Fusarium verticillioides GAP1, a gene encoding a putative glycolipid-anchored surface protein, participates in conidiation and cell wall structure but not virulence

Genetic Characteristics and Metabolic Interactions between Pseudocercospora fijiensis and Banana: Progress toward Controlling Black Sigatoka

The international importance of banana and severity of black Sigatoka disease have led to extensive investigations into the genetic characteristics and metabolic interactions between the Dothideomycete Pseudocercospora fijiensis and its banana host. P. fijiensis was shown to have a greatly expanded genome compared to other Dothideomycetes, due to the proliferation of retrotransposons. Genome analysis suggests the presence of dispensable chromosomes that may aid in fungal adaptation as well as pathogenicity. Genomic research has led to the characterization of genes and metabolic pathways involved in pathogenicity, including: secondary metabolism genes such as PKS10-2, genes for mitogen-activated protein kinases such as Fus3 and Slt2, and genes for cell wall proteins such as glucosyl phosphatidylinositol (GPI) and glycophospholipid surface (Gas) proteins. Studies conducted on resistance mechanisms in banana have documented the role of jasmonic acid and ethylene pathways. With the development of banana transformation protocols, strategies for engineering resistance include transgenes expressing antimicrobial peptides or hydrolytic enzymes as well as host-induced gene silencing (HIGS) targeting pathogenicity genes. Pseudocercospora fijiensis has been identified as having high evolutionary potential, given its large genome size, ability to reproduce both sexually and asexually, and long-distance spore dispersal. Thus, multiple control measures are needed for the sustainable control of black Sigatoka disease.

FvmnSOD is involved in oxidative stress defence, mitochondrial stability and apoptosis prevention in Fusarium verticillioides

Superoxide dismutases are key enzymes in elimination of the superoxide anion radical (O₂ ^•- ) generated intracellularly or by exogenous oxidative stress eliciting agents, like menadione. In this study, we investigated the physiological role of the manganese superoxide dismutase-encoding gene in Fusarium verticillioides via the construction of a gene deletion mutant, ΔFvmnSOD and comparing its phenotype with that of the wild-type parental strain and a ΔFvmnSOD' C strain, complemented with the functional manganese superoxide dismutase gene. Deletion of FvmnSOD had no effect on the relative intracellular superoxide ratio but increased the sensitivity of the fungus to menadione sodium bisulphite on Czapek-Dox stress agar plates. The lack of FvmnSOD caused changes in mitochondrial morphology and physiology: The volumetric ratio of these cell organelles in the second hyphal segment, as well as the total, the KCN-sensitive cytochrome c-dependent and the KCN+SHAM (salicylhidroxamic acid)-resistant residual respiration rates, were higher in the mutant as compared to the wild-type and the complemented strains. Nevertheless, changes in the respiration rates were attributable to the higher volumetric ratio of mitochondria found in the gene deletion mutant. Changes in the mitochondrial functions also brought about higher sensitivity to apoptotic cell death elicited by the Penicillium chrysogenum antifungal protein. The gene deletion mutant developed significantly thinner hyphae in comparison to the wild-type strain. Deletion of FvmnSOD had no effect on fumonisin B₁ and B₂ production of the fungus grown in Myro medium as a static culture.

FvatfA regulates growth, stress tolerance as well as mycotoxin and pigment productions in Fusarium verticillioides

FvatfA from the maize pathogen Fusarium verticillioides putatively encodes the Aspergillus nidulans AtfA and Schizasaccharomyces pombe Atf1 orthologous bZIP-type transcription factor, FvAtfA. In this study, a ΔFvatfA deletion mutant was constructed and then genetically complemented with the fully functional FvatfA gene. Comparing phenotypic features of the wild-type parental, the deletion mutant and the restored strains shed light on the versatile regulatory functions played by FvAtfA in (i) the maintenance of vegetative growth on Czapek-Dox and Potato Dextrose agars and invasive growth on unwounded tomato fruits, (ii) the preservation of conidiospore yield and size, (iii) the orchestration of oxidative (H₂O₂, menadione sodium bisulphite) and cell wall integrity (Congo Red) stress defences and (iv) the regulation of mycotoxin (fumonisins) and pigment (bikaverin, carotenoid) productions. Expression of selected biosynthetic genes both in the fumonisin (fum1, fum8) and the carotenoid (carRA, carB) pathways were down-regulated in the ΔFvatfA strain resulting in defected fumonisin production and considerably decreased carotenoid yields. The expression of bik1, encoding the polyketide synthase needed in bikaverin biosynthesis, was not up-regulated by the deletion of FvatfA meanwhile the ΔFvatfA strain produced approximately ten times more bikaverin than the wild-type or the genetically complemented strains. The abolishment of fumonisin production of the ΔFvatfA strain may lead to the development of new-type, biology-based mycotoxin control strategies. The novel information gained on the regulation of pigment production by this fungus can be interesting for experts working on new, Fusarium-based biomass and pigment production technologies.

