The Molecular Mechanism of Perillaldehyde Inducing Cell Death in Aspergillus flavus by Inhibiting Energy Metabolism Revealed by Transcriptome Sequencing

Unraveling the antifungal and anti-aflatoxin B1 mechanisms of piperitone on Aspergillus flavus

Aspergillus flavus infects important crops and produces carcinogenic aflatoxins, posing a serious threat to food safety and human health. Biochemical analysis and RNA-seq were performed to investigate the effects and mechanisms of piperitone on A. flavus growth and aflatoxin B1 biosynthesis. Piperitone significantly inhibited the growth of A. flavus, AFB1 production, and its pathogenicity on peanuts and corn flour. Differentially expressed genes (DEGs) associated with the synthesis of chitin, glucan, and ergosterol were markedly down-regulated, and the ergosterol content was reduced, resulting in a disruption in the integrity of the cell wall and cell membrane. Moreover, antioxidant genes were down-regulated, the correspondingly activities of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase were reduced, and levels of superoxide anion and hydrogen peroxide were increased, leading to a burst of reactive oxygen species (ROS). Accompanied by ROS accumulation, DNA fragmentation and cell autophagy were observed, and 16 aflatoxin cluster genes were down-regulated. Overall, piperitone disrupts the integrity of the cell wall and cell membrane, triggers the accumulation of ROS, causes DNA fragmentation and cell autophagy, ultimately leading to defective growth and impaired AFB1 biosynthesis.

The photoactivated antifungal activity and possible mode of action of sodium pheophorbide a on Diaporthe mahothocarpus causing leaf spot blight in Camellia oleifera

Introduction

Sodium pheophorbide a (SPA) is a natural plant-derived photosensitizer, with high photoactivated antifungal activity against some phytopathogenic fungi. However, its fungicidal effect on Diaporthe mahothocarpus, a novel pathogen that causes Camellia oleifera leaf spot blight, is unclear.

Methods

In the present study, we explored its inhibitory effects on spore germination and mycelial growth of D. mahothocarpus. Then we determined its effects on the cell membrane, mycelial morphology, redox homeostasis, and cell death through bioassay. Finally, RNA-seq was used further to elucidate its mode of action at the transcriptional level.

Results

We found that SPA effectively inhibited the growth of D. mahothocarpus, with half-maximal effective concentrations to inhibit mycelial growth and spore germination of 1.059 and 2.287 mg/mL, respectively. After 1.0 mg/mL SPA treatment, the conductivity and malondialdehyde content of D. mahothocarpus were significantly increased. Scanning electron microscopy and transmission electron microscopy indicated that SPA significantly affected the morphology and ultrastructure of D. mahothocarpus hyphae, revealing that SPA can destroy the mycelial morphology and cell structure, especially the cell membrane of D. mahothocarpus. Furthermore, transcriptome analysis revealed that SPA significantly suppressed the expression of genes involved in morphology, cell membrane permeability, and oxidative stress. Then, we also found that SPA significantly promoted the accumulation of reactive oxygen species (ROS) in of D. mahothocarpus, while it decreased the content of reduced glutathione, inhibited the enzyme activities of superoxide dismutase and catalase, and exacerbated DNA damage. Annexin V-FITC/PI staining also confirmed that 1.0 mg/mL SPA could significantly induce apoptosis and necrosis.

Discussion

Generally, SPA can induce ROS-mediated oxidative stress and cell death, thus destroying the cell membrane and hyphal morphology, and ultimately inhibiting mycelial growth, which indicates that SPA has multiple modes of action, providing a scientific basis for the use of SPA as an alternative plant-derived photoactivated fungicide against C. oleifera leaf spot blight.

Application of essential oils as slow-release antimicrobial agents in food preservation: Preparation strategies, release mechanisms and application cases

Food spoilage caused by foodborne microorganisms will not only cause significant economic losses, but also the toxins produced by some microorganisms will also pose a serious threat to human health. Essential oil (EOs) has significant antimicrobial activity, but its application in the field of food preservation is limited because of its volatile, insoluble in water and sensitive to light and heat. Therefore, in order to solve these problems effectively, this paper first analyzed the antibacterial effect of EOs as an antimicrobial agent on foodborne bacteria and its mechanism. Then, the application strategies of EOs as a sustained-release antimicrobial agent in food preservation were reviewed. On this basis, the release mechanism and application cases of EOs in different antibacterial composites were analyzed. The purpose of this paper is to provide technical support and solutions for the preparation of new antibacterial packaging materials based on plant active components to ensure food safety and reduce food waste.

