Tobacco Rhizosphere Research Articles

Effects of tobacco plant residue return on rhizosphere soil microbial community

PurposeBased on the recycling principle, returning of tobacco crop residues into the field is a common agronomic practice. However, comprehensive knowledge about the effects of tobacco plant residue return on the rhizosphere soil microbial community is very limited.MethodsAfter tobacco crop residue returning into the potted soil, 16S ribosomal RNA (rRNA) and internal transcribed spacer (ITS) amplicon sequencing were employed to investigate the bacterial and fungal communities, respectively, from the tobacco rhizosphere soils.ResultsThe results showed that tobacco residue returning increased the diversity of microbial communities (bacteria and fungi) and changed the species composition. It further increased the relative abundance of beneficial microorganisms. After tobacco residue returning, the structure of the rhizosphere soil microbial community network was found more complex with strong interactions among microbial species. In addition, the keystone species of bacterial and fungal communities associated with tobacco rhizosphere soil were altered. This had exerted a driving force to the beneficial bacteria such as Sphiningomonas and Psathyrella to the keystone microorganisms which played important roles in microbial species interaction.ConclusionsTobacco residue return into soil showed significant effects on the microbial diversity, community composition, network structure, keystone microorganisms, and ecosystem functions of tobacco rhizosphere soils. This study provides a scientific basis for the improvement of tobacco field ecosystem functioning and ensuring soil health.

Analysis of rhizosphere bacterial communities of tobacco resistant and non-resistant to bacterial wilt in different regions

Tobacco bacterial wilt has seriously affected tobacco production. Ethyl methanesulfonate (EMS) induced tobacco bacterial wilt resistant mutants are important for the control of tobacco bacterial wilt. High-throughput sequencing technology was used to study the rhizosphere bacterial community assemblages of bacterial wilt resistant mutant tobacco rhizosphere soil (namely KS), bacterial wilt susceptible tobacco rhizosphere soil (namely GS) and bulk soil (namely BS) in Xuancheng, Huanxi, Yibin and Luzhou. Alpha analysis showed that the bacterial community diversity and richness of KS and GS in the four regions were not significantly different. However, analysis of intergroup variation in the top 15 bacterial communities in terms of abundance showed that the bacterial communities of KS and GS were significantly different from BS, respectively. In addition, pH, alkali-hydrolysable nitrogen (AN) and soil organic carbon (SOC) were positively correlated with the bacterial community of KS and negatively correlated with GS in the other three regions except Huanxi. Network analysis showed that the three soils in the four regions did not show a consistent pattern of network complexity. PICRUSt functional prediction analysis showed that the COG functions were similar in all samples. All colonies were involved in RNA processing and modification, chromatin structure and dynamics, etc. In conclusion, our experiments showed that rhizosphere bacterial communities of tobacco in different regions have different compositional patterns, which are strongly related to soil factors.

Effect of Altitude on the Release of Metabolites in the Tobacco Rhizosphere Soil

Effect of Altitude on the Release of Metabolites in the Tobacco Rhizosphere Soil

Soil Enzyme Activity and Nutrient Uptake as Influenced by Arbuscular Mycorrhizal Fungi in Tobacco Rhizosphere under Parasitic Weed Stress Condition

The present study assessed the effect of Arbuscular mycorrhizal fungi on nutrient uptake and soil enzyme activity of tobacco rhizosphere under parasitic weed stress condition. The experiment was carried out in Orobanche infested soils of Belagavi district during 2018-19. During the investigation different inoculation methods of Arbuscular mycorrhizal fungi were screened for their ability to improve the nutrient uptake and soil enzyme activity i.e. (pre inoculation of nursery seedlings; direct soil application and the combination of both). The present experimental results revealed that soil enzyme activities like dehydrogenase, phosphates and urease activity was highest in treatments received planting of pre colonized tobacco seedling with STD AMF along with soil application at the time of planting and second highest was recorded in the treatment received pre colonized UASDAMFT plus soil application. Furthermore nutrient uptake like N, P, K and micronutrients were found to be the highest in the plots received mycorrhization compare to the rhizosphere of non mycorrhized plants. In mycorrhizosphere the activity of mycorrhizal fungi and other beneficial microorganisms could be a source for different soil enzyme needed for biochemical reaction in the plant rhizosphere.

