The mechanism of antifungal action of a new polyene macrolide antibiotic antifungalmycin 702 from Streptomyces padanus JAU4234 on the rice sheath blight pathogen Rhizoctonia solani

New Polyene Macrolide Compounds from Mangrove-Derived Strain Streptomyces hiroshimensis GXIMD 06359: Isolation, Antifungal Activity, and Mechanism against Talaromyces marneffei

Mangrove-derived actinomycetes represent a rich source of novel bioactive natural products in drug discovery. In this study, four new polyene macrolide antibiotics antifungalmycin B-E (1-4), along with seven known analogs (5-11), were isolated from the fermentation broth of the mangrove strain Streptomyces hiroshimensis GXIMD 06359. All compounds from this strain were purified using semi-preparative HPLC and Sephadex LH-20 gel filtration while following an antifungal activity-guided fractionation. Their structures were elucidated through spectroscopic techniques including UV, HR-ESI-MS, and NMR. These compounds exhibited broad-spectrum antifungal activity against Talaromyces marneffei with minimum inhibitory concentration (MIC) values being in the range of 2-128 μg/mL except compound 2. This is the first report of polyene derivatives produced by S. hiroshimensis as bioactive compounds against T. marneffei. In vitro studies showed that compound 1 exerted a significantly stronger antifungal activity against T. marneffei than other new compounds, and the antifungal mechanism of compound 1 may be related to the disrupted cell membrane, which causes mitochondrial dysfunction, resulting in leakage of intracellular biological components, and subsequently, cell death. Taken together, this study provides a basis for compound 1 preventing and controlling talaromycosis.

Tolerance and adaptation characteristics of sugar beet (Beta vulgaris L.) to low nitrogen supply

Nitrogen (N) is an essential element required for sugar beet growth. Sugar beets with low N (LN) tolerance and high N use efficiency are excellent materials for breeding. Here, we comprehensively evaluated the morphological and physiological responses of nine sugar beet genotypes to LN supply. It was found that 0.5 mmol·L^-1 N (LN) significantly influenced the performance of leaves and the topology of roots by reducing the bioproduction of chlorophyll a (Chl a) and soluble protein (SP) and the accumulation of N in leaves and roots (LNA and RNA), thus differentially restricting the growth (hypocotyl diameter, HD; root length, RL) and biomass (leaf and root fresh weight; LFW and RFW; leaf dry weight, LDW) of these sugar beets. Principal component and cluster analyses showed that 780016B/12 superior (F) exhibited excellent tolerance to LN; it had higher SOD activity (62.70%) and APX activity (188.92%) and a higher proline content (131.82%) than 92011 (G, LN sensitive). These attributes helped 780016B/12 superior (F) to better endure LN stress, and the morphology and N distribution changed to adapt to N deficiency, such that the root length increased by 112.48%, leaf area increased by 101.23%, and leaf nitrogen accumulation reached a peak of 14.13 g/plant. It seems that LN-tolerant genotypes increased their root length and surface area by reducing the difference in biomass, thereby expanding the contact between roots and soil, which was conducive to the absorption of nutrients (N) by sugar beets and helped distribute more assimilation products to the roots.

Polyenic Antibiotics and Other Antifungal Compounds Produced by Hemolytic Streptomyces Species

Streptomyces are of great interest in the pharmaceutical industry as they produce a plethora of secondary metabolites that act as antibacterial and antifungal agents. They may thrive on their own in the soil, or associate with other organisms, such as plants or invertebrates. Some soil-derived strains exhibit hemolytic properties when cultivated on blood agar, raising the question of whether hemolysis could be a virulence factor of the bacteria. In this work we examined hemolytic compound production in 23 β-hemolytic Streptomyces isolates; of these 12 were soil-derived, 10 were arthropod-associated, and 1 was plant-associated. An additional human-associated S. sp. TR1341 served as a control. Mass spectrometry analysis suggested synthesis of polyene molecules responsible for the hemolysis: candicidins, filipins, strevertene A, tetrafungin, and tetrin A, as well as four novel polyene compounds (denoted here as polyene A, B, C, and D) in individual liquid cultures or paired co-cultures. The non-polyene antifungal compounds actiphenol and surugamide A were also identified. The findings indicate that the ability of Streptomyces to produce cytolytic compounds (here manifested by hemolysis on blood agar) is an intrinsic feature of the bacteria in the soil environment and could even serve as a virulence factor when colonizing available host organisms. Additionally, a literature review of polyenes and non-polyene hemolytic metabolites produced by Streptomyces is presented.

