Ambient pH stress inhibits spore germination of Penicillium expansum by impairing protein synthesis and folding: a proteomic-based study

Inhibitory effects of phytic acid on the in vitro and in vivo growth of Trichothecium roseum in apple fruit and the underlying mechanisms involved in its action

This study evaluated the inhibitory impacts of phytic acid on the growth of T. roseum both in vitro and in apple fruit, as well as elucidated the potential mechanisms underlying its action. Results showed that phytic acid suppressed the lesion diameter caused by T. roseum inoculation in apples, as well as spore germination and mycelial growth of T. roseum in vitro. Phytic acid reduced intracellular conductivity and soluble sugar content, while increasing malondialdehyde and soluble protein contents. Phytic acid treatment inhibited the activities of pectin lyase, pectin methyl polygalacturonase, β-glucosidase, cellulase, xylanase, pectin methyl trans-eliminase, polygalacturonase, and polygalacturonase both in vitro and in apples. In contrast, inoculation of control and phytic acid-treated fruit with T. roseum resulted in increased enzyme activity. These findings suggest that phytic acid decrease the occurrence of heart rot in apples through inducing disruption of the cell membrane of T. roseum and mediating cell wall metabolism.

Effects of Ambient pH on the Growth and Development, Pathogenicity, and Diacetoxyscirpenol Accumulation of Muskmelon Fruit Caused by Fusarium sulphureum

Ambient pH, an important environmental factor, affects the growth, pathogenicity, and mycotoxin production of pathogenic fungus. Fusarium sulphureum is one of the predominant causal agents causing fusarium rot of muskmelon. In this study, we investigated the effects of ambient pH on fusarium rot development and diacetoxyscirpenol (DAS) accumulation in muskmelon infected with F. sulphureum, then analyzed the possible mechanisms in vitro and in vivo. The results suggested that ambient pH 6 was more conducive to the growth, pathogenicity, and mycotoxin production of F. sulphureum in vitro. Ambient pH 6 was also more favorable for secretion of cell wall-degrading enzymes for the pathogen to degrade the cell wall of the host plant and up-regulated the relative expression of genes involved in DAS biosynthesis, thus aggravating fruit disease and DAS accumulation. However, when the pH of the inoculated spore suspension was too acidic or too alkaline, the opposite results were observed.

Set1/COMPASS regulates growth, pathogenicity, and patulin biosynthesis of Penicillium expansum via H3K4 methylation and the interaction with PeVelB

INTRODUCTION

Penicillium expansum is a harmful plant fungal pathogen that causes blue mold disease and produces mycotoxin patulin, leading to huge economic losses and food safety hazard. Set1 associated complex Set1/COMPASS deposits the methylation at lysine 4 of histone H3, which is associated with gene expression in diverse biological processes of fungi. However, the function and underlying mechanisms of Set1/COMPASS are poorly defined in P. expansum.

OBJECTIVES

The study aimed to identify Set1/COMPASS and investigate its regulation mechanisms on growth, pathogenicity, and patulin biosynthesis of P. expansum.

METHODS

Analyses of phylogenetic relationship, conserved structural domain, and gene deletion were used to identify components of Set1/COMPASS. Phenotype analysis and stress tolerance test of gene deletion mutants were conducted to analyze the function of these components. Yeast two-hybrid, Co-Immunoprecipitation (Co-IP), and point mutation were performed to verify the protein interaction. Western blot was conducted for detection of H3K4 methylation levels.

RESULTS

P. expansum owns six components of Set1/COMPASS besides PeSet1. Absence of each component resulted in reduction of H3K4 methylation levels and impaired growth, pathogenicity, and patulin biosynthesis, as well as altered stress responses of P. expansum. One component PeBre2p was found to interact with the conserved global regulator PeVelB (VelvetLike protein B) at Asp294 of PeBre2p. This interaction affected fungal growth and utilization of fructose, lactose, glycine, and proline in P. expansum.

CONCLUSION

This study revealed the important roles of Set1/COMPASS in P. expansum and clarified for the first time the combined regulation of PeBre2p and PeVelB in fungal growth and nutrition utilization. These results will provide potential targets for the control of blue mold disease.

Combined transcriptomic and metabolomic analysis revealed that pH changes affected the expression of carbohydrate and ribosome biogenesis-related genes in Aspergillus niger SICU-33

The process of carbohydrate metabolism and genetic information transfer is an important part of the study on the effects of the external environment on microbial growth and development. As one of the most significant environmental parameters, pH has an important effect on mycelial growth. In this study, the effects of environmental pH on the growth and nutrient composition of Aspergillus niger (A. niger) filaments were determined. The pH values of the medium were 5, 7, and 9, respectively, and the molecular mechanism was further investigated by transcriptomics and metabolomics methods. The results showed that pH 5 and 9 significantly inhibited filament growth and polysaccharide accumulation of A. niger. Further, the mycelium biomass of A. niger and the crude polysaccharide content was higher when the medium's pH was 7. The DEGs related to ribosome biogenesis were the most abundant, and the downregulated expression of genes encoding XRN1, RRM, and RIO1 affected protein translation, modification, and carbohydrate metabolism in fungi. The dynamic changes of pargyline and choline were in response to the oxidative metabolism of A. niger SICU-33. The ribophorin_I enzymes and DL-lactate may be important substances related to pH changes during carbohydrate metabolism of A.niger SICU-33. The results of this study provide useful transcriptomic and metabolomic information for further analyzing the bioinformatic characteristics of A. niger and improving the application in ecological agricultural fermentation.

Effects of temperature, pH, and relative humidity on the growth of Penicillium paneum OM1 isolated from pears and its patulin production

Patulin is a mycotoxin produced by several species of Penicillium sp., Aspergillus sp., and Byssochlamys sp. on apples and pears. Most studies have been focused on Penicillium expansum, a common postharvest pathogen, but little is known about the characteristics of Penicillium paneum. In the present study, we evaluated the effects of temperature, pH, and relative humidity (RH) on the growth of P. paneum OM1, which was isolated from pears, and its patulin production. The fungal strain showed the highest growth rate at 25 °C and pH 4.5 on pear puree agar medium (PPAM) under 97 % RH, while it produced the highest amount of patulin at 20 °C and pH 4.5 on PPAM under 97 % RH. Moreover, RT-qPCR analysis of relative expression levels of 5 patulin biosynthetic genes (patA, patE, patK, patL, and patN) in P. paneum OM1 exhibited that the expression of the 4 patulin biosynthetic genes except patL was up-regulated in YES medium (patulin conducive), while it was not in PDB medium (patulin non-conducive). Our data demonstrated that the 3 major environmental parameters had significant impact on the growth of P. paneum OM1 and its patulin production. These results could be exploited to prevent patulin contamination by P. paneum OM1 during pear storage.