Nematicidal Activity of Burkholderia arboris J211 Against Meloidogyne incognita on Tobacco.

Root-knot nematode (Meloidogyne incognita) is the most widespread nematode affecting Solanaceae crops. Due to the lack of effective measures to control this nematode, its management can be achieved, using biocontrol agents. This study investigated in vitro efficacy of the antagonistic bacterial strain J211 isolated from tobacco rhizosphere soil against M. incognita, and further assessed its role in controlling nematodes, both in pot and field trials. Phylogenetic analysis of the 16S rRNA gene sequence of strain J211 assigned to Burkholderia arboris. Culture filtrates B. arboris J211 exhibited anematicidal activity against the second-stage juveniles (J2s) of M. incognita, with a 96.6% mortality after 24 h exposure. Inoculation of J211 in tobacco roots significantly reduced the root galling caused by M. incognita, both in pot and field trials. Meanwhile, plant growth-promoting (PGP) traits results showed that J211 had outstanding IAA-producing activity, and the IAA production reached 66.60 mg L−1. In the field study, B. arboris J211 also promoted tobacco growth and increase flue-cured tobacco yield by 8.7–24.3%. Overall, B. arboris J211 as a high-yielding IAA nematicidal strain effectively controlled M. incognita and improved tobacco yield making it a promising alternative bionematocide.

Soil Conditioner Affects Tobacco Rhizosphere Soil Microecology.

Reasonable fertilization management can increase nutrient content and enzyme activity in rhizosphere soil, and even increase soil microbial richness. However, different fertilizers could raise distinct influences on the soil properties, including soil environmental factors (physicochemical properties and enzymatic activities) and microbial community. Here, the effects of two soil amendments (microbial fertilizer and woody peat) on environmental factors and microbial community structure in tobacco rhizosphere soil were evaluated, with the correlations between microbes and environmental factors explored. As the results, microbial fertilizer could effectively alleviate soil acidification, increase available potassium and organic matter contents in soil, and was also beneficial to increase nitrate reductase activity in rhizosphere soil. Fertilizers cause changes in the abundance of certain microbes in the soil. Besides, it was shown that the candidate phyla Gal15, Acidobacterota, Latescibacterota, Mortierellommycota, Basidiomycota, and Rozellomycota in tobacco rhizosphere soil had significant correlation with soil environmental factors. Through the functional analysis of these populations, it can be deduced that the changes in the abundance of certain microorganisms may be an important reason for the differences in environmental factors. All these indicated that the differences of environmental factors in different treatments are closely related to the abundance of some special soil microorganisms. Studying the life activities of these microbes would provide good guidance for exploring the interaction among crops, soil, and microorganisms and improving crop yields.

Fungal Inhibition of Agricultural Soil Pathogen Stimulated by Nitrogen-Reducing Fertilization.

