Discovery of (+)-Methyl Nonactate as a Potential Fungicide against Gummy Stem Blight

Gummy stem blight (GSB), which is caused by Stagonosporopsis cucurbitacearum, threatens pumpkin yields and agriculture. Effective, safe antifungal agents are urgently needed. In this study, fermentation broth supernatant of Streptomyces sp. NEAU-T55 demonstrated considerable antifungal activity against S. cucurbitacearum. Activity-guided isolation identified 2 new and 14 known compounds, with (+)-methyl nonactate (10) determined as the main active ingredient. This compound exhibited strong antifungal activity (EC50 = 0.12 μg mL-1), outperforming difenoconazole (EC50 = 0.17 μg mL-1), and achieved 74.1% control efficacy in the pot experiments. Microscopy revealed that (+)-methyl nonactate impeded mycelial growth and induced morphological alterations. Transcriptomic analysis indicated that (+)-methyl nonactate may inhibit acetolactate synthase, thereby disrupting amino acid metabolism and diminishing precursor availability for the tricarboxylic acid cycle. This research represents the first application of (+)-methyl nonactate for GSB control and provides insights into its antifungal mechanisms, laying the groundwork for its potential development as a novel agricultural antibiotic.

AflaILVB/G/I and AflaILVD are involved in mycelial production, aflatoxin biosynthesis, and fungal virulence in Aspergillus flavus

Aflatoxins (AFs) are produced by fungi such as Aspergillus flavus and A. parasiticus and are one of the most toxic mycotoxins found in agricultural products and food. Aflatoxin contamination, which requires the control of A. flavus, remains problematic because of the lack of effective strategies and the exploration of new compounds that can inhibit A. flavus growth and mycotoxin production is urgently required to alleviate potential deleterious effects. Acetohydroxy acid synthase (AHAS) and dihydroxy acid dehydratase are important enzymes in the biosynthetic pathways of branched-chain amino acids (BCAAs), including isoleucine, leucine, and valine. Enzymes involved in BCAA biosynthesis are present in bacteria, plants, and fungi, but not in mammals, and are therefore, attractive targets for antimicrobial and herbicide development. In this study, we characterized AflaILVB/G/I and AflaILVD, which encode the catalytic and regulatory subunits of AHAS and dihydroxy acid dehydratase, from the pathogenic fungus Aspergillus flavus. The AflaILVB/G/I and AflaILVD deletion mutant grew slower and produced smaller colonies than the wild-type strain when grown on glucose minimal medium, potato dextrose agar, and yeast extract medium for three days at 28°C, and disruption of AflaILVB/G/I caused a significant reduction in conidia production when grown on all kinds of media. Cellular stress assays determined that all strains were sensitive to H2O2. Importantly, the pathogenicity and aflatoxin production were affected when AflaILVB/G/I and AflaILVD were knocked out, particularly AflaILVB/G/I. A series of genes that encoded enzymes involved in aflatoxin synthesis were downregulated, meaning that the knockout of AflaILVB/G/I influenced aflatoxin synthesis in A. flavus strain WT. Collectively, our results demonstrate the potential value of antifungals targeting AflaILVB/G/I in A. flavus.

Branched-chain amino acid biosynthesis in fungi

Branched-chain amino acids (BCAAs)-isoleucine, leucine, and valine-are synthesized by fungi. These amino acids are important components of proteins and secondary metabolites. The biochemical pathway for BCAA biosynthesis is well-characterized in the yeast Saccharomyces cerevisiae. The biosynthesis of these three amino acids is interconnected. Different precursors are metabolized in multiple steps through shared enzymes to produce isoleucine and valine, and the valine biosynthesis pathway branches before the penultimate step to a series of leucine biosynthesis-specific steps to produce leucine. Recent efforts have made advances toward characterization of the BCAA biosynthesis pathway in several fungi, revealing diversity in gene duplication and functional divergence in the genes for these enzymatic steps in different fungi. The BCAA biosynthesis pathway is regulated by the transcription factor LEU3 in S. cerevisiae, and LeuB in Aspergillus nidulans and Aspergillus fumigatus, and the activity of these transcription factors is modulated by the leucine biosynthesis pathway intermediate α-isopropylmalate. Herein, we discuss recent advances in our understanding of the BCAA pathway and its regulation, focusing on filamentous ascomycete fungi and comparison with the well-established process in yeast.