VmPacC-mediated pH regulation of Valsa mali confers to host acidification identified by comparative proteomics analysis

Apple valsa canker caused by the Ascomycete fungus Valsa mali is one of the most serious diseases of apple, resulting in huge economic losses in the apple-growing area of China. Previous study found that the pathogen could acidify the infected tissues to make lower ambient pH (from 6.0 to 3.5) for their successfully colonization. The pH signaling transcription factor VmPacC is required for acidification of its environment and for full virulence in V. mali. It is known that the functional cooperation of proteins secreted by V. mali plays pivotal role in its successful colonization of host plants. In this study, we used tandem mass tag (TMT) labeling coupled with LC-MS/MS-based quantitative proteomics to analyze the VmPacC-mediated pH regulation in V. mali, focusing on differentially expressed proteins (DEPs). We identified 222 DEPs specific to VmPacC deletion, and 921 DEPs specific to different pH conditions (pH 6.0 and 3.4). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses indicated that these DEPs were mainly involved in pathways associated with carbon metabolism, biosynthesis of antibiotics, citrate cycle (TCA cycle), glycolysis/gluconeogenesis, glutathione metabolism, ribosomes, and pentose phosphate pathways. Additionally, we identified 119 DEPs that were shared among the VmPacC deletion mutant and different pH conditions, which were mainly related to energy metabolism pathways, providing the energy required for the hyphal growth and responses to environmental stresses. A protein-protein interaction (PPI) network analysis indicated that most of the shared proteins were mapped to an interaction network with a medium confidence score of 0.4. Notably, one uncharacterized protein (KUI69106.1), and two known proteins (heat shock protein 60 (KUI73579.1), aspartate aminotransferase (KUI73864.1)) located in the core of the network were highly connected (with ≥ 38 directed edges) with the other shared DEPs. Our results suggest that VmPacC participates in the pathogen's regulation to ambient pH through the regulation of energy metabolism pathways such as the glycolysis/gluconeogenesis pathway and TCA cycle. Finally, we proposed a sophisticated molecular regulatory network to explain pH decrease in V. mali. Our study, by providing insights into V. mali regulating pH, helps to elucidate the mechanisms of host acidification during pathogen infection.

Application of nickel chitosan nanoconjugate as an antifungal agent for combating Fusarium rot of wheat

Agro-researchers are endlessly trying to derive a potential biomolecule having antifungal properties in order to replace the application of synthetic fungicides on agricultural fields. Rot disease often caused by Fusarium solani made severe loss of wheat crops every year. Chitosan and its metallic nano-derivatives hold a broad-spectrum antifungal property. Our interdisciplinary study deals with the application of nickel chitosan nanoconjugate (NiCNC) against Fusarium rot of wheat, in comparison with chitosan nanoparticles (CNPs) and commercial fungicide Mancozeb. CNPs and NiCNC were characterized on the basis of UV–Vis spectrophotometry, HR-TEM, FESEM, EDXS and FT-IR. Both CNPs and NiCNC were found effective against the fungal growth, of which NiCNC at 0.04 mg/mL showed complete termination of F. solani grown in suitable medium. Ultrastructural analysis of F. solani conidia treated with NiCNC revealed pronounced damages and disruption of the membrane surface. Fluorescence microscopic study revealed generation of oxidative stress in the fungal system upon NiCNC exposure. Moreover, NiCNC showed reduction in rot disease incidence by 83.33% of wheat seedlings which was further confirmed through the observation of anatomical sections of the stem. NiCNC application helps the seedling to overcome the adverse effect of pathogen, which was evaluated through stress indices attributes.

Inhibitory Activity of Shrimp Waste Extracts on Fungal and Oomycete Plant Pathogens

(1) Background: This study was aimed at determining the in vitro inhibitory effect of new natural substances obtained by minimal processing from shrimp wastes on fungi and oomycetes in the genera Alternaria, Colletotrichum, Fusarium, Penicillium, Plenodomus and Phytophthora; the effectiveness of the substance with the highest in vitro activity in preventing citrus and apple fruit rot incited by P. digitatum and P. expansum, respectively, was also evaluated. (2) Methods: The four tested substances, water-extract, EtOAc-extract, MetOH-extract and nitric-extract, were analyzed by HPLC-ESI-MS-TOF; in vitro preliminary tests were carried out to determine the minimal inhibitory/fungicidal concentrations (MIC and MFC, respectively) of the raw dry powder, EtOAc-extract, MetOH-extract and nitric-extract for each pathogen. (3) Results: in the agar-diffusion-assay, nitric-extract showed an inhibitory effect on all pathogens, at all concentrations tested (100, 75, 50 and 25%); the maximum activity was on Plenodomus tracheiphilus, C. gloeosporioides and Ph. nicotianae; the diameters of inhibition halos were directly proportional to the extract concentration; values of MIC and MFC of this extract for all pathogens ranged from 2 to 3.5%; the highest concentrations (50 to 100%) tested in vivo were effective in preventing citrus and apple fruit molds. (4) Conclusions: This study contributes to the search for natural and ecofriendly substances for the control of pre- and post-harvest plant pathogens.

