Changes in fatty acid composition of Neurospora crassa accompany sexual development and ascospore germination

Role of cell membrane homeostasis in the pathogenicity of pathogenic filamentous fungi

The cell membrane forms a fundamental part of all living cells and participates in a variety of physiological processes, such as material exchange, stress response, cell recognition, signal transduction, cellular immunity, apoptosis, and pathogenicity. Here, we review the mechanisms and functions of the membrane structure (lipid components of the membrane and the biosynthesis of unsaturated fatty acids), membrane proteins (transmembrane proteins and proteins contributing to membrane curvature), transcriptional regulation, and cell wall components that influence the virulence and pathogenicity of filamentous fungi.

Lessons on fruiting body morphogenesis from genomes and transcriptomes of Agaricomycetes

Fruiting bodies (sporocarps, sporophores or basidiomata) of mushroom-forming fungi (Agaricomycetes) are among the most complex structures produced by fungi. Unlike vegetative hyphae, fruiting bodies grow determinately and follow a genetically encoded developmental program that orchestrates their growth, tissue differentiation and sexual sporulation. In spite of more than a century of research, our understanding of the molecular details of fruiting body morphogenesis is still limited and a general synthesis on the genetics of this complex process is lacking. In this paper, we aim at a comprehensive identification of conserved genes related to fruiting body morphogenesis and distil novel functional hypotheses for functionally poorly characterised ones. As a result of this analysis, we report 921 conserved developmentally expressed gene families, only a few dozens of which have previously been reported to be involved in fruiting body development. Based on literature data, conserved expression patterns and functional annotations, we provide hypotheses on the potential role of these gene families in fruiting body development, yielding the most complete description of molecular processes in fruiting body morphogenesis to date. We discuss genes related to the initiation of fruiting, differentiation, growth, cell surface and cell wall, defence, transcriptional regulation as well as signal transduction. Based on these data we derive a general model of fruiting body development, which includes an early, proliferative phase that is mostly concerned with laying out the mushroom body plan (via cell division and differentiation), and a second phase of growth via cell expansion as well as meiotic events and sporulation. Altogether, our discussions cover 1 480 genes of Coprinopsis cinerea, and their orthologs in Agaricus bisporus, Cyclocybe aegerita, Armillaria ostoyae, Auriculariopsis ampla, Laccaria bicolor, Lentinula edodes, Lentinus tigrinus, Mycena kentingensis, Phanerochaete chrysosporium, Pleurotus ostreatus, and Schizophyllum commune, providing functional hypotheses for ~10 % of genes in the genomes of these species. Although experimental evidence for the role of these genes will need to be established in the future, our data provide a roadmap for guiding functional analyses of fruiting related genes in the Agaricomycetes. We anticipate that the gene compendium presented here, combined with developments in functional genomics approaches will contribute to uncovering the genetic bases of one of the most spectacular multicellular developmental processes in fungi. Citation: Nagy LG, Vonk PJ, Künzler M, Földi C, Virágh M, Ohm RA, Hennicke F, Bálint B, Csernetics Á, Hegedüs B, Hou Z, Liu XB, Nan S, M. Pareek M, Sahu N, Szathmári B, Varga T, Wu W, Yang X, Merényi Z (2023). Lessons on fruiting body morphogenesis from genomes and transcriptomes of Agaricomycetes. Studies in Mycology 104: 1-85. doi: 10.3114/sim.2022.104.01.

Unfolded Protein Response and Scaffold Independent Pheromone MAP Kinase Signaling Control Verticillium dahliae Growth, Development, and Plant Pathogenesis

Differentiation, growth, and virulence of the vascular plant pathogen Verticillium dahliae depend on a network of interconnected cellular signaling cascades. The transcription factor Hac1 of the endoplasmic reticulum-associated unfolded protein response (UPR) is required for initial root colonization, fungal growth, and vascular propagation by conidiation. Hac1 is essential for the formation of microsclerotia as long-time survival resting structures in the field. Single endoplasmic reticulum-associated enzymes for linoleic acid production as precursors for oxylipin signal molecules support fungal growth but not pathogenicity. Microsclerotia development, growth, and virulence further require the pheromone response mitogen-activated protein kinase (MAPK) pathway, but without the Ham5 scaffold function. The MAPK phosphatase Rok1 limits resting structure development of V.dahliae, but promotes growth, conidiation, and virulence. The interplay between UPR and MAPK signaling cascades includes several potential targets for fungal growth control for supporting disease management of the vascular pathogen V.dahliae.

