The function and properties of the transcriptional regulator COS1 in Magnaporthe oryzae

Involvement of the Cell Wall-Integrity Pathway in Signal Recognition, Cell-Wall Biosynthesis, and Virulence in Magnaporthe oryzae

The fungal cell wall is the first layer exposed to the external environment. The cell wall has key roles in regulating cell functions, such as cellular stability, permeability, and protection against stress. Understanding the structure of the cell wall and the mechanism of its biogenesis is important for the study of fungi. Highly conserved in fungi, including Magnaporthe oryzae, the cell wall-integrity (CWI) pathway is the primary signaling cascade regulating cell-wall structure and function. The CWI pathway has been demonstrated to correlate with pathogenicity in many phytopathogenic fungi. In the synthesis of the cell wall, the CWI pathway cooperates with multiple signaling pathways to regulate cell morphogenesis and secondary metabolism. Many questions have arisen regarding the cooperation of different signaling pathways with the CWI pathway in regulating cell-wall synthesis and pathogenicity. In this review, we summarized the latest advances in the M. oryzae CWI pathway and cell-wall structure. We discussed the CWI pathway components and their involvement in different aspects, such as virulence factors, the possibility of the pathway as a target for antifungal therapies, and crosstalk with other signaling pathways. This information will aid in better understanding the universal functions of the CWI pathway in regulating cell-wall synthesis and pathogenicity in M. oryzae. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

The Fusarium oxysporum species complex (FOSC) includes both plant and human pathogens that cause devastating plant vascular wilt diseases and threaten public health. Each F. oxysporum genome comprises core chromosomes (CCs) for housekeeping functions and accessory chromosomes (ACs) that contribute to host-specific adaptation. This study inspects global transcription factor profiles (TFomes) and their potential roles in coordinating CC and AC functions to accomplish host-specific interactions. Remarkably, we found a clear positive correlation between the sizes of TFomes and the proteomes of an organism. With the acquisition of ACs, the FOSC TFomes were larger than the other fungal genomes included in this study. Among a total of 48 classified TF families, 14 families involved in transcription/translation regulations and cell cycle controls were highly conserved. Among the 30 FOSC expanded families, Zn2-C6 and Znf_C2H2 were most significantly expanded to 671 and 167 genes per family including well-characterized homologs of Ftf1 (Zn2-C6) and PacC (Znf_C2H2) that are involved in host-specific interactions. Manual curation of characterized TFs increased the TFome repertoires by 3% including a disordered protein Ren1. RNA-Seq revealed a steady pattern of expression for conserved TF families and specific activation for AC TFs. Functional characterization of these TFs could enhance our understanding of transcriptional regulation involved in FOSC cross-kingdom interactions, disentangle species-specific adaptation, and identify targets to combat diverse diseases caused by this group of fungal pathogens.

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

The Fusarium oxysporum species complex (FOSC) includes both plant and human pathogens that cause devastating plant vascular wilt diseases and threaten public health. Each F. oxysporum genome comprises core chromosomes (CCs) for housekeeping functions and accessory chromosomes (ACs) that contribute to host-specific adaptation. This study inspected global transcription factor profiles (TFomes) and their potential roles in coordinating CCs and ACs functions to accomplish host-specific pathogenicity. Remarkably, we found a clear positive correlation between the sizes of TFome and proteome of an organism, and FOSC TFomes are larger due to the acquisition of ACs. Among a total of 48 classified TF families, 14 families involved in transcription/translation regulations and cell cycle controls are highly conserved. Among 30 FOSC expanded families, Zn2-C6 and Znf_C2H2 are most significantly expanded to 671 and 167 genes per family, including well-characterized homologs of Ftf1 (Zn2-C6) and PacC (Znf_C2H2) involved in host-specific interactions. Manual curation of characterized TFs increased the TFome repertoires by 3%, including a disordered protein Ren1. Expression profiles revealed a steady expression of conserved TF families and specific activation of AC TFs. Functional characterization of these TFs could enhance our understanding of transcriptional regulation involved in FOSC cross-kingdom interactions, disentangle species-specific adaptation, and identify targets to combat diverse diseases caused by this group of fungal pathogens.

