The catalytic subunit of type 2A protein phosphatase negatively regulates conidiation and melanin biosynthesis in Setosphaeria turcica

Northern corn leaf blight caused by Setosphaeria turcica is a major fungal disease responsible for significant reductions in maize yield worldwide. Eukaryotic type 2A protein phosphatase (PP2A) influences growth and virulence in a number of pathogenic fungi, but little is known about its roles in S. turcica. Here, we functionally characterized S. turcica StPP2A-C, which encodes the catalytic C subunit of StPP2A. StPP2A-C deletion slowed colony growth, conidial germination, and appressorium formation but increased conidiation, melanin biosynthesis, glycerol content, and disease lesion size on maize. These effects were associated with expression changes in genes related to calcium signaling, conidiation, laccase activity, and melanin and glycerol biosynthesis, as well as changes in intra- and extracellular laccase activity. A pull-down screen for candidate StPP2A-c interactors revealed an interaction between StPP2A-c and StLac1. Theoretical modeling and yeast two-hybrid experiments confirmed that StPP2A-c interacted specifically with the copper ion binding domain of StLac1 and that Cys267 of StPP2A-c was required for this interaction. StPP2A-C expression thus appears to promote hyphal growth and reduce pathogenicity in S. turcica, at least in part by altering melanin synthesis and laccase activity; these insights may ultimately support the development of novel strategies for biological management of S. turcica.

Genome-wide transcriptome analysis of toxigenic Fusarium verticillioides in response to variation of temperature and water activity on maize kernels

Fusarium verticillioides is one of the important mycotoxigenic pathogens of maize since it causes severe yield losses and produces fumonisins (FBs) to threaten human and animal health. Previous studies showed that temperature and water activity (aw) are two pivotal environmental factors affecting F. verticillioides growth and FBs production during maize storage. However, the genome-wide transcriptome analysis of differentially expressed genes (DEGs) in F. verticillioides under the stress combinations of temperature and aw has not been studied in detail. In this study, DEGs of F. verticillioides and their related regulatory pathways were analyzed in response to the stress of temperature and aw combinations using RNA-Seq. The results showed that the optimal growth conditions for F. verticillioides were 0.98 aw and 25 °C, whereas the highest per-unit yield of the fumonisin B1 (FB1) was observed at 0.98 aw and 15 °C. The RNA-seq analysis showed that 9648 DEGs were affected by temperature regardless of aw levels, whereas only 218 DEGs were affected by aw regardless of temperature variations. Gene Ontology (GO) analysis revealed that a decrease in temperature at both aw levels led to a significant upregulation of genes associated with 24 biological processes, while three biological processes were downregulated. Furthermore, when aw was decreased at both temperatures, seven biological processes were significantly upregulated and four were downregulated. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that the genes, whose expression was upregulated when the temperature decreased, were predominantly associated with the proteasome pathway, whereas the genes, whose expression was downregulated when the aw decreased, were mainly linked to amino acid metabolism. For the FB1, except for the FUM15 gene, the other 15 biosynthetic-related genes were highly expressed at 0.98 aw and 15 °C. In addition, the expression pattern analysis of other biosynthetic genes involved in secondary metabolite production and regulation of fumonisins production was conducted to explore how this fungus responds to the stress combinations of temperature and aw. Overall, this study primarily examines the impact of temperature and aw on the growth of F. verticillioides and its production of FB1 using transcriptome data. The findings presented here have the potential to contribute to the development of novel strategies for managing fungal diseases and offer valuable insights for preventing fumonisin contamination in food and feed storage.

