillaene, bacillibactin, subtilosin A, bacilysin, and luminmide and has four gene clusters of unknown function. Quantitative real-time PCR (qRT-PCR) analysis verified that yvmC and cypX, key genes involved in the production of pulcherrimin, were highly expressed in CTXW 7-6-2. This study elucidates the mechanism by which B. subtilis strain CTXW 7-6-2 inhibits pathogenic fungi that cause kiwifruit soft rot, suggesting the benefit of further studying its antifungal active substances.

Evaluating the adhesive potential of the newly isolated bacterial strains in research exploitation of plant microbial interaction

Bacterial adhesion potential constitutes the transition of bacteria from the planktonic to the static phase by promoting biofilm formation, which plays a significant role in plant-microbial interaction in the agriculture industry. In present study, the adhesion potential of five soil-borne bacterial strains belonging to different genera was studied. All bacterial strains were capable of forming colonies and biofilms of different levels of firmness on polystyrene. Significant variation was observed in hydrophobicity and motility assays. Among the five bacterial strains (SH-6, SH-8, SH-9, SH-10, and SH-19), SH-19 had a strong hydrophobic force, while SH-10 showed the most hydrophilic property. SH-6 showed great variability in motility; SH-8 had a swimming diffusion diameter of 70 mm, which was three times higher than that of SH-19. In the motility assay, SH-9 and SH-10 showed diffusion diameters of approximately 22 mm and 55 mm, respectively. Furthermore, among the five strains, four are predominately electron donors and one is electron acceptors. Overall, positive correlation was observed among Lewis acid base properties, hydrophobicity, and biofilm forming ability. However, no correlation of motility with bacterial adhesion could be found in present experimental work. Scanning electron microscopy images confirmed the adhesion potential and biofilm ability within extra polymeric substances. Research on the role of adhesion in biofilm formation of bacteria isolated from plants is potentially conducive for developing strategies such as plant-microbial interaction to mitigate the abiotic stress.

Bacillus subtilis Impact on Plant Growth, Soil Health and Environment: Dr. Jekyll and Mr. Hyde

The increased dependence of farmers on chemical fertilizers poses a risk to soil fertility and ecosystem stability. Plant growth-promoting rhizobacteria (PGPR) are at the forefront of sustainable agriculture, providing multiple benefits for the enhancement of crop production and soil health. Bacillus subtilis is a common PGPR in soil that plays a key role in conferring biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation, and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Although it harbors several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota. In some instances, it affects the beneficial microbes of the rhizosphere. External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH, and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity. The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics, and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota. Bacillus subtilis has more positive attributes similar to the character of Dr. Jekyll and some negative attributes on plant growth, soil health, and the environment akin to the character of Mr. Hyde.

Cyanobacterial inoculation as resource conserving options for improving the soil nutrient availability and growth of maize genotypes

Harnessing the benefits of plant-microbe interactions towards better nutrient mobilization and plant growth is an important challenge for agriculturists globally. In our investigation, the focus was towards analyzing the soil-plant-environment interactions of cyanobacteria-based formulations (Anabaena-Nostoc consortium, BF1-4 and Anabaena-Trichoderma biofilm, An-Tr) as inoculants for ten maize genotypes (V1-V10). Field experimentation using seeds treated with the formulations illustrated a significant increase of 1.3- to 3.8-fold in C-N mobilizing enzyme activities in plants, along with more than five- to six-fold higher values of nitrogen fixation in rhizosphere soil samples. An increase of 22-30% in soil available nitrogen was also observed at flag leaf stage, and 13-16% higher values were also recorded in terms of cob yield of V6 with An-Tr biofilm inoculation. Savings of 30 kg N ha^-1 season^-1 was indicative of the reduced environmental pollution, due to the use of microbial options. The use of cyanobacterial formulations also enhanced the economic, environmental and energy use efficiency. This was reflected as 37-41% reduced costs lowered GHG emission by 58-68 CO₂ equivalents and input energy requirement by 3651-4296 MJ, over the uninoculated control, on hectare basis. This investigation highlights the superior performance of these formulations, not only in terms of efficient C-N mobilization in maize, but also making maize cultivation a more profitable enterprise. Such interactions can be explored as resource-conserving options, for future evaluation across ecologies and locations, particularly in the global climate change scenario.

