Antagonistic potential of native strain Streptomyces aurantiogriseus VSMGT1014 against sheath blight of rice disease

Antagonistic potential of rhizobacterial isolates against fungal pathogens causing rhizome rot in turmeric

The study aims to select potent bacterial antagonists to be used as biocontrol agents against rhizome rot disease in turmeric (Curcuma longa L.). A total of 48 bacterial isolates were isolated from the rhizosphere of turmeric. These isolates were screened for their in vitro antagonism against Fusarium solani FS-01 and Pythium aphanidermatum (ITCC 7908). Production of volatile organic compounds and chitinase activity were also performed. Among the tested isolates, two bacterial isolates (IJ2 and IJ10) showed the highest inhibitory activity against these fungal pathogens. GC/MS analysis of the crude extract produced by Pseudomonas sp. IJ2 and B. subtilis IJ10 was found to contain many bioactive compounds with antifungal and antimicrobial activities. The rhizome treatment with these isolates exhibited the lowest percent disease severity with high biocontrol efficacy against the tested pathogens. These isolates with promising antagonistic potential, therefore, can be used as biocontrol agents against rhizome rot in turmeric.

Actinobacteria as Effective Biocontrol Agents against Plant Pathogens, an Overview on Their Role in Eliciting Plant Defense

Pathogen suppression and induced systemic resistance are suitable alternative biocontrol strategies for integrated plant disease management and potentially comprise a sustainable alternative to agrochemicals. The use of Actinobacteria as biocontrol agents is accepted in practical sustainable agriculture, and a short overview on the plant-beneficial members of this phylum and recent updates on their biocontrol efficacies are the two topics of this review. Actinobacteria include a large portion of microbial rhizosphere communities and colonizers of plant tissues that not only produce pest-antagonistic secondary metabolites and enzymes but also stimulate plant growth. Non-pathogenic Actinobacteria can also induce systemic resistance against pathogens, but the mechanisms are still poorly described. In the absence of a pathogen, a mild defense response is elicited under jasmonic acid and salicylic acid signaling that involves pathogenesis-related proteins and secondary plant metabolites. Priming response partly includes the same compounds as the response to a sole actinobacterium, and the additional involvement of ethylene signaling has been suggested. Recent amplicon sequencing studies on bacterial communities suggest that future work may reveal how biocontrol active strains of Actinobacteria can be enriched in plant rhizosphere.

Characterization and Assessment of 2, 4-Diacetylphloroglucinol (DAPG)-Producing Pseudomonas fluorescens VSMKU3054 for the Management of Tomato Bacterial Wilt Caused by Ralstonia solanacearum

Microbial bio-products are becoming an appealing and viable alternative to chemical pesticides for effective management of crop diseases. These bio-products are known to have potential to minimize agrochemical applications without losing crop yield and also restore soil fertility and productivity. In this study, the inhibitory efficacy of 2,4-diacetylphloroglucinol (DAPG) produced by Pseudomonas fluorescens VSMKU3054 against Ralstonia solanacearum was assessed. Biochemical and functional characterization study revealed that P. fluorescens produced hydrogen cyanide (HCN), siderophore, indole acetic acid (IAA) and hydrolytic enzymes such as amylase, protease, cellulase and chitinase, and had the ability to solubilize phosphate. The presence of the key antimicrobial encoding gene in the biosynthesis of 2,4-diacetylphloroglucinol (DAPG) was identified by PCR. The maximum growth and antimicrobial activity of P. fluorescens was observed in king’s B medium at pH 7, 37 °C and 36 h of growth. Glucose and tryptone were found to be the most suitable carbon and nitrogen sources, respectively. DAPG was separated by silica column chromatography and identified by various methods such as UV-Vis, FT-IR, GC-MS and NMR spectroscopy. When R. solanacearum cells were exposed to DAPG at 90 µg/mL, the cell viability was decreased, reactive oxygen species (ROS) were increased and chromosomal DNA was damaged. Application of P. fluorescens and DAPG significantly reduced the bacterial wilt incidence. In addition, P. fluorescens was also found effective in promoting the growth of tomato seedlings. It is concluded that the indigenous isolate P. fluorescens VSMKU3054 could be used as a suitable biocontrol agent against bacterial wilt disease of tomato.