The STRIPAK complex orchestrates cell wall integrity signalling to govern the fungal development and virulence of Fusarium graminearum

Striatin-interacting phosphatases and kinases (STRIPAKs) are evolutionarily conserved supramolecular complexes that control various important cellular processes such as signal transduction and development. However, the role of the STRIPAK complex in pathogenic fungi remains elusive. In this study, the components and function of the STRIPAK complex were investigated in Fusarium graminearum, an important plant-pathogenic fungus. The results obtained from bioinformatic analyses and the protein-protein interactome suggested that the fungal STRIPAK complex consisted of six proteins: Ham2, Ham3, Ham4, PP2Aa, Ppg1, and Mob3. Deletion mutations of individual components of the STRIPAK complex were created, and observed to cause a significant reduction in fungal vegetative growth and sexual development, and dramatically attenuae virulence, excluding the essential gene PP2Aa. Further results revealed that the STRIPAK complex interacted with the mitogen-activated protein kinase Mgv1, a key component in the cell wall integrity pathway, subsequently regulating the phosphorylation level and nuclear accumulation of Mgv1 to control the fungal stress response and virulence. Our results also suggested that the STRIPAK complex was interconnected with the target of rapamycin pathway through Tap42-PP2A cascade. Taken together, our findings revealed that the STRIPAK complex orchestrates cell wall integrity signalling to govern the fungal development and virulence of F. graminearum and highlighted the importance of the STRIPAK complex in fungal virulence.

Protein Phosphatase CgPpz1 Regulates Potassium Uptake, Stress Responses, and Plant Infection in Colletotrichum gloeosporioides

Protein phosphatases play important roles in the regulation of various cellular processes in eukaryotes. The ascomycete Colletotrichum gloeosporioides is a causal agent of anthracnose disease on some important crops and trees. In this study, CgPPZ1, a protein phosphate gene and a homolog of yeast PPZ1, was identified in C. gloeosporioides. Targeted gene deletion showed that CgPpz1 was important for vegetative growth and asexual development, conidial germination, and plant infection. Cytological examinations revealed that CgPpz1 was localized to the cytoplasm. The ΔCgppz1 mutant was hypersensitive to osmotic stresses, cell wall stressors, and oxidative stressors. Taken together, our results indicated that CgPpz1 plays an important role in the fungal development and virulence of C. gloeosporioides and the multiple stress responses generated.

An Efficient Strategy for Obtaining Mutants by Targeted Gene Deletion in Ophiostoma novo-ulmi

The dimorphic fungus Ophiostoma novo-ulmi is the highly aggressive pathogen responsible for the current, highly destructive, pandemic of Dutch elm disease (DED). Genome and transcriptome analyses of this pathogen previously revealed that a large set of genes expressed during dimorphic transition were also potentially related to plant infection processes, which seem to be regulated by molecular mechanisms different from those described in other dimorphic pathogens. Then, O. novo-ulmi can be used as a representative species to study the lifestyle of dimorphic pathogenic fungi that are not shared by the "model species" Candida albicans and Ustilago maydis. In order to gain better knowledge of molecular aspects underlying infection process and symptom induction by dimorphic fungi that cause vascular wilt disease, we developed a high-throughput gene deletion protocol for O. novo-ulmi. The protocol is based on transforming a Δmus52 O. novo-ulmi mutant impaired for non-homologous end joining (NHEJ) as the recipient strain, and transforming this strain with the latest version of OSCAR plasmids. The latter are used for generating deletion constructs containing the toxin-coding Herpes simplex virus thymidine kinase (HSVtk) gene which prevents ectopic integration of the T-DNA in Ophiostoma DNA. The frequency of gene deletion by homologous recombination (HR) at the ade1 locus associated with purine nucleotide biosynthesis was up to 77.8% in the Δmus52 mutant compared to 2% in the wild-type (WT). To validate the high efficiency of our deletion gene methodology we deleted ade7, which also belongs to the purine nucleotide pathway, as well as bct2, ogf1, and opf2 which encode fungal binuclear transcription factors (TFs). The frequency of gene replacement by HR for these genes reached up to 94%. We expect that our methodology combining the use of NHEJ deficient strains and OSCAR plasmids will function with similar high efficiencies for other O. novo-ulmi genes and other filamentous fungi.