Pseudomonas stutzeri MJL19, a rhizosphere-colonizing bacterium that promotes plant growth under saline stress

AIMS

The aim of this study was to find and use rhizobacteria able to confer plants advantages to deal with saline conditions.

METHODS AND RESULTS

We isolated 24 different bacterial species from the rhizosphere of halophyte plants growing in Santiago del Estero, Argentina salt flat. Four strains were selected upon their ability to grow in salinity and their biochemical traits associated with plant growth promotion. Next, we tested the adhesion on soybean seeds surface and root colonization with the four selected isolates. Isolate 19 stood out from the rest and was selected for further experiments. This strain showed positive chemotaxis towards soybean root exudates and a remarkable ability to form biofilm both in vitro conditions and on soybean roots. Interestingly, this trait was enhanced in high saline conditions, indicating the extremely adapted nature of the bacterium to high salinity. In addition, this strain positively impacted on seed germination, plant growth and general plant health status also under saline stress.

CONCLUSIONS

A bacterium isolate with outstanding ability to promote seed germination and plant growth under saline conditions was found.

SIGNIFICANCE AND IMPACT OF THE STUDY

The experimental approach allowed us to find a suitable bacterial candidate for a biofertilizer intended to alleviate saline stress on crops. This would allow the use of soil now considered inadequate for agriculture and thus prevent further advancement of agriculture frontiers into areas of environmental value.

Quorum Sensing System Affects the Plant Growth Promotion Traits of Serratia fonticola GS2

Quorum sensing (QS) enables bacteria to organize gene expression programs, thereby coordinating collective behaviors. It involves the production, release, and population-wide detection of extracellular signaling molecules. The cellular processes regulated by QS in bacteria are diverse and may be used in mutualistic coordination or in response to changing environmental conditions. Here, we focused on the influence of the QS-dependent genes of our model bacterial strain Serratia fonticola GS2 on potential plant growth promoting (PGP) activities including indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, and biofilm formation. Based on genomic and phenotypic experimental data we identified and investigated the function of QS genes in the genome of the model strain. Our gene deletion study confirmed the biological functionality of the QS auto-inducer (gloI) and receptor (gloR) on potential PGP activities of GS2. A transcriptomic approach was also undertaken to understand the role of QS genes in regulation of genes primarily involved in PGP activities (IAA, ACC deaminase activity, and biofilm formation). Both transcriptomic and phenotypic data revealed that the QS-deletion mutants had considerably less PGP activities, as compared to the wild type. In addition, in vivo plant experiments showed that plants treated with GS2 had significantly higher growth rates than plants treated with the QS-deletion mutants. Overall, our results showed how QS-dependent genes regulate the potential PGP activities of GS2. This information may be helpful in understanding the relationship between QS-dependent genes and the PGP activity of bacteria, which aid in the production of practical bio-fertilizers for plant growth promotion.

Screening for Candidate Genes Associated with Biocontrol Mechanisms of Bacillus pumilus DX01 Using Tn5 Transposon Mutagenesis and a 2-DE-Based Comparative Proteomic Analysis

A total of 1467 mutants of the biocontrol bacterium Bacillus pumilus DX01 were obtained by Tn5 insertional mutagenesis and subjected to the determination of antagonistic capabilities. Compared with the wild-type strain DX01, the mutant M25 was identified to have the most significant reduction in antagonistic capability against the phytopathogen Bipolaris maydis and extracellular proteinase activity. The integration site of the exogenous T-DNA in the genome of mutant M25 was revealed in the coding region of malony CoA-ACP transacylase (MCAT) gene (mcat), which belongs to a polyketide synthase (PKS) gene cluster, DX01pks of B. pumilus DX01. Furthermore, the whole DX01pks gene cluster was cloned using Illumina Solexa sequencing technology, and it has a modular framework different from the other two gene clusters involved in polyketide synthesis in B. amyloliquefaciens FZB42 (pks1) and B. subtilis 168 (pksX). Finally, in order to gain more insights into the molecular mechanisms of biocontrol of B. pumilus DX01, the changes in the relative level of expression of total proteins between the original strain DX01 and the mutant M25 were detected by 2-DE-based proteomic analysis. A total of twenty differentially expressed proteins were identified upon the mcat gene transposition mutagenesis. Of these proteins, seven proteins were up-regulated in expression level and the other proteins were down-regulated. Taken together, the results in this study showed that Tn5 transposon mutagenesis of B. pumilus DX01 can lead to a significant change of antiphytopathogen ability, and the DX01pks gene cluster possibly play a potential role in the biocontrol processes of this bacterium.

