Functional analysis of the homoserine O-acetyltransferase gene and its identification as a selectable marker in Gibberella zeae

Methionine biosynthesis enzyme MoMet2 is required for rice blast fungus pathogenicity by promoting virulence gene expression via reducing 5mC modification

The emergence of fungicide resistance severely threatens crop production by limiting the availability and application of established fungicides. Therefore, it is urgent to identify new fungicidal targets for controlling plant diseases. Here, we characterized the function of a conserved homoserine O-acetyltransferase (HOA) from the rice blast fungus Magnaporthe oryzae that could serve as the candidate antifungal target. Deletion of the MoMET2 and MoCYS2 genes encoding HOAs perturbed the biosynthesis of methionine and S-adenyl methionine, a methyl group donor for epigenetic modifications, and severely attenuated the development and virulence of M. oryzae. The ∆Momet2 mutant is significantly increased in 5-methylcytosine (5mC) modification that represses the expression of genes required for pathogenicity, including MoGLIK and MoCDH-CYT. We further showed that host-induced gene silencing (HIGS) targeting MoMET2 and MoCYS2 effectively controls rice blasts. Our studies revealed the importance of HOA in the development and virulence of M. oryzae, which suggests the potential feasibility of HOA as new targets for novel anti-rice blast measurements.

Strategies for Controlling the Sporulation in Fusarium spp

Fusarium species are the most destructive phytopathogenic and toxin-producing fungi, causing serious diseases in almost all economically important plants. Sporulation is an essential part of the life cycle of Fusarium. Fusarium most frequently produces three different types of asexual spores, i.e., macroconidia, chlamydospores, and microconidia. It also produces meiotic spores, but fewer than 20% of Fusaria have a known sexual cycle. Therefore, the asexual spores of the Fusarium species play an important role in their propagation and infection. This review places special emphasis on current developments in artificial anti-sporulation techniques as well as features of Fusarium's asexual sporulation regulation, such as temperature, light, pH, host tissue, and nutrients. This description of sporulation regulation aspects and artificial anti-sporulation strategies will help to shed light on the ways to effectively control Fusarium diseases by inhibiting the production of spores, which eventually improves the production of food plants.

A novel antisense long non-coding RNA participates in asexual and sexual reproduction by regulating the expression of GzmetE in Fusarium graminearum

Fusarium graminearum is an important worldwide pathogen that causes Fusarium head blight in wheat, barley, maize and other grains. LncRNAs play important roles in many biological processes, but little is known about their functions and mechanisms in filamentous fungi. Here, we report that a natural antisense RNA, GzmetE-AS, is transcribed from the opposite strand of GzmetE. GzmetE encodes a homoserine O-acetyltransferase, which is important for sexual development and plant infection. The expression of GzmetE-AS was increased significantly during the conidiation stage, while GzmetE was upregulated in the late stage of sexual reproduction. Overexpression of GzmetE-AS inhibited the transcription of GzmetE. In contrast, the expression of GzmetE was significantly increased in GzmetE-AS transcription termination strain GzmetE-AS-T. Furthermore, GzmetE-AS-T produced more perithecia and facilitated the ascospore discharge, resembling the phenotype of GzmetE overexpressing strains. However, overexpression of GzmetE-AS in ∆dcl1/2 strain cannot inhibit the expression of GzmetE, and the GzmetE nat-siRNA is also significantly reduced in ∆dcl1/2 mutant. Taken together, we have identified a novel antisense lncRNA GzmetE-AS, which is involved in asexual and sexual reproduction by regulating its antisense gene GzmetE through RNAi pathway. Our findings reveal that the lncRNA plays critical roles in the development of F. graminearum.