Identification and Characterization of an Antifungal Gene Mt1 from Bacillus subtilis by Affecting Amino Acid Metabolism in Fusarium graminearum

Fusarium head blight is a devastating disease that causes significant economic losses worldwide. Fusarium graminearum is a crucial pathogen that requires close attention when controlling wheat diseases. Here, we aimed to identify genes and proteins that could confer resistance to F. graminearum. By extensively screening recombinants, we identified an antifungal gene, Mt1 (240 bp), from Bacillus subtilis 330-2. We recombinantly expressed Mt1 in F. graminearum and observed a substantial reduction in the production of aerial mycelium, mycelial growth rate, biomass, and pathogenicity. However, recombinant mycelium and spore morphology remained unchanged. Transcriptome analysis of the recombinants revealed significant down-regulation of genes related to amino acid metabolism and degradation pathways. This finding indicated that Mt1 inhibited amino acid metabolism, leading to limited mycelial growth and, thus, reduced pathogenicity. Based on the results of recombinant phenotypes and transcriptome analysis, we hypothesize that the effect of Mt1 on F. graminearum could be related to the metabolism of branched-chain amino acids (BCAAs), the most affected metabolic pathway with significant down-regulation of several genes. Our findings provide new insights into antifungal gene research and offer promising targets for developing novel strategies to control Fusarium head blight in wheat.

Integrated physiologic and proteomic analysis of Stropharia rugosoannulata mycelia in response to Cd stress

Soil Cd pollution seriously threatens environment and human health. Due to its ability to absorb and accumulate Cd in mycelia, Stropharia rugosoannulata could be a potential candidate for bioremediation of Cd-contaminated soils; however, the response mechanism of mycelia to Cd stress is still unclear. In this study, the physiologic and proteomic differences of S. rugosoannulata mycelia under 0.2 mg/L (low) and 2 mg/L (high) Cd stress were investigated. The results showed that Cd accumulation and mycelial growth inhibition exhibited a concentration-depended trend. Analysis of antioxidant system indicated that SOD, GR, GSH, GSSG and ASA played key roles in resisting the toxic effects of Cd. Via proteome analysis, 24 and 267 differentially expressed proteins (DEPs) were observed under low and high Cd stress, respectively. GO and KEGG analysis found that the mycelial growth inhibition might due to the down-regulation of some DEPs involved in "valine, leucine and isoleucine biosynthesis" and "tyrosine metabolism"; the certain tolerance to high Cd stress might attribute to the regulation of DEPs referred to energy metabolism and antioxidant system-related pathways, maintaining cellular energy homeostasis and removing ROS. These results provide a theoretical basis for further elucidation of response mechanisms in S. rugosoannulata to Cd stress.

Proximity-dependent biotinylation identifies a suite of candidate effector proteins from Fusarium graminearum

Fusarium graminearum is a fungal pathogen that causes Fusarium head blight in cereal crops. The identification of proteins secreted from pathogens to overcome plant defenses and cause disease, collectively known as effectors, can reveal the etiology of a disease process. Proximity-dependent biotin identification (BioID) was used to identify potential effector proteins secreted in planta by F. graminearum during the infection of Arabidopsis. Mass spectrometry analysis of streptavidin affinity-purified proteins revealed over 300 proteins from F. graminearum, of which 62 were candidate effector proteins (CEPs). An independent analysis of secreted proteins from axenic cultures of F. graminearum showed a 42% overlap with CEPs, thereby assuring confidence in the BioID methodology. The analysis also revealed that 19 out of 62 CEPs (approx. 30%) had been previously characterized with virulence function in fungi. The functional characterization of additional CEPs was undertaken through deletion analysis by the CRISPR/Cas9 method, and by overexpression into Triticum aestivum (wheat) leaves by the Ustilago hordei delivery system. Deletion studies of 12 CEPs confirmed the effector function of three previously characterized CEPs and validated the function of another four CEPs on wheat inflorescence or vegetative tissues. Lastly, overexpression in wheat showed that all seven CEPs enhanced resistance against the bacterial pathogen Pseudomonas syringae DC3000.

