Mutation of AREA affects growth, sporulation, nitrogen regulation, and pathogenicity in Colletotrichum gloeosporioides

Regulation of nitrogen utilization and mycotoxin biosynthesis by the GATA transcription factor AaAreA in Alternaria alternata

Alternaria alternata is a prevalent postharvest pathogen that generates diverse mycotoxins, notably alternariol (AOH) and alternariol monomethyl ether (AME), which are recurrent severe contaminants. Nitrogen sources modulate fungal growth, development, and secondary metabolism, including mycotoxin production. The GATA transcription factor AreA regulates nitrogen source utilization. However, little is known about its involvement in the regulation of nitrogen utilization in A. alternata. To examine the regulatory mechanism of AaAreA on AOH and AME biosynthesis in A. alternata, we analyzed the impact of diverse nitrogen sources on the fungal growth, conidiation and mycotoxin production. The use of a secondary nitrogen source (NaNO3) enhanced mycelial elongation and sporulation more than the use of a primary source (NH4Cl). NaNO3 favored greater mycotoxin accumulation than did NH4Cl. The regulatory roles of AaAreA were further clarified through gene knockout. The absence of AaAreA led to an overall reduction in growth in minimal media containing any nitrogen source except NH4Cl. AaAreA positively regulates mycotoxin biosynthesis when both NH4Cl and NaNO3 are used as nitrogen sources. Subcellular localization analysis revealed abundant nuclear transport when NaNO3 was the sole nitrogen source. The regulatory pathway of AaAreA was systematically revealed through comprehensive transcriptomic analyses. The deletion of AaAreA significantly impedes the transcription of mycotoxin biosynthetic genes, including aohR, pksI and omtI. The interaction between AaAreA and aohR, a pathway-specific transcription factor gene, demonstrated that AaAreA binds to the aohR promoter sequence (5'-GGCTATGGAAA-3'), activating its transcription. The expressed AohR regulates the expression of downstream synthase genes in the cluster, ultimately impacting mycotoxin production. This study provides valuable information to further understand how AreA regulates AOH and AME biosynthesis in A. alternata, thereby enabling the effective design of control measures for mycotoxin contamination.

Lignin-degrading enzyme production was enhanced by the novel transcription factor Ptf6 in synergistic microbial co-culture

Synergistic microbial co-culture has been an efficient and energy-saving strategy to produce lignin-degrading enzymes (LDEs), including laccase, manganese peroxidase, and versatile peroxidase. However, the regulatory mechanism of microbial co-culture is still unclear. Herein, the extracellular LDE activities of four white-rot fungi were significantly increased by 88-544% over monoculture levels when co-cultured with Rhodotorula mucilaginosa. Ptf6 was demonstrated from the 9 million Y1H clone library to be a shared GATA transcription factor in the four fungi, and could directly bind to the laccase gene promoter. Ptf6 exists in two alternatively spliced isoforms under monoculture, namely Ptf6-α (1078 amino acids) containing Cys2/Cys2-type zinc finger and Ptf6-β (963 amino acids) lacking the complete domain. Ptf6 responded to co-culture by up-regulation of both its own transcripts and the proportion of Ptf6-α. Ptf6-α positively activated the production of most LDE isoenzymes and bound to four GATA motifs on the LDEs' promoter with different affinities. Moreover, Ptf6-regulation mechanism can be applicable to a variety of microbial co-culture systems. This study lays a theoretical foundation for further improving LDEs production and providing an efficient way to enhance the effects of biological and enzymatic pretreatment for lignocellulosic biomass conversion.

Elicitation of Fruit Fungi Infection and Its Protective Response to Improve the Postharvest Quality of Fruits

Fruit diseases brought on by fungus infestation leads to postharvest losses of fresh fruit. Approximately 30% of harvested fruits do not reach consumers’ plates due to postharvest losses. Fungal pathogens play a substantial part in those losses, as they cause the majority of fruit rots and consumer complaints. Understanding fungal pathogenic processes and control measures is crucial for developing disease prevention and treatment strategies. In this review, we covered the presented pathogen entry, environmental conditions for pathogenesis, fruit’s response to pathogen attack, molecular mechanisms by which fungi infect fruits in the postharvest phase, production of mycotoxin, virulence factors, fungal genes involved in pathogenesis, and recent strategies for protecting fruit from fungal attack. Then, in order to investigate new avenues for ensuring fruit production, existing fungal management strategies were then assessed based on their mechanisms for altering the infection process. The goal of this review is to bridge the knowledge gap between the mechanisms of fungal disease progression and numerous disease control strategies being developed for fruit farming.