STRIPAK, a Key Regulator of Fungal Development, Operates as a Multifunctional Signaling Hub

The striatin-interacting phosphatases and kinases (STRIPAK) multi subunit complex is a highly conserved signaling complex that controls diverse developmental processes in higher and lower eukaryotes. In this perspective article, we summarize how STRIPAK controls diverse developmental processes in euascomycetes, such as fruiting body formation, cell fusion, sexual and vegetative development, pathogenicity, symbiosis, as well as secondary metabolism. Recent structural investigations revealed information about the assembly and stoichiometry of the complex enabling it to act as a signaling hub. Multiple organellar targeting of STRIPAK subunits suggests how this complex connects several signaling transduction pathways involved in diverse cellular developmental processes. Furthermore, recent phosphoproteomic analysis shows that STRIPAK controls the dephosphorylation of subunits from several signaling complexes. We also refer to recent findings in yeast, where the STRIPAK homologue connects conserved signaling pathways, and based on this we suggest how so far non-characterized proteins may functions as receptors connecting mitophagy with the STRIPAK signaling complex. Such lines of investigation should contribute to the overall mechanistic understanding of how STRIPAK controls development in euascomycetes and beyond.

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.

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.

Fusarium verticillioides: Advancements in Understanding the Toxicity, Virulence, and Niche Adaptations of a Model Mycotoxigenic Pathogen of Maize

The importance of understanding the biology of the mycotoxigenic fungus Fusarium verticillioides and its various microbial and plant host interactions is critical given its threat to maize, one of the world's most valuable food crops. Disease outbreaks and mycotoxin contamination of grain threaten economic returns and have grave implications for human and animal health and food security. Furthermore, F. verticillioides is a member of a genus of significant phytopathogens and, thus, data regarding its host association, biosynthesis of secondary metabolites, and other metabolic (degradative) capabilities are consequential to both basic and applied research efforts across multiple pathosystems. Notorious among its secondary metabolites are the fumonisin mycotoxins, which cause severe animal diseases and are implicated in human disease. Additionally, studies of these mycotoxins have led to new understandings of F. verticillioides plant pathogenicity and provide tools for research into cellular processes and host-pathogen interaction strategies. This review presents current knowledge regarding several significant lines of F. verticillioides research, including facets of toxin production, virulence, and novel fitness strategies exhibited by this fungus across rhizosphere and plant environments.

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.

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.

Functional analysis of the Fusarium graminearum phosphatome

Phosphatases are known to play important roles in the regulation of various cellular processes in eukaryotes. However, systematic characterization of the phosphatome has not been reported in phytopathogenic fungi. The wheat scab fungus Fusarium graminearum contains 82 putative phosphatases. The biological functions of each phosphatase were investigated in this study. Although 11 phosphatase genes appeared to be essential, deletion mutants of the other 71 phosphatase genes were obtained and characterized for changes in 15 phenotypes, including vegetative growth, nutrient response and virulence. Overall, the deletion of 63 phosphatase genes resulted in changes in at least one of the phenotypes assayed. Interestingly, the deletion of four genes (Fg06297, Fg03333, Fg03826 and Fg07932) did not dramatically affect hyphal growth, but led to strongly reduced virulence. Western blot analyses showed that three phosphatases (Fg10516, Fg03333 and Fg12867) functioned as negative regulators of the mitogen-activated protein kinase signaling pathways. In addition, we found, for the first time, that FgCdc14 is dispensable for growth, but plays an important role in ribosome biogenesis. Overall, in this first functional characterization of the fungal phosphatome, phosphatases important for various aspects of hyphal growth, development, plant infection and secondary metabolism were identified in the phytopathogenic fungus F. graminearum.

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.

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.

A serine/threonine-protein phosphatase PP2A catalytic subunit is essential for asexual development and plant infection in Magnaporthe oryzae

Protein phosphatase 2A is a subgroup of widely conserved serine/threonine phosphatases and plays diverse roles in transcription, translation, differentiation, cell cycle, and signal transduction in many organisms. However, its roles in biotrophic and hemi-biotrophic phytopathogenic fungi remain to be investigated. In this study, we isolated an insertional mutant of the rice blast fungus Magnaporthe oryzae that was defective in vegetative hyphal growth. In the mutant, the T-DNA fragment was found to be inserted in the promoter region of a putative serine/threonine protein phosphatase 2A catalytic subunit (PP2Ac) gene MoPPG1. Deletion of MoPPG1 leads to severe defects in vegetative hyphal growth and conidiation. Conidia of the ∆Moppg1 null mutants were misshaped, and most of them were two-celled. The deletion mutants of MoPPG1 did not penetrate into host plant cells and failed to cause any disease lesions on rice leaves. Interestingly, significant reduction was found in the ∆Moppg1 null mutants in expression levels of several Rho GTPase family genes including MgCDC42, MgRHO3, and MgRAC1, which were important for pathogenesis of M. oryzae. Taken together, our results indicated that PP2Ac plays vital roles in asexual development and plant infection by regulating Rho GTPases in the rice blast fungus and perhaps other plant pathogenic fungi.

