Kinetics and regioselectivity of three GH62 α-L-arabinofuranosidases from plant pathogenic fungi.

BACKGOUND: Xylan is the second most abundant plant cell wall polysaccharide after cellulose with α-L-arabinofuranose (L-Araf) as one of the major side substituents. Capacity to degrade xylan is characteristic of many plant pathogens; and corresponding enzymes that debranch arabinoxylan provide tools to tailor xylan functionality or permit its full hydrolysis. METHOD: Three GH62_2 family α-arabinofuranosidases (Abfs) from plant pathogenic fungi, NhaAbf62A from Nectria haematococca, SreAbf62A from Sporisorium reilianum and GzeAbf62A from Gibberella zeae, were recombinantly produced in Escherichia coli. Their biochemical properties and substrate specificities were characterized in detail. Particularly with 1H NMR, the regioselectivity and debranching preference of the three Abfs were directly compared. RESULTS: The activities of selected Abfs towards arabinoxylan were all optimal at pH 6.5. Their preferred substrates were wheat arabinoxylan, followed by soluble oat spelt xylan. The Abfs displayed selectivity towards either α-(1 → 2) or α-(1 → 3)-L- Araf mono-substituents in arabinoxylan. Specifically, SreAbf62A and GzeAbf62A removed m-α-(1 → 3)-L-Araf and m-α-(1 → 2)-L-Araf substituents with a similar rates, whereas NhaAbf62A released m-α-(1 → 3)-L-Araf 1.9 times faster than m-α-(1 → 2)-L-Araf. MAJOR CONCLUSIONS: Building upon the known selectivity of GH62 family α-arabinofuranosidases towards L-Araf mono-substituents in xylans, the current study uncovers enzyme-dependent preferences towards m-α-(1 → 3)-L-Araf and m-α-(1 → 2)-L-Araf substitutions. Comparative sequence-structure analyses of Abfs identified an arginine residue in the xylose binding +2R subsite that was correlated to the observed enzyme-dependent L-Araf debranching preferences. GENERAL SIGNIFICANCE: This study expands the limited pool of characterized GH62 Abfs particularly those from plant pathogenic fungi, and provides biochemical details and methodology to evaluate regioselectivity within this glycoside hydrolase family.

Exploring the influence of phospholipid monolayer conformation and environmental conditions on the interfacial binding of Gibberella Zeae lipase.

The involvement of different parameters on Gibberella zeae lipase (GZEL) membrane binding were characterized by using monomolecular film technology and circular dichroism spectroscopy. Among four kinds of phospholipid monolayers, 1,2­dimyristoyl­sn­glycero­3­phosphoethanolamine have the highest maximum insertion pressure (MIP) value. Comparing the GZEL adsorption to phosphatidylcholine monolayers with different acyl chains in sn-1 and sn-2 positions, the higher MIP values were found for 1,2­dilauroyl­sn­glycero­3­phosphocholine. Significantly improvement between 1,2­dioleoyl­sn­glycero­3­phosphocholine and 1,2­distearoyl­sn­glycero­3­phosphocholine suggested that the presence of fatty acid unsaturation may affect protein adsorption by changing the chemical structure in each phospholipid. The MIP value was shown higher (48.6 mN m-1) at pH 5 and pH 6 (47.5 ±â€¯1.9 mN m-1) but decreased significantly (34.2 mN m-1) at pH 9. This may indicate that the proportion of helices in the protein decreases with the alteration of the catalytic center, thus affecting the binding of the protein to its substrate. The MIP values obviously decreased with increasing salt ion concentration, suggesting that excessive salt ion concentration may destabilize the secondary and tertiary structures of the protein, thereby affecting the characteristics of its adsorption at the interfaces. Present studies improve our understanding on the protein-membrane interaction of this enzyme.

Function of C-terminal peptides on enzymatic and interfacial adsorption properties of lipase from Gibberella zeae.