Key points • FvatfA regulates vegetative and invasive growths of F. verticillioides. • FvatfA also orchestrates oxidative and cell wall integrity stress defenses. • The ΔFvatfA mutant was deficient in fumonisin production. • FvatfA deletion resulted in decreased carotenoid and increased bikaverin yields.

FvSTUA is a Key Regulator of Sporulation, Toxin Synthesis, and Virulence in Fusarium verticillioides

Fusarium verticillioides is one of the most important pathogens of maize, causing rot and producing fumonisin mycotoxins during infection. Ingestion of fumonisin-contaminated corn causes underperformance and even fatal toxicity in livestock and is associated with neural tube birth defects, growth stunting in children, and some cancers. StuA, an APSES-class transcription factor, is a major developmental transcriptional regulator in fungi. It has been shown to regulate crucial developmental processes, such as sporulation, virulence, and mycotoxin synthesis among others. In this study, the role of FvSTUA in F. verticillioides was examined by characterizing ∆FvstuA deletion mutants functionally and transcriptomally. The deletion mutants exhibited reduced vegetative growth, stunted aerial hyphae, and significant reductions in microconidiation. Macroconidiation and hydrophobicity of the deletion strains were reduced as well. Additionally, fumonisin production and virulence of the deletion mutants were greatly reduced. Transcriptomic analysis revealed downregulation of expression of several genes in the fumonisin and fusarin C biosynthetic clusters and differential expression of genes involved in conidiation and virulence. Nuclear localization of FvSTUA supported its likely function as a transcription factor. Together, our results indicate that FvSTUA plays a global role in transcriptional regulation in F. verticillioides influencing morphogenesis, toxin production, and virulence.

Fusarium verticillioides SNARE protein FvSyn1 harbours two key functional motifs that play selective roles in fungal development and virulence

Fusarium verticillioides is one of the key fungal pathogens responsible for maize stalk rot. While stalk rot pathogens are prevalent worldwide, our understanding of the stalk rot virulence mechanism in pathogenic fungi is still very limited. We previously identified the F. verticillioides FvSYN1 gene, which was demonstrated to play an important role in maize stalk rot virulence. FvSyn1 belongs to a family of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that play critical roles in a variety of developmental processes. In this study, we further characterized the cellular features of the FvSyn1 protein, namely how different motifs contribute to development and virulence in F. verticillioides by generating motif-specific deletion mutants. Microscopic observation showed that the ∆Fvsyn1 mutant exhibits rough and hyper-branched hyphae when compared to the wild-type progenitor. Moreover, the ∆Fvsyn1 mutant was sensitive to cell wall stress agents, resulting in vegetative growth reduction. We showed that the FvSyn1::GFP protein is associated with the endomembrane, but this did not clarify why the deletion of FvSyn1 led to stress sensitivity and aberrant hyphal development. Characterization of the FvSyn1 domains indicated that both the syntaxin N-terminus (SynN) domain and the SNARE C-terminus domain play distinct roles in fungal development, but also function collectively in the context of virulence. We also determined that two domains in FvSyn1 are not required for fumonisin production. Interestingly, these two domains were involved in carbon nutrient utilization, including pectin, starch and sorbitol. This study further characterized the role of FvSyn1 domains in hyphal growth, cell wall stress response and virulence in F. verticillioides.

Characterizing co-expression networks underpinning maize stalk rot virulence in Fusarium verticillioides through computational subnetwork module analyses

Fusarium verticillioides is recognized as an important stalk rot pathogen of maize worldwide, but our knowledge of genetic mechanisms underpinning this pathosystem is limited. Previously, we identified a striatin-like protein Fsr1 that plays an important role in stalk rot. To further characterize transcriptome networks downstream of Fsr1, we performed next-generation sequencing (NGS) to investigate relative read abundance and also to infer co-expression networks utilizing the preprocessed expression data through partial correlation. We used a probabilistic pathway activity inference strategy to identify functional subnetwork modules likely involved in virulence. Each subnetwork modules consisted of multiple correlated genes with coordinated expression patterns, but the collective activation levels were significantly different in F. verticillioides wild type versus fsr1 mutant. We also identified putative hub genes from predicted subnetworks for functional validation and network robustness studies through mutagenesis, virulence and qPCR assays. Our results suggest that these genes are important virulence genes that regulate the expression of closely correlated genes, demonstrating that these are important hubs of their respective subnetworks. Lastly, we used key F. verticillioides virulence genes to computationally predict a subnetwork of maize genes that potentially respond to fungal genes by applying cointegration-correlation-expression strategy.

