Herbicidal Characteristics and Structural Identification of the Potential Active Compounds from Streptomyces sp. KRA17-580

Biotechnological potential of actinomycetes in the 21st century: a brief review

This brief review aims to draw attention to the biotechnological potential of actinomycetes. Their main uses as sources of antibiotics and in agriculture would be enough not to neglect them; however, as we will see, their biotechnological application is much broader. Far from intending to exhaust this issue, we present a short survey of the research involving actinomycetes and their applications published in the last 23 years. We highlight a perspective for the discovery of new active ingredients or new applications for the known metabolites of these microorganisms that, for approximately 80 years, since the discovery of streptomycin, have been the main source of antibiotics. Based on the collected data, we organize the text to show how the cosmopolitanism of actinomycetes and the evolutionary biotic and abiotic ecological relationships of actinomycetes translate into the expression of metabolites in the environment and the richness of biosynthetic gene clusters, many of which remain silenced in traditional laboratory cultures. We also present the main strategies used in the twenty-first century to promote the expression of these silenced genes and obtain new secondary metabolites from known or new strains. Many of these metabolites have biological activities relevant to medicine, agriculture, and biotechnology industries, including candidates for new drugs or drug models against infectious and non-infectious diseases. Below, we present significant examples of the antimicrobial spectrum of actinomycetes, which is the most commonly investigated and best known, as well as their non-antimicrobial spectrum, which is becoming better known and increasingly explored.

Streptomyces herbicida sp. nov., a novel actinomycete with antibacterial and herbicidal activity isolated from soil

A novel actinobacterial strain (NEAU-HV9T) showing antibacterial activity against Ralstonia solanacearum and herbicidal activity against Amaranthus retroflexus L. was isolated from soil sampled in Bama yao Autonomous County, Hechi City, Guangxi Zhuang Autonomous Region. The strain is aerobic and Gram-positive. Phylogenetic analysis based on 16S rRNA gene sequence indicated that strain NEAU-HV9T belonged to the genus Streptomyces and showed high 16S rRNA sequence similarity to Streptomyces panaciradicis 1MR-8T (98.90 %), Streptomyces sasae JR-39T (98.89 %) and Streptomyces barringtoniae JA03T (98.69 %) and less than 98.5 % similarity to other members of the genus Streptomyces. The cell wall of strain NEAU-HV9T contained ll-diaminopimelic acid and the whole-cell hydrolysates were galactose, mannose and ribose. The predominant menaquinones were composed of MK-9(H2) and MK-9(H8). The major polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylinositol. The major fatty acids were C16 : 0, iso-C16 : 0 and C17 : 1  ω8c. The genomic DNA G+C content of strain NEAU-HV9T was 70.6 mol%. Furthermore, the strain could be clearly distinguished from its closely related type strains by the combination of DNA-DNA hybridization results and some phenotypic characteristics. Meanwhile, strain NEAU-HV9T displayed herbicidal activity. Therefore, strain NEAU-HV9T represents a novel species within the genus Streptomyces, for which the name Streptomyces herbicida sp. nov. is proposed, with strain NEAU-HV9T (=CCTCC AA 2019088T=DSM 113364T) as the type strain.

Herbicidal Activity of Secondary Metabolites Isolated from Streptomyces sp. NEAU-HV44 on Selected Weeds and Crops

