Lipopeptides from a novel Bacillus methylotrophicus 39b strain suppress Agrobacterium crown gall tumours on tomato plants

Antifungal potential of lipopeptides produced by the Bacillus siamensis Sh420 strain against Fusarium graminearum

Biological control is a more sustainable and environmentally friendly alternative to chemical fungicides for controlling Fusarium spp. infestations. In this work, Bacillus siamensis Sh420 isolated from wheat rhizosphere showed a high antifungal activity against Fusarium graminearum as a secure substitute for fungicides. Sh420 was identified as B. siamensis using phenotypic evaluation and 16S rDNA gene sequence analysis. An in vitro antagonistic study showed that Sh420's lipopeptide (LP) extract exhibited strong antifungal properties and effectively combated F. graminearum. Meanwhile, lipopeptides have the ability to decrease ergosterol content, which has an impact on the overall structure and stability of the plasma membrane. The PCR-based screening revealed the presence of antifungal LP biosynthetic genes in this strain's genomic DNA. In the crude LP extract of Sh420, we were able to discover several LPs such as bacillomycin, iturins, fengycin, and surfactins using ultra-high-performance liquid chromatography-quadrupole time-of-flight mass spectrometry. Microscopic investigations (fluorescent/transmission electron microscopy) revealed deformities and alterations in the morphology of the phytopathogen upon interaction with LPs. Sh420 LPs have been shown in grape tests to be effective against F. graminearum infection and to stimulate antioxidant activity in fruits by avoiding rust and gray lesions. The overall findings of this study highlight the potential of Sh420 lipopeptides as an effective biological control agent against F. graminearum infestations.IMPORTANCEThis study addresses the potential of lipopeptide (LP) extracts obtained from the strain identified as Bacillus siamensis Sh420. This Sh420 isolate acts as a crucial player in providing a sustainable and environmentally friendly alternative to chemical fungicides for suppressing Fusarium graminearum phytopathogen. Moreover, these LPs can reduce ergosterol content in the phytopathogen influencing the overall structure and stability of its plasma membrane. PCR screening provided confirmation regarding the existence of genes responsible for biosynthesizing antifungal LPs in the genomic DNA of Sh420. Several antibiotic lipopeptide compounds were identified from this bacterial crude extract using ultra-high-performance liquid chromatography-quadrupole time-of-flight mass spectrometry. Microscopic investigations revealed deformities and alterations in the morphology of F. graminearum upon interaction with LPs. Furthermore, studies on fruit demonstrated the efficacy of Sh420 LPs in mitigating F. graminearum infection and stimulating antioxidant activity in fruits, preventing rust and gray lesions.

Bacterial endophytes inhabiting desert plants provide protection against seed rot caused by Fusarium verticillioides and promote growth in maize

BACKGROUND

Fusarium maize ear and root rot disease caused by Fusarium verticillioides has become one of the most serious fungal diseases associated with maize production. Due to their abilities to promote plant development and manage diseases, bacterial endophytes provide a more promising approach for treating this vascular disease.

RESULTS

This work was undertaken for the selection and identification of promising isolates as plant growth promoters and biocontrol agents against F. verticillioides in maize agroecosystems. A screening procedure consisting of in vitro and in situ tests was applied to 27 endophytic strains originating from desert plants: Euphorbia antiquorum, Calotropis procera, and Alcasia albida. In vitro studies indicated that the bacteria exhibited variable results in biocontrol, endophytism, and plant growth-promoting traits. In addition, in situ plant growth promotion and biocontrol experiments allowed the identification of the most promising bacterial endophytes. In vitro and in situ comparative study results indicated a low correlation. Our data revealed that in situ screening must be used as the method of selection of biocontrol agents against Fusarium ear and root rot disease. Based on in situ results, seven potent strains were selected and identified as Bacillus subtilis, Bacillus velezensis, Bacillus tequilensis, Stenotrophomonas maltophilia, and Klebsiella pneumoniae.

CONCLUSION

The results of this study showed that the selected strains seem to be promising candidates to be exploited as biofertilizers and biocontrol agents against Fusarium maize ear and root rot disease. © 2023 Society of Chemical Industry.

Characterization of Bacillus velezensis 32a metabolites and their synergistic bioactivity against crown gall disease

Crown gall disease caused by Agrobacterium tumefaciens is considered to be the main bacterial threat of stone fruit plants in Mediterranean countries. In a previous study, Bacillus velezensis strain 32a was isolated from Tunisian rhizosphere soil and revealed high antagonistic potential against A. tumefaciens strains. In order to better characterize the antagonistic activity of this strain against this important plant pathogen, the production of secondary metabolites was analyzed using liquid chromatography coupled with mass spectrometry. The results revealed the production of different compounds identified as surfactins, fengycins, iturins and bacillibactin belonging to the lipopeptide group, three polyketides (macrolactins, oxydifficidin and bacillaenes), bacilysin and its chlorinated derivative; chlorotetaine. The involvement of lipopeptides in this antagonistic activity was ruled out by performing agar and broth dilution tests with pure molecules. Thus, the construction of B. velezensis 32a mutants defective in polyketides and bacilysin biosynthesis and their antagonistic activity was performed and compared to a set of derivative mutants of a comparable strain, B. velezensis GA1. The defective difficidin mutants (△dfnA and △dfnD) were unable to inhibit the growth of A. tumefaciens, indicating the high-level contribution of difficidin in the antagonism process. While the macrolactin deficient mutant (∆mlnA) slightly decreased the activity, suggesting a synergetic effect with difficidin. Remarkably, the mutant △dhbC only deficient in bacillibactin production showed significant reduction in its capacity to inhibit the growth of Agrobacterium.Taken collectively, our results showed the strong synergetic effect of difficidin and macrolactins and the significant implication of siderophore to manage crown gall disease.