The CfMcm1 Regulates Pathogenicity, Conidium Germination, and Sexual Development in Colletotrichum fructicola

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola, is a severe disease worldwide on apple, causing defoliation, leaf and fruit spot, and substantial yield loss. However, little is known about its molecular mechanisms of pathogenesis. Previous transcriptome analysis revealed that a transcription factor, CfMcm1, was induced during leaf infection. In the present work, expression pattern analysis verified that the CfMcm1 gene was strongly expressed in conidia and early infection. Phenotypic analysis revealed that the gene deletion mutant ΔCfMcm1 lost pathogenicity to apple leaves by inhibiting conidial germination and appressorium formation. In addition to appressorium-mediated pathogenicity, ΔCfMcm1 colonization and hyphal extension in wounded apple fruit was also reduced, and conidial germination mode and conidial color were altered. ΔCfMcm1 displayed impairment of cell wall integrity and response to stress caused by oxidation, osmosis, and an acid environment. Furthermore, the deletion mutant produced fewer and smaller perithecia and no ascospores. In contrast, melanin deposition in mycelia of ΔCfMcm1 was strengthened. Further comparative transcriptome and quantitative PCR analysis revealed that CfMcm1 modulated expression of genes related to conidial development (CfERG5A, CfERG5B, CfHik5, and CfAbaA), appressorium formation (CfCBP1 and CfCHS7), pectin degradation (CfPelA and CfPelB), sexual development (CfMYB, CfFork, CfHMG, and CfMAT1-2-1), and melanin biosynthesis (CfCmr1, CfPKS1, CfT4HR1, CfTHR1, and CfSCD1). Our results demonstrated that CfMcm1 is a pivotal regulator possessing multiple functions in pathogenicity, asexual and sexual reproduction, and melanin biosynthesis.

Characterization of BbKlf1 as a novel transcription factor vital for asexual and infection cycles of Beauveria bassiana

The large family of C2H2-type zinc finger transcription factors (TFs) comprise the Kruppel-like factors (KLFs) that evolved relatively late in eukaryotes but remain unexplored in filamentous fungi. Here, we report that an orthologue (BbKlf1) of yeast Klf1 mediating cell wall integrity (CWI) is a wide-spectrum TF evidently localized in nucleus and cytoplasm in Beauveria bassiana. BbKlf1 features conserved domains and multiple DNA-binding motifs predicted to bind multiple promoter DNA fragments of target genes across asexual developmental and stress-responsive pathways. Despite limited impact on normal colony growth, deletion of Bbklf1 resulted in impaired CWI and hypersensitivity to Congo red-induced cell wall stress. Also, the deletion mutant was severely compromised in tolerance to oxidative and osmotic stresses, hyphal septation and differentiation, conidiation capacity (reduced by 95%), conidial quality (viability and hydrocarbon epitope pattern) and virulence. Importantly, these phenotypes correlated well with sharply repressed or nearly abolished expressions of those genes required for or involved in chitin biosynthesis, antioxidant activity, cell division and differentiation, aerial conidiation and conidial maturation. These findings indicate an essentiality of BbKlf1 for the asexual and insect-pathogenic lifecycles of B. bassiana and a novel scenario much beyond the yeast orthologue-mediated CWI, suggesting important roles of its orthologues in filamentous fungi.

Melanin Promotes Spore Production in the Rice Blast Fungus Magnaporthe oryzae

The rice blast pathogen, Magnaporthe oryzae, spreads through spores and invades rice through appressoria. Melanin is necessary for an appressorium to penetrate plant cells, but there are many unknown aspects of its role in fungal conidiation. In this study, we confirmed that melanin promotes spore production in M. oryzae, and that this effect is related to the background melanin content of wild-type strains. In the wild-type 70-15 strain with low melanin content of aerial hyphae, increased melanin synthesis promoted sporulation. In contrast, increased melanin synthesis in the wild-type Guy11 strain, which has higher melanin content, did not promote sporulation. The transcription factor Cnf1 (conidial production negative regulatory factor 1), which negatively regulates melanin synthesis, has opposite effects in conidiophore differentiation of Guy11 and 70-15. Deletion of CNF1 did not abolish the defects of Δcos1 and Δhox2 (where COS1/conidiophore stalk-less 1 or HOX2/homeodomain protein 2 was deleted) in conidiation, while increased the conidiation of Δgcc1 and Δgcf3 (where GCC1/growth, conidiation and cell wall regulatory factor 1, or GCF3/growth and conidiation regulatory factor 3 was deleted). Pig1 (pigment of Magnaporthe 1) regulates the melanin synthesis of hyphae but not of conidiophores, spores, or appressoria. Deletion of the same gene in different wild-type strains can lead to different phenotypes, partly because of differences in melanin content between fungal strains. Overall, this study reveals the functional diversity and complexity of melanin in different M. oryzae strains.

