Crinipellins A and I, Two Diterpenoids from the Basidiomycete Fungus Crinipellis rhizomaticola, as Potential Natural Fungicides

Concise Total Syntheses of (-)-Crinipellins A and B Enabled by a Controlled Cargill Rearrangement

Herein, we report concise total syntheses of diterpene natural products (-)-crinipellins A and B with a tetraquinane skeleton, three adjacent all-carbon quaternary centers, and multiple oxygenated and labile functional groups. Our synthesis features a convergent Kozikowski β-alkylation to unite two readily available building blocks with all the required carbon atoms, an intramolecular photochemical [2 + 2] cycloaddition to install three challenging and adjacent all-carbon quaternary centers and a 5-6-4-5 tetracyclic skeleton, and a controlled Cargill rearrangement to rearrange the 5-6-4-5 tetracyclic skeleton to the desired tetraquinane skeleton. These strategically enabling transformations allowed us to complete total syntheses of (-)-crinipellins A and B in 12 and 13 steps, respectively. The results of quantum chemical computations revealed that the Bronsted acid-catalyzed Cargill rearrangements likely involve stepwise paths to products and the AlR3-catalyzed Cargill rearrangements likely involve a concerted path with asynchronous alkyl shifting events to form the desired product.

Discovery of Novel Antioomycete Metabolites from the Marine-Derived Fungus Paraconiothyrium sporulosum

In our screening program for natural products that are effective in controlling plant diseases, we found that the culture filtrate of Paraconiothyrium sporulosum SFC20160907-M11 effectively suppressed the development of tomato late blight disease caused by Phytophthora infestans. Using a bioassay-guided fractionation of antioomycete activity, 12 active compounds (1-12) were obtained from an ethyl acetate extract of the culture filtrate. Chemical structures of five new compounds 1-5 were determined by the extensive analyses of nuclear magnetic resonance (NMR), high resolution mass spectrometry (HRMS), and circular dichroism (CD) data. Interestingly, mycosporulonol (1) and botrallin (8) completely inhibited the growth of P. infestans at concentrations of 8 and 16 μg/mL, respectively. Furthermore, the spray treatment of 1 and 8 (500 μg/mL) successfully protected tomato seedlings against P. infestans with disease control values of 92%. Taken together, these results suggest that the culture filtrates of P. sporulosum SFC20160907-M11 and their bioactive metabolites can be used as new antioomycete agents for Phytophthora late blight control.

A review of biocontrol agents in controlling late blight of potatoes and tomatoes caused by Phytophthora infestans and the underlying mechanisms

Phytophthora infestans causes late blight on potatoes and tomatoes, which has a significant economic impact on agriculture. The management of late blight has been largely dependent on the application of synthetic fungicides, which is not an ultimate solution for sustainable agriculture and environmental safety. Biocontrol strategies are expected to be alternative methods to the conventional chemicals in controlling plant diseases in the integrated pest management (IPM) programs. Well-studied biocontrol agents against Phytophthora infestans include fungi, oomycetes, bacteria, and compounds produced by these antagonists, in addition to certain bioactive metabolites produced by plants. Laboratory and glasshouse experiments suggest a potential for using biocontrol in practical late blight disease management. However, the transition of biocontrol to field applications is problematic for the moment, due to low and variable efficacies. In this review, we provide a comprehensive summary on these biocontrol strategies and the underlying corresponding mechanisms. To give a more intuitive understanding of the promising biocontrol agents against Phytophthora infestans in agricultural systems, we discuss the utilizations, modes of action and future potentials of these antagonists based on their taxonomic classifications. To achieve a goal of best possible results produced by biocontrol agents, it is suggested to work on field trials, strain modifications, formulations, regulations, and optimizations of application. Combined biocontrol agents having different modes of action or biological adaptation traits may be used to strengthen the biocontrol efficacy. More importantly, biological control agents should be applied in the coordination of other existing and forthcoming methods in the IPM programs. © 2023 Society of Chemical Industry.