Histone H3 lysine 9 methyltransferase FvDim5 regulates fungal development, pathogenicity and osmotic stress responses in Fusarium verticillioides

Histone methylation plays important biological roles in eukaryotic cells. Methylation of lysine 9 at histone H3 (H3K9me) is critical for regulating chromatin structure and gene transcription. Dim5 is a lysine histone methyltransferase (KHMTase) enzyme, which is responsible for the methylation of H3K9 in eukaryotes. In the current study, we identified a single ortholog of Neurospora crassa Dim5 in Fusarium verticillioides. In this study, we report that FvDim5 regulates the trimethylation of H3K9 (H3K9me3). The FvDIM5 deletion mutant (ΔFvDim5) showed significant defects in conidiation, perithecium production and fungal virulence. Unexpectedly, we found that deletion of FvDIM5 resulted in increased tolerance to osmotic stresses and upregulated FvHog1 phosphorylation. These results indicate the importance of FvDim5 for the regulation of fungal development, pathogenicity and osmotic stress responses in F. verticillioides.

Fsr1, a striatin homologue, forms an endomembrane-associated complex that regulates virulence in the maize pathogen Fusarium verticillioides

Fsr1, a homologue of mammalian striatin, containing multiple protein-binding domains and a coiled-coil (CC) domain, is critical for Fusarium verticillioides virulence. In mammals, striatin interacts with multiple proteins to form a STRIPAK (striatin-interacting phosphatase and kinase) complex that regulates a variety of developmental processes and cellular mechanisms. In this study, we identified the homologue of a key mammalian STRIPAK component STRIP1/2 (striatin-interacting proteins 1 and 2) in F. verticillioides, FvStp1, which interacts with Fsr1 in vivo. Gene deletion analysis indicates that FvStp1 is critical for F. verticillioides stalk rot virulence. In addition, we identified three proteins, designated FvCyp1, FvScp1 and FvSel1, which interact with the Fsr1 CC domain via a yeast two-hybrid screen. Importantly, FvCyp1, FvScp1 and FvSel1 co-localize to endomembrane structures, each having a preferred localization in the cell, and they are all required for F. verticillioides stalk rot virulence. Moreover, these proteins are necessary for the correct localization of Fsr1 to the endoplasmic reticulum (ER) and nuclear envelope. Thus, we identified several novel components in the STRIPAK complex that regulates F. verticillioides virulence, and propose that the correct organization and localization of Fsr1 are critical for STRIPAK complex function.

Identification of membrane proteome of Paracoccidioides lutzii and its regulation by zinc

Aim:

During infection development in the host, Paracoccidioides spp. faces the deprivation of micronutrients, a mechanism called nutritional immunity. This condition induces the remodeling of proteins present in different metabolic pathways. Therefore, we attempted to identify membrane proteins and their regulation by zinc in Paracoccidioides lutzii.

Materials & methods:

Membranes enriched fraction of yeast cells of P. lutzii were isolated, purified and identified by 2D LC–MS/MS detection and database search.

Results & conclusion:

Zinc deprivation suppressed the expression of membrane proteins such as glycoproteins, those involved in cell wall synthesis and those related to oxidative phosphorylation. This is the first study describing membrane proteins and the effect of zinc deficiency in their regulation in one member of the genus Paracoccidioides.

The methodology of protein identification allows the characterization of biological processes performed by those molecules. Therefore, we performed a membrane proteomic analysis of Paracoccidioides lutzii and further evaluated the responses of the fungus to zinc deprivation. The results obtained in the work allowed the characterization of membrane proteins present in organelles that are related to different cellular functions. Zinc deprivation changes processes related to cellular physiology and metabolism. These results help us to understand the process of pathogen–host interaction, since zinc deprivation is a condition present during infection.

The novel fungal-specific gene FUG1 has a role in pathogenicity and fumonisin biosynthesis in Fusarium verticillioides

Fusarium verticillioides is a globally important pathogen of maize, capable of causing severe yield reductions and economic losses. In addition, F. verticillioides produces toxic secondary metabolites during kernel colonization that pose significant threats to human and animal health. Fusarium verticillioides and other plant-pathogenic fungi possess a large number of genes with no known or predicted function, some of which could encode novel virulence factors or antifungal targets. In this study, we identified and characterized the novel gene FUG1 (Fungal Unknown Gene 1) in F. verticillioides through functional genetics. Deletion of FUG1 impaired maize kernel colonization and fumonisin biosynthesis. In addition, deletion of FUG1 increased sensitivity to the antimicrobial compound 2-benzoxazolinone and to hydrogen peroxide, which indicates that FUG1 may play a role in mitigating stresses associated with host defence. Transcriptional profiling via RNA-sequencing (RNA-seq) identified numerous fungal genes that were differentially expressed in the kernel environment following the deletion of FUG1, including genes involved in secondary metabolism and mycelial development. Sequence analysis of the Fug1 protein provided evidence for nuclear localization, DNA binding and a domain of unknown function associated with previously characterized transcriptional regulators. This information, combined with the observed transcriptional reprogramming in the deletion mutant, suggests that FUG1 represents a novel class of fungal transcription factors or genes otherwise involved in signal transduction.