AwAreA Regulates Morphological Development, Ochratoxin A Production, and Fungal Pathogenicity of Food Spoilage Fungus Aspergillus westerdijkiae Revealed by an Efficient Gene Targeting System

Aspergillus westerdijkiae, the producer of ochratoxin A (OTA), which is of worldwide concern, is an import fungal species in agriculture, food, and industry. Here, we got the uridine auxotrophic mutant of A. westerdijkiae by deleting AwpyrG. The ΔAwpyrG could be used for bio-transformation with exogenous AfpyrG expression cassette as a selection marker. In order to enhance the efficiency of gene targeting, Awku70 and Awlig4 were homologously deleted from ΔAwpyrG. The efficiencies of homologous replacement for ΔAwku70 and ΔAwlig4 were 95.7 and 87.0% in the deletion of AwAreA, respectively, demonstrating a drastic increase from 4.3% of the wild type (WT) strain. Furthermore, the function of AwAreA was identified with AwAreA deletion mutant and the control strain ΔAwku70. AwAreA regulated the growth and conidiation of A. westerdijkiae in response to nitrogen sources. The concentration of OTA for ΔAwku70 was in the range of 19.4 to 186.9 ng/cm2 on all kinds of nitrogen sources. The OTA production influenced by the deletion of AwAreA was different based on nitrogen sources. Pathogenicity assays on pears, grapes, salted meat, and cheese showed that AwAreA acted as a negative regulator in the infection of food substrates. Therefore, the genetic methods and engineered strains enable us to substantially expand the use of A. westerdijkiae, one of more than twenty OTA-producing fungi, in the study of mycotoxin biosynthesis and regulation, and consequently to aim at providing new ways for controlling this pathogen.

PeMetR-mediated sulfur assimilation is essential for virulence and patulin biosynthesis in Penicillium expansum

Penicillium expansum, as the causal agent of blue mould and a main producer of mycotoxin patulin, is a global concern for economic and food safety. To date, the nutritional requirements of the pathogen during infection and patulin biosynthesis are poorly understood. Here, we genetically characterized the role of the bZIP transcription factor PeMetR in sulfur metabolism, virulence and patulin biosynthesis of P. expansum. The PeMetR regulator is crucial for normal germination and growth on inorganic S-sources but dispensable for utilization of organic S-sources. Accordingly, it is involved in regulating the expression of genes in sulfur assimilation pathway rather than methionine metabolic processes. Disruption of PeMetR resulted in a complete loss of virulence on various fruits. Additionally, the mutant showed a remarkably reduced ability to produce patulin. Exogenous methionine could partially or completely rescue the impaired phenotypes of the mutant. Inactivation of the sulfur assimilation pathway genes, PesA, PesB, PesC, PesF, generated growth, virulence and patulin production defects similar to those of ΔPeMetR. Overall, our study provides evidence that PeMetR-mediated sulfur assimilation is essential for growth and infection and shows for the first time that regulation of sulfur assimilation affects biosynthesis of an important mycotoxin patulin in P. expansum.