Bacillus toyonensis COPE52 Modifies Lipid and Fatty Acid Composition, Exhibits Antifungal Activity, and Stimulates Growth of Tomato Plants Under Saline Conditions

Salinity is one of the most important factors that limit the productivity of agricultural soils. Certain plant growth-promoting bacteria (PGPB) have the ability to stimulate the growth of crop plants even under salt stress. In the present study, we analysed the potential of PGPB Bacillus toyonensis COPE52 to improve the growth of tomato plants and its capacity to modify its membrane lipid and fatty acid composition under salt stress. Thus, strain COPE52 increased the relative amount of branched chain fatty acids (15:0i and 16:1∆9) and accumulation of an unknown membrane lipid, while phosphatidylethanolamine (PE) levels decreased during growth with 100 and 200 mM NaCl. Importantly, direct and indirect plant growth-promoting (PGP) mechanisms of B. toyonensis COPE52, such as indole-3-acetic acid (IAA), protease activity, biofilm formation, and antifungal activity against Botrytis cinerea, remained unchanged in the presence of NaCl in vitro, compared to controls without salt. In a greenhouse experiment, tomato plants (Lycopersicon esculentum 'Saladette') showed increased shoot and root length, higher dry biomass, and chlorophyll content when inoculated with B. toyonensis COPE52 at 0 and 100 mM NaCl. In summary, these results indicate that Bacillus toyonensis COPE52 can modify cell membrane lipid components as a potential protecting mechanism to maintain PGP traits under saline-soil conditions.

Functional Imaging of Microbial Interactions With Tree Roots Using a Microfluidics Setup

Coupling microfluidics with microscopy has emerged as a powerful approach to study at cellular resolution the dynamics in plant physiology and root-microbe interactions (RMIs). Most devices have been designed to study the model plant Arabidopsis thaliana at higher throughput than conventional methods. However, there is a need for microfluidic devices which enable in vivo studies of root development and RMIs in woody plants. Here, we developed the RMI-chip, a simple microfluidic setup in which Populus tremuloides (aspen tree) seedlings can grow for over a month, allowing continuous microscopic observation of interactions between live roots and rhizobacteria. We find that the colonization of growing aspen roots by Pseudomonas fluorescens in the RMI-chip involves dynamic biofilm formation and dispersal, in keeping with previous observations in a different experimental set-up. Also, we find that whole-cell biosensors based on the rhizobacterium Bacillus subtilis can be used to monitor compositional changes in the rhizosphere but that the application of these biosensors is limited by their efficiency at colonizing aspen roots and persisting. These results indicate that functional imaging of dynamic root-bacteria interactions in the RMI-chip requires careful matching between the host plant and the bacterial root colonizer.

Priming maize seeds with cyanobacteria enhances seed vigour and plant growth in elite maize inbreds

Biofilm formation of a nitrogen-fixing cyanobacterium Anabaena torulosa with a beneficial fungus Trichoderma viride (An-Tr) was examined under laboratory conditions. A gradual enhancement in growth over A. torulosa alone was recorded in the biofilm, with 15-20% higher values in nitrogen fixation, IAA and exopolysaccharide production illustrating the synergism among the partners in the biofilm. To investigate the role of such biofilms in priming seed attributes, mesocosm studies using primed seeds of two maize inbred lines (V6, V7) were undertaken. Beneficial effects of biofilm (An-Tr) were recorded, as compared to uninoculated treatment and cyanobacterial consortium (Anabaena-Nostoc; BF 1-4) at both stages (7 and 21 DAS, days after sowing) with a significant increase of more than 20% in seedling attributes, along with 5-15% increment in seed enzyme activities. More than three- to fivefold higher values in nitrogen fixation and C-N mobilizing enzyme activities, and significant increases in leaf chlorophyll, proteins and PEP carboxylase activity were observed with V7-An-Tr biofilm. Cyanobacterial inoculation brought about distinct changes in the soil phospholipid fatty acid profiles (PLFA); particularly, significant changes in those representing eukaryotes and anaerobic bacteria. Principal component analyses illustrated the significant role of dehydrogenase activity and microbial biomass carbon and distinct elicited effects on soil microbial communities, as evidenced by the PLFA. This investigation highlighted the promise of cyanobacteria as valuable priming options to improve mobilization of nutrients at seed stage, modulating the abundance and activities of various soil microbial communities, thereby, enhanced plant growth and vigour of maize plants.