Fungal Quorum-Sensing Molecules and Inhibitors with Potential Antifungal Activity: A Review

The theory of persisting independent and isolated regarding microorganisms is no longer accepted. To survive and reproduce they have developed several communication platforms within the cells which facilitates them to adapt the surrounding environmental changes. This cell-to-cell communication is termed as quorum sensing; it relies upon the cell density and can stimulate several traits of microbes including biofilm formation, competence, and virulence factors secretion. Initially, this sophisticated mode of communication was discovered in bacteria; later, it was also confirmed in eukaryotes (fungi). As a consequence, many quorum-sensing molecules and inhibitors have been identified and characterized in various fungal species. In this review article, we will primarily focus on fungal quorum-sensing molecules and the production of inhibitors from fungal species with potential applications for combating fungal infections.

Lipids and soluble carbohydrates in the mycelium and ascomata of alkaliphilic fungus Sodiomyces alkalinus

Alkaliphilic fungi are fundamentally different from alkalitolerant ones in terms of mechanisms of adaptation. They accumulate trehalose in cytosol and phosphatidic acids (PA) in the membrane lipids, whereas alkalitolerants contain these compounds in low amounts. But it is unclear how the composition of osmolytes and lipids changes during cytodifferentiation. In this article the composition of lipids and soluble cytosol carbohydrates in the mycelium and fruit bodies of the alkaliphilic fungus Sodiomyces alkalinus was studied. In the mycelium, mannitol and trehalose dominated, while in fruit bodies only trehalose was predominant. Phosphatidylcholines (PC), PA and sterols were major membrane lipids of the mycelium, while PC and sterols were predominant in fruit bodies. The degree of fatty acids unsaturation of the main mycelium phospholipids (PC and PA) increased with age, while that of PC did not change regardless of the developmental stage. In young mycelium, storage lipids were represented mainly by free fatty acids, and in mature mycelium and fruit bodies-by triacylglycerols. Fruit bodies contained three times less membrane lipids and twice as many storage lipids as mycelium. Trehalose was the main cytosol carbohydrate in the mycelium and fruit bodies, which confirms its key value for alkaliphily.

Disturbance in biosynthesis of arachidonic acid impairs the sexual development of the onion blight pathogen Stemphylium eturmiunum

The formation of sexual fruiting bodies for plant pathogenic fungi is a key strategy to propagate their progenies upon environmental stresses. Stemphylium eturmiunum is an opportunistic plant pathogen fungus causing blight in onion. This self-fertilizing filamentous ascomycete persists in the soil by forming pseudothecia, the sexual fruiting body which helps the fungus survive in harsh environments. However, the regulatory mechanism of pseudothecial formation remains unknown. To uncover the mechanism for pseudothecial formation so as to find a practical measure to control the propagation of this onion pathogen, we tentatively used DNA methyltransferase inhibitor 5-azacytidine (5-AC) to treat S. eturmiunum. 5-AC treatment silenced the gene-encoding monoacylglycerol lipase (magl) concomitant with the presence of the inheritable fluffy phenotype and defectiveness in pseudothecial development. Moreover, the silence of magl also resulted in a reduction of arachidonic acid (AA) formation from 27 ± 3.1 µg/g to 9.5 ± 1.5 µg/g. To correlate the biosynthesis of AA and pseudothecial formation, we created magl knockdown and overexpression strains. Knockdown of magl reduced AA to 11 ± 2.4 µg/g, which subsequently disabled pseudothecial formation. In parallel, overexpression of magl increased AA to 37 ± 3.4 µg/g, which also impaired pseudothecial formation. Furthermore, exogenous addition of AA to the culture of magl-silenced or magl knockdown strains rescued the pseudothecial formation but failed in the gpr1 knockdown strain of S. eturmiunum, which implicates the involvement of AA in signal transduction via a putative G protein-coupled receptor 1. Thus, AA at a cellular level of 27 ± 3.1 µg/g is essential for sexual development of S. eturmiunum. Disturbance in the biosynthesis of AA by up- and down-regulating the expression of magl disables the pseudothecial development. The specific requirement for AA in pseudothecial development by S. eturmiunum provides a hint to curb this onion pathogen: to impede pseudothecial formation by application of AA.

