An aldo-keto reductase, Bbakr1, is involved in stress response and detoxification of heavy metal chromium but not required for virulence in the insect fungal pathogen, Beauveria bassiana

A transcription factor-mediated regulatory network controls fungal pathogen colonization of insect body cavities

Successful host tissue colonization is crucial for fungal pathogens to cause mycosis and complete the infection cycle, in which fungal cells undergo a series of morphological transition-included cellular events to combat with hosts. However, many transcription factors (TFs) and their mediated networks regulating fungal pathogen colonization of host tissue are not well characterized. Here, a TF (BbHCR1)-mediated regulatory network was identified in an insect pathogenic fungus, Beauveria bassiana, that controlled insect hemocoel colonization. BbHCR1 was highly expressed in fungal cells after reaching insect hemocoel and controlled the yeast (in vivo blastospores)-to-hyphal morphological switch, evasion of immune defense response, and fungal virulence. Comparative analysis of RNA sequencing and chromatin immunoprecipitation sequencing identified a core set of BbHCR1 target genes during hemocoel colonization, in which abaA and brlA were targeted to limit the rapid switch from blastospores to hyphae and fungal virulence. Two targets encoding hypothetical proteins, HP1 and HP2, were activated and repressed by BbHCR1, respectively, which acted as a virulence factor and repressor, respectively, suggesting that BbHCR1 activated virulence factors but repressed virulence repressors during the colonization of insect hemocoel. BbHCR1 tuned the expression of two dominant hemocoel colonization-involved metabolite biosynthetic gene clusters, which linked its regulatory role in evasion of immune response. Those functions of BbHCR1 were found to be collaboratively regulated by Fus3- and Hog1-MAP kinases via phosphorylation. These findings have drawn a regulatory network in which Fus3- and Hog1-MAP kinases phosphorylate BbHCR1, which in turn controls the colonization of insect body cavities by regulating fungal morphological transition and virulence-implicated genes.IMPORTANCEFungal pathogens adopt a series of tactics for successful colonization in host tissues, which include morphological transition and the generation of toxic and immunosuppressive molecules. However, many transcription factors (TFs) and their linked pathways that regulate tissue colonization are not well characterized. Here, we identified a TF (BbHCR1)-mediated regulatory network that controls the insect fungal pathogen, Beauveria bassiana, colonization of insect hemocoel. During these processes, BbHCR1 targeted the fungal central development pathway for the control of yeast (blastospores)-to-hyphae morphological transition, activated virulence factors, repressed virulence repressors, and tuned the expression of two dominant hemocoel colonization-involved immunosuppressive and immunostimulatory metabolite biosynthetic gene clusters. The BbHCR1 regulatory function was governed by Fus3- and Hog1-MAP kinases. These findings led to a new regulatory network composed of Fus3- and Hog1-MAP kinases and BbHCR1 that control insect body cavity colonization by regulating fungal morphological transition and virulence-implicated genes.

The Bbotf1 Zn(Ⅱ)2Cys6 transcription factor contributes to antioxidant response, fatty acid assimilation, peroxisome proliferation and infection cycles in insect pathogenic fungus Beauveria bassiana

The abilities to withstand oxidation and assimilate fatty acids are critical for successful infection by many pathogenic fungi. Here, we characterized a Zn(II)2Cys6 transcription factor Bbotf1 in the insect pathogenic fungus Beauveria bassiana, which links oxidative response and fatty acid assimilation via regulating peroxisome proliferation. The null mutant ΔBbotf1 showed impaired resistance to oxidants, accompanied by decreased activities of antioxidant enzymes including CATs, PODs and SODs, and down-regulated expression of many antioxidation-associated genes under oxidative stress condition. Meanwhile, Bbotf1 acts as an activator to regulate fatty acid assimilation, lipid and iron homeostasis as well as peroxisome proliferation and localization, and the expressions of some critical genes related to glyoxylate cycle and peroxins were down-regulated in ΔBbotf1 in presence of oleic acid. In addition, ΔBbotf1 was more sensitive to osmotic stressors, CFW, SDS and LDS. Insect bioassays revealed that insignificant changes in virulence were seen between the null mutant and parent strain when conidia produced on CZP plates were used for topical application. However, propagules recovered from cadavers killed by ΔBbotf1 exhibited impaired virulence as compared with counterparts of the parent strain. These data offer a novel insight into fine-tuned aspects of Bbotf1 concerning multi-stress responses, lipid catabolism and infection cycles.