Transmission and Management of Pathogenic Agrobacterium tumefaciens and Rhodococcus fascians in Select Ornamentals

Pathogenic Agrobacterium tumefaciens and Rhodococcus fascians are phytobacteria that induce crown gall and leafy gall disease, respectively, resulting in undesirable growth abnormalities. When present in nurseries, plants infected by either bacterium are destroyed, resulting in substantial losses for growers, especially those producing plants valued for their ornamental attributes. There are many unanswered questions regarding pathogen transmission on tools used to take cuttings for propagation and whether products used for bacterial disease control are effective. We investigated the ability to transmit pathogenic A. tumefaciens and R. fascians on secateurs and the efficacy of registered control products against both bacteria in vitro and in vivo. Experimental plants used were Rosa × hybrida, Leucanthemum × superbum, and Chrysanthemum × grandiflorum for A. tumefaciens and Petunia × hybrida and Oenothera 'Siskiyou' with R. fascians. In separate experiments, we found secateurs could convey both bacteria in numbers sufficient to initiate disease in a host-dependent manner and that bacteria could be recovered from secateurs after a single cut through an infected stem. In in vivo assays, none of six products tested against A. tumefaciens prevented crown gall disease, although several products appeared promising in in vitro trials. Likewise, four compounds trialed against R. fascians failed to prevent disease. Sanitation and clean planting material remain the primary means of disease management.

Synthesis of silver nanoparticles using Bacillus velezensis M3-7 lipopeptides: Enhanced antifungal activity and potential use as a biocontrol agent against Fusarium crown rot disease of wheat seedlings

Bacillus velezensis M3-7 is a hyperactive mutant, 12-fold improved in its antifungal activity, obtained during a previous study from the wild strain BLB371 after a combination of random mutagenesis and medium component optimization. This study explores the use of this mutant in synthesizing silver nanoparticles (Ag-NPs) for the control of Fusarium crown rot disease (FCR) in wheat seedlings. LC-MS/MS analysis proved that both strains co-produced different families of lipopeptides and that mutagenesis caused the hyper-production of iturin A C14 and C15, the liberation of iturin A C10 and C12, and the inhibition of fengycin release. Our aim was a further improvement in the antifungal activity of the wild strain and the mutant M3-7 in order to control Fusarium crown rot disease (FCR) in wheat seedlings. Therefore, a nanotechnology approach was adopted, and different lipopeptide concentrations produced by the wild strain and the mutant M3-7 were used as capping agents to synthesize silver nanoparticles (Ag-NPs) with enhanced antifungal activity. Ag-NPs formed using 3 mg·mL-1 of the mutant lipopeptides were found to exhibit a good distribution, improved antifungal activity, a promising potential to be used as a biofortified agent for seed germination, and an effective compound to control FCR in wheat seedlings.

Bacillus subtilis SPB1 lipopeptide biosurfactant: antibacterial efficiency against the phytopathogenic bacteria Agrobacterium tumefaciens and compared production in submerged and solid state fermentation systems

Bacillus subtilis SPB1 derived biosurfactants (BioS) proved its bio-control activity against Agrobacterium tumefaciens using tomato plant. Almost 83% of disease symptoms triggered by Agrobacterium tumefaciens were reduced. Aiming potential application, we studied lipopeptide cost-effective production in both fermentations systems, namely the submerged fermentation (SmF) and the solid-state fermentation (SSF) as well as the use of Aleppo pine waste and confectionery effluent as cheap substrates. Optimization studies using Box-Behnken (BB) design followed by the analysis with response surface methodology were applied. When using an effluent/sea water ratio of 1, Aleppo pine waste of 14.08 g/L and an inoculum size of 0.2, a best production yield of 17.16 ± 0.91 mg/g was obtained for the SmF. While for the SSF, the best production yield of 27.59 ± 1.63 mg/g was achieved when the value of Aleppo pine waste, moisture, and inoculum size were, respectively, equal to 25 g, 75%, and 0.2. Hence, this work demonstrated the superiority of SSF over SmF.

Biocontrol Efficacy of Bacillus methylotrophicus TA-1 Against Meloidogyne incognita in Tomato

Root-knot nematodes (RKNs) are harmful plant-parasitic nematodes of tomatoes which can cause significant yield losses. Therefore, there is increasing interest in exploring the application of bacterial nematicides. The bacterium Bacillus methylotrophicus TA-1 is a broad-spectrum biological control agent; however, its effect on RKNs control remains largely unclear. In this study, the toxicity of B. methylotrophicus TA-1 against Meloidogyne incognita was investigated in vitro, and the potential of B. methylotrophicus TA-1 to decrease infection of RKNs in tomato were evaluated in pot and field trials. Results showed that B. methylotrophicus TA-1 exhibited high nematicidal activity against second-stage juveniles (J2s) and eggs of M. incognita with 50% lethal concentration (LC50) values of 5.80 and 7.00 × 108 colony forming units (CFU)/ml, respectively. In the pot experiments and field trials conducted in 2020 and 2021, tomato plants treated with B. methylotrophicus TA-1 soil drench applied once at 3, 6, and 9 × 108 CFU/plant had significantly higher plant height and greater yield compared with the untreated control. Tomato yields of the treated plots with B. methylotrophicus TA-1 in 2 consecutive years' field trials were between 53.4 to 66.1 and 52.8 to 61.5 t/ha, while they were 49.7 and 48.2 t/ha in the untreated control for each year, respectively. The lowest population densities of M. incognita at 30 and 60 days after treatment were 119 and 135 J2s per 100 g soil in 2020 and 43 and 118 J2s in 2021 in TA-1-treated plots. The lowest gall index of 4.7 and 3.3 in 2020 and 2021, respectively, and the highest yield were all observed in the TA-1 at 9 × 108 CFU/plant treated plants, with no significant differences with the commercial control abamectin. These results provided a basis for further studies of B. methylotrophicus TA-1 formulations, application doses, frequencies, and mechanisms of action, which are necessary before it could be used as a component of integrated management programs to manage RKNs in tomato production.