Arginine Methyltransferase PeRmtC Regulates Development and Pathogenicity of Penicilliumexpansum via Mediating Key Genes in Conidiation and Secondary Metabolism

Penicillium expansum is one of the most common and destructive post-harvest fungal pathogens that can cause blue mold rot and produce mycotoxins in fruit, leading to significant post-harvest loss and food safety concerns. Arginine methylation by protein arginine methyltransferases (PRMTs) modulates various cellular processes in many eukaryotes. However, the functions of PRMTs are largely unknown in post-harvest fungal pathogens. To explore their roles in P. expansum, we identified four PRMTs (PeRmtA, PeRmtB, PeRmtC, and PeRmt2). The single deletion of PeRmtA, PeRmtB, or PeRmt2 had minor or no impact on the P. expansum phenotype while deletion of PeRmtC resulted in decreased conidiation, delayed conidial germination, impaired pathogenicity and pigment biosynthesis, and altered tolerance to environmental stresses. Further research showed that PeRmtC could regulate two core regulatory genes, PeBrlA and PeAbaA, in conidiation, a series of backbone genes in secondary metabolism, and affect the symmetric ω-N^G, N’^G-dimethylarginine (sDMA) modification of proteins with molecular weights of primarily 16–17 kDa. Collectively, this work functionally characterized four PRMTs in P. expansum and showed the important roles of PeRmtC in the development, pathogenicity, and secondary metabolism of P. expansum.

Effect of Ambient pH on Growth, Pathogenicity, and Patulin Production of Penicillium expansum

Penicillium expansum is an important postharvest pathogen of pomaceous fruit and a causal agent of blue mold or soft rot. In this study, we investigated the effect of ambient pH on growth, ultrastructure alteration, and pathogenicity of P. expansum, as well as accumulation of patulin and expression of genes involved in patulin biosynthesis. Under different pH, the fungus was routinely cultured and collected for growth, pathogenicity, patulin production, and gene expression studies using transmission electron microscopy, apple inoculation, HPLC, and RT-qPCR methods. Different ambient pH had significant impact on expression of genes and growth factors involved in patulin biosynthesis. Under same range of pH, gene expression profile, growth factors, and patulin accumulation (in vivo and in vitro) all showed similar changing trends. A well-developed cell was observed in addition to upregulation of genes at pH between pH 5.0 and 7.0, while the opposite was observed when pH was too basic (8.5) or too acid (2.5). Additionally, ambient pH had direct or indirect influence on expression of PecreaA, PelaeA, and PepacC. These findings will help in understanding the effect of ambient pH on growth, pathogenicity, and patulin production and support the development of successful methods for combating P. expansum infection on apple fruits.

Molecular basis of pathogenesis of postharvest pathogenic Fungi and control strategy in fruits: progress and prospect

The disease caused by pathogenic fungi is the main cause of postharvest loss of fresh fruits. The formulation of disease control strategies greatly depends on the understanding of pathogenic mechanism of fungal pathogens and control strategy. In recent years, based on the application of various combinatorial research methods, some pathogenic genes of important postharvest fungal pathogens in fruit have been revealed, and their functions and molecular regulatory networks of virulence have been explored. These progresses not only provide a new perspective for understanding the molecular basis and regulation mechanism of pathogenicity of postharvest pathogenic fungi, but also are beneficial to giving theoretical guidance for the creation of new technologies of postharvest disease control. Here, we synthesized these recent advances and illustrated conceptual frameworks, and identified several issues on the focus of future studies.

Volatile Organic Compounds (VOCs) Produced by Gluconobacter cerinus and Hanseniaspora osmophila Displaying Control Effect against Table Grape-Rot Pathogens

Table grapes (Vitis vinifera) are affected by botrytis bunch rot and summer bunch rot, the latter a complex disease caused by Botrytis cinerea, Aspergillus spp., Penicillium expansum and Rhizopus stolonifer. To search for biocontrol alternatives, a new bioproduct composed of Gluconobacter cerinus and Hanseniaspora osmophila, a consortium called PUCV-VBL, was developed for the control of fungal rots in table grapes. Since this consortium presents new biocontrol species, the effect of their VOCs (volatile organic compounds) was evaluated under in vitro and in vivo conditions. The VOCs produced by the PUCV-VBL consortium showed the highest mycelial inhibition against Botrytis cinerea (86%). Furthermore, H. osmophila was able to inhibit sporulation of A. tubingensis and P. expansum. VOCs' effect in vivo was evaluated using berries from Red Globe, Thompson Seedless and Crimson Seedless grapes cultivars, demonstrating a mycelial inhibition by VOCs greater than 70% for all evaluated fungal species. The VOC identification of the PUCV-VBL consortium was analyzed by solid-phase microextraction coupled to gas chromatography-mass spectrometry (SPME-GCMS). A total 26 compounds were identified, including 1-butanol 3-methyl, propanoic acid ethyl ester, ethyl acetate, phenylethyl alcohol, isobutyl acetate and hexanoic acid ethyl ester. Our results show that VOCs are an important mode of action of the PUCV-VBL biological consortium.

Function of pH-dependent transcription factor PacC in regulating development, pathogenicity, and mycotoxin biosynthesis of phytopathogenic fungi

pH, as one of the most important environmental factors, affects various biological processes in pathogenic fungi. Sensing and responding to fluctuations in ambient pH are essential for these fungi to complete their life cycle. Fungi have evolved a complicated and conserved system, the so-called Pal-pH pathway, to regulate genes and adapt to alterations in ambient pH. PacC is the dominant transcription factor in the Pal-pH pathway and regulates various biological processes. The regulatory mode of PacC has been extensively studied in Aspergillus nidulans and is generally conserved in other fungal species, including numerous phytopathogenic fungi. However, species-specific alterations have been reported. This review summarizes recent advances in the regulatory mechanisms of PacC and its role in controlling development, pathogenicity, and mycotoxin biosynthesis in phytopathogenic fungi. Potential applications of these findings and some unresolved questions are also discussed.

Ribonucleoside Diphosphate Reductase Plays an Important Role in Patulin Degradation by Enterobacter cloacae subsp. dissolvens

Patulin contamination is a worldwide concern due to its significant impact on human health. Several yeast strains have been screened for patulin biodegradation; however, little information is available on bacterial strains and their mechanism of degradation. In the present study, we isolated a bacterial strain TT-09 and identified it as Enterobacter cloacae subsp. dissolvens based on the BioLog system and 16S rDNA phylogenetic analysis. The strain was demonstrated to be able to transform patulin into E-ascladiol. Isobaric tags for relative and absolute quantitation and reverse transcription quantitative polymerase chain reaction analyses provided evidence that ribonucleoside diphosphate reductase (NrdA), an important enzyme involved in DNA biosynthesis, plays a crucial role in patulin degradation. Deletion of nrdA resulted in a total loss in the ability to degrade patulin in TT-09. These results indicate a new function for NrdA in mycotoxin biodegradation. The present study provides evidence for understanding a new mechanism of patulin degradation and information that can be used to develop new approaches for managing patulin contamination.