Exploring the Efficacy of Biocontrol Microbes against the Fungal Pathogen Botryosphaeria dothidea JNHT01 Isolated from Fresh Walnut Fruit

Biological control by antagonistic microorganisms are an effective and environmentally friendly approach in postharvest disease management. In order to develop a biocontrol agent for fresh walnut fruit preservation, the potential biocontrol effects of Bacillus amyloliquefaciens RD.006 and Hanseniaspora uvarum FA.006 against the main fungal pathogen of walnuts were evaluated. Botryosphaeria species showed the highest detection, and the JNHT01 strain showed the strongest pathogenicity. Bot. dothidea JNHT01 caused gray mold and brown rot on fresh walnuts, and its incidence rate reached 100% after an 8 days incubation. The growth of this fungal strain can be promoted by lighting, with a maximum growth rate achieved at a pH of 7 and at 28 °C. B. amyloliquefaciens RD.006 and H. uvarum FA.006 supernatants at a concentration of 1-15% v/v showed antifungal activity. The mycelial growth inhibition rates of Bot. dothidea JNHT01 were 23.67-82.61% for B. amyloliquefaciens RD.006 and 1.45-21.74% for H. uvarum FA.006. During Bot. dothidea JNHT01 growth, the biomass, nucleic acid leakage, and malondialdehyde content gradually increased, while the DPPH scavenging capacity and SOD activity decreased. The B. amyloliquefaciens RD.006 and H. uvarum FA.006 strains showed antifungal activity by damaging fungal cell membranes and reducing fungal antioxidant activity. Moreover, the antifungal effect of B. amyloliquefaciens RD.006 was higher than that of H. uvarum FA.006. Hence, the RD.006 strain of B. amyloliquefaciens can be considered a potential biocontrol agent for the management of postharvest walnut diseases caused by Bot. dothidea.

dinF Elicits Nitric Oxide Signaling Induced by Periplanetasin-4 from American Cockroach in Escherichia coli

Modern antibiotics have been developed with the aim of destroying cellular function; however, the risk of antibiotic-resistance is increasing continuously. As a result, antimicrobial peptide (AMP) is considered a novel strategy to substitute traditional drugs. This study focused on revealing the antibacterial mechanism(s) of periplanetasn-4, an AMP identified from Cockroach. To elucidate whether periplanetasin-4 generates reactive oxygen species (ROS), a crucial stress factor for cell death, intracellular ROS was measured in Escherichia coli. The degree of membrane and DNA damage was determined using the properties that ROS causes oxidative stress to cell components. Unlike normal cell death, membrane depolarization was observed but DNA fragmentation did not occur. In addition, accumulation of nitric oxide (NO), a free radical with high toxicity, was measured and the byproduct of NO also induced severe intracellular damage. Periplanetasin-4-induced NO also impacted on cytosol calcium levels and triggered lipid peroxidation and DNA oxidation. These features were weakened when NO synthesis was interrupted, and this data suggested that perplanetasin-4-induced NO participates in E. coli cell damage. Moreover, this AMP-induced NO stimulates expression of SOS repair proteins and activation of RecA, a bacterial caspase-like protein. Features of nitrosative damage did not occur especially without dinF gene which is associated with oxidative stress. Therefore, it was indicated that when there is a NO signal, dinF promotes cell death. In conclusion, the combined investigations demonstrated that the antibacterial mechanism(s) of periplanetasin-4 was a NO-induced cell death, and dinF gene is closely related to cell death pathway.

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.

Discovery of γ-Lactam Alkaloid Derivatives as Potential Fungicidal Agents Targeting Steroid Biosynthesis

Biological control of plant pathogens is considered as one of the green and effective technologies using beneficial microorganisms or microbial secondary metabolites against plant diseases, and so microbial natural products have played important roles in the research and development of new and green agrochemicals. To explore the potential applications for natural γ-lactam alkaloids and their derivatives, 26 γ-lactams that have flexible substituent patterns were synthesized and characterized, and their in vitro antifungal activities against eight kinds of plant pathogens belonging to oomycetes, basidiomycetes, and deuteromycetes were fully evaluated. In addition, the high potential compounds were further tested using an in vivo assay against Phytophthora blight of pepper to verify a practical application for controlling oomycete diseases. The potential modes of action for compound D1 against Phytophthora capsici were also investigated using microscopic technology (optical microscopy, scanning electron microscopy, and transmission electron microscopy) and label-free quantitative proteomics analysis. The results demonstrated that compound D1 may be a potential novel fungicidal agent against oomycete diseases (EC₅₀ = 4.9748 μg·mL^-1 for P. capsici and EC₅₀ = 5.1602 μg·mL^-1 for Pythium aphanidermatum) that can act on steroid biosynthesis, which can provide a certain theoretical basis for the development of natural lactam derivatives as potential antifungal agents.