Quantitative multiplexed proteomics analysis reveals reshaping of the lysine 2-hydroxyisobutyrylome in Fusarium graminearum by tebuconazole

Backgrounds

Lysine 2-hydroxyisobutyrylation (Khib) is a newly discovered posttranslational modification (PTM) and has been identified in several prokaryotic and eukaryotic organisms. Fusarium graminearum, a major pathogen of Fusarium head blight (FHB) in cereal crops, can cause considerable yield loss and produce various mycotoxins that threaten human health. The application of chemical fungicides such as tebuconazole (TEC) remains the major method to control this pathogen. However, the distribution of Khib in F. graminearum and whether Khib is remodified in response to fungicide stress remain unknown.

Results

Here, we carried out a proteome-wide analysis of Khib in F. graminearum, identifying the reshaping of the lysine 2-hydroxyisobutyrylome by tebuconazole, using the most recently developed high-resolution LC–MS/MS technique in combination with high-specific affinity enrichment. Specifically, 3501 Khib sites on 1049 proteins were identified, and 1083 Khib sites on 556 modified proteins normalized to the total protein content were changed significantly after TEC treatment. Bioinformatics analysis showed that Khib proteins are involved in a wide range of biological processes and may be involved in virulence and deoxynivalenol (DON) production, as well as sterol biosynthesis, in F. graminearum.

Conclusions

Here, we provided a wealth of resources for further study of the roles of Khib in the fungicide resistance of F. graminearum. The results enhanced our understanding of this PTM in filamentous ascomycete fungi and provided insight into the remodification of Khib sites during azole fungicide challenge in F. graminearum.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08372-4.

Functional Analysis of Keto-Acid Reductoisomerase ILVC in the Entomopathogenic Fungus Metarhizium robertsii

Ketol-acid reductoisomerase (ILVC) is the second enzyme in the branched-chain amino acid (BCAA) biosynthesis, which regulates many physiological activities in a variety of organisms from bacteria to fungi and plants. In this work, function mechanisms of ILVC in Metarhizium robertsii Metchnikoff (Hypocreales: Clavicipitaceae) were explored with site-directed mutagenesis, reductase activity assays and transcriptomics analysis. The reductase activity assays showed that ILVC from phytopathogenic fungi exhibited significantly higher activities than those from entomopathogenic fungi but lower than those from yeast. Site-directed mutagenesis and enzymatic activities of MrILVC with different active-site mutants (Arg-113, Ser-118, Asp-152, Asp-260, and Glu-264) confirmed that active sites of MrILVC are conserved with plant and bacterial ILVCs. Deleting MrilvC causes the complete failures of vegetative growth and conidial germination, feeding with branched-chain amino acids (BCAAs) recovers the fungal growth but not conidial germination, while both characteristics are restored when supplemented with yeast extract. Compared to ΔMrilvC cultured in czapek agar (CZA), plenty of genes involved in the biosynthesis of antibiotics and amino acids were up- or down-regulated in the wild type or ΔMrilvC feeding with either BCAAs or yeast extract. Further analysis showed some genes, such as catalase A, participate in mycelial growth and conidial germination was down-regulated in ΔMrilvC from CZA, revealing that MrILVC might control the fungal development by gene regulation and BCAAs or yeast extract could play partial roles of MrILVC. This study will advance our understanding of ILVC function mechanisms in fungi.

