cAMP and ras signalling independently control spore germination in the filamentous fungus Aspergillus nidulans

Carbon source affects PKA-dependent polarity of Neurospora crassa in a CRE-1-dependent and independent manner

A defect in mcb, encoding the cAMP-dependent protein kinase A (PKA) regulatory subunit in Neurospora crassa, which confers an apolar growth phenotype, is accompanied by an increase in PKA activity levels. Both PKA and CRE-1 [a key carbon catabolite repression (CCR) regulator] mediate the cellular response to carbon-source availability. Inactivation of the cre-1 gene resulted in reduced growth rate, abnormal hyphal morphology and altered CCR. Both PKA and CRE-1 affected morphology in a carbon-dependent manner, as fructose suppressed the apolar morphology of the mcb strain and enabled faster growth of the Deltacre-1 mutant. An increase in cre-1 transcript abundance was observed in mcb and a reduction in PKA activity levels was measured in Deltacre-1. CRE-1 is involved in determining PKA-dependent polarity, as an mcb;Deltacre-1 strain displayed partial reestablishment of hyphal polarity. Taken together, our results demonstrate regulatory interactions between PKA and CRE-1 that affect cell polarity in a filamentous fungus.

A p21-activated kinase is required for conidial germination in Penicillium marneffei

Asexual spores (conidia) are the infectious propagules of many pathogenic fungi, and the capacity to sense the host environment and trigger conidial germination is a key pathogenicity determinant. Germination of conidia requires the de novo establishment of a polarised growth axis and consequent germ tube extension. The molecular mechanisms that control polarisation during germination are poorly understood. In the dimorphic human pathogenic fungus Penicillium marneffei, conidia germinate to produce one of two cell types that have very different fates in response to an environmental cue. At 25 degrees C, conidia germinate to produce the saprophytic cell type, septate, multinucleate hyphae that have the capacity to undergo asexual development. At 37 degrees C, conidia germinate to produce the pathogenic cell type, arthroconidiating hyphae that liberate uninucleate yeast cells. This study shows that the p21-activated kinase pakA is an essential component of the polarity establishment machinery during conidial germination and polarised growth of yeast cells at 37 degrees C but is not required for germination or polarised growth at 25 degrees C. Analysis shows that the heterotrimeric G protein alpha subunit GasC and the CDC42 orthologue CflA lie upstream of PakA for germination at both temperatures, while the Ras orthologue RasA only functions at 25 degrees C. These findings suggest that although some proteins that regulate the establishment of polarised growth in budding yeast are conserved in filamentous fungi, the circuitry and downstream effectors are differentially regulated to give rise to distinct cell types.

Heterotrimeric G protein signaling in filamentous fungi

Filamentous fungi are multicellular eukaryotic organisms known for nutrient recycling as well as for antibiotic and food production. This group of organisms also contains the most devastating plant pathogens and several important human pathogens. Since the first report of heterotrimeric G proteins in filamentous fungi in 1993, it has been demonstrated that G proteins are essential for growth, asexual and sexual development, and virulence in both animal and plant pathogenic filamentous species. Numerous G protein subunit and G protein-coupled receptor genes have been identified, many from whole-genome sequences. Several regulatory pathways have now been delineated, including those for nutrient sensing, pheromone response and mating, and pathogenesis. This review provides a comparative analysis of G protein pathways in several filamentous species, with discussion of both unifying themes and important unique signaling paradigms.

Ras GTPase-activating protein regulation of actin cytoskeleton and hyphal polarity in Aspergillus nidulans

Aspergillus nidulans gapA1, a mutation leading to compact, fluffy colonies and delayed polarity establishment, maps to a gene encoding a Ras GTPase-activating protein. Domain organization and phylogenetic analyses strongly indicate that GapA regulates one or more "true" Ras proteins. A gapADelta strain is viable. gapA colonies are more compact than gapA1 colonies and show reduced conidiation. gapADelta strains have abnormal conidiophores, characterized by the absence of one of the two layers of sterigmata seen in the wild type. gapA transcript levels are very low in conidia but increase during germination and reach their maximum at a time coincident with germ tube emergence. Elevated levels persist in hyphae. In germinating conidiospores, gapADelta disrupts the normal coupling of isotropic growth, polarity establishment, and mitosis, resulting in a highly heterogeneous cell population, including malformed germlings and a class of giant cells with no germ tubes and a multitude of nuclei. Unlike wild-type conidia, gapADelta conidia germinate without a carbon source. Giant multinucleated spores and carbon source-independent germination have been reported in strains carrying a rasA dominant active allele, indicating that GapA downregulates RasA. gapADelta cells show a polarity maintenance defect characterized by apical swelling and subapical branching. The strongly polarized wild-type F-actin distribution is lost in gapADelta cells. As GapA-green fluorescent protein shows cortical localization with strong predominance at the hyphal tips, we propose that GapA-mediated downregulation of Ras signaling at the plasma membrane of these tips is involved in the polarization of the actin cytoskeleton that is required for hyphal growth and, possibly, for asexual morphogenesis.

cAMP signalling is involved in growth, germination, mycoparasitism and secondary metabolism in Trichoderma virens

An adenylate-cyclase-encoding gene, tac1, of Trichoderma virens, a soil fungus used in the biocontrol of plant pathogens, has been cloned and sequenced. The tac1 ORF spanned 7032 bp, encoding a protein of 2153 aa, which shared an identity of 65 % with the adenylate cyclase of Colletotrichum lagenarium. Deletion of tac1, through double-crossover homologous recombination, lowered the intracellular cAMP levels to below the detection limit. The mutants showed only 5-6 % of the wild-type growth rate on agar, but grew normally in shake culture. The mutants did not sporulate in darkness, and the spores failed to germinate in water. In the confrontation assay, the mutants did not overgrow the test plant pathogens Sclerotium rolfsii, Rhizoctonia solani and Pythium sp. Against Pythium sp., the mutants produced a clear zone of inhibition in the confrontation assay. HPLC analysis and bioassay showed reduced secondary metabolite production in the mutants. Using suppression subtractive hybridization (SSH), the genes that were underexpressed in the mutants were identified. Based on an array of 53 SSH library clones, 11 clones were identified as strongly downregulated in the Deltatac1 mutants; of these 11 clones, nine sequences were homologous to secondary metabolism-related gene sequences. Therefore, cAMP signalling positively regulates secondary metabolism in T. virens. This is believed to be the first direct genetic study on the role of cAMP signalling in a Trichoderma sp. Tac1 is also believed to be the first regulatory protein to be identified in T. virens that is involved in growth, germination, mycoparasitism and secondary metabolism.