The pH-Responsive Transcription Factor PacC Governs Pathogenicity and Ochratoxin A Biosynthesis in Aspergillus carbonarius

Pathogenic fungi must respond effectively to changes in environmental pH for successful host colonization, virulence and toxin production. Aspergillus carbonarius is a mycotoxigenic pathogen with the ability to colonize many plant hosts and secrete ochratoxin A (OTA). In this study, we characterized the functions and addressed the role of PacC-mediated pH signaling in A. carbonarius pathogenicity using designed pacC gene knockout mutant. ΔAcpacC mutant displayed an acidity-mimicking phenotype, which resulted in impaired fungal growth at neutral/alkaline pH, accompanied by reduced sporulation and conidial germination compared to the wild type (WT) strain. The ΔAcpacC mutant was unable to efficiently acidify the growth media as a direct result of diminished gluconic and citric acid production. Furthermore, loss of AcpacC resulted in a complete inhibition of OTA production at pH 7.0. Additionally, ΔAcpacC mutant exhibited attenuated virulence compared to the WT toward grapes and nectarine fruits. Reintroduction of pacC gene into ΔAcpacC mutant restored the WT phenotype. Our results demonstrate important roles of PacC of A. carbonarius in OTA biosynthesis and in pathogenicity by controlling transcription of genes important for fungal secondary metabolism and infection.

The Pattern and Function of DNA Methylation in Fungal Plant Pathogens

To successfully infect plants and trigger disease, fungal plant pathogens use various strategies that are dependent on characteristics of their biology and genomes. Although pathogenic fungi are different from animals and plants in the genomic heritability, sequence feature, and epigenetic modification, an increasing number of phytopathogenic fungi have been demonstrated to share DNA methyltransferases (MTases) responsible for DNA methylation with animals and plants. Fungal plant pathogens predominantly possess four types of DNA MTase homologs, including DIM-2, DNMT1, DNMT5, and RID. Numerous studies have indicated that DNA methylation in phytopathogenic fungi mainly distributes in transposable elements (TEs), gene promoter regions, and the repetitive DNA sequences. As an important and heritable epigenetic modification, DNA methylation is associated with silencing of gene expression and transposon, and it is responsible for a wide range of biological phenomena in fungi. This review highlights the relevant reports and insights into the important roles of DNA methylation in the modulation of development, pathogenicity, and secondary metabolism of fungal plant pathogens. Recent evidences prove that there are massive links between DNA and histone methylation in fungi, and they commonly regulate fungal development and mycotoxin biosynthesis.

Ultra-Structural Alterations in Botrytis cinerea-The Causal Agent of Gray Mold-Treated with Salt Solutions

Potassium bicarbonate (PB), calcium chelate (CCh), and sodium silicate (SSi) have been extensively used as antifungal generally recognized as safe (GRAS) compounds against plant pathogenic fungi. In this research, in in vitro tests, the radial growth, conidial germination, and germ tube elongation of Botrytis cinerea was completely inhibited at 0.3% of PB, SSi, and CCh. In in vivo tests, application of PB, SSi, and CCh completely inhibited the occurrence of gray mold incidence of inoculated 'Italia' grape berries at concentrations of 1.0, 0.8, and 0.8%, respectively. In order to investigate the detailed mechanisms by which salts exhibited antifungal activity, we analyzed their influence on morphological changes by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and also on reactive species of oxygen (ROS), mitochondrial membrane potential (MMP), and adenosine triphosphate (ATP) content. Defects such as malformation and excessive septation were detected on salt-treated hyphae morphology observed by SEM. The internal structure of conidia treated or not with salt solutions was examined by TEM. In treated conidia, most of the conidia were affected and cellular vacuolization and cytoplasmic disorganization was observed. For ROS accumulation, a higher increase was observed in fluorescent conidia in presence of PB, SSi, and CCh by 75, 68, and 70% as compared to control, respectively. MMP was significantly decreased after salt application indicating a loss of mitochondria function. Also, luminescence showed that B. cinerea-conidia treated with salts contained less ATP than the untreated conidia. The results obtained herein are a step towards a comprehensive understanding of the mode of action by which salts act as antifungal agents against B. cinerea.

The pH-responsive PacC transcription factor plays pivotal roles in virulence and patulin biosynthesis in Penicillium expansum

The PacC (loss or reduction in phosphatase activity at acid but not at alkaline pH [Pac]) transcription factor regulates environmental adaptation, secondary metabolism and virulence in many fungal pathogens. Here, we report the functions of PacC in Penicillium expansum, a postharvest pathogenic fungus in horticultural crops, and ascertain that the gene expression and proteolytic processing of PePacC are strictly pH-dependent. Loss of PePacC resulted in an obvious decrease in growth and conidiation of P. expansum cultured in both acidic and alkaline conditions. The ΔPePacC mutant lost the ability of patulin production at pH values above 6.0 because expressions of all the genes in patulin cluster were significantly down-regulated. Additionally, virulence of the ΔPePacC mutant was obviously reduced in pear and apple fruits. Proteome analysis revealed that PePacC could function as an activator or repressor for different target proteins, including calreticulin (PeCRT) and sulfate adenylyltransferase (PeSAT), which were further proved to be involved in virulence of P. expansum. Our results demonstrate important roles for PePacC in patulin biosynthesis via limiting expressions of the genes in the cluster, and in pathogenesis via mediating a known virulence factor glucose oxidase (PeGOD) and new virulence factors, such as PeCRT and PeSAT.

Ethyl pyruvate (EP) suppressed post-harvest blue mold of sweet cherry fruit by inhibiting the growth of Penicillium oxalicum

BACKGROUND

The shelf-life of fresh sweet cherry is relatively short due to fungal decay during post-harvest storage. To investigate the effect and the mechanism of action of ethyl pyruvate (EP) against blue mold of sweet cherry fruit caused by Penicillium oxalicum, the spores were treated with 25 mg L^-1 EP. The spore germination rate of P. oxalicum, the integrities of the cell wall and plasma membrane, reactive oxygen species (ROS) and malondialdehyde (MDA) were evaluated.