Transcription factor control of virulence in phytopathogenic fungi

Plant-pathogenic fungi are a significant threat to economic and food security worldwide. Novel protection strategies are required and therefore it is critical we understand the mechanisms by which these pathogens cause disease. Virulence factors and pathogenicity genes have been identified, but in many cases their roles remain elusive. It is becoming increasingly clear that gene regulation is vital to enable plant infection and transcription factors play an essential role. Efforts to determine their regulatory functions in plant-pathogenic fungi have expanded since the annotation of fungal genomes revealed the ubiquity of transcription factors from a broad range of families. This review establishes the significance of transcription factors as regulatory elements in plant-pathogenic fungi and provides a systematic overview of those that have been functionally characterized. Detailed analysis is provided on regulators from well-characterized families controlling various aspects of fungal metabolism, development, stress tolerance, and the production of virulence factors such as effectors and secondary metabolites. This covers conserved transcription factors with either specialized or nonspecialized roles, as well as recently identified regulators targeting key virulence pathways. Fundamental knowledge of transcription factor regulation in plant-pathogenic fungi provides avenues to identify novel virulence factors and improve our understanding of the regulatory networks linked to pathogen evolution, while transcription factors can themselves be specifically targeted for disease control. Areas requiring further insight regarding the molecular mechanisms and/or specific classes of transcription factors are identified, and direction for future investigation is presented.

MaAreB, a GATA Transcription Factor, Is Involved in Nitrogen Source Utilization, Stress Tolerances and Virulence in Metarhizium acridum

The nitrogen catabolite repression (NCR) pathway is involved in nitrogen utilization, in which the global GATA transcription factor AreA plays an indispensable role and has been reported in many fungi. However, relatively few studies are focused on AreB, another GATA transcription factor in the NCR pathway and the functions of AreB are largely unknown in entomopathogenic fungi. Here, we characterized MaAreB in the model entomopathogenic fungus Metarhizium acridum. Sequence arrangement found that MaAreB had a conserved GATA zinc finger DNA binding domain and a leucine zipper domain. Disruption of MaAreB affected the nitrogen utilization and led to decelerated conidial germination and hyphal growth, decreased conidial yield, and lower tolerances to UV-B irradiation and heat-shock. Furthermore, the MaAreB mutant (ΔMaAreB) exhibited increased sensitivity to CFW (Calcofluor white), decreased cell wall contents (chitin and β-1,3-glucan) and reduced expression levels of some genes related to cell wall integrity, indicating that disruption of MaAreB affected the cell wall integrity. Bioassays showed that the virulence of the ΔMaAreB strain was decreased in topical inoculation but not in intra-hemocoel injection. Consistently, deletion of MaAreB severely impaired the appressorium formation and reduced the turgor pressure of appressorium. These results revealed that MaAreB regulated fungal nitrogen utilization, cell wall integrity and biological control potential, which would contribute to the functional characterization of AreB homologous proteins in other insect fungal pathogens, and even filamentous fungi.

The relationship between features enabling SDHI fungicide binding to the Sc-Sdh complex and its inhibitory activity against Sclerotinia sclerotiorum

BACKGROUND

A new generation of succinate dehydrogenase inhibitors (SDHIs) with high efficiency and broad-spectrum antifungal activity has been frequently used in crop production. Sclerotinia stem rot is a major disease of various plants and crops caused by Sclerotinia sclerotiorum. Although benzovindiflupyr and isopyrazam reportedly have high activity against S. sclerotiorum, little is known about the bioactivity of different SDHIs classes against S. sclerotiorum or the mechanism of their differential antifungal activity.

RESULTS

The in vitro tests revealed that the pyrazole-4-carboxamides of SDHIs (benzovindiflupyr, isopyrazam, fluxapyroxad, pydiflumetofen) had the highest activity against S. sclerotiorum followed by pyridine carboxamides (boscalid), pyridinyl-ethyl benzamides (fluopyram) and thiazole carboxamides (thifluzamide), and of these thifluzamide showed poor antifungal activity with EC₅₀ values greater than 6.01 mg L^-1 . The pyrazole-4-carboxamides of SDHIs showed satisfactory protective and curative activity against Sclerotinia stem rot. After treatment with the pyrazole-4-carboxamides of SDHIs, mitochondrial function in S. sclerotiorum decreased significantly. The enzyme activity assays revealed a lower affinity between thifluzamide and the Sc-Sdh complex than was observed for the other six fungicides, with IC₅₀ values ranging from 0.0036 to 1.2088 μmol L^-1 . Additionally, the docking positions of fungicides were similar, yet binding energies were different in the docking study with the Sdh complex. The correspondingly weaker hydrogen bonds may be responsible for the poor activity of thifluzamide against S. sclerotiorum.

