Cell Wall Synthesis in Apical Hyphal Growth

CgEnd3 Regulates Endocytosis, Appressorium Formation, and Virulence in the Poplar Anthracnose Fungus Colletotrichum gloeosporioides

The hemibiotrophic ascomycete fungus Colletotrichum gloeosporioides is the causal agent of anthracnose on numerous plants, and it causes considerable economic losses worldwide. Endocytosis is an essential cellular process in eukaryotic cells, but its roles in C. gloeosporioides remain unknown. In our study, we identified an endocytosis-related protein, CgEnd3, and knocked it out via polyethylene glycol (PEG)-mediated protoplast transformation. The lack of CgEnd3 resulted in severe defects in endocytosis. C. gloeosporioides infects its host through a specialized structure called appressorium, and ΔCgEnd3 showed deficient appressorium formation, melanization, turgor pressure accumulation, penetration ability of appressorium, cellophane membrane penetration, and pathogenicity. CgEnd3 also affected oxidant adaptation and the expression of core effectors during the early stage of infection. CgEnd3 contains one EF hand domain and four calcium ion-binding sites, and it is involved in calcium signaling. A lack of CgEnd3 changed the responses to cell-wall integrity agents and fungicide fludioxonil. However, CgEnd3 regulated appressorium formation and endocytosis in a calcium signaling-independent manner. Taken together, these results demonstrate that CgEnd3 plays pleiotropic roles in endocytosis, calcium signaling, cell-wall integrity, appressorium formation, penetration, and pathogenicity in C. gloeosporioides, and it suggests that CgEnd3 or endocytosis-related genes function as promising antifungal targets.

Schizophyllum commune: An unexploited source for lignocellulose degrading enzymes

Lignocellulose represents the most abundant source of carbon in the Earth. Thus, fraction technology of the biomass turns up as an emerging technology for the development of biorefineries. Saccharification and fermentation processes require the formulation of enzymatic cocktails or the development of microorganisms (naturally or genetically modified) with the appropriate toolbox to produce a cost‐effective fermentation technology. Therefore, the search for microorganisms capable of developing effective cellulose hydrolysis represents one of the main challenges in this era. Schizophyllum commune is an edible agarical with a great capability to secrete a myriad of hydrolytic enzymes such as xylanases and endoglucanases that are expressed in a high range of substrates. In addition, a large number of protein‐coding genes for glycoside hydrolases, oxidoreductases like laccases (Lacs; EC 1.10.3.2), as well as some sequences encoding for lytic polysaccharide monooxygenases (LPMOs) and expansins‐like proteins demonstrate the potential of this fungus to be applied in different biotechnological process. In this review, we focus on the enzymatic toolbox of S. commune at the genetic, transcriptomic, and proteomic level, as well as the requirements to be employed for fermentable sugars production in biorefineries. At the end the trend of its use in patent registration is also reviewed.

Role of BGT-1 and BGT-2, two predicted GPI-anchored glycoside hydrolases/glycosyltransferases, in cell wall remodeling in Neurospora crassa

Neurospora crassa BGT-1 (NCU06381) and BGT-2 (NCU09175) are two putative glycoside hydrolases (GHs) with additional predicted glycosyltransferase activity and binding sites for a glycosyl phosphatidyl inositol (GPI) anchor that would facilitate their attachment to the plasma membrane (PM). To discern their role in key morphogenetic events during vegetative development of N. crassa, BGT-1 and BGT-2 were labeled with the green fluorescent protein (GFP). The gfp was inserted immediately after the signal peptide sequence, within the bgt-1 encoding sequence, or directly before the GPI-binding site in the case of bgt-2. Both BGT-1-GFP and BGT-2-GFP were observed at the PM of the hyphal apical dome, excluding the foremost apical region and the Spitzenkörper (Spk), where chitin and β-1,3-glucan synthases have been previously found. These and previous studies suggest a division of labor of the cell wall synthesizing machinery at the hyphal dome: at the very tip, glucans are synthesized by enzymes that accumulate at the Spk, before getting incorporated into the PM, whereas at the subtending zone below the apex, glucans are presumably hydrolyzed, producing amenable ends for further branching and crosslinking with other cell wall polymers. Additionally, BGT-1-GFP and BGT-2-GFP were observed at the leading edge of new developing septa, at unreleased interconidial junctions, at conidial poles, at germling and hyphal fusion sites, and at sites of branch emergence, all of them processes that seemingly involve cell wall remodeling. Even though single and double mutant strains for the corresponding genes did not show a drastic reduction of growth rate, bgt-2Δ and bgt-1Δ::bgt-2Δ strains exhibited an increased resistance to the cell wall stressors calcofluor white (CW) and congo red (CR) than the reference strain, which suggests they present significant architectural changes in their cell wall. Furthermore, the conidiation defects observed in the mutants indicate a significant role of BGT-1 and BGT-2 on the re-arrangement of glucans needed at the conidiophore cell wall to allow conidial separation.