BACKGROUND: The crystal structure of lipase from Gibberella zeae (GZEL) indicates that its C-terminal extension is composed of a loop and a α-helix. This structure is unique, possibly providing novel evidence on lipase mechanisms. METHODS: Two C-terminally truncated mutants (GZEL-Δ(α-helix) and GZEL-Δ(α-helix+loop)) were constructed. The role of these secondary structure segments on enzymatic activities and interfacial binding properties of GZEL was investigated by using conventional pH-stat method and monomolecular film techniques. In addition, inactive variants (Ser144Ala) of wild-type GZEL and two truncated mutants were constructed and produced specifically for interfacial binding experiments. RESULTS: Compared to the wild-type GZEL, lipase and phospholipase activities were significantly decreased in the two mutants. Deletion of the α-helix had great influence on the lipase activity of GZEL, resulting in residual 7.3% activity; the additional deletion of the loop led to 8.1% lipase activity. As for the phospholipase function, residual activities of 63.0% and 35.4% were maintained for GZEL-Δ(α-helix) and GZEL-Δ(α-helix+loop), respectively. Findings obtained with monomolecular film experiments further indicated that the reduction in phospholipase activity occurred with the anionic phospholipid as substrate, but was not seen with zwitterionic phospholipid. Results of the maximum insertion pressure, synergy factor and binding kinetic parameters documented that the α-helix structure of GZEL strongly influence the binding and insertion of enzyme to the phospholipid monolayer. Moreover, the interfacial binding function of α-helix was partly conformed by connecting to the C-terminal of Aspergillus oryzae lipase. GENERAL SIGNIFICANCE: Our results provide important information on the understanding of the structure-function relationship of GZEL.

Recombinant Lipase from Gibberella zeae Exhibits Broad Substrate Specificity: A Comparative Study on Emulsified and Monomolecular Substrate.

Using the classical emulsified system and the monomolecular film technique, the substrate specificity of recombinant Gibberella zeae lipase (rGZEL) that originates from Gibberella zeae was characterized in detail. Under the emulsified reaction system, both phospholipase and glycolipid hydrolytic activities were observed, except for the predominant lipase activity. The optimum conditions for different activity exhibition were also determined. Compared with its lipase activity, a little higher ratio of glycolipid hydrolytic activity (0.06) than phospholipase activity (0.02) was found. rGZEL preferred medium chain-length triglycerides, while lower activity was found for the longer-chain triglyceride. Using the monomolecular film technique, we found that the preference order of rGZEL to different phospholipids was 1,2-diacyl-sn-glycero-3-phospho-l-serine (PS) > 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (PG) > 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) > l-α-phosphatidylinositol (PI) > cardiolipin (CL) > 3-sn-phosphatidic acid sodium salt (PA) > l-α-phosphatidylethanolamine (PE), while no hydrolytic activity was detected for sphingomyelin (SM). Moreover, rGZEL showed higher galactolipase activity on 1,2-distearoyimonoglactosylglyceride (MGDG). A kinetic study on the stereo- and regioselectivity of rGZEL was also performed by using three pairs of pseudodiglyceride enantiomers (DDGs). rGZEL presented higher preference for distal DDG enantiomers than adjacent ester groups, however, no hydrolytic activity to the sn-2 position of diglyceride analogs was found. Furthermore, rGZEL preferred the R configuration of DDG enantiomers. Molecular docking results were in concordance with in vitro tests.

Production of a Highly Protease-Resistant Fungal α-Galactosidase in Transgenic Maize Seeds for Simplified Feed Processing.

Raffinose-family oligosaccharide (RFO) in soybeans is one of the major anti-nutritional factors for poultry and livestocks. α-Galactosidase is commonly supplemented into the animal feed to hydrolyze α-1,6-galactosidic bonds on the RFOs. To simplify the feed processing, a protease-resistant α-galactosidase encoding gene from Gibberella sp. strain F75, aga-F75, was modified by codon optimization and heterologously expressed in the embryos of transgentic maize driven by the embryo-specific promoter ZM-leg1A. The progenies were produced by backcrossing with the commercial inbred variety Zheng58. PCR, southern blot and western blot analysis confirmed the stable integration and tissue specific expression of the modified gene, aga-F75m, in seeds over four generations. The expression level of Aga-F75M reached up to 10,000 units per kilogram of maize seeds. In comparison with its counterpart produced in Pichia pastoris strain GS115, maize seed-derived Aga-F75M showed a lower temperature optimum (50 °C) and lower stability over alkaline pH range, but better thermal stability at 60 °C to 70 °C and resistance to feed pelleting inactivation (80 °C). This is the first report of producing α-galactosidase in transgenic plant. The study offers an effective and economic approach for direct utilization of α-galactosidase-producing maize without any purification or supplementation procedures in the feed processing.