Characterization of the major antifungal extrolite from rice endophyte Lysinibacillus sphaericus against Rhizoctonia solani

Sheath blight of rice caused by Rhizoctonia solani is regarded as one of the most widely distributed diseases of rice, and is one of the major production constraints for rice in India and most rice-growing countries of Asia. Biological control of plant diseases using antagonistic bacteria is now considered as a promising alternative to the use of hazardous chemical fungicides or bactericides. Several bacterial endophytes have been reported to support growth and improve the health of the plants and therefore, may be important as biocontrol agents. In the present study, putative antifungal metabolites were extracted from rice foliage endophyte Lysinibacillus sphaericus KJ872548 by solvent extraction methods and purified using HPTLC techniques. Separated bands were subjected to assess the in vitro antagonistic activity toward rice sheath blight pathogen Rhizoctonia solani using a dual culture method. Partially purified active fraction B2 obtained from HPTLC analysis showed the highest percentage of inhibition (76.9%). GC MS and FTIR analyses of B2 revealed the compound as1, 2-Benzenedicarboxylic acid butyl 2-Ethylhexyl ester, a strong antifungal volatile organic compound. Light microscopic analysis of the fungal mycelium from the dual culture plate of both culture filtrate and 1, 2-Benzenedicarboxylic acid butyl 2-Ethylhexyl ester disclosed strong mycolytic activity as evident by mycelial distractions and shrinkage. This is the first report on antifungal production by endophytic Lysinibacillus sphaericus against R. solani, the rice sheath blight pathogen. The findings of this study biologically prospect the endophyte L. sphaericus as an inexpensive broad spectrum bioagent for eco-friendly, economic and sustainable agriculture.

Partial purification and characterization of 2, 4-diacetylphloroglucinol producing Pseudomonas fluorescens VSMKU3054 against bacterial wilt disease of tomato

We find out the antimicrobial potential of partially purified 2,4-diacetylphloroglucinol (DAPG) against Ralstonia solanacearum and fungal plant pathogens isolated from tomato rhizobacterium Pseudomonas fluorescens VSMKU3054. The present study is mainly focused on the control of wilt disease of tomato by our isolate VSMKU3054 and DAPG. The cell free culture filtrate of P. fluorescens VSMKU3054 was significantly arrested the growth of R. solanacearum and fungal pathogens such as Rhizoctonia solani, Sclerotium rolfsii, Macrophomina phaseolina and Fusarium oxysporum compared to control. The existence of DAPG from the crude metabolites of P. fluorescens VSMKU3054 was confirmed on TLC with Rf value 0.34, which is coincide with that of authentic phloroglucinol. The partially purified DAPG exhibited much higher activity against R. solanacearum at 30 µg/ml than the fungal plant pathogens compared to control. The antimicrobial partially purified compound was identified as DAPG by UV, FT-IR and GC-MS analysis. The percentage of live cells of R. solanacearum when supplemented with DAPG at 30 µg/ml, significantly controlled the living nature of R. solanacearum up to 68% compared to tetracycline and universal control observed under high content screening analysis. The selected isolate P. fluorescens VSMKU3054 and DAPG significantly controlled wilt disease of tomato up to 59.5% and 42.12% on 3rd and 7th days compared to positive and negative control by detached leaf assay. Further, in silico analysis revealed that high interaction of DAPG encoding protease with lectin which is associated with R. solanacearum. Based on our findings, we confirmed that P. fluorescens VSMKU3054 and DAPG could be used a potential bio inoculants for the management of bacterial wilt disease of tomato.

Field Sanitation and Foliar Application of Streptomyces padanus PMS-702 for the Control of Rice Sheath Blight