Computational analysis of AnmK-like kinase: New insights into the cell wall metabolism of fungi

1,6-Anhydro-N-acetylmuramic acid kinase (AnmK) is the unique enzyme that marks the recycling of the cell wall of Escherichia coli. Here, 81 fungal AnmK-like kinase sequences from 57 fungal species were searched in the NCBI database and a phylogenetic tree was constructed. The three-dimensional structure of an AnmK-like kinase, levoglucosan kinase (LGK) of the yeast Lipomyces starkeyi, was modeled; molecular docking revealed that AnmK and LGK are conserved proteins, and 187Asp, 212Asp are enzymatic residues, respectively. Analysis suggests that 1,6-anhydro-N-acetylglucosamine (anhGlcNAc) and/or 1,6-anhydro-β-d-glucosamine (anhGlcN) would be the appropriate substrates of AnmK-like kinases. Also, the counterparts of other characteristic enzymes of cell wall recycling of bacteria were found in fungi. Taken together, it is proposed that a putative recycling of anhGlcNAc/anhGlcN, which is associated with the hydrolysis of cell walls, exists in fungi. This computational analysis will provide new insights into the metabolism of fungal cell walls.

Stipe cell wall architecture varies with the stipe elongation of the mushroom Coprinopsis cinerea

A large amount of granular protrusions overlie the outer cell wall surfaces in both elongating and non-elongating stipe regions but overlie the inner cell wall surfaces only in non-elongating stipe regions. Removal of granular protrusions using alkali, amorphous materials overlying on both the inner and outer cell wall surfaces were explored in the non-elongating stipe regions. β-1,3-Glucanase treatment not only removed above those granular protrusions and underlying amorphous materials on the wall surfaces but also removed wall matrices embedding chitin microfibrils on the cell walls of most stipe regions, except for the outer cell wall surfaces of the non-elongating stipe regions where most of the wall matrices remained. The chitin microfibrils were closely and transversely arranged on both the inner and outer cell wall surfaces in the elongating apical stipe region, whereas they were loosely and transversely arranged on the inner cell wall surfaces and further became sparser and even randomly arranged on the outer cell wall surface in the non-elongating stipe regions. We propose that the surface deposition of granular protrusions and amorphous materials and the change of microfibril architecture and wall matrices may cause loss of wall plasticity and cessation of stipe elongation.

Structural and functional organization of growing tips of Neurospora crassa Hyphae

Data are presented on a variety of intracellular structures of the vegetative hyphae of the filamentous fungus Neurospora crassa and the involvement of these structures in the tip growth of the hyphae. Current ideas on the molecular and genetic mechanisms of tip growth and regulation of this process are considered. On the basis of comparison of data on behaviors of mitochondria and microtubules and data on the electrical heterogeneity of the hyphal apex, a hypothesis is proposed about a possible supervisory role of the longitudinal electric field in the structural and functional organization of growing tips of the N. crassa hyphae.

Biocontrol activity of an alkaline serine protease from Aureobasidium pullulans expressed in Pichia pastoris against four postharvest pathogens on apple

The yeast-like fungus Aureobasidium pullulans PL5 is a microbial antagonist against postharvest pathogens of fruits. The strain is able to produce hydrolases, including glucanases, chitinases and proteases. The alkaline serine protease gene ALP5 from A. pullulans was cloned, inserted into the vector pPIC9 to construct pPIC9/ALP5, and then expressed in Pichia pastoris strain KM71. ALP5 had a molecular mass of 42.9kDa after 5days growth with 1% methanol induction at 28°C. The recombinant protease expressed in P. pastoris showed its highest activity under alkaline conditions (at pH10) and a temperature of 50°C. The antifungal activity of the recombinant protease was investigated against Penicillium expansum, Botrytis cinerea, Monilinia fructicola and Alternaria alternata in vitro and on apple. The recombinant protease reduced significantly the spore germination and the germ tube length of the tested pathogens in PDB medium. The highest level of protease efficacy was observed against M. fructicola and B. cinerea, whereas a lower efficacy was observed against P. expansum and A. alternata indicating a possible effect of the pathogen cell wall composition on the proteolytic activity of the recombinant protease. The presence of protease was able to cause the swelling of the hyphae of B. cinerea, under an optical microscope. The recombinant protease expressed in P. pastoris was more active against the pathogens in vitro than the same enzyme expressed in E. coli in previous studies. The efficacy of ALP5 was also evaluated against the pathogens in vivo on cv Golden Delicious apples. The protease was more efficient in controlling M. fructicola, B. cinerea and P. expansum than A. alternata. However, the extent of the activity was dependent on the enzyme concentration and the length of fruit storage. This study demonstrated the capacity of the alkaline serine protease to keep its enzymatic activity for some days in the unfavorable environment of the fruit wounds. The alkaline serine protease could be developed as a postharvest treatment with antimicrobial activity for fruit undergoing a short storage period.