A network of transcription factors in complex with a regulating cell cycle cyclin orchestrates fungal oxidative stress responses

BACKGROUND

Response to oxidative stress is universal in almost all organisms and the mitochondrial membrane protein, BbOhmm, negatively affects oxidative stress responses and virulence in the insect fungal pathogen, Beauveria bassiana. Nothing further, however, is known concerning how BbOhmm and this phenomenon is regulated.

RESULTS

Three oxidative stress response regulating Zn2Cys6 transcription factors (BbOsrR1, 2, and 3) were identified and verified via chromatin immunoprecipitation (ChIP)-qPCR analysis as binding to the BbOhmm promoter region, with BbOsrR2 showing the strongest binding. Targeted gene knockout of BbOsrR1 or BbOsrR3 led to decreased BbOhmm expression and consequently increased tolerances to free radical generating compounds (H2O2 and menadione), whereas the ΔBbOsrR2 strain showed increased BbOhmm expression with concomitant decreased tolerances to these compounds. RNA and ChIP sequencing analysis revealed that BbOsrR1 directly regulated a wide range of antioxidation and transcription-associated genes, negatively affecting the expression of the BbClp1 cyclin and BbOsrR2. BbClp1 was shown to localize to the cell nucleus and negatively mediate oxidative stress responses. BbOsrR2 and BbOsrR3 were shown to feed into the Fus3-MAPK pathway in addition to regulating antioxidation and detoxification genes. Binding motifs for the three transcription factors were found to partially overlap in the promoter region of BbOhmm and other target genes. Whereas BbOsrR1 appeared to function independently, co-immunoprecipitation revealed complex formation between BbClp1, BbOsrR2, and BbOsrR3, with BbClp1 partially regulating BbOsrR2 phosphorylation.

CONCLUSIONS

These findings reveal a regulatory network mediated by BbOsrR1 and the formation of a BbClp1-BbOsrR2-BbOsrR3 complex that orchestrates fungal oxidative stress responses.

Sterol Regulatory Element-Binding Protein, BbSre1, Controls Oxidative Stress Response, Peroxisome Division, and Lipid Homeostasis in an Insect Fungal Pathogen

Sterol regulatory element-binding protein, Sre1, regulates sterol biosynthesis, lipid metabolism, hypoxia adaptation, and virulence in some fungi, even though its roles are varied in fungal species. However, few studies report its other functions in fungi. Here, we report novel roles of Sre1 homolog, BbSre1, in the insect fungal pathogen, Beauveria bassiana, that regulates oxidative stress response, peroxisome division, and redox homeostasis. The gene disruption stain showed increased sensitivity to oxidative stress, which was in line with oxidative stress-induced-BbSre1 nuclear import and control of antioxidant and detoxification-involved genes. The gene mutation also inhibited peroxisome division, affected redox homeostasis, and impaired lipid/fatty acid metabolism and sterol biosynthesis, which was verified by downregulation of their associated genes. These data broaden our understanding of role of Sre1, which regulates peroxisome division, antioxidant, and detoxification-involved genes for control of redox homeostasis and oxidative stress response that links to lipid/fatty acid metabolism and sterol biosynthesis.