Reorganization of plasma membrane lipid domains during conidial germination

Neurospora crassa, a filamentous fungus, in the unicellular conidial stage has ideal features to study sphingolipid (SL)-enriched domains, which are implicated in fundamental cellular processes ranging from antifungal resistance to apoptosis. Several changes in lipid metabolism and in the membrane composition of N. crassa occur during spore germination. However, the biophysical impact of those changes is unknown. Thus, a biophysical study of N. crassa plasma membrane, particularly SL-enriched domains, and their dynamics along conidial germination is prompted. Two N. crassa strains, wild-type (WT) and slime, which is devoid of cell wall, were studied. Conidial growth of N. crassa WT from a dormancy state to an exponential phase was accompanied by membrane reorganization, namely an increase of membrane fluidity, occurring faster in a supplemented medium than in Vogel's minimal medium. Gel-like domains, likely enriched in SLs, were found in both N. crassa strains, but were particularly compact, rigid and abundant in the case of slime cells, even more than in budding yeast Saccharomyces cerevisiae. In N. crassa, our results suggest that the melting of SL-enriched domains occurs near growth temperature (30°C) for WT, but at higher temperatures for slime. Regarding biophysical properties strongly affected by ergosterol, the plasma membrane of slime conidia lays in between those of N. crassa WT and S. cerevisiae cells. The differences in biophysical properties found in this work, and the relationships established between membrane lipid composition and dynamics, give new insights about the plasma membrane organization and structure of N. crassa strains during conidial growth.

Peroxisomes and sexual development in fungi

Peroxisomes are versatile and dynamic organelles that are essential for the development of most eukaryotic organisms. In fungi, many developmental processes, such as sexual development, require the activity of peroxisomes. Sexual reproduction in fungi involves the formation of meiotic-derived sexual spores, often takes place inside multicellular fruiting bodies and requires precise coordination between the differentiation of multiple cell types and the progression of karyogamy and meiosis. Different peroxisomal functions contribute to the orchestration of this complex developmental process. Peroxisomes are required to sustain the formation of fruiting bodies and the maturation and germination of sexual spores. They facilitate the mobilization of reserve compounds via fatty acid β-oxidation and the glyoxylate cycle, allowing the generation of energy and biosynthetic precursors. Additionally, peroxisomes are implicated in the progression of meiotic development. During meiotic development in Podospora anserina, there is a precise modulation of peroxisome assembly and dynamics. This modulation includes changes in peroxisome size, number and localization, and involves a differential activity of the protein-machinery that drives the import of proteins into peroxisomes. Furthermore, karyogamy, entry into meiosis and sorting of meiotic-derived nuclei into sexual spores all require the activity of peroxisomes. These processes rely on different peroxisomal functions and likely depend on different pathways for peroxisome assembly. Indeed, emerging studies support the existence of distinct import channels for peroxisomal proteins that contribute to different developmental stages.

Proteomic analysis of Trichoderma atroviride reveals independent roles for transcription factors BLR-1 and BLR-2 in light and darkness

The genus Trichoderma is one of the most widely used biological control agents of plant-pathogenic fungi. The main mechanism for survival and dispersal of Trichoderma is through the production of asexual spores (conidia). The transition from filamentous growth to conidiation can be triggered by light, nutrient deprivation, and mechanical damage of the mycelium. We conducted proteomic profiling analyses of Trichoderma atroviride after a blue light pulse. The use of two-dimensional electrophoresis (2-DE) and mass spectrometry (MS) analysis allowed us to identify 72 proteins whose expression was affected by blue light. Functional category analysis showed that the various proteins are involved in metabolism, cell rescue, and protein synthesis. We determined the relationship between mRNA levels of selected genes 30 min after a light pulse and protein expression levels at different times after the pulse and found this correlation to be very weak. The correlation was highest when protein and mRNA levels were compared for the same time point. The transcription factors BLR-1 and BLR-2 are vital to the photoconidiation process; here we demonstrate that both BLR proteins are active in darkness and affect several elements at both the transcript and protein levels. Unexpectedly, in darkness, downregulation of proteins prevailed in the Δblr-1 mutant, while upregulation of proteins predominated in the Δblr-2 mutant. Our data demonstrate that the BLR proteins play roles individually and as a complex.