How carbohydrates sculpt cells: chemical control of morphogenesis in the yeast cell wall

In budding yeast, the neck that connects the mother and daughter cell is the site of essential functions such as organelle trafficking, septum formation and cytokinesis. Therefore, the morphology of this region, which depends on the surrounding cell wall, must be maintained throughout the cell cycle. Growth at the neck is prevented, redundantly, by a septin ring inside the cell membrane and a chitin ring in the cell wall. Here, we describe recent work supporting the hypothesis that attachment of the chitin ring, which forms at the mother-bud neck during budding, to β-1,3-glucan in the cell wall is necessary to stop growth at the neck. Thus, in this scenario, chemistry controls morphogenesis.

Involvement of the anucleate primary sterigmata protein FgApsB in vegetative differentiation, asexual development, nuclear migration, and virulence in Fusarium graminearum

The protein ApsB has been shown to play critical roles in the migration and positioning of nuclei and in the development of conidiophores in Aspergillus nidulans. The functions of ApsB in Fusarium graminearum, a causal agent of Fusarium head blight in China, are largely unknown. In this study, we used the blastp program at the Broad Institute to identify FgApsB, an F. graminearum homolog of A. nidulansApsB. The functions of FgApsB were evaluated by constructing a deletion mutant of FgApsB, designated ΔFgApsB-28. Conidiation and mycelial growth rate are reduced in ΔFgApsB-28. The hyphae of ΔFgApsB-28 are thinner than those of the wild type and have a different branching angle. ΔFgApsB-28 exhibited reduced aerial hyphae formation, but increased production of rubrofusarin. Whereas nuclei are evenly distributed in germ tubes and hyphae of the wild type, they are clustered and irregularly distributed in ΔFgApsB-28. The mutant exhibited increased resistance to cell wall-damaging agents, but reduced virulence on flowering wheat heads, which is consistent with its reduced production of the toxin deoxynivalenol. All of the defects in ΔFgApsB-28 were restored by genetic complementation with the parental FgApsB gene. Taken together, the results indicate that FgApsB is important for vegetative differentiation, asexual development, nuclear migration, and virulence in F. graminearum.

The response of dark septate endophytes (DSE) to heavy metals in pure culture

Dark septate endophytes (DSE) occur widely in association with plants exposed to heavy metal stress. However, little is known about the response of DSE exposed to heavy metals. In this study, five DSE were isolated from the roots of Astragalus adsurgens Pall. seedlings growing on lead-zinc mine tailings in China. Based on morphological characteristics and DNA sequence analyses, the isolates were identified as Gaeumannomyces cylindrosporus, Paraphoma chrysanthemicola, Phialophora mustea, Exophiala salmonis, and Cladosporium cladosporioides. G. cylindrosporus was selected to explore responses to Pb stress. Scanning electron microscopic observations of G. cylindrosporus grown on solid medium revealed curling of hyphae and formation of hyphal coils in response to Pb. In contrast, in liquid medium, hyphae became thick and swollen with an increase in Pb (II) concentration. We interpret that these changes are related to the variation in cell wall components. We also demonstrated that fungal melanin content increased with the addition of Pb(II). Melanin, as an important component in the cell wall, is known to be an essential antioxidant responsible for decreasing heavy metal toxicity. We also measured the total soluble protein content and glutathione (GSH) concentrations in G. cylindrosporus and found that they initially increased and then decreased with the increase of Pb(II) concentrations. The antioxidant enzyme activities were also examined, and the results showed that superoxide dismutase (SOD) activity was significantly positively correlated with Pb(II) concentrations (r = 0.957, P<0.001). Collectively, our observations indicate that the intracellular antioxidant systems, especially fungal melanin, play an important role in abating the hazards of heavy metals.