Duplication and Functional Divergence of Branched-Chain Amino Acid Biosynthesis Genes in Aspergillus nidulans

Fungi, bacteria, and plants, but not animals, synthesize the branched-chain amino acids: leucine, isoleucine, and valine. While branched-chain amino acid (BCAA) biosynthesis has been well characterized in the yeast Saccharomyces cerevisiae, it is incompletely understood in filamentous fungi. The three BCAAs share several early biosynthesis steps before divergence into specific pathways. In Aspergillus nidulans, the genes for the first two dedicated steps in leucine biosynthesis have been characterized, but the final two have not. We used sequence searches of the A. nidulans genome to identify two genes encoding β-isopropylmalate dehydrogenase, which catalyzes the penultimate step of leucine biosynthesis, and six genes encoding BCAA aminotransferase, which catalyzes the final step in biosynthesis of all three BCAA. We have used combinations of gene knockouts to determine the relative contribution of each of these genes to BCAA biosynthesis. While both β-isopropylmalate dehydrogenase genes act in leucine biosynthesis, the two most highly expressed BCAA aminotransferases are responsible for BCAA biosynthesis. We have also characterized the expression of leucine biosynthesis genes using reverse transcriptase-quantitative PCR and found regulation in response to leucine availability is mediated through the Zn(II)₂Cys₆ transcription factor LeuB. IMPORTANCE Branched-chain amino acid (BCAA) biosynthesis is important for pathogenic fungi to successfully cause disease in human and plant hosts. The enzymes for their production are absent from humans and, therefore, provide potential antifungal targets. While BCAA biosynthesis is well characterized in yeasts, it is poorly understood in filamentous fungal pathogens. Developing a thorough understanding of both the genes encoding the metabolic enzymes for BCAA biosynthesis and how their expression is regulated will inform target selection for antifungal drug development.

Regulatory Mechanisms of L-Lactic Acid and Taste Substances in Chinese Acid Rice Soup (Rice-Acid) Fermented With a Lacticaseibacillus paracasei and Kluyveromyces marxianus

Rice-acid has abundant taste substances and health protection function due to the various bioactive compounds it contains, including organic acids. L-lactic acid is the most abundant organic acid in rice-acid, but the regulatory mechanisms of L-lactic acid accumulation in rice-acid are obscure. In this study, we analyzed the dynamic changes in organic acids and taste substances in rice-acid in various fermentation phases and different inoculation methods. We identified the key genes involved in taste substance biosynthesis by RNA-Seq analysis and compared the data of four experimental groups. We found that the interaction of the differences in key functional genes (L-lactate dehydrogenase and D-lactate dehydrogenase) and key metabolism pathways (glycolysis, pyruvate metabolism, TCA cycle, amino acid biosynthesis, and metabolism) might interpret the accumulation of L-lactic acid, other organic acids, and taste substances in rice-acid fermented with Lacticaseibacillus paracasei. The experimental data provided the basis for exploring regulatory mechanisms of taste substance accumulation in rice-acid.

The Intermediates in Branched-Chain Amino Acid Biosynthesis Are Indispensable for Conidial Germination of the Insect-Pathogenic Fungus Metarhizium robertsii

Metarhizium spp. are well-known biocontrol agents used worldwide to control different insect pests. Keto-acid reductoisomerase (ILVC) is a key enzyme for branched-chain amino acid (BCAA) biosynthesis, and it regulates many physiological activities. However, its functions in insect-pathogenic fungi are poorly understood. In this work, we identified MrilvC in M. robertsii and dissected its roles in fungal growth, conidiation, germination, destruxin biosynthesis, environmental stress response, and insecticidal virulence. BCAA metabolism affects conidial yields and germination. However, BCAAs cannot recover the conidial germination of an MrilvC-deficient strain. Further feeding assays with intermediates showed that some conidia of the ΔMrilvC mutant start to germinate. Therefore, it is the germination defect that causes the complete failures of conidial penetration and pathogenicity in the ΔMrilvC mutant. In conclusion, we found intermediates in BCAA biosynthesis are indispensable for Metarhizium robertsii conidial germination. This study will advance our understanding of the fungal germination mechanism.IMPORTANCE Branched-chain amino acid (BCAA) metabolism plays a significant role in many biological activities beyond protein synthesis. Spore germination initiates the first stage of vegetative growth, which is critical for the virulence of pathogenic fungi. In this study, we demonstrated that the keto-acid reductoisomerase MrILVC, a key enzyme for BCAA biosynthesis, from the insect-pathogenic fungus Metarhizium robertsii is associated with conidial germination and fungal pathogenicity. Surprisingly, the germination of the ΔMrilvC mutant was restored when supplemented with the intermediates of BCAA metabolism rather than three BCAAs. The result was significantly different from that of plant-pathogenic fungi. Therefore, this report highlights that the intermediates in BCAA biosynthesis are indispensable for conidial germination of M. robertsii.