Rational design and identification of novel thiosemicarbazide derivatives as laccase inhibitors

BACKGROUND

Laccase is a key enzyme in the fungal 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis pathway, which is a potential target for the control of pathogenic fungi. In our previous work, compound a2 was found with higher inhibition activity against laccase and antifungal activity than laccase inhibitor PMDD-5Y. The introduction of hydrogen-bonded receptors in the amino part was found to be beneficial in improving laccase inhibitory activity by target-based-biological rational design. In this work, the hydrogen-bonded receptors morpholine and piperazine were introduced for structure optimization to enhancing biological activity.

RESULTS

Enzyme activity tests indicated that all target compounds had inhibitory activity against laccase, and some compounds exhibited better activity against laccase than a2, it was further verified that the introduction of hydrogen-bonded receptors in the amino portion could enhance the laccase inhibitory activity of target compounds. Most compounds showed excellent antifungal activities in vitro. Compound m14 displayed good activity against Magnaporthe oryzae both in vitro and in vivo. The scanning electron microscopy (SEM) analysis showed that the mycelium of M. oryzae treated with m14 were destroyed. Molecular docking revealed the binding mode between laccase and target compounds.

CONCLUSION

Thirty-eight compounds were synthesized and showed good inhibitory activity against laccase, the introduction of morpholine and piperazine in the amino part was beneficial to improve antifungal activity and laccase activity. Further validation of laccase as a potential target for rice blast control, while m14 can be used as a candidate compound for the control of rice blast. © 2023 Society of Chemical Industry.

Design, Synthesis, and Biological Activity of Novel Laccase Inhibitors as Fungicides against Rice Blast

Laccase is a potential target for novel agricultural fungicide discovery. PMDD-5Y was the first agent reported with high activity against laccase to control phytopathogenic fungi. Thirty-two novel agents containing cinnamaldehyde thiosemicarbazide were synthesized with PMDD-5Y as the lead compound, with most of the target compounds exhibiting excellent activity in vitro. Compound a2 (EC₅₀ = 9.71 μg/mL) exhibited greater potency against Magnaporthe oryzae than the commercial fungicide isoprothiolane (EC₅₀ = 18.62 μg/mL). The curative and protective effects of a2 against M. oryzae on rice were more than those of PMDD-5Y. Scanning electron microscopy indicated that a2 could cause mycelial growth atrophy and malformation. Furthermore, a2 (IC₅₀ = 0.18 mmol/L) showed higher activity against laccase than PMDD-5Y (IC₅₀ = 0.33 mmol/L) and cysteine (IC₅₀ = 0.30 mmol/L). Molecular docking analysis revealed the nature of interaction between these compounds and laccase. This research identified a novel laccase inhibitor a2 as a fungicide candidate to control rice blast in agriculture.

Antimicrobial Diterpenes: Recent Development From Natural Sources

Antimicrobial resistance has been posing an alarming threat to the treatment of infectious diseases over the years. Ineffectiveness of the currently available synthetic and semisynthetic antibiotics has led the researchers to discover new molecules with potent antimicrobial activities. To overcome the emerging antimicrobial resistance, new antimicrobial compounds from natural sources might be appropriate. Secondary metabolites from natural sources could be prospective candidates in the development of new antimicrobial agents with high efficacy and less side effects. Among the natural secondary metabolites, diterpenoids are of crucial importance because of their broad spectrum of antimicrobial activity, which has put it in the center of research interest in recent years. The present work is aimed at reviewing recent literature regarding different classes of natural diterpenes and diterpenoids with significant antibacterial, antifungal, antiviral, and antiprotozoal activities along with their reported structure-activity relationships. This review has been carried out with a focus on relevant literature published in the last 5 years following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A total of 229 diterpenoids from various sources like plants, marine species, and fungi are summarized in this systematic review, including their chemical structures, classification, and significant antimicrobial activities together with their reported mechanism of action and structure-activity relationships. The outcomes herein would provide researchers with new insights to find new credible leads and to work on their synthetic and semisynthetic derivatives to develop new antimicrobial agents.