Thioredoxin Reductase Is Involved in Development and Pathogenicity in Fusarium graminearum

Fusarium graminearum is one of the causal agents of Fusarium head blight and produces the trichothecene mycotoxin, deoxynivalenol (DON). Thioredoxin reductases (TRRs) play critical roles in the recycling of oxidized thioredoxin. However, their functions are not well known in plant pathogenic fungi. In this study, we characterized a TRR orthologue FgTRR in F. graminearum. The FgTRR-GFP fusion protein localized to the cytoplasm. FgTRR gene deletion demonstrated that FgTRR is involved in hyphal growth, conidiation, sexual reproduction, DON production, and virulence. The ΔTRR mutants also exhibited a defect in pigmentation, the expression level of aurofusarin biosynthesis-related genes was significantly decreased in the FgTRR mutant. Furthermore, the ΔTRR mutants were more sensitive to oxidative stress and aggravated apoptosis-like cell death compared with the wild type strain. Taken together, these results indicate that FgTRR is important in development and pathogenicity in F. graminearum.

ELP3 is involved in sexual and asexual development, virulence, and the oxidative stress response in Fusarium graminearum

Fusarium graminearum is an important fungal plant pathogen that causes serious losses in cereal crop yields and mycotoxicoses in humans and livestock. In this study, we characterized an insertion mutant, Z39R9282, with pleiotropic defects in sexual development and virulence. We determined that the insertion occurred in a gene encoding an ortholog of yeast elongator complex protein 3 (ELP3). Deletion of elp3 led to significant defects in sexual and asexual development in F. graminearum. In the elp3 deletion mutant, the number of perithecia formed was reduced and maturation of perithecia was delayed. This mutant also produced morphologically abnormal ascospores and conidia. Histone acetylation in the elp3 deletion mutant was reduced compared with the wild type, which likely caused the developmental defects. Trichothecenes were not produced at detectable levels, and expression of trichothecene biosynthesis genes were significantly reduced in the elp3 deletion mutant. Infection of wheat heads revealed that the elp3 deletion mutant was unable to spread from inoculated florets to neighboring spikelets. Furthermore, the elp3 deletion mutant was more sensitive to oxidative stress than the wild type, and the expression of putative catalase genes was reduced. We demonstrate that elp3 functions in sexual and asexual development, virulence, and the oxidative stress response of F. graminearum by regulating the expression of genes involved in these various developmental processes.

Inhibitors of amino acids biosynthesis as antifungal agents

Fungal microorganisms, including the human pathogenic yeast and filamentous fungi, are able to synthesize all proteinogenic amino acids, including nine that are essential for humans. A number of enzymes catalyzing particular steps of human-essential amino acid biosynthesis are fungi specific. Numerous studies have shown that auxotrophic mutants of human pathogenic fungi impaired in biosynthesis of particular amino acids exhibit growth defect or at least reduced virulence under in vivo conditions. Several chemical compounds inhibiting activity of one of these enzymes exhibit good antifungal in vitro activity in minimal growth media, which is not always confirmed under in vivo conditions. This article provides a comprehensive overview of the present knowledge on pathways of amino acids biosynthesis in fungi, with a special emphasis put on enzymes catalyzing particular steps of these pathways as potential targets for antifungal chemotherapy.

Genome and transcriptome analysis of the fungal pathogen Fusarium oxysporum f. sp. cubense causing banana vascular wilt disease

BACKGROUND

The asexual fungus Fusarium oxysporum f. sp. cubense (Foc) causing vascular wilt disease is one of the most devastating pathogens of banana (Musa spp.). To understand the molecular underpinning of pathogenicity in Foc, the genomes and transcriptomes of two Foc isolates were sequenced.

