Greek indigenous streptomycetes as biocontrol agents against the soil-borne fungal plant pathogen Rhizoctonia solani

Evaluation of Soil Streptomyces spp. for the Biological Control of Fusarium Wilt Disease and Growth Promotion in Tomato and Banana

Fusarium oxysporum f. sp. lycopersici (Fol) and Fusarium oxysporum f. sp. cubense (Foc), are the causal agent of Fusarium wilt disease of tomato and banana, respectively, and cause significant yield losses worldwide. A cost-effective measure, such as biological control agents, was used as an alternative method to control these pathogens. Therefore, in this study, six isolates of the Streptomyces-like colony were isolated from soils and their antagonistic activity against phytopathogenic fungi and plant growth-promoting (PGP) activity were assessed. The results showed that these isolates could inhibit the mycelial growth of Fol and Foc. Among them, isolate STRM304 showed the highest percentage of mycelial growth reduction and broad-spectrum antagonistic activity against all tested fungi. In the pot experiment study, the culture filtrate of isolates STRM103 and STRM104 significantly decreased disease severity and symptoms in Fol inoculated plants. Similarly, the culture filtrate of the STRM304 isolate significantly reduced the severity of the disease and symptoms of the disease in Foc inoculated plants. The PGP activity test presents PGP activities, such as indole acetic acid production, phosphate solubilization, starch hydrolysis, lignin hydrolysis, and cellulase activity. Interestingly, the application of the culture filtrate from all isolates increased the percentage of tomato seed germination and stimulated the growth of tomato plants and banana seedlings, increasing the elongation of the shoot and the root and shoot and root weight compared to the control treatment. Therefore, the isolate STRM103 and STRM104, and STRM304 could be used as biocontrol and PGP agents for tomato and banana, respectively, in sustainable agriculture.

Streptomyces, Greek Habitats and Novel Pharmaceuticals: A Promising Challenge

Bacteria of the genus Streptomyces produce a very large number of secondary metabolites, many of which are of vital importance to modern medicine. There is great interest in the discovery of novel pharmaceutical compounds derived from strepomycetes, since novel antibiotics, anticancer and compounds for treating other conditions are urgently needed. Greece, as proven by recent research, possesses microbial reservoirs with a high diversity of Streptomyces populations, which provide a rich pool of strains with potential pharmaceutical value. This review examines the compounds of pharmaceutical interest that have been derived from Greek Streptomyces isolates. The compounds reported in the literature include antibiotics, antitumor compounds, biofilm inhibitors, antiparasitics, bacterial toxin production inhibitors and antioxidants. The streptomycete biodiversity of Greek environments remains relatively unexamined and is therefore a very promising resource for potential novel pharmaceuticals.

Isolation, Identification, Biocontrol Activity, and Plant Growth Promoting Capability of a Superior Streptomyces tricolor Strain HM10

Streptomyces is a genus with known biocontrol activity, producing a broad range of biologically active substances. Our goal was to isolate local Streptomyces species, evaluate their capacity to biocontrol the selected phytopathogens, and promote the plant growth via siderophore and indole acetic acid (IAA) production and phosphate solubilization. Eleven isolates were obtained from local soil samples in Saudi Arabia via the standard serial dilution method and identified morphologically by scanning electron microscope (SEM) and 16S rRNA amplicon sequencing. The biocontrol of phytopathogens was screened against known soil-borne fungi and bacteria. Plant growth promotion capacity was evaluated based on siderophore and IAA production and phosphate solubilization capacity. From eleven isolates obtained, one showed 99.77% homology with the type strain Streptomyces tricolor AS 4.1867, and was designated S. tricolor strain HM10. It showed aerial hyphae in SEM, growth inhibition of ten known phytopathogens in in vitro experiments, and the production of plant growth promoting compounds such as siderophores, IAA, and phosphate solubilization capacity. S. tricolor strain HM10 exhibited high antagonism against the fungi tested (i.e., Colletotrichum gloeosporides with an inhibition zone exceeding 18 mm), whereas the lowest antagonistic effect was against Alternaria solani (an inhibition zone equal to 8 mm). Furthermore, the most efficient siderophore production was recorded to strain HM8, followed by strain HM10 with 64 and 22.56 h/c (halo zone area/colony area), respectively. Concerning IAA production, Streptomyces strain HM10 was the most effective producer with a value of 273.02 μg/ml. An autochthonous strain S. tricolor HM10 should be an important biological agent to control phytopathogens and promote plant growth.