RESULTS

EP treatment significantly suppressed post-harvest blue mold of sweet cherry fruit. We found that the treatment of 25 mg L^-1 EP significantly suppressed blue mold of post-harvest sweet cherry fruit by directly inhibiting germination and the mycelial growth of P. oxalicum. After co-inoculation with EP for 30 min, the spore germination rate of P. oxalicum was reduced by 83.5%. In addition, the pH of the EP solution was found to affect its antimicrobial activity. After treatment with EP, the cell surface structures of the spores of P. oxalicum were much more incomplete, and higher ROS and MDA values were recorded in the spores.

CONCLUSIONS

The results suggested that EP treatment destroyed the integrities of the cell surface structures and caused oxidative damage of the spores of P. oxalicum. © 2019 Society of Chemical Industry.

Proteomic analysis of the inhibitory effect of oligochitosan on the fungal pathogen, Botrytis cinerea

BACKGROUND

The fungal pathogen Botrytis cinerea infects a broad range of horticultural plants worldwide, resulting in significant economic losses. A derivative of chitosan, oligochitosan, has been reported to be an eco-friendly alternative to synthetic fungicides.

RESULTS

Oligochitosan can greatly inhibit B. cinerea spore germination and induce protein carbonylation. To further investigate the molecular mechanism underlying the inhibitory effect, a comparative proteome analysis was conducted of oligochitosan-treated versus non-treated B. cinerea spores. The cellular proteins were obtained from B. cinerea spore samples and subjected to two-dimensional gel electrophoresis. In total, 21 differentially expressed proteins (DEPs) were identified. Three DEPs were up-regulated in the oligochitosan-treated versus the untreated spores, including scytalone dehydratase and a serine carboxypeptidase III precursor. By contrast, seven DEPs, including Hsp 88 and cell division cycle protein 48, were down-regulated by oligochitosan treatment. Notably, 10 DEPs, including phosphatidylserine decarboxylase proenzyme and ATP-dependent molecular chaperone HSC82, were only detected in the control spores, whereas one DEP, a non-annotated predicted protein, was only detected in the oligochitosan-treated spores.

CONCLUSION

Oligochitosan may affect the spore germination of B. cinerea by impairing protein function. These findings have practical implications with respect to the use of oligochitosan for controlling fungal pathogens. © 2018 Society of Chemical Industry.

Sugar Protectants Improve the Thermotolerance and Biocontrol Efficacy of the Biocontrol Yeast, Candida oleophila

A variety of sugar compounds have been used as additives to protect various biocontrol yeasts from adverse environmental stresses. However, studies on maltose and lactose as sugar protectants are limited, and their protective effect is not clear. In the present study, exposure of the biocontrol yeast Candida oleophila cells to 45°C for 10 min, while immersed in either 5 or 10% (w/v) maltose or lactose, provided a significant protective effect. The addition of maltose and lactose significantly enhanced enzyme activity and gene expression of catalase, thioredoxin reductase, and glutathione reductase, relative to cells that have been immersed in sterile distilled water (controls) exposed to 45°C. In addition, C. oleophila cells suspended in maltose and lactose solutions also exhibited higher viability and ATP levels, relative to control cells. Notably, the biocontrol efficacy of C. oleophila against postharvest diseases of apple fruit was maintained after the yeast was exposed to the high temperature treatment while immersed in maltose and lactose solutions. These results demonstrate the potential of maltose and lactose as sugar protectants for biocontrol agent against heat stress.

Patulin in Apples and Apple-Based Food Products: The Burdens and the Mitigation Strategies

Apples and apple-based products are among the most popular foods around the world for their delightful flavors and health benefits. However, the commonly found mold, Penicillium expansum invades wounded apples, causing the blue mold decay and ensuing the production of patulin, a mycotoxin that negatively affects human health. Patulin contamination in apple products has been a worldwide problem without a satisfactory solution yet. A comprehensive understanding of the factors and challenges associated with patulin accumulation in apples is essential for finding such a solution. This review will discuss the effects of the pathogenicity of Penicillium species, quality traits of apple cultivars, and environmental conditions on the severity of apple blue mold and patulin contamination. Moreover, beyond the complicated interactions of the three aforementioned factors, patulin control is also challenged by the lack of reliable detection methods in food matrices, as well as unclear degradation mechanisms and limited knowledge about the toxicities of the metabolites resulting from the degradations. As apple-based products are mainly produced with stored apples, pre- and post-harvest strategies are equally important for patulin mitigation. Before storage, disease-resistance breeding, orchard-management, and elicitor(s) application help control the patulin level by improving the storage qualities of apples and lowering fruit rot severity. From storage to processing, patulin mitigation strategies could benefit from the optimization of apple storage conditions, the elimination of rotten apples, and the safe and effective detoxification or biodegradation of patulin.

Apple Intrinsic Factors Modulating the Global Regulator, LaeA, the Patulin Gene Cluster and Patulin Accumulation During Fruit Colonization by Penicillium expansum

The mycotoxin patulin is produced in colonized tissue by Penicillium expansum during storage of apples and is significantly affected by environmental factors that contribute to its accumulation. Few reports have, however, examined the effect of natural intrinsic factors associated with the fruit on the production of patulin. Here, we find that with advancing maturity, Golden Delicious apples show increased concentrations of total soluble solids (TSS) from 14 to 17% associated with the increased expression of the global transcription factor involved in regulation of secondary metabolite biosynthesis in filamentous fungi, laeA expression and patulin accumulation. However, the apple cultivar Granny Smith, with similar TSS values but differing in pH levels and malic acid concentrations, showed reduced expression levels of laeA and the patulin biosynthesis gene cluster (pat genes) and patulin accumulation, suggesting a complexity of host factors contribution to patulin accumulation during P. expansum colonization. To start elucidating these apple intrinsic factors, we examined their in vitro impact on laeA and pat gene expression concomitant with patulin synthesis. Increasing sucrose concentrations from 15 to 175 mM repressed laeA and pat gene expression and patulin production. However, this affect was modified and often reversed and sometimes accentuated by changes in pH, or the addition of malic acid or the major apple phenolic compounds, chlorogenic acid and epicatechin. While the increase in malic acid from 0 to 1% increased laeA and pat gene expression, the decrease in pH from 3.5 to 2.5 reduced their expression. Also the increased laeA and pat genes expressions at increasing epicatechin concentrations from 0 to 1 mM, was reversed by increasing sucrose concentrations, all together suggesting the complexity of the interactions in vivo.