Cloning and heterologous expression of SS10, a subtilisin-like protease displaying antifungal activity from Trichoderma harzianum

Trichoderma harzianum parasitizes a large variety of phytopathogenic fungi. Trichoderma harzianum mycoparasitic activity depends on the secretion of complex mixtures of hydrolytic enzymes able to degrade the host cell wall. A gene (SS10) encoding a subtilisin-like protease was cloned from T. harzianum T88, a biocontrol agent effective against soil-borne fungal pathogens. The full-length cDNA was isolated by 5' and 3' rapid amplification of the cDNA ends. The coding region of the gene is 1302 bp long, encoding 433 amino acids of a predicted protein with a molecular mass of 45 kDa and a pI of 6.1. Analysis of the deduced amino acid sequence revealed that this protein had homology to the serine proteases of the subtilisin-like superfamily (subtilases) (EC 3.4.21.) and had a predicted active site made up of the catalytic residues Asp 187, His 218 and Ser 376. Northern experiments demonstrated that SS10 was induced in response to different fungal cell walls. Subtilisin-like protease gene SS10 was expressed in Saccharomyces cerevisiae under control of the GAL1 promoter. The enzyme activity culminates (17.8 U mL(-1)) 60 h after induction with galactose. The optimal enzyme reaction temperature was 50 degrees C and the optimal pH was 8. The subtilisin-like protease exerted broad-spectrum antifungal activity against Alternaria alternata, Fusarium oxysporum, Rhizoctonia solani, Sclerotinia sclerotiorum and Cytospora chrysosperma.

Ultrastructural study on acibenzolar-S-methyl-induced scab resistance in epidermal pectin layers of Japanese pear leaves

The infection behavior of Japanese pear scab pathogen Venturia nashicola race 1 was studied ultrastructurally in acibenzolar-S-methyl (ASM)-pretreated susceptible Japanese pear (cv. Kousui) leaves to determine the mechanism of ASM-induced scab resistance. On ASM-pretreated leaf surfaces, the infection behavior (conidial germination and appressorial formation) was similar to that on distilled water (DW)-pretreated leaves prior to cuticle penetration by the pathogen. However, after penetration, differentiated behavior was found in epidermal pectin layers and middle lamellae of the ASM-pretreated leaves. Subcuticular hyphae in epidermal pectin layers and middle lamellae of ASM-pretreated pear leaves were observed at lower frequency than in DW-treated leaves. The results indicated that fungal growth was suppressed in ASM-pretreated pear leaves. In the pectin layers of ASM- and DW-pretreated leaves, some hyphae showed morphological modifications, which were used as criteria to judge collapse of hyphal cells, including plasmolysis, necrotic cytoplasm, and cell wall destruction. More hyphae had collapsed in ASM-pretreated leaves than in DW-treated ones. In addition, the cell walls of collapsed hyphae broke into numerous fibrous and amorphous pieces, suggesting that ASM-induced scab resistance might be associated with cell-wall-degrading enzymes from pear plants. In addition, results from morphometrical analysis suggested that the activity or production of pectin-degrading enzyme from hyphae were inhibited by ASM application when compared with DW treatment.

Yeast and fungal morphogenesis from an evolutionary perspective

Cellular morphogenesis is a complex process and molecular studies in the last few decades have amassed a large amount of information that is difficult to grasp in any completeness. Fungal systems, in particular the budding and fission yeasts, have been important players in unravelling the basic structural and regulatory elements involved in a wide array of cellular processes. In this article, we address the design principles underlying the various processes of yeast and fungal morphogenesis. We attempt to explain the apparent molecular complexity from the perspective of the evolutionary theory of "facilitated variation". Following a summary of some of the most studied morphogenetic phenomena, we discuss, using recent examples, the underlying core processes and their associated "weak" regulatory linkages that bring about variation in morphogenetic phenotypes.

The cell wall: a carbohydrate armour for the fungal cell

The cell wall is composed of a polysaccharide-based three-dimensional network. Considered for a long time as an inert exoskeleton, the cell wall is now seen as a dynamic structure that is continuously changing as a result of the modification of culture conditions and environmental stresses. Although the cell wall composition varies among fungal species, chemogenomic comparative analysis have led to a better understanding of the genes and mechanisms involved in the construction of the common central core composed of branched beta1,3 glucan-chitin. Because of its essential biological role, unique biochemistry and structural organization and the absence in mammalian cells of most of its constitutive components, the cell wall is an attractive target for the development of new antifungal agents. Genomic as well as drug studies have shown that the death of the fungus can result from inhibition of cell wall polysaccharide synthases. To date, only beta1,3 glucan synthase inhibitors have been launched clinically and many more targets remain to be explored.