Genome-wide gene expression profiling of fertilization competent mycelium in opposite mating types in the heterothallic fungus Podospora anserina

Background

Mating-type loci in yeasts and ascomycotan filamentous fungi (Pezizomycotina) encode master transcriptional factors that play a critical role in sexual development. Genome-wide analyses of mating-type-specification circuits and mating-type target genes are available in Saccharomyces cerevisiae and Schizosaccharomyces pombe; however, no such analyses have been performed in heterothallic (self-incompatible) Pezizomycotina. The heterothallic fungus Podospora anserina serves as a model for understanding the basic features of mating-type control. Its mat+ and mat− mating types are determined by dissimilar allelic sequences. The mat− sequence contains three genes, designated FMR1, SMR1 and SMR2, while the mat+ sequence contains one gene, FPR1. FMR1 and FPR1 are the major regulators of fertilization, and this study presents a genome-wide view of their target genes and analyzes their target gene regulation.

Methodology/Principal Findings

The transcriptomic profiles of the mat+ and mat− strains revealed 157 differentially transcribed genes, and transcriptomic analysis of fmr1⁻ and fpr1⁻ mutant strains was used to determine the regulatory actions exerted by FMR1 and FPR1 on these differentially transcribed genes. All possible combinations of transcription repression and/or activation by FMR1 and/or FPR1 were observed. Furthermore, 10 additional mating-type target genes were identified that were up- or down-regulated to the same level in mat+ and mat− strains. Of the 167 genes identified, 32 genes were selected for deletion, which resulted in the identification of two genes essential for the sexual cycle. Interspecies comparisons of mating-type target genes revealed significant numbers of orthologous pairs, although transcriptional profiles were not conserved between species.

Conclusions/Significance

This study represents the first comprehensive genome-wide analysis of mating-type direct and indirect target genes in a heterothallic filamentous fungus. Mating-type transcription factors have many more target genes than are found in yeasts and exert a much greater diversity of regulatory actions on target genes, most of which are not directly related to mating.

Multi-targeted priming for genome-wide gene expression assays

BACKGROUND

Complementary approaches to assaying global gene expression are needed to assess gene expression in regions that are poorly assayed by current methodologies. A key component of nearly all gene expression assays is the reverse transcription of transcribed sequences that has traditionally been performed by priming the poly-A tails on many of the transcribed genes in eukaryotes with oligo-dT, or by priming RNA indiscriminately with random hexamers. We designed an algorithm to find common sequence motifs that were present within most protein-coding genes of Saccharomyces cerevisiae and of Neurospora crassa, but that were not present within their ribosomal RNA or transfer RNA genes. We then experimentally tested whether degenerately priming these motifs with multi-targeted primers improved the accuracy and completeness of transcriptomic assays.

RESULTS

We discovered two multi-targeted primers that would prime a preponderance of genes in the genomes of Saccharomyces cerevisiae and Neurospora crassa while avoiding priming ribosomal RNA or transfer RNA. Examining the response of Saccharomyces cerevisiae to nitrogen deficiency and profiling Neurospora crassa early sexual development, we demonstrated that using multi-targeted primers in reverse transcription led to superior performance of microarray profiling and next-generation RNA tag sequencing. Priming with multi-targeted primers in addition to oligo-dT resulted in higher sensitivity, a larger number of well-measured genes and greater power to detect differences in gene expression.

CONCLUSIONS

Our results provide the most complete and detailed expression profiles of the yeast nitrogen starvation response and N. crassa early sexual development to date. Furthermore, our multi-targeting priming methodology for genome-wide gene expression assays provides selective targeting of multiple sequences and counter-selection against undesirable sequences, facilitating a more complete and precise assay of the transcribed sequences within the genome.