Plant health is the fundamental of agricultural production, which is threatened by plant pathogens severely. The previous studies exhibited the effects of different pathogen control strategies (physical, chemical, and microbial methods), which resulted from bringing in exogenous additives, on microbial community structures and functions. Nevertheless, few studies focused on the potential inhibitory abilities of native microbial community in the soil, which could be activated or enhanced by different fertilization strategies. In this study, three plant diseases (TMV, TBS, and TBW) of tobacco, fungal community of tobacco rhizosphere soil, and the correlation between them were researched. The results showed that nitrogen-reducing fertilization strategies could significantly decrease the occurrence rate and the disease index of three tobacco diseases. The results of bioinformatics analyses revealed that the fungal communities of different treatments could differentiate the nitrogen-reducing fertilization group and the control group (CK). Furthermore, key genera which were responsible for the variation of fungal community were explored by LEfSe analysis. For instance, Tausonia and Trichocladium increased, while Naganishia and Fusicolla decreased under nitrogen-reducing fertilization conditions. Additionally, the correlation between tobacco diseases and key genera was verified using the Mantel test. Moreover, the causal relationship between key genera and tobacco diseases was deeply explored by PLS–PM analysis. These findings provide a theoretical basis for a nitrogen-reducing fertilization strategy against tobacco diseases without exogenous additives and make contributions to revealing the microbial mechanism of native-valued fungal key taxa against tobacco diseases, which could be stimulated by agricultural fertilization management.

A epsB mutant of Ralstonia solanacearum as novel biocontrol agent of tobacco bacterial wilt via activating salicylic acid signalling

Extracellular polysaccharide (EPS) is the most important virulence factor of R. solanacearum and promotes rapid systemic colonization and induces plant wilting symptoms. In this study, biological control mechanism and effect of EPS biosynthesis genes mutants (ΔepsB, ΔepsC and ΔwecC) on bacterial wilt were evaluated. The expression level of virulence-associated genes, such as xpsR, epsE, vsrB, vsrC and phcA, were significantly decreased in the Δeps strains. Furthermore, application of ΔepsB strain was observed to delay the development of wilt progress for two days and the WT population in rhizosphere soil was significantly reduced by 46.77-fold after ΔepsB strain treatment. If the ΔepsB was applied earlier than WT population, the control efficiencies of ΔepsB were 84.67%, 72.74%, 60.68%, 56.38% and 53.29% at 8, 10, 12, 14, and 16 d after inoculation, respectively. What ‘s more, the ΔepsB mutant significantly induced the expression of plant defence associated genes involved in salicylic acid pathway, but did not affect the expression of genes associated with the jasmonic acid and ethylene pathways. Compared with plant inoculated with WT treatment, the expression of PR1a/c, PR2 and NPR1 was induced 4.5-fold, 6.9-fold and 2.2-fold, respectively, in the plant colonized by ΔepsB mutant. Meanwhile, application of ΔepsB strain in tobacco rhizosphere significantly induced the accumulation of salicylic acid, with increases of 1.58-fold compared with the control. Taken together, the results suggested that the ΔepsB mutant suppressed bacterial wilt disease by activating salicylic acid signalling and has the potential to be used as biocontrol agents for controlling bacterial wilt caused by R. solanacearum.

Effects of Meloidogyne incognita on the fungal community in tobacco rhizosphere

The often widespread and serious Root-Knot Nematode (RKN) disease is an important soil-borne disease affecting tobacco production. This study aimed to understand micro-ecological changes caused by RKN disease and interactions between disease and rhizosphere soil fungal communities. The 18S rRNA gene sequencing was used to study changes in rhizosphere fungal community of tobacco plants having RKN disease. In June 2018, a paired comparison was performed between rhizosphere fungal community structures of healthy tobacco plants and those with RKN disease [...]

Tobacco Root Microbial Community Composition Significantly Associated With Root-Knot Nematode Infections: Dynamic Changes in Microbiota and Growth Stage.