CONCLUSION

Understanding different binding features of various SDHIs classes with the Sc-Sdh complex might be beneficial for the design and development of highly effective broad-spectrum fungicides to ensure high yield and quality in crops by reducing fungicide use. © 2020 Society of Chemical Industry.

The bZIP transcription factor VdAtf1 regulates virulence by mediating nitrogen metabolism in Verticillium dahliae

The fungus Verticillium dahliae causes vascular wilt disease on hundreds of plant species. Homologs of the bZIP transcription factor Atf1 are required for virulence in most pathogenic fungi, but the molecular basis for their involvement is largely unknown. We performed targeted gene deletion, expression analysis, biochemistry and pathogenicity assays to demonstrate that VdAtf1 governs pathogenesis via the regulation of nitrosative resistance and nitrogen metabolism in V. dahliae. VdAtf1 controls pathogenesis via the regulation of nitric oxide (NO) resistance and inorganic nitrogen metabolism rather than oxidative resistance and is important for penetration peg formation in V. dahliae. VdAtf1 affects ammonium and nitrate assimilation in response to various nitrogen sources. VdAtf1 may be involved in regulating the expression of VdNut1. VdAtf1 responds to NO stress by strengthening the fungal cell wall, and by causing over-accumulation of methylglyoxal and glycerol, which in turn impacts NO detoxification. We also verified that the VdAtf1 ortholog in Fusarium graminearum mediates nitrogen metabolism, suggesting conservation of this function in related plant pathogenic fungi. Our findings revealed new functions of VdAtf1 in pathogenesis, response to nitrosative stress and nitrogen metabolism in V. dahliae. The results provide novel insights into the regulatory mechanisms of the transcription factor VdAtf1 in virulence.

Regulation of Morphology, Aflatoxin Production, and Virulence of Aspergillus flavus by the Major Nitrogen Regulatory Gene areA

Aspergillus flavus is a renowned plant, animal and human pathogen. areA is a global nitrogen regulatory gene of the GATA transcription factor family, shown to be the major nitrogen regulator. In this study, we identified areA in A. flavus and studied its function. The AreA protein contained a signatory zinc finger domain, which is extremely conserved across fungal species. Gene deletion (ΔareA) and over-expression (OE::areA) strains were constructed by homologous recombination to elucidate the role of areA in A. flavus. The ΔareA strain was unable to efficiently utilize secondary nitrogen sources for growth of A. flavus, and it had poorly developed conidiophores, when observed on complete medium, resulting in the production of significantly less conidia than the wild-type strain (WT). Aflatoxin B1 (AFB1) production was reduced in ΔareA compared with the WT strain in most conditions tested, and ΔareA had impaired virulence in peanut seeds. areA also played important roles in the sensitivity of A. flavus to osmotic, cell wall and oxidative stresses. Hence, areA was found to be important for the growth, aflatoxin production and pathogenicity of A. flavus. This work sheds light on the function of areA in the regulation of the nitrogen metabolism of A. flavus, and consequently aims at providing new ways for controlling the crossover pathogen, A. flavus.

The GATA-Type Transcriptional Factor Are1 Modulates the Expression of Extracellular Proteases and Cellulases in Trichoderma reesei