Antifungal Activity of an Abundant Thaumatin-Like Protein from Banana against Penicillium expansum, and Its Possible Mechanisms of Action

Thaumatin-like protein from banana (designated BanTLP) has been purified by employing a simple protocol consisting of diethylaminoethyl Sephadex (DEAE⁻Sephadex) chromatography, gel filtration on Sephadex G50, and reversed-phase chromatography. The purified protein was identified by MALDI-TOF mass spectrometry, with an estimated molecular weight of 22.1 kDa. BanTLP effectively inhibited in vitro spore germination of Penicillium expansum, one of the main postharvest pathogens in fruits. This study further investigated the antifungal properties and underlying mechanisms of BanTLP against P. expansum. Results demonstrated that BanTLP exhibited antifungal activity in a wide pH range (4.0⁻10.0) at 20⁻50 °C. Propidium iodide (PI) influx and potassium release confirmed that BanTLP induced membrane disruption of the test pathogen, increasing the membrane permeability and disintegration of the cell. This led to cell death, as evidenced by the assays of thiobarbituric acid-reactive species (TBARS) content, the production of reactive oxygen species (ROS), and 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence integrity. Ultrastructural alterations in P. expansum conidia after BanTLP treatment revealed severe damage to the cell wall. These results suggest that BanTLP purified from banana exerts antifungal activity against P. expansum by inducing plasma membrane disturbance and cell wall disorganization.

Identification of differentially expressed genes involved in spore germination of Penicillium expansum by comparative transcriptome and proteome approaches

In this study, Penicillium expansum, a common destructive phytopathogen and patulin producer was isolated from naturally infected apple fruits and identified by morphological observation and rDNA-internal transcribed spacer analysis. Subsequently, a global view of the transcriptome and proteome alteration of P. expansum spores during germination was evaluated by RNA-seq (RNA sequencing) and iTRAQ (isobaric tags for relative and absolute quantitation) approaches. A total of 3,026 differentially expressed genes (DEGs), 77 differentially expressed predicted transcription factors and 489 differentially expressed proteins (DEPs) were identified. The next step involved screening out 130 overlapped candidates through correlation analysis between the RNA-seq and iTRAQ datasets. Part of them showed a different expression trend in the mRNA and protein levels, and most of them were involved in metabolism and genetic information processing. These results not only highlighted a set of genes and proteins that were important in deciphering the molecular processes of P. expansum germination but also laid the foundation to develop effective control methods and adequate environmental conditions.

pH-Signaling Transcription Factor AopacC Regulates Ochratoxin A Biosynthesis in Aspergillus ochraceus

In Aspergillus and Penicillium species, an essential pH-response transcription factor pacC is involved in growth, pathogenicity, and toxigenicity. To investigate the connection between ochratoxin A (OTA) biosynthesis and ambient pH, the AopacC in Aspergillus ochraceus was functionally characterized using a loss-of-function mutant. The mycelium growth was inhibited under pH 4.5 and 10.0, while the sporulation increased under alkaline condition. A reduction of mycelium growth and an elevation of sporulation was observed in Δ AopacC mutant. Compared to neutral condition, OTA contents were respectively reduced by 71.6 and 79.8% under acidic and alkaline conditions. The expression of AopacC increased with the elevated pH, and deleting AopacC dramatically decreased OTA production and biosynthetic genes Aopks expression. Additionally, the Δ AopacC mutant exhibited attenuated infection ability toward pear fruits. These results suggest that AopacC is an alkaline-induced regulator responsible for growth and OTA biosynthesis in A. ochraceus and this regulatory mechanism might be pH-dependent.

The impact of the postharvest environment on the viability and virulence of decay fungi

Postharvest decay of fruits, vegetables, and grains by fungal pathogens causes significant economic losses. Infected produce presents a potential health risk since some decay fungi produce mycotoxins that are hazardous to human health. Infections are the result of the interplay between host resistance and pathogen virulence. Both of these processes, however, are significantly impacted by environmental factors, such as temperature, UV, oxidative stress, and water activity. In the present review, the impact of various physical postharvest treatments (e.g., heat and UV) on the viability and virulence of postharvest pathogens is reviewed and discussed. Oxidative injury, protein impairment, and cell wall degradation have all been proposed as the mechanisms by which these abiotic stresses reduce fungal viability and pathogenicity. The response of decay fungi to pH and the ability of pathogens to modulate the pH of the host environment also affect pathogenicity. The effects of the manipulation of the postharvest environment by ethylene, natural edible coatings, and controlled atmosphere storage on fungal viability are also discussed. Lastly, avenues of future research are proposed.

The Possible Mechanisms Involved in Degradation of Patulin by Pichia caribbica

In this work, we examined the mechanisms involved in the degradation of patulin by Pichia caribbica. Our results indicate that cell-free filtrate of P. caribbica reduced patutlin content. The heat-killed cells could not degrade patulin. However, the live cells significantly reduced the concentration of the patulin. In furtherance to this, it was observed that patulin was not detected in the broken yeast cells and cell wall. The addition of cycloheximide to the P. caribbica cells decreased the capacity of degradation of patulin. Proteomics analyses revealed that patulin treatment resulted in an upregulated protein which was involved in metabolism and stress response processes. Our results suggested that the mechanism of degradation of patulin by P. caribbica was not absorption; the presence of patulin can induce P. caribbica to produce associated intracellular and extracellular enzymes, both of which have the ability to degrade patulin. The result provides a new possible method that used the enzymes produced by yeast to detoxify patulin in food and feed.