Molecules into cells: specifying spatial architecture

A living cell is not an aggregate of molecules but an organized pattern, structured in space and in time. This article addresses some conceptual issues in the genesis of spatial architecture, including how molecules find their proper location in cell space, the origins of supramolecular order, the role of the genes, cell morphology, the continuity of cells, and the inheritance of order. The discussion is framed around a hierarchy of physiological processes that bridge the gap between nanometer-sized molecules and cells three to six orders of magnitude larger. Stepping stones include molecular self-organization, directional physiology, spatial markers, gradients, fields, and physical forces. The knowledge at hand leads to an unconventional interpretation of biological order. I have come to think of cells as self-organized systems composed of genetically specified elements plus heritable structures. The smallest self that can be fairly said to organize itself is the whole cell. If structure, form, and function are ever to be computed from data at a lower level, the starting point will be not the genome, but a spatially organized system of molecules. This conclusion invites us to reconsider our understanding of what genes do, what organisms are, and how living systems could have arisen on the early Earth.

The mechanics of surface expansion anisotropy in Medicago truncatula root hairs

Wall expansion in tip-growing cells shows variations according to position and direction. In Medicago truncatula root hairs, wall expansion exhibits a strong meridional gradient with a maximum near the pole of the cell. Root hair cells also show a striking expansion anisotropy, i.e. over most of the dome surface the rate of circumferential wall expansion exceeds the rate of meridional expansion. Concomitant measurements of expansion rates and wall stresses reveal that the extensibility of the cell wall must vary abruptly along the meridian of the cell to maintain the gradient of wall expansion. To determine the mechanical basis of expansion anisotropy, we compared measurements of wall expansion with expansion patterns predicted from wall structural models that were either fully isotropic, transversely isotropic, or fully anisotropic. Our results indicate that a model based on a transversely isotropic wall structure can provide a good fit of the data although a fully anisotropic model offers the best fit overall. We discuss how such mechanical properties could be controlled at the microstructural level.

Independent regulation of chitin synthase and chitinase activity in Candida albicans and Saccharomyces cerevisiae

Chitin is an essential structural polysaccharide in fungi that is required for cell shape and morphogenesis. One model for wall synthesis at the growing cell surface suggests that the compliance that is necessary for turgor-driven expansion of the cell wall involves a delicate balance of wall synthesis and lysis. Accordingly,de novochitin synthesis may involve coordinated regulation of members of theCHSchitin synthase andCHTchitinase gene families. To test this hypothesis, the chitin synthase and chitinase activities of cell-free extracts were measured, as well as the chitin content of cell walls isolated from isogenic mutant strains that contained single or multiple knock-outs in members of these two gene families, in bothCandida albicansandSaccharomyces cerevisiae. However, deletion of chitinase genes did not markedly affect specific chitin synthase activity, and deletion of singleCHSgenes had little effect onin vitrospecific chitinase activity in either fungus. Chitin synthesis and chitinase production was, however, regulated inC. albicansduring yeast–hypha morphogenesis. InC. albicans, the total specific activities of both chitin synthase and chitinase were higher in the hyphal form, which was attributable mainly to the activities of Chs2 and Cht3, respectively. It appeared, therefore, that chitin synthesis and hydrolysis were not coupled, but that both were regulated during yeast–hypha morphogenesis inC. albicans.

Interactions in the tomato rhizosphere of two Pseudomonas biocontrol strains with the phytopathogenic fungus Fusarium oxysporum f. sp. radicis-lycopersici

The fungus Fusarium oxysporum f. sp. radicis-lycopersici causes foot and root rot of tomato plants, which can be controlled by the bacteria Pseudomonas fluorescens WCS365 and P. chlororaphis PCL1391. Induced systemic resistance is thought to be involved in biocontrol by P. fluorescens WCS365. The antifungal metabolite phenazine-1-carboxamide (PCN), as well as efficient root colonization, are essential in the mechanism of biocontrol by P. chlororaphis PCL1391. To understand the effects of bacterial strains WCS365 and PCL1391 on the fungus in the tomato rhizosphere, microscopic analyses were performed using different autofluorescent proteins as markers. Tomato seedlings were inoculated with biocontrol bacteria and planted in an F. oxysporum f. sp. radicis-lycopersici-infested gnotobiotic sand system. Confocal laser scanning microscope analyses of the interactions in the tomato rhizosphere revealed that i) the microbes effectively compete for the same niche, and presumably also for root exudate nutrients; ii) the presence of either of the two bacteria negatively affects infection of the tomato root by the fungus; iii) both biocontrol bacteria colonize the hyphae extensively, which may represent a new mechanism in biocontrol by these pseudomonads; and iv) the production of PCN by P. chlororaphis PCL1391 negatively affects hyphal growth and branching, which presumably affects the colonization and infecting ability of the fungus.