The root-knot nematode (RKN) is an important pathogen that affects the growth of many crops. Exploring the interaction of biocontrol bacteria-pathogens-host root microbes is the theoretical basis for improving colonization and controlling the effect of biocontrol bacteria in the rhizosphere. Therefore, 16S and 18S rRNA sequencing technology was used to explore the microbial composition and diversity of tobacco roots (rhizosphere and endophytic) at different growth stages in typical tobacco RKN-infected areas for 2 consecutive years. We observed that RKN infection changed the α-diversity and microbial composition of root microorganisms and drove the transformation of microorganisms from bacteria to fungi. The abundance of Sphingomonas decreased significantly from 18% to less than 3%, while the abundance of Rhizobiaceae increased from 4 to 15% at the early growth stage during the first planting year, and it promoted the proliferation of Chryseobacterium at the late growth stage in rhizosphere microorganisms with the highest abundance of 17%. The overall trend of rhizosphere microorganisms changed in the early growth stage with increasing growth time. The specific results were as follows: (1) Rhizobiaceae and Chryseobacterium increased rapidly after 75 days, became the main abundant bacteria in the rhizosphere microorganisms. (2) The dominant flora in fungi were Fusarium and Setophoma. (3) Comparing the root microbes in 2017 and 2018, RKN infection significantly promoted the proliferation of Pseudomonas and Setophoma in both the rhizosphere and endophytes during the second year of continuous tobacco planting, increasing the relative abundance of Pseudomonas from 2 to 25%. Pseudomonas was determined to play an important role in plant pest control. Finally, a total of 32 strains of growth-promoting bacteria were screened from tobacco rhizosphere bacteria infected with RKN through a combination of 16S rRNA sequencing and life-promoting tests. The results of this research are helpful for analyzing the relationship between RKNs and bacteria in plants, providing reference data for elucidating the pathogenesis of RKNs and new ideas for the biological control of RKNs. GRAPHICAL ABSTRACT

Production of Agrocinopine A by Ipomoea batatas Agrocinopine Synthase in Transgenic Tobacco and Its Effect on the Rhizosphere Microbial Community.

Agrobacterium tumefaciens is a bacterial pathogen that causes crown gall disease on a wide range of eudicot plants by genetic transformation. Besides T-DNA integrated by natural transformation of plant vegetative tissues by pathogenic Agrobacterium spp., previous reports have indicated that T-DNA sequences originating from an ancestral Agrobacterium sp. are present in the genomes of all cultivated sweet potato (Ipomoea batatas) varieties analyzed. Expression of an Agrobacterium-derived agrocinopine synthase (ACS) gene was detected in leaf and root tissues of sweet potato, suggesting that the plant can produce agrocinopine, a sugar-phosphodiester opine considered to be utilized by some strains of Agrobacterium spp. in crown gall. To validate the product synthesized by Ipomoea batatas ACS (IbACS), we introduced IbACS into tobacco under a constitutive promoter. High-voltage paper electrophoresis followed by alkaline silver nitrate staining detected the production of an agrocinopine-like substance in IbACS1-expressing tobacco, and further mass spectrometry and nuclear magnetic resonance analyses of the product confirmed that IbACS can produce agrocinopine A from natural plant substrates. The partially purified compound was biologically active in an agrocinopine A bioassay. A 16S ribosomal RNA amplicon sequencing and meta-transcriptome analysis revealed that the rhizosphere microbial community of tobacco was affected by the expression of IbACS. A new species of Leifsonia (actinobacteria) was isolated as an enriched bacterium in the rhizosphere of IbACS1-expressing tobacco. This Leifsonia sp. can catabolize agrocinopine A produced in tobacco, indicating that the production of agrocinopine A attracts rhizosphere bacteria that can utilize this sugar-phosphodiester. These results suggest a potential role of IbACS conserved among sweet potato cultivars in manipulating their microbial community.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Microbial Diversity in Tobacco Rhizosphere Soil at Different Growth Stages

Rhizosphere microorganisms are the main participants of material transformation and energy cycle in soil. To further explore its composition and variation, the tobacco rhizosphere soil were sequenced by Illumina MiSeq, the microbial community at different growth stages were analyzed and compared. The analysis of Alpha diversity showed that, the Chao1 index, Shannon index of bacteria and Chao1 index of fungi in rhizosphere soil were the highest in tobacco budding stage, while the peak of Shannon index of fungi appeared in tobacco material stage. Principal component analysis (PCA) further showed that at different growth stages, Proteobacteria was the dominant, followed by Actinobacteria, Acidobacteria and Gemmatimonadetes for bacterials; Ascomycota was the dominant, followed by Zygomycota and Basidiomycota for fungi. Under field conditions, the microbial abundance changed with the growth of tobacco, and the microbial diversity reached the peak at budding stage. The bacterial community and abundance between budding and mature stages was highly similar, while the bacterial community in vigorous growth stage is quite different. The similarity of fungal community in budding stage was very low, compared with the other stages; while in other stages was high. This study provides a theoretical basis for further understanding the relationship between tobacco rhizosphere soil microbial diversity and variation, tobacco growth and soil diseases.