Insect fungal pathogens secrete a cell wall-associated glucanase that acts to help avoid recognition by the host immune system

Fungal insect pathogens have evolved diverse mechanisms to evade host immune recognition and defense responses. However, identification of fungal factors involved in host immune evasion during cuticular penetration and subsequent hemocoel colonization remains limited. Here, we report that the entomopathogenic fungus Beauveria bassiana expresses an endo-β-1,3-glucanase (BbEng1) that functions in helping cells evade insect immune recognition/ responses. BbEng1 was specifically expressed during infection, in response to host cuticle and hemolymph, and in the presence of osmotic or oxidative stress. BbEng1 was localized to the fungal cell surface/ cell wall, where it acts to remodel the cell wall pathogen associated molecular patterns (PAMPs) that can trigger host defenses, thus facilitating fungal cell evasion of host immune defenses. BbEng1 was secreted where it could bind to fungal cells. Cell wall β-1,3-glucan levels were unchanged in ΔBbEng1 cells derived from in vitro growth media, but was elevated in hyphal bodies, whereas glucan levels were reduced in most cell types derived from the BbEng1 overexpressing strain (BbEng1OE). The BbEng1OE strain proliferated more rapidly in the host hemocoel and displayed higher virulence as compared to the wild type parent. Overexpression of their respective Eng1 homologs or of BbEng1 in the insect fungal pathogens, Metarhizium robertsii and M. acridum also resulted in increased virulence. Our data support a mechanism by which BbEng1 helps the fungal pathogen to evade host immune surveillance by decreasing cell wall glucan PAMPs, promoting successful fungal mycosis.

Fus3/Kss1-MAP kinase and Ste12-like control distinct biocontrol-traits besides regulation of insect cuticle penetration via phosphorylation cascade in a filamentous fungal pathogen

BACKGROUND

Homolog of the yeast Fus3/Kss1 mitogen-activated protein kinase (MAPK) pathway and its target transcription factor, Ste12-like, are involved in penetration of host cuticle/pathogenicity in many ascomycete pathogens. However, details of their interaction during fungal infection, as well as their controlled other virulence-associated traits, are unclear.

RESULTS

Ste12-like (BbSte12) and Fus3/Kss1 MAPK homolog (Bbmpk1) interacted in nucleus, and phosphorylation of BbSte12 by Bbmpk1 was essential for penetration of insect cuticle in an insect fungal pathogen, Beauveria bassiana. However, some distinct biocontrol-traits were found to be mediated by Ste12 and Bbmpk1. In contrast to ΔBbmpk1 colony that grew more rapid than wild-type strain, inactivation of BbSte12 resulted in the opposite phenotype, which was consistent with their different proliferation rates in insect hemocoel after direct injection of conidia bypass the cuticle. Reduced conidial yield with decreased hydrophobicity was examined in both mutants, however they displayed distinct conidiogenesis, accompanying with differently altered cell cycle, distinct hyphal branching and septum formation. Moreover, ΔBbmpk1 showed increased tolerance to oxidative agent, whereas the opposite phenotype was seen for ΔBbSte12 strain. RNA sequencing analysis revealed that Bbmpk1 controlled 356 genes depending on BbSte12 during cuticle penetration, but 1077 and 584 genes were independently controlled by Bbmpk1 and BbSte12.

CONCLUSION

BbSte12 and Bbmpk1 separately participate in additional pathways for control of conidiation, growth and hyphal differentiation, as well as oxidative stress response besides regulating cuticle penetration via phosphorylation cascade. © 2023 Society of Chemical Industry.

Two Secretory T2 RNases Act as Cytotoxic Factors Contributing to the Virulence of an Insect Fungal Pathogen

RNase T2 members are secreted by several pathogens or parasites during infection, playing various roles in pathogen-host interaction. However, functions of those members in biocontrol microbes targeting their hosts are still unknown. Here, we report that an insect fungal pathogen, Beauveria bassiana, produces two secretory RNase T2 members that act as cytotoxic factors, which were examined by insect bioassays using the targeted gene(s) disruption and overexpression strains. Overexpression strains displayed dramatically increased virulence, which was concurrent with few fungal cells and hemocytes in hemocoel, suggesting a cytotoxicity of the overexpressed gene products. In vitro assays using yeast-expressed proteins verified the cytotoxicity of the two members against insect cells, to which the cytotoxic effect was dependent on their RNases enzyme activities and glycosylation modification. Moreover, the excessive humoral immune responses triggered by the two ribonucleases were examined. These results suggested prospects of these two T2 ribonucleases for improvement of biocontrol agents.