Variation in Growth, Colonization of Maize, and Metabolic Parameters of GFP- and DsRed-Labeled Fusarium verticillioides Strains

Autofluorescent proteins are frequently applied as visual markers in the labeling of filamentous fungi. Genes gfp and DsRed were transformed into the genome of Fusarium verticillioides via the Agrobacterium tumefaciens-mediated transformation method. The selected transformants displayed a bright green or red fluorescence in all the organelles of the growing fungal mycelia and spores (except for the vacuoles) both in cultures and in the maize (Zea mays) roots they colonized. The results of gene-specific polymerase chain reaction (PCR) analysis and the thermal asymmetrical interlaced (TAIL)-PCR analysis demonstrated that gfp and DsRed were integrated on different chromosomes of the fungus. Reductions in the colony growth on the plates at pH 4.0 and 5.5 was observed for the green fluorescent protein (GFP)-transformant G3 and the DsRed-transformant R4, but transformants G4 and R1 grew as well as the wild-type strain at pH 4.0. The speed of growth of all the transformants was similar to the wild-type strain at pH ≥ 7. The insertion of gfp and DsRed did not alter the production of extracellular enzymes and fumonisin B by F. verticillioides. The transformants expressing GFP and DsRed proteins were able to colonize maize roots. However, the four transformants examined produced fewer CFU in the root samples than the wild-type strain during a sampling period of 7 to 28 days after inoculation.

FvSO regulates vegetative hyphal fusion, asexual growth, fumonisin B1 production, and virulence in Fusarium verticillioides

Hyphal anastomosis is a hallmark of filamentous fungi and plays vital roles including cellular homoeostasis, interhyphal communication and nutrient translocation. Here we identify a gene, FvSO, in Fusarium verticillioides, a filamentous ascomycete causing maize ear and stalk rot and producing fumonisin mycotoxins. FvSO, like its Neurospora crassa homologue SO, is required for vegetative hyphal fusion. It is also essential for normal vegetative growth, sporulation, and pathogenesis. FvSO encodes a predicted WW domain protein and shares 70 % protein sequence identity with N. crassa SO. FvSO deletion mutants (ΔFvSO) had abnormal distribution of conidia size, and conidia of ΔFvSO germinated much later and slower than wild type. ΔFvSO was deficient in hyphal anastomosis, had slower radial growth and produced less fungal biomass than wild type. ΔFvSO were unable to perform anastomosis, a key feature of filamentous fungi. Interestingly, production of fumonisin B1 by ΔFvSO was significantly reduced compared to wild type. Additionally, ΔFvSO was nonpathogenic to corn ears, stalks and seedlings, likely due to defective growth and development. In conclusion, FvSO is essential for vegetative hyphal fusion and is required for normal vegetative growth and sporulation, normal levels of fumonisin production and pathogenicity in F. verticillioides. The pleiotropic nature of ΔFvSO phenotypes suggests that FvSO is likely involved in certain signalling pathways that regulate multiple cellular functions.

Combination of driselase and lysing enzyme in one molar potassium chloride is effective for the production of protoplasts from germinated conidia of Fusarium verticillioides

Various cell wall degrading enzymes and the protoplasting media were evaluated for the production of protoplast in Fusarium verticillioides. Among the various enzymes tested, driselase at 12.5 mg/ml in 1 M KCl protoplasting medium produced the maximum number of protoplast. Next to driselase, lysing enzyme at 10 mg/ml in 1.2 M MgSO4 protoplasting medium was found to be the second best enzyme for the production of protoplast. More interestingly, the combined use of driselase @ 12.5 mg/ml and lysing enzyme @ 10 mg/ml in 1 M KCl exhibited the additive effect on protoplast formation. Germinated conidia of F. verticillioides are the most susceptible fungal material for protoplast production. The use of sucrose at 1.2 M in the regeneration medium supported the maximum regeneration of protoplast. From the present study, we recommend driselase (12.5 mg/ml) and lysing enzyme (10 mg/ml) in 1 M KCl protoplasting medium and germinated conidia of F. verticillioides for the maximum production of protoplasts and 1.2 M sucrose is the best osmoticum for the regeneration of protoplasts.

Proteomics of Fusarium oxysporum race 1 and race 4 reveals enzymes involved in carbohydrate metabolism and ion transport that might play important roles in banana Fusarium wilt

Banana Fusarium wilt is a soil-spread fungal disease caused by Fusarium oxysporum. In China, the main virulence fungi in banana are F. oxysporum race 1 (F1, weak virulence) and race 4 (F4, strong virulence). To date, no proteomic analyses have compared the two races, but the difference in virulence between F1 and F4 might result from their differentially expressed proteins. Here we report the first comparative proteomics of F1 and F4 cultured under various conditions, and finally identify 99 protein species, which represent 59 unique proteins. These proteins are mainly involved in carbohydrate metabolism, post-translational modification, energy production, and inorganic ion transport. Bioinformatics analysis indicated that among the 46 proteins identified from F4 were several enzymes that might be important for virulence. Reverse transcription PCR analysis of the genes for 15 of the 56 proteins revealed that their transcriptional patterns were similar to their protein expression patterns. Taken together, these data suggest that proteins involved in carbohydrate metabolism and ion transport may be important in the pathogenesis of banana Fusarium wilt. Some enzymes such as catalase-peroxidase, galactosidase and chitinase might contribute to the strong virulence of F4. Overexpression or knockout of the genes for the F4-specific proteins will help us to further understand the molecular mechanism of Fusarium-induced banana wilt.