Mechanisms of methyl 2-methylbutyrate suppression on Aspergillus flavus growth and aflatoxin B1 biosynthesis

Aspergillus flavus and subsequently produced carcinogenic aflatoxins frequently contaminate postharvest food crops, resulting in a threat to global food safety. Chemical preservatives are currently the main antifungal agents. However, fungal resistance effect, biological toxicity, and environmental contamination limit their practical applications. The application of natural volatile organic compounds has great potential for controlling fungal and mycotoxin contamination of postharvest food crops. This study therefore investigated the antifungal and anti-aflatoxigenic activities of the volatile compound, methyl 2-methylbutyrate (M2M), against Aspergillus flavus and its potential mechanisms. M2M effectively inhibited A. flavus mycelia growth, with a minimum inhibitory concentration of 2.0 μL/mL. Moreover, M2M also suppressed aflatoxin production, sclerotia production, and the pathogenicity on peanut and corn flour. RNA-Seq results showed that 2899 differentially expressed genes (DEGs), and DEGs involved in ergosterol synthesis, cell wall structure, glycolysis, citric acid cycle, mitogen activated protein kinase signaling pathway, DNA replication, and aflatoxin biosynthesis, were down-regulated in A. flavus. Further studies showed that M2M strongly damaged the cell membrane and cell wall integrity, reduced ATP levels, and induced reactive oxygen species (ROS) accumulation and DNA damage. Notably, a GATA type zinc finger transcription factor, AfSreA (AFLA_132440), which is essential for A. flavus growth and aflatoxin production, was identified. The growth and aflatoxin yield in the ΔAfSreA strain decreased by 94.94 % and 71.82 %, respectively. Additionally, deletion of AfSreA destroyed cell wall integrity and decreased expressions of genes involved in aflatoxin biosynthesis. Taken together, our results identified the antifungal and anti-aflatoxigenic mechanisms of M2M against A. flavus, and confirmed the potential of M2M in protecting peanut and corn from fungal contamination.

Recent advancements on use of essential oils as preservatives against fungi and mycotoxins spoiling food grains

Spoilage of grains by mycotoxigenic fungi poses a great threat to food security and human health. Conventionally used chemical agents to prevent grain fungi contamination cause increasingly significant problems such as microbial resistance, residual toxicity and environmental unfriendliness. In recent years, plant essential oils (EOs) have become a hot spot in the research of control of grain fungi and mycotoxins, due to their extensive sources, non-toxicity, environmental friendliness and good antifungal efficiency. The current review aims to provide an overview of the prevention of fungi and mycotoxins in grain through EOs. The antifungal and toxin inhibition efficiency of different EOs and their effective components are investigated. The inhibition mechanism of EOs on fungi and mycotoxins in grains is introduced. The influence of EOs treatment on the change of grain quality is also discussed. In addition, the formulations and techniques used to overcome the disadvantages of EOs application are introduced. The results of recent studies have confirmed that EOs provide great potential for controlling common fungi and mycotoxins in grains, and enhancing quantity and quality safety of grains.

Inhibitory effect of cinnamaldehyde on Fusarium solani and its application in postharvest preservation of sweet potato

Root rot caused by Fusarium solani is one of major postharvest diseases limiting sweet potato production. Antifungal effect and possible mode of action of cinnamaldehyde (CA) against F. solani were investigated. CA concentration of 0.075 g/L inhibited conidial viability of F. solani. CA vapor of 0.3 g/L in air completely controlled the F. solani development in sweet potatoes during storage for 10 days at 28 °C, and protected soluble sugar and starch in the flesh from depletion by the fungus. Further results demonstrated that CA induced reduction in mitochondrial membrane potential (Δψ_m), ROS accumulation, and cell apoptosis characterized by DNA fragmentation in F. solani. Moreover, CA facilitated decomposition of mitochondria-specific cardiolipin (CL) into its catabolites by the catalytic action of phospholipases. Altogether, the results revealed a specific antifungal mechanism of CA against F. solani, and suggest that CA holds promise as a preservative for postharvest preservation of sweet potato.