Complete genome analysis of pathogenic Metschnikowia bicuspidata strain MQ2101 isolated from diseased ridgetail white prawn, Exopalaemon carinicauda

BACKGROUND

Metschnikowia bicuspidata is a pathogenic yesst that can cause disease in many different economic aquatic animal species. In recent years, there was a new disease outbreak in ridgetail white prawn (Exopalaemon carinicauda) in coastal areas of Jiangsu Province China that was referred to as zombie disease by local farmers. The pathogen was first isolated and identified as M. bicuspidata. Although the pathogenicity and pathogenesis of this pathogen in other animals have been reported in some previous studies, research on its molecular mechanisms is still very limited. Therefore, a genome-wide study is necessary to better understand the physiological and pathogenic mechanisms of M. bicuspidata.

RESULT

In this study, we obtained a pathogenic strain, MQ2101, of M. bicuspidata from diseased E. carinicauda and sequenced its whole genome. The size of the whole genome was 15.98 Mb, and it was assembled into 5 scaffolds. The genome contained 3934 coding genes, among which 3899 genes with biological functions were annotated in multiple underlying databases. In KOG database, 2627 genes were annotated, which were categorized into 25 classes including general function prediction only, posttranslational modification, protein turnover, chaperones, and signal transduction mechanisms. In KEGG database, 2493 genes were annotated, which were categorized into five classes, including cellular processes, environmental information processing, genetic information processing, metabolism and organismal systems. In GO database, 2893 genes were annotated, which were mainly classified in cell, cell part, cellular processes and metabolic processes. There were 1055 genes annotated in the PHI database, accounting for 26.81% of the total genome, among which 5 genes were directly related to pathogenicity (identity ≥ 50%), including hsp90, PacC, and PHO84. There were also some genes related to the activity of the yeast itself that could be targeted by antiyeast drugs. Analysis based on the DFVF database showed that strain MQ2101 contained 235 potential virulence genes. BLAST searches in the CAZy database showed that strain MQ2101 may have a more complex carbohydrate metabolism system than other yeasts of the same family. In addition, two gene clusters and 168 putative secretory proteins were predicted in strain MQ2101, and functional analysis showed that some of the secretory proteins may be directly involved in the pathogenesis of the strain. Gene family analysis with five other yeasts revealed that strain MQ2101 has 245 unique gene families, including 274 genes involved in pathogenicity that could serve as potential targets.

CONCLUSION

Genome-wide analysis elucidated the pathogenicity-associated genes of M. bicuspidate while also revealing a complex metabolic mechanism and providing putative targets of action for the development of antiyeast drugs for this pathogen. The obtained whole-genome sequencing data provide an important theoretical basis for transcriptomic, proteomic and metabolic studies of M. bicuspidata and lay a foundation for defining its specific mechanism of host infestation.

The AreA Nitrogen Catabolite Repression Activator Balances Fungal Nutrient Utilization and Virulence in the Insect Fungal Pathogen Beauveria bassiana

In many fungi, the AreA GATA-type transcription factor mediates nitrogen catabolite repression affecting fungal development and, where applicable, virulence. Here, we investigated the functions of AreA in the fungal entomopathogen and plant endophyte Beauveria bassiana using knockdown of gene expression. The antiAreA mutants were impaired in nitrogen utilization and showed increased sensitivities to osmotic stressors but increased tolerances to oxidative/hypoxia stresses. Repression of BbAreA caused overall minimal effects on fungal virulence. The minor effects on virulence appeared to be due in part to competing secondary effects where host defense phenoloxidase activity was significantly decreased, but production of the fungal metabolite oosporein was increased and hyphal body development was impaired. Knockdown of BbAreA expression also resulted in impairment in ability of the fungus to associate with host plants. These data implicate that BbAreA likely acts as a regulator to balance fungal nutrient utilization, pathogenicity, and mutualism, facilitating the fungal occupation of host niches.