1-Aminocyclopropane-1-Carboxylate: A Novel and Strong Chemoattractant for the Plant Beneficial Rhizobacterium Pseudomonas putida UW4

Plant growth-promoting rhizobacteria (PGPR) and fungi-bacterial biofilms are both important biofertilizer inoculants for sustainable agriculture. However, the strongest chemoattractant for bacteria to colonize the rhizosphere and mycelia is not clear. Coincidentally, almost all the PGPRs possess 1-aminocyclopropane-1-carboxylate (ACC) deaminase (AcdS) and can utilize ACC as the sole nitrogen source. Here, we found that ACC was a novel, metabolic dependent and methyl-accepting chemoreceptor-involved chemoattractant for Pseudomonas putida UW4. The chemotactic response of UW4 to ACC is significantly greater than that to the amino acids and organic acids identified in the plant root and fungal hyphal exudates. The colonization counts of the UW4 acdS or cheR deletion mutants in the wheat rhizosphere and on Agaricus bisporus mycelia were reduced one magnitude compared with those of UW4. The colonization counts of UW4 on A. bisporus antisense ACC oxidase mycelia with a high ACC production significantly increased compared with A. bisporus, followed by the UW4 cheR complementary strain and the ethylene chemoreceptor gene-deletion mutant. The colonization counts of the UW4 strains on A. bisporus acdS⁺ mycelia with a low ACC production decreased significantly compared with A. bisporus wild type. These results suggested that ACC and not ethylene should be the strongest chemoattractant for the PGPR that contain AcdS.

Identification of Soil Bacterial Isolates Suppressing Different Phytophthora spp. and Promoting Plant Growth

Bacterial isolates obtained from the rhizosphere of Arabidopsis and a plantless compost potting mix was screened for anti-oomycete activity against Phytophthora capsici, Phytophthora citricola, Phytophthora palmivora, and Phytophthora cinnamomi. Three out of 48 isolates exhibited more than 65% inhibition against all tested Phytophthora species and were selected for further studies. These strains, named UQ154, UQ156, and UQ202, are closely related to Bacillus amyloliquefaciens, Bacillus velezensis, and Acinetobacter sp., respectively, based on 16S rDNA sequence analysis. The isolates were evaluated for their ability to fix nitrogen, solubilize phosphate, as well as for siderophore, indoleacetic acid, cell wall degrading enzymes and biofilm production. Their plant growth promoting activities were evaluated by measuring their effect on the germination percentage, root and shoot length, and seedling vigor of lettuce plants. All of these traits were significantly enhanced in plants grown from seeds inoculated with the isolates compared with control plants. Moreover, bacteria-inoculated P. capsici-infected chili plants exhibited improved productivity based on CO2 assimilation rates. Both real-time quantitative PCR and disease severity index revealed significant decreases in pathogen load in infected chili root tissues when plants were previously inoculated with the isolates. Biocontrol activity may result from the secretion of diketopiperazines as identified by Gas chromatography-mass spectrometry analysis of bacterial cultures' extracts. Collectively, this work demonstrates the potential of bacterial isolates to control Phytophthora infection and promote plant growth. They can, therefore be considered as candidate microbial biofertilizers and biopesticides.