The MADS-Box transcription factor Bcmads1 is required for growth, sclerotia production and pathogenicity of Botrytis cinerea

MADS-box transcription factors are highly conserved in eukaryotic species and involved in a variety of biological processes. Little is known, however, regarding the function of MADS-box genes in Botrytis cinerea, a fungal pathogen with a wide host range. Here, the functional role of the B. cinerea MADS-box gene, Bcmads1, was characterized in relation to the development, pathogenicity and production of sclerotia. The latter are formed upon incubation in darkness and serve as survival structures during winter and as the female parent in sexual reproduction. Bcmads1 is indispensable for sclerotia production. RT-qPCR analysis suggested that Bcmads1 modulated sclerotia formation by regulating the expression of light-responsive genes. Bcmads1 is required for the full virulence potential of B. cinerea on apple fruit. A comparative proteomic analysis identified 63 proteins, representing 55 individual genes that are potential targets of Bcmads1. Among them, Bcsec14 and Bcsec31 are associated with vesicle transport. Deletion of Bcsec14 and Bcsec31 resulted in a reduction in the virulence and protein secretion of B. cinerea. These results suggest that Bcmads1 may influence sclerotia formation by modulating light responsive gene expression and regulate pathogenicity by its effect on the protein secretion process.

The Role of Aquaporins in pH-Dependent Germination of Rhizopus delemar Spores

Rhizopus delemar and associated species attack a wide range of fruit and vegetables after harvest. Host nutrients and acidic pH are required for optimal germination of R. delemar, and we studied how this process is triggered. Glucose induced spore swelling in an acidic environment, expressed by an up to 3-fold increase in spore diameter, whereas spore diameter was smaller in a neutral environment. When suspended in an acidic environment, the spores started to float, indicating a change in their density. Treatment of the spores with HgCl2, an aquaporin blocker, prevented floating and inhibited spore swelling and germ-tube emergence, indicating the importance of water uptake at the early stages of germination. Two putative candidate aquaporin-encoding genes-RdAQP1 and RdAQP2-were identified in the R. delemar genome. Both presented the conserved NPA motif and six-transmembrane domain topology. Expressing RdAQP1 and RdAQP2 in Arabidopsis protoplasts increased the cells' osmotic water permeability coefficient (Pf) compared to controls, indicating their role as water channels. A decrease in R. delemar aquaporin activity with increasing external pH suggested pH regulation of these proteins. Substitution of two histidine (His) residues, positioned on two loops facing the outer side of the cell, with alanine eliminated the pH sensing resulting in similar Pf values under acidic and basic conditions. Since hydration is critical for spore switching from the resting to activate state, we suggest that pH regulation of the aquaporins can regulate the initial phase of R. delemar spore germination, followed by germ-tube elongation and host-tissue infection.

Aquaporin8 regulates cellular development and reactive oxygen species production, a critical component of virulence in Botrytis cinerea

Aquaporins (AQPs) are ubiquitous in nearly all organisms, mediating selective and rapid flux of water across biological membranes. The role of AQPs in phytopathogenic fungi is poorly understood. Orthologs of AQP genes in Botrytis cinerea were identified and knocked out. The effects of AQPs on hyphal growth and conidiation, formation of infection structures and virulence on plant hosts were examined. The role of AQP8 in reactive oxygen species (ROS) production, distribution and transport were further determined. Among eight AQPs, only AQP8 was essential for the ability of B. cinerea to infect plants. AQP8 was demonstrated to be an intrinsic plasma membrane protein, which may function as a channel and mediate hydrogen peroxide uptake. Deletion of AQP8 in B. cinerea completely inhibited the development of conidia and infection structures, and significantly affected noxR expression. Further observations revealed that both AQP8 and noxR impacted ROS distribution in the hyphal tips of B. cinerea. Moreover, AQP8 affected the expression of a mitochondrial protein, NQO1. A knockout mutant of NQO1 was observed to display reduced virulence. These data lead to a better understanding of the important role of AQP8 in the development and pathogenesis of plant pathogens.

Ecofriendly hot water treatment reduces postharvest decay and elicits defense response in kiwifruit

Hot water treatment (HWT) of fruit is an effective approach for managing postharvest decay of fruits and vegetables. In the present study, the effects of HWT (45 °C for 10 min) on the growth of Botrytis cinerea and Penicillium expansum in vitro, and gray (B. cinerea) and blue mold (P. expansum) development in kiwifruit were investigated. HWT effectively inhibited spore germination and germ tube elongation of B. cinerea and P. expansum. Reactive oxygen species accumulation and protein impairment in the fungi triggered by HWT contributed to the inhibitory effect. Results of in vivo studies showed that HWT controlled gray and blue mold in kiwifruit stored at 4 and 25 °C. HWT induced a significant increase in the activity of antioxidant enzymes, including catalase and peroxidase, and the level of total phenolic compounds in kiwifruit. These findings indicate that the inhibition of postharvest decay in kiwifruit by HWT is associated with the inhibition of spore germination of both fungal pathogens and the elicitation of defense response in the kiwifruit host. Moreover, HWT used in this study did not impair fruit quality. HWT appears to represent a potential non-chemical alternative for the effective management of postharvest decay of kiwifruit.

The CarO rhodopsin of the fungus Fusarium fujikuroi is a light-driven proton pump that retards spore germination

Rhodopsins are membrane-embedded photoreceptors found in all major taxonomic kingdoms using retinal as their chromophore. They play well-known functions in different biological systems, but their roles in fungi remain unknown. The filamentous fungus Fusarium fujikuroi contains two putative rhodopsins, CarO and OpsA. The gene carO is light-regulated, and the predicted polypeptide contains all conserved residues required for proton pumping. We aimed to elucidate the expression and cellular location of the fungal rhodopsin CarO, its presumed proton-pumping activity and the possible effect of such function on F. fujikuroi growth. In electrophysiology experiments we confirmed that CarO is a green-light driven proton pump. Visualization of fluorescent CarO-YFP expressed in F. fujikuroi under control of its native promoter revealed higher accumulation in spores (conidia) produced by light-exposed mycelia. Germination analyses of conidia from carO(-) mutant and carO(+) control strains showed a faster development of light-exposed carO(-) germlings. In conclusion, CarO is an active proton pump, abundant in light-formed conidia, whose activity slows down early hyphal development under light. Interestingly, CarO-related rhodopsins are typically found in plant-associated fungi, where green light dominates the phyllosphere. Our data provide the first reliable clue on a possible biological role of a fungal rhodopsin.