Diterpenes Specially Produced by Fungi: Structures, Biological Activities, and Biosynthesis (2010–2020)

Fungi have traditionally been a very rewarding source of biologically active natural products, while diterpenoids from fungi, such as the cyathane-type diterpenoids from Cyathus and Hericium sp., the fusicoccane-type diterpenoids from Fusicoccum and Alternaria sp., the guanacastane-type diterpenoids from Coprinus and Cercospora sp., and the harziene-type diterpenoids from Trichoderma sp., often represent unique carbon skeletons as well as diverse biological functions. The abundances of novel skeletons, biological activities, and biosynthetic pathways present new opportunities for drug discovery, genome mining, and enzymology. In addition, diterpenoids peculiar to fungi also reveal the possibility of differing biological evolution, although they have similar biosynthetic pathways. In this review, we provide an overview about the structures, biological activities, evolution, organic synthesis, and biosynthesis of diterpenoids that have been specially produced by fungi from 2010 to 2020. We hope this review provides timely illumination and beneficial guidance for future research works of scholars who are interested in this area.

Biocontrol Potential of Aspergillus Species Producing Antimicrobial Metabolites

Microbial metabolites have been recognized as an important source for the discovery of new antifungal agents because of their diverse chemical structures with novel modes of action. In the course of our screening for new antifungal agents from microbes, we found that culture filtrates of two fungal species Aspergillus candidus SFC20200425-M11 and Aspergillus montenegroi SFC20200425-M27 have the potentials to reduce the development of fungal plant diseases such as tomato late blight and wheat leaf rust. From these two Aspergillus spp., we isolated a total of seven active compounds, including two new compounds (4 and 6), and identified their chemical structures based on the NMR spectral analyses: sphaeropsidin A (1), (R)-formosusin A (2), (R)-variotin (3), candidusin (4), asperlin (5), montenegrol (6), and protulactone A (7). Based on the results of the in vitro bioassays of 11 plant pathogenic fungi and bacteria, sphaeropsidin A (1), (R)-formosusin A (2), (R)-variotin (3), and asperlin (5) exhibited a wide range of antimicrobial activity. Furthermore, when plants were treated with sphaeropsidin A (1) and (R)-formosusin A (2) at a concentration of 500 μg/ml, sphaeropsidin A (1) exhibited an efficacy disease control value of 96 and 90% compared to non-treated control against tomato late blight and wheat leaf rust, and (R)-formosusin A (2) strongly reduced the development of tomato gray mold by 82%. Asperlin (5) at a concentration of 500 μg/ml effectively controlled the development of tomato late blight and wheat leaf rust with a disease control value of 95%. Given that culture filtrates and active compounds derived from two Aspergillus spp. exhibited disease control efficacies, our results suggest that the Aspergillus-produced antifungal compounds could be useful for the development of new natural fungicides.

Clerodane Diterpenoids Identified from Polyalthia longifolia Showing Antifungal Activity against Plant Pathogens

In the search for new natural resources showing plant disease control effects, we found that the methanol extract of Polyalthia longifolia suppressed fungal disease development in plants. To identify the bioactive substances, the methanol extract of P. longifolia was extracted by organic solvents, and consequently, four new 2-oxo-clerodane diterpenes (1-4), a new 4(3 → 2)-abeo-clerodane diterpene (5), together with ten known compounds (6-16) were isolated and identified from the extracts. Of the new compounds, compound 2 showed a broad spectrum of antifungal activity with moderated minimum inhibitory concentration (MIC) values in a range of 50-100 μg/mL against tested fungal pathogens. Considering with the known compounds, compound 6 showed the most potent antifungal activity with an MIC value in the range of 6.3-12.5 μg/mL. When compound 6 was evaluated for an in vivo antifungal activity against rice blast, tomato late blight, and pepper anthracnose, compound 6 reduced the plant disease by at least 60% compared to the untreated control at concentrations of 250 and 500 μg/mL. Together, our results suggested that the methanol extract of twigs and leaves of P. longifolia and its major compound 6 could be used as a source for the development of eco-friendly plant protection agents.