Fungal nucleobase transporters

Early genetic and physiological work in bacteria and fungi has suggested the presence of highly specific nucleobase transport systems. Similar transport systems are now known to exist in algae, plants, protozoa and metazoa. Within the last 15 years, a small number of microbial genes encoding nucleobase transporters have been cloned and studied in great detail. The sequences of several other putative proteins submitted to databases are homologous to the microbial nucleobase transporters but their physiological functions remain largely undetermined. In this review, genetic, biochemical and molecular data are described concerning mostly the nucleobase transporters of Aspergillus nidulans and Saccharomyces cerevisiae, the two model ascomycetes from which the great majority of data come from. It is also discussed as to what is known on the nucleobase transporters of the two most significant pathogenic fungi: Candida albicans and Aspergillus fumigatus. Apart from highlighting how a basic process such as nucleobase recognition and transport operates, this review intends to highlight features that might be applicable to antifungal pharmacology.

The Aspergillus nidulans cetA and calA genes are involved in conidial germination and cell wall morphogenesis

The Aspergillus nidulans genes cetA (AN3079.2) and calA (AN7619.2) encode a novel class of fungal thaumatin-like proteins of unknown function. Deletion of cetA does not result in an observable phenotype [Greenstein, S., Shadkchan, Y., Jadoun, J., Sharon, C., Markovich, S., Osherov, N., 2006. Analysis of the Aspergillus nidulans thaumatin-like cetA gene and evidence for transcriptional repression of pyr4 expression in the cetA-disrupted strain. Fungal Genet. Biol. 43, 42-53]. We prepared knockout calA and calA/cetA A. nidulans strains. The calA mutants were phenotypically identical to the wild-type. In contrast, the cetA/calA double mutant showed a synthetic lethal phenotype suggesting that the two genes affect a single function or pathway: most of its conidia were completely inhibited in germination. Many collapsed and underwent lysis. A few showed abnormal germination characterized by short swollen hyphae and abnormal hyphal branching. Nongerminated conidia contained a single condensed nucleus suggesting a block in early germination. This is the first functional analysis of the novel cetA/calA family of thaumatin-like genes and their role in A. nidulans conidial germination. We show that CETA and CALA are secreted proteins that together play an essential role in early conidial germination.

The use of global transcriptional analysis to reveal the biological and cellular events involved in distinct development phases of Trichophyton rubrum conidial germination

Background

Conidia are considered to be the primary cause of infections by Trichophyton rubrum.

Results

We have developed a cDNA microarray containing 10250 ESTs to monitor the transcriptional strategy of conidial germination. A total of 1561 genes that had their expression levels specially altered in the process were obtained and hierarchically clustered with respect to their expression profiles. By functional analysis, we provided a global view of an important biological system related to conidial germination, including characterization of the pattern of gene expression at sequential developmental phases, and changes of gene expression profiles corresponding to morphological transitions. We matched the EST sequences to GO terms in the Saccharomyces Genome Database (SGD). A number of homologues of Saccharomyces cerevisiae genes related to signalling pathways and some important cellular processes were found to be involved in T. rubrum germination. These genes and signalling pathways may play roles in distinct steps, such as activating conidial germination, maintenance of isotropic growth, establishment of cell polarity and morphological transitions.

Conclusion

Our results may provide insights into molecular mechanisms of conidial germination at the cell level, and may enhance our understanding of regulation of gene expression related to the morphological construction of T. rubrum.

Induction of glutathione S-transferase in biofilms and germinating spores of Mucor hiemalis strain EH5 from cold sulfidic spring waters

The occurrence and activation of glutathione S-transferase (GST) and the GST activities in biofilms in cold sulfidic spring waters were compared to the occurrence and activation of GST and the GST activities of the aquatic fungal strains EH5 and EH7 of Mucor hiemalis isolated for the first time from such waters. Using fluorescently labeled polyclonal anti-GST antibodies and GST activity measurements, we demonstrated that a high level of GST occurred in situ in natural biofilms and pure cultures of strain EH5. Measurement of microsomal and cytosolic soluble GST activities using different xenobiotic substrates, including 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene, 1,2-epoxy-3-(4-nitrophenoxy)propane, 1-iodo-2,4-dinitrobenzene, and fluorodifen, showed that the overall biotransforming abilities of biofilms were at least sixfold greater than that of strain EH5 alone. Increasing the level of sodium thiosulfate (STS) in the medium stimulated the microsomal and cytosolic GST activities with CDNB of strain EH5 about 44- and 94-fold, respectively, compared to the activities in the control. The induction of microsomal GST activity with fluorodifen by STS was strongly linear, but the initial strong linear increase in cytosolic GST activity with fluorodifen showed saturation-like effects at STS concentrations higher than approximately 1 mM. Using laser scanning confocal and conventional fluorescence microscopy, abundant fluorescently labeled GST proteins were identified in germinating sporangiospores of strain EH5 after activation by STS. High-performance size exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of at least two main GSTs ( approximately 27.8- and approximately 25.6-kDa subunits) in the cytosol of EH5, whereas the major 27.8-kDa subunit was the only GST in microsomes. We suggest that differential cellular GST expression takes place in strain EH5 depending on spore and hyphal development. Our results may contribute to our understanding of induction of GST by sulfurous compounds, as well as to the immunofluorescence visualization of GST in aquatic fungus and fungus-bacterium biofilms.

GPR-4 is a predicted G-protein-coupled receptor required for carbon source-dependent asexual growth and development in Neurospora crassa

The filamentous fungus Neurospora crassa is able to utilize a wide variety of carbon sources. Here, we examine the involvement of a predicted G-protein-coupled receptor (GPCR), GPR-4, during growth and development in the presence of different carbon sources in N. crassa. Deltagpr-4 mutants have reduced mass accumulation compared to the wild type when cultured on high levels of glycerol, mannitol, or arabinose. The defect is most severe on glycerol and is cell density dependent. The genetic and physical relationship between GPR-4 and the three N. crassa Galpha subunits (GNA-1, GNA-2, and GNA-3) was explored. All three Galpha mutants are defective in mass accumulation when cultured on glycerol. However, the phenotypes of Deltagna-1 and Deltagpr-4 Deltagna-1 mutants are identical, introduction of a constitutively activated gna-1 allele suppresses the defects of the Deltagpr-4 mutation, and the carboxy terminus of GPR-4 interacts most strongly with GNA-1 in the yeast two-hybrid assay. Although steady-state cyclic AMP (cAMP) levels are normal in Deltagpr-4 strains, exogenous cAMP partially remediates the dry mass defects of Deltagpr-4 mutants on glycerol medium and Deltagpr-4 strains lack the transient increase in cAMP levels observed in the wild type after addition of glucose to glycerol-grown liquid cultures. Our results support the hypothesis that GPR-4 is coupled to GNA-1 in a cAMP signaling pathway that regulates the response to carbon source in N. crassa. GPR-4-related GPCRs are present in the genomes of several filamentous ascomycete fungal pathogens, raising the possibility that a similar pathway regulates carbon sensing in these organisms.