Biocontrol potential of lipopeptides produced by the novel Bacillus subtilis strain Y17B against postharvest Alternaria fruit rot of cherry

The use of synthetic fungicides against postharvest Alternaria rot adversely affects human health and the environment. In this study, as a safe alternative to fungicides, Bacillus subtilis strain Y17B isolated from soil exhibited significant antifungal activity against Alternaria alternata. Y17B was identified as B. subtilis based on phenotypic identification and 16S rRNA sequence analysis. To reveal the antimicrobial activity of this strain, a PCR-based study detected the presence of antifungal lipopeptide (LP) biosynthetic genes from genomic DNA. UPLC Q TOF mass spectrometry analysis detected the LPs surfactin (m/z 994.64, 1022.68, and 1026.62), iturin (m/z 1043.56), and fengycin (m/z 1491.85) in the extracted LP crude of B. subtilis Y17B. In vitro antagonistic study demonstrated the efficiency of LPs in inhibiting A. alternata growth. Microscopy (SEM and TEM) studies showed the alteration of the morphology of A. alternata in the interaction with LPs. In vivo test results revealed the efficiency of LPs in reducing the growth of the A. alternata pathogen. The overall results highlight the biocontrol potential of LPs produced by B. subtilis Y17B as an effective biological control agent against A. alternata fruit rot of cherry.

Virulence and Biofilm Inhibition of 3-Methoxycinnamic Acid against Agrobacterium tumefaciens

AIMS

In the current study the anti-virulence and anti-biofilm activities of the cinnamic acid derivative, 3-methoxycinnamic acid, was investigated against Agrobacterium tumefaciens.

METHODS AND RESULTS

Based on the disc diffusion test and β-galactosidase activity assay, 3-methoxycinnamic acid was shown to interfere with the quorum sensing (QS) system of A. tumefaciens. Crystal violet staining assay, phenol-sulfuric acid method, Bradford protein assay and confocal laser scanning microscopy (CLSM) revealed that the biofilm formation of A. tumefaciens was inhibited after the treatment of 3-methoxycinnamic acid. Employing high performance liquid chromatography (HPLC) analysis of culture supernatant revealed that the production of 3-oxo-octanoylhomoserine lactone (3-oxo-C8-HSL) decreased concentration-dependently after treatment with 3-methoxycinnamic acid. Swimming and chemotaxis assays also indicated that 3-methoxycinnamic acid had a good effect on reducing the motility and chemotaxis of A. tumefaciens. In addition, the RT-qPCR, molecular docking and simulations further demonstrated that 3-methoxycinnamic acid could competitively inhibit the binding of 3-oxo-C8-HSL to TraR and down-regulate virulence-related genes.

CONCLUSIONS

3-Methoxycinnamic acid is proved to have good anti-virulence and anti-biofilm activities against A. tumefaciens.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study that investigates the anti-virulence and anti-biofilm activities of 3-methoxycinnamic acid against A. tumefaciens. With its potential QS-related virulence and biofilm inhibitory activities, 3-methoxycinnamic acid is expected to be developed as a potent pesticide or adjuvant for the prevention and treatment of crown gall caused by A. tumefaciens.

RNA-Seq Provides Insights into the Mechanisms Underlying Ilyonectria robusta Responding to Secondary Metabolites of Bacillus methylotrophicus NJ13

(1) Background: Ilyonectria robusta can cause ginseng to suffer from rusty root rot. Secondary metabolites (SMs) produced by Bacillus methylotrophicus NJ13 can inhibit the mycelial growth of I. robusta. However, the molecular mechanism of the inhibition and response remains unclear. (2) Methods: Through an in vitro trial, the effect of B. methylotrophicus NJ13’s SMs on the hyphae and conidia of I. robusta was determined. The change in the physiological function of I. robusta was evaluated in response to NJ13’s SMs by measuring the electrical conductivity, malondialdehyde (MDA) content, and glucose content. The molecular interaction mechanism of I. robusta’s response to NJ13’s SMs was analyzed by using transcriptome sequencing. (3) Results: NJ13’s SMs exhibited antifungal activity against I. robusta: namely, the hyphae swelled and branched abnormally, and their inclusions leaked out due to changes in the cell membrane permeability and the peroxidation level; the EC50 value was 1.21% (v/v). In transcripts at 4 dpi and 7 dpi, the number of differentially expressed genes (DEGs) (|log2(fold change)| > 1, p adj ≤ 0.05) was 1960 and 354, respectively. NJ13’s SMs affected the glucose metabolism pathway, and the sugar-transporter-related genes were downregulated, which are utilized by I. robusta for energy production. The cell wall structure of I. robusta was disrupted, and chitin-synthase-related genes were downregulated. (4) Conclusions: A new dataset of functional responses of the ginseng pathogenic fungus I. robusta was obtained. The results will benefit the development of targeted biological fungicides for I. robusta and the study of the molecular mechanisms of interaction between biocontrol bacteria and phytopathogenic fungi.

Microbial Biopesticides against Bacterial, Fungal and Oomycete Pathogens of Tomato, Cabbage and Chickpea

Biological control is an environmentally friendly approach that holds promise to complement or replace chemicals to effectively protect crop plants against pests and pathogens. Environmental samples with highly diverse and competitive microbiomes that harbor antagonistic microbes with diverse modes-of-action can provide a rich source of microbial biopesticides. In the current study, bacteria isolated from rhizosphere soil and food spoilage samples were subsequently screened against various plant fungal and oomycete pathogens in growth inhibition assays. These included the new potential biocontrol bacteria Corynebacterium flavescens, Sporosarcina aquimarina and Sporosarcina saromensis with anti-fungal and antioomycete activities. Potential candidates selected by preliminary screening in plant assays were then applied to tomato, cabbage and chickpea plants to control bacterial (Pseudomonas syringae pv. tomato), fungal (Alternaria brassicicola) and oomycete (Phytophtora medicaginis) phytopathogens. Ten potential microbial biopesticides were demonstrated to be effective against these diseases, and led to significant (p < 0.05) reductions in symptoms and/or pathogen DNA compared to mock-treated diseased plants. We conclude that new and effective microbial biopesticides to control crop pathogens can be rapidly isolated from biodiverse microbiomes, where bacteria may employ these features to effectively compete against each other.