Identification and verification of rhizosphere indicator microorganisms in tobacco root rot

AbstractThis study aimed to investigate the characteristics of the microbial community in the rhizosphere of tobacco (Nicotiana tabacum L.) and explore the potential microbiological indicators of tobacco root rot Rhizosphere soils of healthy and diseased tobacco over different continuous cropping years were sampled, and total DNA was extracted from the soils. Afterward, 16S rRNA combined with internal transcribed spacer gene sequencing was used to study the differences in microbial communities in the rhizosphere. A pot experiment was conducted to evaluate the effects of Paenibacillus polymyxa and Trichoderma harzianum on root rot, and the relative abundance of Fusarium, Rubrobacter, and Talaromyces spp. as the potential indicators of tobacco health were identified by sequencing data Bacterial and fungal diversity decreased with the increase in years of continuous cropping. The bacterial and fungal diversity in the rhizosphere with tobacco root rot was higher than that in the healthy rhizosphere. Rubrobacter and Talaromyces spp. were more abundant in the rhizosphere soil of healthy tobacco than in that of diseased tobacco. Paenibacillus polymyxa and T. harzianum treatment led to an increase in the relative abundance of Rubrobacter and Talaromyces spp., implying that this treatment could have a plant‐promoting effect Rubrobacter and Talaromyces spp. may serve as beneficial microbiological indicators of tobacco root rot, and P. polymyxa and T. harzianum may serve as antagonistic strains for the biocontrol of tobacco root rot.

Pseudomonas sp. TK35-L enhances tobacco root development and growth by inducing HRGPnt3 expression in plant lateral root formation

Rhizosphere colonization is a key requirement for the application of plant growth-promoting rhizobacteria (PGPR) as a biofertilizer. Signaling molecules are often exchanged between PGPR and plants, and genes in plants may respond to the action of PGPR. Here, the luciferase luxAB gene was electrotransformed into Pseudomonas sp. strain TK35, a PGPR with an affinity for tobacco, and the labelled TK35 (TK35-L) was used to monitor colonization dynamics in the tobacco rhizosphere and evaluate the effects of colonization on tobacco growth and root development. The transcript levels of the hydroxyproline-rich glycoprotein HRGPnt3 gene, a lateral root induction indicator, in tobacco roots were examined by qPCR. The results showed that TK35-L could survive for long periods in the tobacco rhizosphere and colonize new spaces in the tobacco rhizosphere following tobacco root extension, exhibiting significant increases in root development, seedling growth and potassium accumulation in tobacco plants. The upregulation of HRGPnt3 transcription in the inoculated tobacco suggested that TK35-L can promote tobacco root development by upregulating the transcript levels of the HRGPnt3 gene, which promotes tobacco seedling growth. These findings lay a foundation for future studies on the molecular mechanism underlying the plant growth-promoting activities of PGPR. Furthermore, this work provided an ideal potential strain for biofertilizer production.