METHODOLOGY/PRINCIPAL FINDINGS

Genome analysis revealed that the genome structures of race 1 and race 4 isolates were highly syntenic with those of F. oxysporum f. sp. lycopersici strain Fol4287. A large number of putative virulence associated genes were identified in both Foc genomes, including genes putatively involved in root attachment, cell degradation, detoxification of toxin, transport, secondary metabolites biosynthesis and signal transductions. Importantly, relative to the Foc race 1 isolate (Foc1), the Foc race 4 isolate (Foc4) has evolved with some expanded gene families of transporters and transcription factors for transport of toxins and nutrients that may facilitate its ability to adapt to host environments and contribute to pathogenicity to banana. Transcriptome analysis disclosed a significant difference in transcriptional responses between Foc1 and Foc4 at 48 h post inoculation to the banana 'Brazil' in comparison with the vegetative growth stage. Of particular note, more virulence-associated genes were up regulated in Foc4 than in Foc1. Several signaling pathways like the mitogen-activated protein kinase Fmk1 mediated invasion growth pathway, the FGA1-mediated G protein signaling pathway and a pathogenicity associated two-component system were activated in Foc4 rather than in Foc1. Together, these differences in gene content and transcription response between Foc1 and Foc4 might account for variation in their virulence during infection of the banana variety 'Brazil'.

CONCLUSIONS/SIGNIFICANCE

Foc genome sequences will facilitate us to identify pathogenicity mechanism involved in the banana vascular wilt disease development. These will thus advance us develop effective methods for managing the banana vascular wilt disease, including improvement of disease resistance in banana.

Serine O-acetyltransferase is important, but not essential for cysteine-methionine synthesis in Fusarium graminearum

O-acetyltransferase (SAT) is a key enzyme converting serine into O-acetylserine in the synthesis of sulphur-containing amino acids. To characterize the function of FgSAT in Fusarium graminearum, three deletion mutants of FgSAT (ΔFgSAT-1, -2 and -18) were obtained using a gene replacement strategy. The three mutants did not show recognizable phenotypic changes on potato dextrose agar medium, but exhibited a very weak growth on fructose gelatin agar (FGA) medium containing SO₄²⁻ as sole sulfur source. Supplementation of O-acetylserine, cysteine, or methionine, but not serine, rescued the defect of mycelial growth in FgSAT deletion mutants, indicating that FgSAT is involved in conversion of serine into O-acetylserine. The three mutants had a decrease in conidiation in mung bean liquid, but not in carboxymethyl cellulose. Virulence, deoxynivalenol production and fungicide sensitivity assays found that the three mutants showed no significant difference from wild-type progenitor PH-1. Real-time PCR assays detected an increase in expression levels of FgOAHS, FgCBS and FgCGL genes involved in the alternative pathway in FgSAT deletion mutants, suggesting that the alternative pathway in F. graminearum is present and can operate. Addition of homoserine, the upstream substrate of the alternative pathway, also restored the normal mycelial growth of FgSAT deletion mutants on FGA, indicating that the alternative pathway in F. graminearum might be positively regulated by homoserine.

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.

Identification and functional characterization of genes involved in the sexual reproduction of the ascomycete fungus Gibberella zeae

We previously reported that G protein alpha subunit 1 (GPA1) is essential for sexual reproduction in the homothallic ascomycete fungus Gibberella zeae. In this study we performed microarray analyses on a GPA1 deletion mutant of G. zeae (Δgpa1) to identify genes involved in the sexual reproduction of this fungus. In the Δgpa1 strain, 645 genes were down-regulated and 550 genes were up-regulated during sexual reproduction when compared to the wild-type strain. One hundred of the down-regulated genes were selected for further investigation based on orthologous group clusters and differences in transcript levels. Quantitative real time-PCR was used to determine transcriptional profiles of these genes at various sexual and vegetative stages. We observed that transcript levels of 78 of these genes were dramatically increased in the wild-type strain during sexual reproduction compared to levels observed during vegetative growth, and were down-regulated in Δgpa1 compared to the wild-type strain. We deleted 57 of these genes and found that four of the deletion mutants lost self-fertility and five produced fewer perithecia compared to the wild-type strain. Two mutants produced wild-type numbers of perithecia, but maturation of perithecia and ascospores was delayed. In all we identified 11 genes that are involved in sexual reproduction of G. zeae and present evidence that some of these genes function at distinct stages during sexual reproduction in the fungus.

