A fast and simple method to estimate relative, hyphal tensile-strength of filamentous fungi used to assess the effect of autophagy

Fungal hyphal strength is an important phenotype which can have a profound impact on bioprocess behavior. Until now, there is not an efficient method which allows its characterization. Currently available methods are very time consuming, thus, compromising their applicability in strain selection and process development. To overcome this issue, a method for fast and easy, statistically verified quantification of relative hyphal tensile strength was developed. It involves off-line fragmentation in a high shear mixer followed by quantification of fragment size using laser diffraction. Particle size distribution (PSD) is determined, with analysis time on the order of minutes. Plots of PSD 90th percentile versus time allow estimation of the specific fragmentation rate. This novel method is demonstrated by estimating relative hyphal strength during growth in control conditions and rapamycin-induced autophagy for Aspergillus nidulans (parental strain) and a mutant strain (ΔAnatg8) lacking an important autophagy gene. Both strains were grown in shake flasks and relative hyphal tensile strength was compared. The mutant strain grown in control conditions appears to be weaker than the parental strain, suggesting that Anatg8 may play a role in other processes involving cell wall biosynthesis. Furthermore, rapamycin-induced autophagy resulted in apparently weaker cells even for the mutant strain. These findings confirm the utility of the developed method in strain selection and process development.

Synthesis and Evaluation of Novel Oxyalkylated Derivatives of 2',4'-Dihydroxychalcone as Anti-Oomycete Agents against Bronopol Resistant Strains of Saprolegnia sp

A series of novel oxyalkylchalcones substituted with alkyl groups were designed and synthesized, and the antioomycete activity of the series was evaluated in vitro against Saprolegnia strains. All tested O-alkylchalcones were synthesized by means of nucleophilic substitution from the natural compound 2′,4′-dihydroxychalcone (1) and the respective alkyl bromide. The natural chalcone (1) and 10 synthetic oxyalkylchalcones (2–11) were tested against Saprolegnia parasitica and Saprolegnia australis. Among synthetic analogs, 2-hydroxy,4-farnesyloxychalcone (11) showed the most potent activity against Saprolegnia sp., with MIC and MOC values of 125 µg/mL (similar to bronopol at 150 µg/mL) and 175 µg/mL, respectively; however, 2′,4′-dihydroxychalcone (1) was the strongest and most active molecule, with MIC and MOC values of 6.25 µg/mL and 12.5 µg/mL.

Characterization of an extracellular subtilisin protease of Rhizopus microsporus and evidence for its expression during invasive rhinoorbital mycosis

An endoprotease Arp (alkaline Rhizopus protease) was identified and purified to virtual homogeneity from the culture supernatant of an isolate of Rhizopus microsporus var. rhizopodiformis recovered from a non-fatal case of rhinoorbital mucormycosis. N-terminal sequencing of the mature native enzyme was obtained for the first 20 amino acids and revealed high homology to serine proteases of the subtilisin subfamily. Arp migrated in SDS-PAGE with an estimated molecular mass of 33 kDa and had a pI determined to be at pH 8.8. Arp is proteolytically active against various substrates, including elastin, over a broad pH range between 6 and 12 with an optimum at pH 10.5. After invasive mucormycosis, specific antibodies against Arp were detected in stored serum samples taken from the patient from whom the R. microsporus strain of this study had been isolated. Furthermore, in search of factors involved in thrombosis as a typical complication of mucormycosis, a procoagulatory effect of the enzyme has recently been shown. Altogether, these data substantiate the expression of Arp during human rhinoorbital mucormycosis and suggest a role of the enzyme in pathogenesis.

Invasive hyphal growth: An F-actin depleted zone is associated with invasive hyphae of the oomycetes Achlya bisexualis and Phytophthora cinnamomi

We have compared F-actin patterns in invasive and non-invasive oomycete hyphae. In Achlya bisexualis an F-actin depleted zone is present in 70% of invasive but only 9% of non-invasive hyphae. In Phytophthora cinnamomi these figures are 74 and 20%, respectively. Thus, the F-actin depleted zone appears to be associated with invasive growth. TEM images indicate that it is unlikely to represent areas of vesicle accumulation. Measurements of turgor indicate no significant increase under invasive conditions (0.65 MPa (invasive) and 0.63 MPa (non-invasive)). Similarly we found no difference in burst pressures (1.04 MPa (invasive) and 1.06 MPa (non-invasive)), although surrounding agarose may lead to overestimates of invasive tip strength. An F-actin depleted zone has the potential, along with wall softening, to increase protrusive force in the absence of turgor increases. Staining of F-actin in hyphae under hyperosmotic conditions suggests that decreases in F-actin at growing tips may also enable non-invasive growth at very low turgor.