Concerted responses between the chitin-binding protein secreting Streptomyces olivaceoviridis and Aspergillus proliferans

Streptomycetes belong to the ecologically important bacterial population within soil, which is also inhabited by many fungi. The highly chitinolytic Streptomyces olivaceoviridis and the ascomycete Aspergillus proliferans were chosen as models to test for interactions among bacteria and fungi. In medium lacking a soluble carbon source, individually cultivated spores of the bacterium S. olivaceoviridis and the fungus A. proliferans do not germinate. However, as shown by viability tests, cultivation of a mixture of both spore types provokes successive events: (i) stimulation of the germination of S. olivaceoviridis spores, (ii) initiation of the outgrowth of some fungal spores to which the S. olivaceoviridis chitinase ChiO1 adheres, (iii) massive extension of viable networks of S. olivaceoviridis hyphae at the expense of fungal hyphae and (iv) balanced proliferation of closely interacting fungal and S. olivaceoviridis hyphae. The replacement of the S. olivaceoviridis wild-type strain by a chromosomal disruption mutant (DeltaC), lacking production of the extracellular chitin-binding protein CHB1 but still secreting the chitinase ChiO1, provokes (v) germination of each spore type, (vi) retarded development of both partners, followed by (vii) preferential proliferation of the fungus. Together with biochemical and immunomicroscopy studies, the data support the conclusion that CHB1 molecules aggregate to an extracellular matrix, maintaining a close contact, followed by several concerted responses of the bacterium and the fungus.

Protein kinase A subunits of the ascomycete pathogen Mycosphaerella graminicola regulate asexual fructification, filamentation, melanization and osmosensing

SUMMARY As in many fungi, asexual reproduction of Mycosphaerella graminicola in planta is a complex process that requires proper differentiation of the infectious hyphae in the substomatal cavities of foliar tissue before pycnidia with conidia can be formed. In this study, we have investigated the role of the cAMP signalling pathway in development and pathogenicity of this pathogen by disruption of the genes encoding the catalytic (designated MgTpk2) and regulatory subunit (designated MgBcy1) of protein kinase A. The MgTpk2 and MgBcy1 mutants showed altered phenotypes in vitro when grown under different growth conditions. On potato dextrose agar (PDA), MgBcy1 mutants showed altered osmosensitivity and reduced melanization, whereas the MgTpk2 mutants showed accelerated melanization when compared with the M. graminicola IPO323 wild-type strain and ectopic transformants. MgTpk2 mutants also secreted a dark-brown pigment into yeast glucose broth medium. In germination and microconidiation assays, both mutants showed a germination pattern similar to that of the controls on water agar, whereas on PDA filamentous growth of MgTpk2 mutants was impaired. Pathogenicity assays showed that the MgTpk2 and MgBcy1 mutants were less virulent as they caused only limited chlorotic and necrotic symptoms at the tips of the inoculated leaves. Further analyses of the infection process showed that MgTpk2 and MgBcy1 mutants were able to germinate, penetrate and colonize mesophyll tissue, but were unable to produce the asexual fructifications, which was particularly due to inappropriate differentiation during the late stage of this morphogenesis-related process.

Systems biology of AGC kinases in fungi

Sch9 appears to be the Saccharomyces cerevisiae homolog of protein kinase B and is involved in the control of numerous nutrient-sensitive processes, including regulation of cell size, cell cycle progression, and stress resistance. Sch9 has also been implicated in the regulation of replicative and chronological life span. Systematic comparison of the phenotypes of sch9 and other AGC kinase mutants in fungal species with their counterparts in model eukaryotic organisms provides insight into the functions of AGC kinases. The availability of data from global studies of protein-protein interactions now makes it possible to predict and validate functional connections between Sch9, its putative substrates, and other proteins. This review highlights several emerging paradigms of AGC kinase signaling that are relevant for growth, development, and aging.

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.

The Zn(II)2Cys6 putative transcription factor NosA controls fruiting body formation in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans reproduces asexually with conidiospores and sexually with ascospores, both of which are the result of complex morphogenetic pathways. The developmental decisions for both ways of reproduction largely depend on the action of stage-specific transcription factors. Here we have characterized the putative Zn(II)(2)Cys(6) transcription factor NosA (number of sexual spores), a protein of 675 aa, which shares 44% sequence identity to Pro1 from Sordaria macrospora and 43% identity to A. nidulans RosA, a second protein of that class. The nosA gene was constitutively expressed during the life cycle of A. nidulans and was upregulated during late asexual development and upon carbon starvation. The NosA protein localized to nuclei. Both, NosA and RosA, regulate sexual development. Whereas RosA plays a role in early decisions and represses sexual development, NosA activity is required for primordium maturation. Interestingly, the two factors are genetically linked, because RosA repressed NosA expression. This illustrates that the balance of these two Zn(II)(2)Cys(6) proteins determines the fate of vegetative hyphae to undergo sexual development.

Different signalling pathways involving a Galpha protein, cAMP and a MAP kinase control germination of Botrytis cinerea conidia

Conidial germination of the grey mould fungus Botrytis cinerea was found to be induced by different chemical and physical signals, namely the amount and quality of nutrients as well as the hydrophobicity and rigidity of the surface. A B. cinerea Deltabcg3 mutant disrupted in the Galpha3 subunit of the heterotrimeric G protein was specifically defective in germination induced by carbon sources. A similar germination defect of an adenylate cyclase mutant, and the complementing effect of cAMP addition to conidia of these mutants confirmed the involvement of cAMP. In contrast, a Deltabmp1 MAP kinase mutant was delayed in carbon source-induced germination, but completely unable to germinate on hydrophobic surfaces. Based on these data, it is proposed that the germination response of B. cinerea conidia is controlled by three signalling pathways: Germination induction by rich media is weakly dependent on BMP1; induction by carbon sources requires BCG3, cAMP and BMP1; and induction by contact to hydrophobic surfaces is absolutely dependent on BMP1. Other defects of the Deltabcg3 mutant, such as low conidiation, excessive formation of sclerotia and delayed host infection, were also restored by cAMP. Microscopical studies of germling growth and differentiation on host cuticles revealed that the delayed infection of the Deltabcg3 mutant was due to a surface sensing defect leading to a reduced penetration. Thus, in addition to their role in germination, Galpha3, cAMP as well as BMP1 are required also for proper host surface recognition and penetration ability of germinated conidia.