Improving the catalytic potential and substrate tolerance of Gibberella intermedia nitrilase by whole-cell immobilization.

Comparative studies of immobilized and free cells of Gibberella intermedia CA3-1 in bioconversion of 3-cyanopyridine to nicotinic acid were performed. Entrapping method was chosen based on the advantages in enzymatic activity recovery, mechanical strength and preparation procedure. Four entrapment matrices were investigated and sodium alginate was screened to be the most suitable material. Maximal nitrilase activity of alginate immobilized cells was obtained under conditions of 2 % alginate, 0.6 % CaCl2, 0.4 g cell/g alginate, 1.8 mm bead size. The immobilized cells showed excellent substrate tolerance even when the 3-cyanopyridine concentration was 700 mM. The half-lives of immobilized cells at 30, 40 and 50 °C were 315, 117.5 and 10.9 h, respectively, correspondingly 1.4, 1.6 and 1.7-fold compared with that of the free cells. Efficient reusability of immobilized cells up to 28 batches was achieved and 205.7 g/(g dcw) nicotinic acid was obtained with 80.55 % enzyme activity preserved.

[Biocatalytic desymmetric hydrolysis of 3-(4-chlorophenyl)-glutaronitrile to the key precursor of optically pure baclofen].

We produced (S)-4-cyano-3-(4-chlorophenyl)-butyrate by highly stereoselective biocatalyst in this study. A nitrilase-producing strain, named Gibberella intermedia WX12, was isolated by 3-(4-chlorophenyl)-glutaronitrile as substrate in the screening with phenol-sodium hypochlorite method. The fermentation conditions and catalytic properties of this strain were investigated. The preferred carbon and nitrogen sources for nitrilase production were lactose (30 g/L) and peptone (20 g/L). After being cultivated for 96 h, the cells were collected for use in biotransformation. The hydrolysis of 3-(4-chlorophenyl)-glutaronitrile was performed at 30 degrees C in phosphate buffer (pH 8.0, 50 mmol/L) for 24 h to give (S)-4-cyano-3-(4-chlorophenyl)-butyric acid with 90% yield and > 99% of ee, which can be used for the synthesis of (R)- and (S)-baclofen. The configuration of product was determined by chemically converting it to baclofen and comparison with the authentic sample by chiral HPLC analysis.

Isolation and characterization of Gibberella intermedia CA3-1, a novel and versatile nitrilase-producing fungus.

Nitrilase-mediated biocatalysis has attracted substantial attention for its application in carboxylic acid production in recent years. In the present study, the fungus CA3-1 was isolated and identified as Gibberella intermedia based on its morphology, its 18S ribosomal DNA (rDNA), and internal transcribed spacer (ITS) sequences. The enzymatic properties of G. intermedia resting cells were determined, and the optimum activity was achieved at 40 °C with pH 7.6. The half-lives of the nitrilase at 30, 40, and 50 °C were 231.1, 72.9, and 6.4 h, respectively. This Gibberella nitrilase showed a wide substrate spectrum with high specificity for heterocyclic and aliphatic nitriles. It remained extremely active in 5% propanol. The presence of Ag(+), Hg(2+), and excess substrate inhibited the nitrilase activity, whereas Fe(2+), Mn(2+), and Li(+) improved enzyme activity. 3-Cyanopyridine (50 mM) was hydrolyzed into nicotinic acid within 30 min, whereas only <5% of nicotinamide was detected. The results show that this fungal nitrilase is a promising candidate for commercial application in nicotinic acid production.