Functional characterization of the gene FoOCH1 encoding a putative α-1,6-mannosyltransferase in Fusarium oxysporum f. sp. cubense

Fusarium oxysporum f. sp. cubense (FOC) is the causal agent of banana Fusarium wilt and has become one of the most destructive pathogens threatening the banana production worldwide. However, few genes related to morphogenesis and pathogenicity of this fungal pathogen have been functionally characterized. In this study, we identified and characterized the disrupted gene in a T-DNA insertional mutant (L953) of FOC with significantly reduced virulence on banana plants. The gene disrupted by T-DNA insertion in L953 harbors an open reading frame, which encodes a protein with homology to α-1,6-mannosyltransferase (OCH1) in fungi. The deletion mutants (ΔFoOCH1) of the OCH1 orthologue (FoOCH1) in FOC were impaired in fungal growth, exhibited brighter staining with fluorescein isothiocyanate (FITC)-Concanavalin A, had less cell wall proteins and secreted more proteins into liquid media than the wild type. Furthermore, the mutation or deletion of FoOCH1 led to loss of ability to penetrate cellophane membrane and decline in hyphal attachment and colonization as well as virulence to the banana host. The mutant phenotypes were fully restored by complementation with the wild type FoOCH1 gene. Our data provide a first evidence for the critical role of FoOCH1 in maintenance of cell wall integrity and virulence of F. oxysporum f. sp. cubense.

The N-terminus region of the putative C2H2 transcription factor Ada1 harbors a species-specific activation motif that regulates asexual reproduction in Fusarium verticillioides

Fusarium verticillioides is an important plant pathogenic fungus causing maize ear and stalk rots. In addition, the fungus is directly associated with fumonisin contamination of food and feeds. Here, we report the functional characterization of Ada1, a putative Cys2-His2 zinc finger transcription factor with a high level of similarity to Aspergillus nidulans FlbC, which is required for the activation of the key regulator of conidiation brlA. ADA1 is predicted to encode a protein with two DNA binding motifs at the C terminus and a putative activator domain at the N terminus region. Deletion of the flbC gene in A. nidulans results in "fluffy" cotton-like colonies, with a defect in transition from vegetative growth to asexual development. In this study we show that Ada1 plays a key role in asexual development in F. verticillioides. Conidia production was significantly reduced in the knockout mutant (Δada1), in which aberrant conidia and conidiophores were also observed. We identified genes that are predicted to be downstream of ADA1, based on A. nidulans conidiation signaling pathway. Among them, the deletion of stuA homologue, FvSTUA, resulted in near absence of conidia production. To further investigate the functional conservation of this transcription factor, we complemented the Δada1 strain with A. nidulans flbC, F. verticillioides ADA1, and chimeric constructs. A. nidulans flbC failed to restore conidia production similar to the wild-type level. However, the Ada1N-terminal domain, which contains a putative activator, fused to A. nidulans FlbC C-terminal motif successfully complemented the Δada1 mutant. Taken together, Ada1 is an important transcriptional regulator of asexual development in F. verticillioides and that the N-terminus domain is critical for proper function of this transcription factor.

The role of MADS-box transcription factors in secondary metabolism and sexual development in the maize pathogen Fusarium verticillioides

MADS-box transcription factors (TFs) regulate functionally diverse gene targets in eukaryotes. In select ascomycetes, MADS-box TFs have been shown to play a role in virulence, and vegetative and sexual development. Here, we characterized Fusarium verticillioides MADS-box TFs, Mads1 and Mads2, in terms of their roles in secondary metabolism and sexual mating. Sequence analyses showed that MADS1 and MADS2 encode TFs with a SRF-type dimerization domain and a MEF2-type dimerization domain, respectively. The MADS1 and MADS2 knockout mutants (Fmt1 and Fmt2 strains, respectively) exhibited decreased vegetative growth and FB1 production when compared to the wild-type. Fmt1 showed reduced expression of 14 polyketide synthase (PKS) genes present in the organism, whereas Fmt2 did not display a change in PKS gene expression. Significantly, the deletion of MADS1 and MADS2 in the MAT1-2 genotype (Fmt4 and Fmt5 strains, respectively) led to strains that failed to produce perithecia and ascospores when crossed with the MAT1-1 wild-type strain. Notably, deletion of either gene did not have an effect on the ability of the fungus to colonize maize stalk or kernels. FB1 production and PKS expression data suggest that Mads1 is a broad regulator of secondary metabolism in F. verticillioides, and may target regulons upstream of Mads2 to influence FB1 production. In addition, MADS-box TFs in F. verticillioides play a critical role in the perithecia development.