A conserved homeobox transcription factor Htf1 is required for phialide development and conidiogenesis in Fusarium species

Conidia are primary means of asexual reproduction and dispersal in a variety of pathogenic fungi, and it is widely recognized that they play a critical role in animal and plant disease epidemics. However, genetic mechanisms associated with conidiogenesis are complex and remain largely undefined in numerous pathogenic fungi. We previously showed that Htf1, a homeobox transcription factor, is required for conidiogenesis in the rice pathogen Magnaporthe oryzae. In this study, our aim was to characterize how Htf1 homolog regulates common and also distinctive conidiogenesis in three key Fusarium pathogens: F. graminearm, F. verticillioides, and F. oxysporum. When compared to wild-type progenitors, the gene-deletion mutants in Fusarium species failed to form conventional phialides. Rather, they formed clusters of aberrant phialides that resembled elongated hyphae segments, and it is conceivable that this led to the obstruction of conidiation in phialides. We also observed that mutants, as well as wild-type Fusaria, can initiate alternative macroconidia production directly from hyphae through budding-like mechanism albeit at low frequencies. Microscopic observations led us to conclude that proper basal cell division and subsequent foot cell development of macroconidia were negatively impacted in the mutants. In F. verticillioides and F. oxysporum, mutants exhibited a 2- to 5- microconidia complex at the apex of monophialides resulting in a floral petal-like shape. Also, prototypical microconidia chains were absent in F. verticillioides mutants. F. graminearum and F. verticillioides mutants were complemented by introducing its native HTF1 gene or homologs from other Fusarium species. These results suggest that Fusarium Htf1 is functionally conserved homeobox transcription factor that regulates phialide development and conidiogenesis via distinct signaling pathways yet to be characterized in fungi.

Identification of pathogenesis-associated genes by T-DNA-mediated insertional mutagenesis in Botrytis cinerea: a type 2A phosphoprotein phosphatase and an SPT3 transcription factor have significant impact on virulence

Agrobacterium tumefaciens-mediated transformation (ATMT) was used to generate an insertional mutant library of the gray mold fungus Botrytis cinerea. From a total of 2,367 transformants, 68 mutants showing significant reduction in virulence on tomato and bean plants were analyzed in detail. As reported for other fungal ATMT libraries, integrations were mostly single copy, occurred preferentially in noncoding (regulatory) regions, and were frequently accompanied by small deletions of the target sequences and loss of parts of the border sequence. Two T-DNA integration events that were found to be linked to virulence were characterized in more detail: a catalytic subunit of a PP2A serine/threonine protein phosphatase (BcPP2Ac) and the SPT3 subunit of a Spt-Ada-Gcn5-acetyltransferase (SAGA-like) transcriptional regulator complex. Gene replacement and silencing approaches revealed that both Bcpp2Ac and SPT3 are crucial for virulence, growth, and differentiation as well as for resistance to H(2)O(2) in B. cinerea.

Gene encoding a c-type cyclin in Mycosphaerella graminicola is involved in aerial mycelium formation, filamentous growth, hyphal swelling, melanin biosynthesis, stress response, and pathogenicity

Mycosphaerella graminicola is an important wheat pathogen causing Septoria tritici blotch. To date, an efficient strategy to control M. graminicola has not been developed. More significantly, we have a limited understanding of the molecular mechanisms of M. graminicola pathogenicity. In this study, we attempted to characterize an MCC1-encoding c-type cyclin, a gene homologous to FCC1 in Fusarium verticillioides. Four independent MCC1 knock-out mutants were generated via Agrobacterium tumefaciens-mediated transformation. All of the MCC1 mutants showed consistent multiple phenotypes. Significant reductions in radial growth on potato dextrose agar (PDA) were observed in all of the MCC1 mutants. In addition, MCC1 gene-deletion mutants produced less aerial mycelium on PDA, showed delayed filamentous growth, had unusual hyphal swellings, produced more melanin, showed an increase in their stress tolerance response, and were reduced significantly in pathogenicity. These results indicate that the MCC1 gene is involved in multiple signaling pathways, including those involved in pathogenicity in M. graminicola.