Meiotic regulators Ndt80 and ime2 have different roles in Saccharomyces and Neurospora

Meiosis is a highly regulated process in eukaryotic species. The filamentous fungus Neurospora crassa has been shown to be missing homologs of a number of meiotic initiation genes conserved in Saccharomyces cerevisiae, but has three homologs of the well-characterized middle meiotic transcriptional regulator NDT80. In this study, we evaluated the role of all three NDT80 homologs in the formation of female reproductive structures, sexual development, and meiosis. We found that none of the NDT80 homologs were required for meiosis and that even the triple mutant was unaffected. However, strains containing mutations in NCU09915 (fsd-1) were defective in female sexual development and ascospore maturation. vib-1 was a major regulator of protoperithecial development in N. crassa, and double mutants carrying deletions of both vib-1 (NCU03725) and fsd-1 exhibited a synergistic effect on the timing of female reproductive structure (protoperithecia) formation. We further evaluated the role of the N. crassa homolog of IME2, a kinase involved in initiation of meiosis in S. cerevisiae. Strains containing mutations in ime-2 showed unregulated development of protoperithecia. Genetic analysis indicated that mutations in vib-1 were epistatic to ime-2, suggesting that IME-2 may negatively regulate VIB-1 activity. Our data indicate that the IME2/NDT80 pathway is not involved in meiosis in N. crassa, but rather regulates the formation of female reproductive structures.

Light regulation of metabolic pathways in fungi

Light represents a major carrier of information in nature. The molecular machineries translating its electromagnetic energy (photons) into the chemical language of cells transmit vital signals for adjustment of virtually every living organism to its habitat. Fungi react to illumination in various ways, and we found that they initiate considerable adaptations in their metabolic pathways upon growth in light or after perception of a light pulse. Alterations in response to light have predominantly been observed in carotenoid metabolism, polysaccharide and carbohydrate metabolism, fatty acid metabolism, nucleotide and nucleoside metabolism, and in regulation of production of secondary metabolites. Transcription of genes is initiated within minutes, abundance and activity of metabolic enzymes are adjusted, and subsequently, levels of metabolites are altered to cope with the harmful effects of light or to prepare for reproduction, which is dependent on light in many cases. This review aims to give an overview on metabolic pathways impacted by light and to illustrate the physiological significance of light for fungi. We provide a basis for assessment whether a given metabolic pathway might be subject to regulation by light and how these properties can be exploited for improvement of biotechnological processes.

Genetic analysis of the role of peroxisomes in the utilization of acetate and fatty acids in Aspergillus nidulans

Peroxisomes are organelles containing a diverse array of enzymes. In fungi they are important for carbon source utilization, pathogenesis, development, and secondary metabolism. We have studied Aspergillus nidulans peroxin (pex) mutants isolated by virtue of their inability to grow on butyrate or by the inactivation of specific pex genes. While all pex mutants are able to form colonies, those unable to import PTS1 proteins are partially defective in asexual and sexual development. The pex mutants are able to grow on acetate but are affected in growth on fatty acids, indicating a requirement for the peroxisomal localization of beta-oxidation enzymes. However, mislocalization of malate synthase does not prevent growth on either fatty acids or acetate, showing that the glyoxylate cycle does not require peroxisomal localization. Proliferation of peroxisomes is dependent on fatty acids, but not on acetate, and on PexK (Pex11), expression of which is activated by the FarA transcription factor. Proliferation was greatly reduced in a farADelta strain. A mutation affecting a mitochodrial ketoacyl-CoA thiolase and disruption of a mitochondrial hydroxy-acyl-CoA dehydrogenase gene prevented growth on short-chain but not long-chain fatty acids. Together with previous results, this is consistent with growth on even-numbered short-chain fatty acids requiring a mitochondrial as well as a peroxisomal beta-oxidation pathway. The mitochondrial pathway is not required for growth on valerate or for long-chain fatty acid utilization.

Spore germination in Mortierella alpina is associated with a transient depletion of arachidonic acid and induction of fatty acid desaturase gene expression

Mortierella alpina is an oleaginous filamentous fungus whose vegetative mycelium is known to accumulate triglyceride oil containing large amounts of arachidonic acid (ARA 20:4, n - 6). We report that the spores of Mortierella alpina also contain a large proportion of ARA, comprising 50% of total fatty acid. Fatty acid desaturase genes were not expressed in dormant spores but were induced during germination, following a significant drop in the level of ARA (down from 50% of total fatty acid to 12%) prior to germ-tube emergence. We propose that ARA serves as a reserve supply of carbon and energy that is utilised during the early stages of spore germination in Mortierella alpina.