Pectin and the role of the physical properties of the cell wall in pollen tube growth of Solanum chacoense

The cell wall is one of the structural key players regulating pollen tube growth, since plant cell expansion depends on an interplay between intracellular driving forces and the controlled yielding of the cell wall. Pectin is the main cell wall component at the growing pollen tube apex. We therefore assessed its role in pollen tube growth and cytomechanics using the enzymes pectinase and pectin methyl esterase (PME). Pectinase activity was able to stimulate pollen germination and tube growth at moderate concentrations whereas higher concentrations caused apical swelling or bursting in Solanum chacoense Bitt. pollen tubes. This is consistent with a modification of the physical properties of the cell wall affecting its extensibility and thus the growth rate, as well as its capacity to withstand turgor. To prove that the enzyme-induced effects were due to the altered cell wall mechanics, we subjected pollen tubes to micro-indentation experiments. We observed that cellular stiffness was reduced and visco-elasticity increased in the presence of pectinase. These are the first mechanical data that confirm the influence of the amount of pectins in the pollen tube cell wall on the physical parameters characterizing overall cellular architecture. Cytomechanical data were also obtained to analyze the role of the degree of pectin methyl-esterification, which is known to exhibit a gradient along the pollen tube axis. This feature has frequently been suggested to result in a gradient of the physical properties characterizing the cell wall and our data provide, for the first time, mechanical support for this concept. The gradient in cell wall composition from apical esterified to distal de-esterified pectins seems to be correlated with an increase in the degree of cell wall rigidity and a decrease of visco-elasticity. Our mechanical approach provides new insights concerning the mechanics of pollen tube growth and the architecture of living plant cells.

Characterization of 1,3-beta-glucanase and 1,3;1,4-beta-glucanase genes from Phytophthora infestans

Three putative exo-1,3-beta-glucanase genes (Piexo1, Piexo2, Piexo3), one endo-1,3-beta-glucanase (Piendo1) and one endo-1,3;1,4-beta-glucanase (Piendo2) gene were cloned and characterized from the oomycete Phytophthora infestans. Southern hybridization revealed that Piexo1, Piexo2, Piexo3, and Piendo2 are single copy, and that Piendo1 is encoded by two copies. Furthermore, the analyses showed that for each gene, one or two closely related gene family members were present. The genes contain no introns. Nucleotide sequence analysis of the promoter regions (200nt upstream of ATG start codon) showed that the regions have 56-81% similarity to a 16-nt core sequence hypothesized to be the initiation of transcription point in oomycetes. The predicted molecular weights (32-83kDa), iso-electric points (4.2-6.7) and amino acid sequences of the five proteins are diverse. All the genes are expressed in in vitro grown mycelia and sporangia, as well as during infection of potatoes. Further, Piendo1 and Piendo2 are also expressed in germinating cysts, and Piendo2 in zoospores.

Force and compliance: rethinking morphogenesis in walled cells

In the turgid cells of plants, protists, fungi, and bacteria, walls resist swelling; they also confer shape on the cell. These two functions are not unrelated: cell physiologists have generally agreed that morphogenesis turns on the deformation of existing wall and the deposition of new wall, while turgor pressure produces the work of expansion. In 1990, I summed up consensus in a phrase: "localized compliance with the global force of turgor pressure." My purpose here is to survey the impact of recent discoveries on the traditional conceptual framework. Topics include the recognition of a cytoskeleton in bacteria; the tide of information and insight about budding in yeast; the role of the Spitzenkörper in hyphal extension; calcium ions and actin dynamics in shaping a tip; and the interplay of protons, expansins and cellulose fibrils in cells of higher plants.

Force exertion in fungal infection

Fungal pathogens of plants or animals invade their hosts either by secretion of lytic enzymes, exerting force, or by a combination of both. Although many fungi are thought to rely mostly on lysis of the host tissue, some plant pathogenic fungi differentiate complex infection cells that develop enormous turgor pressure, which in turn is translated into force used for invasion. In order to understand mechanisms of fungal infection in detail, methods have been developed that indirectly or directly measure turgor pressure and force. In this article, these methods are described and critically discussed, and their importance in analysis of fungal infection are outlined.