Putative Protein Discovery from Microalgal Genomes as a Synthetic Biology Protein Library for Heavy Metal Bio-Removal

Simple Summary

Nowadays, heavy metal polluted wastewater is one of the global challenges that leads to an insufficient supply of clean water. Taking advantage of what nature has to offer, several organisms, including microalgae, can natively bioremediate these heavy metals. However, the effectiveness of such processes does not meet expectations, especially with the increasing amount of pollution in today’s world. Therefore, with the goal of creating effective strains, synthetic biology via bioengineering is widely used as a strategy to enhance the heavy metal bio-removing capability, either by directly engineering the native ability of organisms or by transferring the ability to a more suitable host. In order to do so, a list of genes or proteins involved in the processes is crucial for stepwise engineering. Yet, a large amount of information remains to be discovered. In this work, a comprehensive library of putative proteins that are involved in heavy metal bio-removal from microalgae was constructed. Moreover, with the development of machine learning, the 3D structures of these proteins are also predicted, using machine learning-based methods, to aid the use of synthetic biology further.

Abstract

Synthetic biology is a principle that aims to create new biological systems with particular functions or to redesign the existing ones through bioengineering. Therefore, this principle is often utilized as a tool to put the knowledge learned to practical use in actual fields. However, there is still a great deal of information remaining to be found, and this limits the possible utilization of synthetic biology, particularly on the topic that is the focus of the present work—heavy metal bio-removal. In this work, we aim to construct a comprehensive library of putative proteins that might support heavy metal bio-removal. Hypothetical proteins were discovered from Chlorella and Scenedesmus genomes and extensively annotated. The protein structures of these putative proteins were also modeled through Alphafold2. Although a portion of this workflow has previously been demonstrated to annotate hypothetical proteins from whole genome sequences, the adaptation of such steps is yet to be done for library construction purposes. We also demonstrated further downstream steps that allow a more accurate function prediction of the hypothetical proteins by subjecting the models generated to structure-based annotation. In conclusion, a total of 72 newly discovered putative proteins were annotated with ready-to-use predicted structures available for further investigation.

Secretome Analysis of Macrophomina phaseolina Identifies an Array of Putative Virulence Factors Responsible for Charcoal Rot Disease in Plants

Macrophomina phaseolina is a global devastating necrotrophic fungal pathogen. It causes charcoal rot disease in more than 500 host plants including major food crops, pulse crops, fiber crops, and oil crops. Despite having the whole-genome sequence of M. phaseolina, understanding the M. phaseolina genome-based plant–pathogen interactions is limited in the absence of direct experimental proof of secretion. Thus, it is essential to understand the host–microbe interaction and the disease pathogenesis, which can ensure global agricultural crop production and security. An in silico–predicted secretome of M. phaseolina is unable to represent the actual secretome. We could identify 117 proteins present in the secretome of M. phaseolina using liquid chromatography–electrospray ionization–tandem mass spectrometry. Data are available via ProteomeXchange with identifier PXD032749. An array of putative virulence factors of M. phaseolina were identified in the present study using solid-state culture. Similar virulence factors have been reported in other plant pathogenic fungi also. Among the secretory fungal proteins with positive economic impacts, lignocellulolytic enzymes are of prime importance. Further, we validated our results by detecting the cell wall–degrading enzymes xylanase, endoglucanase, and amylase in the secretome of M. phaseolina. The present study may provide a better understanding about the necrotrophic fungi M. phaseolina, which modulate the host plant defense barriers using secretory proteins.