Streptomyces is an effective source of new natural bioherbicides. In this study, a novel isolated strain NEAU-HV44 showed strong inhibitory activity against Amaranthus retroflexus L. and was concluded to the genus Streptomyces. Strain NEAU-HV44 fermentation conditions were optimized to maximize the herbicidal activity. The supernatant of strain NEAU-HV44 could significantly control the growth of weeds (A. retroflexus L., Setaria viridis, Portulaca oleracea L., and Chenopodium album) and crops (maize, soybean, wheat, Chinese cabbage, cucumber, tomato, and romaine lettuce) with dose-dependent in preemergence. Notably, weeds were more sensitive to a low-concentration supernatant extract than crops in preemergence. In postemergence, the 2 mg mL-1 supernatant extract could significantly reduce the height and >50% biomass (fresh weight) of tested weeds. The supernatant extract could cause cell membrane destabilization and the cell death of weeds. In addition, the growth of tomato was also inhibited at a high concentration, but no obvious symptoms were observed on soybean and romaine lettuce after spraying the supernatant extract. Then two novel julichrome monomers, julichromes Q12 (1) and Q13 (2), and two known julichromes, julichrome Q3.3 (3) and julichrome Q3.5 (4), were isolated from the supernatant extract of strain NEAU-HV44 by bioactivity-guided approach. This is the first report of the herbicidal activity of julichromes. These four herbicidal compounds could inhibit the shoot and root growth of weeds at 0.2 mg mL-1, and compound 4 could completely inhibit the growth of P. oleracea L. Thus, julichromes (Q12, Q13, Q3.3, Q3.5 1-4) may be new bioherbicidal candidates.

Biomolecules in modern and sustainable agriculture

This review presents scientific findings which indicate biomolecules are excellent candidates for the development of biopesticides. Efforts are being done to find routes to increase their concentrations in the cultivation media because this concentration facilitates applications, storage, and transportation. Some of these routes are co-fermentation and ultrasound-assisted fermentation. Ultrasonication increases metabolite production and growth rates by improvement of cell permeability and nutrient uptake rates through cell membranes. For example, 24% increase in the enzymatic activity of cellulases produced by Trichoderma reesei in solid-state fermentation was achieved with ultrasonication. Also, chitinase and β-1,3-glucanase productions were stimulated by ultrasound in Beauveria bassiana cultivation, presenting positive results. The common parameters evaluated in the production of biomolecules by ultrasound-assisted fermentation are the duty cycle, time of application, power, energetic density, and how long the sonication is maintained in the fermentation media. Many successful cases are reported and discussed, which include the final formulation of bioproducts for agricultural applications. In this trend, nanotechnology is a promising tool for the development of nanoformulations. Nanoemulsification, green synthesis, biosynthesis, or biogenic synthesis are technologies used to produce such nanoformulations, allowing the controlled release of control agents, as well as the delivery of biomolecules to specific targets.

Herbicidal characteristics and structural identification of a potential active compound produced by Streptomyces sp. KRA18-249

The KRA18-249 strain, isolated from a natural recreational forest near Jeongseon, Gangwon-do, when applied to plants induced signs of wilting within 24 h, leading to plant death. The isolated actinomycete was identified as Streptomyces gardneri based on 16S rRNA gene homogeneity analysis. The culture filtrate was solvent fractionated to obtain the active substance, and the active compound 249-Y1 was isolated from the purified fractions via a herbicide activity test using Digitaria ciliaris. NMR and ESI-MS analyses revealed that the molecular formula of 249-Y1 is C₂₀H₁₆O₆ [MW = 352.0947] and is an anthraquinone (rubiginone D2) produce by polyketide synthetase system. The active compound 249-Y1 showed strong (100%) herbicidal activity against several weeds at 500 μg mL^-1 concentration. Twisting symptoms began to appear within 24 h of treatment and intensified over time. The KRA18-249 strain produced the herbicidal compound under specific culture conditions, that is, at 200 rpm, 35 °C, for eight days at an initial pH of 10. We also found that 249-Y1 inhibited chlorophyll, but was not a radical generator. Overall, the secondary metabolite 249-Y1, produced by KRA18-249, can be used as a new biological agent for weed control.