Antifungal Activity of Perillaldehyde on Fusarium solani and Its Control Effect on Postharvest Decay of Sweet Potatoes

Root rot caused by Fusarium solani is one of the major postharvest diseases limiting sweet potato production. Here, antifungal activity and the action mode of perillaldehyde (PAE) against F. solani were investigated. A PAE concentration of 0.15 mL/L in air (mL/L air) markedly inhibited the mycelial growth, spore reproduction and spore viability of F. solani. A PAE vapor of 0.25 mL/L in air could control the F. solani development in sweet potatoes during storage for 9 days at 28 °C. Moreover, the results of a flow cytometer demonstrated that PAE drove an increase in cell membrane permeability, reduction of mitochondrial membrane potential (MMP) and accumulation of reactive oxygen species (ROS) in F. solani spores. Subsequently, a fluorescence microscopy assay demonstrated that PAE caused serious damage to the cell nuclei in F. solani by inducing chromatin condensation. Further, the spread plate method showed that the spore survival rate was negatively correlated with the level of ROS and nuclear damage, of which the results indicated that PAE-driven ROS accumulation plays a critical role in contributing to cell death in F. solani. In all, the results revealed a specific antifungal mechanism of PAE against F. solani, and suggest that PAE could be a useful fumigant for controlling the postharvest diseases of sweet potatoes.

ROS Stress and Cell Membrane Disruption are the Main Antifungal Mechanisms of 2-Phenylethanol against Botrytis cinerea

2-Phenylethanol (2-PE), a common compound found in plants and microorganisms, exhibits broad-spectrum antifungal activity. Using Botrytis cinerea, we demonstrated that 2-PE suppressed mycelium growth in vitro and in strawberry fruit and reduced natural disease without adverse effects to fruit quality. 2-PE caused structural damage to mycelia, as shown by scanning and transmission electron microscopy. From RNA sequencing analysis we found significantly upregulated genes for enzymatic and nonenzymatic reactive oxygen species (ROS) scavenging systems including sulfur metabolism and glutathione metabolism, indicating that ROS stress was induced by 2-PE. This was consistent with results from assays demonstrating an increase ROS and hydrogen peroxide levels, antioxidant enzyme activities, and malondialdehyde content in treated cells. The upregulation of ATP-binding cassette transporter genes, the downregulation of major facilitator superfamily transporters genes, and the downregulation of ergosterol biosynthesis genes indicated a severe disruption of cell membrane structure and function. This was consistent with results from assays demonstrating compromised membrane integrity and lipid peroxidation. To summarize, 2-PE exposure suppressed B. cinerea growth through ROS stress and cell membrane disruption.

Antimicrobial mechanisms of spice essential oils and application in food industry

Spice essential oils (SEOs) are commonly used in food flavoring and are considered an effective food preservative. It has a broad range of applications and promising development prospects. As a natural food additive, SEOs' antimicrobial effects have been widely studied and utilized towards food preservation. Many SEOs have exhibited significant antimicrobial activities against food-borne pathogenic and food spoilage microorganisms. We reviewed the antibacterial and antifungal properties of SEOs, the active components, their corresponding mechanisms of actions, as well as their application in the food industry, providing a theoretical basis for SEOs' further development and application as natural preservatives.