Fungal His-tagged nitrilase from Gibberella intermedia: gene cloning, heterologous expression and biochemical properties.

BACKGROUND: Nitrilase is an important member of the nitrilase superfamiliy. It has attracted substantial interest from academia and industry for its function of converting nitriles directly into the corresponding carboxylic acids in recent years. Thus nitrilase has played a crucial role in production of commercial carboxylic acids in chemical industry and detoxification of nitrile-contaminated wastes. However, conventional studies mainly focused on the bacterial nitrilase and the potential of fungal nitrilase has been far from being fully explored. Research on fungal nitrilase gene expression will advance our understanding for its biological function of fungal nitrilase in nitrile hydrolysis. METHODOLOGY/PRINCIPAL FINDINGS: A fungal nitrilase gene from Gibberella intermedia was cloned through reverse transcription-PCR. The open reading frame consisted of 963 bp and potentially encoded a protein of 320 amino acid residues with a theoretical molecular mass of 35.94 kDa. Furthermore, the catalytic triad (Glu-45, Lys-127, and Cys-162) was proposed and confirmed by site-directed mutagenesis. The encoding gene was expressed in Escherichia coli Rosetta-gami (DE3) and the recombinant protein with His(6)-tag was purified to electrophoretic homogeneity. The purified enzyme exhibited optimal activity at 45°C and pH 7.8. This nitrilase was specific towards aliphatic and aromatic nitriles. The kinetic parameters V(max) and K(m) for 3-cyanopyridine were determined to be 0.81 µmol/min·mg and 12.11 mM through Hanes-Woolf plot, respectively. 3-Cyanopyridine (100 mM) could be thoroughly hydrolyzed into nicotinic acid within 10 min using the recombinant strain with the release of about 3% nicotinamide and no substrate was detected. CONCLUSIONS/SIGNIFICANCE: In the present study, a fungal nitrilase was cloned from the cDNA sequence of G. intermedia and successfully expressed in E. coli Rosetta-gami (DE3). The recombinant strain displayed good 3-cyanopyridine degradation efficiency and wide substrate spectrum. This fungal nitrilase might be a potential candidate for industrial applications in carboxylic acids production.

New PCR assays for the identification of Fusarium verticillioides, Fusarium subglutinans, and other species of the Gibberella fujikuroi complex.

Fusarium verticillioides and Fusarium subglutinans are important fungal pathogens of maize and other cereals worldwide. In this study, we developed PCR-based protocols for the identification of these pathogens targeting the gaoB gene, which codes for galactose oxidase. The designed primers recognized isolates of F. verticillioides and F. subglutinans that were obtained from maize seeds from several producing regions of Brazil but did not recognize other Fusarium spp. or other fungal genera that were either obtained from fungal collections or isolated from maize seeds. A multiplex PCR protocol was established to simultaneously detect the genomic DNA from F. verticillioides and F. subglutinans. This protocol could detect the DNA from these fungi growing in artificially or naturally infected maize seeds. Another multiplex reaction with a pair of primers developed in this work combined with a pre-existing pair of primers has allowed identifying F. subglutinans, F. konzum, and F. thapsinum. In addition, the identification of F. nygamai was also possible using a combination of two PCR reactions described in this work, and another described in the literature.

Differential roles of pyruvate decarboxylase in aerial and embedded mycelia of the ascomycete Gibberella zeae.

The pyruvate-acetaldehyde-acetate (PAA) pathway has diverse roles in eukaryotes. Our previous study on acetyl-coenzyme A synthetase 1 (ACS1) in Gibberella zeae suggested that the PAA pathway is important for lipid production, which is required for perithecia maturation. In this study, we deleted all three pyruvate decarboxylase (PDC) genes, which encode enzymes that function upstream of ACS1 in the PAA pathway. Results suggest PDC1 is required for lipid accumulation in the aerial mycelia, and deletion of PDC1 resulted in highly wettable mycelia. However, the total amount of lipids in the PDC1 deletion mutants was similar to that of the wild-type strain, likely due to compensatory lipid production processes in the embedded mycelia. PDC1 was expressed both in the aerial and embedded mycelia, whereas ACS1 was observed only in the aerial mycelia in a PDC1-dependent manner. PDC1 is also involved in vegetative growth of embedded mycelia in G. zeae, possibly through initiating the ethanol fermentation pathway. Thus, PDC1 may function as a key metabolic enzyme crucial for lipid production in the aerial mycelia, but play a different role in the embedded mycelia, where it might be involved in energy generation by ethanol fermentation.