Rice sheath blight (ShB), caused by Rhizoctonia solani Kühn AG1-IA, is one of the destructive rice diseases worldwide. The aims of this study were to develop biocontrol strategies focusing on field sanitation and foliar application with a biocontrol agent for ShB management. Streptomyces padanus PMS-702 showed a great antagonistic activity against R. solani. Fungichromin produced by S. padanus PMS-702, at 3.07 mg/l inhibited 50% mycelial growth, caused leakage of cytoplasm, and inhibited the formation of infection structures of R. solani. Fungichromin could reach to 802 mg/l when S. padanus PMS-702 was cultured in MACC broth for 6 days. Addition of 0.5% S. padanus PMS-702 broth into soil decreased the survival rate of the pathogen compared to the control. Soil amended with 0.5% S. padanus broth and 0.5% tea seed pomace resulted in the death of R. solani mycelia in the infested rice straws, and the germination of sclerotia was inhibited 21 days after treatment. Greenhouse trials revealed that S. padanus cultured in soybean meal-glucose (SMGC-2) medium after mixing with different surfactants could enhance its efficacy for inhibiting the pathogen. Of six surfactants tested, the addition of 2% tea saponin was the most effective in suppressing the pathogen. S. padanus broth after being fermented in SMGC-2, mixed with 2% tea saponin, diluted 100 fold, and sprayed onto rice plants significantly reduced ShB disease severity. Thus, S. padanus PMS-702 is an effective biocontrol agent. The efficacy of S. padanus PMS-702 for disease control could be improved through formulation.

Characterization of antifungal metabolite phenazine from rice rhizosphere fluorescent pseudomonads (FPs) and their effect on sheath blight of rice

We have shown, the outcome of antifungal activity of phenazine derivatives which is produced by fluorescent pseudomonads (FPs) for the control of sheath blight of rice. A total of 50 fluorescent pseudomonads (FPs) were isolated from rice rhizosphere. Off which, 36 FPs exhibited antagonistic activity against Rhizoctonia solani, Macrophomina phaseolina, Fusarium oxysporum, Alternaria alternata and Sclerotium rolfsii up to 70–80% compared to control by dual culture method. BOX-PCR analyses of antagonistic isolates indicated that two phylogenetic group, where group I consisted of 28 isolates and eight isolates belongs to group II. Among 36 FPs, a total of 10 FPs revealed that the presence of phenazine derivatives on thin layer chromatography (TLC), which is coincided with that of authentic phenazine with R_f value 0.57. Similar to TLC analysis, antibiotic encoding gene phenazine-1-carboxamide (PCN) was detected in 10 FPs by PCR analysis with respective primer. Among, PCN detected isolates of FPs, a significant biocontrol potential possessing isolate designated as VSMKU1 and it was showed prominent antifungal activity against R. solani and other tested fungal pathogens. Hence, the isolate VSMKU1 was selected for further studies. The selected isolate VSMKU1 was identified as Pseudomonas aeruginosa by 16S rDNA sequence analysis. The antifungal metabolite phenazine like compound produced by VSMKU1 was confirmed by UV, FT-IR and HPLC analysis. The phenazine compound from VSMKU1 significantly arrest the growth of R. solani compared to carbendazim by well diffusion method. The detached leaf assay showed remarkable inhibition of lesion height 80 to 85% by the treatments of culture (VSMKU1), cell free culure filtrate and phenazine like compound compared to control and other treatments was observed in detached leaves of rice. These results emphasized that VSMKU1 isolate can be used as an alternative potential biocontrol agent against sheath blight of rice, instead of using commercial fungicide such as validamycin and carbendazim which cause environmental pollution and health hazards.

Antagonistic activity of a novel antifungalmycin N2 from Streptomyces sp. N2 and its biocontrol efficacy against Rhizoctonia solani

Antifungalmycin N2 (3-methyl-3,5-amino-4-vinyl-2-pyrone, C6H7O2N) is a novel bioactive substance produced by Streptomyces sp. N2. In this present work, the antagonistic activity of antifungalmycin N2 and its biocontrol efficacy on Rhizoctonia solani were carried out to evaluate its potential as a biocontrol agent against fungal plant diseases. By using potato dextrose agar media for in vitro cultivation of phytopathogenic fungi, the results showed that antifungalmycin N2 not only displayed broad-spectrum antifungal activities against various plant pathogenic fungi, but also had a strong antagonism to the sclerotial germination of R. solani. In a detached leaf assay, it was found that antifungalmycin N2 could effectively protect the rice leaves form the infection of R. solani, resulting in a significantly reduced sheath blight severity on the surfaces of rice leaves. In the pot experiments, the results also revealed that significantly lower sheath blight infections occurred in the tissues of the treated rice plants, which further confirmed that antifungalmycin N2 had a favorable biocontrol efficacy on rice sheath blight. In conclusion, the above results indicated that the novel antifungalmycin N2 was one of promising biocontrol agents for plant disease control.