The multifunctional beta-oxidation enzyme is required for full symptom development by the biotrophic maize pathogen Ustilago maydis

The transition from yeast-like to filamentous growth in the biotrophic fungal phytopathogen Ustilago maydis is a crucial event for pathogenesis. Previously, we showed that fatty acids induce filamentation in U. maydis and that the resulting hyphal cells resemble the infectious filaments observed in planta. To explore the potential metabolic role of lipids in the morphological transition and in pathogenic development in host tissue, we deleted the mfe2 gene encoding the multifunctional enzyme that catalyzes the second and third reactions in beta-oxidation of fatty acids in peroxisomes. The growth of the strains defective in mfe2 was attenuated on long-chain fatty acids and abolished on very-long-chain fatty acids. The mfe2 gene was not generally required for the production of filaments during mating in vitro, but loss of the gene blocked extensive proliferation of fungal filaments in planta. Consistent with this observation, mfe2 mutants exhibited significantly reduced virulence in that only 27% of infected seedlings produced tumors compared to 88% tumor production upon infection by wild-type strains. Similarly, a defect in virulence was observed in developing ears upon infection of mature maize plants. Specifically, the absence of the mfe2 gene delayed the development of teliospores within mature tumor tissue. Overall, these results indicate that the ability to utilize host lipids contributes to the pathogenic development of U. maydis.

Signalling pathways connecting mycotoxin production and sporulation

SUMMARY Mycotoxin contamination of food and feed presents a serious food safety issue on a global scale, causing tremendous yield and economic losses. These toxins, produced largely by members of the genera Aspergillus and Fusarium, represent a subset of the impressive array of secondary metabolites produced by filamentous fungi. Some secondary metabolites are associated temporally and functionally with sporulation. In Aspergillus and Fusarium, sporulation and mycotoxin production are both regulated by G protein signalling pathways. G protein signalling pathways commonly regulate fungal development, stress response and expression of virulence traits. In addition, fungal development is influenced by external factors. Among these are lipids, and in particular, oxylipin signals, which may be derived from either the fungus or infected seeds. Regardless of origin, oxylipins have the potential to elicit profound changes in both sporulation and mycotoxin production in the fungus. Signal transduction via G protein signalling pathways represents one mechanism by which oxylipin signals might elicit these changes. Therefore, in this review we integrate discussion of oxylipin signals and of G protein signalling cascades as regulators of fungal development.

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.

Three putative oxylipin biosynthetic genes integrate sexual and asexual development in Aspergillus nidulans

Oxylipins called psi factors have been shown to alter the ratio of asexual to sexual sporulation in the filamentous fungusAspergillus nidulans. Analysis of theA. nidulansgenome has led to the identification of three fatty acid oxygenases (PpoA, PpoB and PpoC) predicted to produce psi factors. Here, it is reported that deletion ofppoB(ΔppoB) reduced production of the oleic-acid-derived oxylipin psiBβand increased the ratio of asexual to sexual spore development. Generation of the triple mutant ΔppoAΔppoBΔppoCresulted in a strain deficient in producing oleic- and linoleic-acid-derived 8′-hydroxy psi factor and caused increased and mis-scheduled activation of sexual development. Changes in asexual to sexual spore development were positively correlated to alterations in the expression ofbrlAandveA, respectively. PpoB and/or its products antagonistically mediate the expression levels ofppoAandppoC, thus revealing regulatory feedback loops among these three genes. Phylogenetic analyses showed thatppogenes are present in both saprophytic and pathogenic Ascomycetes and Basidiomycetes, suggesting a conserved role for Ppo enzymes in the life cycle of fungi.