Effects of Bacillus methylotrophicus SY200 Supplementation on Growth Performance, Antioxidant Status, Intestinal Morphology, and Immune Function in Broiler Chickens

The present study was focused on evaluating the effects of Bacillus methylotrophicus SY200 in broiler production. A total of 120 healthy 7-day-old broiler chicks were randomly assigned to four dietary treatments, which included basal diet supplemented with 0%, 0.10%, 0.25%, or 0.50% (w/w) B. methylotrophicus SY200 preparation (1.0 × 10⁹ cfu/g), regarded as negative control group (NC), low-dose group (BML), medium-dose group (BMM), and high-dose group (BMH), respectively. Each treatment was fed the corresponding experimental diet for 35 days. Results showed that dietary supplementation of B. methylotrophicus SY200 could improve broiler weight gain, especially the finisher phase. Further studies suggested that a certain amount of B. methylotrophicus SY200 enhanced the broiler antioxidant status and improved the morphological development of jejunum. Besides, dietary supplementation of B. methylotrophicus SY200 especially in 0.50% levels significantly increased the relative weight of immune organs and Newcastle disease virus antibody titer, similarly, increased mRNA expression levels of claudin-1, claudin-3, zonula occluden-1, and zonula occluden-2 were observed in the jejunum of BMM group. Moreover, B. methylotrophicus SY200 also showed beneficial effects in improving broilers microbiota homeostasis by increasing the number of beneficial bacteria. Conclusively, B. methylotrophicus SY200 could effectively improve the antioxidant status, modulate the intestinal structure, enhance the intestinal mucosal barrier function, and regulate the immune function of broilers, which finally improves the performance of the chicken in the finisher period.

Contribution of macrolactin in Bacillus velezensis CLA178 to the antagonistic activities against Agrobacterium tumefaciens C58

Beneficial rhizobacteria can inhibit soilborne pathogens by secreting an array of polyketides, lipopeptides and dipeptides, but the effect of polyketides on crown gall disease caused by Agrobacterium tumefaciens C58 is unclear. In this study, the antagonistic compounds of the plant growth-promoting rhizobacterium Bacillus velezensis CLA178 was sorted with different organic phases, purified by high-pressure liquid chromatography, and detected by a liquid chromatography ionization-mass spectrometry system. Macrolactins were found to be the compounds with antagonistic activity against A. tumefaciens C58. When the macrolactin synthesis pathway was disrupted, the mutant △mlnA only showed slight antagonistic activity against A. tumefaciens C58. Transmission electron microscopy showed that the inhibition of C58 cell division by cell-free culture from the mutant △mlnA was weaker than that by cell-free culture from CLA178. The mutant deficient in production of macrolactin showed a weaker transcription of genes involved in attachment of C58 to plant and lower biocontrol of crown gall disease in rose than the wild-type strain CLA178. The effect of macrolactins on pathogen C58 has been also confirmed by the purified macrolactins. These results reveal that macrolactins contribute to the biocontrol activity of C58 by inhibiting cell division and downregulating the transcription of chvB and chvE.

Immunomodulatory Role of Microbial Surfactants, with Special Emphasis on Fish

Microbial surfactants (biosurfactants) are a broad category of surface-active biomolecules with multifunctional properties. They self-assemble in aqueous solutions and are adsorbed on various interfaces, causing a decrease in surface tension, as well as interfacial tension, solubilization of hydrophobic compounds, and low critical micellization concentrations. Microbial biosurfactants have been investigated and applied in several fields, including bioremediation, biodegradation, food industry, and cosmetics. Biosurfactants also exhibit anti-microbial, anti-biofilm, anti-cancer, anti-inflammatory, wound healing, and immunomodulatory activities. Recently, it has been reported that biosurfactants can increase the immune responses and disease resistance of fish. Among various microbial surfactants, lipopeptides, glycolipids, and phospholipids are predominantly investigated. This review presents the various immunological activities of biosurfactants, mainly glycolipids and lipopeptides. The applications of biosurfactants in aquaculture, as well as their immunomodulatory activities, that make them novel therapeutic candidates have been also discussed in this review.

Biological Control of Verticillium Wilt on Olive Trees by the Salt-Tolerant Strain Bacillus velezensis XT1

Verticillium wilt, caused by the pathogen Verticillium dahliae, is extremely devastating to olive trees (Olea europea). Currently, no successful control measure is available against it. The objective of this work was to evaluate the antifungal activity of Bacillus velezensis XT1, a well-characterized salt-tolerant biocontrol strain, against the highly virulent defoliating V. dahliae V024. In vitro, strain XT1 showed to reduce fungal mycelium from 34 to 100%, depending on if the assay was conducted with the supernatant, volatile compounds, lipopeptides or whole bacterial culture. In preventive treatments, when applied directly on young olive trees, it reduced Verticillium incidence rate and percentage of severity by 54 and ~80%, respectively. It increased polyphenol oxidase (PPO) activity by 395%, indicating an enhancement of disease resistance in plant tissues, and it decreased by 20.2% the number of fungal microsclerotia in soil. In adult infected trees, palliative inoculation of strain XT1 in the soil resulted in a reduction in Verticillium symptom severity by ~63%. Strain XT1 is biosafe, stable in soil and able to colonize olive roots endophytically. All the traits described above make B. velezensis XT1 a promising alternative to be used in agriculture for the management of Verticillium wilt.