Casein Kinase 2 Mediates Degradation of Transcription Factor Pcf1 during Appressorium Formation in the Rice Blast Fungus

The appressorium is a specialized structure that is differentiated from Magnaporthe oryzae spores that can infect host cells. In the process of cellular transformation from spore to appressorium, the contents inside the spores are transferred into appressoria, accompanied by major differences in the gene expression model. In this study, we reported a transcription factor (TF), Pcf1, which was depressed at the transcription level and degraded at the protein level in nuclei of incipient appressoria at four hpi (hours post inoculation). To investigate its degradation mechanism, the interacting proteins of Pcf1 were identified using an immunoprecipitation-mass spectrometry (IP-MS) assay. Yeast two-hybrid (Y2H) and co-IP (co-immunoprecipitation) assays confirmed that Pcf1 interacted with the casein kinase 2 (CK2) holoenzyme through direct combination with the CKb2 subunit. Moreover, Pcf1 was ubiquitinated in the hyphae. These changes in Pcf1 protein levels in nuclei provide a new clue of how TFs are degraded during appressorium formation: temporarily unnecessary TFs in spores are phosphorylated through interacting with CK2 enzyme and are then ubiquitinated and digested by the ubiquitin proteasome system (UPS).

Role refinement of melanin synthesis genes by gene knockout reveals their functional diversity in Pyricularia oryzae strains

Pyricularia oryzae is a plant pathogenic fungus that severely affects rice production. Past studies, primarily using mutants generated by spontaneous mutations or artificial physical and chemical mutagenesis, have determined that melanin is required for appressorium turgor, penetration, and virulence of P. oryzae. However, these roles need to be verified by gene knockout and/or overexpression in different strains considering the potential differences in the level of virulence. Here, we confirmed the indispensable roles of melanin in the development and virulence of P. oryzae by knocking out and over-expressing three melanin synthesis genes (ALB1, RSY1, and BUF1) in two wild-type strains (Guy11 and 70-15). Deletion of ALB1, RSY1, or BUF1 led to loss of melanin and virulence in both strains. ALB1, RSY1 and BUF1 in Guy11, and BUF1 in 70-15 were required for conidiation, respectively. ALB1, RSY1, and BUF1 were required for conidial resistance to environmental stresses (UV exposure, oxidization, and freezing damage) in both strains. Guy11 cells had greater amounts of melanin and more transcripts of melanin synthesis genes than 70-15 cells. Paired culture experiments between the deletion or over-expression mutants of melanin synthesis genes suggested that the reaction catalyzed by Buf1, but not Alb1 and Rsy1, was likely a rate-limiting step in melanin biosynthesis in 70-15. These results expand our understanding on melanin and its synthesis genes in P. oryzae as well as its responses to biotic and abiotic environments.

Synergistic deletion of RGS1 and COS1 may reduce the pathogenicity of Magnaporthe oryzae

Rice blast, caused by Magnaporthe oryzae, is a serious threat to global rice production. In recent years, many pathogenicity genes of M. oryzae have been identified, although most of their functions remain unknown. In this study, we report the synergistic deletion of RGS1 and COS1 that may reduce the pathogenicity of M. oryzae. The investigation involved comparing ΔMorgs1, ΔMocos1, and ΔMorgs1/ΔMocos1 mutants. The ΔMorgs1/ΔMocos1 mutant showed a weak reduction in vegetative growth, and the colonies displayed fewer and smoother aerial hyphae. The ΔMorgs1/ΔMocos1 mutant exhibited delayed appressorium-like structure formation and 'low pathogenicity' on detached rice seedling leaves when compared with ΔMocos1. Moreover, the melanin content of the single and double mutants was remarkably lower than that of the WT type. Thus, our results indicate that the synergy between RGS1 and COS1 may be crucial in the pathogenicity of M. oryzae.

Glycerol-3-Phosphate Shuttle Is Involved in Development and Virulence in the Rice Blast Fungus Pyricularia oryzae