A fungus-specific ras homolog contributes to the hyphal growth and virulence of Aspergillus fumigatus

The Ras family of GTPase proteins has been shown to control morphogenesis in many organisms, including several species of pathogenic fungi. In a previous study, we identified a gene encoding a fungus-specific Ras subfamily homolog, rasB, in Aspergillus fumigatus. Here we report that deletion of A. fumigatus rasB caused decreased germination and growth rates on solid media but had no effect on total biomass accumulation after 24 h of growth in liquid culture. The DeltarasB mutant had an irregular hyphal morphology characterized by increased branching. Expression of rasBDelta113-135, a mutant transgene lacking the conserved rasB internal amino acid insertion, did not complement the deletion phenotype of delayed growth and germination rates and abnormal hyphal morphology. Virulence of the rasB deletion strain was diminished; mice infected with this strain exhibited approximately 65% survival compared to approximately 10% with wild-type and reconstituted strains. These data support the hypothesis that rasB homologs, which are highly conserved among fungi that undergo hyphal growth, control signaling modules important to the directional growth of fungal hyphae.

The pkaB gene encoding the secondary protein kinase A catalytic subunit has a synthetic lethal interaction with pkaA and plays overlapping and opposite roles in Aspergillus nidulans

Filamentous fungal genomes contain two distantly related cyclic AMP-dependent protein kinase A catalytic subunits (PKAs), but only one PKA is found to play a principal role. In Aspergillus nidulans, PkaA is the primary PKA that positively functions in vegetative growth and spore germination but negatively controls asexual sporulation and production of the mycotoxin sterigmatocystin. In this report, we present the identification and characterization of pkaB, encoding the secondary PKA in A. nidulans. Although deletion of pkaB alone does not cause any apparent phenotypic changes, the absence of both pkaB and pkaA is lethal, indicating that PkaB and PkaA are essential for viability of A. nidulans. Overexpression of pkaB enhances hyphal proliferation and rescues the growth defects caused by DeltapkaA, indicating that PkaB plays a role in vegetative growth signaling. However, unlike DeltapkaA, deletion of pkaB does not suppress the fluffy-autolytic phenotype resulting from DeltaflbA. While upregulation of pkaB rescues the defects of spore germination resulting from DeltapkaA in the presence of glucose, overexpression of pkaB delays spore germination. Furthermore, upregulation of pkaB completely abolishes spore germination on medium lacking a carbon source. In addition, upregulation of pkaB enhances the level of submerged sporulation caused by DeltapkaA and reduces hyphal tolerance to oxidative stress. In conclusion, PkaB is the secondary PKA that has a synthetic lethal interaction with PkaA, and it plays an overlapping role in vegetative growth and spore germination in the presence of glucose but an opposite role in regulating asexual sporulation, germination in the absence of a carbon source, and oxidative stress responses in A. nidulans.

Analysis of the Aspergillus nidulans thaumatin-like cetA gene and evidence for transcriptional repression of pyr4 expression in the cetA-disrupted strain

The asexual spore or conidium plays a critical role in the life cycle of many filamentous fungi. However, the process of conidial germination remains surprisingly obscure. To better understand this process at the molecular level we characterized the Aspergillus nidulans cetA gene which is uniquely transcribed in conidiating cultures and whose transcript is significantly enriched in mature conidia. CetA is a member of a novel family of fungal genes of unknown function with homology to plant thaumatin-like (PR-5) defense proteins. We demonstrate by Northern analysis that cetA is a glucose-repressible gene. Transcriptional repression is dependent on the presence of protein kinase A. Western analysis indicates that the CETA protein is absent from conidia but is highly expressed during the first 6h of germination and is secreted into the medium. Disruption of the cetA gene seemingly results in delayed germination, slow growth, abnormal hyphal branching, and cell-wall defects. However, further analysis shows that the mutant phenotype is the result of glucose-dependent transcriptional repression of the pyr4 selectable marker used to disrupt the cetA gene. This is the first time that repression of a selectable marker ("position effect") has been reported in A. nidulans, a finding that may well be of significance in the analysis and interpretation of mutant phenotypes in this organism.

Colony sectorization of Metarhizium anisopliae is a sign of ageing

Spontaneous phenotypic degeneration resulting in sterile sectors is frequently observed when culturing filamentous fungi on artificial medium. Sterile sectors from two different strains of the insect pathogenic fungus Metarhizium anisopliae were investigated and found to contain reduced levels of cAMP and destruxins (insecticidal peptides). Microarray analysis using slides printed with 1730 clones showed that compared to wild-type, sterile sectors down-regulated 759 genes and upregulated 27 genes during growth in Sabouraud glucose broth or on insect cuticle. The differentially expressed genes are largely involved in cell metabolism (18.8 %), cell structure and function (13.6 %) and protein metabolism (8.8 %). Strong oxidative stress was demonstrated in sectorial cultures using the nitro blue tetrazolium assay and these cultures show other syndromes associated with ageing, including mitochondrial DNA alterations. However, genes involved in deoxidation and self-protection (e.g. heat-shock proteins, HSPs) were also upregulated. Further evidence of physiological adaptation by the degenerative sectorial cultures included cell-structure reorganization and the employment of additional signalling pathways. In spite of their very similar appearance, microarray analysis identified 181 genes differentially expressed between the two sectors, and the addition of exogenous cAMP only restored conidiation in one of them. Most of the differentially expressed genes were involved in catabolic or anabolic pathways, but the latter included genes for sporulation. Compared to the mammalian ageing process, sectorization in M. anisopliae showed many similarities, including similar patterns of cAMP production, oxidative stress responses and the involvement of HSPs. Thus, a common molecular machinery for ageing may exist throughout the eukaryotes.

Generation and phenotypic characterization of Aspergillus nidulans methylisocitrate lyase deletion mutants: methylisocitrate inhibits growth and conidiation

Propionate is a very abundant carbon source in soil, and many microorganisms are able to use this as the sole carbon source. Nevertheless, propionate not only serves as a carbon source for filamentous fungi but also acts as a preservative when added to glucose containing media. To solve this contradiction between carbon source and preservative effect, propionate metabolism of Aspergillus nidulans was studied and revealed the methylcitrate cycle as the responsible pathway. Methylisocitrate lyase is one of the key enzymes of that cycle. It catalyzes the cleavage of methylisocitrate into succinate and pyruvate and completes the alpha-oxidation of propionate. Previously, methylisocitrate lyase was shown to be highly specific for the substrate (2R,3S)-2-methylisocitrate. Here, the identification of the genomic sequence of the corresponding gene and the generation of deletion mutants is reported. Deletion mutants did not grow on propionate as sole carbon and energy source and were severely inhibited during growth on alternative carbon sources, when propionate was present. The strongest inhibitory effect was observed, when glycerol was the main carbon source, followed by glucose and acetate. In addition, asexual conidiation was strongly impaired in the presence of propionate. These effects might be caused by competitive inhibition of the NADP-dependent isocitrate dehydrogenase, because the K(i) of (2R,3S)-2-methylisocitrate, the product of the methylcitrate cycle, on NADP-dependent isocitrate dehydrogenase was determined as 1.55 microM. Other isomers had no effect on enzymatic activity. Therefore, methylisocitrate was identified as a potential toxic compound for cellular metabolism.