Sda1, a Cys2-His2 zinc finger transcription factor, is involved in polyol metabolism and fumonisin B1 production in Fusarium verticillioides

The ubiquitous ascomycete Fusarium verticillioides causes ear rot and stalk rot of maize, both of which reduce grain quality and yield. Additionally, F. verticillioides produces the mycotoxin fumonisin B1 (FB1) during infection of maize kernels, and thus potentially compromises human and animal health. The current knowledge is fragmentary regarding the regulation of FB1 biosynthesis, particularly when considering interplay with environmental factors such as nutrient availability. In this study, SDA1 of F. verticillioides, predicted to encode a Cys-2 His-2 zinc finger transcription factor, was shown to play a key role in catabolizing select carbon sources. Growth of the SDA1 knock-out mutant (Δsda1) was completely inhibited when sorbitol was the sole carbon source and was severely impaired when exclusively provided mannitol or glycerol. Deletion of SDA1 unexpectedly increased FB1 biosynthesis, but reduced arabitol and mannitol biosynthesis, as compared to the wild-type progenitor. Trichoderma reesei ACE1, a regulator of cellulase and xylanase expression, complemented the F. verticillioides Δsda1 mutant, which indicates that Ace1 and Sda1 are functional orthologs. Taken together, the data indicate that Sda1 is a transcriptional regulator of carbon metabolism and toxin production in F. verticillioides.

Protein phosphatase 2A regulatory subunits perform distinct functional roles in the maize pathogen Fusarium verticillioides

Fusarium verticillioides is a pathogen of maize causing ear rot and stalk rot. The fungus also produces fumonisins, a group of mycotoxins linked to disorders in animals and humans. A cluster of genes, designated FUM genes, plays a key role in the synthesis of fumonisins. However, our understanding of the regulatory mechanism of fumonisin biosynthesis is still incomplete. We have demonstrated previously that Cpp1, a protein phosphatase type 2A (PP2A) catalytic subunit, negatively regulates fumonisin production and is involved in cell shape maintenance. In general, three PP2A subunits, structural A, regulatory B and catalytic C, make up a heterotrimer complex to perform regulatory functions. Significantly, we identified two PP2A regulatory subunits in the F. verticillioides genome, Ppr1 and Ppr2, which are homologous to Saccharomyces cerevisiae Cdc55 and Rts1, respectively. In this study, we hypothesized that Ppr1 and Ppr2 are involved in the regulation of fumonisin biosynthesis and/or cell development in F. verticillioides, and generated a series of mutants to determine the functional role of Ppr1 and Ppr2. The PPR1 deletion strain (Δppr1) resulted in drastic growth defects, but increased microconidia production. The PPR2 deletion mutant strain (Δppr2) showed elevated fumonisin production, similar to the Δcpp1 strain. Germinating Δppr1 conidia formed abnormally swollen cells with a central septation site, whereas Δppr2 showed early hyphal branching during conidia germination. A kernel rot assay showed that the mutants were slow to colonize kernels, but this is probably a result of growth defects rather than a virulence defect. Results from this study suggest that two PP2A regulatory subunits in F. verticillioides carry out distinct roles in the regulation of fumonisin biosynthesis and fungal development.

Genome-wide in silico identification of GPI proteins in Mycosphaerella fijiensis and transcriptional analysis of two GPI-anchored β-1,3-glucanosyltransferases

The hemibiotrophic fungus Mycosphaerella fijiensis is the causal agent of black Sigatoka (BS), the most devastating foliar disease in banana (Musa spp.) worldwide. Little is known about genes that are important during M. fijiensis-Musa sp. interaction. The fungal cell wall is an attractive area of study because it is essential for maintenance of cellular homeostasis and it is the most external structure in the fungal cell and therefore mediates the interaction of the pathogen with the host. In this manuscript we describe the in silico identification of glycosyl phosphatidylinositol-protein (GPI) family in M. fijiensis, and the analysis of two β-1,3-glucanosyltrans-ferases (Gas), selected by homology with fungal pathogenicity factors. Potential roles in pathogenesis were evaluated through analyzing expression during different stages of black Sigatoka disease, comparing expression data with BS symptoms and fungal biomass inside leaves. Real-time quantitative RT-PCR showed nearly constant expression of MfGAS1 with slightly increases (about threefold) in conidia and at speck-necrotrophic stage during banana-pathogen interaction. Conversely, MfGAS2 expression was increased during biotrophy (about seven times) and reached a maximum at speck (about 23 times) followed by a progressive decrease in next stages, suggesting an active role in M. fijiensis pathogenesis.