Reduction of Fusarium rot and maintenance of fruit quality in melon using eco-friendly hot water treatment

Significant losses in harvested fruit can be directly attributable to decay fungi and quality deterioration. Hot water treatment (HWT) has been demonstrated to be an effective and economic environment-friendly approach for managing postharvest decay and maintaining fruit quality. In this study, the effects of HWT (45 °C for 10, 15, 20, and 25 min) on in vitro growth of Fusarium oxysporum, in vivo Fusarium rot, and natural decay of melon were investigated. HWT inhibited spore germination and germ tube elongation of F. oxysporum. Protein impairment and ATP consumption triggered by HWT contributed to the inhibitory effect. Results of in vivo studies showed that HWT effectively controlled Fusarium rot and natural decay of melon. Correspondingly, HWT induced a significant increase in content of total phenolic compounds and lignin of melon. These findings indicate that the effects of HWT on Fusarium rot may be associated with the direct fungal inhibition and the elicitation of defense responses in fruit. Importantly, HWT used in this study had beneficial effects on fruit quality as well. HWT may represent an effective non-chemical approach for management of postharvest Fusarium rot.

Ascorbic acid enhances oxidative stress tolerance and biological control efficacy of Pichia caribbica against postharvest blue mold decay of apples

The effect of ascorbic acid (VC) on improving oxidative stress tolerance of Pichia caribbica and biocontrol efficacy against blue mold caused by Penicillium expansum on apples was investigated. P. caribbica showed susceptibility to the oxidative stress in vitro test, and 250 μg/mL VC treatment improved its oxidative stress tolerance. The higher viability exhibited by VC-treated yeast was associated with a lower intracellular ROS level. The activities of antioxidant enzymes of P. caribbica were improved by VC treatment, including catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPX). Additionally, VC-treated yeast exhibited greater biocontrol activity against P. expansum and faster growth when stored at 25 and 4 °C, respectively, compared to the performance of the non-VC-treated yeast. In response to the VC treatment under oxidative stress, several differentially expressed proteins were identified in P. caribbica, and most of the poteins were confirmed to be related to basic metabolism. Therefore, the application of ascorbic acid is a useful approach to improve oxidative stress tolerance of P. caribbica and its biocontrol efficacy on apples.

Ca(2+)-CaM regulating viability of Candida guilliermondii under oxidative stress by acting on detergent resistant membrane proteins

Reactive oxygen species (ROS) play a vital role in reducing viability of yeast cells. The Ca(2+)-CaM signaling pathways are involved in regulating the intracellular ROS level in yeast cells under stress. Detergent resistant membranes (DRMs), the sterol-rich microdomains, participate in a wide range of cellular processes including growth, trafficking and death in yeast cells. In the present study, we found that Trifluoperazine (TFP), an antagonist of CaM, could increase the viability of Candida guilliermondii cells under H2O2 stress. Based on comparative analysis of DRM sub proteomics, a total number of 29 differentially expressed protein spots were identified, among which 8 protein spots belong to the electron transport chain and 7 protein spots belong to transporters. It is suggested that TFP treatment could modulate the intracellular ROS generation in yeast cells. We additionally ascertained that TFP treatment could effectively alleviate the ROS accumulation and protein damage in C. guilliermondii cells under H2O2 stress, via investigating the intracellular ROS levels and protein oxidative damage in yeast cells. These findings firstly revealed that the Ca(2+)-CaM signaling pathway is related to the viability of yeast cells under H2O2 stress, and provide novel evidences for exploring Ca(2+)-CaM's role in regulating this viability via acting on DRM proteins.

BIOLOGICAL SIGNIFICANCE

Detergent-resistant membranes (DRMs), which are more resistant to extraction with cold non-ionic detergents, have been considered the functional microdomains in the plasma membrane. In yeast, DRMs are involved in a wide range of additional cellular processes including cell growth and death. The Ca(2+)-CaM signaling pathways could regulate the stress tolerance of yeast cells by modulating the intracellular ROS generation. In this study, we found that trifluoperazine (TFP), a calmodulin antagonist, could increase the viability of C. guilliermondii under H2O2 stress. Based on comparative analysis of DRM sub proteomics, electron transport chain proteins and transporters were identified to be associated with the Ca(2+)-CaM transduction. We proved that TFP treatment decreases the intracellular ROS accumulation and alleviates oxidative damage to cellular proteins. These results ascertain that Ca(2+)-CaM is involved in regulating the viability of C. guilliermondii under oxidative stress via acting on the DRM proteins.

Mechanism of Penicillium expansum in response to exogenous nitric oxide based on proteomics analysis

Penicillium expansum is an important fungal pathogen, which causes blue mold rot in various fruits and produces a mycotoxin (patulin) with potential damage to public health. Here, we found that nitric oxide (NO) donor could significantly inhibit germinability of P. expansum spores, resulting in lower virulence to apple fruit. Based on two dimension electrophoresis (2-DE) and mass spectrometry (MS) analysis, we identified ten differentially expressed proteins in response to exogenous NO in P. expansum. Among of them, five proteins, such as glutamine synthetase (GS), amidohydrolase, nitrilases, nitric oxide dioxygenase (NOD) and heat shock protein 70, were up-regulated. Others including tetratricopeptide repeat domain, UDP-N-acetylglucosamine pyrophosphorylase, enolase (Eno), heat shock protein 60 and K homology RNA-binding domain were down-regulated. The expression of three genes associated with the identified proteins (GS, NOD, and Eno) was evaluated at the mRNA level by RT-PCR. Our results provide the novel evidence for understanding the mechanism, by which NO regulates growth of P. expansum and its virulence.

BIOLOGICAL SIGNIFICANCE

Crop diseases caused by fungal pathogens lead to huge economic losses every year in the world. Application of chemical fungicides to control diseases brings the concern about food and environmental safety. Screening new antimicrobial compounds and exploring involved mechanisms have great significance to development of new disease management strategies. Nitric oxide (NO), as an important intracellular signaling molecule, has been proved to be involved in many physiological processes and defense responses during plant-pathogen interactions. In this study, we firstly found that NO at high concentration could distinctly delay spore germination and significantly reduce virulence of P. expansum to fruit host, identified some important proteins in response to NO stress and characterized the functions of these proteins. These results provide novel evidence for understanding the mechanism of NO regulating virulence of the fungal pathogen, but are beneficial for screening new targets of antifungal compounds.