Aspergillus fumigatus conidial pigment and cAMP signal transduction: significance for virulence

The conidial pigment of Aspergillus fumigatus contains 1,8-dihydroxynaphthalene (DHN)-like pentaketide melanin. It plays a major role in the protection of the fungus against immune effector cells; for example, it is able to scavenge reactive oxygen species generated by alveolar macrophages and neutrophiles. The polyketide synthase PKSP (ALB1) is a key-enzyme of the biosynthesis pathway; its structural gene is part of a gene cluster. Furthermore, the presence of a functional pksP (albl) gene in A. fumigatus conidia is associated with an inhibition of phagolysosome fusion in human monocyte-derived macrophages. Moreover, the analysis of mutants that are defective in elements of the cAMP signaling pathway found that they are almost avirulent in an optimized low dose murine inhalation model. Taken together, our results indicate that the cAMP/PKA signal transduction pathway is required for A. fumigatus pathogenicity. In addition, we showed that the expression of the pksP gene is, at least in part, controlled by the cAMP/ PKA signal transduction pathway. Currently, we hypothesize that pentaketide melanin is important for defence against ROS. However, besides its contribution to the biosynthesis of DHN-like melanin, PKSP also appears to be involved in the formation of another compound which is immunosuppressive.

Cross-talk between cAMP and calcium signalling in Aspergillus niger

Very little is known about cross-talk between cAMP and calcium signalling in filamentous fungi. The aim of this study was to analyse the influence of cAMP and protein kinase A (PKA)-dependent phosphorylation on calcium signalling in Aspergillus niger. For this purpose, cytosolic free calcium ([Ca2+]c) was measured in living hyphae expressing codon-optimized aequorin. The calcium signature following mechanical perturbation was analysed after applying dibutryl-cAMP or IBMX which increased intracellular cAMP, or H7 which inhibited phosphorylation by PKA. Calcium signatures were also measured in mutant strains in which phosphorylation by PKA was increased or lacking. The results indicated that calcium channels were activated by cAMP-mediated, PKA-dependent phosphorylation. Further evidence for cross-talk between cAMP and calcium signalling came from the analysis of a mutant in which the catalytic subunit of PKA was under the control of an inducible promoter. The consequence of PKA induction was a transient increase in [Ca2+]c which correlated with a polar-apolar transition in hyphal morphology. A transient increase in [Ca2+]c was not observed in this mutant when the morphological shift was in the opposite direction. The [Ca2+]c signatures in response to mechanical perturbation by polarized and unpolarized cells were markedly different indicating that these two cell types possessed different calcium signalling capabilities. These results were consistent with PKA-dependent phosphorylation increasing [Ca2+]c to induce a polar to apolar shift in hyphal morphology.

Molecular and functional characterization of a fructose specific transporter from the gray mold fungus Botrytis cinerea

In the gray mold fungus Botrytis cinerea, spore germination and plant infection are stimulated in the presence of nutrients, in particular sugars. Applied at micromolar concentrations, fructose is a more potent inducer of germination than glucose. To test whether preferred fructose uptake is responsible for this effect, and to study the mechanism of fructose transport in B. cinerea, a gene (frt1) encoding a fructose transporter was cloned. FRT1 is highly similar to recently identified fructose transporters of yeasts, but much less to other fungal hexose transporters characterized so far. By using a hexose uptake deficient yeast strain for expression, FRT1 was found to be a high affinity proton coupled symporter specific for fructose but not for glucose. B. cinerea frt1 disruption mutants were created and showed normal vegetative growth and plant infection, but a delay in fructose-induced germination when compared to wild-type. Sugar uptake experiments with both wild-type and mutant conidia showed a higher affinity for glucose than for fructose. Thus, we propose that the specific effect of fructose on germination is not due to transport but rather to an as yet unknown intracellular sensing.

The involvement of cAMP in the growth inhibition of filamentous fungus Rhizopus nigricans by steroids

Several steroids, in particular progesterone, are toxic for the filamentous fungus Rhizopus nigricans and, at high concentrations, inhibit its growth. Previous studies on this microorganism revealed progesterone specific receptors coupled to G proteins at the plasma membrane. In this study, the next step of steroid signalling in R. nigricans following G protein activation is investigated, together with the possible impact of this pathway on fungal growth inhibition. The intracellular level of cAMP decreased in the presence of steroids, demonstrating the probable involvement of cAMP signalling in the response of R. nigricans to steroids. Results of the growth analysis in the presence of cAMP increasing agents suggest that the role of cAMP in fungal growth inhibition by steroids cannot be ruled out, but it would appear to be minor and not make a major contribution to growth inhibition.

The Ras and Rho GTPases genetically interact to co-ordinately regulate cell polarity during development in Penicillium marneffei

Ras and Rho GTPases have been examined in a wide variety of eukaryotes and play varied and often overlapping roles in cell polarization and development. Studies in Saccharomyces cerevisiae and mammalian cells have defined some of the central activities of these GTPases. However, these paradigms do not explain the role of these proteins in all eukaryotes. Unlike yeast, but like more complex eukaryotes, filamentous fungi have Rac-like proteins in addition to Ras and Cdc42. To investigate the unique functions of these proteins and determine how they interact to co-ordinately regulate morphogenesis during growth and development we undertook a genetic analysis of GTPase function by generating double mutants of the Rho GTPases cflA and cflB and the newly isolated Ras GTPase rasA from the dimorphic pathogenic fungus, Penicillium marneffei. P. marneffei growth at 25 degrees C is as multinucleate, septate, branched hyphae which are capable of undergoing asexual development (conidiation), while at 37 degrees C, uninucleate pathogenic yeast cells which divide by fission are produced. Here we show that RasA (Ras) acts upstream of CflA (Cdc42) to regulate germination of spores and polarized growth of both hyphal and yeast cells, while also exhibiting CflA-independent activities. CflA (Cdc42) and CflB (Rac) co-ordinately control hyphal cell polarization despite also having unique roles in regulating conidial germination and polarized growth of yeast cells (CflA) and polarized growth of conidiophore cell types and hyphal branching (CflB).