Mechanics of solid tissue invasion by the mammalian pathogen Pythium insidiosum

The relative significance of mechanical penetration versus the action of substrate-degrading enzymes during solid tissue invasion has not been established for any fungal disease. Pythium insidiosum is an oomycete fungus (or stramenopile) that causes a rare, but potentially lethal infection in humans and other mammalian hosts. Experiments with miniature strain gauges showed that single hyphal apices of this pathogen exert forces of up to 6.9 microN, corresponding to maximum pressures of 0.3 microN microm(-2) or MPa. Samples of cutaneous and subcutaneous tissue from fresh human cadavers displayed a mean strength (resistance to needle puncture) of 24 microN microm(-2), and a mean pressure of 30 microN microm(-2) was necessary to penetrate skin strips from slaughtered horses. These experiments demonstrate that P. insidiosum does not exert sufficient pressure to penetrate undamaged skin by mechanics alone, but must effect a decisive reduction in tissue strength by proteinase secretion.

Molecular cloning and targeted deletion of PEP2 which encodes a novel aspartic proteinase from Aspergillus fumigatus

An aspartic proteinase PEP2 [EC 3.4.23.25] was purified from a cell wall fraction of Aspergillus fumigatus. The enzyme, which showed a broad range of activity from pH 2.0 to 7.0 and migrated as a single band of 39 kDa in SDS-PAGE, was not detected in the culture supernatant. A specific gene probe was designed on the basis of the N-terminal sequence of the native protein, and the PEP2 genomic and cDNA were isolated from corresponding libraries. The deduced amino acid sequence of PEP2 consists of 398 amino acids. A signal sequence of 18 amino acids and a proregion of another 52 amino acids were identified. The mature protein consists of 328 amino acids which include the two DTG-motifs of the active site common to almost all pepsin-like enzymes. PEP2 showed a 64% identity with the vacuolar proteinase A (PrA), of Saccharomyces cerevisiae, and an 88% identity with PEPE, an aspartic proteinase of Aspergillus niger. Recombinant PEP2 was overexpressed in Pichia pastoris and the active enzyme was secreted into the culture supernatant. Targeted deletion of PEP2 did not affect vegetative growth or cell and colony morphology. Identification of proteinases, such as PEP2, which are apparently associated with the Aspergillus cell wall raises new interest in these molecules with respect to their possible function in the pathogenesis of invasive aspergillosis.

Mechanisms of hyphal tip growth: tube dwelling amebae revisited

Over 100 years ago, Reinhardt suggested that hyphal tip growth is comparable to ameboid movement inside a tube; the apical cytoplasm being protruded like a pseudopodium with the wall assembled on its surface. There are increasing data from hyphae which are explicable by this model. Fungi produce pseudopodia-like structures and their cytoplasm contains all of the major components implicated in pseudopodium production in animal cells. Most of these components are concentrated in hyphal tips and tip growth involves actin, a major component of pseudopodia. Together these data indicate that the essence of the ameboid model is still tenable. However, detailed mechanisms of tip growth remain too poorly known to provide definitive proof of the model and the behavior of the trailing cytoplasm indicates differences which are probably a response to the walled lifestyle.

Glucans, walls, and morphogenesis: On the contributions of J. G. H. Wessels to the golden decades of fungal physiology and beyond

This is a collection of impressions on the career of J. G. H. Wessels and his work in the areas of cell wall metabolism and apical morphogenesis. It highlights the finding of massive cell wall glucan metabolism during differentiation, the discovery of covalent linkages between wall polymers, the changes in chemical and physical properties of the wall at the fungal apex, and the steady-state model for tip growth. A tandem VSC-SS model for hyphal morphogenesis is proposed that combines the spatial control of wall synthesis provided by the vesicle supply center model with the temporal regulation intrinsic in Wessels's steady-state model.

Evolution of spore release mechanisms in the saprolegniaceae (Oomycetes): evidence from a phylogenetic analysis of internal transcribed spacer sequences

Classical studies on spore release within the Saprolegniaceae (Oomycetes) led to the proposition that different mechanisms of sporangial emptying represent steps in an evolutionary transition series. We have reevaluated this idea in a phylogenetic framework using internal transcribed spacer sequences of four genera. These data were compared with the response to osmotic stress exhibited by each taxon. Saprolegnia emerges as the most basal genus, sister to Achlya, Thraustotheca, and Dictyuchus. Achlya and Thraustotheca are most closely related, while Dictyuchus appears to have evolved along a separate evolutionary lineage. The resulting phylogenetic framework is consistent with the idea that the mechanism of sporangial emptying exhibited by Saprolegnia represents the plesiomorphic condition from which the other mechanisms were derived independently. These alternative mechanisms of spore release may have resulted from a small number of mutations that inhibited axonemal development and altered the temporal and spatial expression of lytic enzymes that degrade the sporangial wall. Copyright 1998 Academic Press.