Quantification of fungal colonization, sporogenesis, and production of mycotoxins using kernel bioassays

The rotting of grains by seed-infecting fungi poses one of the greatest economic challenges to cereal production worldwide, not to mention serious risks to human and animal health. Among cereal production, maize is arguably the most affected crop, due to pathogen-induced losses in grain integrity and mycotoxin seed contamination. The two most prevalent and problematic mycotoxins for maize growers and food and feed processors are aflatoxin and fumonisin, produced by Aspergillus flavus and Fusarium verticillioides, respectively. Recent studies in molecular plant-pathogen interactions have demonstrated promise in understanding specific mechanisms associated with plant responses to fungal infection and mycotoxin contamination(1,2,3,4,5,6). Because many labs are using kernel assays to study plant-pathogen interactions, there is a need for a standardized method for quantifying different biological parameters, so results from different laboratories can be cross-interpreted. For a robust and reproducible means for quantitative analyses on seeds, we have developed in-lab kernel assays and subsequent methods to quantify fungal growth, biomass, and mycotoxin contamination. Four sterilized maize kernels are inoculated in glass vials with a fungal suspension (10(6)) and incubated for a predetermined period. Sample vials are then selected for enumeration of conidia by hemocytometer, ergosterol-based biomass analysis by high performance liquid chromatography (HPLC), aflatoxin quantification using an AflaTest fluorometer method, and fumonisin quantification by HPLC.

LongSAGE gene-expression profiling of Botrytis cinerea germination suppressed by resveratrol, the major grapevine phytoalexin

The ascomycetes Botrytis cinerea is one of the most studied necrotrophic phytopathogens and one of the main fungal parasites of grapevine. As a defense mechanism, grapevine produces a phytoalexin compound, resveratrol, which inhibits germination of the fungal conidium before it can penetrate the plant barriers and lead to host cell necrotrophy. To elucidate the effect of resveratrol on transcriptional regulation in B. cinerea germlings, two LongSAGE (long serial analysis of gene expression) libraries were generated in vitro for gene-expression profiling: 41 428 tags and among them, 15 665 unitags were obtained from resveratrol-treated B. cinerea germlings and 41 358 tags, among them, 16 362 unitags were obtained from non-treated B. cinerea germlings. In-silico analysis showed that about half of these unitags match known genes in the complete B. cinerea genome sequence. Comparison of unitag frequencies between libraries highlighted 110 genes that were transcriptionally regulated in the presence of resveratrol: 53 and 57 genes were significantly down- and upregulated, respectively. Manual curation of their putative functional categories showed that primary metabolism of germinating conidia appears to be markedly affected under resveratrol treatment, along with changes in other putative metabolic pathways, such as resveratrol detoxification and virulence-effector secretion, in B. cinerea germlings. We propose a hypothetical model of cross talk between B. cinerea germinating conidia and resveratrol-producing grapevine at the very early steps of infection.

Contribution of the gas1 gene of the entomopathogenic fungus Beauveria bassiana, encoding a putative glycosylphosphatidylinositol-anchored beta-1,3-glucanosyltransferase, to conidial thermotolerance and virulence

Beauveria bassiana is a mycoinsecticide alternative to chemicals for use in biological pest control. The fungus-insect interaction is also an emerging model system to examine unique aspects of the development, pathogenesis, and diversity of fungal lifestyles. The glycoside hydrolase 72 (GH72) family includes β-1,3-glucanosyltransferases that are glycosylphosphatidylinositol (GPI)-anchored cell wall-modeling enzymes affecting fungal physiology. A putative B. bassiana GPI-anchored β-1,3-glucanosyltransferase (Bbgas1) was isolated and characterized. B. bassiana targeted gene knockouts lacking Bbgas1 were affected in Congo red and salt sensitivity but displayed minor growth defects in the presence of sorbitol, SDS, or calcofluor white. Lectin and antibody mapping of surface carbohydrates revealed increased exposure of carbohydrate epitopes, including β-1,3-glucans, in the ΔBbgas1 strain. Transmission electron micrographs revealed localized destabilization of the cell wall in ΔBbgas1 conidia, in which fraying of the outer cell wall was apparent. Heat shock temperature sensitivity profiling showed that in contrast to the wild-type parent, ΔBbgas1 conidial spores displayed decreased germination after 1 to 4 h of heat shock at temperatures >40°C, and propidium iodide exclusion assays revealed decreased membrane stability in the knockout strain at temperatures >50°C. The ΔBbgas1 knockout showed reduced virulence in Galleria mellonella insect bioassays in both topical and intrahemocoel-injection assays. B. bassiana ΔBbgas1 strains complemented with the complete Bbgas1 open reading frame were indistinguishable from the wild-type parent in all phenotypes examined. The Bbgas1 gene did not complement the phenotype of a Saccharomyces cerevisiae β-1,3-glucanosyltransferase Δgas1 mutant, indicating that this family of enzymes likely possess discrete cellular functions.