The heterotrimeric G-protein GanB(alpha)-SfaD(beta)-GpgA(gamma) is a carbon source sensor involved in early cAMP-dependent germination in Aspergillus nidulans

The role of heterotrimeric G-proteins in cAMP-dependent germination of conidia was investigated in the filamentous ascomycete Aspergillus nidulans. We demonstrate that the G alpha-subunit GanB mediates a rapid and transient activation of cAMP synthesis in response to glucose during the early period of germination. Moreover, deletion of individual G-protein subunits resulted in defective trehalose mobilization and altered germination kinetics, indicating that GanB(alpha)-SfaD(beta)-GpgA(gamma) constitutes a functional heterotrimer and controls cAMP/PKA signaling in response to glucose as well as conidial germination. Further genetic analyses suggest that GanB plays a primary role in cAMP/PKA signaling, whereas the SfaD-GpgA (G betagamma) heterodimer is crucial for proper activation of GanB signaling sensitized by glucose. In addition, the RGS protein RgsA is also involved in regulation of the cAMP/PKA pathway and germination via attenuation of GanB signaling. Genetic epistatic analyses led us to conclude that all controls exerted by GanB(alpha)-SfaD(beta)-GpgA(gamma) on conidial germination are mediated through the cAMP/PKA pathway. Furthermore, GanB may function in sensing various carbon sources and subsequent activation of downstream signaling for germination.

The GanB Galpha-protein negatively regulates asexual sporulation and plays a positive role in conidial germination in Aspergillus nidulans

We isolated the ganB gene encoding the Galpha-protein homolog from Aspergillus nidulans. To investigate the cellular function of GanB, various mutant strains were isolated. Deletion of constitutively inactive ganB mutants showed conidiation and derepressed brlA expression in a submerged culture. Constitutive activation of GanB caused a reduction in hyphal growth and a severe defect in asexual sporulation. We therefore propose that GanB may negatively regulate asexual sporulation through the BrlA pathway. In addition, deletion or constitutive inactivation of GanB reduced germination rate while constitutive activation led to precocious germination. Furthermore, conidia of a constitutively active mutant could germinate even without carbon source. Taken together, these results indicated that GanB plays a positive role during germination, possibly through carbon source sensing, and negatively regulates asexual conidiation in A. nidulans.

Regulation of aflatoxin synthesis by FadA/cAMP/protein kinase A signaling in Aspergillus parasiticus

Analysis of fadA and pkaA mutants in the filamentous fungus Aspergillus nidulans demonstrated that FadA (Galpha) stimulates cyclic AMP (cAMP)-dependent protein kinase A (PKA) activity resulting, at least in part, in inhibition of conidiation and sterigmatocystin (ST) biosynthesis. In contrast, cAMP added to the growth medium stimulates aflatoxin (AF) synthesis in Aspergillus parasiticus. Our goal was to explain these conflicting reports and to provide mechanistic detail on the role of FadA, cAMP, and PKA in regulation of AF synthesis and conidiation in A. parasiticus. cAMP or dibutyryl-cAMP (DcAMP) were added to a solid growth medium and intracellular cyclic nucleotide levels, PKA activity, and nor-1 promoter activity were measured in A. parasiticus D8D3 (nor1::GUS reporter) and TJYP1-22 (fadAGA2R, activated allele). Similar to Tice and Buchanan [34], cAMP or DcAMP stimulated AF synthesis (and conidiation) associated with an AflR-dependent increase in nor-1 promoter activity. However, treatment resulted in a 100-fold increase in intracellular cAMP/DcAMP accompanied by a 40 to 80 fold decrease in total PKA activity. ThefadAG42R allele in TJYP1-22 decreased AF synthesis and conidiation, increased basal PKA activity 10 fold, and decreased total PKA activity 2 fold. In TJYP1-22, intracellular cAMP increased 2 fold without cAMP or DcAMP treatment; treatment did not stimulate conidiation or AF synthesis. Based on these data, we conclude that: (1) FadA/PKA regulate toxin synthesis and conidiation via similar mechanisms in Aspergillus spp.; and (2) intracellular cAMP levels, at least in part, mediate a PKA-dependent regulatory influence on conidiation and AF synthesis.

Pka, Ras and RGS protein interactions regulate activity of AflR, a Zn(II)2Cys6 transcription factor in Aspergillus nidulans

Sterigmatocystin (ST) is a carcinogenic polyketide produced by several filamentous fungi including Aspergillus nidulans. Expression of ST biosynthetic genes (stc genes) requires activity of a Zn(II)2Cys6 transcription factor, AflR. aflR is transcriptionally and post-transcriptionally regulated by a G-protein/cAMP/protein kinase A (PkaA) signaling pathway involving FlbA, an RGS (regulator of G-protein signaling) protein. Prior genetic data showed that FlbA transcriptional regulation of aflR was PkaA dependent. Here we show that mutation of three PkaA phosphorylation sites in AflR allows resumption of stc expression in an overexpression pkaA background but does not remediate stc expression in a deltaflbA background. This demonstrates negative regulation of AflR activity by phosphorylation and shows that FlbA post-transcriptional regulation of aflR is PkaA independent. AflR nucleocytoplasmic location further supports PkaA-independent regulation of AflR by FlbA. GFP-tagged AflR is localized to the cytoplasm when pkaA is overexpressed but nuclearly located in a deltaflbA background. aflR is also transcriptionally and post-transcriptionally regulated by RasA. RasA transcriptional control of aflR is PkaA independent but RasA post-transcriptional control of AflR is partially mediated by PkaA.

The cyclic AMP-dependent protein kinase a network regulates development and virulence in Aspergillus fumigatus

Aspergillus fumigatus is an important pathogen of immunocompromised hosts, causing pneumonia and invasive disseminated disease with high mortality. To determine the importance of the cyclic AMP (cAMP) signaling pathway for virulence, the pkaC1 gene encoding a protein kinase A (PKA) catalytic subunit was cloned and characterized. Deletion of pkaC1 led to reduced conidiation and growth. PKA activity was not detectable in DeltapkaC1, DeltagpaB, and DeltaacyA mutant strains. gpaB and acyA encode a G protein alpha subunit involved in cAMP signal transduction and adenylate cyclase, respectively. Addition of cAMP led to PKA activity in crude extracts of both the DeltagpaB and DeltaacyA strains but not in crude extracts of the DeltapkaC1 strain. These findings provide evidence that PKAC1 represents the predominant form of PKA under the conditions tested, and GPAB and ACYA are members of the cAMP signaling cascade. Analysis of a pksPp-lacZ gene fusion indicated that the expression of the pathogenicity determinant-encoding pksP gene was reduced in DeltapkaC1 mutant strains compared with the expression of the gene fusion in the parental strain. In a low-dose murine inhalation model, conidia of both the DeltapkaC1 and DeltagpaB mutant strains were almost avirulent. Taken together, these findings indicate that the cAMP-PKA signal transduction pathway is required for A. fumigatus pathogenicity.