Functional analyses of two acetyl coenzyme A synthetases in the ascomycete Gibberella zeae.

Acetyl coenzyme A (acetyl-CoA) is a crucial metabolite for energy metabolism and biosynthetic pathways and is produced in various cellular compartments with spatial and temporal precision. Our previous study on ATP citrate lyase (ACL) in Gibberella zeae revealed that ACL-dependent acetyl-CoA production is important for histone acetylation, especially in sexual development, but is not involved in lipid synthesis. In this study, we deleted additional acetyl-CoA synthetic genes, the acetyl-CoA synthetases (ACS genes ACS1 and ACS2), to identify alternative acetyl-CoA production mechanisms for ACL. The ACS1 deletion resulted in a defect in sexual development that was mainly due to a reduction in 1-palmitoyl-2-oleoyl-3-linoleoyl-rac-glycerol production, which is required for perithecium development and maturation. Another ACS coding gene, ACS2, has accessorial functions for ACS1 and has compensatory functions for ACL as a nuclear acetyl-CoA producer. This study showed that acetate is readily generated during the entire life cycle of G. zeae and has a pivotal role in fungal metabolism. Because ACSs are components of the pyruvate-acetaldehyde-acetate pathway, this fermentation process might have crucial roles in various physiological processes for filamentous fungi.

ATP citrate lyase is required for normal sexual and asexual development in Gibberella zeae.

Adenosine triphosphate (ATP) citrate lyase (ACL) is a key enzyme in the production of cytosolic acetyl-CoA, which is crucial for de novo lipid synthesis and histone acetylation in mammalian cells. In this study, we characterized the mechanistic roles of ACL in the homothallic ascomycete fungus Gibberella zeae, which causes Fusarium head blight in major cereal crops. Deletion of ACL in the fungus resulted in a complete loss of self and female fertility as well as a reduction in asexual reproduction, virulence, and trichothecene production. When the wild-type strain was spermatized with the ACL deletion mutants, they produced viable ascospores, however ascospore delimitation was not properly regulated. Although lipid synthesis was not affected by ACL deletion, histone acetylation was dramatically reduced in the ACL deletion mutants during sexual development, suggesting that the defects in sexual reproduction were caused by the reduction in histone acetylation. This study is the first report demonstrating a link between sexual development and ACL-mediated histone acetylation in fungi.

Genome mining for the discovery of new nitrilases in filamentous fungi.

PURPOSE OF WORK: our aim is to describe new fungal nitrilases whose sequences were published but whose catalytic properties were unknown. We adapted for expression in E. coli three of the genes and confirmed that the enzymes acted on organic nitriles. The genome mining approach was used to search for nitrilases in filamentous fungi. Synthetic genes encoding nitrilases in Aspergillus niger, Gibberella moniliformis and Neurospora crassa were expressed in Escherichia coli. This is the first heterologous expression of fungal enzymes of this type. The recombinant enzyme derived from G. moniliformis was an aromatic nitrilase with an activity of 390 U l(-1) culture with benzonitrile as substrate. This was much less than the activities of the recombinant enzymes derived from A. niger and N. crassa that had activities of 2500 and 2700 U l(-1) culture, respectively, with phenylacetonitrile as substrate.

Histidine kinase two-component response regulator proteins regulate reproductive development, virulence, and stress responses of the fungal cereal pathogens Cochliobolus heterostrophus and Gibberella zeae.