Integrated Metabolomics and Transcriptomics Unravel the Metabolic Pathway Variations for Different Sized Beech Mushrooms

Beech mushrooms (Hypsizygus marmoreus) are largely relished for their characteristic earthy flavor, chewy-texture, and gustatory and nutritional properties in East Asian societies. Intriguingly, the aforementioned properties of beech mushroom can be subsumed under its elusive metabolome and subtle transcriptome regulating its various stages of growth and development. Herein, we carried out an integrated metabolomic and transcriptomic profiling for different sized beech mushrooms across spatial components (cap and stipe) to delineate their signature pathways. We observed that metabolite profiles and differentially expressed gene (DEGs) displayed marked synergy for specific signature pathways according to mushroom sizes. Notably, the amino acid, nucleotide, and terpenoid metabolism-related metabolites and genes were more abundant in small-sized mushrooms. On the other hand, the relative levels of carbohydrates and TCA intermediate metabolites as well as corresponding genes were linearly increased with mushroom size. However, the composition of flavor-related metabolites was varying in different sized beech mushrooms. Our study explores the signature pathways associated with growth, development, nutritional, functional and organoleptic properties of different sized beech mushrooms.

Differences in protein profiles between Malassezia pachydermatis strains obtained from healthy and infected dogs

Malassezia pachydermatis causes infections of the skin and mucous membranes, especially in individuals with metabolic, hormonal, and immunological disorders. The search for M. pachydermatis properties that differentiate isolates from healthy and infected animals may result in the identification of typically commensal and potentially pathogenic strains within the entire species. We aimed to determine and compare protein profiles of M. pachydermatis strains isolated from 30 dogs with clinical symptoms of otitis externa and 34 dogs without symptoms of any disease. Two-dimensional gel electrophoresis was applied, and proteins distinguishing the two groups of strains were identified by liquid chromatography coupled with tandem mass spectrometry. Significant differences were found between potentially pathogenic and commensal isolates. The most significant finding was the presence of nicotinamide adenine dinucleotide phosphate (NADP)-dependent mannitol dehydrogenase and ketol-acid reductoisomerase among M. pachydermatis strains obtained from dogs with otitis externa. Nevertheless, it is not clear whether they are associated directly with the pathogenicity or they play the role of fungal allergen. On the basis of these findings, we can conclude that there may be two distinct groups of M. pachydermatis strains-one typically commensal and the other with properties that enhance the infection process. These results may be used for more precise diagnosis and identification of potentially pathogenic strains in the future.

FgIlv3a is crucial in branched-chain amino acid biosynthesis, vegetative differentiation, and virulence in Fusarium graminearum

Dihydroxyacid dehydratase (DHAD), encoded by ILV3, catalyses the third step in the biosynthetic pathway of branched-chain amino acids (BCAAs), which include isoleucine (Ile), leucine (Leu), and valine (Val). Enzymes involved in BCAA biosynthesis exist in bacteria, plants, and fungi but not in mammals and are therefore attractive targets for antimicrobial or herbicide development. In this study, three paralogous ILV3 genes (FgILV3A, FgILV3B, and FgILV3C) were identified in the genome of Fusarium graminearum, the causal agent of Fusarium head blight (FHB). Deletion of FgILV3A alone or combined with FgILV3B or FgILV3C indicated an important role for FgILV3A in BCAA biosynthesis. FgILV3A deletion mutants lost the ability to grow on medium lacking amino acids. Exogenous supplementation of 1 mM Ile and Val rescued the auxotrophy of ΔFgIlv3A, though 5 mM was required to recover the growth defects in ΔFgIlv3AB and ΔFgIlv3AC strains, indicating that FgIlv3b and FgIlv3c exhibit redundant but accessory roles with FgIlv3a in BCAA biosynthesis. The auxotrophy of ΔFgIlv3A resulted in pleiotropic defects in aerial hyphal growth, in conidial formation and germination, and in aurofusarin accumulation. In addition, the mutants showed reduced virulence and deoxynivalenol production. Overall, our study demonstrates that FgIlv3a is crucial for BCAA biosynthesis in F. graminearum and a candidate fungicide target for FHB management.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