Identification and comprehensive evaluation of a novel biocontrol agent Bacillus atrophaeus JZB120050

Bacillus spp. have long been used as biocontrol agents because of their efficient broad-spectrum antimicrobial activity. We identified a novel strain of Bacillus atrophaeus, named JZB120050, from soil. B. atrophaeus JZB120050 had a strong inhibitory effect against Botrytis cinerea and many other phytopathogens. Gas chromatography-mass spectrometry showed that B. atrophaeus JZB120050 produced many secondary metabolites, such as alkanes, alkenes and acids; some of which were related to pathogen inhibition. Enzyme activity analysis showed that B. atrophaeus JZB120050 secreted cell-wall-degrading enzymes, including chitinase, glucanase and protease, which degraded fungal cell walls. Both the novel glucanase gene bglu and chitinase gene chit1 were cloned and heterologously expressed in Escherichia coli and the products showed strong enzyme activity. In addition, B. atrophaeus JZB120050 secreted siderophores and formed a significant biofilm. Future studies should focus on these antimicrobial factors to facilitate widespread application in the field of agricultural biocontrol.

Developing climate-smart restoration: Can plant microbiomes be hardened against heat waves?

Heat waves are increasing in frequency and intensity, presenting a challenge for the already difficult practice of ecological restoration. We investigated whether pre-heating locally sourced rhizosphere soil (inoculum) could acclimatize plants to a field-imposed heat wave in a restoration setting. Soil heating in the laboratory caused a marked shift in rhizosphere bacterial community composition, accompanied by an increase in species evenness. Furthermore, pre-heated rhizosphere soil reduced plant height, number of leaves, and shoot mass of the C₄ grass, blue grama (Bouteloua gracilis), and it reduced the shoot mass of the C₃ grass, Arizona fescue (Festuca arizonica) in the glasshouse. Following transplantation and the application of a field heat wave, pre-heated inoculum did not influence heat wave survival for either plant species. However, there were strong species-level responses to the field heat wave. For instance, heat wave survivorship was over four times higher in blue grama (92%) than in Arizona fescue (22%). These results suggest that the use of C₄ seeds may be preferable for sites exhibiting high heat wave risk. Further research is needed to understand whether inocula are more effective in highly degraded soil in comparison with partially degraded soils.

The Endophytic Strain Klebsiella michiganensis Kd70 Lacks Pathogenic Island-Like Regions in Its Genome and Is Incapable of Infecting the Urinary Tract in Mice

Klebsiella spp. have been isolated from many different environmental habitats but have mainly been associated with nosocomial acquired diseases in humans. Although there are many recently published sequenced genomes of members of this genus, there are very few studies on whole genome comparisons between clinical and non-clinical isolates, and it is therefore still an open question if a strain found in nature is capable of infecting humans/animals. Klebsiella michiganensis Kd70 was isolated from the intestine of larvae of Diatraea saccharalis but genome analysis revealed multiple genes associated with colonization and growth promotion in plants suggesting an endophytic lifestyle. Kd70 cells labeled with gfp confirmed capability of root colonization and soil application of Kd70 promoted growth in greenhouse grown sugarcane. Further genomic analysis showed that the Kd70 genome harbored fewer mammalian virulence factors and no pathogen island-like regions when compared to clinical isolates of this species, suggesting attenuated animal/human pathogenicity. This postulation was corroborated by in vivo experiments in which it was demonstrated that Kd70 was unable to infect the mouse urinary tract. This is to the best of our knowledge the first experimental example of a member of a pathogenic Klebsiella spp. unable to infect a mammalian organism. A proteomic comparison deduced from the genomic sequence between Kd70 and several other K. michiganensis strains showed a high similarity with isolates from many different environments including clinical strains, and demonstrated the existence of conserved genetic lineages within this species harboring members from different ecological niches and geographical locations. Furthermore, most genetic differences were found to be associated with genomic islands of clinical isolates, suggesting that evolutionary adaptation of animal pathogenicity to a large extent has depended on horizontal gene transfer. In conclusion our results demonstrate the importance of conducting thorough in vivo pathogenicity studies before presupposing animal/human virulence of non-clinical bacterial isolates.