Crop Protection against Botrytis cinerea by Rhizhosphere Biological Control Agent Bacillus velezensis XT1

This study aims to evaluate the use of Bacillus velezensis strain XT1 as a plant growth-promoting rhizobacterium (PGPR) and biocontrol agent against B. cinerea in tomato and strawberry plants. Foliar and radicular applications of strain XT1 increased plant total biomass as compared to the control and B. cinerea-infected plants, with root applications being, on the whole, the most effective mode of treatment. Applications of the bacterium were found to reduce infection parameters such as disease incidence and severity by 50% and 60%, respectively. We analyzed stress parameters and phytohormone content in order to evaluate the capacity of XT1 to activate the defense system through phytohormonal regulation. Overall, the application of XT1 reduced oxidative damage, while the H₂O₂ and malondialdehyde (MDA) content was lower in XT1-treated and B. cinerea-infected plants as compared to non-XT1-treated plants. Moreover, treatment with XT1 induced callose deposition, thus boosting the response to pathogenic infection. The results of this study suggest that the signaling and activation pathways involved in defense mechanisms are mediated by jasmonic acid (JA) and ethylene hormones, which are induced by preventive treatment with XT1. The study also highlights the potential of preventive applications of strain XT1 to activate defense mechanisms in strawberry and tomato plants through hormone regulation.

Forest Tree Associated Bacterial Diffusible and Volatile Organic Compounds against Various Phytopathogenic Fungi

Plant growth-promoting rhizobacteria (PGPR) can potentially be used as an alternative strategy to control plant diseases. In this study, strain ST-TJ4 isolated from the rhizosphere soil of a healthy poplar was found to have a strong antifungal activity against 11 phytopathogenic fungi in agriculture and forestry. Strain ST-TJ4 was identified as Pseudomonas sp. based on 16S rRNA-encoding gene sequences. The bacterium can produce siderophores, cellulase, and protease, and has genes involved in the synthesis of phenazine, 1-phenazinecarboxylic acid, pyrrolnitrin, and hydrogen cyanide. Additionally, the volatile compounds released by strain ST-TJ4 can inhibit the mycelial growth of plant pathogenic fungi more than diffusible substances can. Based on volatile compound profiles of strain ST-TJ4 obtained from headspace collection and GC-MS/MS analysis, 1-undecene was identified. In summary, the results suggested that Pseudomonas sp. ST-TJ4 can be used as a biocontrol agent for various plant diseases caused by phytopathogenic fungi.

Plant growth-promoting activity and quorum quenching-mediated biocontrol of bacterial phytopathogens by Pseudomonas segetis strain P6

Given the major threat of phytopathogenic bacteria to food production and ecosystem stability worldwide, novel alternatives to conventional chemicals-based agricultural practices are needed to combat these bacteria. The objective of this study is to evaluate the ability of Pseudomonas segetis strain P6, which was isolated from the Salicornia europaea rhizosphere, to act as a potential biocontrol agent given its plant growth-promoting (PGP) and quorum quenching (QQ) activities. Seed biopriming and in vivo assays of tomato plants inoculated with strain P6 resulted in an increase in seedling height and weight. We detected QQ activity, involving enzymatic degradation of signal molecules in quorum sensing communication systems, against a broad range of N-acylhomoserine lactones (AHLs). HPLC-MRM data and phylogenetic analysis indicated that the QQ enzyme was an acylase. The QQ activity of strain P6 reduced soft rot symptoms caused by Dickeya solani, Pectobacterium atrosepticum and P. carotovorum on potato and carrot. In vivo assays showed that the PGP and QQ activities of strain P6 protect tomato plants against Pseudomonas syringae pv. tomato, indicating that strain P6 could have biotechnological applications. To our knowledge, this is the first report to show PGP and QQ activities in an indigenous Pseudomonas strain from Salicornia plants.

Biological control of tomato bacterial wilt by oxydifficidin and difficidin-producing Bacillus methylotrophicus DR-08

Bacillus methylotrophicus DR-08 exhibited strong antibacterial activity against Ralstonia solanacearum, a causal agent of tomato bacterial wilt. This study aimed to identify the antibacterial metabolites and evaluate the efficacy of the strain as a biocontrol agent for tomato bacterial wilt. A butanol extract of the DR-08 broth culture completely inhibited the growth of 14 phytopathogenic bacteria with minimum inhibitory concentration (MIC) values of 1.95-500 μg/mL. R. solanacearum was highly sensitive to the DR-08 extract, with an MIC value of 12.62 μg/mL. Two antibacterial metabolites were isolated and identified as difficidin and oxydifficidin derivatives through bioassay-guided fractionation and instrumental analyses. Both metabolite derivatives inhibited the growth of most of the phytopathogenic bacteria tested and the oxydifficidin derivatives generally presented a stronger antibacterial activity than the difficidin derivatives. A 30% suspension concentrate of DR-08, at a 500-fold dilution, effectively suppressed the development of tomato bacterial wilt in pot and field experiments. It also effectively reduced the development of bacterial leaf spot symptoms on peach and red pepper. The results of this study suggests that B. methylotrophicus DR-08 can be utilized as a biocontrol agent for various bacterial plant diseases including tomato bacterial wilt.

Mecanismos de acción de <i>Bacillus</i> spp. (Bacillaceae) contra microorganismos fitopatógenos durante su interacción con plantas

Algunos Bacillus spp. promotores de crecimiento vegetal son microorganismos reconocidos como agentes de control biológico que forman una estructura de resistencia denominada endospora, que les permite sobrevivir en ambientes hostiles y estar en casi todos los agroecosistemas. Estos microorganismos han sido reportados como alternativa al uso de agroquímicos. Sus mecanismos de acción se pueden dividir en: producción de compuestos antimicrobianos, como son péptidos de síntesis no ribosomal (NRPs) y policétidos (PKs); producción de hormonas, capacidad de colonización, formación de biopelículas y competencia por espacio y nutrientes; síntesis de enzimas líticas como quitinasas, glucanasas, protesasas y acil homoserin lactonasas (AHSL); producción de compuestos orgánicos volátiles (VOCs); e inducción de resistencia sistémica (ISR). Estos mecanismos han sido reportados en la literatura en diversos estudios, principalmente llevados a cabo a nivel in vitro. Sin embargo, son pocos los estudios que contemplan la interacción dentro del sistema tritrófico: planta – microorganismos patógenos – Bacillus sp. (agente biocontrolador), a nivel in vivo. Es importante destacar que la actividad biocontroladora de los Bacillus es diferente cuando se estudia bajo condiciones de laboratorio, las cuales están sesgadas para lograr la máxima expresión de los mecanismos de acción. Por otra parte, a nivel in vivo, la interacción con la planta y el patógeno juegan un papel fundamental en la expresión de dichos mecanismos de acción, siendo esta más cercana a la situación real de campo. Esta revisión se centra en los mecanismos de acción de los Bacillus promotores de crecimiento vegetal, expresados bajo la interacción con la planta y el patógeno.