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

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

FcRav2, a gene with a ROGDI domain involved in Fusarium head blight and crown rot on durum wheat caused by Fusarium culmorum

Fusarium culmorum is a soil-borne fungal pathogen which causes foot and root rot and Fusarium head blight on small-grain cereals, in particular wheat and barley. It causes significant yield and quality losses and results in the contamination of kernels with type B trichothecene mycotoxins. Our knowledge of the pathogenicity factors of this fungus is still limited. A transposon tagging approach based on the mimp1/impala double-component system has allowed us to select a mutant altered in multiple metabolic and morphological processes, trichothecene production and virulence. The flanking regions of mimp1 were used to seek homologies in the F. culmorum genome, and revealed that mimp1 had reinserted within the last exon of a gene encoding a hypothetical protein of 318 amino acids which contains a ROGDI-like leucine zipper domain, supposedly playing a protein-protein interaction or regulatory role. By functional complementation and bioinformatic analysis, we characterized the gene as the yeast Rav2 homologue, confirming the high level of divergence in multicellular fungi. Deletion of FcRav2 or its orthologous gene in F. graminearum highlighted its ability to influence a number of functions, including virulence, trichothecene type B biosynthesis, resistance to azoles and resistance to osmotic and oxidative stress. Our results indicate that the FcRav2 protein (and possibly the RAVE complex as a whole) may become a suitable target for new antifungal drug development or the plant-mediated resistance response in filamentous fungi of agricultural interest.

Crystal Structures of Staphylococcus aureus Ketol-Acid Reductoisomerase in Complex with Two Transition State Analogues that Have Biocidal Activity

Ketol-acid reductoisomerase (KARI) is an NAD(P)H and Mg²⁺ -dependent enzyme of the branched-chain amino acid (BCAA) biosynthesis pathway. Here, the first crystal structures of Staphylococcus aureus (Sa) KARI in complex with two transition state analogues, cyclopropane-1,1-dicarboxylate (CPD) and N-isopropyloxalyl hydroxamate (IpOHA) are reported. These compounds bind competitively and in multi-dentate manner to KARI with K_i values of 2.73 μm and 7.9 nm, respectively; however, IpOHA binds slowly to the enzyme. Interestingly, intact IpOHA is present in only ≈25 % of binding sites, whereas its deoxygenated form is present in the remaining sites. This deoxy form of IpOHA binds rapidly to Sa KARI, but with much weaker affinity (K_i =21 μm). Thus, our data pinpoint the origin of the slow binding mechanism of IpOHA. Furthermore, we propose that CPD mimics the early stage of the catalytic reaction (preceding the reduction step), whereas IpOHA mimics the late stage (after the reduction took place). These structural insights will guide strategies to design potent and rapidly binding derivatives of these compounds for the development of novel biocides.

A Gin4-Like Protein Kinase GIL1 Involvement in Hyphal Growth, Asexual Development, and Pathogenesis in Fusarium graminearum

Fusarium graminearum is the main causal agent of Fusarium head blight (FHB) on wheat and barley. In a previous study, a GIN4-like protein kinase gene, GIL1, was found to be important for plant infection and sexual reproduction. In this study we further characterized the functions of GIL1 kinase in different developmental processes. The Δgil1 mutants were reduced in growth, conidiation, and virulence, and formed whitish and compact colonies. Although phialide formation was rarely observed in the mutants, deletion of GIL1 resulted in increased hyphal branching and increased tolerance to cell wall and cell membrane stresses. The Δgil1 mutants produced straight, elongated conidia lacking of distinct foot cells and being delayed in germination. Compared with the wild type, some compartments in the vegetative hyphae of Δgil1 mutants had longer septal distances and increased number of nuclei, suggesting GIL1 is related to cytokinesis and septation. Localization of the GIL1-GFP fusion proteins to the septum and hyphal branching and fusion sites further supported its roles in septation and branching. Overall, our results indicate that GIL1 plays a role in vegetative growth and plant infection in F. graminearum, and is involved in septation and hyphal branching.

