Antifungal Activity of Sesamol on Pestalotiopsis neglecta: Interfering with Cell Membrane and Energy Metabolism

This paper investigated the inhibitory effect of Sesamol (Ses) on Pestalotiopsis neglecta. The potential inhibitory mechanisms were explored by observing changes in cell morphology, measuring alterations in cell membrane-related indices, as well as energy metabolism-related indices and changes in enzyme activities related to virulence. The results show that Ses completely inhibited the growth of P. neglecta at 600 μg/mL (minimum inhibitory concentration and minimum fungicidal concentration), with an EC50 of 142 ± 13.22 μg/mL. As observed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), Ses treatment resulted in the breakage and crumpling of P. neglecta cell membrane and organelle lysis. Ergosterol content and the total lipid in P. neglecta treated with 300 μg/mL Ses was 91.52% and 54% of that in the control groups, respectively. In addition, spores were stained, increased leakage of intracellular constituents at 260 nm, and decreased extracellular pH. This suggests damage to the cell membrane integrity and permeability. Furthermore, Ses decreased the ATP levels and key enzymes in the tricarboxylic acid (TCA) cycle, indicating interference with the fungal energy metabolism. Moreover, the activities of polygalacturonase (PG) and endoglucanase (EG) of P. neglecta treated with 300 μg/mL of Ses were only 28.20% and 29.13% of that in the control groups, respectively, indicating that Ses can reduce the virulence of P. neglecta. In conclusion, our results show that Ses should be considered as a potential plant-derived fungicide due to its ability to disrupt the morphology of P. neglecta, damage cell membrane integrity and permeability in P. neglecta, interfere with energy metabolism, and reduce its virulence, ultimately affecting the fungal growth.

Suppression of anthracnose disease by orsellinaldehyde isolated from the mushroom Coprinus comatus

AIMS

Anthracnose caused by Colletotrichum species is one of the most devastating diseases of fruits and crops. We isolated and identified an antifungal compound from the mushroom Coprinus comatus and investigated its inhibitory potential against anthracnose disease-causing fungi with the goal of discovering natural products that can suppress anthracnose-caused plant disease.

METHODS AND RESULTS

The culture filtrate of C. comatus was subjected to a bioassay-guided isolation of antifungal compounds. The active compound was identified as orsellinaldehyde (2,4-dihydroxy-6-methylbenzaldehyde) based on mass spectroscopy and nuclear magnetic resonance analyses. Orsellinaldehyde displayed broad-spectrum inhibitory activity against different plant pathogenic fungi. Among the tested Colletotrichum species, it exhibited the lowest IC50 values on conidial germination and germ tube elongation of Colletotrichum orbiculare. The compound also showed remarkable inhibitory activity against Colletotrichum gloeosporiodes. The staining of Colletotrichum conidia with fluorescein diacetate and propidium iodide demonstrated that the compound is fungicidal. The postharvest in-vivo detached fruit assay indicated that orsellinaldehyde suppressed anthracnose lesion symptoms on mango and cucumber fruits caused by C. gloeosporioides and C. orbiculare, respectively.

CONCLUSIONS

Orsellinaldehyde was identified as a potent antifungal compound from the culture filtrate of C. comatus. The inhibitory and fungicidal activities of orsellinaldehyde against different Colletotrichum species indicate its potential as a fungicide for protecting various fruits against anthracnose disease-causing fungi.

2-Hydroxy-4-methoxybenzaldehyde, a more effective antifungal aroma than vanillin and its derivatives against Fusarium graminearum, destroys cell membranes, inhibits DON biosynthesis, and performs a promising antifungal effect on wheat grains

Fusarium graminearum (F. graminearum) is a severe pathogen threatening the safety of agriculture and food. This study aimed to explore the antifungal efficacies of several plant-derived natural compounds (vanillin and its derivatives) against the growth of F. graminearum and investigate the antifungal mechanism of 2-hydroxy-4-methoxybenzaldehyde (HMB), the strongest one. The minimum inhibitory concentration (MIC) of HMB in inhibiting mycelial growth was 200 μg/mL. HMB at MIC damaged cell membranes by increasing the permeability by about 6-fold (p < 0.05) as evidenced by propidium iodide (PI) staining. Meanwhile, the content of malondialdehyde (MDA) and glycerol was increased by 45.91 and 576.19% by HMB treatment at MIC, respectively, indicating that lipid oxidation and osmotic stress occurred in the cell membrane. Furthermore, HMB exerted a strong antitoxigenic role as the content of deoxynivalenol (DON) was remarkably reduced by 93.59% at MIC on 7th day. At last, the antifungal effect of HMB against F. graminearum was also confirmed on wheat grains. These results not only revealed the antifungal mechanism of HMB but also suggested that HMB could be applied as a promising antifungal agent in the preservation of agricultural products.

Mechanism of inactivation of Aspergillus flavus spores by dielectric barrier discharge plasma

Dielectric barrier discharge plasma (DBDP) displays strong against fungal spores, while its precise mechanism of spore inactivation remains inadequately understood. In this study, we applied morphological, in vivo and in vitro experiments, transcriptomics, and physicochemical detection to unveil the potential molecular pathways underlying the inactivation of Aspergillus flavus spores by DBDP. Our findings suggested that mycelium growth was inhibited as observed by SEM after 30 s treatment at 70 kV, meanwhile spore germination ceased and clustering occurred. It led to the release of cellular contents and subsequent spore demise by disrupting the integrity of spore membrane. Additionally, based on the transcriptomic data, we hypothesized that the induction of spore inactivation by DBDP might be associated with downregulation of genes related to cell membranes, organelles (mitochondria), oxidative phosphorylation, and the tricarboxylic acid cycle. Subsequently, we validated our transcriptomic findings by measuring the levels of relevant enzymes in metabolic pathways, such as superoxide dismutase, acetyl-CoA, total dehydrogenase, and ATP. These physicochemical indicators revealed that DBDP treatment resulted in mitochondrial dysfunction, redox imbalance, and inhibited energy metabolism pathways. These findings were consistent with the transcriptomic results. Hence, we concluded that DBDP accelerated spore rupture and death via ROS-mediated mitochondrial dysfunction, which does not depend on cell membranes.