Bacillus Methylotrophicus Has Potential Applications Against Monilinia Fructicola

Biocontrol is a cost-effective and environmentally friendly technique used in agricultural production. We isolated and screened a bacterial strain from the soils of a peach orchard with high yield. Using biochemical and physiological analysis as well as phylogenetic sequencing data, we identified a strain of Bacillus methylotrophicus, strain XJ-C. The results of our screening trials showed that XJ-C was able to suppress M. fructicola at an inhibition rate of 81.57%. Following the application of a 1×10⁹ CFU/mL XJ-C strain suspension to the fruits, leaves, and shoots of peach trees infected with M. fructicola, the inhibition rate reached 64.31%, 97.34%, and 64.28%, respectively. Using OM and SEM, we observed that, under the inhibition of strain XJ-C, M. fructicola mycelium and spores were abnormally shaped. Under TEM, cell walls were transparent, organelles had disappeared, and the intracellular vacuole was deformed. Thus, XJ-C has the potential to be used in biocontrol.

Synergistic Effect of Combined Application of a New Fungicide Fluopimomide with a Biocontrol Agent Bacillus methylotrophicus TA-1 for Management of Gray Mold in Tomato

Gray mold caused by Botrytis cinerea can be a severe disease of tomato infecting leaves and fruits of tomato plants. Chemical control is currently the most effective and reliable method; however, application of fungicides has many drawbacks. The combination of biological control agents with newly developed fungicides may be a practicable method to control B. cinerea. Fluopimomide is a newly developed fungicide with a novel mode of action. Bacillus methylotrophicus TA-1, isolated from rhizosphere soil of tomato, is a bacterial strain with a broad spectrum of antimicrobial activities. Little information is currently available about the effect of fluopimomide and its integrated effect on B. cinerea. Therefore, laboratory, pot, and field experiments were carried out to determine the effects of fluopimomide alone and in combination with B. methylotrophicus TA-1 against gray mold on tomato. The in vitro growth of B. methylotrophicus TA-1 was unaffected by 100 mg liter^-1 fluopimomide. Inhibition of B. cinerea mycelial growth was significantly increased under combined treatment of fluopimomide and B. methylotrophicus TA-1. In greenhouse experiments, efficacy against gray mold was significantly greater by an integration of fluopimomide and B. methylotrophicus TA-1 than by either alone; control efficacy of fluopimomide at 50 and 100 g ha^-1 in combination with B. methylotrophicus TA-1 at 10⁸ colony-forming units (cfu) ml^-1 reached 70.16 and 69.32%, respectively, compared with the untreated control. In both field trials during 2017 and 2018, control efficacy was significantly higher for the combination of fluopimomide at 50 and 100 g ha^-1 in combination with B. methylotrophicus TA-1 than for either treatment alone. The results from this study indicated that integration of the new fungicide fluopimomide with the biocontrol agent B. methylotrophicus TA-1 synergistically increased control efficacy of the fungicide against gray mold of tomato.

Screening of Endophytic Antagonistic Bacterium from Phellodendron amurense and Their Biocontrol Effects against Canker Rot

Thirty-four strains of bacteria were isolated from Phellodendron amurense. Using Nectria haematococca as an indicator strain, the best strain, B18, was obtained by the growth rate method. The morphological, physiological and biochemical characteristics of strain B18 and its 16S DNA gene sequence were identified, and the biocontrol effect of strain B18 was assessed in pot and field tests, as well as in a field-control test. Drilling methods were used to determine the antibacterial activity of metabolites from strain B18 and their effects on the growth of pathogen mycelia and spores. The best bacteriostatic rate was 85.4%. B18 can hydrolyse starch and oxidize glucose but does not produce gas; a positive result was obtained in a gelatine liquefaction test. According to 16S DNA gene sequencing, strain B18 is Bacillus methylotrophicus (GenBank accession number: MG457759). The results of pot and field-control trials showed 98% disease control when inoculating 10⁸ cfu/ml of the strain. The disease control effect of the B18 culture liquid (concentrations of 10⁸, 2 × 10⁶, 10⁶, 5 × 10⁵ and 2.5 × 10⁵ cfu/ml) in the field-control test was higher than 80%, and the cure rate of the original delivery solution was 96%. Therefore, in the practical forestry production, a 2.5 × 10⁵ cfu/ml culture liquidshould be applied in advance to achieve good control effects.