Enhanced tolerance to low temperature in tobacco by over-expression of a new maize protein phosphatase 2C, ZmPP2C2

Low temperature is one of the most common environmental stresses affecting plant growth and agricultural production. Serine/threonine protein phosphatases 2C (PP2Cs) have been suggested to play an important role in stress signaling. To identify potential new member of the PP2C proteins in maize and investigate its functions for stress responses, the ZmPP2C2 gene, encoding a new PP2C protein from maize roots, was cloned by RT-PCR and RACE-PCR. Its constitutive expression in roots, stems and leaves of maize seedlings was detected by RNA gel blot, and its regulation in response to cold stress was also examined. To further evaluate its function in the cold stress response, we over-expressed the ZmPP2C2 gene in tobacco under the control of the Cauliflower Mosaic Virus (CaMV) 35S promoter, and assessed a series of physiological changes in wild type and transgenic plants under low temperatures. Compared with wild type tobacco under cold stress, plants that over-expressed ZmPP2C2 displayed higher germination speed and rate, higher antioxidant enzyme (SOD, POD, CAT) activities, with lower cold-induced electrolyte leakage and malondialdehyde (MDA) contents. These results show that over-expression of ZmPP2C2 in tobacco enhanced tolerance to cold stress, suggesting that this new gene, ZmPP2C2, may act as a positive regulator of cold resistance in plants.

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.

Adenylyl cyclase regulates heavy metal sensitivity, bikaverin production and plant tissue colonization in Fusarium proliferatum

A homologue of the adenylyl cyclase (AC) gene of Neurospora crassa, named Fpacy1 was cloned from the genomic library of Fusarium proliferatum ITEM 2287 by screening the library with a DNA fragment amplified by using PCR primers designed from conserved sequences of the catalytic domain of AC genes from other fungi. The deduced FPACY1 protein had 53-77% identity with the AC proteins of other fungi. DeltaFpacy1 mutants obtained by targeted gene disruption showed retarded vegetative growth, increased conidiation and delayed conidial germination. Colonization capability of the mutants, assessed on maize seedlings and tomato fruits also was adversely affected. In sexual crosses the AC mutants retained full male fertility, but their female fertility decreased significantly. Disruption of Fpacy1 abolished vegetative self-incompatibility, suggesting that the AC gene is involved in multiple developmental processes related to vegetative growth, as well as sexual and parasexual events. The elevated thermo- and H(2)O(2)-tolerance of the DeltaFpacy1 mutants was coupled to an increased sensitivity towards Cd and Cu, indicating that the cAMP signaling pathway may have both negative and positive regulatory roles on the stress response mechanisms of fungal cells. When grown under nitrogen limitation conditions, the DeltaFpacy1 mutants produced an average of approximately 274 microg g(-1) bikaverin, whereas only traces of this metabolite was detected in the wild type. This finding provides further evidence of the role of the cAMP-PKA pathway in regulating bikaverin production.