Histidine kinase (HK) phosphorelay signaling is a major mechanism by which fungi sense their environment. The maize pathogen Cochliobolus heterostrophus has 21 HK genes, 4 candidate response regulator (RR) genes (SSK1, SKN7, RIM15, REC1), and 1 gene (HPT1) encoding a histidine phosphotransfer domain protein. Because most HKs are expected to signal through RRs, these were chosen for deletion. Except for pigment and slight growth alterations for rim15 mutants, no measurable altered phenotypes were detected in rim15 or rec1 mutants. Ssk1p is required for virulence and affects fertility and proper timing of sexual development of heterothallic C. heterostrophus. Pseudothecia from crosses involving ssk1 mutants ooze masses of single ascospores, and tetrads cannot be found. Wild-type pseudothecia do not ooze. Ssk1p represses asexual spore proliferation during the sexual phase, and lack of it dampens asexual spore proliferation during vegetative growth, compared to that of the wild type. ssk1 mutants are heavily pigmented. Mutants lacking Skn7p do not display any of the above phenotypes; however, both ssk1 and skn7 mutants are hypersensitive to oxidative and osmotic stresses and ssk1 skn7 mutants are more exaggerated in their spore-type balance phenotype and more sensitive to stress than single mutants. ssk1 mutant phenotypes largely overlap hog1 mutant phenotypes, and in both types of mutant, the Hog1 target gene, MST1, is not induced. ssk1 and hog1 mutants were examined in the homothallic cereal pathogen Gibberella zeae, and pathogenic and reproductive phases of development regulated by Ssk1 and Hog1 were found to mirror, but also vary from, those of C. heterostrophus.

Gibberellin biosynthesis and gibberellin oxidase activities in Fusarium sacchari, Fusarium konzum and Fusarium subglutinans strains.

Several isolates of three Fusarium species associated with the Gibberella fujikuroi species complex were characterized for their ability to synthesize gibberellins (GAs): Fusarium sacchari (mating population B), Fusarium konzum (mating population I) and Fusarium subglutinans (mating population E). Of these, F. sacchari is phylogenetically related to Fusarium fujikuroi and is grouped in the Asian clade of the complex, while F. konzum and F. subglutinans are only distantly related to Fusarium fujikuroi and belong to the American clade. Variability was found between the different F. sacchari strains tested. Five isolates (B-12756; B-1732, B-7610, B-1721 and B-1797) were active in GA biosynthesis and accumulated GA(3) in the culture fluid (2.76-28.4 microg/mL), while two others (B-3828 and B-1725) were inactive. GA(3) levels in strain B-12756 increased by 2.9 times upon complementation with ggs2 and cps-ks genes from F. fujikuroi. Of six F. konzum isolates tested, three (I-10653; I-11616; I-11893) synthesized GAs, mainly GA(1), at a low level (less than 0.1 microg/mL). Non-producing F. konzum strains contained no GA oxidase activities as found for the two F. subglutinans strains tested. These results indicate that the ability to produce GAs is present in other species of the G. fujikuroi complex beside F. fujikuroi, but might differ significantly in different isolates of the same species.

Functional analyses of two syntaxin-like SNARE genes, GzSYN1 and GzSYN2, in the ascomycete Gibberella zeae.

We identified two syntaxin-like SNARE genes, named GzSYN1 and GzSYN2, from the plant pathogenic ascomycete Gibberella zeae, and characterized the functions and cellular localization of these genes. The GzSYN1 deletion mutant (Deltagzsyn1) had 71% reduced hyphal growth compared to the wild-type strain, but produced perithecia with normal ascospores. Deltagzsyn2 had the same hyphal growth rate as the wild-type, but completely lost both self and female fertility. When Deltagzsyn2 was spermatized for Deltamat1-1 or Deltamat1-2 strains, it retained its male fertility, but the ascus shape was abnormal and ascospore delimitation was delayed. The Deltagzsyn1 and Deltagzsyn2 virulence on barley was reduced by 67% and 75%, respectively, compared to the wild-type. The GFP::GzSYN1 fusion protein was localized in vesicles, vacuoles, plasma membranes, and septa, whereas GFP::GzSYN2 was found only in plasma membranes and septa. These results suggest that syntaxins have key roles in fungal development and virulence in G. zeae.