Endophytic Bacterium Serratia plymuthica From Chinese Leek Suppressed Apple Ring Rot on Postharvest Apple Fruit

Apple ring rot caused by Botryosphaeria dothidea is an economically significant plant disease that spreads across the apple production areas in China. The pathogen infects apple fruits during the growing season and results in postharvest fruits rot during storage, which brings about a huge loss to plant growers. The study demonstrated that an endophytic bacterium Serratia plymuthica isolated from Chinese leek (Allium tuberosum) significantly suppressed the mycelial growth, severely damaging the typical morphology of B. dothidea, and exerted a high inhibition of 84.64% against apple ring rot on postharvest apple fruit. Furthermore, S. plymuthica significantly reduced the titratable acidity (TA) content, enhanced the soluble sugar (SS) content, vitamin C content, and SS/TA ratio, and maintained the firmness of the fruits. Furthermore, comparing the transcriptomes of the control and the S. plymuthica treated mycelia revealed that S. plymuthica significantly altered the expressions of genes related to membrane (GO:0016020), catalytic activity (GO:0003824), oxidation-reduction process (GO:0055114), and metabolism pathways, including tyrosine metabolism (ko00280), glycolysis/gluconeogenesis (ko00010), and glycerolipid metabolism (ko00561). The present study provided a possible way to control apple ring rot on postharvest fruit and a solid foundation for further exploring the underlying molecular mechanism.

Comparative proteomics and secretomics revealed virulence, and coresistance-related factors in non O1/O139 Vibrio cholerae recovered from 16 species of consumable aquatic animals

Vibrio cholerae can cause pandemic cholera in humans. The bacterium resides in aquatic environments worldwide. Identification of risk factors of V. cholerae in aquatic products is imperative for assuming food safety. In this study, we determined virulence-associated genes, cross-resistance between antibiotics and heavy metals, and genome fingerprinting profiles of non O1/O139 V. cholerae isolates (n = 20) recovered from 16 species of consumable aquatic animals. Secretomes and proteomes of V. cholerae with distinct genotypes and phenotypes were obtained by using two-dimensional gel electrophoresis (2D-GE) and/or liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques. Comparative secretomic analysis revealed 4 common and 45 differential extracellular proteins among 20 V. cholerae strains, including 13 virulence- and 8 resistance-associated proteins. A total of 21,972 intracellular proteins were identified, and comparative proteomic analysis revealed 215 common and 913 differential intracellular proteins, including 22 virulence- and 8 resistance-associated proteins. Additionally, different secretomes and proteomes were observed between V. cholerae isolates of fish and shellfish origins. A number of novel proteins with unknown function and strain-specific proteins were also discovered in the V. cholerae isolates. SIGNIFICANCE: V. cholerae can cause pandemic cholera in humans. The bacterium is distributed in aquatic environments worldwide. Identification of risk factors of V. cholerae in aquatic products is imperative for assuming food safety. Non-O1/O139 V. cholerae has been reported to cause sporadic cholera-like diarrhea and bacteremia diseases, which indicates virulence factors rather than the major cholera toxin (CT) exist. This study for the first time investigated proteomes and secretomes of non-O1/O139 V. cholerae originating from aquatic animals. This resulted in the identification of a number of virulence and coresistance-related factors, as well as novel proteins and strain-specific proteins in V. cholerae isolates recovered from 16 species of consumable aquatic animals. These results fill gaps for better understanding of pathogenesis and resistance of V. cholerae, and also support the increasing need for novel diagnosis and vaccine targets against the leading waterborne pathogen worldwide.

Identification of a key protein set involved in Moniliophthora perniciosa necrotrophic mycelium and basidiocarp development