Linking rhizosphere bacterial diversity and soil fertility in tobacco plants under different soil types and cropping pattern in Tanzania: A pilot study

Tobacco (Nicotiana tabacum L.), one of the major crop plants in Tanzania, cropping affects the level of soil fertility, but the reason has not been known. Plant rhizosphere plays an important role in affecting soil fertility through changing microbial composition. We planned a pilot study to understand the changes in microbial composition and soil nutrients in the rhizosphere soils of tobacco in three agro-ecological zone, namely Sikonge, Tabora and Urambo in Tanzania. This study assessed bacteriota composition using 16S rRNA sequencing and soil fertility in the rhizosphere of tobacco plants. The results showed that bacterial diversity in tobacco rhizosphere soils belonged to Proteobacteria phyla, associated significantly (p < 0.05) with solubilization of insoluble P, K and S. The solubilization of P, K and S in soils facilitates the availability of these nutrients to the tobacco plants (a heavy feeder crop) allows low levels of these nutrients in the soils for the subsequent crop. The Proteobacteria phyla also associated with an increase in soil N content through fixation. Therefore, bacteria diversity in tobacco rhizosphere influence solubilities of macronutrients (P, K, S) and quickly up taken by the tobacco plant and reduces their levels in soils, some bacteria involved in fixing N and increases total N in the soil.

Genomic insights into a plant growth-promoting Pseudomonas koreensis strain with cyclic lipopeptide-mediated antifungal activity.

Strain S150 was isolated from the tobacco rhizosphere as a plant growth‐promoting rhizobacterium. It increased plant fresh weight significantly and lateral root development, and it antagonized plant pathogenic fungi but not phytobacteria. Further tests showed that strain S150 solubilized organic phosphate and produced ammonia, siderophore, protease, amylase, and cellulase, but it did not produce indole‐3‐acetic acid. Using morphology, physiological characteristics, and multi‐locus sequence analysis, strain S150 was identified as Pseudomonas koreensis. The complete genome of strain S150 was sequenced, and it showed a single circular chromosome of 6,304,843 bp with a 61.09% G + C content. The bacterial genome contained 5,454 predicted genes that occupied 87.7% of the genome. Venn diagrams of the identified orthologous clusters of P. koreensis S150 with the other three sequenced P. koreensis strains revealed up to 4,167 homologous gene clusters that were shared among them, and 21 orthologous clusters were only present in the genome of strain S150. Genome mining of the bacterium P. koreensis S150 showed that the strain possessed 10 biosynthetic gene clusters for secondary metabolites, which included four clusters of non‐ribosomal peptide synthetases (NRPSs) involved in the biosynthesis of cyclic lipopeptides (CLPs). One of the NRPSs possibly encoded lokisin, a cyclic lipopeptide produced by fluorescent Pseudomonas. Genomic mutation of the lokA gene, which is one of the three structural NRPS genes for lokisin in strain S150, led to a deficiency in fungal antagonism that could be restored fully by gene complementation. The results suggested that P. koreensis S150 is a novel plant growth‐promoting agent with specific cyclic lipopeptides and contains a lokisin‐encoding gene cluster that is dominant against plant fungal pathogens.

The Effects of Tetracycline Residues on the Microbial Community Structure of Tobacco Soil in Pot Experiment

To evaluate the micro-ecological effects of tetracycline residues on tobacco soil, high-throughput sequencing technology was used to study the effects of the addition of different concentrations (0, 5, 50, and 500 mg·kg−1) of tetracycline on the abundance, diversity, and structure of bacterial and fungal communities in the rhizosphere and non-rhizosphere soil of flue-cured tobacco in China. Results showed that the presence of tetracycline had an important but varying effect on soil bacterial and fungal community richness, diversity, and structure. Changes in the diversity indices (Chao index and Shannon index) of soil bacterial and fungal communities showed a similar pattern after the addition of tetracycline; however, a few differences were found in the effects of tetracycline in the rhizosphere and non-rhizosphere soil, suggesting an evident rhizosphere-specific effect. The bacterial community at the phylum level in the rhizosphere closely clustered into one group, which might be the result of tobacco root secretions and rhizodeposition. Tetracycline showed a concentration-dependent effect on the soil bacterial community structure. The soil bacterial community structures observed after treatments with higher concentrations of tetracycline (50 and 500 mg·kg−1) were found to be closely related. Moreover, the effects of the treatments with higher concentrations of tetracycline, on the soil bacterial community at the phylum level, were different from those with lower concentrations of tetracycline (5 mg·kg−1), and CK treatments. This might have resulted from the induction of increasing selective pressure with increasing antibiotic concentration. Tetracycline continued to affect the soil bacterial community throughout the experiment. Tetracycline was found to have a varying impact on the community structure of soil fungi compared to that of soil bacteria, and the addition of an intermediate concentration of tetracycline (50 mg·kg−1) significantly increased the soil fungal diversity in the non-rhizosphere soil. The biological effects of tetracycline on the soil fungal community and the fungal-bacterial interactions, therefore, require further elucidation, warranting further research.