Molecular and morphological characterization of rice phylloplane fungi and determination of the antagonistic activity against rice pathogens

Cladosporium spp. is a cosmopolitan fungal genus. In the literature, it has been reported as a biological agent for controlling several plant diseases, but its mechanism of action has never been clarified. The present study aims to identify Cladosporium spp. based on the DNA phylogeny of nine isolates obtained from the phylloplane of rice and their potential antagonistic activity against the main fungal pathogens that affect rice crop. Nine isolates of Cladosporium spp. were identified based on DNA phylogeny, molecular and morphological characterization, and their antagonistic effects with the rice pathogens C. miyabeanus, M. oryzae, M. albescens and S. oryzae. Four isolates were selected to study lytic enzymes such as β-1,3-glucanase, chitinase and protease, and only one isolate was selected for a conidial germination and appressoria formation assay. The nine isolates were identified as C. cladosporioides, C. tenuissimum and C. subuliforme. Four isolates, identified as C. cladosporioides, inhibited the mycelial growth of rice pathogens such as C1H (68.59%) of S. oryzae, C5 G (74.32%) of C. miyabeanus, C11 G (75.97%) of M. oryzae and C24 G (77.39%) of M. albescens. C24 G showed a high activity of lytic enzymes, was tested against C. miyabeanus and M. oryzae, and inhibited conidial germination and appressorium formation by more than 59.36%. The characterization of C. cladosporioides suggested this species as a potential bioagent for the management of several rice diseases, especially rice blast. This is the first time that a potential biological agent from the genus Cladosporium identified at the species level was isolated from the rice phylloplane, and some of its mechanisms of action were demonstrated, such as increasing lytic enzyme activity against rice pathogens.

Design, synthesis, fungicidal property and QSAR studies of novel β-carbolines containing urea, benzoylthiourea and benzoylurea for the control of rice sheath blight

BACKGROUND

Rice sheath blight is a globally important rice disease. Unfortunately, this critical disease has not been effectively controlled, and the intensive and continuous use of the same fungicide might increase the risk of resistance development in the pathogen. To discover new active agents against rice sheath blight, in this study, three series of β-carboline urea, benzoylurea and benzoylthiourea derivatives were designed, synthesized and evaluated for in vitro and in vivo fungicidal activity against Rhizoctonia solani.

RESULTS

All these compounds (EC₅₀ : 0.131-1.227 mmol L^-1 ) exhibited better fungicidal activity than harmine itself (EC₅₀ : 2.453 mmol L^-1 ). Significantly, compound 17c (EC₅₀ : 0.131 mmol L^-1 ) displayed the best efficacy in vitro and superior fungicidal activity compared with validamycin A (EC₅₀ : 0.397 mmol L^-1 ). Moreover, the in vivo bioassay also indicated that compound 17c could be effective for the control of rice sheath blight.

CONCLUSION

Based on the bioassay result and quantitative structure-activity relationship (QSAR) information, structure modification in β-carboline warrants further investigation and its benzoylurea derivative 17c, which showed the best fungicidal activities, could emerge as a potential fungicide against rice sheath blight. © 2018 Society of Chemical Industry.

Plant Growth Promoting and Biocontrol Activity of Streptomyces spp. as Endophytes

There has been many recent studies on the use of microbial antagonists to control diseases incited by soilborne and airborne plant pathogenic bacteria and fungi, in an attempt to replace existing methods of chemical control and avoid extensive use of fungicides, which often lead to resistance in plant pathogens. In agriculture, plant growth-promoting and biocontrol microorganisms have emerged as safe alternatives to chemical pesticides. Streptomyces spp. and their metabolites may have great potential as excellent agents for controlling various fungal and bacterial phytopathogens. Streptomycetes belong to the rhizosoil microbial communities and are efficient colonizers of plant tissues, from roots to the aerial parts. They are active producers of antibiotics and volatile organic compounds, both in soil and in planta, and this feature is helpful for identifying active antagonists of plant pathogens and can be used in several cropping systems as biocontrol agents. Additionally, their ability to promote plant growth has been demonstrated in a number of crops, thus inspiring the wide application of streptomycetes as biofertilizers to increase plant productivity. The present review highlights Streptomyces spp.-mediated functional traits, such as enhancement of plant growth and biocontrol of phytopathogens.