The genetic basis of cellular morphogenesis in the filamentous fungus Neurospora crassa

Cellular polarity is a fundamental property of every cell. Due to their extremely fast growth rate (>/=1 microm/s) and their highly elongated form, filamentous fungi represent a prime example of polarized growth and are an attractive model for the analysis of fundamental mechanisms underlying cellular polarity. To identify the critical components that contribute to polarized growth, we developed a large-scale genetic screen for the isolation of conditional mutants defective in this process in the model fungus Neurospora crassa. Phenotypic analysis and complementation tests of ca. 950 mutants identified more than 100 complementation groups that define 21 distinct morphological classes. The phenotypes include polarity defects over the whole hypha, more specific defects localized to hyphal tips or subapical regions, and defects in branch formation and growth directionality. To begin converting this mutant collection into meaningful biological information, we identified the defective genes in 45 mutants covering all phenotypic classes. These genes encode novel proteins as well as proteins which 1) regulate the actin or microtubule cytoskeleton, 2) are kinases or components of signal transduction pathways, 3) are part of the secretory pathway, or 4) have functions in cell wall formation or membrane biosynthesis. These findings highlight the dynamic nature of a fungal hypha and establish a molecular model for studies of hyphal growth and polarity.

The autolysis of industrial filamentous fungi

Fungal autolysis is the natural process of self-digestion of aged hyphal cultures, occurring as a result of hydrolase activity, causing vacuolation and disruption of organelle and cell wall structure. Previously, authors have considered individual aspects of fungal lysis, in terms of either an enzyme, a process or an organism. This review considers both the physiology and morphology of fungal autolysis, with an emphasis on correlations between enzymological profiles and the morphological changes occurring during culture degeneration. The involvement of the main groups of autolytic hydrolases is examined (i.e., proteases, glucanases, and chitinases), in addition to the effects of autolysis on the morphology and products of industrial bioprocesses. We call for a concerted approach to the study of autolysis, as this will be fundamental for research to progress in this field. Increased understanding will allow for greater control of the prevention, or induction of fungal autolysis. Such advances will be applicable in the development of antifungal medicines and enable increased productivity and yields in industrial bioprocesses. Using paradigms in existing model systems, including mammalian cell death and aging in yeast, areas for future study are suggested in order to advance the study of fungal cell death.

Physiological characterisation of recombinant Aspergillus nidulans strains with different creA genotypes expressing A. oryzae alpha-amylase

The physiology of three strains of Aspergillus nidulans was examined--a creA deletion strain, a wild type creA genotype and a strain containing extra copies of the creA gene, all producing Aspergillus oryzae alpha-amylase. The strains were cultured in batch and continuous cultivations and the biomass formation and alpha-amylase production was characterised. Overexpression of the creA gene resulted in a lower maximum specific growth rate and a slightly higher repression of the alpha-amylase production during conditions with high glucose concentration. No expression of creA also resulted in a decreased maximum specific growth rate, but also in drastic changes in morphology. Furthermore, the expression of alpha-amylase was completely derepressed and creA thus seems to be the only regulatory protein responsible for glucose repression of alpha-amylase expression. The effect of different carbon sources on the alpha-amylase production in the creA deletion strain was investigated and it was found that starch was the best inducer. The degree of induction by starch increased almost linearly with the concentration of starch in starch/glucose mixtures. High-density batch cultivation was performed with the creA deletion strain and a final titre of 6.0 g l(-1) of alpha-amylase was reached after 162 h of cultivation.

Hydrophobins: multipurpose proteins

Class I and class II hydrophobins are small secreted fungal proteins that play a role in a broad range of processes in the growth and development of filamentous fungi. For instance, they are involved in the formation of aerial structures and in the attachment of hyphae to hydrophobic surfaces. The mechanisms by which hydrophobins fulfill these functions are based on their property to self-assemble at hydrophilic-hydrophobic interfaces into a 10 nm-thin highly amphipathic film. Complementation studies have shown that class I hydrophobins belong to a closely related group of morphogenetic proteins, but that they have evolved to function at specific interfaces. Recent evidence indicates that hydrophobins do not only function by self-assembly. Monomeric hydrophobin has been implicated in cell-wall assembly, but the underlying mechanism is not yet clear. In addition, hydrophobin monomers could act as toxins and elicitors.