Isolation of genes differentially expressed during the fruit body development ofPleurotus ostreatusby differential display of RAPD

To analyze genes involved in fruit body development of Pleurotus ostreatus, mRNAs from three different developmental stages: i.e., vegetative mycelium, primordium, and mature fruit body, were isolated and reverse-transcribed to cDNAs. One hundred and twenty random PCR amplifications were performed with the cDNAs, which generated 382, 394, 393 cDNA fragments from each developmental stage. From these fragments, four cDNA clones specifically expressed in primordium or mature fruit body were detected. Sequence analysis and database searches revealed significant similarity with triacylglycerol lipase, cytochrome P450 sterol 14 alpha-demethylase and developmentally regulated genes of other fungi. Northern blot analyses confirmed that all of the four cDNAs were unexpressed in mycelium, thus stage-specific genes for fruit body formation of P. ostreatus were successfully isolated.

Deletion of the Delta12-oleic acid desaturase gene of a nonaflatoxigenic Aspergillus parasiticus field isolate affects conidiation and sclerotial development

AIMS

To investigate how linoleic acid affects conidial production and sclerotial development in a strictly mitotic Aspergillus parasiticus field isolate as related to improving biocompetitivity of atoxigenic Aspergillus species.

METHODS AND RESULTS

We disrupted A. parasiticusDelta12-oleic acid desaturase gene (odeA) responsible for the conversion of oleic acid to linoleic acid. We examined conidiation and sclerotial development of SRRC 2043 and three isogenic mutant strains deleted for the odeA gene (DeltaodeA), either with or without supplementing linoleic acid, on one complex potato dextrose agar (PDA) medium and on two defined media: nitrate-containing Czapek agar (CZ) and Cove's ammonium medium (CVN). The DeltaodeA mutants produced less conidia than the parental strain on all media. Linoleic acid supplementation (as sodium linoleate at 0.3 and 1.2 mg ml(-1)) restored the DeltaodeA conidial production comparable to or exceeding the unsupplemented parental level, and the effect was medium dependent, with the highest increase on CVN and the least on PDA. SRRC 2043 and the DeltaodeA mutants were unable to produce sclerotia on CVN. On unsupplemented PDA and CZ, DeltaodeA sclerotial mass was comparable to that of SRRC 2043, but sclerotial number increased significantly to two- to threefold. Supplementing linoleic acid to media, in general, tended to decrease wild type and DeltaodeA sclerotial mass and sclerotial number.

CONCLUSIONS

Linoleic acid stimulates conidial production but has an inhibitory effect on sclerotial development. The relationship between the two processes in A. parasiticus is complex and affected by multiple factors, such as fatty acid composition and nitrogen source.

SIGNIFICANCE AND IMPACT OF STUDY

Conditions that promote sclerotial development differ from those required to promote maximum conidial production. Manipulation of content and availability of linoleic acid at different fungal growth phases might optimize conidial and sclerotial production hence increasing the efficacy of biocompetitive Aspergillus species.

Lipid-induced filamentous growth in Ustilago maydis

The phytopathogenic fungus Ustilago maydis is obligately dependent on infection of maize to complete the sexual phase of its life cycle. Mating interactions between haploid, budding cells establish an infectious filamentous cell type that invades the host, induces large tumours and eventually forms large masses of black spores. The ability to switch from budding to filamentous growth is therefore critical for infection and completion of the life cycle, although the signals that influence the transition have not been identified from the host or the environment. We have found that growth in the presence of lipids promotes a filamentous phenotype that resembles the infectious cell type found in planta. In addition, the ability of the fungus to respond to lipids is dependent on both the cAMP signalling pathway and a Ras/MAPK pathway; these pathways are known to regulate mating, filamentous growth and pathogenesis in U. maydis. Overall, these results lead us to hypothesize that lipids may represent one of the signals that promote and maintain the filamentous growth of the fungus in the host environment.

Differential expression of desaturases and changes in fatty acid composition during sporangiospore germination and development in Mucor rouxii