Trichoderma reesei is a biotechnologically important filamentous fungus with the remarkable ability to secrete large amounts of enzymes, whose production is strongly affected by both the carbon and nitrogen sources. While the carbon metabolism regulators are extensively studied, the regulation of enzyme production by the nitrogen metabolism regulators is still poorly understood. In this study, the GATA transcription factor Are1, which is an orthologue of the Aspergillus global nitrogen regulator AREA, was identified and characterized for its functions in regulation of both protease and cellulase production in T. reesei. Deletion of the are1 gene abolished the capability to secrete proteases, and complementation of the are1 gene rescued the ability to produce proteases. Quantitative RT-PCR analysis revealed that the transcripts of protease genes apw1 and apw2 were also significantly reduced in the Δare1 strain when grown in the medium with peptone as the nitrogen source. In addition, deletion of are1 resulted in decreased cellulase production in the presence of (NH₄)₂SO₄. Consistent with the reduction of cellulase production, the transcription levels of the major cellulase genes, including cbh1, cbh2, egl1, and egl2, were dramatically decreased in Δare1. Sequence analysis showed that all promoter regions of the tested protease and cellulase genes contain the consensus GATA elements. However, the expression levels of the major cellulase transcription activator Xyr1 and the repressor Cre1 had no significant difference between Δare1 and the parental strain QM9414, indicating that the regulatory mechanism deserves further investigation. Taken together, these results demonstrate the important role of Are1 in the regulation of protease and cellulase production in T. reesei, although these processes depend on the kind of nitrogen sources. The findings in this study contribute to the understanding of the regulation network of carbon and nitrogen sources in filamentous fungi.

Genome-wide identification and analysis of the GATA transcription factor gene family in Ustilaginoidea virens

In filamentous fungi, the conserved transcription factors play important roles in multiple cellular and developmental processes. The GATA proteins, a family of GATA-binding zinc finger transcription factors, play diverse functions in fungi. Ustilaginoidea virens is an economically important pathogen-causing rice false smut worldwide. To gain additional insight into the cellular and molecular mechanisms of this pathogen, in this study, we identified and functionally characterized seven GATA proteins from the U. virens genome (UvGATA). Sequences analysis indicated that these GATA proteins are divided into seven clades. The proteins in each clade contained conserved clade-specific sequences and structures, thus leading to the same motif serving different purposes in various contexts. The expression profiles of UvGATA genes at different infection stages and under H₂O₂ stress were detected. Results showed that the majority of UvGATA genes performed functions at both processes, thereby confirming the roles of these genes in pathogenicity and reactive oxygen species stress tolerance. This study provided an important starting point to further explore the biological functions of UvGATA genes and increased our understanding of their potential transcriptional regulatory mechanisms in U. virens.

CgGCS, Encoding a Glucosylceramide Synthase, Is Required for Growth, Conidiation and Pathogenicity in Colletotrichum gloeosporioides

Fungal glucosylceramide plays important role in cell division, hyphal formation and growth, spore germination and the modulation of virulence and has recently been considered as target for small molecule inhibitors. In this study, we characterized CgGCS, a protein encoding a glucosylceramide synthase (GCS) in Colletotrichum gloeosporioides. Disruption of CgGCS resulted in a severe reduction of mycelial growth and defects in conidiogenesis. Sphingolipid profile analysis revealed large decreases in glucosylceramide production in the mutant strains. Pathogenicity assays indicated that the ability of the ΔCgGCS mutants to invade both tomato and mango hosts was almost lost. In addition, the expression levels of many genes, especially those related to metabolism, were shown to be affected by the mutation of CgGCS via transcriptome analysis. Overall, our results demonstrate that C. gloeosporioides glucosylceramide is an important regulatory factor in fungal growth, conidiation, and pathogenesis in hosts.

Colletotrichum higginsianum as a Model for Understanding Host⁻Pathogen Interactions: A Review

Colletotrichum higginsianum is a hemibiotrophic ascomycetous fungus that causes economically important anthracnose diseases on numerous monocot and dicot crops worldwide. As a model pathosystem, the Colletotrichum⁻Arabidopsis interaction has the significant advantage that both organisms can be manipulated genetically. The goal of this review is to provide an overview of the system and to point out recent significant studies that update our understanding of the pathogenesis of C. higginsianum and resistance mechanisms of Arabidopsis against this hemibiotrophic fungus. The genome sequence of C. higginsianum has provided insights into how genome structure and pathogen genetic variability has been shaped by transposable elements, and allows systematic approaches to longstanding areas of investigation, including infection structure differentiation and fungal⁻plant interactions. The Arabidopsis-Colletotrichum pathosystem provides an integrated system, with extensive information on the host plant and availability of genomes for both partners, to illustrate many of the important concepts governing fungal⁻plant interactions, and to serve as an excellent starting point for broad perspectives into issues in plant pathology.