Insight into the Bacterial Endophytic Communities of Peach Cultivars Related to Crown Gall Disease Resistance

Crown gall disease caused by Agrobacterium tumefaciens severely impacts the production of peach and other fruit trees. Several peach cultivars are partially resistant to A. tumefaciens, but little is known about the roles of endophytic microbiota in disease resistance. In the present study, the endophytic bacterial communities of resistant and susceptible peach cultivars "Honggengansutao" and "Okinawa" were analyzed using universal 16S rRNA gene amplicon sequencing in parallel with the cultivation and characterization of bacterial isolates. A total of 1,357,088 high-quality sequences representing 3,160 distinct operational taxonomic units (OTUs; Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes) and 1,200 isolates of 20 genera and 305 distinct ribotypes were collected from peach roots and twigs. It was found that factors including plant developmental stage, cultivar, and A. tumefaciens invasion strongly influenced the peach endophytic communities. The community diversity of endophytic bacteria and the abundance of culturable bacteria were both higher in the roots of the resistant cultivar, particularly after inoculation. Strikingly, the pathogen antagonists Streptomyces and Pseudomonas in roots and Rhizobium in twigs were most frequently detected in resistant plants. Our results suggest that the higher abundance and diversity of endophytic bacteria and increased proportions of antagonistic bacteria might contribute to the natural defense of the resistant cultivar against A. tumefaciens This work reveals the relationships between endophytic bacteria and disease resistance in peach plants and provides important information for microbiome-based biocontrol of crown gall disease in fruit trees.IMPORTANCE Agrobacterium tumefaciens as the causal agent of peach crown gall disease can be controlled by planting resistant cultivars. This study profiles the endophytic bacteria in susceptible and resistant peach cultivars, advancing our understanding of the relationships between endophytic bacterial communities and peach crown gall disease, with potential implications for other complex microbiome-plant-pathogen interactions. The resistant cultivar may defend itself by increasing the diversity and abundance of beneficial endophytic bacteria. The antagonists identified among the genera Streptomyces, Pseudomonas, and Rhizobium may have application potential for biocontrol of crown gall disease in fruit trees.

Bacillus methylotrophicus ASWU-C2, a strain inhabiting hot desert soil, a new source for antibacterial bacillopyrone, pyrophen, and cyclopeptides

A strain of Bacillus methylotrophicus was isolated from a soil sample collected in Aswan eastern desert, which is known for its extremely arid climate. After fermentation of the strain in liquid culture and subsequent extraction, a bioassay-guided isolation procedure yielded five compounds: 2-benzyl-4H-pyran-4-one, named bacillopyrone (1), pyrophen (2), macrolactin A (3) and the cyclopeptides malformin A1 (4), and bacillopeptin A (5). The structures were determined by interpretation of nuclear magnetic resonance (NMR) spectroscopy and high resolution mass spectrometry (HR-MS) data. This is the first report on the isolation of compounds 1 and 2 from Bacillus species; compound 1 was reported previously as synthetic product. Bacillopyrone (1) exhibited moderate activity against the Gram-negative Chromobacterium violaceum with minimum inhibitory concentration 266.6 μg/mL, while macrolactin A (3) and malformin A1 (4) inhibited Staphylococcus aureus (minimum inhibitory concentrations 13.3 and 133.3 μg/mL, respectively).

Lipopeptides from Bacillus amyloliquefaciens strain 32a as promising biocontrol compounds against the plant pathogen Agrobacterium tumefaciens

Despite the potential biological importance of lipopeptides from Bacillus amyloliquefaciens as antimicrobial compounds, their effects on Agrobacterium tumefaciens biofilms have not been previously studied. These latter are important virulence factors for the development and re-occurrence of crown gall disease. As part of the development of a new biopesticide acting as anti-biofilm and biocontrol agent, we investigated for the first time the ability of a mixture of lipopeptides produced by B. amyloliquefaciens strain 32a to inhibit the tumor formation on plants and to reduce the formation of biofilms by the phytopathogenic A. tumefaciens strains C58 and B6. The mixture was found to display a strong biosurfactant activity as well as bactericidal activity against planktonic Agrobacterium cells. Moreover, the lipopeptide treatment inhibited biofilm formation in the range of 79.58 ± 0.60-100.00 ± 0.00% and dislodged 43.42 ± 0.91-93.89 ± 2.70% of preformed biofilm. For these assays, fluorescence microscopy did not show any adherent cell in the anti-adhesive assay and only few ones in the cell-dislodging assay. More importantly, lipopeptide-enriched extract inhibits tumor formation on tomato stem when treatments were applied after pathogen adhesion to wounded tissues. By virtue of its ability to inhibit biofilms formed on biotic and abiotic surfaces and to control efficiently tumor development, the 32a lipopeptide mixture may represent an excellent new tool for an efficient biocontrol of crown gall disease.

Antifungal Activity of Lipopeptides From Bacillus XT1 CECT 8661 Against Botrytis cinerea

This work aims to explore the capacity of a Bacillus methylotrophicus (later heterotypic synonym of Bacillus velezensis) strain named XT1 CECT 8661 against the necrotrophic plant pathogen Botrytis cinerea and to identify the compounds responsible for its activity. Q_TOF electrospray mass spectrometry analysis allows us to detect several lipopeptides – surfactin, bacillomycin, and fengycin – in XT1 cultures. In vitro antibiosis studies demonstrated the efficiency of the lipopeptide fraction for the inhibition of fungal growth. In fact, microscopy studies (SEM/TEM) revealed, an alteration of the morphology of the phytopathogen in interaction with lipopeptides, with resistance structures appearing in the early stages of growth of the fungus. Our studies, carried out with tomatoes, grapes, and strawberries have demonstrated the efficiency of Bacillus XT1 CECT 8661 lipopeptides against B. cinerea infection and it capability to trigger the antioxidant activity in fruit. Overall, the results of this study highlight the potential of lipopeptides of this strain as an effective biological control agent against the colonisation of B. cinerea.