Moniliophthora perniciosa is a hemibiotrophic fungus that causes witches' broom disease in cacao (Theobroma cacao L.). The biotrophic fungal phase initiates the disease and is characterized by a monokaryotic mycelium, while the necrotrophic phase is characterized by a dikaryotic mycelium and leads to necrosis of infected tissues. A study of the necrotrophic phase was conducted on bran-based solid medium, which is the only medium that enables basidiocarp and basidiospore production. Six different fungal developmental phases were observed according to the mycelium colour or the organ produced: white, yellow, pink, dark pink, primordium and basidiocarp. In this study, we identified notable proteins in each phase, particularly those accumulated prior to basidiocarp formation. Proteins were analysed by proteomics; 2-D gels showed 300-550 spots. Statistically differentially accumulated spots were sequenced by mass spectrometry and 259 proteins were identified and categorized into nine functional classes. Proteins related to energy metabolism, protein folding and morphogenesis that were potentially involved in primordium and basidiocarp formation were identified; these proteins may represent useful candidates for further analysis related to the spread and pathogenesis of this fungus. To the best of our knowledge, this report describes the first proteomic analysis of the developmental phases of Moniliophthora perniciosa.

Development of Molecular Markers Associated with Resistance to Gray Mold Disease in Onion (Allium cepa L.) through RAPD-PCR and Transcriptome Analysis

Onions (Allium cepa L.) are one of the most consumed vegetable crops worldwide and are damaged by several fungal diseases in the field or during storage. Gray mold disease caused by the necrotrophic pathogens Botrytis cinerea and Botrytis squamosa is a disease that reduces the productivity and storage life in onions. However, it is difficult to control gray mold disease in onions by using physical and chemical methods. Breeding resistant onions against gray mold disease can reduce the damage caused by pathogens, reduce the labor required for control, and reduce environmental pollution caused by fungicides. However, onions have a large genome size (16Gb), making them difficult to analyze, and have a biennial cycle, resulting in a very long breeding period. Therefore, in this study, markers were developed to shorten the onion breeding period. First, random amplified polymorphic DNA (RAPD) was performed to confirm the genetic relationship between the gray mold disease-resistant and -susceptible lines through a dendrogram. In addition, the sequence characterized amplified region (SCAR)-OPAN1 marker to select resistant lines was developed using a polymorphic RAPD fragment. Second, the RNA-seq of the gray mold-resistant and -susceptible onion lines were analyzed using NGS technology. Using the RNA-seq results and DEG and GO analyses were performed, and the variants, such as SNPs and indels, were analyzed to develop a selectable marker for the resistant line. This study developed the SNP-3 HRM marker for selecting gray mold disease-resistant lines by using the SNPs present in the aldo-keto reductase (AKR) gene with high expression levels in these lines. The SCAR-OPAN1 and SNP-3 HRM markers developed in this study could be used to select gray mold disease-resistant onions in breeding programs to reduce the damage caused by gray mold disease.

Mycoremediation of heavy metals: processes, mechanisms, and affecting factors

Industrial processes and mining of coal and metal ores are generating a number of threats by polluting natural water bodies. Contamination of heavy metals (HMs) in water and soil is the most serious problem caused by industrial and mining processes and other anthropogenic activities. The available literature suggests that existing conventional technologies are costly and generated hazardous waste that necessitates disposal. So, there is a need for cheap and green approaches for the treatment of such contaminated wastewater. Bioremediation is considered a sustainable way where fungi seem to be good bioremediation agents to treat HM-polluted wastewater. Fungi have high adsorption and accumulation capacity of HMs and can be potentially utilized. The most important biomechanisms which are involved in HM tolerance and removal by fungi are bioaccumulation, bioadsorption, biosynthesis, biomineralisation, bioreduction, bio-oxidation, extracellular precipitation, intracellular precipitation, surface sorption, etc. which vary from species to species. However, the time, pH, temperature, concentration of HMs, the dose of fungal biomass, and shaking rate are the most influencing factors that affect the bioremediation of HMs and vary with characteristics of the fungi and nature of the HMs. In this review, we have discussed the application of fungi, involved tolerance and removal strategies in fungi, and factors affecting the removal of HMs.