Τhe Nematicidal Potential of Bioactive Streptomyces Strains Isolated from Greek Rhizosphere Soils Tested on Arabidopsis Plants of Varying Susceptibility to Meloidogyne spp

A total of 461 indigenous Streptomycetes strains recovered from various Greek rhizosphere habitats were tested for their bioactivity. All isolates were examined for their ability to suppress the growth of 12 specific target microorganisms. Twenty-six were found to exert antimicrobial activity and were screened for potential nematicidal action. S. monomycini ATHUBA 220, S. colombiensis ATHUBA 438, S. colombiensis ATHUBA 431, and S. youssoufensis ATHUBA 546 were proved to have a nematicidal effect and thus were further sequenced. Batch culture supernatants and solvent extracts were assessed for paralysis on Meloidogyne javanica and Meloidogyne incognita second-stage juveniles (J2). The solvent extracts of S. monomycini ATHUBA 220 and S. colombiensis ATHUBA 438 had the highest paralysis rates, so these Streptomycetes strains were further on tested for nematodes' biological cycle arrest on two Arabidopsis thaliana plants; the wild type (Col-0) and the katanin mutant fra2, which is susceptible to M. incognita. Interestingly, S. monomycini ATHUBA 220 and S. colombiensis ATHUBA 438 were able to negatively affect the M. incognita biological cycle in Col-0 and fra2 respectively, and increased growth in Col-0 upon M. incognita infection. However, they were ineffective against M. javanica. Fra2 plants were also proved susceptible to M. javanica infestation, with a reduced growth upon treatments with the Streptomyces strains. The nematicidal action and the plant-growth modulating abilities of the selected Streptomycetes strains are discussed.

Densities and inhibitory phenotypes among indigenous Streptomyces spp. vary across native and agricultural habitats

Streptomyces spp. perform vital roles in natural and agricultural soil ecosystems including in decomposition and nutrient cycling, promotion of plant growth and fitness, and plant disease suppression. Streptomyces densities can vary across the landscape, and inhibitory phenotypes are often a result of selection mediated by microbial competitive interactions in soil communities. Diverse environmental factors, including those specific to habitat, are likely to determine microbial densities in the soil and the outcomes of microbial species interactions. Here, we characterized indigenous Streptomyces densities and inhibitory phenotypes from soil samples (n = 82) collected in 6 distinct habitats across the Cedar Creek Ecosystem Science Reserve (CCESR; agricultural, prairie, savanna, wetland, wet-woodland, and forest). Significant variation in Streptomyces density and the frequency of antagonistic Streptomyces were observed among habitats. There was also significant variation in soil chemical properties among habitats, including percent carbon, percent nitrogen, available phosphorus, extractable potassium, and pH. Density and frequency of antagonists were significantly correlated with one or more environmental parameters across all habitats, though relationships with some parameters differed among habitats. In addition, we found that habitat rather than spatial proximity was a better predictor of variation in Streptomyces density and inhibitory phenotypes. Moreover, habitats least conducive for Streptomyces growth and proliferation, as determined by population density, had increased frequencies of inhibitory phenotypes. Identifying environmental parameters that structure variation in density and frequency of antagonistic Streptomyces can provide insight for determining factors that mediate selection for inhibitory phenotypes across the landscape.

Composting of Vegetable Waste Using Microbial Consortium and Biocontrol Efficacy of Streptomyces Sp. Al-Dhabi 30 Isolated from the Saudi Arabian Environment for Sustainable Agriculture

Thirty-seven root-associated Actinomycetes were isolated from the tomato plant for plant growth promoting activity. Among these, ten were selected for phosphate solubilisation, the production of siderophores, and indole acetic acid. Out of ten, eight Actinomycetes solubilised phosphate, whereas, Streptomyces sp. Al-Dhabi 30 showed better activity (43.1 mg/dL). Actinomycetes produced siderophore and the concentration ranged between 1.6 and 42.1 μg/mL. Streptomyces sp. Al-Dhabi 30 showed the ability to produce a maximum amount of indole acetic acid (IAA) (43 μg/mL), chitinase (43.1 U/mL), cellulase (67 U/mL), and protease (121 U/mL) than other strains. Further, vegetable waste was used as the bulk material for composting using Streptomyces sp. Al-Dhabi 30 along with microbial consortium. Total nitrogen content was 3.8% in Streptomyces sp. Al-Dhabi 30 inoculated compost, whereas 2.7% organic nitrogen was detected in the control. In the compost vegetable waste, the C:N ratio was 10.07, whereas it was 17.51 in the control. The vegetable waste composted with Streptomyces sp. Al-Dhabi 30, Lactobacillus plantarum ATCC 33222, and Candidautilis ATCC 9950 showed antagonistic activity and the supplemented compost enhanced shoot, root height, and total weightin tomato plants. These findings clearly suggest the use of Streptomyces sp. Al-Dhabi 30 as a potential biocontrol agent.