Antifungal activity and mechanism of tetramycin against Alternaria alternata, the soft rot causing fungi in kiwifruit

Kiwifruit rot caused by the fungus Alternaria alternata occurs in many countries, leading to considerable losses during kiwifruit production. In this study, we evaluated the antifungal activity and mechanism of tetramycin against kiwifruit soft rot caused by Alternaria alternata. Tetramycin exerted antifungal effects through the suppression of mycelial growth, conidial germination, and the pathogenicity of A. alternata. Scanning electron microscopic observations revealed that tetramycin destroyed the mycelial structure, causing the mycelia to twist, shrink, and even break. Furthermore, transmission electron microscopy revealed that tetramycin caused severe plasmolysis and a decrease in cell inclusions, and the cell wall appeared thinner with blurred boundaries. In addition, tetramycin destroyed cell membrane integrity, resulting in the leakage of cellular components such as nucleic acids and proteins in mycelial suspensions. Moreover, tetramycin also caused cell wall lysis by enhancing the activities of chitinase and β-1,3-glucanase and inducing the overexpression of related chitinase gene (Chit) and β-1,3-glucanase gene (β-1,3-glu) in A. alternata. In field trials, tetramycin not only decreased the incidence of kiwifruit rot but also create a beneficial living space for kiwifruit growth. Overall, this study indicated that the application of tetramycin could serve as an alternative measure for the management of kiwifruit rot.

Paeonol Disrupts the Integrity of Aspergillus flavus Cell Walls via Releasing Surface Proteins, Inhibiting the Biosynthesis of β-1,3-Glucan and Promoting the Degradation of Chitin, and an Identification of Cell Surface Proteins

Paeonol can effectively inhibit Aspergillus flavus (A. flavus) via damaging cell walls. In this work, paeonol treatment remarkably destroyed both the outer amorphous layer and the inner fibrous layer of cell walls. Furthermore, FT-IR and XPS characterization showed that OH functional groups were altered and proteins in the outer layer were released. According to proteomic analysis, 605 proteins have been identified and annotated. The activities of β-1,3-glucan synthase and chitinase were prohibited and promoted, respectively, by paeonol treatment, however, the activities of β-1,3-glucanase and chitin synthase were not influenced. QRT-PCR results suggested that FKSP, CHIIII, and CHIV genes might be the antifungal targets of paeonol. In addition, paeonol can effectively restrain the pathogenicity of A. flavus on peanut butter. This study provided a new elucidation on the mode of action of paeonol against cell walls of A. flavus, facilitating the application of paeonol in the preservation of agricultural products.

Hexanal induces early apoptosis of Aspergillus flavus conidia by disrupting mitochondrial function and expression of key genes

Aspergillus flavus is a notorious saprophytic fungus that compromises the quantity and quality of postharvest grains and produces carcinogenic aflatoxins. The natural compound hexanal disrupts cell membrane synthesis and mitochondrial function and induces apoptosis in A. flavus; here, we investigated the molecular mechanisms underlying these effects. The minimum inhibition and fungicidal concentration (MIC and MFC) of hexanal against A. flavus spores were 3.2 and 9.6 μL/mL, respectively. Hexanal exposure resulted in abnormal spore morphology and early spore apoptosis. These changes were accompanied by increased reactive oxygen species production, reduced mitochondrial membrane potential, and DNA fragmentation. Transcriptomic analysis revealed that hexanal treatment greatly altered the metabolism of A. flavus spores, including membrane permeability, mitochondrial function, energy metabolism, DNA replication, oxidative stress, and autophagy. This study provides novel insights into the mechanism underlying the antifungal activity of hexanal, suggesting that hexanal can be used an anti-A. flavus agent for agricultural applications. KEY POINTS: • Hexanal exposure resulted in abnormal spore morphology. • The apoptotic characteristics of A. flavus were induced after hexanal treatment. • Hexanal could change the expression of key A. flavus growth-related genes.

o-Vanillin, a promising antifungal agent, inhibits Aspergillus flavus by disrupting the integrity of cell walls and cell membranes

o-Vanillin is a natural product that has been widely applied in the food and pharmaceutical industries. In this study, we determined that o-vanillin can strongly inhibit the growth of Aspergillus flavus mycelia. However, the inhibition mechanism of o-vanillin is still elusive. The ultrastructural morphology of mycelia was injured, and the cell walls were destroyed. The OH functional groups on cell walls were altered, and the content of protein in mycelial cell walls was reduced by o-vanillin. The content of β-1,3-glucan in cell walls was significantly (P < 0.05) reduced by o-vanillin in a dose-dependent manner, while chitin was not markedly affected. Moreover, o-vanillin led to an increase in the permeability of cell membranes. o-Vanillin also exhibited a promising antifungal effect on contaminated corn kernels. Therefore, o-vanillin inhibited the growth of mycelia by disrupting the integrity of cell walls and cell membranes. This study not only sheds light on the antifungal mechanism of o-vanillin but also indicates that it is a promising agent for the control of A. flavus infection. KEY POINTS: • o-Vanillin has strong inhibitory effects on A. flavus. • o-Vanillin destroyed the integrity of cell walls and cell membranes. • o-Vanillin could effectively inhibit the growth of A. flavus on corn kernels.