Polyunsaturated fatty acids (PUFAs), namely, oleic (C18:1), linoleic (C18:2), and gamma-linolenic acid (C18:3), constituted the majority in the total fatty acid content (44%) of sporangiospores of Mucor rouxii. At 30 degrees C, the germination begins within 1h at which time spore swelling occurs, followed by germ tube emergence within 3-4h. Throughout germination, an increase in gamma-linolenic acid (GLA) was observed and its content was highest at germ tube emergence. It took longer for sporangiospores of M. rouxii to germinate at sub-optimal temperatures (15 and 35 degrees C). However, the content of GLA was higher at the germ tube initiation than at the mycelial stage at all temperatures, suggesting the association of GLA and germination of sporangiospores. This finding was substantially confirmed by differential expression of delta9-, delta12-, and delta6-desaturase genes measured during spore germination. The expression of three desaturase genes parallels the pattern of GLA synthesis. By using RT-PCR techniques to follow gene expression, we found that mRNA of delta12- and delta6-desaturase genes were translated as soon as the spores were introduced into a fresh medium while the mRNA of delta9-desaturase gene could not be detected until 2h after introduction. A sharp increase in mRNA of delta6-desaturase genes correlated well with an increase in GLA content at germ tube emergence (4h). These results demonstrated that changes in fatty acid composition of sporangiospore of M. rouxii and differential expression of desaturase genes occurred during germination, and that extensive changes in GLA synthesis associated with some events in germination process.

Genetic Connection between Fatty Acid Metabolism and Sporulation in Aspergillus nidulans

In the Ascomycete fungus Aspergillus nidulans, the ratio of conidia (asexual spores) to ascospores (sexual spores) is affected by linoleic acid moieties including endogenous sporogenic factors called psi factors. Deletion of odeA (Delta odeA), encoding a Delta-12 desaturase that converts oleic acid to linoleic acid, resulted in a strain depleted of polyunsaturated fatty acids (18:2 and 18:3) but increased in oleic acid (18:1) and total percent fatty acid content. Linoleic acid-derived psi factors were absent in this strain but oleic acid-derived psi factors were increased relative to wild type. The Delta odeA strain was reduced in conidial production and mycelial growth; these effects were most noticeable when cultures were grown at 26 degrees C in the dark. Under these environmental conditions, the Delta odeA strain was delayed in ascospore production but produced more ascospores than wild type over time. This suggests a role for oleic acid-derived psi factors in affecting the asexual to sexual spore ratio in A. nidulans. Fatty acid composition and spore development were also affected by veA, a gene previously shown to control light driven conidial and ascospore development. Taken together our results indicate an interaction between veA and odeA alleles for fatty acid metabolism and spore development in A. nidulans.

Cell differentiation during sexual development of the fungus Sordaria macrospora requires ATP citrate lyase activity

During sexual development, mycelial cells from most filamentous fungi differentiate into typical fruiting bodies. Here, we describe the isolation and characterization of the Sordaria macrospora developmental mutant per5, which exhibits a sterile phenotype with defects in fruiting body maturation. Cytological investigations revealed that the mutant strain forms only ascus precursors without any mature spores. Using an indexed cosmid library, we were able to complement the mutant to fertility by DNA-mediated transformation. A single cosmid clone, carrying a 3.5-kb region able to complement the mutant phenotype, has been identified. Sequencing of the 3.5-kb region revealed an open reading frame of 2.1 kb interrupted by a 66-bp intron. The predicted polypeptide (674 amino acids) shows significant homology to eukaryotic ATP citrate lyases (ACLs), with 62 to 65% amino acid identity, and the gene was named acl1. The molecular mass of the S. macrospora ACL1 polypeptide is 73 kDa, as was verified by Western blot analysis with a hemagglutinin (HA) epitope-tagged ACL1 polypeptide. Immunological in situ detection of the HA-tagged polypeptide demonstrated that ACL is located within the cytosol. Sequencing of the mutant acl1 gene revealed a 1-nucleotide transition within the coding region, resulting in an amino acid substitution within the predicted polypeptide. Further evidence that ACL1 is essential for fruiting body maturation comes from experiments in which truncated and mutated versions of the acl1 gene were used for transformation. None of these copies was able to reconstitute the fertile phenotype in transformed per5 recipient strains. ACLs are usually involved in the formation of cytosolic acetyl coenzyme A (acetyl-CoA), which is used for the biosynthesis of fatty acids and sterols. Protein extracts from the mutant strain showed a drastic reduction in enzymatic activity compared to values obtained from the wild-type strain. Investigation of the time course of ACL expression suggests that ACL is specifically induced at the beginning of the sexual cycle and produces acetyl-CoA, which most probably is a prerequisite for fruiting body formation during later stages of sexual development. We discuss the contribution of ACL activity to the life cycle of S. macrospora.