The glycerol-3-phosphate (G-3-P) shuttle is an important pathway for delivery of cytosolic reducing equivalents into mitochondrial oxidative phosphorylation, and plays essential physiological roles in yeast, plants, and animals. However, its role has been unclear in filamentous and pathogenic fungi. Here, we characterize the function of the G-3-P shuttle in Pyricularia oryzae by genetic and molecular analyses. In P. oryzae, a glycerol-3-phosphate dehydrogenase 1 (PoGpd1) is involved in NO production, conidiation, and utilization of several carbon sources (pyruvate, sodium acetate, glutamate, and glutamine). A glycerol-3-phosphate dehydrogenase 2 (PoGpd2) is essential for glycerol utilization and fungal development. Deletion of PoGPD2 led to delayed aerial hyphal formation, accelerated aerial hyphal collapse, and reduced conidiation on complete medium (CM) under a light-dark cycle. Aerial mycelial surface hydrophobicity to water and Tween 20 was decreased in ΔPogpd2. Melanin synthesis genes required for cell wall construction and two transcription factor genes (COS1 and CONx2) required for conidiation and/or aerial hyphal differentiation were down-regulated in the aerial mycelia of ΔPogpd2 and ΔPogpd1. Culturing under continuous dark could complement the defects of aerial hyphal differentiation of ΔPogpd2 observed in a light-dark cycle. Two light-sensitive protein genes (PoSIR2 encoding an NAD⁺-dependent deacetylase and TRX2 encoding a thioredoxin 2) were up-regulated in ΔPogpd2 cultured on CM medium in a light-dark cycle. ΔPogpd2 showed an increased intracellular NAD⁺/NADH ratio and total NAD content, and alteration of intracellular ATP production. Culturing on minimal medium also could restore aerial hyphal differentiation of ΔPogpd2, which is deficient on CM medium in a light-dark cycle. Two glutamate synthesis genes, GDH1 and PoGLT1, which synthesize glutamate coupled with oxidation of NADH to NAD⁺, were significantly up-regulated in ΔPogpd2 in a light-dark cycle. Moreover, deletion of PoGpd1 or PoGpd2 led to reduced virulence of conidia or hyphae on rice. The glycerol-3-phosphate shuttle is involved in cellular redox, fungal development, and virulence in P. oryzae.

Characterization of 47 Cys2 -His2 zinc finger proteins required for the development and pathogenicity of the rice blast fungus Magnaporthe oryzae

The Cys2 -His2 (C2H2) zinc finger protein family is the second-largest family of transcription factors (TFs) in Magnaporthe oryzae, the causal fungus responsible for the destructive rice blast disease. However, little is known about the roles of most C2H2 TFs in the development and pathogenicity of M. oryzae. The roles of 47 C2H2 genes in development and pathogenicity were investigated by gene deletion in M. oryzae. The TF-dependent genes in mycelia or appressoria were analyzed with RNA sequencing and quantitative PCR (qPCR). Forty-four C2H2 genes are involved in growth (20 genes), conidiation (28 genes), appressorium formation (four genes) and pathogenicity (22 genes) in M. oryzae. Of these, MGG_14931, named as VRF1, is required for pathogenicity, specifically controlling appressorium maturation by affecting the expression of genes related to appressorial structure and function, including melanin biosynthesis, chitin catabolism, lipid metabolism, proteolysis, transmembrane transport, and response to oxidative stress; MGG_01776, named as VRF2, is required for plant penetration and invasive growth; conidiation-related gene CON7 is required for conidial differentiation; and MoCREA, encoding a carbon catabolite repression protein, is a novel repressor of lipid catabolism when glucose obtainable in M. oryzae. This study provides many insights into the regulation of growth, asexual development, appressorium formation, and pathogenicity by C2H2 TFs in M. oryzae.

Light governs asexual differentiation in the grey mould fungus Botrytis cinerea via the putative transcription factor BcLTF2

Botrytis cinerea is a plant pathogenic fungus known for its utilization of light as environmental cue to regulate asexual differentiation: conidia are formed in the light, while sclerotia are formed in the dark. As no orthologues of known regulators of conidiation (e.g., Aspergillus nidulans BrlA, Neurospora crassa FL) exist in the Leotiomycetes, we initiated a de novo approach to identify the functional counterpart in B. cinerea. The search revealed the light-responsive C2H2 transcription factor BcLTF2 whose expression - usually restricted to light conditions - is necessary and sufficient to induce conidiation and simultaneously to suppress sclerotial development. Light-induced expression of bcltf2 is mediated via a so far unknown pathway, and is attenuated in a (blue) light-dependent fashion by the White Collar complex, BcLTF1 and the VELVET complex. Mutation of either component leads to increased bcltf2 expression and causes light-independent conidiation (always conidia phenotype). Hence, the tight regulation of bcltf2 governs the balance between vegetative growth that allows for the colonization of the substrate and subsequent reproduction via conidia in the light. The orthologue ssltf2 in the closely related species Sclerotinia sclerotiorum is not significantly expressed suggesting that its deregulation may cause the lack of the conidiation program in this fungus.