Insecticidal characteristics and structural identification of the potential active compounds from Streptomyces sp. KR0006: Strain improvement through mutagenesis

Pest control by biological means is an effective, eco-friendly, and promising method that typically involves compounds naturally derived from actinomycetes. Thus, the present study aimed to screen, characterize, and identify the structure of insecticidal compounds from Streptomyces sp. KR0006 and increase the activity through mutagenesis. In the examination of the insecticidal activity level of the isolates, Streptomyces sp. KR0006 metabolite showed significant activity against larvae and moths of Plutella xylostella. Taxonomic analyses of the 16S rRNA gene sequences revealed that the isolated KR0006 strain tended to be 99% consistent with Streptomyces cinereoruber strain NBRC 12756. Three active compounds isolated from the culture filtrate of KR0006 were purified by solvent partition, mid-pressure liquid chromatography (MPLC), Sephadex LH20 column chromatography, and high-performance liquid chromatography (HPLC). By performing ¹H-NMR, ¹³C-NMR, and 2D-NMR experiments, and high-resolution electrospray ionization mass spectrometry analysis, the 316-HP2, 316-HP3, and 316-HP5 compounds were inferred as antimycin A3a (MW, 519.; C₂₆H₃₆N₂O₉), antimycin A8a (MW, 534; C₂₇H₃₈N₂O₉), and antimycin A1a (MW, 548; C₂₈H₄₀N₂O₉) respectively. Mutant U67 obtained from exposure to ultraviolet (UV) irradiation (254 nm, height 17 cm) for 70 seconds resulted in a 70% more larval mortality than that of the initial wild culture. The second mutation of the culture broth enhanced insecticidal activity by 80 and 100% compared with the first mutation and initial medium, respectively. Our study found that Streptomyces sp. KR0006 strain produces insecticidal active compounds and could be used for practical pest management.

Macrolides from Streptomyces sp. SN5452 and Their Antifungal Activity against Pyricularia oryzae

Pyricularia oryzae causes rice blast, the major destructive disease in nearly all rice fields. In order to obtain highly active compounds against P. oryzae, four new 20-membered macrolides named venturicidins G–J (1–4) were isolated from the culture broth of Streptomyces sp. SN5452 along with two known ones, venturicidins A (5) and B (6). Their structures were determined by the cumulative analyses of nuclear magnetic resonance (NMR) spectroscopy and high-resolution electrospray ionization mass spectrometry (HRESIMS) data. All isolated compounds were evaluated for their antifungal activity against P. oryzae. Interestingly, these compounds exhibited obvious inhibition to mycelial growth and conidial germination of P. oryzae. Remarkably, the EC₅₀ values of venturicidins A (5), B (6), and I (3) against mycelial growth were 0.11, 0.15 and 0.35 µg/mL, and their EC₅₀ values of conidial germination were 0.27, 0.39 and 1.14 µg/mL, respectively. The analysis of structure-activity relationships (SARs) revealed that the methylated positions might be involved in the antifungal activity of venturicidins. These results indicate that the venturicidins are prospective candidates for novel fungicides that can be applied in controlling rice blast.

Recently Discovered Secondary Metabolites from Streptomyces Species

The Streptomyces genus has been a rich source of bioactive natural products, medicinal chemicals, and novel drug leads for three-quarters of a century. Yet studies suggest that the genus is capable of making some 150,000 more bioactive compounds than all Streptomyces secondary metabolites reported to date. Researchers around the world continue to explore this enormous potential using a range of strategies including modification of culture conditions, bioinformatics and genome mining, heterologous expression, and other approaches to cryptic biosynthetic gene cluster activation. Our survey of the recent literature, with a particular focus on the year 2020, brings together more than 70 novel secondary metabolites from Streptomyces species, which are discussed in this review. This diverse array includes cyclic and linear peptides, peptide derivatives, polyketides, terpenoids, polyaromatics, macrocycles, and furans, the isolation, chemical structures, and bioactivity of which are appraised. The discovery of these many different compounds demonstrates the continued potential of Streptomyces as a source of new and interesting natural products and contributes further important pieces to the mostly unfinished puzzle of Earth’s myriad microbes and their multifaceted chemical output.