Nutritional influence on fungal colony growth and biomass distribution in response to toxic metals

This work examines nutritional influence on fungal colony growth and biomass distribution in response to toxic metals. In low-substrate solid medium, 0.1 mM Cd, Cu and Zn caused a decrease in radial expansion of both Trichoderma viride and Rhizopus arrhizus. However, as the amount of available carbon source (glucose) increased, the apparent toxicity of the metals decreased. These metals also affected the overall length of the fungal mycelium and branching patterns. In low-nutrient conditions, T. viride showed a decrease in overall mycelial length and number of branches in response to Cu, resulting in an extremely sparsely branched colony. Conversely, although Cd also reduced overall mycelial length to about one-third of the control length, the number of branches decreased only slightly which resulted in a highly branched colony with many aberrant features. Cu and Cd induced similar morphological changes in R. arrhizus. A large-scale mycelial-mapping technique showed that disruption of normal growth by Cu and Cd resulted in altered biomass distribution within the colony. When grown on metal-free low-substrate medium, T. viride showed an even distribution of biomass within the colony with some allocation to the periphery. However, Cu caused most of the biomass to be allocated to the colony periphery, while in the presence of Cd, most biomass was located at the interior of the colony. These results imply that such alterations of growth and resource allocation by Cu and Cd may influence success in locating nutrients as well as survival, and that these metals have individual and specific effects on the growing fungus.

Autolysis and aging of Penicillium chrysogenum cultures under carbon starvation: Chitinase production and antifungal effect of allosamidin

In carbon-depleted cultures of Penicillium chrysogenum, age-related chitinases were shown to play a crucial role in both autolysis and fragmentation as indicated by in vivo enzyme inhibition experiments using allosamidin. This pseudotrisaccharide even hindered significantly the outgrowth of new hyphal tips from the surviving yeastlike fragments after glucose supplementation. The antifungal effect of allosamidin on autolyzing P. chrysogenum mycelia was fungistatic rather than fungicidal. In growing hyphae, membrane-bound microsomal chitinase zymogen(s) were detected, which may be indicative of some compartmentalization of these hydrolases. Later, during autolysis, no zymogenic chitinase was detected in any enzyme fraction studied, including microsomes. These observations may explain the different sensitivity of growing and autolyzing mycelia to allosamidin. Chitinases taking part in the age-related fragmentation of hyphae and the outgrowth of surviving hyphal fragments seem to be potent targets for future antifungal drug research.

Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

Molecular and physiological characterization of the NAD-dependent glycerol 3-phosphate dehydrogenase in the filamentous fungus Aspergillus nidulans

In filamentous fungi, glycerol biosynthesis has been proposed to play an important role during conidiospore germination and in response to a hyperosmotic shock, but little is known about the genes involved. Here, we report on the characterization of the major Aspergillus nidulans glycerol 3-phosphate dehydrogenase (G3PDH)-encoding gene, gfdA. G3PDH is responsible for the conversion of dihydroxyacetone phosphate (DHAP) into glycerol 3-phosphate (G3P), which is subsequently converted into glycerol by an as yet uncharacterized phosphatase. Inactivation of gfdA does not abolish glycerol biosynthesis, showing that the other pathway from DHAP, via dihydroxyacetone (DHA), to glycerol is also functional in A. nidulans. The gfdA null mutant displays reduced G3P levels and an osmoremediable growth defect on various carbon sources except glycerol. This growth defect is associated with an abnormal hyphal morphology that is reminiscent of a cell wall defect. Furthermore, the growth defect at low osmolarity is enhanced in the presence of the chitin-interacting agent calcofluor and the membrane-destabilizing agent sodium dodecyl sulphate (SDS). As inactivation of gfdA has no impact on phospholipid biosynthesis or glycolytic intermediates levels, as might be expected from reduced G3P levels, a previously unsuspected link between G3P and cell wall integrity is proposed to occur in filamentous fungi.