A novel gene, ROA, is required for normal morphogenesis and discharge of ascospores in Gibberella zeae

Head blight, caused by Gibberella zeae, is a significant disease among cereal crops, including wheat, barley, and rice, due to contamination of grain with mycotoxins. G. zeae is spread by ascospores forcibly discharged from sexual fruiting bodies forming on crop residues. In this study, we characterized a novel gene, ROA, which is required for normal sexual development. Deletion of ROA (Δroa) resulted in an abnormal size and shape of asci and ascospores but did not affect vegetative growth. The Δroa mutation triggered round ascospores and insufficient cell division after spore delimitation. The asci of the Δroa strain discharged fewer ascospores from the perithecia but achieved a greater dispersal distance than those of the wild-type strain. Turgor pressure within the asci was calculated through the analysis of osmolytes in the epiplasmic fluid. Deletion of the ROA gene appeared to increase turgor pressure in the mutant asci. The higher turgor pressure of the Δroa mutant asci and the mutant spore shape contributed to the longer distance dispersal. When the Δroa mutant was outcrossed with a Δmat1-2 mutant, a strain that contains a green fluorescence protein (GFP) marker in place of the MAT1-2 gene, unusual phenotypic segregation occurred. The ratio of GFP to non-GFP segregation was 1:1; however, all eight spores had the same shape. Taken together, the results of this study suggest that ROA plays multiple roles in maintaining the proper morphology and discharge of ascospores in G. zeae.

Development of a conditional gene expression system using a zearalenone-inducible promoter for the ascomycete fungus Gibberella zeae

The ascomycete fungus Gibberella zeae is an important plant pathogen that causes fusarium head blight on small grains. Molecular studies of this fungus have been performed extensively to uncover the biological mechanisms related to pathogenicity, toxin production, and sexual reproduction. Molecular methods, such as targeted gene deletion, gene overexpression, and gene fusion to green fluorescent protein (GFP), are relatively easy to perform with this fungus; however, conditional expression systems have not been developed. The purpose of this study was to identify a promoter that could be induced by zearalenone (ZEA) for the development of a conditional expression system in G. zeae. Through microarray analysis, we isolated one zearalenone response gene (ZEAR) whose expression was increased more than 50 times after ZEA treatment. Northern blot analysis showed that the ZEAR transcript dramatically increased after 1 h of ZEA treatment. To determine the utility of the ZEAR promoter, called Pzear, in a conditional expression system, we transformed a Pzear::GFP fusion construct into G. zeae. Our data showed a ZEA concentration-dependent increase in GFP expression. We also replaced the promoter of G. zeae metE (GzmetE), an essential gene for methionine biosynthesis, with the Pzear promoter. The growth of the Pzear-GzmetE mutant on minimal medium was dependent on the ZEA concentration supplemented in the medium and showed that GzMetE expression was induced by ZEA. This study is the first report of an inducible promoter in G. zeae. Our system will be useful for the characterization of essential gene functions in this fungus through differential and ZEA-dependent gene expression. In addition, the Pzear promoter may be applicable as a biosensor for the detection of ZEA contamination in agricultural products.

Comparative analysis of expressed sequence tags from the white-rot fungi (Phanerochaete chrysosporium)

Comprehensive analysis of the transcriptome of the P. chrysosporium is a useful approach to improve our understanding of its special and unique enzyme system and fungal evolution in molecular and industrial aspects. In order to unveil the functional diversity of this white-rot fungus in gene level and the expression patterns of its genes, in this study we carried out sequencing and annotation of 4,917 P. chrysosporium expressed sequence tags (ESTs). Through our bioinformatic ESTs analysis, we elucidated that 1,751 genes were derived from the present dataset of 4,917 ESTs, based on clustering and comparative genomic analyses of the ESTs. Of the 1,751 unique ESTs, 1,006 (57.5%) had homologues and orthologues in similarity searches. Our P. chrysosporium ESTs showed many genes for encoding 23 secreted proteins, many proteins for the degradation of cellulose and hemicelluloses, and heat shock proteins for stress resistance, which explain the reason why P. chrysosporium is very important and unique white-rot fungus in dealing with contaminated resources and in degrading lignin and in applying this organism to several industrial aspects.In addition, comparative analysis has shed the fresh light on the mystery about how its unique enzyme system and stress resistance have been evolved differently from its closest relatives.