Two FgLEU2 Genes with Different Roles in Leucine Biosynthesis and Infection-Related Morphogenesis in Fusarium graminearum

3-isopropylmalate dehydrogenase (IPMD) encoded by LEU2 is a key enzyme in leucine (Leu) biosynthetic pathway. Analysis of the genome sequence of Fusarium graminearum revealed two paralogous LEU2 genes (designated as FgLEU2A and FgLEU2B) in this fungus and the deduced amino acid sequences of FgLeu2A and FgLeu2B share 45% identity. Targeted disruption of individual FgLEU2A/B gene in F. graminearum assigned a more crucial role of FgLeu2A in Leu biosynthesis as disruption of FgLEU2A resulted in mutant (ΔFgLeu2A-10) that was Leu-auxotrophic and could not grow in minimal medium limited for amino acids, whereas FgLEU2B deletion mutant ΔFgLeu2B-2 was morphologically indistinguishable from the wild type strain PH-1. The growth defects of ΔFgLeu2A-10 could be overcome by exogenous addition of Leu at 0.25 mM. Double deletion of FgLEU2A and FgLEU2B (ΔFgLeu2AB-8) caused a more severe Leu-auxotrophic phenotype as the concentration of Leu exogenously added to medium to rescue the growth defect of ΔFgLeu2AB-8 should be raised to 1.25 mM, indicating a less important but nonnegligible role of FgLeu2B in Leu biosynthesis. Disturb of Leu biosynthesis caused by FgLEU2A deletion leads to slower growth rate, reduced aerial hyphal formation and red pigmentation on PDA plates and completely blocked conidial production and germination. All of the defects above could be overcome by Leu addition or complementation of the full-length FgLEU2A gene. ΔFgLeu2A-10 also showed significantly increased sensitivity to osmotic and oxidative stresses. Pathogenicity assay results showed that virulence of mutants lacking FgLEU2A were dramatically impaired on wheat heads and non-host cherry tomatoes. Additionally, a low level of deoxynivalenol (DON) production of ΔFgLeu2A-10 and ΔFgLeu2AB-8 in wheat kernels was also detected. Taken together, results of this study indicated a crucial role of FgLeu2A and a less important role of FgLeu2B in Leu biosynthesis and fungal infection-related morphogenesis in F. graminearum and FgLeu2A may serve as a potential target for novel antifungal development.

Acetohydroxyacid synthase FgIlv2 and FgIlv6 are involved in BCAA biosynthesis, mycelial and conidial morphogenesis, and full virulence in Fusarium graminearum

In this study, we characterized FgIlv2 and FgIlv6, the catalytic and regulatory subunits of acetohydroxyacid synthase (AHAS) from the important wheat head scab fungus Fusarium graminearum. AHAS catalyzes the first common step in the parallel pathways toward branched-chain amino acids (BCAAs: isoleucine, leucine, valine) and is the inhibitory target of several commercialized herbicides. Both FgILV2 and FgILV6 deletion mutants were BCAA-auxotrophic and showed reduced aerial hyphal growth and red pigmentation when cultured on PDA plates. Conidial formation was completely blocked in the FgILV2 deletion mutant ΔFgIlv2-4 and significantly reduced in the FgILV6 deletion mutant ΔFgIlv6-12. The auxotrophs of ΔFgIlv2-4 and ΔFgIlv6-12 could be restored by exogenous addition of BCAAs but relied on the designated nitrogen source the medium contained. Deletion of FgILV2 or FgILV6 also leads to hypersensitivity to various cellular stresses and reduced deoxynivalenol production. ΔFgIlv2-4 lost virulence completely on flowering wheat heads, whereas ΔFgIlv6-12 could cause scab symptoms in the inoculated spikelet but lost its aggressiveness. Taken together, our study implies the potential value of antifungals targeting both FgIlv2 and FgIlv6 in F. graminearum.