A novel transcription factor negatively regulates antioxidant response, cell wall integrity and virulence in the fungal insect pathogen, Beauveria bassiana

Genomic data have identified a class of fungal specific transcription factors (FsTFs) that are thought to regulate unique aspects of fungal gene expression, although the functions of many of these proteins remain unknown. Here, a novel FsTF (BbStf1), which features a leucine zipper dimerization domain and a fungal transcription factor regulatory middle homology region, was characterized in Beauveria bassiana, a filamentous insect fungal pathogen. Transcriptional activation and nuclear localization were experimentally confirmed for BbStf1. Disruption of Bbstf1 resulted in increased tolerance to oxidative stress and cell wall perturbation, accompanied by increased peroxidase (POD) and superoxide dismutase (SOD) activities and ratio of reduced/oxidized glutathione (GSH/GSSG), and by thickened cell wall and altered composition. Gene expression profile analysis revealed that transcription patterns of antioxidant enzyme and cell wall integrity-involved genes were altered in the ∆Bbstf1, including some BbStf1-targeted genes clarified with evidence. The ∆Bbstf1 strain displayed greater virulence to Galleria mellonella in the bioassays through both topical infection and intrahaemocoel injection due to more rapid proliferation in the haemocoel as compared to the wild-type strain. Altogether, BbStf1 acts as a negative regulator of antioxidant response, cell wall integrity and virulence in B. bassiana.

Multifunctional role of a fungal pathogen-secreted laccase 2 in evasion of insect immune defense

Laccases are widely present in bacteria, fungi, plants and invertebrates and involved in a variety of physiological functions. Here, we report that Beauveria bassiana, an economic important entomopathogenic fungus, secretes a laccase 2 (BbLac2) during infection that detoxifies insect immune response-generated reactive oxygen species (ROS) and interferes with host immune phenoloxidase (PO) activation. BbLac2 is expressed in fungal cells during proliferation in the insect haemocoel and can be found to distribute on the surface of haemolymph-derived in vivo fungal hyphal bodies or be secreted. Targeted gene-knockout of BbLac2 increased fungal sensitivity to oxidative stress, decreased virulence to insect, and increased host PO activity. Strains overexpressing BbLac2 showed increased virulence, with reduced host PO activity and lowered ROS levels in infected insects. In vitro assays revealed that BbLac2 could eliminate ROS and oxidize PO substrates (phenols), verifying the enzymatic functioning of the protein in detoxification of cytotoxic ROS and interference with the PO cascade. Moreover, BbLac2 acted as a cell surface protein that masked pathogen associated molecular patterns (PAMPs), enabling the pathogen to evade immune recognition. Our data suggest a multifunctional role for fungal pathogen-secreted laccase 2 in evasion of insect immune defenses.

MADS-box transcription factor Mcm1 controls cell cycle, fungal development, cell integrity and virulence in the filamentous insect pathogenic fungus Beauveria bassiana

MADS-box transcription factor Mcm1 plays crucial roles in regulating mating processes and pathogenesis in some fungi. However, its roles are varied in fungal species, and its function remains unclear in entomopathogenic fungi. Here, Mcm1 orthologue, Bbmcm1, was characterized in a filamentous entomopathogenic fungus, Beauveria bassiana. Disruption of Bbmcm1 resulted in a distinct reduction in growth with abnormal conidiogenesis, and a significant decrease in conidial viability with abnormal germination. ΔBbmcm1 displayed impaired cell integrity, with distorted cell wall structure and altered cell wall component. Abnormal cell cycle was detected in ΔBbmcm1 with longer G₂ /M phase but shorter G₁ /G₀ and S phases in unicellular blastospores, and sparser septa in multicellular hyphae, which might be responsible for defects in development and differentiation due to the regulation of cell cycle-involved genes, as well as the corresponding cellular events-associated genes. Dramatically decreased virulence was examined in ΔBbmcm1, with impaired ability to escape haemocyte encapsulation, which was consistent with markedly reduced cuticle-degrading enzyme production by repressing their coding genes, and downregulated fungal effector protein-coding genes, suggesting a novel role of Mcm1 in interaction with host insect. These data indicate that Mcm1 is a crucial regulator of development, cell integrity, cell cycle and virulence in insect fungal pathogens.