Nicotine release at the tobacco rhizosphere and their adsorption capacities in different soil textures

Nicotine release at the tobacco rhizosphere and their adsorption capacities in different soil textures

Characterization of a Versatile Plant Growth-Promoting Rhizobacterium Pseudomonas mediterranea Strain S58.

Plant growth-promoting rhizobacterial strain S58 was isolated from the tobacco rhizosphere. It showed strong antagonism against a battery of plant pathogenic fungi and bacteria, and controlled wheat sharp eyespot and tobacco wildfire diseases efficiently. Further tests showed that strain S58 solubilized organic phosphate and produced siderophore, protease, ammonia, and indole-3-acetic acid. In Arabidopsis thaliana, it promoted plant growth and changed root system architecture by restricting the growth of primary roots and increasing lateral root numbers. We relied on morphological, biochemical, physiological characteristics, and molecular phylogenic analysis to identify strain S58 as Pseudomonas mediterranea. The complete genome of strain S58 has a single circular chromosome of 6,150,838 bp with a 61.06% G+C content. The bacterial genome contained 5,312 predicted genes with an average length of 992.90 bp. A genome analysis suggested that P. mediterranea S58 was a rich cyclic lipopeptide (CLP)-producing strain that possessed seven non-ribosomal peptide gene clusters for CLP synthesis. Leaf inoculation of the bacterial culture and supernatants triggered cell death-like immunity in tobacco. Quantitative real-time PCR assays showed that the strain S58 induced the expression of pattern-triggered immunity and cell death marker genes, but not jasmonic acid marker genes. The results suggested that P. mediterranea S58 is a novel, versatile plant growth-promoting agent with multiple beneficial traits for plants.

The rhizosphere signature on the cell motility, biofilm formation and secondary metabolite production of a plant-associated Lysobacter strain

Lysobacter spp. are common bacterial inhabitants of the rhizosphere of diverse plant species. However, the impact of the rhizosphere conditions on their physiology is still relatively understudied. To provide clues on the behaviour of Lysobacter spp. in this ecological niche, we investigated the physiology of L. capsici AZ78 (AZ78), a biocontrol strain isolated from tobacco rhizosphere, on a common synthetic growth medium (LBA) and on a growth medium containing components of the plant rhizosphere (RMA). The presence of a halo surrounding the AZ78 colony on RMA was a first visible effect related to differences in growth medium composition and it corresponded to the formation of a large outer ring. The lower quantity of nutrients available in RMA as compared with LBA was associated to a higher expression of a gene encoding cAMP-receptor-like protein (Clp), responsible for cell motility and biofilm formation regulation. AZ78 cells on RMA were motile, equipped with cell surface appendages and organised in small groups embedded in a dense layer of fibrils. Metabolic profiling by mass spectrometry imaging revealed increased diversity of analytes produced by AZ78 on RMA as compared with LBA. In particular, putative cyclic lipodepsipeptides, polycyclic tetramate macrolactams, cyclic macrolactams and other putative secondary metabolites with antibiotic activity were identified. Overall, the results obtained in this study shed a light on AZ78 potential to thrive in the rhizosphere by its ability to move, form biofilm and release secondary metabolites.