Plant-Growth Promotion and Biocontrol Properties of Three Streptomyces spp. Isolates to Control Bacterial Rice Pathogens

Bacterial Panicle Blight caused by Burkholderia glumae is a major disease of rice, which has dramatically affected rice production around the world in the last years. In this study we describe the assessment of three Streptomyces isolates as biocontrol agents for B. glumae. Additionally, the presence of other plant-growth promoting abilities and their possible beneficial effects upon their inoculation on rice plants was evaluated as an ecological analysis for their future inoculation in rice crops. Two isolates (A20 and 5.1) inhibited growth of virulent B. glumae strains, as well as a wide range of bacterial and fungal species, while a third strain (7.1) showed only antifungal activity. In vitro tests demonstrated the ability of these strains to produce siderophores, Indoleacetic acid (IAA), extracellular enzymes and solubilizing phosphate. Greenhouse experiments with two rice cultivars indicated that Streptomyces A20 is able to colonize rice plants and promote plant growth in both cultivars. Furthermore, an egfp tagged mutant was generated and colonization experiments were performed, indicating that Streptomyces A20 –GFP was strongly associated with root hairs, which may be related to the plant growth promotion observed in the gnotobiotic experiments. In order to characterize the antimicrobial compounds produced by strain A20 bacteria, mass spectrometry analyses were performed. This technique indicated that A20 produced several antimicrobial compounds with sizes below 3 kDa and three of these molecules were identified as Streptotricins D, E and F. These findings indicate the potential of Streptomyces A20 as a biocontrol inoculant to protect rice plants against bacterial diseases.

Actinomycetes from Eucalyptus and their biological activities for controlling Eucalyptus leaf and shoot blight

In Thailand, Eucalyptus plantations rapidly expand across the country. Leaf and shoot blight caused by Cryptosporiopsis eucalypti, Cylindrocladium sp. and Teratosphaeria destructans is a serious disease in Eucalyptus plantations. In this study, a total of 477 actinomycete strains were successfully isolated from roots and rhizosphere soil of Eucalyptus. Four hundred and thirty nine isolates were classified as streptomycetes and 38 isolates were non-streptomycetes. Among these isolates, 272 (57.0%), 118 (24.7%) and 241 (50.5%) isolates were antagonistic to Cryptosporiopsis eucalypti, Cylindrocladium sp. and Teratosphaeria destructans, respectively. All isolates were tested for their abilities to produce siderophores, indole acetic acid (IAA) and solubilise phosphate. Most isolates (464, 97.3%) produced siderophores. The majority of isolates (345, 72.3%) solubilised phosphate. In addition, almost half of these isolates (237, 49.7%) produced indole acetic acid. Strain EUSKR2S82 which showed the strongest inhibitory effect against all tested fungi with plant growth promoting ability was selected to test with Eucalyptus. This strain could colonize plant roots and increase Eucalyptus roots length. In a detached leaves bioassay, the disease severity of EUSKR2S82-inoculated Eucalyptus leaves was only 30% compared to 95% in the control treatment. The 16S rRNA gene sequence analysis revealed that the strain EUSKR2S82 was related to Streptomyces ramulosus NRRL-B 2714(T) (99.44% similarity). Identification of non-streptomycete isolates using 16S rRNA gene sequences classified them into 9 genera: Actinoallomurus, Actinomadura, Amycolatopsis, Cryptosporangium, Microbispora, Micromonospora, Nocardia, Nonomuraea and Pseudonocardia. It is evident that Eucalyptus tree harbored several genera of actinomycetes. The selected isolate, EUSKR2S82 showed potential as a candidate for biocontrol agent of leaf and shoot blight of Eucalyptus and to promote growth.

Streptomyces rochei ACTA1551, an indigenous Greek isolate studied as a potential biocontrol agent against Fusarium oxysporum f.sp. lycopersici

Many studies have shown that several Greek ecosystems inhabit very interesting bacteria with biotechnological properties. Therefore Streptomyces isolates from diverse Greek habitats were selected for their antifungal activity against the common phytopathogenic fungus Fusarium oxysporum. The isolate encoded ACTA1551, member of Streptomyces genus, could strongly suppress the fungal growth when examined in antagonistic bioassays in vitro. The isolate was found phylogenetically relative to Streptomyces rochei after analyzing its 16S rDNA sequence. The influence of different environmental conditions, such as medium composition, temperature, and pH on the expression of the antifungal activity was thoroughly examined. Streptomyces rochei ACTA1551 was able to protect tomato seeds from F. oxysporum infection in vivo while it was shown to promote the growth of tomato plants when the pathogen was absent. In an initial effort towards the elucidation of the biochemical and physiological nature of ACTA1551 antifungal activity, extracts from solid streptomycete cultures under antagonistic or/and not antagonistic conditions were concentrated and fractionated. The metabolites involved in the antagonistic action of the isolate showed to be more than one and produced independently of the presence of the pathogen. The above observations could support the application of Streptomyces rochei ACTA1551 as biocontrol agent against F. oxysporum.