The FvMK1 mitogen-activated protein kinase gene regulates conidiation, pathogenesis, and fumonisin production in Fusarium verticillioides

Fusarium verticillioides is one of the most important fungal pathogens to cause destructive diseases of maize worldwide. Fumonisins produced by the fungus are harmful to human and animal health. To date, our understanding of the molecular mechanisms associated with pathogenicity and fumonisin biosynthesis in F. verticillioides is limited. Because MAP kinase pathways have been implicated in regulating diverse processes important for plant infection in phytopathogenic fungi, in this study we identified and functionally characterized the FvMK1 gene in F. verticillioides. FvMK1 is orthologous to FMK1 in F. oxysporum and GPMK1 in F. graminearum. The Fvmk1 deletion mutant was reduced in vegetative growth and production of microconidia. However, it was normal in sexual reproduction and increased in the production of macroconidia. In infection assays with developing corn kernels, the Fvmk1 mutant was non-pathogenic and failed to colonize through wounding sites. It also failed to cause stalk rot symptoms beyond the inoculation sites on corn stalks, indicating that FvMK1 is essential for plant infection. Furthermore, the Fvmk1 mutant was significantly reduced in fumonisin production and expression levels of FUM1 and FUM8, two genes involved in fumonisin biosynthesis. The defects of the Fvmk1 mutant were fully complemented by re-introducing the wild type FvMK1 allele. These results demonstrate that FvMK1 plays critical roles in the regulation of vegetative growth, asexual reproduction, fumonisin biosynthesis, and pathogenicity.

Regulation of morphogenesis and biocontrol properties in Trichoderma virens by a VELVET protein, Vel1

Mycoparasitic strains of Trichoderma are applied as commercial biofungicides for control of soilborne plant pathogens. Although the majority of commercial biofungicides are Trichoderma based, chemical pesticides, which are ecological and environmental hazards, still dominate the market. This is because biofungicides are not as effective or consistent as chemical fungicides. Efforts to improve these products have been limited by a lack of understanding of the genetic regulation of biocontrol activities. In this study, using gene knockout and complementation, we identified the VELVET protein Vel1 as a key regulator of biocontrol, as well as morphogenetic traits, in Trichoderma virens, a commercial biocontrol agent. Mutants with mutations in vel1 were defective in secondary metabolism (antibiosis), mycoparasitism, and biocontrol efficacy. In nutrient-rich media they also lacked two types of spores important for survival and development of formulation products: conidia (on agar) and chlamydospores (in liquid shake cultures). These findings provide an opportunity for genetic enhancement of biocontrol and industrial strains of Trichoderma, since Vel1 is very highly conserved across three Trichoderma species.

GAC1, a gene encoding a putative GTPase-activating protein, regulates bikaverin biosynthesis in Fusarium verticillioides

Fusarium verticillioides (teleomorph Gibberella moniliformis) is an ascomycete known to produce a variety of secondary metabolites, including fumonisins, fusaric acid and bikaverin. These metabolites are synthesized when the fungus is under stress, notably nutrient limitations. To date we have limited understanding of the complex regulatory process associated with fungal secondary metabolism. In this study we generated a collection of F. verticillioides mutants by using REMI (restriction enzyme mediated integration) mutagenesis and in the process identified a strain, R647, that carries a mutation in a gene designated GAC1. Mutation in the GACI locus, which encodes a putative GTPase activating protein, resulted in the increased production of bikaverin, suggesting that GAC1 is negatively associated with bikaverin biosynthesis. Complementation of R647 with the wildtype GAC1 gene restored the bikaverin production level to that of the wild-type progenitor, demonstrating that gac1 mutation was directly responsible for the overproduction of bikaverin. We also demonstrated that AREA, encoding global nitrogen regulator, and PKS4, encoding polyketide synthase, are downstream genes that respectively are regulated positively and negatively by GAC1. Our results suggest that GAC1 plays an important role in signal transduction regulating bikaverin production in F. verticillioides.

Characterization of Fusarium graminearum Mes1 reveals roles in cell-surface organization and virulence

The surfaces of fungal hyphae are mosaics of carbohydrates and cell-surface proteins. Presently, very little is known about the role of these proteins and their organization at the cell surface. Here, we characterize two Fusarium graminearum genes implicated in cell-surface organization, mes1 (FGSG_06680) and pls1 (FGSG_08695). Mes1 is a homologue of mesA, which is required for the formation of stable polarity axes in Aspergillus nidulans. Pls1 encodes a tetraspanin, which belongs to a class of proteins that have been shown to aggregate in membrane rafts along with integrins and other signaling proteins. Our results indicate that Pls1 is dispensable for saprophytic growth and wheat head infection by F. graminearum (a pathogen that does not form appressoria). However, deletion of mes1 reduces sexual and asexual reproduction, severely perturbs the shape of macroconidia and hyphae, alters the pattern of cell wall deposition and the organization of sterol-rich rafts, and attenuates virulence on wheat heads. Our results provide a basis for identifying determinants of fungal virulence that may localize to specialized domains at the cell surface.