Dynamics of Aspen Roots Colonization by Pseudomonads Reveals Strain-Specific and Mycorrhizal-Specific Patterns of Biofilm Formation

Rhizosphere-associated Pseudomonas fluorescens are known plant growth promoting (PGP) and mycorrhizal helper bacteria (MHB) of many plants and ectomycorrhizal fungi. We investigated the spatial and temporal dynamics of colonization of mycorrhizal and non-mycorrhizal Aspen seedlings roots by the P. fluorescens strains SBW25, WH6, Pf0-1, and the P. protegens strain Pf-5. Seedlings were grown in laboratory vertical plates systems, inoculated with a fluorescently labeled Pseudomonas strain, and root colonization was monitored over a period of 5 weeks. We observed unexpected diversity of bacterial assemblies on seedling roots that changed over time and were strongly affected by root mycorrhization. P. fluorescens SBW25 and WH6 stains developed highly structured biofilms with internal void spaces forming channels. On mycorrhizal roots bacteria appeared encased in a mucilaginous substance in which they aligned side by side in parallel arrangements. The different phenotypic classes of bacterial assemblies observed for the four Pseudomonas strains were summarized in a single model describing transitions between phenotypic classes. Our findings also reveal that bacterial assembly phenotypes are driven by interactions with mucilaginous materials present at roots.

Bacterial Compatibility in Combined Inoculations Enhances the Growth of Potato Seedlings

The compatibility of strains is crucial for formulating bioinoculants that promote plant growth. We herein assessed the compatibility of four potential bioinoculants isolated from potato roots and tubers (Sphingomonas sp. T168, Streptomyces sp. R170, Streptomyces sp. R181, and Methylibium sp. R182) that were co-inoculated in order to improve plant growth. We screened these strains using biochemical tests, and the results obtained showed that R170 had the highest potential as a bioinoculant, as indicated by its significant ability to produce plant growth-promoting substances, its higher tolerance against NaCl (2%) and AlCl₃ (0.01%), and growth in a wider range of pH values (5.0–10.0) than the other three strains. Therefore, the compatibility of R170 with other strains was tested in combined inoculations, and the results showed that the co-inoculation of R170 with T168 or R182 synergistically increased plant weight over un-inoculated controls, indicating the compatibility of strains based on the increased production of plant growth promoters such as indole-3-acetic acid (IAA) and siderophores as well as co-localization on roots. However, a parallel test using strain R181, which is the same Streptomyces genus as R170, showed incompatibility with T168 and R182, as revealed by weaker plant growth promotion and a lack of co-localization. Collectively, our results suggest that compatibility among bacterial inoculants is important for efficient plant growth promotion, and that R170 has potential as a useful bioinoculant, particularly in combined inoculations that contain compatible bacteria.

Molecular characterization and identification of plant growth promoting endophytic bacteria isolated from the root nodules of pea (Pisum sativum L.)

Root nodule accommodates various non-nodulating bacteria at varying densities. Present study was planned to identify and characterize the non-nodulating bacteria from the pea plant. Ten fast growing bacteria were isolated from the root nodules of cultivated pea plants. These bacterial isolates were unable to nodulate pea plants in nodulation assay, which indicate the non-rhizobial nature of these bacteria. Bacterial isolates were tested in vitro for plant growth promoting properties including indole acetic acid (IAA) production, nitrogen fixation, phosphate solubilization, root colonization and biofilm formation. Six isolates were able to produce IAA at varying level from 0.86 to 16.16 μg ml(-1), with the isolate MSP9 being most efficient. Only two isolates, MSP2 and MSP10, were able to fix nitrogen. All isolates were able to solubilize inorganic phosphorus ranging from 5.57 to 11.73 μg ml(-1), except MSP4. Bacterial isolates showed considerably better potential for colonization on pea roots. Isolates MSP9 and MSP10 were most efficient in biofilm formation on polyvinyl chloride, which indicated their potential to withstand various biotic and abiotic stresses, whereas the remaining isolates showed a very poor biofilm formation ability. The most efficient plant growth promoting agents, MSP9 and MSP10, were phylogenetically identified by 16S rRNA gene sequence analysis as Ochrobactrum and Enterobacter, respectively, with 99% similarity. It is suggested the potential endophytic bacterial strains, Ochrobactrum sp. MSP9 and Enterobacter sp. MSP10, can be used as biofertilizers for various legume and non-legume crops after studying their interaction with the host crop and field evaluation.