Leucine metabolism regulates TRI6 expression and affects deoxynivalenol production and virulence in Fusarium graminearum

TRI6 is a positive regulator of the trichothecene gene cluster and the production of trichothecene mycotoxins [deoxynivalenol (DON)] and acetylated forms such as 15-Acetyl-DON) in the cereal pathogen Fusarium graminearum. As a global transcriptional regulator, TRI6 expression is modulated by nitrogen-limiting conditions, sources of nitrogen and carbon, pH and light. However, the mechanism by which these diverse environmental factors affect TRI6 expression remains underexplored. In our effort to understand how nutrients affect TRI6 regulation, comparative digital expression profiling was performed with a wild-type F. graminearum and a Δtri6 mutant strain, grown in nutrient-rich conditions. Analysis showed that TRI6 negatively regulates genes of the branched-chain amino acid (BCAA) metabolic pathway. Feeding studies with deletion mutants of MCC, encoding methylcrotonyl-CoA-carboxylase, one of the key enzymes of leucine metabolism, showed that addition of leucine specifically down-regulated TRI6 expression and reduced 15-ADON accumulation. Constitutive expression of TRI6 in the Δmcc mutant strain restored 15-ADON production. A combination of cellophane breach assays and pathogenicity experiments on wheat demonstrated that disrupting the leucine metabolic pathway significantly reduced disease. These findings suggest a complex interaction between one of the primary metabolic pathways with a global regulator of mycotoxin biosynthesis and virulence in F. graminearum.

The Sch9 kinase regulates conidium size, stress responses, and pathogenesis in Fusarium graminearum

Fusarium head blight caused by Fusarium graminearum is an important disease of wheat and barley worldwide. In a previous study on functional characterization of the F. graminearum kinome, one protein kinase gene important for virulence is orthologous to SCH9 that is functionally related to the cAMP-PKA and TOR pathways in the budding yeast. In this study, we further characterized the functions of FgSCH9 in F. graminearum and its ortholog in Magnaporthe oryzae. The ΔFgsch9 mutant was slightly reduced in growth rate but significantly reduced in conidiation, DON production, and virulence on wheat heads and corn silks. It had increased tolerance to elevated temperatures but became hypersensitive to oxidative, hyperosmotic, cell wall, and membrane stresses. The ΔFgsch9 deletion also had conidium morphology defects and produced smaller conidia. These results suggest that FgSCH9 is important for stress responses, DON production, conidiogenesis, and pathogenesis in F. graminearum. In the rice blast fungus Magnaporthe oryzae, the ΔMosch9 mutant also was defective in conidiogenesis and pathogenesis. Interestingly, it also produced smaller conidia and appressoria. Taken together, our data indicate that the SCH9 kinase gene may have a conserved role in regulating conidium size and plant infection in phytopathogenic ascomycetes.

Involvement of threonine deaminase FgIlv1 in isoleucine biosynthesis and full virulence in Fusarium graminearum

In this study we characterized FgIlv1, a homologue of the Saccharomyces cerevisiae threonine dehydratase (TD) from the important Fusarium head blight fungus Fusarium graminearum. TD catalyzes the first step in the biosynthesis pathway of isoleucine (Ile) for conversion of threonine (Thr) to 2-ketobutyrate (2-KB). The FgILV1 deletion mutant ΔFgIlv1-3 was unable to grow on minimal medium or fructose gelatin agar which lacked Ile. Exogenous supplementation of Ile or 2-KB but not Thr rescued the mycelial growth defect of ΔFgIlv1-3, indicating the involvement of FgIlv1 in the conversion of Thr to 2-KB in Ile biosynthesis. Additionally, exogenous supplementation of Methionine (Met) could also rescue the mycelial growth defect of ΔFgIlv1-3, indicating a crosstalk between Ile biosynthesis and Met catabolism in F. graminearum. Deletion of FgILV1 also caused defects in conidial formation and germination. In addition, ΔFgIlv1-3 displayed decreased virulence on wheat heads and a low level of deoxynivalenol (DON) production in wheat kernels. Taken together, results of this study indicate that FgIlv1 is an essential component in Ile biosynthesis and is required for various cellular processes including mycelial and conidial morphogenesis, DON biosynthesis, and full virulence in F. graminearum. Our data indicate the potential of targeting Ile biosynthesis for anti-FHB management.