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.

GzSNF1 is required for normal sexual and asexual development in the ascomycete Gibberella zeae

The sucrose nonfermenting 1 (SNF1) protein kinase of yeast plays a central role in the transcription of glucose-repressible genes in response to glucose starvation. In this study, we deleted an ortholog of SNF1 from Gibberella zeae to characterize its functions by using a gene replacement strategy. The mycelial growth of deletion mutants (DeltaGzSNF1) was reduced by 21 to 74% on diverse carbon sources. The virulence of DeltaGzSNF1 mutants on barley decreased, and the expression of genes encoding cell-wall-degrading enzymes was reduced. The most distinct phenotypic changes were in sexual and asexual development. DeltaGzSNF1 mutants produced 30% fewer perithecia, which matured more slowly, and asci that contained one to eight abnormally shaped ascospores. Mutants in which only the GzSNF1 catalytic domain was deleted had the same phenotype changes as the DeltaGzSNF1 strains, but the phenotype was less extreme in the mutants with the regulatory domain deleted. In outcrosses between the DeltaGzSNF1 mutants, each perithecium contained approximately 70% of the abnormal ascospores, and approximately 50% of the asci showed unexpected segregation patterns in a single locus tested. The asexual spores of the DeltaGzSNF1 mutants were shorter and had fewer septa than those of the wild-type strain. The germination and nucleation of both ascospores and conidia were delayed in DeltaGzSNF1 mutants in comparison with those of the wild-type strain. GzSNF1 expression and localization depended on the developmental stage of the fungus. These results suggest that GzSNF1 is critical for normal sexual and asexual development in addition to virulence and the utilization of alternative carbon sources.

Functional analyses of heterotrimeric G protein G alpha and G beta subunits in Gibberella zeae

The homothallic ascomycete fungus Gibberella zeae (anamorph: Fusarium graminearum) is a major toxigenic plant pathogen that causes head blight disease on small-grain cereals. The fungus produces the mycotoxins deoxynivalenol (DON) and zearalenone (ZEA) in infected hosts, posing a threat to human and animal health. Despite its agricultural and toxicological importance, the molecular mechanisms underlying its growth, development and virulence remain largely unknown. To better understand such mechanisms, we studied the heterotrimeric G proteins of G. zeae, which are known to control crucial signalling pathways that regulate various cellular and developmental responses in fungi. Three putative Gα subunits, GzGPA1, GzGPA2 and GzGPA3, and one Gβ subunit, GzGPB1, were identified in the F. graminearum genome. Deletion of GzGPA1, a homologue of the Aspergillus nidulans Gα gene fadA, resulted in female sterility and enhanced DON and ZEA production, suggesting that GzGPA1 is required for normal sexual reproduction and repression of toxin biosynthesis. The production of DON and ZEA was also enhanced in the GzGPB1 mutant, suggesting that both GαGzGPA1 and GβGzGPB1 negatively control mycotoxin production. Deletion of GzGPA2, which encodes a Gα protein similar to A. nidulans GanB, caused reduced pathogenicity and increased chitin accumulation in the cell wall, implying that GzGPA2 has multiple functions. Our study shows that G. zeae heterotrimeric G protein subunits can regulate vegetative growth, sexual development, toxin production and pathogenicity.