Regulators of G-protein signalling in Aspergillus nidulans: RgsA downregulates stress response and stimulates asexual sporulation through attenuation of GanB (Gα) signalling

Regulators of G-protein signalling play a crucial role in controlling the degree of heterotrimeric G-protein signalling. In addition to the previously studied flbA, we have identified three genes (rgsA, rgsB and rgsC) encoding putative RGS proteins in the genome of Aspergillus nidulans. Characterization of the rgsA gene revealed that RgsA downregulates pigment production and conidial germination, but stimulates asexual sporulation (conidiation). Deletion of rgsA (DeltargsA) resulted in reduced colony size with increased aerial hyphae, elevated accumulation of brown pigments as well as enhanced tolerance of conidia and vegetative hyphae against oxidative and thermal stress. Moreover, DeltargsA resulted in conidial germination in the absence of a carbon source. Deletion of both flbA and rgsA resulted in an additive phenotype, suggesting that the G-protein pathways controlled by FlbA and RgsA are different. Morphological and metabolic alterations caused by DeltargsA were suppressed by deletion of ganB encoding a Galpha subunit, indicating that the primary role of RgsA is to control negatively GanB-mediated signalling. Overexpression of rgsA caused inappropriate conidiation in liquid submerged culture, supporting the idea that GanB signalling represses conidiation. Our findings define a second and specific RGS-Galpha pair in A. nidulans, which may govern upstream regulation of fungal cellular responses to environmental changes.

Aspergillus fumigatus rasA and rasB regulate the timing and morphology of asexual development

Expression of rasA plays an important role in conidial germination in Aspergillus nidulans. Conidial germination is required to initiate both infection and asexual development in the opportunistic pathogen Aspergillus fumigatus. Therefore, we sought to determine the requirements for Ras proteins in conidial germination and asexual development of A. fumigatus. A second homolog, rasB, has been identified that characterizes a new subclass of Ras genes. Dominant active (DA) and dominant negative (DN) mutations of each gene were introduced into protoplasts as transgenes. DArasA expression led to reduced conidiation, malformed conidiophores, and altered mitotic progression, whereas expression of DNrasA caused a significant reduction in the rate of conidial germination. In contrast, expression of DNrasB slightly delayed the initiation of germination and caused the development of conidiophores in submerged culture. DArasB expression led to reduced conidiation. RasA and RasB appear to play different, but overlapping, roles in the vegetative growth and asexual development of A. fumigatus.

cAMP regulation of "pathogenic" and "saprophytic" fungal spore germination

We report on the elucidation of two separate pathways of spore germination in a plant pathogenic fungus Colletotrichum gloeosporioides f. sp. aeschynomene. Conidia of the fungus can germinate either from one side or from both sides, depending on external conditions. In shake culture that includes an extract made up from fresh peas, the unicellular conidium divides and one of the two cells develops a germ tube. On a solid surface this germ tube differentiates an appressorium. In rich medium without pea extract, germination is highly similar to Aspergillus spore germination: the conidium swells, forms a single germ tube and then divides and forms a second germ tube. Conidia that germinate in a rich medium do not form appressoria even on a solid surface and are non-pathogenic. In rich medium, cAMP stimulates germination in rich liquid cultures and induces appressoria formation on a hard surface. In pea extract cAMP induces swelling and formation of irregular germ tubes and appressoria. Our results suggest that plant surface signals induce pathogenic-specific spore germination in a cAMP-independent manner. cAMP is required for saprophytic germination and for appressorium formation.

Transcription of purine transporter genes is activated during the isotropic growth phase of Aspergillus nidulans conidia

Aspergillus nidulans possesses three well-characterized purine transporters encoded by the genes uapA, uapC and azgA. Expression of these genes in mycelium is induced by purines and repressed by ammonium or glutamine through the action of the pathway-specific UaY regulator and the general GATA factor AreA respectively. Here, we describe the regulation of expression of purine transporters during conidiospore germination and the onset of mycelium development. In resting conidiospores, mRNA steady-state levels of purine transporter genes and purine uptake activities are undetectable or very low. Both mRNA steady-state levels and purine transport activities increase substantially during the isotropic growth phase of conidial germination. Both processes occur in the absence of purine induction and independently of the nitrogen source present in the medium. The transcriptional activator UaY is dispensable for the germination-induced expression of the three transporter genes. AreA, on the other hand, is essential for the expression of uapA, but not for that of azgA or uapC, during germination. Transcriptional activation of uapA, uapC and azgA during germination is also independent of the presence of a carbon source in the medium. This work establishes the presence of a novel system triggering purine transporter transcription during germination. Similar results have been found in studies on the expression of other transporters in A. nidulans, suggesting that global expression of transporters might operate as a general system for sensing solute availability.

Roles played by Ras subfamily proteins in the cell and developmental biology of microorganisms

The Ras subfamily proteins are monomeric GTPases that function as molecular switches in cellular signal transduction pathways. This review describes our current knowledge of the roles that these proteins play in the growth and differentiation of single celled microorganisms.

The G-protein alpha-subunit GasC plays a major role in germination in the dimorphic fungus Penicillium marneffei

The opportunistic human pathogen Penicillium marneffei exhibits a temperature-dependent dimorphic switch. At 25 degrees, multinucleate, septate hyphae that can undergo differentiation to produce asexual spores (conidia) are produced. At 37 degrees hyphae undergo arthroconidiation to produce uninucleate yeast cells that divide by fission. This work describes the cloning of the P. marneffei gasC gene encoding a G-protein alpha-subunit that shows high homology to members of the class III fungal Galpha-subunits. Characterization of a DeltagasC mutant and strains carrying a dominant-activating gasC(G45R) or a dominant-interfering gasC(G207R) allele show that GasC is a crucial regulator of germination. A DeltagasC mutant is severely delayed in germination, whereas strains carrying a dominant-activating gasC(G45R) allele show a significantly accelerated germination rate. Additionally, GasC signaling positively affects the production of the red pigment by P. marneffei at 25 degrees and negatively affects the onset of conidiation and the conidial yield, showing that GasC function overlaps with functions of the previously described Galpha-subunit GasA. In contrast to the S. cerevisiae ortholog Gpa2, our data indicate that GasC is not involved in carbon or nitrogen source sensing and plays no major role in either hyphal or yeast growth or in the switch between these two forms.