Biocontrol and Action Mechanism of Bacillus subtilis Lipopeptides' Fengycins Against Alternaria solani in Potato as Assessed by a Transcriptome Analysis

Alternaria solani is an airborne fungus and the primary causal agent of potato early blight worldwide. No available fungicides that are both effective and environmentally friendly are usable to control this fungus. Therefore, biological control is a potential approach for its suppression. In this study, Bacillus subtilis strain ZD01's fermentation broth strongly reduced A. solani pathogenicity under greenhouse conditions. The effects of strain ZD01's secondary metabolites on A. solani were investigated. The exposure of A. solani hyphae to the supernatant resulted in swelling and swollen sacs, and the ZD01 supernatant reduced A. solani conidial germination significantly. Matrix-assisted laser desorption/ionization time of flight mass spectrometry and pure product tests revealed that fengycins were the main antifungal lipopeptide substances. To elucidate the molecular mechanism of the fengycins' biological control, RNA sequencing analyses were performed. A transcriptome analysis revealed that 304 and 522 genes in A. solani were differentially expressed after 2-h and 6-h fengycin treatments, respectively. These genes were respectively mapped to 53 and 57 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. In addition, the most enriched KEGG pathway analysis indicated that the inhibitory mechanisms of fengycins against A. solani regulated the expression of genes related to cell wall, cell membrane, transport, energy process, protein synthesis and genetic information. In particular, cell wall and cell membrane metabolism were the main processes affected by fengycin stress. Scanning and transmission electron microscope results revealed hyphal enlargement and a wide range of abnormalities in A. solani cells after exposure to fengycins. Furthermore, fengycins induced chitin synthesis in treated cells, and also caused the capture of cellular fluorescent green labeling and the release of adenosine triphosphate (ATP) from outer membranes of A. solani cells, which may enhance the fengycins ability to alter cell membrane permeability. Thus, this study increases the transcriptome data resources available and supplies a molecular framework for B. subtilis ZD01 inhibition of A. solani HWC-168 through various mechanisms, especially damaging A. solani cell walls and membranes. The transcriptomic insights may lead to an effective control strategy for potato early blight.

Oxidative stress response pathways in fungi

Fungal response to any stress is intricate, specific, and multilayered, though it employs only a few evolutionarily conserved regulators. This comes with the assumption that one regulator operates more than one stress-specific response. Although the assumption holds true, the current understanding of molecular mechanisms that drive response specificity and adequacy remains rudimentary. Deciphering the response of fungi to oxidative stress may help fill those knowledge gaps since it is one of the most encountered stress types in any kind of fungal niche. Data have been accumulating on the roles of the HOG pathway and Yap1- and Skn7-related pathways in mounting distinct and robust responses in fungi upon exposure to oxidative stress. Herein, we review recent and most relevant studies reporting the contribution of each of these pathways in response to oxidative stress in pathogenic and opportunistic fungi after giving a paralleled overview in two divergent models, the budding and fission yeasts. With the concept of stress-specific response and the importance of reactive oxygen species in fungal development, we first present a preface on the expanding domain of redox biology and oxidative stress.

Dysfunction of FadA-cAMP signalling decreases Aspergillus flavus resistance to antimicrobial natural preservative Perillaldehyde and AFB1 biosynthesis

Aspergillus flavus is an opportunistic fungal pathogen that colonizes agriculture crops with aflatoxin contamination. We found that Perillaldehyde (PAE) effectively inhibited A. flavus viability and aflatoxin production by inducing excess reactive oxygen species (ROS). Transcriptome analysis indicated that the Gα protein FadA was significantly induced by PAE. Functional characterization of FadA showed it is important for asexual development and aflatoxin biosynthesis by regulation of cAMP-PKA signalling. The ΔfadA mutant was more sensitive to PAE, while ΔpdeL and ΔpdeH mutants can tolerate excess PAE compared to wild-type A. flavus. Further RNA-sequence analysis showed that fadA was important for expression of genes involved in oxidation-reduction and cellular metabolism. The flow cytometry and fluorescence microscopy demonstrated that ΔfadA accumulated more concentration of ROS in cells, and the transcriptome data indicated that genes involved in ROS scavenging were downregulated in ΔfadA mutant. We further found that FadA participated in regulating response to extracellular environmental stresses by increasing phosphorylation levels of MAPK Kinase Slt2 and Hog1. Overall, our results indicated that FadA signalling engages in mycotoxin production and A. flavus resistance to antimicrobial PAE, which provide valuable information for controlling this fungus and AF biosynthesis in pre- and postharvest of agricultural crops.