Sulfur-containing natural hinduchelins derivatives as potential antifungal agents against Rhizoctonia solani

Aryl-oxazole alkaloids are an important class of heterocyclic natural products, and which has been demonstrated to exhibit broad biological functions. During the course of our research for highly active compounds from natural products, the natural hinduchelins A-D with typical aryl-oxazole unit have been synthesized and investigated. So, in order to develop highly potential functional molecules, a series of novel sulfur-containing aryl-oxazole compounds derived from natural hinduchelins was designed and synthesized, and their in vitro fungicidal activities against four common plant pathogenic fungi (oomycetes Phytophthora capsici, ascomycetes Sclerotinia sclerotiorum, deuteromycetes Botrytis cinerea and basidiomycetes Rhizoctonia solani) were evaluated, the results demonstrated that compounds 7b and 7c displayed good selectivity and specificity in vitro against basidiomycetes R. solani. In addition, the in vivo antifungal activities also indicated compounds 7b and 7c can protect the horsebean against infection by R. solani, and the possible mechanism of antifungal action for these compounds has also been investigated.

Antifungal Action of Antifungalmycin N2 Against Rhizoctonia solani by Disrupting Cell Membrane and Inhibiting Succinate Dehydrogenase

Antifungalmycin N2 (3-methyl-3,5-amino-4-vinyl-2-pyrone, C₆H₇O₂N) was a novel structural antifungal metabolite produced by Streptomyces sp. strain N2. Our previous study reported that the antagonistic interaction between antifungalmycin N2 and Rhizoctonia solani was accompanied by an oxidative stress in R. solani cell, indicating a probable damage occurred in the cell membranes and mitochondria. To verify this, the present study focused on investigating the effects of antifungalmycin N2 on the structure and function of cell membranes and mitochondria of R. solani. Morphological observations in transmission electron microscopy and fluorescence microscope showed that cell membranes of R. solani were damaged, and its cytoplasmic organelles were disorganized when treated with antifungalmycin N2. Meanwhile, the kinetics of membrane-related physiological and biochemical parameters, such as the increased malondialdehyde level, dropped ergosterol formation, and enhanced electrical conductivity in R. solani mycelia, further confirmed that antifungalmycin N2 would disrupt the cell membrane structure and function. More significantly, antifungalmycin N2 had a significantly inhibitory effect on the succinate dehydrogenase (SDH) activity of R. solani, and indicated that the mode and site of action of antifungalmycin N2 against R. solani might be similar to the existing succinate dehydrogenase inhibitors fungicides by binding in the ubiquinone-binding site. In conclusion, the above results demonstrated that the mode and site of action of antifungalmycin N2 targeted to cell membrane and SDH of R. solani, thus exerting the antifungal activity by damaging cell membrane structure and function, together with inhibiting the SDH activity.

Control of human and plant fungal pathogens using pentaene macrolide 32, 33-didehydroroflamycoin

AIMS

The aim of this study was to address the toxicity of recently described polyene macrolide 32, 33-didehydroroflamycoin (DDHR) on a wide range of fungal pathogens and its potential to control plant fungal diseases.

METHODS AND RESULTS

The antifungal activity of DDHR in vitro was examined against common human and plant pathogenic fungi using a broth microdilution assay and a disk diffusion assay. Minimum inhibitory concentrations ranged from 12·5 to 35 μg ml(-1) . A radial growth inhibition assay showed that DDHR inhibited mycelia growth, inducing mycelial necrosis and affecting sporulation. During the in vivo assay on apple fruits administration of DDHR 1 h before fungal inoculation inhibited spreading of the infection. Importantly, DDHR exhibited no phytotoxic effects on the model plant, Capsicum annum, verified by the plant growth rate and chlorophyll content.

CONCLUSIONS

DDHR inhibits growth of various plant pathogens in vitro with the strongest activity against Alternaria alternata, Colletotrichum acutatum and Penicillium expansum, and protects apple fruits from decay.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of the inhibitory effect of DDHR on important pathogenic fungal isolates. DDHR could be a good scaffold for developing new antifungal agents for fruit and vegetable protection.