cAMP signaling in Aspergillus fumigatus is involved in the regulation of the virulence gene pksP and in defense against killing by macrophages

Aspergillus fumigatus is an important pathogen of immunocompromised hosts, causing pneumonia and invasive disseminated disease and resulting in high mortality. In order to determine the importance of the cAMP signaling pathway for virulence, three genes encoding putative elements of the pathway have been cloned and characterized: the adenylate cyclase gene acyA, and gpaA and gpaB, both of which encode alpha subunits of heterotrimeric G proteins. The acyA and gpaB genes were each deleted in A. fumigatus. Both mutants showed reduced conidiation, with the deltaacyA mutant producing very few conidia. The growth rate of the deltaacyA mutant was also reduced, in contrast to that of the deltagpaB mutant. Addition of 10 mM dibutyryl-cAMP to the culture medium completely restored the wild-type phenotype in both mutant strains. To study the influence of GPAB on the expression of the gene pksP, which encodes a virulence factor that is involved in pathogenicity, a pksPp-lacZ gene fusion was generated and integrated as a single copy at the pyrG gene locus of both the parental strain and the deltagpaB mutant strain. The deltagpaB mutant showed reduced expression of the pksPp-lacZ reporter gene relative to that in the parental strain. In mycelia of both the parental strain and the deltagpaB mutant pksPp-lacZ expression was increased when isobutyl-methyl-xanthine, an inhibitor of intracellular phosphodiesterases, was added to the medium. The survival rate of conidia after ingestion by human monocyte-derived macrophages was also determined. The killing rate for conidia from deltaacyA and deltagpaB strains was significantly higher than that for wild-type conidia. Taken together, these findings suggest that cAMP triggers a system that protects A. fumigatus from the effects of immune effector cells of the host.

Quinoxyfen perturbs signal transduction in barley powdery mildew (Blumeria graminis f.sp. hordei)

SUMMARY Quinoxyfen is a protectant fungicide which controls powdery mildew diseases by interfering with germination and/or appressorium formation. Mutants of barley powdery mildew, Blumeria graminis f.sp. hordei, which are resistant to quinoxyfen produce fewer conidia, which germinate and form appressoria more promiscuously than do the prolific numbers of wild-type spores. This suggests that resistance bypasses host recognition signals. RT-PCR profiles of signal transduction genes, recorded during wild-type germling morphogenesis, reveals that quinoxyfen alters the accumulation of Protein Kinase C (pkc), pkc-like and catalytic subunit of Protein Kinase A (cpka) transcripts. Differential display-reverse transcription PCR identified a gene transcript in wild-type conidia that was absent, or much less abundant, in conidia from quinoxyfen-resistant mutants. This mRNA was not detectable 24 h after wild-type conidia were inoculated on to barley. It encodes a GTPase activating protein (GAP), which may interact with a small molecular weight Ras-type GTP binding protein. In the presence of quinoxyfen, the gap mRNA remains throughout germling morphogenesis. The involvement of GAP in resistance suggests that quinoxyfen inhibits mildew infection by disrupting early cell signalling events.

Of smuts, blasts, mildews, and blights: cAMP signaling in phytopathogenic fungi

cAMP regulates morphogenesis and virulence in a wide variety of fungi including the plant pathogens. In saprophytic yeasts such as Saccharomyces cerevisiae, cAMP signaling plays an important role in nutrient sensing. In filamentous saprophytes, the cAMP pathway appears to play an integral role in vegetative growth and sporulation, with possible connections to mating. Infection-related morphogenesis includes sporulation (conidia and teliospores), formation of appressoria, infection hyphae, and sclerotia. Here, we review studies of cAMP signaling in a variety of plant fungal pathogens. The primary fungi to be considered include Ustilago maydis, Magnaporthe grisea, Cryphonectria parasitica, Colletotrichum and Fusarium species, and Erisyphe graminis. We also include related information on Trichoderma species that act as mycoparasites and biocontrol agents of phytopathogenic fungi. We point out similarities in infection mechanisms, conservation of signaling components, as well as instances of cross-talk with other signaling pathways.

The adenylate cyclase (BAC) in Botrytis cinerea is required for full pathogenicity

SUMMARY The grey mould Botrytis cinerea is an economically important plant pathogen. Previously we found that null mutants of bcg1 encoding one of the two Galpha subunits of heterotrimeric GTP-binding proteins differed in colony morphology and showed reduced pathogenicity. To further understand the mechanisms involved in infection, we cloned the bac gene encoding adenylate cyclase, the enzyme that catalyses production of cAMP from ATP. The deduced protein sequence consists of 2300 amino acids, the ORF is interrupted by three conserved introns, and there is a high degree of similarity with the catalytic domains of other fungal adenylate cyclases. Gene replacement resulted in reduced vegetative growth and a morphology similar to that of bcg1 mutants. The wild-type (WT) colony morphology was partially restored by feeding exogenous cAMP. These bac mutants still had a low but constant level of cAMP, despite deletion of the complete catalytic domain of the enzyme. Conidia from bac mutants germinated, penetrated the leaves of Phaseolus vulgaris and caused spreading soft rot lesions (in contrast to bcg1 mutants), although these were slower to develop than in WT controls. Compared to the latter, the most striking difference was that no sporulation occurred on leaves inoculated with bac mutant conidia. These results confirm that the cAMP signalling pathway plays an important role in vegetative growth and pathogenicity in B. cinerea. On the other hand, a much stronger effect of bcg1 mutation on pathogenicity in comparison to the effects of bac mutations suggests that BCG1 controls at least one more signalling component other than adenylate cyclase, and that the cAMP signalling pathway is not the only one responsible for pathogenicity.

Relationship between secondary metabolism and fungal development

Filamentous fungi are unique organisms-rivaled only by actinomycetes and plants-in producing a wide range of natural products called secondary metabolites. These compounds are very diverse in structure and perform functions that are not always known. However, most secondary metabolites are produced after the fungus has completed its initial growth phase and is beginning a stage of development represented by the formation of spores. In this review, we describe secondary metabolites produced by fungi that act as sporogenic factors to influence fungal development, are required for spore viability, or are produced at a time in the life cycle that coincides with development. We describe environmental and genetic factors that can influence the production of secondary metabolites. In the case of the filamentous fungus Aspergillus nidulans, we review the only described work that genetically links the sporulation of this fungus to the production of the mycotoxin sterigmatocystin through a shared G-protein signaling pathway.