Modeling of phytoextraction efficiency of microbially stimulated Salix dasyclados L. in the soils with different speciation of heavy metals

Bioaugmentation of soils with selected microorganisms during phytoextraction can be the key solution for successful bioremediation and should be accurately calculated for different physicochemical soil properties and heavy metal availability to guarantee the universality of this method. Equally important is the development of an accurate prediction tool to manage phytoremediation process. The main objective of this study was to evaluate the role of three metallotolerant siderophore-producing Streptomyces sp. B1-B3 strains in the phytoremediation of heavy metals with the use of S. dasyclados L. growing in four metalliferrous soils as well as modeling the efficiency of this process based on physicochemical and microbiological properties of the soils using artificial neural network (ANN) analysis. The bacterial inoculation of plants significantly stimulated plant biomass and reduced oxidative stress. Moreover, the bacteria affected the speciation of heavy metals and finally their mobility, thereby enhancing the uptake and bioaccumulation of Zn, Cd, and Pb in the biomass. The best capacity for phytoextraction was noted for strain B1, which had the highest siderophore secretion ability. Finally, ANN model permitted to predict efficiency of phytoextraction based on both the physicochemical properties of the soils and the activity of the soil microbiota with high precision.

Biocontrol Potential of Streptomyces hydrogenans Strain DH16 toward Alternaria brassicicola to Control Damping Off and Black Leaf Spot of Raphanus sativus

Biocontrol agents and their bioactive metabolites provide one of the best alternatives to decrease the use of chemical pesticides. In light of this, the present investigation reports the biocontrol potential of Streptomyces hydrogenans DH16 and its metabolites towards Alternaria brassicicola, causal agent of black leaf spot and damping off of seedlings of crucifers. In vitro antibiosis of strain against pathogen revealed complete suppression of mycelial growth of pathogen, grown in potato dextrose broth supplemented with culture supernatant (20% v/v) of S. hydrogenans DH16. Microscopic examination of the fungal growth showed severe morphological abnormalities in the mycelium caused by antifungal metabolites. In vivo studies showed the efficacy of streptomycete cells and culture supernatant as seed dressings to control damping off of Raphanus sativus seedlings. Treatment of pathogen infested seeds with culture supernatant (10%) and streptomycete cells significantly improved seed germination (75-80%) and vigor index (1167-1538). Furthermore, potential of cells and culture supernatant as foliar treatment to control black leaf spot was also evaluated. Clearly visible symptoms of disease were observed in the control plants with 66.81% disease incidence and retarded growth of root system. However, disease incidence reduced to 6.78 and 1.47% in plants treated with antagonist and its metabolites, respectively. Additionally, treatment of seeds and plants with streptomycete stimulated various growth traits of plants over uninoculated control plants in the absence of pathogen challenge. These results indicate that S. hydrogenans and its culture metabolites can be developed as biofungicides as seed dressings to control seed borne pathogens, and as sprays to control black leaf spot of crucifers.

In vitro biocontrol activity of halotolerant Streptomyces aureofaciens K20: A potent antagonist against Macrophomina phaseolina (Tassi) Goid

A halotolerant actinobacterial strain isolated from salinity affected soil of Eastern Indo-Gangetic plains (IGP), Uttar Pradesh, India, was characterised for its antagonistic potential against Macrophomina phaseolina by dual-culture assay. It was shown to effectively inhibit the growth of M. phaseolina with an inhibition zone of 27 ± 1.33 mm. Further the actinobacterial strain was evaluated for its plant growth promoting (PGP) properties and its ability to produce biocontrol related extracellular enzymes viz. amylase, protease, cellulase, chitinase, gelatinase and urease. The results revealed that the actinobacterial strain had PGP potential along with positive assay for amylase, chitinase and urease. The interaction study between antagonist strain and fungal pathogen, performed by scanning electron microscopy technique revealed that the actinobacterium was able to damage fungal mycelia may be due to chitinase, establishing its role as a potential antagonist against M. phaseolina. The actinobacterial isolate was characterised by 16S rDNA gene sequencing, and was identified as Streptomyces genera. The identified gene sequence was deposited to NCBI GenBank with an accession number KP331758.