Rice Phyllosphere Bacillus Species and Their Secreted Metabolites Suppress Aspergillus flavus Growth and Aflatoxin Production In Vitro and In Maize Seeds

The emergence of super-toxigenic strains by recombination is a risk from an intensive use of intraspecific aflatoxin (AF) biocontrol agents (BCAs). Periodical alternation with interspecific-BCAs will be safer since they preclude recombination. We are developing an AF-biocontrol system using rice-associated Bacilli reported previously (RABs). More than 50% of RABs inhibited the growth of multiple A. flavus strains, with RAB4R being the most inhibitory and RAB1 among the least. The fungistatic activity of RAB4R is associated with the lysis of A. flavus hyphal tips. In field trails with the top five fungistatic RABs, RAB4R consistently inhibited AF contamination of maize by Tox4, a highly toxigenic A. flavus strain from Louisiana corn fields. RAB1 did not suppress A. flavus growth, but strongly inhibited AF production. Total and HPLC-fractionated lipopeptides (LPs) isolated from culture filtrates of RAB1 and RAB4R also inhibited AF accumulation. LPs were stable in vitro with little loss of activity even after autoclaving, indicating their potential field efficacy as a tank-mix application. A. flavus colonization and AF were suppressed in RAB1- or RAB4R-coated maize seeds. Since RAB4R provided both fungistatic and strong anti-mycotoxigenic activities in the laboratory and field, it can be a potent alternative to atoxigenic A. flavus strains. On the other hand, RAB1 may serve as an environmentally safe helper BCA with atoxigenic A. flavus strains, due its lack of strong fungistatic and hemolytic activities.

Isolation, Characterization and Amino Acid Composition of a Bacteriocin Produced by Bacillus methylotrophicus Strain BM47

Members of the bacterial genus Bacillus are known as producers of a broad spectrum of antibiotic compounds of proteinaceous nature that possess inhibitory activity against different saprophytic and pathogenic microorganisms. In the current research, a peptide synthesized by Bacillus methylotrophicus strain BM47, previously isolated from a natural thermal spring in Bulgaria, was identified and characterized as a bacteriocin. In vitro antimicrobial screening of the crude bacteriocin substance of B. methylotrophicus BM47 showed activity against the plant pathogenic fungi Fusarium moniliforme, Aspergillus awamori, Penicillium sp., Aspergillus niger and Gram-negative bacterium Pseudomonas aeruginosa. The antimicrobial activity of the crude bacteriocin substance was partially inhibited by the enzymes trypsin, Alcalase®, Savinase®, proteinase K, papain and Esperase®, while catalase was not effective. The crude bacteriocin substance was relatively pH resistant, but sensitive to the action of heat and most organic solvents and detergents tested. To obtain the active protein fractions, crude bacteriocin substance was purified by fast protein liquid chromatography (FPLC) using a strong anion exchange column. Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated that the purified bacteriocin had molecular mass of 19 578 Da. The amino acid analysis performed by high-performance liquid chromatography (HPLC) revealed that the isolated bacteriocin consisted of 17 types of amino acids, with the highest mol fraction expressed as percent of serine (29.3), valine (10.3), alanine (9.8) and tyrosine (7.1).

Silencing of Agrobacterium tumefaciens oncogenes ipt and iaaM induces resistance to crown gall disease in plum but not in apricot

BACKGROUND

In this study, two vectors with short-length chimeric transgenes were used to produce Prunus rootstocks resistant to crown gall disease through RNA-interference-mediated gene silencing of the Agrobacterium tumefaciens oncogenes ipt and iaaM.

RESULTS

Transgenic plum and apricot lines were produced with efficiencies of up to 7.7 and 1.1% respectively. An in vitro evaluation method allowed identification of susceptible lines and reduction in the number of lines to be evaluated in the greenhouse. Five transgenic plum lines, expressing transgene-derived small interfering RNA (siRNA) and low levels of transgene hairpin RNA (hpRNA), showed a significant reduction in the development of the disease after infection with Agrobacterium strains C58 and A281 under greenhouse conditions. However, unexpectedly, all transgenic apricot lines were gall susceptible. The infection of apricot plants with a binary vector containing only the 6b oncogene demonstrated that the expression of this gene is involved in the induction of tumours in the apricot species.

CONCLUSION

RNAi-mediated gene silencing can be used for inducing crown gall resistance in plum rootstocks. These could be used to graft non-genetically modified commercial fruit cultivars reducing, or eliminating, the disease symptoms. © 2017 Society of Chemical Industry.

Antibiosis and bmyB Gene Presence As Prevalent Traits for the Selection of Efficient Bacillus Biocontrol Agents against Crown Gall Disease

This study aimed to improve the screening method for the selection of Bacillus biocontrol agents against crown gall disease. The relationship between the strain biocontrol ability and their in vitro studied traits was investigated to identify the most important factors to be considered for the selection of effective biocontrol agents. In fact, previous selection procedure relying only on in vitro antibacterial activity was shown to be not suitable in some cases. A direct plant-protection strategy was performed to screen the 32 Bacillus biocontrol agent candidates. Moreover, potential in vitro biocontrol traits were investigated including biofilm formation, motility, hemolytic activity, detection of lipopeptide biosynthetic genes (sfp, ituC and bmyB) and production of antibacterial compounds. The obtained results indicated high correlations of the efficiency of the biocontrol with the reduction of gall weight (p = 0.000) and the antibacterial activity in vitro (p = 0.000). Moreover, there was strong correlations of the efficiency of the biocontrol (p = 0.004) and the reduction in gall weight (p = 0.000) with the presence of the bmyB gene. This gene directs the synthesis of the lipopeptide bacillomycin belonging to the iturinic family of lipopeptides. These results were also confirmed by the two-way hierarchical cluster analysis and the correspondence analysis showing the relatedness of these four variables. According to the obtained results a new screening procedure of Bacillus biocontrol agents against crown gall disease could be advanced consisting on two step selection procedure. The first consists on selecting strains with high antibacterial activity in vitro or those harbouring the bmyB gene. Further selection has to be performed on tomato plants in vivo. Moreover, based on the results of the biocontrol assay, five potent strains exhibiting high biocontrol abilities were selected. They were identified as Bacillus subtilis or Bacillus amyloliquefaciens. These strains were found to produce either surfactin or surfactin and iturin lipopeptides. In conclusion, our study presented a new and effective method to evaluate the biocontrol ability of antagonistic Bacillus strains against crown gall disease that could increase the efficiency of screening method of biocontrol agents. Besides, the selected strains could be used as novel biocontrol agents against pathogenic Agrobacterium tumefaciens strains.