Recent development in biological activities and safety concerns of perillaldehyde from perilla plants: A review

Monoterpene Perillaldehyde (PAE) is a major component of the essential oil extracted from perilla plants (Perilla frutescens), which has been used as a leafy vegetable and a medicinal agent. PAE has gained a lot of attention in recent years because of its antifungal and other microbial activities and, human health benefits. PAE has also been used as food additives, perfume ingredients, and traditional medicine concoctions. Biological analyses of PAE have revealed that it has good antioxidant activities and can serve as organic fruit and food preservative. Animal studies indicated potent anticancer, anti-depressant, and anti-inflammatory effects of PAE. Also, PAE is certified "generally recognized as safe" (GRAS) and not mutagenic. However, moderation during usage is advisable, as minor adverse effects are associated with a very high dosage. Despite the newly reported findings, its properties have not been thoroughly summarized and reviewed. Also, clinical trials and official large-scale field applications of PAE in the agricultural sectors are yet to be reported. In this review, updated PAE research progress was provided, focusing on its antifungal and other antimicrobial properties and the mechanisms behind it, phytochemical profile, pharmacological effects, and safety concerns.HighlightsIsolation and recovery techniques of PAE from perilla plants have been developed and improved in recent years.PAE is a potential anti-oxidant and antifungal agent that can be widely used in the food industry.PAE can be developed into drug ingredients for pharmaceutical industries due to its anti-inflammatory, anti-cancer and anti-depressant activities.PAE can be safely used in human when low and moderate dosage is used.

Transcriptome Sequencing Revealed an Inhibitory Mechanism of Aspergillus flavus Asexual Development and Aflatoxin Metabolism by Soy-Fermenting Non-Aflatoxigenic Aspergillus

Aflatoxins (AFs) have always been regarded as the most effective carcinogens, posing a great threat to agriculture, food safety, and human health. Aspergillus flavus is the major producer of aflatoxin contamination in crops. The prevention and control of A. flavus and aflatoxin continues to be a global problem. In this study, we demonstrated that the cell-free culture filtrate of Aspergillus oryzae and a non-aflatoxigenic A. flavus can effectively inhibit the production of AFB1 and the growth and reproduction of A. flavus, indicating that both of the non-aflatoxigenic Aspergillus strains secrete inhibitory compounds. Further transcriptome sequencing was performed to analyze the inhibitory mechanism of A. flavus treated with fermenting cultures, and the results revealed that genes involved in the AF biosynthesis pathway and other biosynthetic gene clusters were significantly downregulated, which might be caused by the reduced expression of specific regulators, such as AflS, FarB, and MtfA. The WGCNA results further revealed that genes involved in the TCA cycle and glycolysis were potentially involved in aflatoxin biosynthesis. Our comparative transcriptomics also revealed that two conidia transcriptional factors, brlA and abaA, were found to be significantly downregulated, which might lead to the downregulation of conidiation-specific genes, such as the conidial hydrophobins genes rodA and rodB. In summary, our research provides new insights for the molecular mechanism of controlling AF synthesis to control the proliferation of A. flavus and AF pollution.

Perillaldehyde: A promising antifungal agent to treat oropharyngeal candidiasis

Perillaldehyde (PAE), a natural monoterpenoid agent extracted from Perilla frutescence, PAE has been reported to present various physiological capabilities, such as anti-inflammation, anti-oxidative and anti-fungal. In this study, we show that PAE exhibits strong antifungal activity against Candida albicans (C. albicans). C. albicans, a fungal pathogen with high incidence of antifungal resistance in clinical settings, is the major cause of oropharyngeal candidiasis (OPC). OPC is characterized by inflammatory immunological responses to fungal infections. Our in vitro results show PAE inhibited several virulence attributes of C. albicans including biofilm formation, yeast-to-hyphal transition and secreted aspartic proteinases (SAPs) gene expression. Using an experimental murine model of OPC, we found that PAE inhibited NLRP3 inflammasome assembly, reduced the excessive accumulation of ROS and prevented the p65 transfer in nuclear; processes all leading to reduced inflammation burden in the host. Together, this supports use PAE as a promising new agent to improve OPC.