Crystal structure of a secreted lipase from Gibberella zeae reveals a novel "double-lock" mechanism.

Fusarium graminearum (sexual stage: Gibberella zeae) is the causative agent of Fusarium Head Blight (FHB), which is one of the most destructive plant disease of cereals, accounting for high grain yield losses, especially for wheat and maize. Like other fungal pathogens, several extracellular enzymes secreted by G. zeae are known to be involved in host infection. Among these secreted lipases, G. zeae lipase (GZEL), which is encoded by the FGL1 gene, was demonstrated to be crucial to G. zeae pathogenicity. However, the precise mechanism of GZEL remains unclear due to a lack of detailed structural information. In this study, we report the crystal structure of GZEL at the atomic level. The structure of GZEL displays distinct structural differences compared to reported homologues and indicates a unique "double lock" enzymatic mechanism. To gain insight into substrate/inhibitor recognition, we proposed a model of GZEL in complex with substrate and the lipase inhibitor ebelactone B (based on the reported structures of GZEL homologues), which defines possible substrate binding sites within the catalytic cleft and suggests an "anti sn-l" binding mode. These results pave the way to elucidating the mechanism of GZEL and thus provide clues for the design of anti-FHB inhibitors.

Roles of the glyoxylate and methylcitrate cycles in sexual development and virulence in the cereal pathogen Gibberella zeae.

The glyoxylate and methylcitrate cycles are involved in the metabolism of two- or three-carbon compounds in fungi. To elucidate the role(s) of these pathways in Gibberella zeae, which causes head blight in cereal crops, we focused on the functions of G. zeae orthologs (GzICL1 and GzMCL1) of the genes that encode isocitrate lyase (ICL) and methylisocitrate lyase (MCL), respectively, key enzymes in each cycle. The deletion of GzICL1 (DeltaGzICL1) caused defects in growth on acetate and in perithecium (sexual fruiting body) formation but not in virulence on barley and wheat, indicating that GzICL1 acts as the ICL of the glyoxylate cycle and is essential for self-fertility in G. zeae. In contrast, the DeltaGzMCL1 strains failed to grow on propionate but exhibited no major changes in other traits, suggesting that GzMCL1 is required for the methylcitrate cycle in G. zeae. Interestingly, double deletion of both GzICL1 and GzMCL1 caused significantly reduced virulence on host plants, indicating that both GzICL1 and GzMCL1 have redundant functions for plant infection in G. zeae. Thus, both GzICL1 and GzMCL1 may play important roles in determining major mycological and pathological traits of G. zeae by participating in different metabolic pathways for the use of fatty acids.

Gibberella zeae chitin synthase genes, GzCHS5 and GzCHS7, are required for hyphal growth, perithecia formation, and pathogenicity.

Gibberella zeae causes Fusarium head blight of cereal crops, and sexual spores of the fungus play an important role as primary inocula. We isolated a restriction enzyme-mediated integration (REMI) transformant, ZH431, of G. zeae with defects in perithecia formation and virulence. Integration of the REMI vector resulted in disruption of GzCHS7 gene, which encodes a putative class VII chitin synthase with high similarity to Fusarium oxysporum ChsVb. A second chitin synthase, GzCHS5, is adjacently located in a head-to-head configuration with GzCHS7, and its deduced protein sequence showed similarity with a class V chitin synthase in F. oxysporum. Neither DeltaGzChs5 nor DeltaGzChs7 mutants produced perithecia or caused disease on barley heads. Microscopic observation revealed that both mutants formed balloon-shaped hyphae and intrahyphal hyphae and that cell wall rigidity of the mutants was weaker than that of the wild-type strain. Transcription profiles of GzCHS5 and GzCHS7 were not altered in DeltaGzChs7 and DeltaGzChs5, respectively, suggesting that transcription regulations of the genes are independent of each other. Our results demonstrate that GzCHS5 and GzCHS7 are indispensable for perithecia formation and pathogenicity as well as normal septa formation and hyphal growth in G. zeae.