A PAK-like protein kinase is required for maturation of young hyphae and septation in the filamentous ascomycete Ashbya gossypii

Filamentous fungi grow by hyphal extension, which is an extreme example of polarized growth. In contrast to yeast species, where polarized growth of the tip of an emerging bud is temporally limited, filamentous fungi exhibit constitutive polarized growth of the hyphal tip. In many fungi, including Ashbya gossypii, polarized growth is reinforced by a process called hyphal maturation. Hyphal maturation refers to the developmental switch from slow-growing hyphae of young mycelium to fast-growing hyphae of mature mycelium. This process is essential for efficient expansion of mycelium. We report for the first time on the identification and characterization of a fungal gene important for hyphal maturation. This novel A. gossypii gene encodes a presumptive PAK (p21-activated kinase)-like kinase. Its closest homolog is the S. cerevisiae Cla4 protein kinase; the A. gossypii protein is therefore called AgCla4p. Agcla4 deletion strains are no longer able to perform the developmental switch from young to mature hyphae, and GFP (green fluorescent protein)-tagged AgCla4p localizes with much higher frequency in mature hyphal tips than in young hyphal tips. Both results support the importance of AgCla4p in hyphal maturation. AgCla4p is also required for septation, indicated by the inability of Agcla4 deletion strains to properly form actin rings and chitin rings. Despite the requirement of AgCla4p for the development of fast-growing hyphae, AgCla4p is not necessary for actin polarization per se, because tips enriched in cortical patches and hyphae with a fully developed network of actin cables can be seen in Agcla4 deletion strains. The possibility that AgCla4p may be involved in regulatory mechanisms that control the dynamics of the actin patches and/or actin cables is discussed.

Hydrophobin gene expression affects hyphal wall composition in Schizophyllum commune

Disruption of the SC3 hydrophobin gene of Schizophyllum commune (DeltaSC3 strain) affected the composition of the cell wall. Compared to a wild-type strain the amount of mucilage (i.e., water-soluble (1-3)beta-glucan with single glucose residues attached by (1-6)beta-linkages) increased considerably, while the amount of alkali-resistant glucan (linked to chitin) decreased. Reintroduction of the SC3 gene or other hydrophobins genes expressed behind the SC3 promotor restored wild-type cell wall composition. However, addition of purified SC3 protein to the medium or growing the DeltaSC3 strain in spent medium of the wild-type strain had no effect. In young cultures of wild-type strains of S.commune, not yet expressing SC3, the amount of mucilage was also relatively high. These data show that hydrophobins not only function at hydrophilic/hydrophobic interfaces, as shown previously, but also affect wall composition.

Chitin biosynthesis and structural organization in vivo

Many organisms utilize chitin as a structural component of the protective cell walls or exoskeletons which surround them. These structures are light and resistant composites with specific structural and mechanical properties which allow them to fulfill their protective role. Chitin, in the form of microfibrils, is immersed in a matrix of proteins and other polysaccharides. Chitin microfibrils provide the high strength which allows them to resist tensions and modulus. The cementing compounds protect chitin from chemical attack; keep the microfibrils separate, preventing fracture; and provide support to tensions. The resulting structures adopt specific forms which are conserved during growth and are transmitted in a hereditary fashion. Synthesis of these complex structures involves the following steps: (i) synthesis of chitin either intracellularly or at the interphase with the extracellular medium; (ii) transport of the chitin molecules to the extracellular space; (iii) chemical modification of part of the noncrystallized chitin and association with other molecules; (iv) crystallization of the unmodified chitin which is covered by the rest of the components. The resulting supramolecular structure acquires viscoelastic mechanical properties; (v) maturation of the composite through formation of secondary covalent bonds among its components, and deposition of different substances.

Brefeldin A affects growth, endoplasmic reticulum, Golgi bodies, tubular vacuole system, and secretory pathway in Pisolithus tinctorius

Brefeldin A (BFA) reduced radial growth in Pisolithus tinctorius at a concentration as low as 2 microM. Use of endoplasmic reticulum (ER)-Tracker dye, unconjugated BFA, and fluorescent BFA (BODIPY-BFA) allowed comparison of the effects of BFA on the endomembrane system of P. tinctorius at the light and electron microscope levels. Both ER-Tracker dye and BODIPY-BFA have been shown previously to label the ER. Unconjugated BFA and BODIPY-BFA modified the ER network and disrupted the tubular vacuole system in the tip region. The ultrastructure in freeze-substituted hyphae showed that BFA treatment resulted in (i) disruption of the Spitzenkörper, (ii) reduction in number of apical vesicles, (iii) redistribution and mild dilation of ER, and (iv) persistence and increased size and complexity of Golgi bodies. The effects of BFA on the ER were only partially reversible in the time period examined. We conclude that in P. tinctorius, BFA as the free metabolite or BODIPY-BFA affects the tubular vacuole system as well as anterograde membrane flow between the ER and the Golgi bodies and post-Golgi transport.