Functional characterization of acetylglutamate synthase and phosphoribosylamine-glycine ligase genes in Gibberella zeae

Gibberella zeae (anamorph, Fusarium graminearum) is an important pathogen of cereal crops found in many regions of the world. In this study, we have characterized two auxotrophic strains, designated S4B1279 and S4B3008, which were discovered from a collection of insertional mutants of G. zeae generated by restriction enzyme-mediated integration (REMI). Both mutant strains exhibited pleiotropic phenotypic changes that include reduction of mycelial growth and virulence and abolished sexual reproduction. Molecular analysis of the REMI mutants has shown that the auxotrophy of S4B1279 is due to a mutation of the ARG2 gene encoding an acetylglutamate synthase, and the auxotrophy of S4B3008 is due to a mutation of the ADE5 gene encoding a phosphoribosylamine-glycine ligase. Subsequent gene disruption and complementation studies have confirmed the functions for ARG2 and ADE5, respectively, in G. zeae. Our study has demonstrated the feasibility of using the REMI technique in studying G. zeae virulence mechanisms, in addition to providing two new selectable markers allowing genetic transformation of the fungus.

FSR1 is essential for virulence and female fertility in Fusarium verticillioides and F. graminearum

Fusarium verticillioides (teleomorph Gibberella moniliformis) and F. graminearum (teleomorph G. zeae) are well known to cause devastating diseases on cereal crops. Despite their importance, our understanding of the molecular mechanisms involved in these host-pathogen interactions is limited. The FSR1 locus in F. verticillioides was identified by screening REMI mutants for loss of virulence in maize stalk rot inoculation studies. FSR1 encodes an 823-codon open reading frame interrupted by two introns. The Fsr1 protein shares 60% sequence identity with the Sordaria macrospora Pro11, a multimodular protein with four putative protein-protein binding domains (caveolin-binding domain, coiled-coil structure, calmodulin-binding motif, and seven-WD40 repeats), which plays a regulatory role in cell differentiation and ascocarp development. Our data demonstrate that FSR1 is essential for female fertility and virulence in F. verticillioides. Significantly, targeted disruption of the FSR1 ortholog in F. graminearum (FgFSR1) reduced virulence on barley and deterred perithecia formation. Cross-complementation experiments demonstrated that the gene function is conserved in the two Fusarium species. FSR1 is expressed constitutively, and we hypothesize that Fsr1 regulates virulence by acting as a scaffold for a signal transduction pathway. A survey of available genome databases indicates Fsr1 homologs are present in a number of filamentous fungi and animal systems but not in budding yeast or plants. A maximum likelihood analysis of this gene family reveals well-supported monophyletic clades associated with fungi and animals.

Microarray analysis of transcript accumulation during perithecium development in the filamentous fungus Gibberella zeae (anamorph Fusarium graminearum)

Gibberella zeae (anamorph Fusarium graminearum) is the causal agent of Fusarium head blight (FHB) of wheat and barley in the United States. Ascospores forcibly discharged from mature fruiting bodies, the perithecia, serve as the primary inoculum for FHB epidemics. To identify genes important for perithecium development and function, a cDNA microarray that covered 11% of the G. zeae genome was constructed. The microarray was used to measure changes in transcription levels of genes expressed during three successive stages of perithecium development. When compared with vegetative mycelia, 651 (31%) cDNA clones showed changes in transcript levels in at least one of the three developmental stages. During perithecium development, 263 (13%) cDNA clones showed temporal changes in transcript profiles. Transcripts that showed the greatest changes in levels in maturing perithecia belonged to genes in the FunCat main functional categories of cell rescue, metabolism, cell type differentiation, energy, and cellular transport. For genes related to metabolism and cell type differentiation, transcripts showed the highest levels in immature perithecia, whereas for cellular transport-related genes, transcripts showed the highest levels in mature perithecia. This study represents the first large-scale investigation of both spatial and temporal changes in transcript levels during perithecium development. It provides clear evidence that the sexual development in fungi is a complex, multigenic process and identifies genes involved in sexual development of this agriculturally important fungus.