In Vitro and In Vivo Antifungal Activity of Sorbicillinoids Produced by Trichoderma longibrachiatum

In the search for antifungal agents from marine resources, we recently found that the culture filtrate of Trichoderma longibrachiatum SFC100166 effectively suppressed the development of tomato gray mold, rice blast, and tomato late blight. The culture filtrate was then successively extracted with ethyl acetate and n-butanol to identify the fungicidal metabolites. Consequently, a new compound, spirosorbicillinol D (1), and a new natural compound, 2',3'-dihydro-epoxysorbicillinol (2), together with 11 known compounds (3-13), were obtained from the solvent extracts. The chemical structures were determined by spectroscopic analyses and comparison with literature values. The results of the in vitro antifungal assay showed that of the tested fungal pathogens, Phytophthora infestans was the fungus most sensitive to the isolated compounds, with MIC values ranging from 6.3 to 400 µg/mL, except for trichotetronine (9) and trichodimerol (10). When tomato plants were treated with the representative compounds (4, 6, 7, and 11), bisvertinolone (6) strongly reduced the development of tomato late blight disease compared to the untreated control. Taken together, our results revealed that the culture filtrate of T. longibrachiatum SFC100166 and its metabolites could be useful sources for the development of new natural agents to control late blight caused by P. infestans.

A critical review on submerged production of mushroom and their bioactive metabolites

Mushrooms are ubiquitous in nature. Even though humankind has been consuming mushrooms for ages, their medicinal and nutraceutical properties are not used to its fullest potential in the present market. Edible mushrooms are not only a cheap and nutritious option to mitigate malnutrition, but they also produce effective biomass. Submerged fermentation (SmF) is not only a cost-effective method to produce biomass along with exquisite bioactive metabolites but it also reduces the chances of contamination and the time of production. Therefore, this study unveils the bioactive metabolites being produced by mushrooms. Moreover, it also showcases the recent advances in the areas of bio-active compounds and their judicious implementations in daily life and pharmaceutical industries. Moreover, there is a distinct lack in utilizing the potential benefits of bioactive compounds from mushroom unless in vivo and in vitro studies are demonstrated.

Plant Phytochemicals in Food Preservation: Antifungal Bioactivity: A Review

Synthetic food additives generate a negative perception in consumers. This fact generates an important pressure on food manufacturers, searching for safer natural alternatives. Phytochemicals (such as polyphenols and thiols) and plant essential oils (terpenoids) possess antimicrobial activities that are able to prevent food spoilage due to fungi (e.g., Aspergillus, Penicillium) and intoxications (due to mycotoxins), both of which are important economic and health problems worldwide. This review summarizes industrially interesting antifungal bioactivities from the three main types of plant nutraceuticals: terpenoids (as thymol), polyphenols (as resveratrol) and thiols (as allicin) as well as some of the mechanisms of action. These phytochemicals are widely distributed in fruits and vegetables and are very useful in food preservation as they inhibit growth of important spoilage and pathogenic fungi, affecting especially mycelial growth and germination. Terpenoids and essential oils are the most abundant group of secondary metabolites found in plant extracts, especially in common aromatic plants, but polyphenols are a more remarkable group of bioactive compounds as they show a broad array of bioactivities.

Discovery of New Triterpenoid Saponins Isolated from Maesa japonica with Antifungal Activity against Rice Blast Fungus Magnaporthe oryzae

While searching for new antifungal compounds, we revealed that a methanol extract of plant species Maesa japonica has a potent antifungal activity in vivo against rice blast fungus Magnaporthe oryzae. To identify the antifungal substances, the methanol extract of M. japonica was extracted by organic solvents, and consequently, six active compounds were isolated from the n-butanol layer. The isolated compounds were five new acylated triterpenoid saponins including maejaposide I (1), maejaposides C-1, C-2, and C-3 (2-4), and maejaposide A-1 (5), along with a known one, maejaposide A (6). These chemical structures were determined by NMR and a comparison of their NMR and MS data with those reported in the literature. Based on the in vitro antifungal bioassay, the five compounds (2-6) exhibited strong antifungal activity against M. oryzae with MIC values ranging from 4 to 32 μg/mL, except for maejaposide I (1) (MIC > 250 μg/mL). When the compounds were evaluated at concentrations of 125, 250, and 500 μg/mL for an in vivo antifungal activity against rice blast, compounds 2-6 strongly reduced the development of blast by at least 85% to 98% compared to the untreated control. However, compound 1 did not show any in vivo antifungal activity up to a concentration of 500 μg/mL. Taken together, our results suggest that the methanol extract of M. japonica and the new acylated triterpenoid saponins can be used as a source for the development of natural fungicides.