The cAMP signaling pathway in Fusarium verticillioides is important for conidiation, plant infection, and stress responses but not fumonisin production

Fusarium verticillioides is one of the most important fungal pathogens of maize. Mycotoxin, fumonisins produced by this pathogen pose a threat to human and animal health. Because cAMP signaling has been implicated in regulating diverse developmental and infection processes in fungal pathogens, in this study, we aimed to elucidate the function of the cAMP-protein kinase A (PKA) pathway in toxin production and plant infection in F. verticillioides. Targeted deletion mutants were generated for the CPK1 and FAC1 genes that encode a catalytic subunit of PKA and the adenylate cyclase, respectively. Defects in radial growth and macroconidiation were observed in both the cpk1 and fac1 deletion mutants. The fac1 mutant also was significantly reduced in virulence and microconidiation but increased in tolerance to heat and oxidative stresses. These phenotypes were not observed in the cpk1 mutant, indicating that additional catalytic subunit of PKA must exist and function downstream from FAC1. The fac1 mutant formed microconidia mainly in false heads. The expression levels of the hydrophobin genes HYD1 and HYD2, which are known to be associated with change in formation of microconidia, were significantly reduced in the fac1 mutant. Expression of F. verticillioides GSY2 and HSP26 genes, two other putative downstream targets of FAC1, was increased in the fac1 mutant and may be associated with its enhanced stress tolerance. Although fumonisin production was normal, biosynthesis of bikaverin was increased in the fac1 mutant, suggesting that FAC1 and cAMP signaling may have pathway-or metabolite-specific regulatory roles in secondary metabolism. Overall, the pleiotropic defects of the fac1 deletion mutant indicate that the cAMP-PKA pathway is involved in growth, conidiation, bikaverin production, and plant infection in F. verticillioides.

Calcineurin is an antagonist to PKA protein phosphorylation required for postmating filamentation and virulence, while PP2A is required for viability in Ustilago maydis

Ustilago maydis is a dimorphic basidiomycete and the causal agent of corn smut disease. It serves as a genetic model for understanding dimorphism, pathogenicity, and mating response in filamentous fungi. Previous studies indicated the importance of regulated cAMP-dependent protein kinase A (PKA) for filamentous growth and pathogenicity in U. maydis. The roles of two protein phosphatases that potentially act antagonistically to PKA were assessed. A reverse genetics approach to mutate the catalytic subunits of calcineurin (CN, protein phosphatase [PP]2B) and PP2A in U. maydis was employed. A mutation in the CN catalytic subunit ucn1 caused a dramatic multiple-budding phenotype and mating between two ucn1 mutants was severely reduced. The pathogenicity of ucn1 mutant strains was also severely reduced, even in a solopathogenic haploid strain. Importantly, mutations disrupting protein phosphorylation by PKA were epistatic to ucn1 mutation, indicating a major role of ucn1 as a PKA antagonistic phosphatase. Genetic and inhibitor studies indicated that the U. maydis PP2A catalytic subunit gene (upa2) was essential.

FDB2 encodes a member of the arylamine N-acetyltransferase family and is necessary for biotransformation of benzoxazolinones by Fusarium verticillioides

AIMS

To clone and characterize genes from the mycotoxigenic fungus, Fusarium verticillioides, which are associated with its ability to biotransform allelopathic benzoxazolinones produced by maize, wheat, and rye.

METHODS AND RESULTS

Suppression subtractive hybridization identified F. verticillioides genes up-regulated in response to 2-benzoxazolinone (BOA), including a cluster of genes along chromosome 3. One of these genes, putatively encoding an arylamine N-acetyltransferase (NAT), was highly represented in the subtracted library and was of particular interest since previous analyses identified the FDB2 locus as possibly encoding transferase activity. The gene was subcloned and complemented a natural fdb2 mutant. Conversely, disruption of the gene eliminated the ability of F. verticillioides to metabolize BOA. Other genes in the cluster also were assessed using a complementation assay. Metabolic profiles of fdb2 mutants suggest that minor acylation activity occurred independently of the NAT activity encoded by FDB2.

CONCLUSIONS

The previously defined FDB2 locus was functionally associated with the gene encoding putative NAT activity, and the FDB2 gene was essential for biotransformation of BOA. The flanking gene FDB3 encodes a putative Zn(II)2Cys6 transcription factor and contributes to efficient BOA biotransformation but was not essential.

SIGNIFICANCE AND IMPACT OF THE STUDY

Biotransformation of benzoxazolinones by F. verticillioides may enhance its ecological fitness in maize field environments and our results provide greater understanding of the genes that modulate the biotransformation process. Additionally, this is the first homologue of the NAT gene family to be characterized in a filamentous fungus.